RocketCat sez

Have you simply had it up to here with these impotent little momma's-boy rockets that take almost a year to crawl to Mars?

Then you want a herculean muscle-rocket, with rippling titanium washboard abs and huge geodesic truck-nuts! You want a Torchship! To heck with John's Law, who cares if the exhaust can evaporate Rhode Island? You wanna rocket with an obscenely high delta V, one that can crank out one g for days at a time. Say goodby to all that fussy Hohmann transfer nonsense, the only navigation you need is point-and-shoot.

It is a pity that torchships are currently science fiction. But they are unobtainium, not handwavium. Ain't no law of physics sez they are impossible, we just don't know how to make one. Yet.

And like all good unobtainium, even though we can't build it yet, we can calculate what it can do just fine.

The basic problem is that rocket engines seem to only come in two types:

Engine Types

Muscle engines have the thrust to perform Hohmann trajectories but the low specific impulse means they have to carry outrageous amounts of propellant. Which really cuts into the payload mass.

Fuel Economy engines need only modest amounts of propellant. Unfortunately the low thrust means they take forever to inject themselves into a Hohmann trajectory.

Some engines can "shift gears" from muscle to fuel-economy and back, but that still just trades one set of liabilities with another.

The technical terms for Muscle high-thrust + low-specific-impulse are Specific-Impulse Limited and High-Thrust Systems. Typical examples are chemical and solid-core nuclear thermal. These usually create the exhaust velocity by thermal means (heat), so they are limited by how hot you can get the exhaust (limited by chemical energy or limited by the melting point of the rocket engine).

The technical terms for Fuel Economy low-thrust + high-specific-impulse are Specific-Mass Limited, Low-Thrust Systems, and Power-Limited Systems. Typical example is an ion drive. "Specific Mass" or "Alpha" (α) is the mass of the propulsion system divided by the thrust power. These are usually electrically powered rockets, which is why they are power-limited.

What rocket designers really want is an engine with both high thrust and high specific impulse. Such engines don't have to use weak Hohmanns, they can use fantastically expensive (but rapid) Brachistochrone trajectories.

Such engines are called "Torch Drives", a spacecraft with a torch drive is called a "Torchship."

The term "Torchship" was coined by Robert Heinlein in 1953 for his short story "Sky Lift", it is also featured in his stories Farmer in the Sky, Tunnel in the Sky, Time for the Stars, and Double Star. Sometimes it is referred to as "Ortega's Torch".

Nowadays it is implied that a Torchship is some kind of high thrust fusion drive, but Heinlein meant it to mean a handwavium total-conversion mass-into-energy type drive.


[radio static noises followed by blaring heavy metal]

I can't thrive on ion drive.
I need that rocket to burn.
Kick the ass, give it the gas.
I've got that need... for I... s... p.

[guitar riff]

Solar sail is an epic fail.
I need that rocket to burn.
Full on thrust is what I lust.
I've got that need... for I... s... p.

(Can you say) Constant acceleration?

by Todd Zircher (2018)

The Commodore’s office was crowded. All present wore the torch, except a flight surgeon and Commodore Berrio himself, who wore the jets of a rocketship pilot.

He held a torcher’s contempt for the vast distance itself. Older pilots thought of interplanetary trips with a rocket-man’s bias, in terms of years — trips that a torch ship with steady acceleration covered in days. By the orbits that a rocketship must use the round trip to Jupiter takes over five years; Saturn is twice as far, Uranus twice again, Neptune still farther. No rocketship ever attempted Pluto; a round trip would take more than ninety years. But torch ships had won a foothold even there: Proserpina Station — cryology laboratory, cosmic radiation station, parallax observatory, physics laboratory, all in one quintuple dome against the unspeakable cold.

From SKY LIFT by Robert Heinlein (1953)

(ed note: The liner Pegasus is a conventional ion-drive spacecraft using finely divided dust as propellant. Like most non-torchships it has barely enough propellant to perform the mission.

The Federal cruiser Acheron is an honest-to-Heinlen torchship, using a hand-waving reactionless drive.)

"That's the background, but now it's only the less important part of the story. There are weapons here as well. Some have been completed and tested, others are waiting for the final adjustments. I'm bringing the key components for what may be the decisive one. That's why Earth may owe you a greater debt than it can ever pay. Don't interrupt—we're nearly there and this is what I really want to tell you. The radio was not telling the truth about that twenty hours of safety. That's what the Federation wants us to believe, and we hope they go on thinking we've been fooled. But we've spotted their ships, and they're approaching ten times as fast as anything that's ever moved through space before. I'm afraid they've got a fundamental new method of propulsion—I only hope it hasn't given them new weapons as well. We've not much more than three hours before they get here, assuming they don't step up their speed still further. You could stay, but for your own safety I advise you to turn around and drive like hell back to the Observatory. If anything starts to happen while you're still out in the open, get under cover as quickly as possible. Go down into a crevasse—anywhere you can find shelter—and stay there until it's all over. Now good-by and good luck. I hope we have a chance of meeting again, when this business is finished."

THE LINER Pegasus, with three hundred passengers arid a crew of sixty, was only four days out from Earth when the war began and ended. For some hours there had been a great confusion and alarm on board, as the radio messages from Earth and Federation were intercepted. Captain Halstead had been forced to take firm measures with some of the passengers, who wished to turn back rather than go on to Mars and an uncertain future as prisoners of war. It was not easy to blame them; Earth was still so close that it was a beautiful silver crescent, with the Moon a fainter and smaller echo beside it. Even from here, more than a million kilometers away, the energies that had just flamed across the face of the Moon had been clearly visible, and had done little to restore the morale of the passengers.

They could not understand that the law of celestial mechanics admit of no appeal. The Pegasus was barely clear of Earth, and still weeks from her intended goal. But she had reached her orbiting speed, and had launched herself like a giant projectile on the path that would lead inevitably to Mars, under the guidance of the sun's all-pervading gravity. There could be no turning back: that would be a maneuver involving an impossible amount of propellant. The Pegasus carried enough dust in her tanks to match velocity with Mars at the end of her orbit, and to allow for reasonable course corrections en route. Her nuclear reactors could provide energy for a dozen voyages—but sheer energy was useless if there was no propellant mass to eject. Whether she wanted to or not, the Pegasus was headed for Mars with the inevitability of a runaway streetcar. Captain Halstead did not anticipate a pleasant trip.

The words MAYDAY, MAYDAY came crashing out of the radio and banished all other preoccupations of the Pegasus and her crew. For three hundred years, in air and sea and space, these words had alerted rescue organizations, had made captains change their course and race to the aid of stricken comrades. But there was so little that the commander of a spaceship could do; in the whole history of astronautics, there have been only three cases of a successful rescue operation in space.

There are two main reasons for this, only one of which is widely advertised by the shipping lines. Any serious disaster in space is extremely rare; almost all accidents occur during planetfall or departure. Once a ship has reached space, and has swung into the orbit that will lead it effortlessly to its destination, it is safe from all hazards except internal, mechanical troubles. Such troubles occur more often than the passengers ever know, but are usually trivial and are quietly dealt with by the crew. All spaceships, by law, are built in several independent sections, any one of which can serve as a refuge in an emergency. So the worst that ever happens is that some uncomfortable hours are spent by all while an irate captain breathes heavily down the neck of his engineering officer.

The second reason why space rescues are so rare is that they are almost impossible, from the nature of things. Spaceships travel at enormous velocities on exactly calculated paths, which do not permit of major alterations—as the passengers of the Pegasus were now beginning to appreciate. The orbit any ship follows from one planet to another is unique; no other vessel will ever follow the same path again, among the changing patterns of the planets. There are no "shipping lanes" in space and it is rare indeed for one ship to pass within a million kilometers of another. Even when this does happen, the difference of speed is almost always so great that contact is impossible.

All these thoughts flashed through Captain Halstead's mind when the message came down to him from Signals. He read the position and course of the distressed ship—the velocity figure must have been garbled in transmission, it was so ridiculously high. Almost certainly, there was nothing he could do—they were too far away, and it would take days to reach them.

Then he noticed the name at the end of the message. He thought he was familiar with every ship in space, but this was a new one to him. He stared in bewilderment for a moment before he suddenly realized just who was calling for his assistance. Enmity vanishes when men are in peril on sea or in space. Captain Halstead leaned over his control desk said: "Signals! Get me their captain."

"He's on circuit, sir. You can go ahead." Captain Halstead cleared his throat. This was a novel experience, and not a pleasant one. It gave him no sort of satisfaction to tell even an enemy that he could do nothing to save him.

"Captain Halstead, Pegasus, speaking," he began. "You're too far away for contact. Our operational reserve is less than ten kilometers a second. I've no need to compute, I can see it's impossible. Have you any suggestions? Please confirm your velocity; we were given an incorrect figure."

The reply, after a four-second time-lag that seemed doubly maddening in these circumstances, was unexpected and astonishing. (distance of 2 light-seconds, about 599,585 kilometers or twice the Terra-Luna distance)

"Commodore Brennan, Federal cruiser Acheron. I can confirm our velocity figure. We can contact you in two hours, and make all course corrections ourselves. (I calculate average speed of 83 km/s) We still have power, but must abandon ship in less than three hours. Our radiation shielding has gone, and the main reactor is becoming unstable. We've got manual control on it, and it will be safe for at least an hour after we reach you. But we can't guarantee it beyond then." Captain Halstead felt the scalp crawl at the back of his neck. He did not know how a reactor could became unstable, but he knew what would happen if one did. There were a good many things about the Acheron he did not understand—her speed, above all—but there was one point that emerged very clearly upon which Commodore Brennan must be left in no doubt.

"Pegasus to Acheron," he replied. "I have three hundred passengers aboard. I cannot hazard my ship if there is danger of an explosion."

"There is no danger, I can guarantee that. We will have at least five minutes' warning, which will give us ample time to get clear of you."

