The Hefty First Step

Lifting your rocket from Terra's surface into circular orbit takes an unreasonably large amount of delta V. As a matter of fact, if your missions use Hohmann trajectories, the lift-off portion will take about the same delta V as does the Hohmann from Terra to the destination planet. As Heinlein put it:

Mr. Heinlein and I were discussing the perils of template stories: interconnected stories that together present a future history. As readers may have suspected, many future histories begin with stories that weren't necessarily intended to fit together when they were written. Robert Heinlein's box came with "The Man Who Sold the Moon." He wanted the first flight to the Moon to use a direct Earth-to-Moon craft, not one assembled in orbit; but the story had to follow "Blowups Happen" in the future history.

Unfortunately, in "Blowups Happen" a capability for orbiting large payloads had been developed. "Aha," I said. "I see your problem. If you can get a ship into orbit, you're halfway to the Moon."

"No," Bob said. "If you can get your ship into orbit, you're halfway to anywhere."

He was very nearly right.

From A Step Farther Out by Jerry Pournelle (1979)

How much delta V does it take to go from Low Terra Orbit to Mars orbit? About 5.6 kilometers per second.

How much delta V does it take to go from the surface of Terra to Low Terra Orbit? 7.6 Freaking kilometers per second, that's what! In other words it takes more delta V to travel the pathetic 360 kilometers up to Low Terra Orbit as it does to travel the 228,000,000 kilometers to Mars!

From Low Terra Orbit, where can you travel to with 7.6 km/s? Oh, only to the Planet Saturn, 1,433,000,000 kilometers towards the edge of the entire solar system.


But the delta V cost breakdown is interesting. Getting into orbit takes just a little bit of delta V. It is making sure you stay in space that takes a freaking lot of delta V.

A little sounding rocket can easily rise from 50 to 1,500 kilometers above Terra's surface, where outer space starts about 150 kilometers up. Then the propellant runs out, and the poor little rocket finds itself unsupported hundreds of kilometers up. So it plummets to its doom.

How do you support the sad little rocket? If it uses propellant it will eventually run out, sooner more than later. You can't build rocket legs that are hundreds of kilometers long. You can't use a helicopter blade because there is no air.

But what you can do is put the rocket in an "orbit". An orbit is a clever way to constantly fall but never hit the ground. The trouble is that entering an orbit takes a freaking lot of delta V, about 8 kilometers per second around Terra.

Of course, once you have torchships you can stop all this child's play with wimpy Hohmann transfers and start doing some big muscular Brachistochrone trajectories. Brachistochrones typically require delta Vs that are hundreds of times more than the equivalent Hohmann. So any ship that can handle a Brachistochrone is not going to even notice the delta V cost for lift-off.

But even with torchships, the real bottle-neck restricting developing space resources remains the cost to boost payloads into Earth orbit.

For some cold hard reality read When Rocket Science Meets The Dismal Science.

SPACE JOCKEY

The traveling-public gripes at the lack of direct Earth-to-Moon service, but it takes three types of rocket ships and two space-station changes to make a fiddling quarter-million-mile jump for a good reason: Money.

The Commerce Commission has set the charges for the present three-stage lift from here to the Moon at thirty dollars a pound. Would direct service be cheaper? A ship designed to blast off from Earth, make an airless landing on the Moon, return and make an atmosphere landing, would be so cluttered up with heavy special equipment used only once in the trip that it could not show a profit at a thousand dollars a pound! Imagine combining a ferry boat, a subway train, and an express elevator. So Trans-Lunar uses rockets braced for catapulting, and winged for landing on return to Earth to make the terrific lift from Earth to our satellite station Supra-New York. The long middle lap, from there to where Space Terminal circles the Moon, calls for comfort-but no landing gear. The Flying Dutchman and the Philip Nolan never land; they were even assembled in space, and they resemble winged rockets like the Skysprite and the Firefly as little as a Pullman train resembles a parachute.

The Moonbat and the Gremlin are good only for the jump from Space Terminal down to Luna . . . no wings, cocoon-like acceleration-and-crash hammocks, fractional controls on their enormous jets.

From SPACE JOCKEY by Robert Heinlein (1949)

There are other ways besides rocket boosters and space shuttles to get payloads into orbit. These might take the form of rockets climbing rails set up the side of a mountain, a laser thermal launching facility (in THE MILLENNIAL PROJECT, Marshall Savage calls this a "Bifrost Bridge", that is, a bridge to space composed of colored light), launching loops, space fountains or the base of a Space Elevator.

FUNDAMENTAL COST OF PUTTING STUFF IN ORBIT

What is the minimum energy of orbit, and how does that compare to the energy in a chemical rocket’s propellant?

Accessing a 150km LEO orbit requires first the energy to get to 150km. That’s roughly (in Energy/mass, or J/kg, aka m^2/s^2, the unit I’ll mostly use here): 150km*9.8m/s^2.

Orbital velocity at 150 km altitude is just v=sqrt(mu/a), where the distance from the center of the Earth a = r_Earth + 150km. Mu is the “standard gravitational parameter” of Earth, or ~3.986*10^14 m^3/s^2.

(BTW, I’ll write numbers like 3.986*10^14 in a more compact notation: 3.986E14.)

So v= sqrt(3.986E14m^3/s^2/(r_Earth+150km)) = 7814m/s ( here is the google calculation: https://www.google.com/webhp?q=sqrt(3.986E14m^3/s^2/(r_Earth%2B150km)) ).

But we can minus the speed from the rotation of the Earth: v= sqrt(3.986E14m^3/s^2/(r_Earth+150km)) – 2*pi*r_Earth/day

Now we need to make this in terms of energy in order to add that potential energy from being 150km high:
E_specific (energy/mass) = .5*(sqrt(3.986E14m^3/s^2/(r_Earth+150km)) – 2*pi*r_Earth/day) + 150km*9.8m/s^2

Which is roughly: 28,480,000 m^2/s^2 or 28.5MJ/kg. That’s 7.9kWh/kg or just under $1 per kg to LEO at typical 10-12 cents per kWh.
And in terms of delta-v, it’s: v = sqrt(2*E) = 7550m/s or so.

That’s zero aero or gravity drag, launching due East on the equator. Imagine a 150km tall tower with a 100% efficient electromagnetic launch mechanism on the top, including the energy required to lift stuff up that tower and assuming no energy loss from the sled, no mass for the encapsulating of the payload, and 100% efficiency for electromagnetic launch. None of these are realistic assumptions.

Let’s compare with chemical launch. Assume a hypothetical stoichiometric methane/oxygen rocket engine operating at 3.7km/s exhaust velocity. This is very aggressive (especially at sea level), would probably melt the engine due to operating stoichiometrically, but it may actually be possible.

A stoich methane/oxygen mix, with methane having 55.5MJ/kg specific energy and the mix having 11.1MJ/kg, would have a theoretical exhaust velocity, if you totally convert chemical energy to jet energy, of 4.712km/s, so 3.7km/s isn’t physically impossible in the least (would be feasible in vacuum, but would require incredibly high pressures at sea level).
Anyway, let’s assume a mass ratio of, say, 25 for each stage. Let’s assume a 100 ton payload. The first stage weighs 120 tons dry (25 times that wet), and the next stage 10 tons dry (etc). That gets us 9km/s delta-v, which we’ll say is good enough, launching on the equator due East to 150km altitude.

Work: 3.7*ln((25*120+(25*10+100))/((25*10+100)+120)+3.7*ln((25*10+100)/(100+10)

We assume the dry mass magically can be recovered at no mass penalty (I will address this in another post…).

Mass of the propellant is: 120*24 + 10*24 = 3120 tons. Or 31.2 kg of propellant per kg to orbit. At 11.1MJ/kg, that’s 346MJ/kg of chemical energy in the form of methane. Natural gas is about $0.30 per therm in bulk. A therm is about 105MJ. So the cost of chemical energy to put stuff in orbit via chemical rocket like I described is actually ALSO $1/kg, and with arguably more realistic (though also aggressive) assumptions.

Moral of the story: It’s not, and never ever has been, about the cost of energy to get to orbit. Such arguments are flawed.

Launch Sites

The two main types of orbit that launch vehicles boost payload into are equatorial orbits and polar orbits.

Polar orbits pass over both the north and south poles, with an inclination close to 90 degrees with respect to the equator. But the important point is a satellite in polar orbit will eventually pass over every single spot on Terra. Heinlein calls these "ball of yarn" orbits, since the path of the satellite resembles wrapping a strand of yarn around a yarn ball. This is why such orbits are used for Earth-mapping, Earth-observation, some weather satellites … and reconnaissance satellites aka "spy" satellites.

For communication satellites, space stations, resupply missions, space exploration, and pretty much everything else, you launch into equatorial orbits.


LAUNCH CORRIDOR

When deciding where to put a launch site, you have to plan around the Launch Corridor. This is the path the rocket will take when launching which will [1] allow the rocket to reach the desired orbit and [2] if the rocket engines fail, the rocket (or the remaining flaming rocket debris) will only fall on uninhabited areas as long as it stays inside the launch corridor. The standard practice is to arrange launch corridors to be over the ocean. Failing that, you need land areas where a rain of flaming rocket bits is unlikely to result in lawsuits or negative publicity. And of course ones that do not violate another nation's sovereign airspace.

During launch, the range safety officer will be watching the rocket like a hawk. If the rocket shows signs of failing to reach orbit, the officer will make a note to dispatch a rescue/cleanup team. If the rocket shows signs of leaving the launch corridor, the officer will hit the panic buttons. Unmanned rockets will shutdown their engines and vent their propellant. Manned rockets will have the on-board pilot take action, but if they are ineffective the range safety officer might have to shoot the rocket out of the sky.


Obviously polar launch corridors have to be along the north-south axis.

The United States uses Vandenberg AFB Space Launch Complex 6 (SLC-6 aka "Slick Six") to launch into polar orbits. Rockets launch due south so the launch corridor is thousands of miles of uninhabited Pacific ocean. The alternative is to launch due north, but that puts the launch corridor right across California, the long way.


EQUATOR BOOST

Equitorial launches have a second consideration besides the launch corridor.

When you are dealing with feeble launch vehicles using chemical propulsion you need to use every trick you can find. They have grotesque mass ratios which really cut into the payload mass. The most important trick is one to reduce the delta V the rocket needs to achieve orbit.

Since Terra is spinning on its axis, when the rocket is sitting on the ground it is actually already moving. At least it is moving relative to the desired orbit, which is the important thing. If you are standing in New York City; you, the ground, the skyscrapers, the taxi cabs, and everything else is moving at 356 meters per second. The only reason everything seems stationary is because everything is moving together. Now remember that on Terra everything is moving due east because that is the direction Terra is spinning on its axis.

The technical term is the tangential velocity of Terra's surface. It is equal to

tangentialVelocity = ((2 * π * planetRadius) / siderialRotation) * cos(latitude)

where

tangentialVelocity = tangential velocity at planet surface (m/s) (Terra = 465 m/s)
π = pi = 3.14159…
planetRadius = radius of the planet (meters) (Terra = 6,371,000)
siderialRotation = siderial rotation period (seconds) (Terra = 86,164 seconds, which is actually 23 hours, 56 minutes, 4 seconds)
cos(x) = cosine of x (do not make the mistake of giving your spreadsheet or calculator "x" in degrees when it is expecting radians or something)
Example

What is the tangential velocity at planet surface for New York, Terra?

tangentialVelocity = ((2 * π * planetRadius) / siderialRotation) * cos(latitude)
tangentialVelocity = ((2 * 3.14159 * 6,371,000) / 86,164) * cos(40°)
tangentialVelocity = (40,030,139.78 / 86,164) * 0.766
tangentialVelocity = 465 * 0.766
tangentialVelocity = 356 m/s

What is the tangential velocity at planet surface for a location on Terra's equator?

tangentialVelocity = ((2 * 3.14159 * 6,371,000) / 86,164) * cos(0°)
tangentialVelocity = 465 * 1.0
tangentialVelocity = 465 m/s

I gave you the entire equation in case you wanted to do the calculations for an extraterrestrial planet. If you are just trying to place launch sites on Terra, the equation is:

tangentialVelocity = 465 * cos(latitude)


The point is that the delta V the launch vehicle needs to achieve orbit is reduced by the tangential velocity of the launch site. Bottom line is the closer you can put the launch site to the equator, the better.

For Terra, the pure orbit delta V is about 9,700 m/s (would be 7,800 m/s except for air-drag, gravity-drag, and vertical acceleration). But when launching from New York the delta V is only 9,700 - 356 = 9,344 m/s. And launching from the equator it is 9,700 - 465 = 9,235 m/s. That kind of delta V reduction can buy you lots of extra payload.

Keep in mind that since Terra is spinning due east, the rocket has to launch in an easterly direction in order to take advantage of the bonus. By the same token, if the stupid rocket launches west, the bonus turns into a liability. Launching westward on Terra's equator means the rocket needs an additional 465 m/s to reach orbit.

The important point is that on Terra the equatorial launch corridor is going to point due east.


The better science fiction novels put Terran equatorial launch sites as close to the equator as possible, and where an eastward launch corridor passes over lots of ocean (i.e., on the east coast, near the equator).

  • The North Maluku province of Indonesia has parts right on the equator. It has pretty much the entire Pacific Ocean to use as a launch corridor, except only scattered tiny islands in the launch corridor. Possible launch site.

  • There is a part of the coast of Brazil that is right on the equator. It has pretty much the entire Atlantic Ocean to use as a launch corridor. Possible launch site.

  • Parts of the Galápagos Islands are right on the equator. Unfortunately it only has 906 km of Pacific Ocean launch corridor before flaming rocket bits start raining down on Ecuador. Possible launch site.

  • In ARTEMIS by Andy Weir the launch site is in Kenya, with parts right on the equator. It has pretty much the entire Indian Ocean to use as a launch corridor. However, the part closest to the equator that does not include Somalia in the launch corridor is located at 1.7° S latitude.

  • In ISLANDS IN SPACE by Arthur C. Clarke the launch site is at New Guinea, with point closest to equator at about 2.6° S latitude. It has pretty much the entire Pacific Ocean to use as a launch corridor, except for the Solomon Islands.

  • The real world Guiana Space Centre in French Guiana is at about 5° N latitude. It has pretty much the entire Atlantic Ocean to use as a launch corridor.

  • Palmyra Atoll is at about 5° N latitude. It has pretty much the entire Pacific Ocean to use as a launch corridor. And it is a US unorganized incorporated territory. Drawbacks include it is pretty much on the opposite side of Terra from the continental US so that logistics is a nightmare, and the highest point is (currently) only 10 meters above sea level.

  • The US Virgin Islands are at about 17.7° N latitude. It has pretty much the entire Atlantic Ocean to use as a launch corridor. Possible launch site.

  • In High Justice by Jerry Pournelle the launch site is at Cabo San Lucas, Mexico. It is at an unhelpful 22.8° N latitude. And it only has 390 kilometers of launch corridor.

  • The real world Kennedy Space Center Launch Complex 39 is at an ugly 28.5° N latitude. But the United States does not get that much closer to the equator. It has pretty much the entire Atlantic Ocean to use as a launch corridor.

  • The real world Baikonur Cosmodrome is at an almost utterly worthless 45.6° N latitude. What's worse it it has to launch at a 51.6° inclination, since China takes a very dim view of being in the launch corridor. Sadly Baikonur is probably located at the best out of Russia's poor selection of launch sites.


THE CURIOUS CASE OF THE ISS INCLINATION

Now one would have expected that the International Space Station (ISS) would be in a 28.5° inclined orbit, which is the orbit you get when launching due East of Kennedy Space Center (latitude 28.5° N).

But it isn't, the ISS is instead in a 51.6° inclined orbit. Why? So that Russian cargo rockets from Baikonur Cosmodrome can reach it. Launching into a different inclination than the space port's latitude costs rocket propellant and reduces payload.

Changing the ISS planned inclination to 51.6° was in retrospect a very good decision. When NASA stupidly cancelled the Space Shuttle program before the replacement vehicle was online, they assured everybody that the replacement would be flying by 2014 at the latest. This would make a small three-year gap in NASA's ISS transport ability. Unfortunately and predictably when 2014 arrived NASA has not even started work on deciding which of the many proposals will be used, much less bending metal and cranking out functional rockets. This leaves NASA at the mercy of the Russians for access to the ISS, but without the Russians there would be no access at all and the station would have long ago burnt up in reentry like Skylab. But I digress.

Clever readers will say but wait! Baikonur Cosmodrome is at latitude 45.6°, should not that be the inclination?. In a perfect world, yes, but there is a problem. When a spacecraft is launched from Kennedy Space Center the lower stages fall into the Atlantic Ocean. And if something goes really wrong, the entire spacecraft can abort and ditch into the ocean as well. If Baikonur Cosmodrome did the same thing, large spent lower stage boosters and/or huge flaming aborting Russian spacecraft would crash into Mainland China, and the political situation would rapidly deteriorate. To avoid that unhappy state of affairs, Russian spacecraft launched from Baikonur go at a 51.6° inclination, so falling rocket bits will miss China.

The Russians already have an annoying problem with the lack of warm-water ports for seagoing vessels. They really dislike having much the same problem with respect to space launches. Therefore they are in negotiations for launch privileges at the ESA's Guiana Space Centre, which is optimally located quite near the Equator and to the West of the Atlantic Ocean.

BRAZIL SPACEPORT

At 5.16 degrees north latitude, Kourou in French Guiana is about as close as you can get to lying directly on the Earth's equator — an almost ideal spot from which to launch a spaceship.

Emphasis on "almost."

To hear Brazil tell it, you see, there's actually an even better place to put a spaceport than Kourou: Namely, the Brazilian city of Alcantara. Located in the state of Maranhao, at 2.40 degrees south latitude, Alcantara is less than half as far away from the equator as Kourou, and Brazil would really like to convince space companies that this makes Alcantara more than twice as attractive a place from which to launch their rockets. (Additionally, like Kourou, Alcantara is conveniently located next to a large body of water to the east, into which falling rocket parts can safely splash down).

(ed note: Kourou tangential velocity = 463 km/s, Alcantara tangential velocity = 465)

Escolha-me! Escolha-me! ("Pick me! Pick me!")

Last year, Brazil made much this same argument to a group of representatives from U.S. space launch companies. Ranging from large to small in size, with names including Boeing and Lockheed Martin and even tiny rocket launcher Vector, American space launch companies made the trek to Brazil.

In an article last week describing Brazil's pitch to the space establishment, Reuters reports that Brazil is offering Alcantara as a budget-priced alternative to launching from Kourou (a favorite launch site of Airbus subsidiary Arianespace). According to the Brazilians, launching from Alcantara can save North American space launch companies as much as "one third" on their fuel costs. This is because such launches get a bigger speed boost from the Earth's rotation, which is fastest at the equator and slows progressively as one moves north or south from it.

Still, it's not entirely clear what launch site Brazil picked as its "control" when making cost comparisons — perhaps the Kodiak Launch Complex in Alaska, from which Vector, for example, plans to make its first orbital launch later this year. In contrast, relative to launches from better-known, more active spaceports such as Cape Canaveral, Vector CEO Jim Cantrell estimates that launching from Alcantara could yield fuel savings closer to 10% to 15%.

Location, location, location

Will 33% savings — or 15%, or 10% — be enough to convince space companies to shift their business south from Florida to Brazil? Maybe not, if that were the only advantage. But as Cantrell told me in a phone conversation over the weekend, there are other advantages to launching close to the equator on top of simple fuel savings.

Because of quirks in the ballistics of missile launch, Cantrell explains, there are certain orbits that simply cannot be achieved with ease when launching from high latitudes but can be easily achieved when launching from near the equator. Over the course of the next five years, the U.S.-based Space Enterprise Council estimates we could see as many as 600 small rocket launches take place. The more satellites go up, the more satellites can be expected to slot into orbits easier-reached from equatorial launching points. This could help Alcantara capture as much as 25% of the market, says the Council.

Caveats and provisos

In order for U.S. space companies like Boeing, Lockheed, and Vector to take advantage of Alcantara's offerings, however, a couple of obstacles must be overcome. First and foremost, a technology safeguards agreement (TSA) must be signed, whereby the United States State Department certifies Brazil's ability to protect U.S. "launching and operating technologies."

Brazil is hoping to finalize such an agreement this year, permitting development of an Alcantara spaceport to move forward, and chances of that look good. After all, the U.S. already approved such a TSA once before, in 2000 — but Brazil's own Senate refused to ratify it. Presumably, if the political will is now present in Brazil to move forward, inking a new deal shouldn't be difficult.

