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.

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.


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:^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.

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

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 Launch Vehicle (HLLV)Payload mass delivered to LEOCost per payload kilogram
Long March 3B (China)13.6 metric tons$4,412/kg
Zenit 2 (Ukraine)13.7 metric tons$3,093/kg
Zenit 3SL (Sea Launch)15.9 metric tons$16,190/kg
Ariane 5G (ESA)18 metric tons$9,167/kg
Proton-M (Russia)20 metric tons$4,302/kg
Space Shuttle (NASA)28.8 metric tons$10,416/kg
Saturn V (NASA)118 metric tons??
Falcon 9 v1.1 (SpaceX)13.15 metric tons$4,654/kg
Falcon Heavy (SpaceX)53 metric tons$1,700/kg

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.


Payload mass delivered to LEOCost per payload kilogram
2.8 metric tons$11/kg (1968 dollars)
Gross Mass97,976 kg
Empty Mass6,668 kg
LEO Payload2,812 kg
Thrust (vac)1,558,100 N
464 s
Diameter6.6 m
Length18.8 m
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.


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.


Payload mass delivered to LEOCost per payload kilogram
450 metric tons$2.30 to $5.40/kg
(1964 dollars)
Gross mass6,363,000 kg
Payload450,000 kg
Height29 m
Diameter24 m
Thrust79,769,000 N
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 LEOCost per payload kilogram
550 metric tons$59/kg to $600/kg

Details here, here, and here.

Sea Dragon was designed by Robert Truax in 1962 to be a low-cost heavy lift launch vehicle. 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. The rocket would then float with the second stage cargo hatch conveniently just above the waterline, ready to be loaded.

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

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.


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 a thin layer of bloody chunky salsa covering the deck plates). 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..."


"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
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.

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.


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). 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 isnt' 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$20/kg

Details about the mechanism of a Lofstrom loop can be found here

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

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.

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.


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.

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)

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)

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

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.


  • 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


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

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

To Be Added

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