Then the ship's speakers announced, in an almost quietly conversational tone: Pegasus to Acheron. We've got all your men out of the locks. No casualties. A few hemorrhages. Give us five minutes to get ready for the next batch."

They lost one man on the last transfer. He panicked and they had to slam the lock shut without him, rather than risk the lives of all the others. It seemed a pity that they could not all have made it, but for the moment everyone was too thankful to worry about that.

There was only one thing still to be done. Commodore Brennan, the last man aboard the Acheron, adjusted the timing circuit that would start the drive in thirty seconds. That would give him long enough; even in his clumsy spacesuit he could get out of the open hatch in half that time. It was cutting it fine, but only he and his engineering officer knew how narrow the margin was.

He threw the switch and dived for the hatch. He had already reached the Pegasus when the ship he had commanded, still loaded with millions of kilowatt-centuries of energy, came to life for the last time and dwindled silently toward the stars of the Milky Way (more than 6.3×1018 joules, equivalent to 1.5 gigaton nuclear weapon).

The explosion was easily visible among all the inner planets. It blew to nothingness the last ambitions of the Federation, and the last fears of Earth.

From EARTHLIGHT by Arthur C. Clarke (1955)

What is a Torchship?

What is the definition of a torchship? Well, it is kind of vague. It more or less boils down to "unreasonably powerful."

With most propulsion systems, there is an inverse relationship between thrust and specific-impulse/exhaust-velocity (if one is high the other is low). So Rick Robinson defines a torch drive as propulsion system with both high acceleration (from high thrust) and high exhaust velocity. Note that whether the drive is "high acceleration" or not depends upon the thrust of the drive and the total mass of the spacecraft, that is, it does not just depend upon the drive.

If you recall, a propulsion systems Thrust Power (Fp) is thrust times exhaust velocity, then divide by two. Which sort of combines Rick's two parameters defining a torch drive (but so does propellant mass flow). The Drive Table is helpfully sorted in order of increasing Thrust Power. Remember that each entry is for a single engine of that type, it is possible to have multiple engines. With multiple engines, the exhaust velocity stays the same, but the thrust is multiplied by the number of engines.

A spacecraft's Specific Power (Fsp) is its propulsion system's Thrust Power divided by the spacecraft's dry mass. Remember that dry mass is the mass of spacecraft fully loaded with cargo and everything, but no propellant.

Rick Robinson's general rule is that a Torch Drive is a propulsion system with both both high acceleration and high exhaust velocity. His further general rule is that a Torchship is a spacecraft with a Torch Drive and a specific power of one megawatt per kilogram or larger.

And as a side note, such a frightful amount of thrust power in your spacecraft exhaust could do severe damange to anything it hosed. One would almost say weapons-grade levels of damage. Clever readers have already mouthed the phrase the Kzinti Lesson. This might lead to torchships being reserved for military vessels only. If not, it will be likely that the Spaceguard will be able to remotely explode any civilian torchship that starts aiming its deadly exhaust in an unauthorized direction.

Another thing to keep in mind that as a general rule, torchships are not subject to the Every gram counts rule, because they are unreasonably powerful.

When it comes to propulsion systems we might actually be able to build in the near future, the list includes Orion drives, Zubrin's nuclear salt water rocket, and maybe Medusa.

Another definition of a torchship is a spacecraft with more than 300 km/s total delta V and an acceleration greater than 0.01 g. Which may or may not fit with Rick's definition.

I am trying to triangulate on Rick's general rule for Torch Drives. I'm toying with defining "high thrust" as 100,000 Newtons or higher, and "high exhaust velocity" as 100,000 m/s or higher.

And a thrust power of 100 gigawatts or higher. Which is a number I pulled out of the air by examining the drive table and picking a dividing line that pleased me.

And a propellant mass flow between 100 and 0.01 kg/sec.

I'm working on it, OK? Let me crunch some numbers and draw a few graphs and I'll get back to you.

Would Rick Robinson consider the good ship Polaris to be a torchship? Let's see.

Our design equips the Polaris with not one, not two, but three freaking Nuclear Salt Water rocket engines. 12,900,000 Newtons of thrust each for a total of 38,700,000 Newtons. And an exhaust velocity of 66,000 meters per second. So NSWRs are definitely torch drives. Is the Polaris a torchship?

  • Fp = ( (F * E) * Ve ) / 2
  • Fp = ( (12,900,000 * 3) * 66,000 ) / 2
  • Fp = ( 38,700,000 * 66,000 ) / 2
  • Fp = 2,554,200,000,000 / 2
  • Fp = 1,280,000,000,000 watts or 1.28 terawatts

The Polaris has a dry mass of 126,000 kilograms (126 metric tons).

  • Fsp = Fp / Me
  • Fsp = 1,280,000,000,000 / 126,000
  • Fsp = 10,200,000 watts per kilogram or 10.2 megawatts per kilogram

Oh, yes, Rick Robinson would say the Polaris is very much a torchship.


(ed note: the article uses the term "starcruiser", which in context more or less means "torchship")

Wars in space will never be like Star Wars. “Starfighters” will not engage in dogfights with unlimited maneuverability and range. An actual conflict in space would be slow and deliberate, requiring prepositioning of weapons and meticulous planning.

Even defense analysts working on the matter understate the physical constraints on warfare in space. In a recent War on the Rocks article, Jeff Becker claims that the era of “starcruisers” — spacecraft whose maneuvering is not principally dictated by orbital mechanics — is closer than people think. While Becker examines some meaningful technological developments, his analysis — like other work on the topic — does not recognize the challenges and physics that would be involved in fighting a conflict in space.

Policymakers and defense planners need to have a realistic understanding of what is physically possible and practical. As we explain in a recent research paper and accompanying video, in any space war physical limitations will constrain both the movement of assets and overall strategy. Space is big and satellites maneuver slowly while following predictable trajectories. Major limitations such as these mean that starcruisers like the Millennium Falcon or Galactica will never exist.

Physical Constraints in Space

Spacecraft and space weapons cannot defy physics. As they develop their ideas about spacepower, strategists should internalize five major points and use them to attain advantages, rather than working against physical realities.

Satellites move quickly. Those in commonly used circular orbits move at speeds of 6,700 to 18,000 miles per hour (three to eight kilometers per second), depending on their altitude. In comparison, an average bullet only travels about 1,700 miles per hour (0.75 kilometers per second). To an Earth-bound observer, satellite speeds might suggest that science fiction is the right way to imagine space war, but speed is not actually the biggest driver of fighting in space.

Satellites move predictably. Due to orbital dynamics, satellites exhibit a strict relationship between altitude, speed, and orbit shape. Gravity dictates that satellites at lower altitudes always move more quickly than those at higher altitudes. This makes their path predictable — and deviating from an orbit is costly and slow — which means that engagements, whether peaceful or aggressive, can be planned long ahead of time regardless of the speed at which satellites are traveling.

Space is big. The volume of space between low Earth orbit and geostationary orbit is about 50 trillion cubic miles (200 trillion cubic kilometers). That is 190 times bigger than the volume of Earth. Even if satellites and spacecraft are designed to have more energy for maneuverability, distances in space are so big that extensive maneuvering will remain painstakingly slow.

Timing is everything. Within the confines of the Earth’s atmosphere, airplanes, tanks, and ships can move in multiple directions. By comparison, satellites are always moving in a circular or elliptical path due to the gravitational pull of Earth. The nature of conflict often requires two competing weapon systems to get close to each other, in this case “on-orbit.” Therefore, timing is everything.

Satellites maneuver slowly. While satellites move quickly along their orbit, purposeful maneuvers to a different orbit are relatively slow. That is because space is big. Since gravity dictates the motion of a satellite, deviating from the prescribed orbit requires the use of an engine to maneuver. However, those deliberate maneuvers can only happen at certain points in an orbit. Adopting a mindset and a strategy of slow and deliberate movements is needed, rather than thinking about flashy “light speed” jumps.

The X-37B: Not a Forerunner to Starcruisers

A starcruiser will not exist because it would have to defy these physical constraints, especially the realities that satellites move predictably and maneuver slowly. Becker argues that two current programs — the X-37B and SpaceX’s Starship — show that starcruisers will be here soon enough that analysts need to begin thinking through the implications for spacepower. But, in reality, there’s no prospect of a technological breakthrough that will allow spacecraft to effectively “beat” physics. Neither the X-37B nor the Starship show that the fundamentals of spacepower will change anytime soon. Those programs should not give false hope to those who dream big about space.

The X-37B spacecraft offers real benefits because of its overall reusability and its ability to conduct experiments and then return them to Earth. It is unique in that it operates in low Earth orbit and leverages the atmosphere to maneuver. That allows it to move in unpredictable ways, but it is not a precursor to starcruisers. To understand why, consider the concept of “delta-v,” which is the change in velocity required to alter a spacecraft’s trajectory. Since there is a strict relationship between a satellite’s velocity and its orbit, a change in velocity will necessarily change the orbit. To deviate from the path determined by orbital dynamics, delta-v is required. The amount of delta-v carried by a satellite is known as its “delta-v budget.” This metric is independent of the satellite’s size and propellant type. Thus, contrary to what Becker suggests, there is no concept of delta-v efficiency. What matters is the overall delta-v capacity of a satellite and how quickly it can be applied to accelerate the satellite.

The X-37B has a total delta-v budget of 3,100 meters per second for a mission. That is not radically different to the delta-v budget of a typical low-thrust geostationary orbit communications satellite. That type of satellite spends 1,800 meters per second of delta-v just to get to its orbit and then requires about 55 meters per second per year to maintain its orbit. If we also include what is needed for re-positioning, disposal, and other maneuvers, a typical geostationary orbit satellite’s delta-v budget approaches 4,000 meters per second. Even more strikingly, the X-37B’s delta-v budget is small compared to the commercially available Boeing 702SP satellite bus, which first flew in 2012 and carries up to 6,800 meters per second of delta-v. Nobody thinks these satellites are forerunners to starcruisers. Neither is the X-37B.