Another issue is cost — not the cost of rocket fuel this time, but of diesel fuel. Alcantara is, after all, more than 3,200 miles distant from Cape Canaveral, which is a long lateral detour to ask companies to make just to lift their sats a few hundred miles above the Earth's surface! Brazil's distance would add significantly to shipping costs for any rockets (and satellites) that U.S. companies want to build in the U.S. and ship to Brazil for launch.

Granted, such factors don't prevent Airbus from using French Guiana as its preferred launch site, but it's still a factor to consider when gauging Brazil's chances of success. Cantrell estimates that shipping one of its small Vector-R rockets from the U.S. to Brazil might cost anywhere from $200,000 to $300,000 — a significant percentage of the company's estimated $1.5 million cost to put a small satellite in orbit.

Even if Alcantara turns out to be economically inadvisable for U.S. space launch companies, however, there's always the possibility that Brazil will try to use it as a springboard to launch its own space industry. As Reuters notes, Brazil is already working to develop a small rocket of its own, and intends to fly it out of Alcantara when ready — perhaps as early as next year.

If and when that happens, expect the market for space launch to become even more crowded.

Boosters: Present and Proposed

For comparison purposes, here are the masses of a few sample payloads. This is to give you a mental image of the capabilities of the following booster systems.

Sample Payloads

PayloadMass
GPS satellite0.8 metric ton
Communication satellite1 metric ton
Weather satellite1 metric ton
Hubble Space Telescope11 metric tons
KH-11 spy satellite13 metric tons
TransHab habitat module34 metric tons
Skylab77 metric tons
Space Station Mir124 metric tons
International Space Station287 metric tons
1 gW Solar Power Satellite1,900 metric tons
Lunar Mass Driver2,750 metric tons
Lunar Base (150 crew)17,050 metric tons
10 gW Solar Power Satellite19,000 metric tons
5 gW Solar Power Satellite (Rockwell International estimate)37,000 metric tons
2001 Space Odyssey Station V145,000 metric tons
1 tW Solar Power Satellite1,900,000 metric tons
1.5 tW Solar Power Satellite2,800,000 metric tons
L5 Colony10,000,000 metric tons

Existing Heavy Lift Launch Vehicles

"Heavy Lift" is defined as 12 metric tons or more into LEO.

Heavy Lift Launch Vehicle (HLLV)Payload mass delivered to LEOCost per payload kilogram
Zenit 2 (Ukraine)13.7 metric tons$3,093/kg
Zenit 3SL (Sea Launch)15.9 metric tons$5,354/kg
Japan H2B16.5 metric tons?/kg
Ariane 5G (ESA)18 metric tons$9,167/kg
Atlas V 55118.51 metric tons?/kg
Ariane 5 ES (ESA)20 metric tons?/kg
Titan IV-B21.69 metric tons?/kg
Falcon 9 v1.2 (SpaceX)22.8 metric tons$2,720/kg
Delta IV Heavy (ULA)28.79 metric tons?/kg
Proton-M (Russia)23 metric tons$4,302/kg
Space Shuttle (NASA)24 metric tons$10,416/kg
Falcon Heavy (SpaceX)63.8 metric tons$2,968/kg
Saturn V (NASA)118 metric tons??

Proposed STO Solutions

SystemPayload mass delivered to LEOCost per payload kilogram
The Rocket Company DH-12.2 metric tons$440/kg
SASSTO2.8 metric tons$11/kg (1968 dollars)
Collier's space ferry25 metric tons??
Star-Raker91 metric tons$22/kg to $33/kg
Nuclear DC-X100 metric tons$150/kg
Rombus450 metric tons$2.30 to $5.40/kg (1964 dollars)
Sea Dragon550 metric tons$59/kg to $600/kg
GCNR Liberty Ship1,000 metric tons??
Uprated GCNR Nexus1,500 metric tons??
Space Elevator x12,000 metric tons/year$3,000/kg
Planetary Orion3,000 metric tons??
Laser Launch (HX)3,000 metric tons/year$550/kg
Space Elevator x24,000 metric tons/year$1,900/kg
Super Nexus4,600 metric tons??
Space Elevator x36,000 metric tons/year$1,600/kg
Aldebaran27,000 metric tons??
Lofstrom loop small40,000 metric tons/year$300/kg
Rocket Sled (StarTram)150,000 metric tons/year$43/kg
Bifrost Bridge175,200 metric tons/year$20/kg
Verne Gun280,000 metric tons??
Lofstrom loop large6,000,000 metric tons/year$3/kg
Super Orion8,000,000 metric tons??

The Rocket Company DH-1

Payload mass delivered to LEOCost per payload kilogram
2.2 metric tons$440/kg

The DH-1 is a fictional two stage to orbit re-useable rocket described in the book The Rocket Company (ISBN 1-56347-696-7). There are some sample chapters here. I recommend this book.

While the design is fictional, it would actually work. The authors have patented it. The small payload means the rocket is intended more for "space access" instead of heavy lift to orbit. The business model for the developers was more to sell the rockets (at an attractive price of $250 million) rather than selling cargo boost services.

There are DH-1 plug-ins for the spacecraft simulation Orbiter.

SASSTO

Payload mass delivered to LEOCost per payload kilogram
2.8 metric tons$11/kg (1968 dollars)
SASSTO
Gross Mass97,976 kg
Empty Mass6,668 kg
LEO Payload2,812 kg
Thrust (vac)1,558,100 N
Specific
Impulse
464 s
Diameter6.6 m
Length18.8 m
EngineChemical
Plug-nozzle
LOX/LH2
Num Engines36

The Saturn Application Single-Stage-to-Orbit (SASSTO) is from Frontiers of Space by Philip Bono and Kenneth Gartland (1969)

In 1966 when winged space shuttle designs were being studied, the Douglas Aircraft Company was doing a cost-benefit analysis. They were comparing reusable space shuttle costs to throwaway two-stage ballistic boosters. Somewhere along the line they took a look at whether it was possible to make a reusable single stage ballistic booster. The SASSTO was the result. The payload was not much, but it was enough for a Gemini space capsule. A Gemini would transform the SASSTO into a space taxi or even a space fighter, capable of satellite inspection missions. Without the Gemini it could deliver supplies and propellant to space stations and spacecraft in LEO.

Bono pointed out how inoperative satellites could become space hazards (although the concept of the Kessler Syndrome would not be created until 1978). A SASSTO could deal with such satellites in LEO (Bono called this Saturn Application Retrieval and Rescue Apparatus or SARRA). Even better, such satellites could be grabbed and brought back to Terra for refurbishment and re-launch. This would be much cheaper than building an entire new satellite from scratch, which would interest satellite corporations. Only satellites in LEO though, communication satellites in geostationary orbit would be out of reach.

The interesting part was on the base. Conventional spacecraft trying to do an aerobraking landing need a large convex heat shield on the base (for example the Apollo command module.). Unfortunately a reusable spacecraft has a large concave exhaust nozzle on the bottom, exactly the opposite of what you want. Tinsley's artist conception for the "Mars Snooper" had petals that would close over the exhaust nozzle sticking out of the heat shield, but that was impractical.

Douglas' solution was to use an aerospike engine with the spike truncated (which they confusingly call a "plug nozzle", contrary to modern terminology). The truncated part became the heat shield, the untruncated part around the edge was the aerospike engine.

Collier's space ferry

Payload mass delivered to LEOCost per payload kilogram
25 metric tons??

Nuclear DC-X

Payload mass delivered to LEOCost per payload kilogram
100 metric tons$150/kg

This is from a report called AFRL-PR-ED-TR-2004-0024 Advanced Propulsion Study (2004). It is a single stage to orbit vehicle using a LANTR for propulsion.

Star-Raker

Payload mass delivered to LEOCost per payload kilogram
91 metric tons$22/kg to $33/kg

Star-Raker is from a 1970's Rockwell International study, one of the many proposals on how to boost into orbit the outrageous payload requirements of a multi-kilometer solar power satellite (SPS). They were figuring on about 37,000 meric tons per SPS, and they wanted a constellation of 60 of them. For the project they estimated boosting 74,000 metric tons per year (2 SPS/year).

Star-Raker was a single-stage-to orbit airbreathing horizontal takeoff and landing craft (HTO-SSTO). The gross mass would be about 2,268 metric tons, the payload mass was about 91 metric tons, and it was claimed it would have a boost turnaround time of about a day and be really really cheap. Keeping in mind that at the time Rockwell was also claiming that the Space Shuttle would have a two-week turnaround and be really really cheap, which turned out to be somewhere between irrationally optimistic and an assurance from a used-car dealer. It was to be capable of delivering its payload into a 550 kilometer equatorial orbit.

To manage the proposed schedule of boosting the payload for two SPS per year would need about 815 flight per year, or 2.2 flights per day. This assumes a fleet of more than one Star-Raker.

Horizontal takeoff and landing, and single-stage were design choices due to the need for rapid turnaround. Having to fish stages out of the ocean, haul them to the launch site, refurbish, and re-stack them would make it impossible to have a single-day turnaround. To save mass the take-off wheels would be jettisoned at the end of the runway and recovered. For landing lighter internal landing gear is used, since by then the craft will be lighter by many metric tons of absent payload and burnt fuel.

It has a "wet-wing" design, that is, the wing is the fuel tank. The body of the craft is reserved for the payload. It was to be capable of taking off and landing on a 2,500 meter runway.

It is an air-breather using atmosphere for oxidizer up to the point where the air is too thin at thirty kilometers altitude (ten supersonic-turbofan/airturbo-exchanger/ramjet engines with a combined thrust of 6.2×107 newtons thrust). For the last portion of the boost it switches over to rocket engines (three rockets with 1.4×107 newtons thrust each). The jet engine air inlets will be closed by retractable ramps while the craft is under rocket flight and during ballistic re-entry. From zero to 1,800 m/s it will be using airbreathing propulsion, from 1,800 to 2,200 m/s it will use both airbreathing and rocket propulsion, and from 2,200 m/s to orbit it will use only rocket propulsion.

It would also be capable of making trips as a conventional cargo aircraft. For instance, from the launch site to a site where the payload had been assembled, and back to the launch site. It saves on having to ship the payload to the launch site, but I question the wisdom of risking an expensive HTO-SSTO craft when a less expensive and more expendable cargo plane would suffice. The entire nose (including crew compartment) swings open to expose the cargo hatch (which must be scary for the crew when the playload is released into orbit). This allows it to be loaded from a conventional cargo platform. Cargo floor is designed similar to a C5-A military transport aircraft.

There was another design tailored for delivering payload into polar orbits, which would reduce the payload mass. Polar orbits are expensive in terms of delta V, but are necessary for Department of Defense spy satellites.

Report can be found here.

Rombus

Payload mass delivered to LEOCost per payload kilogram
450 metric tons$2.30 to $5.40/kg
(1964 dollars)
Rombus
Gross mass6,363,000 kg
Payload450,000 kg
Height29 m
Diameter24 m
Thrust79,769,000 N
EngineChemical
Plug-nozzle
LOX/LH2
Specific Impulse455 s
Num nozzles×36

The Reusable Orbital Module-Booster & Utility Shuttle (ROMBUS) is from Frontiers of Space by Philip Bono and Kenneth Gartland (1969). This is a reusable plug-nozzle powered booster. It used an aerospike engine with the spike truncated and turned into an aerobraking heat shield.

Bono also created a passenger carrying variant named Pegasus, and a military troop carrier called Ithacus. When the concept lost support at NASA, Philip Bono designed a more modest concept, adding an aerospike engine to a Saturn V to create the SASSTO concept.

The vehicle is staged in the sense that it jettisons external hydrogen fuel tanks during the ascent phase. The tanks have parachutes to increase the chance they can be reused.

After delivering its payload, the vehicle would typically spend 24 hours in orbit before the ground track passes close enough to the landing site. It lands using parachutes and rockets, with the final touchdown burn delivered by four engines running at 25% thrust for twelve seconds. The vehicle turnaround time would be about 76 days.



1. Payload 0.8 to 1.0 million pounds to orbit
2. Roll-control nozzle pairs
3. Vent lines for liquid hydrogen tanks (8)
4. Propellant utilization probes (8)
5. Booster centre body
6. Fuel tank support fittings (16)
7. guidance and electronic package
8. Attitude-control propellant tanks
9. Spherical oxidizer tank
10. Anti-slosh baffles
11. Fuel feed lines (18)
12. Quick-disconnect fittings (8)
13. Propellant turbopumps (18)
14. Peripherally arranged combustion chambers (36)
15. Oxidizer feed lines (18)
16. Liquid hydrogen tank for entry cooling
17. Turbine discharge lines (18)
18. Turbine discharge port
19. Oxidizer-tank-pressurization helium bottles (4)
20. Propellant tank for retro-thrust
21. Isentropic-expansion plug nozzle
22. Retractable landing legs (4)
23. Regeneration-cooling tubes
24. Liquid Oxygen Tank sump
25. Solid motors for thrust augmentation (4)
26. Liquid hydrogen manifold
27. Fuel manifold valve for liquid hydrogen tanks (8)
28. Attitude-control propellant tanks (4)
29. Centrebody recovery components
30. Cylindrical liquid hydrogen fuel tanks (8)
31. Tank recovery thermal protection (4)

Sea Dragon

Payload mass
delivered to LEO
Cost per
payload kilogram
550 metric tons$59/kg to $600/kg
(1960 dollars)
Sea Dragon
(1963 design)
(recoverable)
Payload499,000 kg
Launch Cost$300,000,000
(1960 dollars)
Height150 m
Diameter23 m
Stages2
Stage 1
OxidizerLO2
FuelRP-1
Thrust360,000,000 N
Wet Mass12,799,000 kg
Dry Mass1,333,000 kg
ΔV1,800 m/s
Max Accel4.21 g
Stage 2
OxidizerLO2
FuelLH2
Thrust62,800,000 N
Wet Mass4,823,000 kg
Dry Mass465,000 kg
ΔV5,400 m/s
Max Accel5.2 g
Mass Budget
Payload499,000 kg
Stage 112,799,000 kg
Stage 24,823,000 kg
Total18,121,000 kg

Details here, here, and here. Most of the illustrations here (and the data block at left) are from NASA-CR-52817 and NASA-CR-51034.

Sea Dragon was designed by Robert Truax in 1962 to be a low-cost heavy lift launch vehicle. A "big dumb booster", emphasis on "big". To reduce costs for launch pads and gantries, the vehicle was to be launched from the ocean. It would be towed out to the watery launch site, and the ballast tank in the first stage exhaust nozzle would be flooded. This would drag the tail down and the nose up, orienting the rocket into launch position.

At 150 m long and 23 m in diameter, Sea Dragon would have been the largest rocket ever built. To lower the cost of the rocket itself, it was designed to be build of inexpensive materials, specifically 8 mm steel sheeting.

The contruction techniques would be quite different than modern-day rockets. The latter are horribly damaged if they are touched by sea water, especially rocket engines. This is why SpaceX goes to the trouble of landing their reusable rockets on robot barges instead of letting them splash down in the ocean.

The design ground rules mandated a minimum payload of 450 metric tons delivered to a 600 kilometer orbit. For the reusable version of the vehicle, a 10 year useful life for the system was assumed.

The Sea Dragon project was shut down by NASA in the mid-1960's due to budget cuts.

Nuclear Thermal Turbo

Payload mass
delivered to LEO
Cost per
payload kilogram
13 metric tons$13,000
(with zero re-use)
Nuclear Thermal
Turbo Rocket
Wet Mass72,600 kg
Propellant Mass35,700 kg
Inert Mass36,900 kg
Structural Mass7,260 kg
TSP Mass7,260 kg
Payload Mass13,000
Payload Fraction19%
Dry Fraction50.9%
Average Isp1,662 sec
NTR and shield7,080 kg
Air breathing2,270 kg

This is from The Nuclear Thermal Turbo Rocket: A Conceptual High-Performance Earth To Orbit Propulsion System by John R. Bucknell. John Bucknell was Senior Propulsion Engineer for the Raptor full-flow staged combustion methalox rocket at Spacex and is currently the Senior Propulsion Scientist for Divergent3D in Torrance, CA developing additively manufactured vehicle technologies. Slides from his talk are here.

Mr. Bucknell notes that the only practical method of dramatically bringing down the cost of boosting payloads into low Earth orbit (LEO) is to lower investment and realize a large return on that investment. The implication is you want a low dry mass Single Stage to Orbit Resuable Launch Vehicle with a high payload mass fraction. This is challenging.

Nuclear thermal rockets (NTR) have the highest specific impulse and thrust of available rockets. But the thrust-to-weight (T/W) ratio is poor since the blasted thing needs heavy radiations shielding. This really cuts into the payload fraction.

NERVA had a T/W of 5:1, particle bed had T/W of 15:1, and Miniature Reactor Engine (MITEE) managed 23:1. Unfortunately chemical LOX/RP-1 engines can achieve 150:1 easy.

Air-breathing propulsion has much higher specific impulse than NTR. But air-breathing propulsion don't work if there isn't any air. Long before LEO is reached the air pressure will drop below the level required for the air-breathing engine. Air breathers can only operate for the first 25% of the ascent, after that you need a rocket.

Therefore Mr. Bucknell's concept is to have a hybrid engine that can start in air-breathing hypersonic turbine mode and switch to NTR mode when the air runs out. This is called Nuclear Thermal Turbo Rocket (NTTR).


From Mach 0 to 8 the engine is in air-breathing subsonic ramjet mode. Combustion is subsonic. The nuclear rocket hot-hydrogen thrust is used to spin the fan rotor, driving the turbines. The hydrogen escapes via the trailing edge of the thrust fan blades. The turbine thrust fan blade vary their pitch and the variable nozzle throat geometry adapt to the changing atmospheric conditions. The turbine compresses the atmosphere from the inlet cone and the hydrogen from the thrust fan blades into the combustor, where they are burned for ramjet thrust.


From Mach 8 to 14 the engine is in air-breathing scramjet mode. Combustion is supersonic. The thrust fan blades lock into the neutral position aligned with the vehicle axis (depitches). The variable inlet cone expands, as does the PYBB variable nozzle.


From Mach 15 on up, the engine is in nuclear thermal rocket mode. The variable inlet cone contracts shut. The only thrust is rocket thrust from hot hydrogen escaping the trailing edge of the thrust fan blades.


GCNR Liberty Ship

Payload mass delivered to LEOCost per payload kilogram
1,000 metric tons??

Anthony Tate has an interesting solution to the heavy lift problem. In his essay, he says that if we can grow up and stop panicking when we hear the N-word a reusable closed-cycle gas-core nuclear thermal rocket can boost huge amounts of payload into orbit. He calls it a "Liberty Ship." His design has a cluster of seven nuclear engines, with 1,200,000 pounds of thrust (5,340,000 newtons) each, from a thermal output of approximately 80 gigawatts. Exhaust velocity of 30,000 meters per second, which is a specific impulse of about 3060 seconds. Thrust to weight ratio of 10. Engine with safety systems, fuel storage, etc. masses 120,000 pounds or 60 short tons (54 metric tons ).

Using a Saturn V rocket as a template, the Liberty Ship has a wet mass of six million pounds (2,700,000 kilograms). Mr. Tate designs a delta V of 15 km/s, so it can has powered descent. It can take off and land. This implies a propellant mass of 2,400,000 pounds (1,100,000 kilograms). Using liquid hydrogen as propellant, this will make the propellant volume 15,200 cubic meters, since hydrogen is inconveniently non-dense. Say 20 meters in diameter and 55 meters long. It will be plump compared to a Saturn V.

Design height of 105 meters: 15 meters to the engines, 55 meters for the hydrogen tank, 5 meters for shielding and crew space, and a modular cargo area which is 30 meters high and 20 meters in diameter (enough cargo space for a good sized office building).

A Saturn V has a dry mass of 414,000 pounds (188,000 kilograms).

The Liberty Ship has seven engines at 120,000 pounds each, for a total of 840,000 pounds. Mr. Tate splurges and gives it a structural mass of 760,000 pounds, so it has plenty of surplus strength and redundancy. Add 2,400,000 pounds for reaction mass, and the Liberty Ship has a non-payload wet mass of 4,000,000 pounds.

Since it is scaled as a Saturn V, it is intended to have a total mass of 6,000,000 pounds. Subtract the 4,000,000 pound non-payload wet mass, and we discover that this brute can boost into low earth orbit a payload of Two Million Pounds. Great galloping galaxies! That's about 1000 metric tons, or eight times the boost of the Saturn V.

The Space Shuttle can only boost about 25 metric tons into LEO. The Liberty Ship could carry three International Space Stations into orbit in one trip.

Having said all this, it is important to keep in mind that a closed-cycle gas-core nuclear thermal rocket is a hideously difficult engineering feat, and we are nowhere near possessing the abilty to make one. An open-cycle gas-core rocket is much easier, but there is no way it would be allowed as a surface to orbit vehicle. Spray charges of fissioning radioactive plutonium death out the exhaust nozzle at fifty kilometers per second? That's not a lift off rocket, that's a weapon of mass destruction. However, see the Nexus.

There is an interesting analysis of the Liberty Ship on Next Big Future.

Uprated GCNR Nexus

Payload mass delivered to LEOCost per payload kilogram
1,500 metric tons??

This is from some fragmentary circa 1964 documents uncovered by The Unwanted Blog.

A Convair concept for an all-chemical Nexus SSTO launch vehicle with a second stage using open-cycle gas-core nuclear thermal rockets. Presumably the designers thought that the chemical stage would loft the second stage high enough so that the twin plumes of incandescent radioactive death would be diluted into plausible deniabilty.

Super Nexus

Payload mass delivered to LEOCost per payload kilogram
4,600 metric tons??

This is from some fragmentary circa 1964 documents uncovered by The Unwanted Blog.

This monster is the Uprated GCNR Nexus grown to three times the size. The document says that it can deliver 453 metric tons not to LEO, but to Lunar orbit. Doing some calculations on the back of an envelope with my slide rule, I estimate that it can loft 4,600 metric tons into LEO. And also with a proportional increase in radioactive exhaust.

A bit over 122 meters tall with the second stage having a diameter of 37 meters. Total wet mass of 10,900 metric tons. Second (nuclear) stage wet mass 5,900 metric tons for the Lunar orbit configuration. Dry second stage at Lunar orbit has a mass of 450 metric tons. The LEO configuration will be different.

The chemical stage has a total delta V capacity of 2.4 km/s. The gas core engines have a specific impulse rating of 2,220 seconds. The gas core stage in Lunar orbit configuration has a total delta V capacity of 21.8 km/s.

Orion

TypePayload mass delivered to LEOCost per payload kilogram
Planetary3,000 metric tons??
Super8,000,000 metric tons??

Verne Gun

Payload mass delivered to LEOCost per payload kilogram
280,000 metric tons??

Brian Wang has come up with an innovative concept. He mulled over a couple of his articles from his blog The Next Big Future (specifically this one and this one) and had an idea. Remember that one of the best propulsion systems for boosting huge payloads into orbit is the Orion drive; were it not for the fallout, the EMP, and the Nuclear Test Ban Treaty.

Then Mr. Wang thought about Jules Verne's novel From The Earth To The Moon, and the giant cannon Columbiad.

You set off one solitary ten megaton nuclear device in a deep underground salt dome. Perched on top is an Orion type spacecraft. All the EMP and radiation is contained in the underground cave (as has been done with historical underground nuclear tests). And 280,000 TONS of payload sails into low Earth orbit. Not pounds. Tons.

I say "sails into orbit", but of course it is more like "slammed by thousands of gs of acceleration", so this has to be unmanned (any human beings on board would instantly be converted into wall-gazpacho). But 280,000 tons? That's about one thousand International Space Stations, an entire Space Elevator (see below), an entire Lunar colony, an orbital fuel depot that would make future NASA missions ten times cheaper, a space station the size of the one in the movie 2001 A Space Odyssey, or about one-tenth of a ecologically clean 1.5 terawatt solar power station.

I know that nuclear-phobes will have a screaming fit, but this concept deserves close consideration.

Karl Schroeder analyzes the concept here.

"Okay, okay, just a suggestion," Ross assured him. He was quiet for a moment, then added, "But there's one thing that bothers me..."

"What?"

"Well, if I've read it once, I've read it a thousand times, that you have to go seven miles per second to get away from the earth. Yet here we are going only 3300 miles per hour."

"We're moving, aren't we?"

"Yeah, but-"

"As a matter of fact we are going to build up a lot more speed before we start to coast. We'll make the first part of the trip much faster than the last part. But suppose we just held our present speed -- how long would it take to get to the moon?"

Ross did a little fast mental arithmetic concerning the distance of the moon from the earth, rounding the figure off to 240,000 miles. "About three days."

"What's wrong with that? Never mind," Cargraves went on. "I'm not trying to be a smart-Aleck. The misconception is one of the oldest in the book, and it keeps showing up again, every time some non-technical man decides to do a feature story on the future of space travel. It comes from mixing up shooting with rocketry. If you wanted to fire a shot at the moon, the way Jules Verne proposed, it would have to go seven miles per second when it left the gun or it would fall back. But with a rocket you could make the crossing at a slow walk if you had enough power and enough fuel to keep on driving just hard enough to keep from falling back. Of course it would raise Cain with your mass-ratio. But we're doing something of that sort right now. We've got tower to spare; I don't see why we should knock ourselves out with higher acceleration than we have to just to get there a little sooner. The moon will wait. It's waited a long time.

"Anyhow," he added, "no matter what you say and no matter how many physics textbooks are written and studied, people still keep mixing up gunnery and rocketry.

(ed. note: of course the reason the Galileo can take its good time getting up to seven miles a second is because it is a species of torchship, and thus does not have to worry as much about mass ratios.)

From Rocket Ship Galileo by Robert Heinlein (1947). Thanks to Thomas Gagnon for suggesting this.

Space Elevator

Number of
Elevators
Payload mass delivered to LEOCost per payload kilogram
12,000 metric tons$3,000/kg
24,000 metric tons$1,900/kg
36,000 metric tons$1,600/kg

You can find details about space elevators here.

The big limitations are: it must be sited exactly on the equator, and it is absurdly vulvnerable to sabotage.

You can read all about the complicated equations required to calculate the annual payload lifitng capacity of a space elevator here. A baseline Edwards-Westling 20 metric ton space elevator powered by a bank of solar panels could boost about 272 metric tons a year. If powered by a large nuclear reactor it could boost about 2,720,000 metric tons a year.

Aldebaran

Payload mass delivered to LEOCost per payload kilogram
27,000 metric tons??

This extreme heavy lift vehicle appears in Beyond Tomorrow by Dandridge Cole of "Macrolife" fame (Amherst Press 1965). The best place to watch lift-off is from an adjacent continent. That engine looks like it could accidentally vaporize Florida. They better work on the cargo handling system, though. Loading it crate by crate by helicopter is too much like eating a bowl of rice with tweezers one grain at a time.

Mr. Cole assumes that the economies of scale would dictate such a huge rocket to keep up with the orbital boost demands of the far-flung futurstic year 1990. The wet mass would be 50,000 tons. If the propulsion system had a specific impulse of 3,000 seconds, it would have a propellant fraction of 0.7 and a payload mass of 60 million pounds (27,000 metric tons). Or it could soft-land a smaller payload mass of 20,000 metric tons on Luna. If the propulsion system was weaker, say a specific impulse of 1,500 seconds, it would have a propellant fraction of 0.5 and a payload of 20 million pounds (9,000 metric tons). That propellant fraction doesn't make sense to me, I'll have to do the math.

The design is winged, for controlled aerodynamic Earth landing (now that would be a sight to see). Water take off and landing because there isn't a runway in the world that could survive that monster.

Lofstrom loop

TypePayload mass delivered to LEOCost per payload kilogram
Small40,000 metric tons/year$300/kg
Large6,000,000 metric tons/year$3/kg

This was invented by Keith Lofstrom in 1981. Details about the mechanism of a Lofstrom loop can be found here and here, don't miss the paper here.

In science fiction, Lofstrom loops are featured in Heechee Rendezvous by Frederik Pohl, The Last Theorem by Arthur C. Clarke and Frederik Pohl, and Starquake by Robert Forward.

LAUNCH LOOP

A launch loop or Lofstrom loop is a proposed system for launching objects into space orbit using a moving cable-like system situated inside a sheath attached to the Earth at two ends and suspended above the atmosphere in the middle. The design concept was published by Keith Lofstrom and describes an active structure maglev cable transport system that would be around 2,000 km (1,240 mi) long and maintained at an altitude of up to 80 km (50 mi). A launch loop would be held up at this altitude by the momentum of a belt that circulates around the structure. This circulation, in effect, transfers the weight of the structure onto a pair of magnetic bearings, one at each end, which support it.

Launch loops are intended to achieve non-rocket spacelaunch of vehicles weighing 5 metric tons by electromagnetically accelerating them so that they are projected into Earth orbit or even beyond. This would be achieved by the flat part of the cable which forms an acceleration track above the atmosphere.

The system is designed to be suitable for launching humans for space tourism, space exploration and space colonization, and provides a relatively low 3g acceleration.

History

Launch loops were described by Keith Lofstrom in November 1981 Reader's Forum of the American Astronautical Society News Letter, and in the August 1982 L5 News.

In 1982, Paul Birch published a series of papers in Journal of the British Interplanetary Society which described orbital rings and described a form which he called Partial Orbital Ring System (PORS). The launch loop idea was worked on in more detail around 1983–1985 by Lofstrom. It is a fleshed-out version of PORS specifically arranged to form a mag-lev acceleration track suitable for launching humans into space; but whereas the orbital ring used superconducting magnetic levitation, launch loops use electromagnetic suspension (EMS).

Description

A launch loop is proposed to be a structure 2,000 km long and 80 km high. The loop runs along at 80 km above the earth for 2000 km then descends to earth before looping back on itself rising back to 80 km above the earth to follow the reverse path then looping back to the starting point. The loop would be in the form of a tube, known as the sheath. Floating within the sheath is another continuous tube, known as the rotor which is a sort of belt or chain. The rotor is an iron tube approximately 5 cm (2 inches) in diameter, moving around the loop at 14 km/s (31,000 miles per hour).

Although the overall loop is very long, at around 4,000 km circumference, the rotor itself would be thin, around 5 cm diameter and the sheath is not much bigger.

Ability to stay aloft

When at rest, the loop is at ground level. The rotor is then accelerated up to speed. As the rotor speed increases, it curves to form an arc. The structure is held up by the force from the rotor, which attempts to follow a parabolic trajectory. The ground anchors force it to go parallel to the earth upon reaching the height of 80 kilometers. Once raised, the structure requires continuous power to overcome the energy dissipated. Additional energy would be needed to power any vehicles that are launched.

Launching payloads

To launch, vehicles are raised up on an 'elevator' cable that hangs down from the West station loading dock at 80 km, and placed on the track. The payload applies a magnetic field which generates eddy currents in the fast-moving rotor. This both lifts the payload away from the cable, as well as pulls the payload along with 3g (30 m/s²) acceleration. The payload then rides the rotor until it reaches the required orbital velocity, and leaves the track.

If a stable or circular orbit is needed, once the payload reaches the highest part of its trajectory then an on-board rocket engine ("kick motor") or other means is needed to circularize the trajectory to the appropriate Earth orbit.

The eddy current technique is compact, lightweight and powerful, but inefficient. With each launch the rotor temperature increases by 80 kelvins due to power dissipation. If launches are spaced too close together, the rotor temperature can approach 770 °C (1043 K), at which point the iron rotor loses its ferromagnetic properties and rotor containment is lost.

Capacity and capabilities

Closed orbits with a perigee of 80 km quite quickly decay and re-enter, but in addition to such orbits, a launch loop by itself would also be capable of directly injecting payloads into escape orbits, gravity assist trajectories past the Moon, and other non closed orbits such as close to the Trojan points.

To access circular orbits using a launch loop a relatively small 'kick motor' would need to be launched with the payload which would fire at apogee and would circularise the orbit. For GEO insertion this would need to provide a delta-v of about 1.6 km/s, for LEO to circularise at 500 km would require a delta-v of just 120 m/s. Conventional rockets require delta-vs of roughly 10 and 14 km/s to reach LEO and GEO respectively.

Launch loops in Lofstrom's design are placed close to the equator and can only directly access equatorial orbits. However other orbital planes might be reached via high altitude plane changes, lunar perturbations or aerodynamic techniques.

Launch rate capacity of a launch loop is ultimately limited by the temperature and cooling rate of the rotor to 80 per hour, but that would require a 17 GW power station; a more modest 500 MW power station is sufficient for 35 launches per day.

Economics

For a launch loop to be economically viable it would require customers with sufficiently large payload launch requirements.

Lofstrom estimates that an initial loop costing roughly $10 billion with a one-year payback could launch 40,000 metric tons per year, and cut launch costs to $300/kg. For $30 billion, with a larger power generation capacity, the loop would be capable of launching 6 million metric tons per year, and given a five-year payback period, the costs for accessing space with a launch loop could be as low as $3/kg.

Comparisons

Advantages of launch loops

Compared to space elevators, no new high-tensile strength materials have to be developed, since the structure resists Earth's gravity by supporting its own weight with the kinetic energy of the moving loop, and not by tensile strength.

Lofstrom's launch loops are expected to launch at high rates (many launches per hour, independent of weather), and are not inherently polluting. Rockets create pollution such as nitrates in their exhausts due to high exhaust temperature, and can create greenhouse gases depending on propellant choices. Launch loops as a form of electric propulsion can be clean, and can be run on geothermal, nuclear, wind, solar or any other power source, even intermittent ones, as the system has huge built-in power storage capacity.

Unlike space elevators which would have to travel through the Van Allen belts over several days, launch loop passengers can be launched to low earth orbit, which is below the belts, or through them in a few hours. This would be a similar situation to that faced by the Apollo astronauts, who had radiation doses 200 times lower than the space elevator would give.

Unlike space elevators which are subjected to the risks of space debris and meteorites along their whole length, launch loops are to be situated at an altitude where orbits are unstable due to air drag. Since debris does not persist, it only has one chance to impact the structure. Whereas the collapse period of space elevators is expected to be of the order of years, damage or collapse of loops in this way is expected to be rare. In addition, launch loops themselves are not a significant source of space debris, even in an accident. All debris generated has a perigee that intersects the atmosphere or is at escape velocity.

Launch loops are intended for human transportation, to give a safe 3g acceleration which the vast majority of people would be capable of tolerating well, and would be a much faster way of reaching space than space elevators.

Launch loops would be quiet in operation, and would not cause any sound pollution, unlike rockets.

Finally, their low payload costs are compatible with large-scale commercial space tourism and even space colonisation.

Difficulties of launch loops

A running loop would have an extremely large amount of energy in its linear momentum. While the magnetic suspension system would be highly redundant, with failures of small sections having essentially no effect, if a major failure did occur the energy in the loop (1.5×1015 joules or 1.5 petajoules) would be approaching the same total energy release as a nuclear bomb explosion (350 kilotons of TNT equivalent), although not emitting nuclear radiation.

While this is a large amount of energy, it is unlikely that this would destroy very much of the structure due to its very large size, and because most of the energy would be deliberately dumped at preselected places when the failure is detected. Steps might need to be taken to lower the cable down from 80 km altitude with minimal damage, such as parachutes.

Therefore, for safety and astrodynamic reasons, launch loops are intended to be installed over an ocean near the equator, well away from habitation.

The published design of a launch loop requires electronic control of the magnetic levitation to minimise power dissipation and to stabilise the otherwise under-damped cable.

The two main points of instability are the turnaround sections and the cable.

The turnaround sections are potentially unstable, since movement of the rotor away from the magnets gives reduced magnetic attraction, whereas movements closer gives increased attraction. In either case, instability occurs. This problem is routinely solved with existing servo control systems that vary the strength of the magnets. Although servo reliability is a potential issue, at the high speed of the rotor, very many consecutive sections would need to fail for the rotor containment to be lost.

The cable sections also share this potential issue, although the forces are much lower. However, an additional instability is present in that the cable/sheath/rotor may undergo meandering modes (similar to a Lariat chain) that grow in amplitude without limit. Lofstrom believes that this instability also can be controlled in real time by servo mechanisms, although this has never been attempted.

Competing and similar designs

In works by Alexander Bolonkin it is suggested that Lofstrom's project has many non-solved problems and that it is very far from a current technology. For example, the Lofstrom project has expansion joints between 1.5 meter iron plates. Their speeds (under gravitation, friction) can be different and Bolonkin claims that they could wedge in the tube; and the force and friction in the ground 28 km diameter turnaround sections are gigantic. In 2008, Bolonkin proposed a simple rotated close-loop cable to launch the space apparatus in a way suitable for current technology.

Another project, the space cable, is a smaller design by John Knapman that is intended for launch assist for conventional rockets and suborbital tourism. The space cable design uses discrete bolts rather than a continuous rotor, as with the launch loop architecture. John Knapman has also mathematically shown that the meander instability can be tamed.

The skyhook is another launch system concept. Skyhook could be either rotating or non-rotating. The non-rotating skyhook hangs from a low Earth orbit down to just above the Earth's atmosphere (skyhook cable is not attached to Earth). The rotating skyhook changes this design to decrease the speed of the lower end; the entire cable rotates around its center of gravity. The advantage of this is an even greater velocity reduction for the launch vehicle flying to the bottom end of the rotating skyhook which makes for an even larger payload and a lower launch cost. The two disadvantages of this are: the greatly reduced time available for the arriving launch vehicle to hook up at the lower end of the rotating skyhook (approximately 3 to 5 seconds), and the lack of choice regarding the destination orbit.

From the Wikipedia entry for LAUNCH LOOP
LAUNCH LOOP 1

A Business Trip, 2005

The elevator ride from the ground has taken almost an hour, and the ride to NBC-1, a geosynchronous transmission satellite serving the eastern seaboard, will take another four. That isn’t nearly as long as the airplane trip from New York, but at least the plane had windows, and you weren’t strapped in and “plumbed.” The small cabin that you share with five other passengers has no portholes; you have just a video screen built into the seat in front of you. This is your first trip, so you are tuned into the outside camera system. The screen shows the “Can”‘ you are in, the cradle carrying it, and the slender cables disappearing into the black sky above and the ocean below. The woman strapped into the seat beside you is reading technical reports on her screen, which is configured as a terminal. The man in front is playing a video game. They are apparently experienced space travelers.

There is a queasy feeling in your stomach as the Can slows its ascent; you are nearing west station, now less than three kilometers above you. West station is the terminus of the 12O-kilometer-high elevator system, and the start of the 2000-kilometer-long acceleration track that will hurl you into space. As you get closer, you begin to make out details: the light, open structure of west station, its long support pillar, and the small observation cabin on top, bristling with radar and communication antennas. The cradle rack above you holds two other Cans similar to yours, streamlined and covered with heat shields. The rack also holds eight box-like cargo containers, probably from the container ship you saw this morning on your way in.

If the carriers above you are standard five-metric-ton vehicles , your Can should be lowered over the ribbon and launched in about 30 minutes. This is happening to the top vehicle now. The crane has lifted it out of its cradle and is lowering it over the track, carefully positioning it sothe magnets in the channel on its belly are on top of the high-speed ribbon. The container starts moving towards the east, picks up speed rapidly, and vanishes into the rising sun.

Getting Out of the Hole

Moving from the Earth’s surface to useful orbits requires momentum and energy. By Newton’s laws, the momentum must be removed from something else, whether it is rocket exhaust, a beam of light, the atmosphere, or the Earth itself. Energy can be applied in several ways, but the amount of energy that must be turned into payload energy is constant. If the energy is applied less efficiently, more is needed to begin with. This change of momentum and energy can be expressed as a change in velocity, or delta v.

Vehicles are launched to higher orbits in elliptical transfer orbits. At the bottom of the ellipse—at the point closest to the Earth—the vehicle is moving faster than circular orbital velocity at that altitude. A very high delta v is necessary to put the vehicle into this faster orbit. At the top of the transfer orbit (apogee), the height of the vehicle’s circular orbit destination, the vehicle is moving slower than circular orbital velocity. Velocity has been lost as the vehicle traveled up out of the gravity well. More delta v must be added to make the orbit circular, but this velocity change is small compared to the delta v needed at launch.

The velocity change at launch time is the largest and most expensive. From the Earth’s equator to the Moon, the delta v at the start of the transfer orbit is 10.6 kilometers per second. The transfer orbit to geosynchronous altitudes requires 9.95 kilometers per second of delta v.

An Earth launch system should accelerate a vehicle to transfer orbit velocity without crushing acceleration or dropping it back to the ground. A low acceleration requires a long acceleration path. Accelerating a vehicle to 10.6 kilometers per second at 3 g’s requires an acceleration path 1900 kilometers long, almost5 percent of the Earth’s circumference.

The energy is proportional to the mass and the velocity squared; for a one-kilogram mass, a delta v of 10.6 kilometers per second requires the addition of 56 million joules. This looks smaller if measured electrically; 3.6 million joules equals one kilowatt-hour (1000 watts for one hour, and a watt is one joule per second), and one kilowatt-hour costs about 4 cents in the Northwest. That comes to about 60 cents’ worth of electricity, and that’s Why electrically powered launch systems are starting to get a lot of attention.