The Starship: Evolutionary Technology

SpaceX’s Starship is very exciting, but for the purposes of space warfare it is evolutionary not revolutionary. Starship is a huge, reusable rocket and spacecraft. It is able to refuel on-orbit, which provides it with a delta-v budget of 6,900 meters per second and the maneuverability that comes with it. If Starship was fully topped off outside of the Earth’s atmosphere, this uniquely large spacecraft would be able to perform more maneuvers and enjoy a longer lifetime. While these gains are real, they do not eliminate the constraints on maneuvering due to orbital mechanics. Making decisions about the number and location of fueling depots would also be a complex challenge, precisely because space is so big.

We are all for gaining refueling capabilities on-orbit, but for different reasons than one might think. Once the Space Force can have hundreds of millions of pounds of propellant in space waiting to refuel spacecraft, it will have the means to greatly improve freedom of movement and prolong the lifetime of satellites. But that will not make space operations analogous to air operations. Even Starship’s large delta-v budget is too small to enable it to match the orbit of many satellites. We calculated that if a fully loaded Starship is in an equatorial orbit (zero-degree inclination) at an altitude of 250 miles (400 kilometers), it would barely have enough delta-v to match the International Space Station’s orbit, at 51 degrees inclination. It definitely could not match satellites in sun-synchronous orbits, which have an inclination of about 98 degrees and are a common orbit for Earth observation satellites.

Among its major capabilities, the Space Force needs the ability to quickly launch payloads into their desired orbits and Starship could play an important role in performing that mission. But that need can also be met better by other systems. Recently, the Space Force demonstrated its ability to rapidly put satellites into orbit by using the air-launched Pegasus rocket. That rocket has a critical advantage over Starship: While Starship can currently only launch from two locations — Boca Chica, Texas, and Cape Canaveral, Florida — Pegasus can already be launched from a number of locations worldwide.

Science-fiction movies have conditioned people to believe that advanced propulsion systems enable travel in a straight line through space, unconstrained by orbital mechanics. However, our calculations show that in order to travel in a straight line from an altitude of 400 kilometers to 1,000 kilometers, a spacecraft would need at least 40,000 meters per second of delta-v split into two equal, instantaneous bursts about 30 seconds apart — one to start the maneuver and one to stop it. To go from 400 kilometers to geostationary orbit — an altitude of 35,786 kilometers — a spacecraft would need at least 2,400,000 meters per second of delta-v split into two equal, instantaneous bursts about 30 seconds apart. The spacecraft would also need to survive the immense forces generated by accelerating that fast. This is not happening in our lifetime. Nor is it clear why this would be desirable. At the very least, the Starship, just like the X-37B, is nowhere close to achieving this type of maneuver. Even if they were, supporting infrastructure would be required to fight in space. Currently, the Space Force does not have the communications, navigation, and other capabilities necessary for the effective use of a starcruiser or other advanced spacecraft. That should not alarm American policymakers, precisely because starcruisers will not exist.

Starcruisers: The Stuff of Science Fiction

Without having delta-v budgets orders of magnitude greater than that currently being assumed for Starship, orbital mechanics bound what spacecraft can do. As we have written elsewhere, large applications of delta-v by a spacecraft in a low Earth orbit can move it into orbits that either hit Earth or that enter the Van Allen belts, a region of charged particles that can reduce the spacecraft’s lifetime or permanently damage it. By trying to be fast, a starcruiser would be forced to make many large corrective burns to avoid these fates, destroying any “efficiency” provided by the large delta-v load.

Discussions focused on delta-v overshadow another key component of conflict in space: time. Since space is big, maneuvering in space is slow, even with large delta-v budgets. Maneuvering to rendezvous with another satellite, whether to refuel or attack, requires the approaching satellite to match orbital planes and phasing with the other one. Both satellites will need to end up in the same spot, at the same time, with no velocity difference. Larger delta-v budgets certainly enable faster transfers to different orbits, but it may still take days for a satellite to reach its target. As the Space Force’s capstone doctrine emphasizes:

Constrained freedom of maneuver is a defining attribute of space operations. … Due to extreme velocities, the amount of energy required to reach a different orbit may be significant enough to render the option unfeasible or impractical.

By embracing the constrained nature of space movement, the Space Force and other space operators can use Starship-like capabilities to enhance their warfighting abilities, rather than fight needless battles against natural forces that will inevitably win.

Looking Ahead

Emerging space technologies are exciting. They promise to create more responsive and reusable launch vehicles and to provide refueling and repair opportunities on-orbit. However, contrary to the hopes of some defense analysts, they will not allow the Space Force to defy physics. Policy planning and strategizing should be based on reality and built upon an understanding of astrodynamics. Having spacecraft with massive amounts of fuel will certainly be beneficial to space operations, but that will not take space warfare into the realm of science fiction. Any conflict in space would be a slow and deliberate affair. An effective spacepower strategy should be developed with physics-based constraints in mind, not based on dreams of starcruisers.


With ion engines, chemical engines, and nuclear torches we're facing a classic Newton's Third Law problem. Somehow the exhaust needs to have sufficient momentum for the opposite reaction to give the ship a good acceleration.

Chemical rockets solve the problem by expelling a ton of mass at a relatively low velocity. (high propellant mass flow but low exhaust velocity: SUV)

Ion drives expel a tiny amount of mass, so to get anywhere they get it moving FAST, but even at gigawatts of power they get a measly 0.0001g. (low propellant mass flow but high exhaust velocity: Economy)

Torch drives take a small-to-moderate amount of mass and use nuclear destruction to get it moving insanely fast. (medium propellant mass flow and high exhaust velocity: Torch) They're the only ones (insert disclaimer) with enough power per unit of reaction mass to get 0.3g constant acceleration conveniently. Even a perfect ion drive would need a phenomenal (read: impossible) amount of power input to match the performance of a nuclear explosion.

(A low propellant mass flow and low exhaust velocity engine would be utterly worthless)

From ON TORCHSHIPS comment by Eric (2010)

Why So Hard To Make?

A torchship has high thrust (so it can rapidly increase the ship's deltaV) and a high specific impulse / exhaust velocity (so it has enough gas mileage to accelerate for weeks at a time).

Why is this so hard to do? Well for starters, the two requirements are sort of opposed to each other.

F = * Isp * g0

Isp = F / ( * g0)


F = thrust (Newtons)
= propellant mass flow (kg/sec)
Isp = specific impulse (seconds)
g0 = acceleration due to gravity (m/sec) = 9.81 m/s

What is sort of boils down to is:

  • to increase the Thrust you raise the propellant mass flow
  • to increase the Specific Impulse you lower the propellant mass flow

This is because the more propellant mass you hurl out the exhaust per second, the more Newton's law of action and reaction will accelerate the spacecraft. And the more propellant you use per second, the worse is the engine's gas mileage.

Torchships are hard because it is difficult to figure out how to simultaneously raise and lower the propellant mass flow.

Torchship Performance

Fast Torchship
High GearLow Gear
Wet Mass1,000,000 kg
Dry Mass500,000 kg
Mass Ratio2
300,000 m/s50,000 m/s
ΔV200,000 m/s40,000 m/s
Thrust3,000,000 N14,700,000 N
3 m/s2
(0.3 g)
14.7 m/s2
(1.5 g)
Thrust Power450 GW370 GW

Let us say that our ship has a mass of 1000 tons, and a modest exhaust velocity of 300 km/s, a mere 0.001 c. In the name of further modesty we will set our acceleration at less than one third g, 3 meters per second squared. This ship is very much on the low end of torchships; Father Heinlein would hardly recognize her. If our torch has VASIMR style capability, we can trade specific impulse for thrust; dial exhaust velocity down to 50 km/s to develop 1.5 g, enough to lift from Earth. (but see below)

(ed note: trading specific impulse for thrust is shifting from high gear to low gear)

Setting surface lift aside, let's look at travel performance from low Earth orbit. We will reach escape velocity about 15 minutes after lighting up, a good Oberth boot, but with this ship it hardly matters.

(ed note: 15 minutes of 1.5 g is 900 seconds of 14.7 m/s2 which is a ΔV cost of about 13,000 m/s)

Suppose half our departure mass is propellant, giving us about 180 km/s of delta v in the tanks

(ed note: 200,000 m/s total - 13,000 m/s liftoff = 187,000 m/s.)

Since we must make departure and arrival burns, our transfer speed (relative to Earth) is about 90 km/s.

(ed note: 180,000 / 2 = 90,000 m/s.)

Holding acceleration to 0.3 g as we burn off mass, we'll reach 90 km/s in less than eight hours,

(ed note: 90 km/s divided by 0.3 g is 90,000 m/s / 3 m/s2 = 30,000 seconds. 30,000 / 3,600 = 8.3 hours.) about three times lunar distance from Earth. (If we were going to the Moon, we'd need to swing around three hours out for our deceleration burn.)

For any deep space mission we'll end up coasting most of the way, and 90 km/s is some pretty fast coasting. Even this low end torchship lets you take retrograde hyperboloid orbits, which is Isaac Newton's way of saying you can pretty much point & scoot.

You will truck along at 1 AU every three weeks, reaching Mars in little more than a week at opposition, though up to two months if you travel off season. The near side of the asteroid belt is a month from Earth, Jupiter three months in season. For Saturn and beyond things do get prolonged.

Still, 90 days for the inner system out to Jupiter is nothing shabby, and there's no apparent High Magic involved.

How much power output does out modest torch drive have? The answer, skipping simple but tedious math, is not quite half a terawatt, 450 GW.

(ed note: (3,000,000 N thrust * 300,000 m/s exhaust velocity) / 2 = 450,000,000,000 watts = 450 GW.)