Present rocket launch systems cost much more than this, because of their enormous complexity and the vast amounts of fuel they consume. Most of the thrust a rocket generates lifts the fuel, tanks, and engines it will need later in the flight. A fully loaded space shuttle orbiter weighs 100 metric tons (a metric ton is 2205 pounds, a little more than an English short ton). The assemblage of tanks and solid boosters that lifts from Kennedy Space Center weighs over 20()0 metric tons, most of which is fuel. The orbiter, surely one of the most marvelous machines ever built, is nevertheless an incredibly expensive vehicle. Optimistic estimates suggest more than two months between each shuttle re-use, a slow way to pay back a multibillion-dollar investment. The maximum payload is 30 metric tons to low Earth orbit, or 5 metric tons to geosynchronous orbit, a tiny fraction of the launch weight. A shuttle launch costs more than 30 million dollars, and this doesn’t include the purchase of the shuttle itself, or the expensive shuttle ground support systems left over from the Apollo program. A greatly expanded space program based on rockets may prove much too costly.

The idea of a fixed structure on Earth or in space, electromagnetically driven and capable of handling many vehicles per hour, is not a new one. The skyhook was suggested by Yuri Artsutanov in 1960, and independently by Isaacs et al in 1966. The skyhook is a long cable reaching up from the Earth’s surface far into space, its downward weight balanced by centrifugal force as it follows the rotation of the Earth; Artsutanov’s idea has been expanded on by others, with tapered cables, rotating cables, and other refinements intended to lower the mass of the system or ease construction. Incredibly strong materials are required that will not be commercially available for many years, making these systems impractical at present. Most designs must be built from orbit, which requires a large existing space launch capability as well.

Mass drivers use moving magnetic fields to accelerate vehicles equipped with electrically conducting coils or shells. In the November 1979 Analog Roger Amold and Donald Kingsbury suggested an orbiting mass driver for vehicle capture, the Spaceport. The Spaceport is an orbiting platform 500 kilometers long that captures vehicles from the Earth or high orbit along its length. Energy is extracted from the velocity difference between the vehicle and the Spaceport and stored in rotating coils. The vehicle is accelerated to the same speed as the Spaceport, and the Spaceport changes velocity slightly. The stored energy may be used to eject vehicles from the Spaceport. The Spaceport has a mass of 50,000 metric tons and must be assembled in orbit, while the vehicles it handles are too small (230 kilograms, or 500 pounds) to transport human beings or larger machines. A Spaceport that can move people must be much larger. While this system may be the most economical in the long term, it would be very expensive to ship up with rockets.

Earth-based mass drivers’ are capable of reaching orbital velocities, but the Earth’s atmosphere is a major problem. Lf vehicles are launched horizontally, they must travel through hundreds of kilometers of atmosphere before reaching space, and the air drag is enormous. If launched vertically, the accelerator must be very tall and the g forces are much too high for people or complex machinery. Such a system might be useful for some raw materials, but only if people and machinery get into orbit by some other means.

Such accelerators also must handle enormous pulsed power. A five-metric-ton vehicle accelerating at only three g’s and moving at eight kilometers per second requires 1.2 billion watts of power from the segment of the accelerator immediately around it. This is the power level of a large power generating plant, and this power-handling capability must be repeated many times down the accelerator. Accelerators such as this could be very expensive.

A good Earth-based launch system should be built from the ground up, operate in vacuum, and deliver energy and momentum to the vehicle without expensive power-handling circuitry.

By Your Own Bootstraps

A ball tossed into the air, a stream of water from a hose, and a planet in its orbit are all governed by Newton’s Laws, and follow paths that balance the external forces on them with their own accelerations. An orbit can be viewed as a balance between centrifugal acceleration and gravity; if the centrifugal force is higher than the gravitational force, the orbiting body moves upwards. If a stream of material moves faster than orbital velocity, it will also move upwards, unless extra downwards force is added. This force could be provided by stationary weight somehow “hung” on the stream.

The centrifugal force on the stream is proportional to the velocity squared. A stream moving twice as fast as orbital velocity generates a centrifugal force four times that of gravity, and such a stream could support the weight of a stationary mass three times its own mass. Altitude actually improves the lifting action, which can be used to support long, light structures far above the Earth’s surface.

If the moving stream is an iron strip or ribbon, it will be attracted to a magnetic field. Consider an electromagnet with long pole faces parallel to the direction of the moving iron ribbon, as shown in Figure 1. The magnetic flux travels through the coils and into one pole. It then passes up through the gap into the ribbon, across it, and back down through the other gap to the other pole, completing the magnetic circuit. The poles are attracted to the ribbon, attempting to close the gap. The control electronics sense the spacing and adjust the current in the control coils to change the field, and thus the upwards force on the poles.

The magnets, electronics, and poles are called the “track.” The track does not need to move with the ribbon. If the magnetic field between the track and ribbon is uniform, the ribbon can move at very high speed without friction between it and the stationary track, even while the ribbon supports the track against gravity. This is a form of "magnetic levitation,” which is being considered for high-speed trains, supporting the train cars without the rolling friction of wheels.

We will use an iron ribbon 5 centimeters wide and 2.6 millimeters thick (about 2 inches by 0.1 inches) with a mass of 1 kilogram per meter. If the ribbon is moving at 12 kilometers per second relative to the Earth’s surface, and at an altitude of 120 kilometers above it, the upwards centrifugal force is capable of supporting 2.35 kilograms of mass per meter against gravity. The ribbon itself has a mass of 1 kilogram per meter. The centrifugal force can thus support the ribbon and a stationary mass of 1.35 kilograms per meter.

If the poles are 1 centimeter wide, the force per area on the poles will be equivalent to the weight of 70 kilograms per square meter. This force can be generated with a small magnetic field of about 0.04 tesla (a metric unit of magnetic field strength). For comparison, the Earth’s magnetic field is 0.0001 teslas, a good permanent magnet generates about 1 tesla, and superconducting magnets can go beyond 20 teslas. This small field is deflecting the ribbon around the radius of the Earth. To deflect the ribbon around a tighter radius, more force is needed.

If a fast-moving stream is deflected by a small angle, a force is generated in opposition, proportional to the angle times the mass per length times the velocity squared. To deflect the moving ribbon described above by an angle of l degree, a force of 260 metric tons is needed. This deflection does not stretch or strain the ribbon, or change its speed; the force simply changes the stream’s direction. In this way a very small, fast-moving ribbon can support huge loads.

A stream of matter can also carry enormous power, equal to half the mass per length times the velocity cubed. A 1 kilogram-per-meter ribbon moving at 12 kilometers per second carries 864 billion watts, more than three times present U.S. electrical capacity; yet an iron ribbon of that size can easily travel through a 10-centimeter pipe.

A high-speed iron strip can transmit power, support heavy loads, and suspend stationary mass at high altitudes. These features make possible the Launch Loop, a new kind of electromagnetic launch system.

The Launch Loop

The Launch Loop, illustrated in Figure 2, is a very large, stationary, gossamer structure built around a 6000-kilometer-long recirculating loop of iron ribbon moving at 12 kilometers per second. The structure is 3000 kilometers long and 120 kilometers high. This seems gigantic, but the mass per length is only 5 to l0 kilograms per meter, and most cross sections can be measured in centimeters. The entire above-ground weight of the Launch Loop is 35,000 metric tons, about the weight of a large ship. It may be constructed from modest quantities of materials that are now corrnnercially available.

The ribbon follows a path shaped like one side of a sawhorse, parallel to the equator. The ribbon travels up one leg (the west incline), across the top (the launch path), and down the east incline. At the bottom of the east end, it is deflected back to return along the same path. At the west end, it is deflected again to complete the loop. The ribbon completes one circuit of the Loop, 6000 kilometers, in 8 minutes.

The 2000-kilometer-long launch path in the center of the Loop accounts for most of its length. Fortunately, it is also the cheapest part of the system. This section is 120 kilometers high, and runs parallel to the equator. It is supported by the centrifugal force generated by the moving ribbon. The ribbon runs both directions through this section, with the “forward” ribbon moving from west to east and the “return” ribbon running east to west. A track is suspended underneath each ribbon, consisting of magnet structures, control electronics, position sensors, stabilizing cables, and parachutes to protect the track if the system falls down. The return ribbon and track run a few hundred meters below the forward track, and the two tracks are coupled with thin cables. The launch path is at high altitude to minimize air drag on the ribbon and the vehicles launched from it.

The weight of the launch path is entirely supported by the centrifugal acceleration on the ribbon; if it were extended at the ends, it could circle the Earth without support. In practice, small stabilizing forces are required to control the system. A very small deflection of the ribbon at west station could result in the ribbon leaving the track at east station.

The “east” and “west” stations are at either end of the launch path. The stations are built on 1200-meter-long, curved sections of electromagnets that deflect the ribbons about 7 degrees downwards to provide the upwards force needed to support the station. Long Kevlar® cables to the ground stabilize the station and relieve horizontal forces. The stations are equipped-with vehicle storage racks, repair bays, communication systems, and large, fast motors that adjust cable tensions. The elevators on the west station haul the vehicles up from the surface for launching. Each station weighs 2500 metric tons, and the cables are tensioned to a total of 1200 metric tons. West station is illustrated in Figure 3.

Before and after the stations are the inclines, two sections sloping down to the Earth’s surface at a 7- to 20-degree angle. The inclines connect the launch path to structures on the surface. The inclines travel through the atmosphere, so a vacuum sheath is required to protect the ribbon from air friction. Horizontal forces and wind stresses are relieved by Kevlar® cables running diagonally to the ground. The inclines have a mass of about 5 kilograms per meter, and curve more than the Earth’s surface, drooping near the top.

The ribbon reaches the bottom of the inclined section at a 20-degree angle, and is forced to horizontal with electromagnets on a curved ramp. This ramp changes height by 600 meters, and may be cheaper if built in a narrow tunnel under the surface, resulting in the “S” shaped kink shown at the bottom of the incline in Figure 4.

Traveling parallel to the Earth’s surface, the ribbon passes through a 2-kilometer-long, high-efficiency linear motor. Four of these linear motors, two on each end of the Launch Loop, restore energy removed by friction and vehicle launches.

Once the ribbon is horizontal, and restored to speed by the motors, it must be deflected 180 degrees and sent back the other way. A force of nearly 30,000 metric tons is necessary to do this, equal to two times the ribbon mass per length times the velocity squared. This will be done with the “D” magnets, a magnetic track like that of the launch path, but much more powerful. The ribbon is rotated so that the flat surface is pointing sideways, and the flat surface is pulled toward the magnets, deflecting the ribbon in the horizontal plane. The magnets deflect the ribbon in a 20-kilometer-diameter semicircle, with a force of about 1.5 metric tons per meter and a magnetic field of about 2 tesla. These magnets will weigh about 200 metric tons, andtheir windings will consume 60 megawatts. They must be firmly anchored to the ground to absorb the deflection stress. With a deflector at either end of the Loop, the Earth itself can be viewed as a giant structural member, holding the ends of the Loop together.

The ribbon changes height from the top to the bottom of the Loop; as it travels downwards, its speed increases by 100 meters per second, causing the ribbon to stretch by 0.8%. Iron will fracture with this much strain, so the ribbon will be built in 1-meter segments, with sliding joints between them. Although this weakens the ribbon along its length, the ribbon is never under tension or compression during normal operation.

The Launch Loop should be located along the equator for optimum launching and weather conditions. Most of the interesting places in space are on or near the plane of the equator and are most easily reached from there. Violent storms and high winds are aided by Coriolis forces, which result from the Earth’s rotation. These forces are minimized at the equator, causing milder winds.

Launching Vehicles

The Launch Loop is a large stable structure, reaching from the Earth’s surface into space. How is this device used to launch vehicles?

There are two common fonns of magnetic levitationes one is based on the attraction of magnets to other magnets or ferromagnetic materials such as iron; the other uses the repulsion between a magnet and induced currents in a conductor.

The attractive levitation process theoretically consumes no power, but is unstable and requires power for stabilizing electromagnets and control circuitry. This process is used in the track support and end deflection magnets, where large forces must be generated with minimum power dissipation.

Vehicles are supported and driven by repulsive levitation. Repulsive levitation uses the eddy currents induced in a conductor by a rapidly changing magnetic field. The eddy currents generate a reverse magnetic field, pushing the originating magnet away. The eddy currents dissipate heat, which appears mechanically as drag between the conductor and the generating magnet. Drag is desirable between the ribbon and the vehicle, as it provides the force to accelerate the vehicle.

This version of the Loop is designed to launch 5-metric-ton vehicles. Payload containers have strips of magnets on their bottom side designed to generate a lift of up to 5 metric tons and a drag of 15 metric tons, accelerating them at 3 g’s. Rocket motors on the bottom of the payload container will provide additional delta v when the vehicle is at the top of its orbit, at apogee. The center of mass of the payload, container, and rockets is on axis with the ribbon, with stability provided by magnetic damping and small rocket thrusters.

Re-enterable payload containers, as shown in Figure 5, are equipped with a lifting shell, a heat shield, and parachutes for reentry of human cargo if the Loop fails. Insurable, inanimate payloads will not need this protection, and will probably burn up if they accidentally re-enter.

The heat removal by the ribbon from the vicinity of the vehicle determines the maximum force the vehicle can put on the ribbon. A drag force of 15 metric tons results in almost 2 billion watts of heat carried from a vehicle near rest; this heats the ribbon from 400 to 620 degrees centigrade. Iron loses its magnetic properties above 770 degrees centigrade, its Curie temperature.

Launching a 5-metric-ton vehicle to 10 kilometers per second removes 600 billion joules of ribbon kinetic energy, of which 350 billion joules is turned into heat, for an energy efficiency of 41%. The initial Loop will be driven by a 500-megawatt gas turbine power plant. Sixty megawatts will be used for deflection magnets, and V40 megawatts for auxiliary equipment, leaving 400 megawatts to drive the motor. About 7.0 megawatts will be lost to air friction and drag in the ribbon, leaving 330 megawatts to restore the energy used to launch vehicles. This can restore the energy used to launch a 5-metric-ton vehicle in about 30 minutes. This is equivalent to 240 metric tons per day, or 87,000 metric tons per year.

The energy storage capacity of the Loop will allow it to launch at high rates for short periods with less than full power plant capacity. Power plants may be brought on and off line as necessary; the Loop can store energy for days. More power plants can be added, and more vehicles launched per hour, until the Loop reaches its thermal limit. This Loop limits at 4 billion watts (4 gigawatts), allowing the launch of 115 metric tons per hour, or l million metric tons per year. It may be years before even one Launch Loop is used at full capacity.

System Startup

The system is stable once it's going, and it can launch a lot of payload, but how is it started up? The ribbon must be started on the ground from a stand-still, and accelerated without stretching it too much. The ribbon sections normally above the atmosphere are now in it, and must be protected from air drag. The east and west stations must be lifted to altitude.

The Loop is started flat on the ground. The ribbon is levitated at rest by the D magnets, and is held underneath the inverted tracks in the launch path. The launch path is surrounded by a temporary vacuum sheath, to be stripped off later. When the ribbon is first started moving, it is stretched by the pull of the motors and compressed after leaving them. This is a slow process, as the ribbons cannot be pulled too hard without breaking the joints between ribbon segments.

Once the ribbon is moving fast enough, the electrical generators are brought up to full power. To get the 6000-metric-ton ribbon moving at 12 kilometers per second requires 120 gigawatt-hours of energy. The Loop will need almost two weeks to get up to speed if this energy is put in at a 400-megawatt rate.

When the ribbon is finally moving at 12 kilometers per second, the stations and the launch path may be raised from the surface. At the start, the inclines have zero length, and the launch path is 500 kilometers longer than normal. The anchoring cables on the stations pull them toward the center, and the structure slowly rises. During this time, air traffic must be guided over or under the Loop; once the Loop is up, only the ends will pose a hazard to navigation.

As the system rises, the launch path in the center gets shorter and the inclined sections get longer. At the stations, the inclines are extended by welding together new sheath over them, while sheath is cut away from the launch path. This is performed in long vacuum chambers running the length of the stations; while the ends of the chambers are not tightly sealed, they are long and equipped with powerful pumps, so that a high vacuum can be maintained where the sheath is opened.

The Loop may have to be re-erected a few times per year. Control failure on too many segments of track, ultra-high winds, meteoroid impact, vehicle magnet failures, and other problems may cause Loop failure. Themajor portions of the system must survive the loss of the ribbon, the kinetic energy of the ribbon must be safely dissipated, and the system should be quickly restorable to service.

The moving ribbon stores 120 million kilowatt-hours. This amount of energy would be produced in heat by the combustion of 10,000 metric tons of oil (modern oil tankers carry 550,000 metric tons). If the Launch Loop fails, this energy is lost, and the ribbon should, be dumped out in a harmless way. Releasing it at the top of the Loop will throw it away from the Earth at escape velocity, creating a cloud of space junk in solar orbit. From the inclined sections of the D magnets it will be thrown into the atmosphere or onto the ground, in line of sight with the Loop. The Loop should be operated in unpopulated areas. A lost ribbon can only land near the equator, and must be slowed to just below orbital velocity by air friction to do so. This much air friction would harmlessly vaporize the ribbon.

System Costs

The Launch Loop can launch vehicles very cheaply, but how much will it cost to build one? As the first prototype of a new kind of launch system, it could be very expensive. Fortunately, however, most of the main components are commercially available or are easily mass produced, and their costs may be calculated.

The beginning power plant will use 11 United Technologies 56-megawatt dual FT4 gas turbine power plants, costing $77 million. Structural material costs include $5 million for 200 metric tons of Union Carbide Thornel® carbon fiber and $25 million for 1000 metric tons of DuPont Kevlar® aramid fiber. The magnets and control systems will use $3 million for 1500 metric tons of copper wire and $16 million for 400 metric tons of formed Alnico 8 magnets. The control electronics and motor drivers will cost around $60 million. These identified costs total less than $200 million.

Unknown costs include sheath and track manufacturing, and the upward deflectors on the ends. If the Loop is built on land, many square kilometers of land must be purchased; if at sea, floats and anchoring cables are needed. Vacuum pumps, storage tanks, security systems, housing, and a myriad of other details must be included.

The first commercial Launch Loop may cost l billion dollars (a guess), and be used at 30% capacity with a 500-megawatt generator (26,000 metric tons per year). If this system was paid back in one year as a high-risk venture it would cost $50 per gross kilogram (including 6 cents per kilowatt-hour for turbine fuel). Later, launching 750,000 metric tons per year with 4 gigawatts power capacity, 5-year amortization, $9 billion capital cost, and 1.3 cents per kilowatt-hour fuel cost, the cost per gross kilogram is $3. At this cost, labor and vehicle systems will probably dominate net payload cost. Total Launch Loop system cost will probably be well below that of Earth-to-high-orbit rocket systems.

Conclusions

The Launch Loop described here was designed for launching 5-metric-ton vehicles to geosynchronous, LaGrange, and lunar destinations, but other applications are possible. Increasing ribbon speed to l6 kilometers per second lowers near-Earth efficiency, but increases Loop range to Mercury and Jupiter.

Loops may be constructed off Earth, for launching from other bodies or vehicle capture in orbit. For example, the 1500-meter-per-second delta v needed for geosynchronous orbit circularization could be provided by a capture ribbon 120 kilometers long, accelerating vehicles at 1 g. This would make apogee boost motors unnecessary, allowing more net mass per vehicle. Momentum may be restored to the capturing system with high-efficiency ion engines, or payload capture from higher orbits.

Loop structures may even be built in evacuated pipes on the Earth’s surface, and used to transmit power. A ribbon with a mass of 1 kilogram per meter moving at 8 kilometers per second carries 250 billion watts of power, and perhaps 20 billion watts can be added or removed as necessary. Energy can be put into the ribbon, transmitted 5000 kilometers, and taken back out with less than 1% loss.

We are now building a 3.7-meter-across, racetrack-shaped, 170-meter-per-second model of the Launch Loop, and are planning larger experiments. It may take l5 to 20 years to scale up to a commercial Loop.

A Business Trip, Continued

The wait is nearly over; the overhead crane is moving over to lift your Can off its cradle and onto the ribbon. The ribbon is so thin, it’s hard to see, extending into the distance. You are reminded of the Loop on NBC-1, which you are going up to help complete. With the new orbital Loop, vehicles can be captured from Earth without onboard rocket engines, pulled up to the speed necessary for geosynchronous orbit by the Loop and the mass of the space platform itself. You will be installing the large ion engines that launched ahead of you, which will be used to restore the momentum removed by arriving vehicles.