This baby has 600 million horses under the hood. That's effective thrust power. Hotel power for the hab is extra, and so is getting rid of the waste heat.

Putting it another way, this drive puts out a tenth of a kiloton per second, plus losses.

(ed note: 450,000,000,000 watts / 4.184 × 1012 = 0.108 kilotons per second.)

And yes, Orion is the one currently semi plausible drive that can deliver this level of power and performance. I say semi plausible because, broadly political issues aside, I suspect that Orion enthusiasts gloss over the engineering details of deliberately nuking yourself thousands of times. Like banging your head on the table, it only feels good when you stop.

Reasonably Fast Torchship
Wet Mass1,000,000 kg
Dry Mass500,000 kg
Mass Ratio2
300,000 m/s
ΔV200,000 m/s
Thrust290,000 N
0.29 m/s2
(30 mg)
Thrust Power45 GW
Really Fast Torchship
Wet Mass1,000,000 kg
Dry Mass500,000 kg
Mass Ratio2
3,000,000 m/s
ΔV2,000,000 m/s
Thrust3,000,000 N
3 m/s2
(0.3 g)
Thrust Power4.5 TW

The good news, for practical, Reasonably Fast space travel, is that we don't need those near-terawatt burns. They don't really save much time on interplanetary missions — we could reduce drive power by 90 percent, and acceleration to 30 milligees — the acceleration of a freight train — and only add three days to travel time.

(Our reduced 'sub-torch' fusion drive is still putting out a non trivial 45 gigawatts of thrust power, which happens to be close to the effective thrust power of the Saturn V first stage.)

(ed note: (290,000 N thrust * 300,000 m/s exhaust velocity) / 2 = 45,000,000,000 watts = 45 GW.)

If you want Really Fast space travel, however, you will need more than this. Go back to our torchship and increase her drive exhaust velocity by tenfold, to 3000 km/s, and mission delta v to 1800 km/s, while keeping the same comfortable 0.3 g acceleration.

A classic brachistochrone orbit, under power using our full delta v, takes a week and carries us 270 million km, 1.8 AU.

Add a week of coasting in the middle and you're at Jupiter. Saturn is about three weeks' travel, and you can reach distant Eris in 6 months.

Drive power output of our upgraded torchship is now 4.5 TW, about a third the current power output of the human race. Which in itself is no argument against it. Controlling the reaction and getting rid of the waste heat are more immediate concerns.

(ed note: (3,000,000 N thrust * 3,000,000 m/s exhaust velocity) / 2 = 4,500,000,000,000 watts = 4.5 TW.)

As for a true, Heinleinian torchship? Heinlein's torch is a mass conversion torch. He sensibly avoids any details of the physics, but apparently the backwash is a mix of radiation, AKA photon drive, and neutrons, probably relativistic.

Torchship Lewis and Clark, pictured above, is about 60 meters in diameter, masses in on the order of 50,000 tons, and in Time for the Stars she begins her relativistic interstellar mission by launching from the Pacific Ocean at 3 g. I don't know how to adjust the rocket equations for relativity, but the naive, relativity-ignoring calculation gives a power output of 225 petawatts, AKA 225,000 TW, AKA 53 megatons per second.

Do not try this trick at your homeworld. I don't know whether Heinlein never checked this calculation, did it and ignored the results, or did it and decided that a few dozen gigatons — 12 torchships are launched, one after another — was no big deal since it was off in the middle of the Pacific somewhere.

Maybe he only did the calculation later, because in other stories the torchships sensibly remain in orbit (served by NERVA style nuke thermal shuttles).

From ON TORCHSHIPS by Rick Robinson (2010)

Rick Robinson:

Those performance stats (for the Project Daedalus) are certainly torchlike, and in fact an exhaust velocity of 10,000 km/s is wasteful for nearly all Solar System travel — on most routes you just don't have time to reach more than a few hundred km/s.

Using STL starship technology on interplanetary routes is like using a jet plane to get around town.

Jean Remy:

There's no such thing as going somewhere "too fast". At least in terms of military strategy you'll want the ability to get somewhere faster than anyone else can, and damn the price at the pump. It is more costly to arrive at a battle late (and for want of a horse)

Rick Robinson:

Oh, I have nothing against speed! A better way to put it is that STL starships are geared all wrong for insystem travel, like driving city streets in 5th gear.

Luke Campbell:

Consider a 1,000 ton spacecraft with a 10,000 km/s exhaust velocity and an acceleration of 0.722 m/s/s. For a 1 AU trip at constant acceleration, flipping at the midpoint, it will take 10.5 days and consume 66 tons of propellant/fuel.

Now let's add extra mass into the exhaust stream, so that the spacecraft uses propellant at 16 times the rate but expells it at 1/4 the exhaust velocity (thus keeping the same power). This brings the acceleration up to 2.89 m/s/s. We will accelerate for 1/10 the distance, drift for 8/10 the distance, and then decelerate for 1/10 the distance. The trip now takes 7 days and uses 240 tons of propellant, of which only 7 tons is fuel.

Bulk inert (non-fuel) propellant is probably cheap (water or hydrogen). Fuel is probably expensive (He-3 and D). The second option gets you there faster and cheaper.

In Rick's analogy, high exhaust velocity, low thrust, low propellant flow corresponds to high gear. Low exhaust velocity, high thrust, high propellant flow is low gear. In this case, a lower gear than the default "interstellar" Daedelus thrust parameters is preferable.

Rick Robinson:

'Gearing' is highly desirable even if the drive won't produce surface lift thrust from any significant body. Each deep space mission also has its own optimum balance of acceleration and delta v, favoring an adjustable drive.

(ed note: given the mission delta V, the optimal exhaust velocity is Δv * 0.72 . See here.)

From ON TORCHSHIPS comments (2010)

Calculating the performance of a spaceship can be complicated. But if the ship is powerful enough, we can ignore gravity fields. It is then fairly easy. The ship will accelerate to a maximum speed and then turn around and slow down at its destination. Fusion or annihilation-drive ships will probably do this. They will apply power all the time, speeding up and slowing down.(ed note: a "brachistochrone" trajectory)

In this simple case, all the important performance parameters can be expressed on a single graph. This one is drawn for the case when 90% of the starting mass is propellant. (ed note: a mass ratio of 10) Jet velocity (exhaust velocity) and starting acceleration are the graph scales. Distance for several bodies are shown. Mars varies greatly; I used 150 million kilometers. Trip times and specific power levels are also shown. "Specific power" expresses how much power the ship generates for each kilogram of its mass, that is, its total power divided by its mass. The propellant the ship will carry is not included in the mass value. (ed note: Specific Power (Fsp) is its propulsion system's Thrust Power divided by the spacecraft's dry mass. On the chart specific power is the diagonal dotted lines.)

An example: Suppose your ship can produce 100 kW/kg of jet power (specific power). You wish to fly to Jupiter. Where the 100 kW/kg and Jupiter lines cross on the graph, read a jet velocity of 300,000 m/s (Isp = 30,000) and an initial acceleration of nearly 0.01g. Your trip will take about two months.

The upper area of the graph shows that high performance is needed to reach the nearest stars. Even generation ships will need, in addition to very high jet velocities, power on the order of 100 kW/kg. The space shuttle orbiter produces about 100 kW/kg with its three engines. The high power needed for starflight precludes its attainment with means such as electric propulsion.

From TO THE STARS by Gordon Woodcock (1983). Collected in Islands In The Sky, edited by Stanley Schmidt and Robert Zubrin (1996).

Torch Drive Heat

The main problem with torch drives is getting rid of the waste heat generated by such a monster.

Remember that Thrust Power (Fp) is thrust times exhaust velocity, then divide by two. Thrust power is rated in watts, and if you look at the ratings for torch drives, well, that's a lotta watts.

Unavoidably some of those watts are going to become waste heat.

So if the torch engine is a hollow metal ball surrounding the torch reaction (with a hole for the exhaust nozzle), the solid walls of the metal ball are going to become real hot real quick.

If you have a Resistojet with a thrust power of 700 watts, and 10% becomes waste heat, the heat will be 70 watts. This is only about as much as emitted by an old style incandescent bulb, not a problem.

But if you have Rick's Fast Torchship with a thrust power of 450 gigawatts, the heat will be 45,000 megawatts. Since your average drive assembly can survive no more than 5 megawatts, it will glow blue-white for a fraction of a second before it vaporizes. Along with most of the spacecraft.

A spacecraft with a chemical engine can have a torchship like specific power of 1MW/kg, but the chemical engine is not a torch drive because of its pathetic exhaust velocity. Chemical engines have no waste heat problem. Yes they have huge amounts of it, but they can easily use their huge propellant mass flow to get rid of it. Basically the exhaust plume acts like a heat radiator. The technical term is "open-cycle cooling".

Unfortunately torch drives cannot use that trick. High propellant flow equals pathetic specific impulse. Torch dives have large specific impulse ratings, which means low propellant flows, which means they will have to rely upon something else to keep themselves from melting. There isn't enough propellant in the exhaust plume to carry away the heat.

mDot = F / Ve


  • F = Thrust (Newtons)
  • mDot = Propellant Mass Flow (kg/s)
  • Ve = Exhaust Velocity (m/s)

As you can see from the above equation, if both thrust and specific impulse is torche-drive high, the propellant mass flow will be small.

Reaction Chamber Size

So open-cycle cooling is out.

If one has no science-fictional force fields, as a general rule the maximum heat load allowed on the drive assembly (the hollow metal ball surrounding the torch reaction) is around 5 MW/m2. This is the theoretical ultimate, for an actual propulsion system it will probably be quite a bit less. For a back of the envelope calculation:

Af = sqrt[(1/El) * (1 / (4 * π))]

Rc = sqrt[H] * Af


Af = Attunation factor. Anthony Jackson says 0.126, Luke Campbell says 0.133
El = Maximum heat load (MW/m2). Anthony Jackson says 5.0, Luke Campbell says 4.5
π = pi = 3.141592...
H = reaction chamber waste heat (megawatts)
Rc = reaction chamber radius (meters)
sqrt[x] = square root of x

This is using the inverse-square law to reduce the heat-per-square-meter load to a point low enough that the drive assembly doesn't vaporize. The larger the reaction chamber radius, the lower the head load.

As a first approximation, for most propulsion systems one can get away with using the thrust power for H. But see magnetic nozzle waste heat below.

Only use this equation if H is above 4,000 MW (4 GW) or so, and if the propulsion system is a thermal type (i.e., fission, fusion, or antimatter). It does not work on electrostatic or electromagnetic propulsion systems.

(this equation courtesy of Anthony Jackson and Luke Campbell)

Science-fictional technologies can cut the value of H to a percentage of thrust power by utilizing handwavium force fields to prevent the waste heat from getting to the chamber walls (e.g., Larry Niven's technobabble crystal-zinc tubes lined with magic force fields). A pity that by definition handwavium does not exist.


Say your propulsion system has an exhaust velocity of 5.4e6 m/s and a thrust of 2.5e6 N. Now Fp=(F*Ve)/2 so the thrust power is 6.7e12 W. So, 6.7e12 watts divided by 1.0e6 watts per megawatt gives us 6.7e6 megawatts.

Assuming Anthony Jackson's more liberal 5.0 MW/m2, this means Af = 0.126

Plugging this into the equation results in sqrt[6.7e6 MW] * 0.126 = drive chamber radius of 326 meters or a diameter of almost half a mile. Ouch.

Equation Derivation

Here is how the above equation was derived. If you could care less, skip over this box.

It is based on the good old Inverse-square law.

The reaction chamber is assumed to be a spherical hollow ball, with a hole for the exhaust nozzle. Obviously the larger the radius of the chamber, the more surface area it has, and the given amount of waste heat has to be spread thinner in order to cover the entire area. If you only have one pat of butter, the more slices of toast you spread it over means the lesser amount of butter each slice gets.

El is the Maximum heat load, or how many megawatts per square meter the engine can take before the blasted thing starts melting. Anthony Jackson says 5.0 MW/m2.

The idea is to expand the radius of the reaction chamber such that the inverse-square law attenuates the waste heat to the point where it is below the maximum heat load. Then the engine won't melt.

The attenuation due to the inverse square law is:

ISLA = (4 * π * Rc2)


ISLA = attenuation due to the inverse square law
π = pi = 3.141592...
Rc = reaction chamber radius (meters)

The heat load on the reaction chamber walls is:

Cl = H / ISLA


H = waste heat (megawatts)
Cl = heat load on chamber wall (MW/m2)

Merging the equations:

Cl = H / (4 * π * Rc2)

Solve for Rc:

Cl = H / (4 * π * Rc2)
4 * π * Rc2 = H / Cl
4 * π * Rc2 = H * (1/Cl)
Rc2 = (H * (1/Cl)) / (4 * π)
Rc2 = H * (1/Cl) * (1 / (4 * π))
Rc = sqrt[H] * sqrt[(1/Cl) * (1 / (4 * π))]

Looking at the last equation, take the right half and swap Cl for El to get:

Af = sqrt[(1/El) * (1 / (4 * π))]

and the entire equation is where we get:

Rc = sqrt[H] * Af

which is what we were trying to derive. QED.

Playing with these figures will show that enclosing a thermal torch drive inside a reaction chamber made of solid matter is impractical. Unless you think a drive chamber a half mile in diameter is reasonable.

Therefore, the main strategy is to try and direct the drive energy with magnetic fields instead of metal walls. See "Magnetic Nozzle" below.

With these propulsion systems, H is not equal to thrust power. It is instead equal to the fraction of thrust power that is being wasted. In other words the reaction energy that cannot be contained and directed by the magnetic nozzle. Which usually boils down to neutrons, x-rays, and any other reaction products that are not charged particles.

For instance, D-T (deuterium-tritium) fusion produces 80% of its energy in the form of uncharged neutrons and 20% in the form of charged particles. The charged particles are directed as thrust by the magnetic nozzle, so they are not counted as wasted energy. The pesky neutrons cannot be so directed, so they do count as wasted energy. Therefore in this case H is equal to 0.8 * thrust power.

Magnetic Nozzles

So enclosing a thermal torch drive inside a reaction chamber made out of soild matter appears to be a dead end.

Therefore, the main strategy is to try and direct the star-core hot exhaust with energy instead of matter: using magnetic fields instead of metal walls. In science fiction terms, this would be a force-field rocket nozzle. The magnetic fields cannot melt in the heat because magnetism doesn't melt. The waste heat just shines in all directions because the reaction chamber and nozzle are transparent.

Rick Robinson points out that the only way we know to make strong sculptured magnetic fields is by using magnetic coils. Which are made of matter. So the torchship drive will have an open latticework to support a magnetic containment nozzle. The coils will be protected from melting by blade shields.

Rick Robinson likes to call subtorch high-end drives a "lantern", because it will glow brilliantly. Which is putting it mildly. Heinlein calls them "torches" and John Lumpkin calls them "candles."


High specific impulse thermodynamic rockets benefit from a nozzle that is not limited by the melting point of its material.

Magnetic nozzles direct the exhausted flow of ions or a conductive plasma by use of magnetic fields instead of walls made of solid materials (see de Laval nozzle). If superconducting coils are used, these must be thermally shielded to remain in the superconducting range.

The design illustrated operates at a throat magnetic field strength of 25 Teslas and a nozzle efficiency of 86%.

From HIGH FRONTIER by Philip Eklund

All advanced concepts have the same problem when they try to produce high thrust at high specific impulse (greater than 1500 s) in a compact engine.

The problem is independent of the type of engine, whether it involves high-power electromagnetic thrusters; atomic hydrogen, metallic hydrogen, or metastable atom fuels; solar, laser, or microwave-heated thrusters; and fission-, fusion-, or antimatter-powered reaction chambers. The high-energy exhaust from any of these processes produces a blazing plasma that melts or evaporates any reaction chamber and nozzle made of ordinary materials.

One solution is to make or shield the reaction chamber and nozzle with magnetic fields. Research in this field is still in its preliminary phases [Gerwin, 1990] and we are not sure a good design is possible.

[Gerwin, 1990] Gerwin, R. A., G. J. Marklin, A. G. Sgro, A. H. Glasser. 1990. Characterization of Plasma Flow Through Magnetic Nozzles. Los Alamos National Laboratory. Final Technical Report AL-TR-89-092, February 1990, on Contract RPL: 69018 with Air Force Astronautics Laboratory, Edwards AFB, CA 93523,1 May 1986 to 30 April 1987.

Simply stated, a magnetic nozzle converts thermal energy of a plasma into directed kinetic energy. This conversion is achieved using a magnetic field contoured similarly to the solid walls of a conventional nozzle (see, for example, Fig. 1). The applied magnetic field in most cases possesses cylindrical symmetry and is formed using permanent magnets or electromagnetic coils, which confines the plasma and acts as an effective "magnetic wall" through which the thermal plasma expands into vacuum. Applications include laboratory simulations of space plasmas, surface processing, and plasma propulsion for spaceflight.

Magnetic nozzle research at the EPPDyL began in 2008. The goal of this research is to understand the fundamental physical processes of the plasma flow through the nozzle and their impact on the nozzle performance. Specifically, we are working towards answering the following important questions:

  • How do the dynamics of the plasma flow through the magnetic nozzle influence the exhaust plume structure?
  • How is the exhausted plasma affected by the relationship between the physical and magnetic goemetries?
  • How does the magnetized plasma detach from the applied magnetic field?
  • These questions will be answered through a combination of theoretical, experimental, and computational research. Ultimately, knowledge of these processes will yield fundamental scaling laws for the performance of magnetic nozzles for plasma propulsion.

    From MAGNETIC NOZZLE PLASMA DYNAMICS AND DETACHMENT by Electric Propulsion and Plasma Dynamics Lab at Princeton University

    Blade Shields

    Magnetic nozzles are created by superconducting coils. These coils typically have to be cooled to subzero temperatures in order to operate. The fact that the coils are being exposed to the star-core heat of the torch reaction means that shortly they ain't going to be at subzero temperature no more. And about the same time the superconductivity will disappear, and so will the magnetic nozzle. It is likely that the waste heat will be so furious that the coils will melt, if not actually vaporize.

    To prevent this unhappy state of affairs, the coils will have to be protected by "blade shield."

    How do they work? Much in the same way that the angle of sunlight makes it hot in the summer and cold in the winter.

    Refer to Figure 1. Consider a beam of light from the sun that is one mile in diameter. In the summer the beam hits the ground at a 90° angle, so the amount of ground that is illuminated is one mile in diameter. This makes it hot in the summer. But in winter the beam hits the round at a slanty 30° angle. The energy in the beam is spread out over a larger area, an oval that is two miles in long axis which has twice the ground area as the one mile diameter circle. The bottom line is that each square meter of ground is getting half the solar energy in winter as it gets in summer, due to the angle. So it is cold in the winter.

    The blade shield tries to angle itself to spread the heat radiation from the torch reaction over as big an area as possible. Except instead of using a 30° angle, it is using more of a 0.29° angle (200-1 angle). At such an extreme angle, the heat flux is spread out over a huge area, and the neutron radiation glances off instead of penetrating.


    And as for torch ships — a bigger problem than creating the hellish inferno of nuclear fire needed to propel your spacecraft is keeping your spacecraft from evaporating under the intense x-ray, gamma-ray, and neutron irradiation.

    My best guess for accomplishing this is keeping the torch flame outside the spacecraft, and couple the plasma to the spacecraft using magnetic fields to give you thrust. The latter trick probably means several conductive — or, more likely, superconductive — loops of cable surrounding the torch flame but at a good healthy radius.