This is only a temporary measure. Materials are being accumulated on the Moon for a lunar base, which will use its own surface-based Launch Loop for shipping ore from the surface to the smelters and foundries being built at L-5. Those materials will be launched back down to NBC-l, and used for building more antennas; the extra momentum of these cargos will compensate for lost momentum from Earth shipments. When that happens, NBC will sell their ion engines to U.S. Steel, who is planning a mining expedition to the asteroids. You are wondering if you will follow them out.

There is a slight jolt as the Can is lowered onto the ribbon and slowly released by the crane. The ribbon in front of the Can is heated to dim incandescence; perhaps that is only your imagination, but the force starting to push you back into your seat is not. The end of west station passes by, and it feels like you are on your back with someone on top of you; strange but not painful.

In six minutes the acceleration will end, followed by four hours of free fall. Thousands have taken this path before you, billions more will follow; but you still feel, and rightly so, like a pioneer.

From THE LAUNCH LOOP by Keith Lofstrom (1983)
LAUNCH LOOP 2

A Space Fountain does not have to go straight up. The projectiles from the Base Station could be sent off at an angle in a large partial orbital arc that intersects the ground some distance away. A second Base Station could then receive the stream of projectiles, turn them around and send them back to the first Base Station, completing the loop. This concept has been studied in detail by Paul Birch and Keith Lofstrom. The Keith Lofstrom design is called a Launch Loop. It has a long straight section on top that is used to launch payloads into low Earth orbit. The projectiles used in the Launch Loop are bars of iron. The ends of the bars are interleaved like tongue and groove boards into a continuous ribbon of iron moving at twelve kilometers a second.

Surrounding the two high-speed projectile streams is a non-moving hollow double-track system that shields the moving projectile stream from the atmosphere. The track contains sensors, cables, control electronics, permanent magnets, electromagnets, and parachutes in case of catastrophic system failure. The track supports itself by hanging one centimeter below the ribbon of iron using the attractive forces from permanent magnets augmented by active electromagnetic control forces to maintain the spacing. The track is also designed to support vehicles that ride on the outside of the stationary track using electromagnetic levitation, while extracting kinetic energy by coupling magnetically to the high speed iron ribbon inside the track. The ribbon of iron bars is launched from the West Turnaround Terminal by a mass driver at about a fifteen degree angle to the surface. The ribbon climbs to about 120 kilometers altitude where it is deflected by the West Deflector Station into a trajectory that follows the Earth's surface below.

The path of the iron ribbon is that of the orbit of a satellite at 120 kilometers altitude modified slightly by the weight of the track that it must support. The twelve kilometer per second "orbital speed" of the iron ribbon is much greater than the true orbital speed of eight kilometers per second at this altitude, so the ribbon has a tendency to fly outward. This net upwards force on the ribbon means it can support a weight of over a kilogram per meter of length of non-moving track while remaining parallel to the Earth's surface. This "straight" portion of the Launch Loop continues on for 2000 kilometers to the East Deflector Station, where the ribbon is deflected downward to the East Turnaround Terminal. There the ribbon of iron bars is turned around, brought up to speed with mass driver and launched on the return path.

The vehicles are hauled up on 120 kilometer long elevator cables to the West Deflector Station and placed on the acceleration track. They are launched from there to the east in order to utilize the rotation of the Earth to aid in reaching the desired terminal velocity. The vehicles slip-couple to the rapidly moving iron ribbon with magnetic fields and accelerate at three Earth gravities. Depending upon their desired final destination, the vehicles can be launched with any velocity up to Earth escape velocity of eleven kilometers per second. The Launch Loop can be used for landing by simply reversing the process, with the kinetic energy of the returning vehicle being put back into the iron ribbon instead of being dissipated as heat. The excess energy can be used to launch another vehicle or turned back into electricity by using the electromagnetic mass drivers as electromagnetic brakes. A single Launch Loop could easily launch a five ton vehicle to escape velocity every hour with an input of 200 megawatts of electrical power. At five cents per electrical kilowatt-hour, that amounts to two dollars per kilogram for launching payloads into space.

From INDISTINGUISHABLE FROM MAGIC by Robert L. Forward (1995)
LAUNCH LOOP 3

As we have described the dynamic beanstalk, the main portions are vertical, with turnaround points at top and bottom. However, when the main portion is horizontal We have a launch loop.

Imagine a closed loop of evacuated tube through which runs a continuous, rapidly moving metal ribbon. The tube has one section that runs from west to east and is inclined at about 20 degrees to the horizontal. This leads to a 2,000-kilometer central section, 80 kilometers above the Earth’s surface and also running west to east. A descending west-to-east third section leads back to the ground, and the fourth section is one at sea level that goes east to west and returns to meet the tube at the lower end of the first section.

The metal ribbon is 5 centimeters wide and only a couple of millimeters thick, but it travels at 12 kilometers a second. Since the orbital velocity at 80 kilometers height is only about 8 kilometers a second, the ribbon will experience a net outward force. This outward force supports the whole structure: ribbon, containing tube, and an electromagnetic launch system along the 2,000 kilometer upper portion of the loop. This upper part is the acceleration section, from which 5-ton payloads are launched into orbit. The whole structure requires about a gigawatt of power to maintain it. Hanging cables from the acceleration section balance the lateral forces produced by the acceleration of the payloads.

Although the launch loop and the dynamic beanstalk both employ materials moving through evacuated tubes, they differ in important ways. In the dynamic beanstalk the upward transfer of momentum is obtained using a decelerating and accelerating particle stream. By contrast, the launch loop contains a single loop of ribbon moving at constant speed and the upper section is maintained in position as a result of centrifugal forces.

From BORDERLANDS OF SCIENCE by Charles Sheffield (1999)

Rocket Sled

SystemPayload mass delivered to LEOCost per payload kilogram
StarTram150,000 metric tons/year43/kg

Details about Rocket sled launch can be found here. Details about StarTram can be found here.


MagLifter

This is from The MagLifter: An Advanced Concept Using Electromagnetic Propulsion in Reducing the Cost of Space Launch (1994)

The report properly points out that NASA's Space Shuttle did many wondeful things, but lowering costs sadly was not one of them. NASA proudly predicted that the proposed shuttle could boost payload into orbit for $260 US per kilogram. In practice the accurséd thing cost $18,000 per kilogram of payload, which was pathetic compared to the $5,000 per kilogram price of the non-resusable simple-as-dirt 1966-vintage Russian Proton booster.

Naturally researchers were motivated to find some alternative boost method that might lower the cost by a couple of orders of magnitude.

In theory electromagnetic acceleration should be far more efficient than using a disintegrating totem pole made of high exposives. However in the past applying electromagnetism to space launch took the form of guns, as in railguns and coilguns. Both of those are still not ready for prime-time, despite the military throwing lots of money at the project of turning them into weapons.

But the study author John Mankins said "What about magnetic levitation trains?" Good old MagLev. You know, the kind that was patented in 19-freaking-37 and which currently holds the speed record for rail vehicles? Technology that is actually being used in the real world in bullet trains is certainly mature technology.

The concept is called "MagLifter".


The bottom line is the MagLifter can provide the launch vehicle with a free 300 m/s of launch delta-V. Granted this is only about 3% of the total delta-V needed, but the cost savings are huge. It cuts the delta-V from the start of the launch, when the propellant cost per meter/sec is at its most expensive.

The paper has an analysis, comparing a sample single-stage to orbit rocket with the same rocket scaled down but using MagLifter. The scaled-down version saved 327 metric tons of wet mass, 24 metric tons of dry mass, and required only 4 rocket engines instead of 6.


Railguns and coilguns are typically short, since they have to fit on some sort of military vehicle. This means all the velocity has to be jammed into the projectile within the short length of the gun, meaning that the acceleration will be strong enough to smash an astronaut like a cockroach. It will also do nasty things to unliving cargo.

On the other hand since MagLifter is based on a railroad train, the accelerating segment can be, say, four kilometers long. This means velocity can be added at a much more leisurely pace and gentler acceleration. The advantage is that the astronauts don't die and the inert payloads do not need expensive reengineering.

Another advantage over railguns is that MagLifter does not expend lots of hardware with each launch (sabot, projectile heat shield, orbit insertion propulsion module).

And unlike the Space Shuttle, MagLifter does not require very high launch rates in order to achieve economical operations. Railgun launches are even worse, some concepts can only bring the price down to the goal by doing four launches per day.

The report estimates that current (1997) maglev train in the 300 miles-per-hour range costs about $10 to $20 million US per mile and $3 to $5 million US per train (payload of about 23 metric tons). Annual operations and maintenance cost around 1% of capital cost.


The MagLifter system has five major elements: Catapult, Structural Support Systems, Power Systems, Supporting Systems, and Launch Vehicles.


Catapult

The catapult has thee major elements: Maglev Guideway, Accelerator-Carrier Vehicle, and Accelerator-Carrier Staging Facility.

Maglev Guideway

This is the "rails" of the maglev railroad. It will be about 3 to 4 miles of maglev rails. 2.5 miles where the payload is accelerated, and 0.5 to 1.0 mile where the accelerator-carrier is frantically decelerated after the payload is launched. You have to be able to reuse the accelerator-carriers, those things are expensive. The acceleration segment is enclosed in a pressurized tube full of helium gas; since helium has low density, low drag forces, and a high speed of sound.

Accelerator-Carrier Vehicle

These are the "cars" that are accelerated by the railroad track. The launch vehicle is strapped to the accelerator-carrier with rapid precisely-controlled release mechanism. If the launch vehicle is extra-long, several accelerator-carriers will have to be linked like cars on a choo-choo train.

Each accelerator-carrier has cradles to give structural support to the launch vehicles during the acceleration phase. Mostly on the "rear" of the launch vehicle, so the carrier does not go shooting ahead while leaving the launch vehicle hovering in mid-air like Wile E. Coyote.

Accelerator-Carrier Staging Facility

This houses the operation control center, and the accelerator-carrier management center. This is where the launch vehicles are strapped to their carrier, and also contains the carrier servicing and maintenance facilities.


Structural Support Systems

This is the part that supports the maglev guideway. It is assumed to be mostly composed of a mountain, since building support towers two kilometers tall is a bit of a challenge. The guideway will either be on trestles set on the exterior of a mountain, inside a 'cut' made into the side of a mountain, or inside a tunnel in the mountain's interior.

It has three elements: Tunnel, Tunnel Environment Monitoring and Control Systems, and Launch / Exit systems.

Tunnel

The acceleration section of the maglev guideway is encased in a tunnel, to smooth things as the launch vehicle furiously accelerates. The deceleration section of the guideway has no encasing tunnel, but still needs trestles or something to support it.

Tunnel Environment Monitoring and Control Systems

The tunnel will be filled with a normal oxygen-nitrogen atmosphere at the start, but near the exit it will be filled with gaseous helium. This will provide a low-density low-drag medium as the launch vehicle exceeds Mach 1. The speed of sound in helium is also about 2.6 times what it is in ordinary atmosphere. This is a good thing because you do not want a sonic boom inside the tube.

The tunnel will need sensors and gas injectors to ensure the gaseous environment is arranged properly and the tube is clear of foreign objects.

Launch / Exit systems

This is the system that manages the separation of acceleration-carrier and launch vehicle, and their exit from the tube gaseous environment.


Power Systems

Energy Storage

This is a bank of batteries, probably a superconducting magnetic energy storage system (SMES). It will be gradually charged up from the local power grid, and used to power the launch. It would be nice to generate the required power during the launch. But since the blasted thing sucks 10 gigawats for a whopping 20 seconds, generating the power during launch is out of the question. Unless you have an antimatter power plant up your sleeve.

Power Management and Distribution

This system has to manage the massive discharge of the SMES and direct each watt to the proper component over the 30 second launch. It better not slip up or the maglev is in for tons of high-voltage fun that will make a thunderbolt look like scuffing your shoes on the carpet.

Thermal Management

The second law of thermodynamics says there will always be waste heat. If the maglev system is 80% efficient, this means the thermal management system has to deal with 40 gigaJoules of waste heat over three miles of catapult. Otherwise the entire thing turns into three miles of molten lava.


Supporting Systems

This is mostly the stuff crammed into the accelerator-carrier staging facility.

Staging Facilites

This handles staging for the launch vehicles, the payload, and the accelerator-carrier. It also handles mating the launch vehicle (including payload) with the accelerator-carrier, and performing maintenace on the launch vehicle following each flight.

Operations Control Center

The crew here control both the staging and launch operations. MagLifter operations rely upon rapid turn-around, low-cost (submarine-style) launch operations.

Installation Facilities

This is in charge of all those behind-the-scenes details vital to the operation. This includes maintenance on the access roads servicing the entire operation and temporary housing for the launch passengers.


Launch Vehicles

The small-, moderate-, and large-scale rockets that transport the payload the rest of the way to orbit. These are winged like the Space Shuttle, so they can return to the launch site and be re-used.

Prelude to Space

Prometheus Beta
Payload60,000 kg
(Prometheus Alpha Dry)
Wet Mass450,000 kg
EngineNuclear Ramjet/
NTR
PropellantAtomspheric gases/
Liquid Methane
Exhaust Velocity6,700 m/s atmo/
6,318 m/s methane
Specific Impulse683 sec atmo/
644 sec methane
Electromagnetic
sled velocity
220 m/s
(min ramjet 165 m/s)

This is from a science fiction novel by Sir Arthur C. Clarke. Keeping in mind that Clarke was the Chairman of the British Interplanetary Society from 1946 – 1947, and again from 1951 – 1953. The performance data for the nuclear stage was taken from the classic paper The Atomic Rocket by A.V. Cleaver and L.R. Shepherd, published in the Journal of the British Interplanetary Society in a series of articles September 1948–March 1949.

For five miles straight as an arrow, the gleaming metal track lay along the face of the desert. It pointed to the northwest across the dead heart of the continent and to the ocean beyond. Over this land, once the home of the aborigines, many strange shapes had risen, roaring, in the last generation. The greatest and strangest of them all lay at the head of the launching track along which it was to hurtle into the sky.

A little town had grown out of the desert in this valley between the low hills. It was a town built for one purpose—a purpose which was embodied in the fuel-storage tanks and the power station at the end of the five-mile-long track. Here had gathered scientists and engineers from all the countries of the world. And here the “Prometheus,” first of all spaceships, had been assembled in the past three years.

The Prometheus of legend had brought fire from heaven down to earth. The Prometheus of the twentieth century was to take atomic fire back into the home of the Gods, and to prove that Man, by his own exertions, had broken free at last from the chains that held him to his world for a million years.

No one seemed to know who had given the spaceship its name. It was, in actuality, not a single ship at all but really consisted of two separate machines. With notable lack of enterprise, the designers had christened the two components “Alpha” and “Beta.” Only the upper component, “Alpha,” was a pure rocket. “Beta,” to give it its full name, was a “hypersonic athodyd (an abbreviation of aero thermodynamic duct).” Most people usually called it an atomic ramjet, which was both simpler and more expressive.

It was a long way from the flying bombs of the Second World War to the two-hundred-ton “Beta,” skimming the top of the atmosphere at thousands of miles an hour. Yet both operated on the same principle—the use of forward speed to provide compression for the jet. The main difference lay in the fuel. V.1 had burned gasoline; “Beta” burned plutonium, and her range was virtually unlimited. As long as her air-scoops could collect and compress the tenuous gas of the upper atmosphere, the white-hot furnace of the atomic pile would blast it out of the jets. Only when at last the air was too thin for power or support need she inject into the pile the methane from her fuel tanks and thus become a pure rocket.

“Beta” could leave the atmosphere, but she could never escape completely from Earth. Her task was twofold. First, she had to carry up fuel tanks into the orbit round the Earth, and set them circling like tiny moons until they were needed. Not until this had been done would she lift “Alpha” into space. The smaller ship would then fuel up in free orbit from the waiting tanks, fire its motors to break away from Earth, and make the journey to the Moon.

Circling patiently, “Beta” would wait until the spaceship returned. At the end of its half-million-mile journey “Alpha” would have barely enough fuel to maneuver into a parallel orbit. The crew and their equipment would then be transferred to the waiting “Beta,” which would still carry sufficient fuel to bring them safely back to Earth.

It was an elaborate plan, but even with atomic energy it was still the only practicable way of making the lunar round-trip with a rocket weighing not less than many thousands of tons. Moreover, it had many other advantages. “Alpha” and “Beta” could each be designed to carry out their separate tasks with an efficiency which no single, all-purpose ship could hope to achieve. It was impossible to combine in one machine the ability to fly through Earth’s atmosphere and to land on the airless Moon.

When the time came to make the next voyage, “Alpha” would still be circling the Earth, to be refuelled in space and used again. No later journey would ever be quite as difficult as the first. In time there would be more efficient motors, and later still, when the lunar colony had been founded, there would be refuelling stations on the Moon. After that it would be easy, and space flight would become a commercial proposition—though this would not happen for half a century or more.

Meanwhile the “Prometheus,” alias “Alpha” and “Beta,” still lay glistening beneath the Australian sun while the technicians worked over her. The last fittings were being installed and tested: the moment of her destiny was drawing nearer. In a few weeks, if all went well, she would carry the hopes and fears of humanity into the lonely deeps beyond the sky.


Dirk examined the array of controls and switches from a respectful distance. He could guess the purpose of some from the labels they bore, but others were quite incomprehensible. The words “Manual” and “Auto” occurred over and over again. Almost as popular were “Fuel,” “Drive Temperature,” “Pressure,” and “Earth Range.” Others, such as “Emergency Cut-out,” “Air Warning,” and “Pile Jettison” had a distinctly ominous flavor. A third and still more enigmatic group provided grounds for endless speculation. “Alt. Trig. Sync.,” “Neut. Count,” and “Video Mix” were perhaps the choicest specimens in this category.

“You’d almost think, wouldn’t you,” said Matthews, “that the house was ready to take off at any moment. It’s a complete mock-up, of course, of ‘Alpha’s’ control room. I’ve seen them training on it, and it’s fascinating to watch even if you don’t quite know what it’s all about.”


     “Yes, if everything goes according to plan. ‘Beta’ should have passed her final full-speed tests by then, and we’ll all be packing our trunks for Australia. By the way, have you seen those films of the first launchings?”
     “No.”
     “Remind me to let you see them—they’re most impressive.”
     “What’s she done so far?”
     “Four and a half miles a second (7.24 km/s) with full load. That’s a bit short of orbital speed (7.8 km/s plus typically 1.5–2 km/s for atmospheric drag and gravity drag), but everything was still working perfectly. It’s a pity, though, that we can’t test ‘Alpha’ before the actual flight.”
     “When will that be?”
     “It’s not fixed yet, but we know that the take-off will be when the Moon’s entering her first quarter. The ship will land in the Mare Imbrium region while it’s still early morning. The return’s scheduled for the late afternoon, so they’ll have about ten Earth-days there.”
     “Why the Mare Imbrium, in particular?”
     “Because it’s flat, very well mapped, and has some of the most interesting scenery on the Moon. Besides, spaceships have always landed there since Jules Verne’s time. I guess that you know that the name means ‘Sea of Rains.’”

Two hundred and seventy miles above the Earth, “Beta” was making her third circuit of the globe. Skirting the atmosphere like a tiny satellite, she was completing one revolution every ninety minutes (implies an altitude of 283 kilometers! I suspect that the "two hundred and seventy miles" is an error, should be "two hundred and seventy kilometers" or "one hundred eight miles"). Unless the pilot turned on her motors again, she would remain here forever, on the frontiers of space.

Yet, “Beta” was a creature of the upper atmosphere rather than the deeps of space. Like those fish which sometimes clamber on to the land, she was venturing outside her true element, and her great wings were now useless sheets of metal burning beneath the savage sun. Not until she returned to the air far beneath would they be of any service again.

Fixed upon “Beta’s” back was a streamlined torpedo that might, at first glance, have been taken for another rocket. But there were no observation ports, no motor nozzles, no signs of landing gear. The sleek metal shape was almost featureless, like a giant bomb awaiting the moment of release. It was the first of the fuel containers for “Alpha,” holding tons of liquid methane which would be pumped into the spaceship’s tanks when it was ready to make its voyage.

“Beta” seemed to be hanging motionless against the ebon sky, while the Earth itself turned beneath her. The technicians aboard the ship, checking their instruments and relaying their findings to the control stations on the planet below, were in no particular hurry. It made little difference to them whether they circled the Earth once or a dozen times. They would stay in their orbit until they were satisfied with their tests—unless, as the chief engineer had remarked, they were forced down earlier by a shortage of cigarettes.

Presently, minute puffs of gas spurted along the line of contact between “Beta” and the fuel tank upon her back. The explosive bolts connecting them had been sheared: very slowly, at the rate of a few feet a minute, the great tank began to drift away from the ship.