    To protect your field generating cable, you will need to shield them, probably with blade-like tungsten structures edge-on to the torch. The large surface area of the blades gives you a lot of radiator surface while only intercepting a small amount of radiation (only what is needed to shield the cables), tungsten does a good job of scattering neutrons away without heating up much, at shallow angles of incidence x-rays (but not neutrons or gammas) are reflected away, and tungsten can heat up to yellow-white hot without evaporating too much.

    An unusual consequence of the latter, and the relatively small emissions from your optically thin thrust plasma, is that visually the tungsten shields will be the brightest part of your spacecraft when under thrust — the intensity will be the same as that of an incandescent filament from a light bulb, but you will have a lot more area to radiate from.

    These resulting torch craft don't end up looking much like conventional rockets. You get loopy filigree and from the support and field cables, and graceful glowing sheets from the heat shields, enclosing a volume that is probably much larger than the passenger/payload section.

    A recent thread on got me thinking about how to protect your lantern structure from the radiation of a Magic Fusion Torch (MFT). here are my current thoughts I figured I'd throw out for the group mind to pick over.

    My design goal was to get a shield that did not require active cooling in a moderately compact package. For illustration, I will use a 10 TW D-3He laser inertial confinement fusion (ICF) torch.

    Materials: You need something that can withstand high temperatures. Briefly, this comes down to graphite, tungsten, and silcon carbide.

    • Graphite has sublimation vapor pressure of 1 atmosphere at about 3900 K, and a vapor pressure of 10 Pa at 2840 K so we would probably want to keep the temperature below 2500 K. Additional properties that will become important later are that high energy neutrons in graphite have a mean free path of about 6 cm before scattering, and each scattering will transfer on average about 14% of the neutron's energy to the graphite.
    • Tungsten melts at 3695 K, but experience with incandescent bulbs tells us it is best to keep the temperature below 3000 K to prevent sublimation (putting the tungsten in a quartz container and flooding it with halogen gas can help re-deposit the evaporated tungsten back on the tungsten, but you need to keep the quartz cool enough that it does not melt). The mean free path of high energy neutrons in tungsten is about 2.5 cm, and each collision between a neutron and a tungsten nucleus imparts about 1% of the neutron's energy to the tungsten. Further, at angles of incidence of 0.5 degrees or less, you get total external reflection of 8 keV x-rays off of a tungsten surface. This critical angle increases as the x-ray energy decreases, so for soft x-rays tungsten at low angles will reflect most of the incident x-ray flux.
    • Silicon carbide sublimates at 3003 K, presumably this is where the vapor pressure is 1 atmosphere. We would thus want to keep the temperature of SiC components at a maximum of somewhere around 2000 to 2500 K. The benefit of SiC is that it is an excellent thermal conductor.

    All data on neutrons was taken for the 2.5 MeV neutrons produced by D-D fusion.

    (ed note: Deuterium-Helium 3 fusion does not produce neutrons. Unfortunately you cannot rely upon the deuterium nuclei to obligingly react only with the Helium-3 nuclei. Some will insist upon reacting with other deuterium nuclei in the vicinity, and that reaction does produce those nasty neutrons)

    My idea, therefore, is to point a very narrow wedge of tungsten at the point of the fusion torch flame. A 0.3 degree angle is a slope of 200:1, so we will take two 2 mm thick sheets of tungsten and join them at an opening angle of 0.6 degrees. Pointed directly at the fusion source, we get a 0.3 angle of incidence. This will specularly reflect most x-rays. Plots I have seen for low angle x-ray reflectance indicates at least 99% of the x-rays will be reflected.

    A neutron encountering the tungsten shield will pass through 40 cm of tungsten for a 2 mm thick sheet at a 200:1 slope. This is on the order of 16 interaction lengths, so essentially all incident neutrons will be scattered. Because the thickness of the sheets is much less than the mean free path of the neutrons, essentially all neutrons will only scatter once. Thus, about 1% of the intercepted neutron energy will be deposited in the tungsten, the rest will be scattered away.

    (ed note: MW is megawatt {millions of watts or 106 watts}, GW is gigawatts {billions of watts or 109 watts}, TW is terawatt {trillions of watts or 1012 watts} )

    We now have a structure that will absorb at most only 1% of the fusion radiation it intercepts. It also has a radiating area of 200 times the cross section it presents to the radiation source. If instead of a wedge, we used a single inclined sheet, we could double this radiating area at the expense of a shield that is twice as long. If the tungsten can withstand a maximum temperature of 3000 K before sublimation becomes problematic, it will be radiating 4.6 MW/m2 of surface area. This corresponds to 200 * 4.6 MW/m2 = 920 MW/m2 of cross section presented to the fusion source. Since only 1% of the incident radiation is absorbed, this structure can withstand radiant intensities of 92 GW/m2 before it suffers from excessive sublimation. If we limit the temperature to 2500 K, the allowable radiant flux is still 44 GW/m2.

    For the 10 TW D-3He torch, we need to deal with about 2 TW of x-rays and about 0.5 TW of neutrons from the D-D side reaction. This gives a total radiant intensity of 200 GW/r2, where r is the distance from the fusion source. This gives us a stand-off distance of 1.5 meters for 3000 K, or 2.1 meters for 2500 K.

    There is the additional consideration of the size of the lantern structures to be protected. If you need cables and support beams 10 cm across, then the wedge will need to stick out 10 meters from the lantern structures. This gives a lantern radius of around 12 meters. If you can get by with 2 cm wide structures, you can build your lantern as compact as 3.5 to 4 meters in radius.

    This analysis neglects radiant heat from one shield being intercepted by another shield. It also ignores how you would protect the optical elements used to focus the laser on the fuel pellet. You would also probably want additional neutron shielding for superconducting cables that need to be cooled (steel support structures wouldn't need this), since some neutrons will be scattered at low angles, and other parts of the lantern structure will be scattering neutrons all over the place. You will probably need active cooling for the superconductors, but at least you will not need to pressurize the coolant at hundreds of atmospheres to contain it at 3000 K.

    Isaac Kuo

    This is assuming the fusion source is a point source of radiation, of course.

    You can also double the radiating area with a wedge by doubling the shield length and halving the sheet thickness.

    Luke Campbell

    Good call. The inverse process, using a thicker sheet at a larger angle, would probably be useful for keeping the length of the wedge down. In particular, you could have the wedge be very sharp near the fusion source, and then have a wider angle as you keep the tungsten at a constant temperature. Otherwise, you end up with a wedge that is 3000 K at 1.5 meters from the fusion source, 2500 K at 2.1 meters, 2000 K at 3.3 meters, 1500 K at 6 meters, and so on.

    Anthony Jackson

    Luke Campbell: We now have a structure that will absorb at most only 1% of the fusion radiation it intercepts. It also has a radiating area of 200 times the cross section it presents to the radiation source.

    Sadly, the tip does not, so what will happen is that the tip will melt/sublimate, becoming blunt, and it will continue sublimating backwards.

    Luke Campbell

    This should depend on the sharpness of the tip and the thermal conductivity of the material. If the thermal conductivity is high enough and the amount of energy absorbed by the tip is low enough due to the tip being very sharp, you should be able to rapidly conduct that heat away to adjacent highly inclined bits and keep the temperature down.

    I think. I'll have to do the analysis later.

    Anthony Jackson

    That may argue for diamond or graphite, for superior thermal conductivity. Also, if you can get the tip to be translucent, its absorption goes down.

    In general, though, it's important to ask what the purpose of the lantern shield is. Usually, it's envisioned as a support structure for the magnetic nozzle and/or igniter mechanism.

    Luke Campbell

    Anthony Jackson: That may argue for diamond or graphite, for superior thermal conductivity. Also, if you can get the tip to be translucent, its absorption goes down.

    True, although carbon cannot withstand as high of temperatures as tungsten in vacuum (C has 10 Pa vapor pressure at 2840 K, W has 10 Pa vapor pressure at 3770 K). Carbon also has the disadvantage that it absorbs an order of magnitude more energy when struck by a neutron. However, since the tip is so small, you probably only need a very thin layer of carbon. Neutrons are so penetrating that most of them will just pass through the thin carbon layer.

    At these temperatures, diamond is not available. It decomposes into graphite at 1700 K. For the same reasons, any delicate nanostructure of carbon will probably decompose into graphite at some temperature less than 2000 K.

    One option would be a tungsten/carbon multilayer mirror. These are currently used for soft x-ray reflection, and can achieve normal incidence soft x-ray reflectivities of around 0.4 to 0.6, with theory predicting normal incidence reflectivities of up to 0.8. This is nowhere near as good as the .99+ reflectivity for grazing incidence, of course, but can still cut down the amount absorbed. It would be a very thin surface structure located only at the tip, to cut down on neutron absorption. If the outer layer were tungsten, it might be able to contain the carbon vapor at temperatures close to 3000 K.

    Anthony Jackson: In general, though, it's important to ask what the purpose of the lantern shield is. Usually, it's envisioned as a support structure for the magnetic nozzle and/or igniter mechanism.

    I was thinking of shielding the superconducting cables (which may need to be at cryogentic temperatures, and even in settings with room temperature superconductors still probably couldn't exceed a few hundred K) from the x-rays and neutrons. In addition, I want to shield the support structures that transmit the thrust from the magloops to the spacecraft. Presumably, these are made out of steel (possibly reactor steel, to handle scattered neutrons), and would weaken around 1000 K or a bit higher and melt at 1800 K. Cryogenic coolant pipes can be run behind the structural members, but these also need to be protected from the intense radiation.