In the hull of “Beta” an airlock door opened and two men floated out in their unwieldy spacesuits. With short bursts of gas from tiny cylinders, they directed themselves toward the drifting fuel tank and began to inspect it carefully. One of them opened a little hatch and started to take instrument readings, while the other began a survey of the hull with a portable leak detector.

Nothing else happened for nearly an hour, apart from occasional spurts of vapor from “Beta’s” auxiliary steering jets. The pilot was turning her so that she pointed against her orbital motion, and was obviously taking his time over the maneuver. A distance of nearly a hundred feet now lay between “Beta” and the fuel tank she had carried up from Earth. It was hard to realize that during their slow separation the two bodies had almost circled the Earth.

The space-suited engineers had finished their task. Slowly they jetted back to the waiting ship and the airlock door closed again behind them. There was another long pause as the pilot waited for the exact moment to begin braking.

Quite suddenly, a stream of unbearable incandescence jetted from “Beta’s” stern. The white-hot gases seemed to form a solid bar of light. To the men in the ship, normal weight would have returned again as the motors started to thrust. Every five seconds, “Beta” was losing a hundred miles an hour of her speed (deceleration 8.94 m/s2). She was breaking her orbit, and would soon be falling back to Earth.

The intolerable flame of the atomic rocket flickered and died. Once more the little controlling jets spurted vapor: the pilot was in a hurry now as he swung the ship round on her axis again. Out in space, one orientation was as good as another—but in a few minutes the ship would be entering atmosphere and must be pointing in the direction of her motion.

It would always be a tense moment, waiting for that first contact. To the men in the ship, it came in the form of a gentle but irresistible tugging of their seatstraps. Slowly it increased, minute by minute, until presently there came the faintest whisper of sound through the insulation of the walls. They were trading altitude for speed—speed which they could only lose against air-resistance. If the rate of exchange was too great, the stubby wings would snap, the hull would turn to molten metal, and the ship would crash in meteoric ruin down through a hundred miles of sky.

The wings were biting again into the thin air streaming past them at eighteen thousand miles an hour (8,100 m/s). Although the control surfaces were still useless, the ship would soon be responding sluggishly to their commands. Even without the use of his engines, the pilot could choose a landing spot almost anywhere on Earth. He was flying a hypersonic glider whose speed had given it world-wide range.

Very slowly, the ship was settling down through the stratosphere, losing speed minute by minute. At little more than a thousand miles an hour (450 m/s), the air-scoops of the ramjets were opened and the atomic furnaces began to glow with deadly life. Streams of burning air were being blasted from the nozzles and in its wake the ship was leaving the familiar reddish-brown tinge of nitric oxides. It was riding the atmosphere again, safely under power, and could turn once more for home.

The final test was over. Almost three hundred miles above, exchanging night and day every forty minutes, the first fuel tank was spinning in its eternal orbit.

(ed note: I originally had some incorrect calculations here, because I misinterpreted the above sentence to mean that the tank was in a 40 minute orbit. Francis Drake‏ and Michael Hutson pointed out my error. "Exchanging night and day" means 40 minutes is one-half an orbit, so the total orbit is 80 minutes. Even then, I still calculate that an object in an equatorial orbit of 80 minutes will have an altitude of 219 kilometers, or only 136 miles. Not three hundred, as stated. I suspect an error on the part of the person charged with translating the British metric system into American imperial units.)

In a few days its companions would be launched in the same path, by the same means. They would be lashed together, awaiting the moment when they would pour their contents into the empty tanks of “Alpha” and speed the spaceship on the journey to the Moon….


“We had, then, to design some kind of atomic reactor which would heat a gas stream to a very high temperature indeed—at least 4,000 degrees Centigrade. Since all known metals melt a long way below this, the problem gave us a bit of a headache! “The answer we produced is called the ‘line-focused reactor.’ It’s a long, thin, plutonium pile, and gas is pumped in at one end and becomes heated as it travels through. The final result is a central core of intensely hot gas into which we can concentrate or focus the heat from the surrounding elements. In the middle the jet temperature is over 6,000 degrees—hotter than the sun—but where it touches the walls it’s only a quarter of this.

“So far, I haven’t said what gas we’re going to use. I think you’ll realize that the lighter it is—strictly speaking, the lower its molecular weight—the faster it will be moving when it comes out of the jet. Since hydrogen is the lightest of all elements, it would be the ideal fuel, with helium a fairly good runner-up. I ought to explain, by the way, that we still use the word ‘fuel,’ even though we don’t actually burn it but simply use it as a working fluid.” (in this website, the plutonium is the "fuel", the hydrogen is the "propellant" or "reaction mass")

“That’s one thing that had me puzzled,” confessed Dirk. “The old chemical rockets carried their own oxygen tanks, and it’s a bit disconcerting to find that the present machines don’t do anything of the sort.”

Collins laughed. “We could even use helium as a ‘fuel,’” he said, “though that won’t burn at all—or indeed take part in any chemical reaction.

“Now although hydrogen’s the ideal working fluid, as I called it, it’s impossible stuff to carry round. In the liquid state it boils at a fantastically low temperature, and it’s so light that a spaceship would have to have fuel tanks the size of gasometers. So we carry it combined with carbon in the form of liquid methane—CH4—which isn’t hard to handle and has a reasonable density. In the reactor it breaks down to carbon and hydrogen. The carbon’s a bit of a nuisance, and tends to clog the works, but it can’t be helped. Every so often we get rid of it by turning off the main jet and flushing out the motor with a draft of oxygen. It makes quite a pretty firework display.

“That, then, is the principle of the spaceship’s motors. They give exhaust speeds three times that of any chemical rocket (say 13,000 m/s or an Isp of 1,350 sec), but even so still have to carry a tremendous amount of fuel. And there are all sorts of other problems I’ve not mentioned: shielding the crew from the pile radiations was the worst.

“‘Alpha,’ the upper component of the ‘Prometheus,’ weighs about three hundred tons of which two hundred and forty are fuel (mass ratio of 5.0). If it starts from an orbit around the Earth, it can just make the landing on the Moon and return with a small reserve (delta-V about 12,400 m/s, assuming about 3,200 m/s of aerobraking on the Moon-LEO leg). It has, as you know, to be carried up to that orbit by ‘Beta.’ ‘Beta’ is a very heavy, super-high-speed flying-wing, also powered by atomic jets. She starts as a ramjet, using air as ‘fuel,’ and only switches over to her methane tanks when she leaves the top of the atmosphere. As you’ll realize, not having to carry any fuel for the first stage of the journey helps things enormously.

At take-off, the ‘Prometheus’ weighs five hundred tons, and is not only the fastest but the heaviest of all flying machines. To get it airborne, Westinghouse have built us a five-mile-long electric launching track out in the desert. It cost nearly as much as the ship itself, but of course it will be used over and over again.

“To sum up, then: we launch the two components together and they climb until the air’s too thin to operate the ramjets any more. ‘Beta’ then switches over to her fuel tanks and reaches circular velocity at a height of about three hundred miles (480 km?). ‘Alpha,’ of course, hasn’t used any fuel at all—in fact, its tanks are almost empty when ‘Beta’ carries it up.

“Once the ‘Prometheus’ has homed on the fuel containers we’ve got circling up there, the two ships separate, ‘Alpha’ couples up to the tanks with pipelines and pumps the fuel aboard. We’ve already practiced this sort of thing and know it can be done. Orbital refuelling, it’s called, and it’s really the key to the whole problem, because it lets us do the job in several stages. It would be quite impossible to build one huge spaceship that would make the journey to the Moon and back on a single load of fuel.

“Once ‘Alpha’s’ tanked up, it runs its motors until it’s built up the extra two miles a second (+3.2 km/s) to get out of its orbit and go to the Moon. It reaches the Moon after four days, stays there a week and then returns, getting back into the same orbit as before. The crew transfers to ‘Beta,’ which is still patiently circling with her very bored pilot (who won’t get any of the publicity) and is brought down to Earth again. And that’s all there is to it. What could be simpler?”


The ‘Prometheus’ is out there, lying under the floodlights. It’s strange to think that she—or rather ‘Beta’—has been up into space a dozen times or more on those fueling runs. Yet ‘Beta’ belongs to our planet, while ‘Alpha,’ which is still earthbound, will soon be up among the stars, never to touch the surface of this world again.


Even when first seen from ground level a mile away, the “Prometheus” was an impressive sight. She stood on her multiple undercarriage at the edge of the great concrete apron around the launcher, the scoops of her air-intakes gaping like hungry mouths. The smaller and lighter “Alpha” lay in its special cradle a few yards away, ready to be hoisted into position. Both machines were surrounded with cranes, tractors and various types of mobile equipment.

A rope barrier was slung round the site, and the truck halted at the opening in the cordon, beneath a large notice which read:

  • WARNING-RADIOACTIVE AREA!
  • No unauthorized persons allowed past this point.
  • Visitors wishing to examine the ship, contact Ext. 47 (Pub. Rel. IIa).
  • THIS IS FOR YOUR PROTECTION!

They were now standing beneath the slim, pointed snout of “Beta” and her great wings, sweeping away from them on either side, made her look like a moth in repose. The dark caverns of the air-scoops looked ominous and menacing, and Dirk was puzzled by the strange fluted objects which protruded from them at various places. Collins noticed his curiosity.

“Shock diffusers,” he explained. “It’s quite impossible to get one kind of air-intake to operate over the whole speed range from five hundred miles an hour at sea level to eighteen thousand miles an hour at the top of the stratosphere. Those gadgets are adjustable and can be moved in and out. Even so the whole thing’s shockingly inefficient and only the fact that we’ve unlimited power makes it possible at all. Let’s see if we can get aboard.”

Her stubby undercarriage made it easy to enter the machine through the airlock door in her side. The rear of the ship, Dirk noticed, had been carefully fenced off with great movable barriers so that no one could approach it. He commented on this to Collins.

“That part of ‘Beta,’ “said the aerodynamicist grimly, “is Strictly Out of Bounds until the year 2000 or so.”

Dirk looked at him blankly. “What do you mean?”

“Just that. Once the atomic drive’s started to operate, and the piles get radioactive, nothing can ever go near them again. They won’t be safe to touch for years.”

Even Dirk, who was certainly no engineer, began to realize the practical difficulties this must involve. “Then how the devil do you inspect the motors, or put things right when they’ve gone wrong? Don’t tell me that your designs are so perfect that there aren’t any breakdowns!”

Collins smiled. “That’s the biggest headache of atomic engineering. You’ll have a chance to see how it’s done later.”

There was surprisingly little to see aboard “Beta,” since most of the ship consisted of fuel tanks and motors, invisible and unapproachable behind their barriers of shielding. The long, thin cabin at the nose might have been the control room of any airliner, but was more elaborately appointed since the crew of pilot and maintenance engineer would be living aboard her for nearly three weeks. They would have a very boring time, and Dirk was not surprised to see that the ship’s equipment included a microfilm library and projector. It would be unfortunate, to say the least, if the two men had incompatible personalities: but no doubt the psychologists had checked this point with meticulous care.


“Alpha” was an even more compact mass of motors and fuel tanks than the bigger ship. It had, of course, no fins or aerofoils of any kind, but there were signs that many oddly-shaped devices had been retracted into the hull. Dirk asked his friend about these.

“Those will be the radio antennae, periscopes, and outriggers for the steering jets,” explained Collins. “Back at the rear you’ll see where the big shock absorbers for the lunar landing have been retracted. When ‘Alpha’s’ out in space they can all be extended and the crew can check ’em over to see if they’re working properly. They can then stay out for good, since there’s no air resistance for the rest of the voyage.”

There was radiation screening around “Alpha’s” rocket units, so it was impossible to get a complete view of the spaceship. It reminded Dirk of the fuselage of an old-fashioned airliner which had lost its wings or was yet to acquire them. In some ways “Alpha” strongly resembled a giant artillery shell, with an unexpected circlet of portholes near the nose. The cabin for the crew occupied less than a fifth of the rocket’s length. Behind it were the multitudinous machines and controls which would be needed on the half-million-mile journey.

Collins roughly indicated the different sections of the machine.

“Just behind the cabin,” he said, “we’ve put the airlock and the main controls which may have to be adjusted in flight. Then come the fuel tanks—six of them—and the refrigeration plant to keep the methane liquid. Next we have the pumps and turbines, and then the motor itself which extends halfway along the ship. There’s a great wad of shielding around it, and the whole of the cabin is in the radiation shadow so that the crew gets the maximum protection. But the rest of the ship’s ‘hot,’ though the fuel itself helps a good deal with the shielding.”

The tiny airlock was just large enough to hold two people, and Collins went ahead to reconnoiter. He warned Dirk in advance that the cabin would probably be too full to admit visitors, but a moment later he emerged again and signaled for him to enter.

“Everyone except Jimmy Richards and Digger Clinton had gone over to the workshops,” he said. “We’re in luck—there’s bags of room.”

That, Dirk soon discovered, was a remarkable exaggeration. The cabin had been designed for three people living under zero gravity, when walls and floor were freely interchangeable and its whole volume could be used for any purpose. Now that the machine was lying horizontally on Earth, conditions were decidedly cramped.

“Don’t look alarmed,” he said as Dirk watched him anxiously. “We won’t take off—there’s nothing in the fuel tanks!”

“I’m getting rather a complex about this,” Dirk confessed. “Next time I come aboard, I’d like to make sure that we’re tied down to a nice, fat anchor.”

“As some anchors go,” laughed Richards, “it needn’t be such a big one. ‘Alpha’ hasn’t much thrust—about a hundred tons. But it can keep it up for a long time!” “Only a hundred tons thrust? But she weighs three times that!”

Collins coughed delicately in the background. “It, I thought we decided,” he remarked. However, Richards seemed willing to adopt the new gender.

“Yes, but she’s in free space when she starts, and when she takes off from the Moon her effective weight will be only about thirty-five tons. So everything’s under control.”

The layout of “Alpha’s” cabin seemed to be the result of a pitched battle between science and surrealism. The design had been determined by the fact that for eight days the occupants would have no gravity at all, and would know nothing of “up” or “down”; while for a somewhat longer period, when the ship was standing on the Moon, there would be a low gravitational field along the axis of the machine. As at the moment the center-line was horizontal, Dirk had a feeling that he should really be walking on the walls or roof.


“There’s some more to see yet. Let’s go over to the launcher.” The launching track was impressive by its very simplicity. Two sets of rails began in the concrete apron—and went straight out to disappear over the horizon. It was the finest example example of perspective that Dirk had ever seen.

The catapult shuttle was a huge metal carriage with arms that would grasp the “Prometheus” until the ship had gained flying speed. It would be just too bad, Dirk thought, if they failed to release at the right time.

“Launching five hundred tons (450 metric tons) at as many m.p.h. (480 km/h) must take quite a generating plant,” he said to Collins. “Why doesn’t the ‘Prometheus’ take off under her own power?”

“Because with that initial loading she stalls at four-fifty, and the ramjets don’t operate until just above that. So we have to get up speed first. The energy for the launch comes from the main power station over there; that smaller building beside it houses a battery of flywheels which are brought up to speed just before the take-off. Then they’re coupled directly to the generators.”

“I see,” said Dirk. “You wind up the elastic, and away she goes.”

“That’s the idea,” Collins replied. “When ‘Alpha’s’ launched, ‘Beta’ isn’t overloaded any more, and can be brought in to land at a reasonable speed—less than two hundred and fifty miles an hour; which is easy to anyone who makes a hobby of flying two-hundred-ton gliders!”


“You’ve all had a chance of selecting observation sites along the launching track. There should be plenty of room for everyone in the first four kilometers. But remember—no one must go past the red barrier at five kilometers. That’s where the jets start firing, and it’s still slightly radioactive from previous launchings. When the blast opens up, it will spray fission products over a wide area. We’ll give the all-clear as soon as it’s safe for you to collect the automatic cameras you have mounted out there.

“A number of people have asked when the radiation shields are being taken away from the ships so that they can be seen properly. We’ll be doing this tomorrow afternoon and you can come and watch then. Bring binoculars or telescopes if you want to look at the jet units—you won’t be allowed closer than a hundred yards. And if anyone thinks this is a lot of nonsense, there are two people in the hospital here who sneaked up to have a good look and now wish they hadn’t.


It was a pity, he meditated, that one had to leave a stand-by crew aboard “Beta” while she circled the Earth. But it could not be avoided, since the instruments and the refrigeration plant for the fuel had to be looked after, and both machines would have to be fully maneuverable in order to make contact again. One school of thought considered that “Beta” should land and take off once more a fortnight later to meet the returning “Alpha.” There had been much argument over this, but the orbital view had finally been accepted. It would be introducing fewer additional hazards to leave “Beta” where she was, already in position just outside the atmosphere.


A roughly triangular area had been roped off, so that the “Prometheus” was at one apex and they were at the base. The nearest they could get to the machine’s driving units was about a hundred yards. Looking into those gaping nozzles, Dirk felt no particular desire to come any closer. Cameras and binoculars were being brought into action, and presently Dirk managed to get his look through the telescope. The rocket motors seemed only a few yards away, but he could see nothing except a metal pit full of darkness and mystery. Out of that nozzle would soon be coming hundreds of tons of radioactive gas at fifteen thousand miles an hour (exhaust velocity of 6,700 m/s, Isp = 683 sec). Beyond it, hidden in shadow, were the pile elements that no human being could ever again approach.


“It’s rather a queer feeling, you know,” he said to Dirk, “looking at a machine you’ve helped build yourself—and which you can never go near again without committing suicide.”

While he spoke, an extraordinary vehicle was approaching across the concrete. It was a very large truck, not unlike those which television companies use for outside broadcasts, and it was towing a machine at which Dirk could only stare in baffled amazement. As it went past, he had a confused impression of jointed levers, small electric motors, chain drives and worm-wheels, and other devices he could not identify.

The two vehicles came to a halt just inside the danger area. A door opened in the big truck, and half a dozen men clambered out. They uncoupled the trailer, and began connecting it up to three large armored cables which they unwound from drums at the front of the van.

The strange machine suddenly came to life. It rolled forward on its little balloon tires, as though testing its mobility. The jointed levers began to flex and unflex, giving a weird impression of mechanical life. A moment later it started to roll purposefully toward the “Prometheus,” the larger machine following behind it at the same speed.

Collins was grinning hugely at Dirk’s amazement and the obvious surprise of the journalists around him.

“That’s Tin Lizzie,” he said, by way of introduction. “She’s not really a true robot, as every movement she makes is controlled directly by the men in the van. It takes a crew of three to run her, and it’s one of the most highly skilled jobs in the world.”

Lizzie was now within a few yards of “Alpha’s” jets, and after some precise foot-work with her bogies she came to a gentle halt. A long, thin arm carrying several obscure pieces of machinery disappeared down that ominous tunnel.

“Remote servicing machinery,” explained Collins to his interested audience, “has always been one of the most important side-lines of atomic engineering. It was first developed on a large scale for the Manhattan Project during the War. Since then it’s become quite an industry in itself. Lizzie is just one of the more spectacular products. She could almost repair a watch—or at least an alarm clock!”

“Just how does the crew control her?” asked Dirk.

“There’s a television camera on that arm, so they can see the work just as if they were watching it directly. All movements are carried out by servo motors controlled through those cables.”

No one could see what Lizzie was now doing, and it was a long time before she slowly backed away from the rocket. She was carrying, Dirk saw, a curiously shaped bar about three feet long which she held firmly in her metal claws. The two vehicles withdrew three-quarters of the way to the barrier, and as they approached the journalists hastily retreated from that drab gray object in the robot’s claws. Collins, however, stood his ground, so Dirk decided it must be safe to remain.

There was a sudden, raucous buzzing from the engineer’s coat-pocket, and Dirk jumped a foot in the air. Collins held up his hand and the robot came to a halt about forty feet away. Its controllers, Dirk guessed, must be watching them through the television eyes.

Collins waved his arms, and the bar slowly rotated in the robot’s claws. The buzzing of the radiation alarm ceased abruptly and Dirk breathed again. “There’s usually some sort of beaming effect from an irregular object like that,” explained Collins. “We’re still in its radiation field, of course, but it’s too weak to be dangerous.”


(ed note: And there is the horrifying scene I will not go into detail here. Some nut-job deluded themselves that the launch of the moon rocket violated his beliefs somehow. He tries to sabotage the rocket. He is of course instantly spotted, and told to surrender to the guards. He panics and tries to escape.

By crawling up the radioactive rocket exhaust nozzles and hiding next to the nuclear reactor.

Tin Lizzie manages to drag him out, but by that time he has suffered a radiation dose of several hundred Grays. He dies a few minutes later.)

From PRELUDE TO SPACE by Arthur C. Clarke (1951)

Mass Driver

Mass Drivers are a way to use electromagnets to hurl, well, pretty much anything. They can be mounted on spacecraft or asteroid and used as propulsion systems (hurling whatever can be put in the buckets as reaction mass). But with respect to Surface To Orbit maneuvers, you generally encounter them in space colonization promos and science fiction novels them deployed on the Lunar surface near a mine, lobing ore at an Lagrange point for the construction of an L5 Colony.

They do have the side effect of turning a spaceport into an impromptu planetary fortress. After all, they are basically huge coil guns. This was popularized in the classic Robert Heinlein novel The Moon Is A Harsh Mistress.

The acceleration track has to be in vacuum, or air friction will do unfortunate things to the cargo cannister. Mass driver launchers on Terra have to be encased in a vacuum chamber, such a in the Bifrost Bridge. On Luna or other airless world you just have place a series of acceleration rings every few meters.

Lunatron

The concept of launching cargoes and passengers off the moon using an electromagnetic track originated with Arthur C. Clarke, who first wrote about it in 1950 in the pages of the Journal of the British Interplanetary Society. The 1954 book The Exploration of the Moon, written by Clarke and illustrated by artist R.A. Smith, depicted such a device (image right). Eight years later (April 1962), Clarke published "Maelstrom II," a science fiction story based on the concept. Escher explained that he was unaware of Clarke's priority when he began his Lunatron work. After learning of it, however, he engaged in a "helpful correspondence" with the British author and spaceflight thinker.

Escher noted a limitation on the Lunatron's speed: "the centripetal acceleration resulting from the circular path imposed on the spacecraft as it is retained upon being accelerated to above circular velocity on the Moon-fixed track." As they passed lunar orbital speed (1.7 kilometers per second), trolley and payload would tend to rise away from the track. Lunar escape speed is, however, 2.4 kilometers per second, so they would need to be held down so acceleration could continue.

As the Lunatrom continued to accelerate the trolley, passengers would feel "down" shift by up to 180°, from toward the moon's center to directly away from it. Escher proposed that they "be mounted in swivel support systems to compensate for this effect." The faster the trolley moved, the more acceleration the passengers would feel in the new "down" direction. In effect, the Lunatron would become a centrifuge and the payload would become its gondola.

Escher calculated that, for a 50-to-500-kilometer-long Lunatron for launching cargoes and passengers from the moon to the Earth, acceleration would top out at a tolerable eight times the pull of Earth's gravity. However, for larger systems — such as the 870-kilometer Lunatron for throwing payloads out of the Solar System — acceleration could reach 60 Earth gravities.

The MSFC engineer proposed siting the Lunatron for launching beyond the Solar System at the center of the moon's Farside hemisphere. Launching there at local midnight would take advantage of the orbital speeds of the moon around the Earth and the Earth around the Sun, slashing the velocity the Lunatron would need to provide from 42.5 kilometers per second to just 12 kilometers per second. This would in turn limit the acceleration to which its passengers would be subjected.

Building a long Lunatron track, Escher wrote, would constitute "an almost overwhelmingly large construction job," with "extensive cuts. . .through mountains [and] fills or bridge structures. . .across low areas." He maintained that the magnitude of the construction task, combined with the large amount of electricity needed to accelerate payloads, would mean that the Lunatron would probably not become available until "well after the start of colonization of the Moon."

"On the Utility of the Moon in Space Transportation: the Lunatron Concept," William J. D. Escher, Engineering Problems of Manned Interplanetary Exploration, pp. 102-112; paper presented in Palo Alto, California, September 30-October 1, 1963.
From Lunatron by David Portree (2009)

Laser Launch

SystemPayload mass delivered to LEOCost per payload kilogram
Pournelle? metric tons/year$1.9/kg plus power plant amortization
Jordin Kare HX Laser Launch3000 metric tons/year$550/kg

Details about Laser Launch can be found here.

Matter Beam points out that the system will also work with an orbiting spacecraft equipped with a powerful laser battery, sending a beam to assist a surface-to-orbit shuttle lifting off. This can come in handy if the planet does not have a ground based laser launch facility, for instance an exploration spacecraft orbiting an uninhabited planet helping one of its landing craft return to the ship. A warship could also use its laser weapon batteries to give a boost to its fighters and missiles during a space battle, but I digress.

An important thing to keep in mind is that a laser-launch site is functionally equivalent to a planetary fortress. It can hurl projectiles and use laser beams directly at any invading spacecraft.

LASER LAUNCH DETAILS

This magic feat is performed by lasers. The basic design of the system comes from A. N. Pirri and R. F. Weiss of Avco-Everett research laboratories (based on a concept from a paper by Arthur (Arky) Kantrowitz also of Avco-Everett). What they propose is an enormous ground-based laser installation consuming about 3,000 megawatts. In practice, there would probably be a number of smaller lasers feeding into mirrors, and the mirrors would then concentrate the beam onto one single (steerable) launching mirror about a meter in diameter. This ground station zaps the spacecraft; the ships themselves carry no rocket motors, but instead have a chamber underneath into which the laser beam is directed.

The spacecraft weigh about a metric ton (1000 kilograms or 2200 pounds) and are accelerated at 30 g's for about 30 seconds; that puts them in orbit. While the capsule is in the atmosphere the laser is pulsed at about 250 hertz (cycles per second when I was in school). Each pulse causes the air in the receiving chamber to expand and be expelled rapidly. The chamber refills and another pulse hits: a laser-powered ramjet. For the final kick outside the atmosphere the laser power is absorbed directly in the chamber and part of the spacecraft itself is ablated off and blown aft to function as reaction mass. Of the 1000 kg. start-weight, about 900 kg. goes into orbit.

Some 80 metric tons can be put into orbit each hour at a total cost of around 3000 megawatt-hours. Figuring electricity at 3¢ a kilowatt hour, that's $150 thousand, less than a dollar a kilogram for fuel costs. Obviously there are operating costs and the spacecraft aren't free, but the whole system is an order of magnitude more economical than anything we have now.

Conventional power plants cost something like $300 a kilowatt; a 3000 megawatt power plant would run close to a billion dollars in construction costs. However, when it isn't being used for space launches it could feed power into the national grid, so some of that is recovered as salable power. The laser installation might easily run $5 billion, and another $5 billion in research may be needed.

The point is that for an investment on the order of what we put out to go to the Moon, we could buy the research and construct the equipment for a complete operating spaceflight system, and then begin to exploit the economic possibilities of cheap spaceflight.

There are a lot of benefits to an economical system for getting into orbit. Some are commercial, things like materials that can only be made in gravity-free environments and such like. Others are not precisely commercial, but highly beneficial. For example, the power/pollution problem is enormously helped. Solar cells can collect sunlight that would have fallen onto the Earth. They convert it to electricity and send it down from orbit by microwave. That's fed into the power grid, and when it's used it becomes heat that would have arrived here anyway; the planetary heat balance isn't affected.

Interestingly enough, it's now believed that orbiting solar power plants can be economically competitive with conventional plants, provided that we get the cost of a pound in orbit down to about $20. The laser-launch system could power itself.

We don't even have to build a permanent power plant to get the laser-launcher into operation. There are a lot of old rocket motors around, and they're very efficient at producing hot ionized gasses. Hot ionized gas is the power source for electricity extracted by magneto-hydro-dynamics, or MHD. MHD is outside the scope of this article, but basically a hot gas is fed down a tube wrapped with conducting coils, and electricity comes out. MHD systems are about as efficient as turbine systems for converting fuel to electricity, and they can burn hydrogen to reduce pollution.

The rocket engines wouldn't last forever, and it takes power to make the hydrogen they'd burn—but we don't have to use the system forever. It needn't last longer than it takes to get the big station built in space and start up a solar-screen power plant.

None of this is fantasy. The numbers work. Avco has done some experiments with small-scale laser powered "rockets," and they fly. There are no requirements for fundamental breakthroughs, only a lot of development engineering, to get a full-scale working system.

(ed note: for all you young whipper-snappers, ¢ is the "cent" sign. It symbolizes the number of pennies, much like the $ symbolizes the number of dollars)

From A STEP FARTHER OUT by Jerry Pournelle (1979)
HIGH JUSTICE

      The tower overlooked a valley ringed by low hills. A forest of cardones, the great sentinel cactus, marched down the sides of the hills to the leveled plain below. Rail lines and huge electric cables snaked through at either end; the plain was filled with concrete blockhouses where the power cables terminated. At the end of each blockhouse was a flat mirror a meter in diameter, and they all pointed toward the installation below them where streamlined cylinders squatted on railroad cars.
     The spacecraft were two meters in diameter and five times that tall, and as they waited in neat lines for their turn they reminded Aeneas of machine gun ammunition grown swollen and pregnant; but their progeny was not war.

     Everyone in the tower had been politely respectful, but harried; now they had no time for visitors. Hansen Enterprises carried no dead weight. There were no explainers, not even when the owner came to watch the operations; perhaps especially when Laurie Jo Hansen was present. Aeneas and Laurie Jo were alone in a small, glass-enclosed room, while below a dozen hard-eyed young men sat at consoles.
     A clock ticked off the seconds. "We have to be very precise," she told him. "The MHD engines give us half the power we need, but we have to draw the rest directly from the line. There'll be dim-outs all over Baja."
     "And it costs," Aeneas said.
     "Yes. Three thousand megawatts for an hour. At twenty cents a kilowatt hour."
     "But you get part of the power directly."
     "From burning hydrogen in old rocket engines and sending it through an MHD system. Yes. But the hydrogen and oxygen have to be made. That part of the operation is less efficient than just taking the power from the line, but we have to do it. We can't take everything off the line when we launch." She looked fondly at the capsules below. "We get a lot for my six hundred thousand dollars, Aeneas. Eighty tons go into orbit in the next hour."

     The first of the capsules moved over the embankment enclosing the launch area. A roar from beyond the low hills signaled the beginning of the rocket engines: giant engines, but they lay on their sides, their exhaust directed down ceramic tubes protecting copper coils that drew power directly from the hot gasses.
     Aeneas couldn't see the launching mirror below the capsule, but suddenly the spacecraft rose and there was a blinding green beam, a solid rod of light over a meter thick extending from the capsule to the ground. The sound rolled past: two hundred and fifty explosions each second as the laser expanded the air in the parabolic chamber below the capsule, and the air rushed out to propel it upward. The two hundred and fifty-cycle note was oddly musical, but very loud at first, then dying away. The spacecraft soon vanished, but the light stayed on for half a minute, tracking the capsule; then it vanished as well.
     The mirrors at each blockhouse pivoted slightly, and a second capsule rose from another launch station. The green light tore through roiled air, and there was a humming roar that vibrated the glass of the observation room until the spacecraft was gone and there was only the silent power of the green light. In the half minute that the second capsule absorbed power, a new spacecraft had been placed on the first launch station. The mirrors pivoted again, and it rose; then another, and another.
     The laser launchings had been impressive on TV; live they were unbelievable. The long lines of capsules moved toward the earth and concrete emplacements protecting the launching mirror; they reached them; and seconds later, each capsule vanished at 30 gees, shoved upward by a meter-thick column that was nothing more than light, but which looked like a great green growing plant.
     "About a thousand kilograms each?" Aeneas asked.
     "Exactly a thousand kilos total weight," she said. "We lose fifty kilos of ablating material. The rest goes into orbit, and that's all payload. Any mass is payload. That's what we need up there, Aeneas, mass, any mass—metal, fuel, gases, tankage, even human wastes. We can convert and modify if we have something to start with."
     "And you can launch eighty thousand kilos in one hour . . ."
     "Yes. We lose some. Each one of those capsules has to be picked up, somehow. That costs mass. We guide some into rendezvous with Heimdall, but they have to go after most. Still it's cheaper this way—once we start launching, the power scheduling's such that it's better to go on for a full hour."

     The lines of capsules had ended; now new ones were brought up. These were longer and slimmer than the others; and when they took their places over the launching mirrors, they rose more slowly.
     "Ten gees," she said. "Crew capsules. Ten gees for a minute and a half."
     "Isn't that close to human tolerance?"
     "Not really." Her voice was cold and distant. "I took it. And if I can—"
     He finished the thought for her. "Hansen Enterprises employees will damn well have to. Or starve."
     "I want no one who goes only for the money."
     They watched the three personnel capsules rise; then the trains brought up more of the unmanned thirty-g cargo capsules, and the pregnant machine gun began again. "And this was what it was all for. Your crusade," he said.
     Her smile was wistful, full of triumph and regret. "Yes. I'm not proud of all I've done, Aeneas. You've seen La Paz. Todos Santos. Cabo. Ugly, changed, not what they were when we—not what they were. But the men in Cabo don't go to the mainland looking for work while their families starve. I've done that."
     "Yes. You've done that."
     "But it was all only fallout, Aeneas. This is what it was for. Heimdall. The rainbow bridge to the stars! And by God it was worth it! You haven't seen the station, Aeneas. And I want you to."
     He said nothing, but he looked out at the launching field. The lasers were off now. The great crippled rocket engines were silent. The power from the reactors was back on line, fed to the Baja industries, to Southern California; to the pumps even now cooling the laser installations. To the water-makers that made Baja fertile, for a while. But all that was incidental, because she hadn't lost the dream they'd shared, a dream she'd learned from him in his anger when America retreated from adventure…

(ed note: The bread-and-butter of Hansen Enterprises is valuable products manufactured in the free-fall environment of the Hansen space station i.e., MacGuffinite. They are delivered back to Terra by a reentry capsule. These have to be captured in mid-air, because once they land in the water the capsules are considered "salvage" under international law. Because the contents of each is worth about seven million US , the drop sites swarm with pirates.)

     They flew high over the Pacific. There were no luxuries in this aircraft; Aeneas and Laurie Jo sat uncomfortably in bucket seats over the wing, and Miguel sat far behind them. Neither the pilot nor the air crew paid them any attention. The pilot was not pleased to have them aboard, no matter that the plane belonged to Laurie Jo Hansen.
     Two armed jets flew high above them. They bore the markings of Hansen Enterprises and were registered in Mexico; and the bribes required to keep permission for a private air force were as staggering as the cost of operating them.
     "Why?" Aeneas asked, pointing to the slim black delta shapes above.
     "Pirates," she said. "Each capsule holds a thousand kilos of cargo." She took papers from her briefcase and handed them to him. "Computer chips, four thousand dollars a kilo. Water-maker membranes, six thousand dollars a kilo if we'd sell them. We won't until we've enough for ourselves. Concentrated vitamins, forty-five hundred dollars a kilo. And other things. Chemicals, vaccines. Some not for sale at any price."
     The value of each capsule in the current drop was nearly seven million dollars ($35,000,000 in 2017 dollars). Even in these inflated times that was enough money to make a man wealthy for life. And there would be no problem selling the cargo . . . .

(ed note: Aeneas MacKenzie is being boosted into orbit by laser launcher.)

     Ten gravities for ninety seconds is easily within the tolerance of a healthy man; but Aeneas had no wish to prolong the experience. He was laid flat on his back in a nylon web, encased in baggy reflective coverall and under that a tight garment resembling a diver's wet suit (a skintight space suit). The neckseal and helmet were uncomfortable, and it was an effort to exhale against the higher pressures in the helmet.
     He had thought waiting for the launch the most unpleasant experience he'd ever had: lying awkwardly on his back, with no control of his destiny, enclosed in steel; then the laser cut in.
     He weighed far too much. His guts ached. Like the worst case of indigestion imaginable, he thought. There was no way to estimate the time. He tried counting, but it was too difficult, and he lost count somewhere. Surely he had been at eighty seconds? He started over again.
     There was noise, the loud, almost musical two-hundred-fifty-cycle tone of the explosions produced as the laser heated the air in the chamber under him—how close? he wondered. That great stabbing beam that could slice through metal aimed directly at him; he squirmed against the high gravity, and the effort was torture.
     The noises changed. The explosion tone drifted down the scale. He was beyond the atmosphere, and the laser was boiling off material from the thrust chamber, reaching closer and closer to him—

     Silence. The crushing weight was gone. He was falling endlessly, with no way to know. Was he in orbit? Or was he plunging downward to his doom? He closed his eyes to wait, and then he felt he was truly falling, with the sick sensations of a boat in motion—he opened his eyes again to orient himself in the capsule.
     Will they pick me up? There was no reason they shouldn't. New crewmen arrived weekly, and he was merely another. He listened for a voice, a signal, anything—
     "Hullo, laddie. All right in there?"
     Aeneas grabbed for the microphone and pressed the talk switch. "That was one hell of a ride." He fought for control of his voice. "I think I'm all right now."

From HIGH JUSTICE by Jerry Pournelle (1974)
LIFE AMONG THE ASTEROIDS

That's the concept, and I think I was the first to use it in a science fiction story. Imagine my surprise, then, when at an AAAS meeting I heard Freeman Dyson of Princeton's Institute for Advanced Studies give a lecture on laser-launched systems as "highways to space."

Dyson is, of course, one of the geniuses of this culture. His Dyson spheres have been used by countless science fiction writers (Larry Niven cheerfully admits that he stole the Ringworld from Dyson). One should never be surprised by Freeman Dyson—perhaps I should rephrase that. One is always surprised by Freeman Dyson. It's just that you shouldn't be surprised to find you've been surprised, so to speak.

Dyson wants the U.S. to build a laser-launching system. It is, he says, far better than the shuttle, because it will give access to space—not merely for government and big corporations, but for a lot of people.

Dyson envisions a time when you can buy, for about the cost of a present-day house and car, a space capsule. The people collectively own the laser-launch system, and you pay a small fee to use it. Your capsule goes into orbit. Once you're in orbit you're halfway to anyplace in the solar system. Specifically, you're halfway to the L-5 points, if you want to go help build O'Neill colonies. You're halfway to the asteroid Belt if you'd like to try your hand at prospecting. You're halfway to Mars orbit if that's your desire.

America, Dyson points out, wasn't settled by big government projects. The Great Plains and California were settled by thousands of free people moving across the plains in their own wagons. There is absolutely no reason why space cannot be settled the same way. All that's required is access.

Dangerous? Of course. Many families will be killed. A lot of pioneers didn't survive the Oregon Trail, either. The Mormons' stirring song "Come Come Ye Saints" is explicit about it: the greatest rewards go to those who dare and whose way is hard.

That kind of Highway to Space would generate more true freedom than nearly anything else we could do; and if the historians who think one of the best features of America was our open frontiers, and that we've lost most of our freedom through loss of frontier—if they're right, we can in a stroke bring back a lot of what's right with the country.

Why don't we get at it?


Dyson envisions a time when individual families can buy a space capsule and, once Out There, do as they like: settle on the Moon, stay in orbit, go find an asteroid; whatever. It will be a while before we can build cheap, self-contained space capsules operable by the likes of you and me; but it may not be anywhere as long as you think.

The problem is the engines, of course; there's nothing else in the space home economy that couldn't, at teast in theory, be built for about the cost of a family home, car, and recreational vehicle. But then most land-based prefabricated homes don't have their own motive power either; they have to hire a truck for towing.

It could make quite a picture: a train of space capsules departing Earth orbit for Ceres and points outward, towed by a ship something like the one I described in "Tinker." Not quite Ward Bond in Wagon Train, but it still could make a good TV series. The capsules don't have to be totally self-sufficient, of course. It's easy enough to imagine way stations along the route, the space equivalent of filling stations in various orbits.

Dyson is fond of saying that the U.S. wasn't settled by a big government settlement program, but by individuals and families who often had little more than courage and determination when they started. Perhaps that dream of the ultimate in freedom is too visionary; but if so, it isn't because the technology won't exist.

However we build our Moonbase, it's a very short step from there to asteroid mines. Obviously the Moon is in Earth orbit: with the shallow Lunar gravity well it's no trick at all to get away from the Moon, and Earth's orbit is halfway to anywhere in the solar system. We don't know what minerals will be available on the Moon. Probably it will take a while before it gets too expensive to dig them up, but as soon as it does, the Lunatics themselves will want to go mine the asteroids.

There's probably more water ice in the Belt than there is on Luna, so for starters there will be water prospectors moving about among the asteroids. The same technology that sends water to Luna will send metals to Earth orbit.

Meanwhile, NERVA or the ion drive I described earlier will do the job. In fact, it's as simple to get refined metals from the Asteroid Belt to near-Earth orbit as it is to bring them down from the Lunar surface. It takes longer, but who cares? If I can promise GM steel at less than they're now paying, they'll be glad to sign a "futures" contract, payment on delivery.