    The igniter mechanism could possibly be protected from the radiation if the ICF used heavy ions. In this case, the beams would be steered with magnetic fields and the steering magnets could be protected with the tungsten shields. For laser ICF, you could use a highly inclined thin sheet of silicon carbide (SiC) in front of the mirror. The SiC would reflect the x-rays and scatter the neutrons like the tungsten, but it would be a transparent window to the laser light. Unfortunately, the transparency means it will not radiate its heat efficiently, and neutrons will transfer more of their energy to the lower mass silicon and carbon atoms that they would to heavy tungsten. Perhaps it could be actively cooled by a transparent molten salt that pumps the heat away to a black body radiator. With a set-up like this you could probably get the mirrors within a few tens of meters of that 10 GW fusion source.

    Anthony Jackson

    Luke Campbell: The igniter mechanism could possibly be protected from the radiation if the ICF used heavy ions.

    Of course, unless you're using self-ignited targets (Mini-Mag Orion) or antimatter, dealing with incidental heat from the method by which you generate the beam may be a bigger problem than shielding the torch. You can probably use your mag-loop to steal energy from the drive (the Mini-Mag Orion does that), probably without a lot of waste heat, but you'll have some, and you'll have some more from your ion beams, so it's still hard to see waste heat less than about 0.1% of drive power, all of it at fairly low temperatures (operating temperature of generator and drive beam).

    Luke Campbell

    That is a good point. I wonder if it is worthwhile to use a heat pump to increase the temperature of the radiator coolant, in order to increase the rate at which entropy is radiated away?

    Anthony Jackson

    Depends on assumptions. For perfect heat pumps, a net efficiency of 25% minimizes radiator area, but the weight penalty elsewhere may be more important than the penalty for radiator area. For example, in Attack Vector: Tactical, going from 25% efficient reactors with 1,500K radiators to 50% efficient reactors with 1,000K radiators would increase radiator size by 69%, but it would reduce reactor fuel consumption by 50% and, while the storage capacity of heat sinks would be reduced (perhaps by as much as 50%), the quantity of heat generated would be reduced by even more (by a factor of 3), and the plumbing would be a lot easier to work with.

    Isaac Kuo

    Luke Campbell: This should depend on the sharpness of the tip and the thermal conductivity of the material.

    You can greatly reduce the area of the tips by using an edge shaped like a sharp sawtooth. This more or less transitions a wedge shape into a series of sharp knife-points. You have grazing angles everywhere except for the tips of the peaks and the saddle-points of the valleys.

    Anthony Jackson: That may argue for diamond or graphite, for superior thermal conductivity. Also, if you can get the tip to be translucent, its absorption goes down.

    Another possibility is to actively cool the edge. Instead of joining the two sides together, the two sides of the wedge simply fit together by spring forces. Between the two sides is a "coolant plate" that slides in/out. When it slides out, the wedge sides are forced apart and thermal conduction transfers heat from the wedge edges to the coolant plate. When it slides in, the wedge sides join together.

    Anthony Jackson: In general, though, it's important to ask what the purpose of the lantern shield is. Usually, it's envisioned as a support structure for the magnetic nozzle and/or igniter mechanism.

    It's meant to shield the superconducting magnetic nozzle loop(s) from neutral radiation.

    But you also need a shield for the bulk of your ship...the ship itself may be protected by a very large wedge shape; the ship's hull being a flat wedge shape. This provides more volume than a cone.

    Think of it like the design of a house where the roof needs to be steeply angled to let snow fall off. A seemingly compact square or circle floorplan is very bad because it requires a very tall roof. In contrast, a long rectangle floorplan can be covered with a shorter roof.

    So, the ship's overall shape could look like a thin slice of cantaloupe, along with a superconducting loop ring. This loop ring has a conical fringe for its shield.

    Luke Campbell

    Isaac Kuo: You can greatly reduce the area of the tips by using an edge shaped like a sharp sawtooth. This more or less transitions a wedge shape into a series of sharp knife-points. You have grazing angles everywhere except for the tips of the peaks and the saddle-points of the valleys.

    I don't think you want any valleys — the sides of the valleys would be radiating their heat on to each other rather than into space.

    I expect you could get the knife-edge of the tungsten wedge to be only a few atoms wide — very sharp steel knives are only a few atoms wide, and some ceramics can get a mono-atomic edge.

    Isaac Kuo: Another possibility is to actively cool the edge. Instead of joining the two sides together, the two sides of the wedge simply fit together by spring forces. Between the two sides is a "coolant plate" that slides in/out. When it slides out, the wedge sides are forced apart and thermal conduction transfers heat from the wedge edges to the coolant plate. When it slides in, the wedge sides join together.

    This adds a considerable amount of complexity over passive cooling. If necessary, it would be best to design it so that the coolant plate were retracted and protected by the tungsten wedge during each fusion pulse. Then it could perform its cooling in between the pulses. It would be best to make it out of heavy elements, to minimize energy transfer from neutrons, since that close to the tip you are likely to get plenty of scattered neutrons hitting it.

    Isaac Kuo

    Luke Campbell: I don't think you want any valleys — the sides of the valleys would be radiating their heat on to each other rather than into space.

    Not these valleys. These valleys are on a sawtooth knife-edge, so the sides still radiate almost perpendicular to the overall plane.

    Geometrically, consider the wedge to be the product of sweeping the straight edge at a 1:200 slope. The alternate sawtooth would be created by sweeping a sharp sawtooth shape at the same slope.

    Rick Robinson


    But I agree with whoever it was upthread who said that the problem here is the knife edge, or even if you sawtooth it the points. No matter how fine you make it there will be a blunt surface facing the full brunt of the fire. As it sublimates away the flat surface will widen, leading to progressive and accelerating failure.

    This does not invalidate the concept, but I think the radius will have to be much more conservative to keep the knife edge within the limits of some form of active cooling.

    The tougher problem in torch design seems to be the one Anthony brought up. If you suppose that about 1 percent of torch output will be in the form of relatively low-temperature heat generated by the various gizmos that keep the torch running, for a 1 TW torch that will be 10 GW of low-grade heat to dump from secondary radiators, probably at much less than 1 MW/m2.

    I tend to handwave a 2-stage control function, where the torch cycle is sustained by pulsations in a plasma (or some such), and the onboard gizmos are required only to stabilize and control that secondary plasma, nominally permitting about another hundredfold reduction in onboard low-grade waste heat, to about 100 MW.

    Luke Campbell

    Torch drive describes a class of thrusters for spacecraft propulsion in which a high energy yield detonation or pulse is initiated at a high rate external to the spacecraft. Magnetic fields are used to deflect the plasma produced by the pulse to generate thrust. By detonating the pulse outside the main structure of the spacecraft, the neutral radiation by-products of the pulse (neutrons, bremmstrahlung x-rays, gamma rays, and thermal radiation) can mostly escape into space, without creating a large thermal load aboard the spacecraft. This allows very high energy thrusters, which can combine both high thrust and high delta-V simultaneously.

    A torch drive requires one or more current carrying loops surrounding the reaction region to produce the magnetic field that deflects the plasma and charged radiation from the drive pulses. Typically, these loops are made of superconductors, since any normal conductor would quickly melt or vaporize under the high currents needed to produce the field. The field coils are backed by a high tensile strength support to withstand the magnet-current back reaction from bursting the superconductor.

    These field coils must be protected from the intense neutral radiation that the drive pulses produce. This is because a superconductor that becomes too hot will cease to superconduct. A common shielding design is a sheet of tungsten with a "V"-shaped cross section at a very narrow opening angle, resembling a knife-edge. The point of the V faces the radiation source, the field coil runs along the open top of the V. At very small angles of incidence, tungsten makes a good reflector of x-rays so that most x-rays are simply reflected at low angles away from the field coils and into space. Since tungsten atoms are much heavier than neutrons, a collision between a tungsten nucleus and a neutron results in the neutron rebounding with most of its original energy, delivering only 1% of its energy on average to the tungsten shield. This scatters the neutrons away from the field coils. The narrow opening angle means that the tungsten knife edge is essentially a sheet perpendicular to the incoming radiation, allowing a large radiator area compared to the cross section exposed to the radiation.

    Rates of tungsten sublimation become problematic at temperatures above 3000 K, so the shield is typically placed far enough away from the drive pulses to keep its temperature at or below this value. At all times, the shield must be kept below 3695 K, the melting point of tungsten. At these temperatures, the tungsten knife-edge sheets are radiating at a blazing yellow-white color, with the intensity of an M-class star or an old style incandescent bulb filament. A torch drive in operation appears as a brilliant flare primarily from the thermal radiation of the tungsten shields — the drive pulses themselves emit relatively little visible light in comparison. With a 200:1 aspect ratio for the length of the knife blade to its width, a heat shield that absorbs 1% of the incident radiation and scatters the rest can withstand an incident intensity of 90 GW/m2.

    A plasma in a magnetic field will expand against the field while the energy density of the plasma is greater than the energy density of the field. In SI units, the energy density of the field in J/m3 is given by


    where B is the magnitude of the magnetic field in tesla and mu_0 is the magnetic constant (mu_0 = 4 pi * 10-7 N/A2). If the field coils produce a uniform field, then a drive pulse with energy E in its plasma will expand against the field until it reaches a volume V such that the following relation approximately holds

    V = 2 mu_0 E/B2

    Assuming the radius is approximately spherical, the pulse's blast will expand into a fireball with a diameter d approximately the cube root of this volume. The time t it will take for the pulse to expand to a stop before it is deflected is approximately

    t = d/V_ex

    where V_ex is the velocity of the torch drive exhaust. If a second pulse is detonated before the first pulse has been fully deflected, it will add its energy to that of the first and require a larger volume to hold the combined fireball.

    The size of the field coils is set by the requirement that the tungsten shield remain cool enough not to sublimate, and for the drive pulse fireballs not to contact the tungsten shield. Since the tungsten shield extends a considerable distance toward the detonation point (the distance from the field coil to the tip of the shield is typically 200 times or more the width of the field coil), the field coil must be set far enough back that the tip of the tungsten shield does not evaporate.