It's going to be colorful out in the Belt, with huge mirrors boiling out chunks from mile-round rocks, big refinery ships moving from rock to rock; mining towns, boom-towns, and probably traveling entertainment vessels. Perhaps a few scenes from the wild west, or the Star Wars bar scene? "Claim jumpers! Grab your rifle—"

Thus from the first Moonbase we'll move rapidly, first to establish other Moon colonies (the Moon's a big place) and out to the Asteroid Belt. After that we'll have fundamental decisions to make. We can either build O'Neill colonies or stay with planets and Moons. I suspect we'll do both. While one group starts constructing flying city-states at the Earth-Moon Trojan points, another will decide to make do with Mars.

From A STEP FARTHER OUT by Jerry Pournelle (1979)

Lightcraft

A Lightcraft is a type of laser launch vessel. Air enters in through vents at the waist. A laser beam is shined at the parabolic mirror on the base where it flash-heats the air there into plasma. The plasma rapidly escapes out of the base creating thrust. More air enters in through the waist vents and the cycles start again.

Since it is using atmospheric gas for propellant instead of on-board propellant, and the mass of the engine is at the spaceport instead of being on-board, most of the mass of the spacecraft will be payload. Instead of being mostly non-payload like most other booster vehicles.

Beamed Energy Propulsion (BEP)

This is from Beamed-Energy Propulsion (BEP) Study. The report looks at three different types of laser launch systems which are reasonably mature. This means the payloads are pretty small, forty to eighty kilograms as most (40 kg ` six cubesats). The payload mass will rise with technological advancement.

The hope was that using optical and millimeter wave lasers to power propulsion systems would give high exhaust velocity and high thrust. Plus the advantage that the power plant is on the ground instead of adding penalty mass to the boost vehicle.

  • LASER OPTICAL: mirrored cowl intercepts and focuses laser light from a ground-based installation to heat atmosphere or water propellant.
  • LASER THERMAL: propellant is circulated inside a large heat exchanger (HX). The exchanger is heated by a visible-light laser beam from a ground installation.
  • MILLIMETER WAVE THERMAL: same as laser thermal, except instead of a visible light laser a microwave laser is used instead.

All the designs use two ground laser installations. The first is the "boost" beaming station, it is optimized to propel the spacecraft from the launch pad to high altitude as fast as possible. The "main" beaming station located downrange is optimized to delta-V the spacecraft up to orbital velocity and orbital height.


LASER OPTICAL

This engine operates in air-breathing laser ramjet mode from launch up to the time it reaches Mach 7 and an altitude of 35 kilometers. Then is switches to rocket mode using water propellant.

In both modes visible laser light is interceped by the mirrored base of the boost vehicle and funneled into the cowl. There the laser energy heats up either atmospheric gases or water propellant. The hot propellant exists through the bottom of the cowl, providing thrust. In ramjet mode the spacecraft forebody directs atmospheric gases into slots on the top of the cowl. The gases are compressed and injected into the laser cavity. In rocket mode the slots are closed, and water from onboard tanks is injected into the cowl.

The vehicle assembly building and the laser boost beaming station are a single building, unlike the other two concepts. This is because the laser beam has to be directed upwards into the base of the vehicle. The other two concepts direct the laser beam at the ventral side of the vehicle.

Risks and issues:

If the mirrored surface of the base and inside the cowl is damaged or degradated, the 3000 watts per square centimeter of laser energy will quickly burn through and destroy the launch vehicle. Mirror damage can come from debris impact or erosion by the propellant plasma.

If the propellant plasma comes witin a few centimeters of the mirror surface, there will be excessive heating. This is because the mirror surface is not as refective to the heat frequency from the plasma as it is to the laser beam frequency.

Laser light reflected off the mirror can possibly reach the surface of Terra, which could damage the eyesight of people on the ground watching the launch.


LASER THERMAL

A large external heat exchanger (HX) is heated by the ground-based laser installation. Liquid propellant (water and liquid hydrogen) from onboard tanks is heated inside the HX, and escapes through a conventional rocket nozzle to provide thrust.

Risks and issues:

In order to achieve the required heat transfer capabilities, the heat exchanger walls are very thin. This means the HX is very fragile. It can be broke by:

  • Aerodynamic loading during ascent
  • Thermal stress due to large temperature variations during launch
  • The temperature gradient across the HX during normal operation
  • The high internal pressure due to the superheated and expanding hydrogen propellant

MILLIMETER WAVE THERMAL

Risks and issues:

Basically the same as the laser thermal: problems with the heat exchanger.

Bifrost Bridge

Payload mass delivered to LEOCost per payload kilogram
175,200 metric tons/year$20/kg

This is a combination of a mass driver and a laser launch system. You can find details here.

Space Fountain

Payload mass delivered to LEOCost per payload kilogram
??

The Space Fountain utilizes fast streams of pellets that the tower structure couples to electromagnetically in order to support itself.

Advantages:

  • Does not require materials with extreme strength
  • Can be located at any point on a planet's surface instead of just the equator
  • Can be raised to heights lower than the level of geostationary orbit

Distadvantages:

  • Requires large constant amounts energy
  • If the power is interrupted, the entire tower comes crashing down
SPACE FOUNTAIN

A space fountain is a proposed form of structure extending into space that, like a space elevator, can extend to geostationary orbit, but does not rely on tensile strength for support. In contrast to the space elevator design, a space fountain is a tremendously tall tower extending up from the ground. Since such a tall tower could not support its own weight using traditional materials, fast-moving pellets are projected upward from the bottom of the tower and redirected back down once they reach the top, so that the force of redirection holds the top of the tower aloft. Payloads ascend or descend by coupling with this stream of pellets or by climbing up the side of the tower. The space fountain has some advantages over a space elevator in that it does not require materials with extreme strength, can be located at any point on a planet's surface instead of just the equator, and can be raised to heights lower than the level of geostationary orbit. Its major disadvantages come about from the fact that it is an extremely-high-energy active structure. It requires constant power input to make up energy losses and remain erect. The high energy content of the kinetic component of the structure also continually threatens to cause the collapse of the tower if the containment systems fail.

History

The concept originated in a conversation on a computer net in the 1980s when scientists Marvin Minsky of MIT, John McCarthy, and Hans Moravec of Stanford, speculated about variations on the skyhook concept with Roderick Hyde and Lowell Wood, scientists at Lawrence Livermore National Laboratory. As a means of supporting the upper end of a traditional space elevator at an altitude much less than geostationary, they proposed a ring of space stations hovering 2,000 kilometers above Earth, motionless relative to the surface. These stations would not be in orbit; they would support themselves by deflecting a ring of fast-moving pellets circling Earth. The pellets would be moving at far greater speed than the orbital velocity for that altitude, so if the stations stopped deflecting them the pellets would move outward and the stations would fall inward.

Robert L. Forward joined the conversation at this point, suggesting that instead of using a pellet stream to support the top of a traditional tensional cable, a vertical pellet stream shot straight up from Earth's surface could support a station and provide a path for payloads to travel without requiring a cable at all. Problems that were initially raised with this proposal were friction of the pellet stream with The Earth's atmosphere at lower altitudes and the Coriolis forces due to the rotation of the Earth, but Roderick Hyde worked out all the engineering design details for a space fountain and showed that these issues could theoretically be overcome.

Design

The space fountain acts as a continuous coil gun with captive projectiles travelling in a closed loop.

In the Hyde design for a space fountain a stream of projectiles is shot up through the bore of a hollow tower at around 14 km/s. As the projectiles travel upward through the tower they are slowed down by electromagnetic drag devices that extract kinetic energy from the upgoing stream and turn it into electricity. As the projectiles are braked they also transfer some of their upward momentum to the tower structure, exerting a lifting force to support some of its weight. When the projectiles reach the station at the top of the tower they are turned around by a large bending magnet. In the turnaround process they exert an upward force on the station at the top of the tower, keeping it levitated above the launch point.

As the projectiles travel back down the tower they are accelerated by coil guns that use the electrical energy extracted from the upgoing stream of projectiles. This provides the rest of the upward lifting force required to support the weight of the tower. The projectiles reach the bottom of the tower with almost the same speed that they had when they were launched, losing a small amount of energy due to inefficiencies in the electromagnetic accelerators and decelerators in the tower. This can be minimized by the use of superconductors.

When the stream of high speed projectiles reaches the bottom of the tower it is then bent through 90 degrees by a magnet at the tower's base so that it is traveling parallel to the Earth's surface, through a large circular underground tunnel similar to a particle accelerator. Electromagnetic accelerators in this tunnel bring the projectiles back up to the original launch speed, and then the stream of projectiles is bent one more time by 90 degrees to send it back up the tower again to repeat the cycle.

The downward force from the weight of the tower is transmitted solely by the stream of projectiles to the bending magnet at the tower's base, and so no materials with extraordinary compressive strength are needed to support the tower itself. The tower's base requires a foundation capable of supporting the weight of the tower, but this can be constructed with conventional materials available cheaply on the Earth's surface. Together, the stressed structure and flowing projectile stream form a rigid, stable structure that is not limited in height by the strength of materials.

The lower parts of the tower would have to be surrounded by an airtight tube to maintain a vacuum for the projectiles to travel through, reducing energy losses due to drag. After the first one hundred kilometers or so the tube would no longer be necessary and the only structure that would be needed is a minimal framework to hold communication and power lines, and the guide tracks for the elevator cars. When the projectiles return to the base of the tower they have nearly the same speed and energy as they started with, only with the opposite momentum (downward instead of upward). As a result, the input power required to support the space fountain is determined by the inefficiency in the electromagnetic motors and air drag on the projectiles.

The elevators that would take payloads up the space fountain could conceivably ride up tracks on the tower structure using electrical power supplied by the tower, treating the space fountain solely as a mechanical support. A more attractive option would be to design the tower structure so the elevator cars can interact directly with the projectile streams themselves, and not couple to the tower structure at all. In this manner the momentum needed to raise the elevator car up against the Earth's gravity would come directly from the projectile stream.

Construction

A space fountain type structure would be built incrementally from the ground up. The driver loop and the bending magnets at the base would be constructed first, then the top station with its turnaround magnets would be constructed right above it. The system could then be loaded with projectiles and turned on at low power, lifting the top station off the ground. The vacuum tube would be built as the top station rises, with the power increasing and more projectiles being added to the loop as the tower gets longer. The rate of construction is entirely controllable, and can be halted at any height. The tower would be capable of lifting payloads throughout its construction as well, including its own construction materials.

Safety measures

To provide redundancy, a space fountain could be built with more than one projectile loop and power supply. In the event of projectile loops failing, the remaining loops would be capable of supporting the structure until the others were repaired. A safety margin would be provided simply due to the extra lifting strength that would be required by the system to raise large payloads to orbit during routine operation. In an emergency, payloads in transit could be jettisoned from the tower to reduce tower loading. Valuable or crewed payloads would likely be in capsules capable of emergency reentry as a matter of course.

Even if all of the tower's power sources failed simultaneously, it would still take a long time for the tower to begin suffering. The kinetic energy stored inside the circulating loop of projectiles is vastly greater than the amount lost to inefficiencies, so it would take many hours or even days for the velocity of the projectiles to drop enough to cause problems in supporting the tower's mass. The round trip time for the projectiles alone provides some safety margin; in Hyde's concept design it takes each projectile over three hours to complete one loop, so even if the projectile stream was completely cut off (by the destruction of the top or base station, for example) there would be some time for evacuation of the remaining tower structure and regions that might be affected by significant pieces of falling debris.

From the Wikipedia entry for SPACE FOUNTAIN

Hanging a cable down from the sky using the tensile strength of materials is just one way of making a magic beanstalk. There is another way. Like Jack's magic beanstalk, this beanstalk grows from the ground up, but unlike a Tower or a Skyhook, it does not depend upon either the compressive or tensile strength of materials. I call it the Space Fountain, for it holds objects up in space in the same way that a water fountain supports a ball bobbing at the top of its vertical jet of water.

The Space Fountain concept originated in early 1980 in the etheric depths of a computer net. Some scientists who usually work in artificial intelligence, Marvin Minsky of MIT, and John McCarthy and Hans Moravec of Stanford were speculating back and forth over the net about variations on the Skyhook concept with some scientists at Lawrence Livermore National Laboratory who usually work on laser fusion, Roderick Hyde and Lowell Wood. One of the ideas was a method of supporting the upper ends of a Skyhook at altitudes that were much less than geostationary orbit altitudes. This would be done with a stream of pellets that would be shot from a space platform hovering motionless up at 2000 kilometers altitude to another platform partway around the Earth. The pellets would be deflected by that platform to the next platform until the polygonal pellet stream made its way around the Earth back to the original station. The deflection of the pellets at each station would be sufficient to support that station in the gravity field of the Earth at that altitude. Since the stations would be only 2000 kilometers from the surface of the Earth instead of 36,000 kilometers, it would be more feasible to find materials strong enough to hang Skyhooks from the stations down to the surface of the Earth. There was still some concern expressed by the computernet debaters whether a strong enough material could be found to make a cable even 2000 kilometers long.

I joined the discussion on the net at about that time and suggested that instead of a dynamic compression hexagonal pellet stream held together with Skyhooks under tension, that a pellet stream be shot straight up from the surface of the Earth to support a pellet deflector station at the upper end that would reflect the pellet stream back down to the surface again. There was initially some skepticism by the others on the net that the idea would work, because of the Earth's atmosphere at the lower altitudes and the Coriolis forces due to the rotation of the Earth. Further hard work and detailed engineering calculations by Rod Hyde showed, however, that the concept was valid. Hyde has now worked out all the engineering design details for a Space Fountain right down to the design of the transistors to switch the currents in the projectile accelerators and decelerators.

In the Hyde design for a Space Fountain, a stream of projectiles is shot up the bore of a hollow tower. As the projectiles travel along the tower they are slowed down by electromagnetic drag devices that extract energy from the upgoing stream and turn it into electricity. As the projectiles are braked, they exert a lifting force on the tower which supports the weight of the tower. When the projectiles reach the top of the tower, they are turned around by a large bending magnet. In the turnaround process they exert an upward force on the station at the top of the tower, keeping it levitated above the launch point. [See Figure 5.]

As the projectiles travel back down the tower they are accelerated by electromagnetic drivers that use the electrical energy extracted from the upgoing stream of projectiles. The push exerted by the tower drivers also acts to support the weight of the tower. The projectiles reach the bottom of the tower with almost the same velocity that they had when they were launched. The stream of high speed projectiles is then bent through 90 degrees by a bending magnet so that it is traveling horizontally to the surface in an underground tunnel. The projectile stream is then turned in a large circle by more bending magnets and energy is added by electromagnetic drivers to bring the projectiles back up to the original launch velocity. The beam of projectiles is then bent one more time by 90 degrees to send it back up the tower again to repeat the cycle. Thus, the Space Fountain acts as a continuous mass driver with captive projectiles. The various parts of the external structure are stressed by the transfer of momentum from the pellet stream. Together, the stressed structure and flowing projectile stream form a rigid, stable structure that is not limited in height by the strength of materials.

Since the projectiles are slowed down or sped up just enough to balance the gravitational force on the tower at every point, there is no requirement anywhere for ultrastrong materials. In the lower parts of the tower there will have to be an airtight pipe supported between the Deflector Stations to keep out the atmosphere so that the drag on the projectiles is negligible. But after the first one hundred kilometers the only structure that would be needed is a minimal framework to hold communication and power lines, and the guide tracks for the elevator cars.

To first order, no energy is needed to support the Space Fountain. When the projectiles return to the base of the tower, they have essentially the same speed and energy as they started with. Their momentum has been changed, but not their energy. As a result, the input power required to support the Space Fountain is determined by the inefficiency in the electromagnetic motors and air drag on the projectiles.

One of the major advantages to the Space Fountain concept is that it can be built slowly from the ground up. The driver loop and the bending magnets in the Base Station are constructed first, then the Top Station with its turnaround magnets is constructed right above it. The system is loaded with projectiles and tested out at full power with the Top Station sitting safely just above the Earth's surface. Once these major components have been thoroughly tested out, then the power is increased, and the projectile velocity rises until the Top Station starts to lift off the ground. More projectiles are added and the Top Station rises up a few hundred meters, pulling up out of the ground a section of vacuum pipe and the first Deflector Station with it. The next Deflector Station and section of pipe are assembled around the exit and entrance tubes to the driver, power is increased, and the Space Fountain rises into the air as fast as the additional sections can be attached.

A Space Fountain should be built with a good deal of redundancy in it. Instead of just one double projectile stream, there should be two, three, or six, each with a separate power supply. Each stream by itself should be able to support the basic Space Fountain structure with a small amount of safety margin. All of them working together would have sufficient power to haul heavy loads up into space while providing adequate safety margin for minor failures and other problems like heavy transverse wind loads at the surface.

Because the circulating power in the projectiles is so much greater than the driving power, and the round trip time for the projectiles is over three hours, the tower will continue to operate for many hours even if the main drive power failed, as long the control circuits were still operating (they can be powered by electricity extracted from the energy in the projectile stream).

The elevators that would take payloads up the Space Fountain could conceivably ride up tracks on the tower structure using electrical power supplied by the tower, treating the Space Fountain solely as a mechanical structure. A more attractive option would be to design the tower structure, the Deflector Stations, and the elevator cars so that the cars can interact directly with the projectile streams themselves rather than coupling to the tower structure at all. In this manner, both the momentum needed to hold the elevator car up in Earth gravity and the energy needed to raise it to a higher level will come directly from the projectile stream.

One straightforward design, which I used in my science fiction novel Starquake, had a Space Fountain with six separate pairs of projectile streams in a hexagonal pattern. Each Deflector Station was hexagonal with two triangular cutouts to let the triangularly shaped upgoing and downgoing elevators pass through. Each elevator rode on three pairs of projectile streams, dragging on the upgoing streams and pushing on the downgoing streams. Their couplers were strong enough that they could decouple from one or more projectile streams and ride on the rest. By doing this sequentially, they could pass over the stream couplers to the Deflector Stations.

What is most amazing about the design studies that Rod Hyde has done for the Space Fountain is that none of the design parameters requires the use of exotic materials. As Rod Hyde likes to point out, this is a Skyhook that we can build now. Yes, the structure is immense in mass and length compared to anything that we build now. Yes, it will take years to power it up and push it into the sky. Yes, it will take a city-worth of power to keep it running. But the payoff is enormous. The Space Fountain can carry a payload at any one time that is two percent of its total mass. If that payload moves at a reasonable speed of one kilometer per second once it gets out in vacuum, it can make the 30,000 kilometer trip up the Space Fountain in eight hours. At that rate, the amount of mass transmitted into space by just one Space Fountain is six million tons per year, just for the cost of the electrical power to run it. This is indeed a magic beanstalk that could open up space for exploration, industrialization, and finally colonization.

From INDISTINGUISHABLE FROM MAGIC by Robert L. Forward (1995)

The dynamic beanstalk has also been called a space fountain and an Indian rope trick. It is another elegant use of momentum transfer.

Consider a continuous stream of objects (say, steel bullets) launched up the center of an evacuated vertical tube. The bullets are fired off faster than Earth’s escape velocity, using an electromagnetic accelerator on the ground. As the bullets ascend, they will be slowed naturally by gravity. However, they will receive an additional deceleration through electromagnetic coupling with coils placed in the walls of the tube. As this happens, the bullets transfer momentum upward to the coils. This continues all the way up the tube.

At the top, which may be at any altitude, the bullets are slowed and brought to a halt by electromagnetic coupling. Then they are reversed in direction and allowed to drop down another parallel evacuated tube. As they fall they are accelerated downward by coils surrounding the tube. This again results in an upward transfer of momentum from bullets to coils.

At the bottom the bullets are slowed, caught, given a large upward velocity, and placed hack in the original tube to be fired up again. We thus have a continuous stream of bullets, ascending and descending in a closed loop.

If we arrange the initial velocity and the bullets’ rate of slowing correctly, the upward force at any height can be made to match the total downward gravitational force of tube, coils, and anything else we attach to them. The whole structure will stand in dynamic equilibrium, and we have no need for any super-strong materials.

The dynamic beanstalk can be made to any length, although there are advantages to extending it to geo- synchronous height. Payloads raised to that point can be left in orbit without requiring any additional boost. However, a prototype could stretch upward just a few hundred kilometers, or even a few hundred meters. Seen from the outside there is no indication as to what is holding up the structure, hence the “Indian rope trick” label.

Note, however, that the word “dynamic” must be in the description, since this type of beanstalk calls for a continuous stream of bullets, with no time out for repair or maintenance. This is in contrast to our static or rotating beanstalks, which can stand on their own without the need for continuously operating drive elements.

From BORDERLANDS OF SCIENCE by Charles Sheffield (1999)

To Be Added

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