    If there are multiple field coils producing the magnetic field, then the knife-blade heat shield of one coil will scatter neutrons and radiate thermal heat onto the heat shields of the other field coils, compromising their ability to shed heat. Consequently, many designs use only a single field coil despite the loss of efficiency. Those torch drives that use multiple field coils typically space them at distances significantly larger than the length of the knife-blade shield.

    As previously mentioned, the tungsten knife-blade heat shield that protects the field coils will glow very brightly. At 3000 K, it will radiate 4.6 MW/m2 of heat as thermal radiation. As an example, consider a torch drive with 2 cm wide field coils, 4 meter long knife-blade heat shields that are exposed to 90 GW/m2, and 10 MW of neutral radiation produced by the drive pulses. At this rated intensity, the tip of the heat shield can be as close as 9 meters to the detonation point. This produces a radiating disk with an outer radius of 13 meters and an inner radius of 9 meters, which will therefore radiate 2.6 GW of heat and 76 Glm of luminous power combined from its front and back. The apparent brightness will depend on the angle of the disk with respect to the observer, but unless the disk is edge-on, the unaided dark-adapted eye of a baseline human could detect the disk at a distance of approximately 1 Gm (gigameter) and would appear as an apparent magnitude 0 point of light at approximately 0.05 Gm. For comparison, the distance between Sol and old Earth is approximately 150 Gm. A dedicated 1 meter aperture early alarm scanning scope could detect the disk at approximately 600 Tm (terameter) with 1 kilosecond exposure time. For comparison, this is over 1% of the distance from Sol to its nearest stellar neighbor, Alpha Centauri.


    Controlling Neutron Flux

    This is pure Unobtainium. Meaning that while it is not actually forbidden by the laws of physics; if you try engineering this into a practical rocket engine, well, good luck with that. You'll need it.

    One annoying problem with using fusion reactions in rocket engines is the problem of neutrons.

    Many proposed designs used Deuterium-Tritium fusion. As you can see in the Fusion Reactions Chart, D-T fusion has a Lawson criterion of just 1. This means (in theory) D-T fusion should be the easiest of all fusion reactions to ignite. Since researchers have been trying for decades to push a fusion reactor up to the point it breaks-even and failing, it was natural to use every advantage possible and use D-T fusion.

    The trouble is that (as you can see in the fusion reaction chart) a whopping 79% of the fusion energy is in the form of worthless and deadly neutrons.

    Worthless because they are randomly emitted in all directions and the uncharged little monsters cannot be directed by electrostatic or electromagnetic fields (and you can forget about antigravity). Which means the neutrons provide zero thrust.

    Deadly because Wonder Neutron helps kill strong bodies 3 ways: Neutron Activation, Neutron Embrittlement, and Neutron Heating. So you need massive shadow shield to protect the crew and massive blade shields to protect the engine (which really cuts into your payload mass). Plus extra heat radiators to prevent the engine from melting from neutron heating.

    So to sum it up, D-T fusion wastes 79% of its energy on generating neutrons that destroy the ship and kill the crew. Only 21% of the energy actually provides thrust. And other fusion reactions either have outrageous Lawson criterions (hydrogen-boron fusion has practically no neutrons but has a Lawson of 500!) or require hard-to-get fusion fuel (I'm looking at you, Helium-3). The closest good source of Helium-3 is the planet Saturn. By contrast Deuterium is common in seawater, and Tritium can be easily manufactured by neutron activation of lithium.

    Makes you wonder if fusion is worth all the trouble.

    Gee, it's just too bad that there is no way to direct neutrons...

    ...or is there?

    In 1998 there came out a very interesting report titled Investigation on the possibility of using Nuclear Magnetic Spin Alignment. Sounds esoteric and dull, don't it?

    In D-T fusion, a deuterium nuclei (one proton plus one neutron) slams into a tritium nuclei (one protons and two neutron) and becomes an excited nucleus. 1 femtosecond later it splits into an alpha particle (two protons plus two neutrons) and a single neutron.

    Fascinating point: due to the law of conservation of momentum the alpha particle and the neutron are always emitted in exactly opposite directions. You are thinking: yawn!

    Utterly riveting point: if the so-called magnetic spin moments of the deuterium and tritium nuclei can be precisely aligned at the moment of fusion, you can direct a stream of emitted neutrons in one direction and a stream of alpha particles in the opposite direction.

    This changes everything

    • The stream of neutron would be directed out the exhaust nozzle
      • Suddenly that 79% of the wasted fusion energy becomes thrust!
      • You do not need any heavy shadow shields because the neutrons are all going in a safe direction
      • There is no neutron activation, neutron embrittlement, nor neutron heating. So you do not need blade shields either
      • There is no need for a cathode neutralizer like on an ion drive because the neutron exhaust is already neutral
    • The stream of alpha particles would be directed in the opposite direction towards the spacecraft
      • where they enter a magnetohydrodynamic generator or something to transform the particle energy directly into electricity
      • only 10% of the electricity would be needed to run the fusion engine. The rest is just gravy
      • But most of the alpha particles will have to be reflected by a magnetic field or used to heat propellant. Otherwise the thrust from the neutrons and the alpha particles will exactly cancel each other out with no net thrust

    Controlling the nuclear magnetic spin moments is not particularly hard, medical MRI scanners do it every day.

    Not so fast! says Dr. Luke Campbell. An MRI scanner controls the magnetic spin moments of a few atoms, but you have to have a freaking powerful magnet to ensure that all of them are aligned. Your average MRI scanner is strong enough to yank the service revolver right out of the hand of a police officer standing in the doorway. It is four times as powerful as magnets used to lift cars in junkyards. But the magnets needed for this neutron drive are so strong they make an MRI look like a refrigerator magnet.



    How do they plan to get the nuclei spin-aligned? The deuteron magnetic moment is 2.7E-8 eV/T, so even in a 10 tesla field the energy difference between spin aligned with the field and anti-aligned with the field is 5.4E-7 eV (currently the largest MRI scanner has a 11.75 tesla field).

    A fusion plasma is about 50 keV temperature, which means by the Boltzmann distribution you would have a difference in population between the spin aligned and anti-aligned states of about 2.7E-7/(2*50,000) = 2.7E-12. Thats 2.7 parts per trillion. The spin polarization of the fusing gas would be negligible (with a 10 tesla field).

    Now nuclei do tend to remain spin polarized for a surprisingly long time before coming to thermal equilibrium with their surroundings, so it may be possible to cool the deuterium (and tritium, but I haven't analyzed the energetics of this yet) down to sub-microkelvin temperatures, get nearly complete spin polarization, and then flash heat it to fusion plasma temperatures and only hold the plasma long enough that the spin doesn't depolarize much before dumping the plasma. It seems kind of iffy to me.

    You can get polarized beams with a particle beam type setup, but while you can get fusion reactions in particle beams it is a horribly inefficient way to do it.


    If I'm reading and did the math correctly (and I wouldn't trust me, if I were you) the neutron gets most of the energy, totalling 14MeV for D-T fusion, which corresponds to about 15% of C (!). Note, though, that 80% of your fuel mass ends up in that slower alpha particle going the other way (meaning that the neutron mass flow and thrust will be low).

    OTOH, this does maybe solve a big problem with magnetic bottle fusion torch ships, which can't redirect the uncharged neutrons and end up having to waste and dissipate all that energy.


    The neutron will be going at 17.1% the speed of light (Michael Earl is pretty close!). The alpha particles will be moving at 4.3% the speed of light, and can be reflected out as part of the exhaust with magnets. It is not a coincidence that they both carry the same momentum — in a reaction with two products, in the center-of-mass frame each product must go away from the reaction with opposite momenta in order to conserve momentum. By re-directing the alpha stream out into the exhaust, both the alphas and neutrons will be contribution equally to the thrust.

    In practice, of course, you will actually be using the alphas to heat up the D-T plasma, and eject the mostly-unburnt plasma as your rocket propellant. In this more realistic case, the neutrons will add only a tiny bit to the thrust, but at least you will be directing them away from your spacecraft and all of its fiddly radiation-sensitive components and crew, as well as eliminating a major source of heating to the structures surrounding the reactor/rocket.

    Ian Mallett took a closer look and thinks this drive still has merit:


    Deuterium–Tritium fusion is easy(-est) to ignite, but it has the major problem that, in-addition to an alpha particle, the reaction produces a 14.1 MeV neutron. Hence, using this reaction for fusion drives is problematic—it requires heavy shielding and regular maintenance (to replace neutron-embrittled reactor parts). The energy wasted in the neutron cuts your efficiency significantly, too.

    The neutron is emitted in a random direction, but it turns out this is because the atoms are oriented in random directions. If we could align them (using a powerful magnetic field), the neutron radiation could be our exhaust, while the alpha particle (which would by conservation of mass then go up into the rocket) could be easily deflected back out to join it.

    This possibility was discussed here

    Unfortunately, the kinds of magnetic fields we can apply to plasmas are insufficient (and in any case, they'll clash with the fields we're using to contain the stuff anyway). Apparently, aligned particle beams are possible, but the amount of fusion produced in their collision is tiny.

    I ask, why is that a problem?

    Picture, if you will, two particle-accelerator loops producing spin-aligned beams—one of deuterium and one of tritium—at an angle to each other. Where they cross (either one or two places), they form an "X", at the intersection of which is a fusion reaction, with neutrons coming out perpendicularly (and alpha particles perpendicularly the other way). You arrange the X so that the neutrons go out the back of the ship, and the alpha particles go toward it. You then deflect the alpha particles so that they go out the back too.

    Because the particle beams are loops, very little energy need be lost. And, while the amount of fusing material for reasonable-density beams might be small, fusion reactors tend to work better as you scale them up. The reaction products are going significant fractions of c, so this functions as a low-thrust, high-ISP drive. You'd probably want the "X" to have a very shallow angle, so that the beams can be lower-energy. It might even let you reuse some of the same magnetic fields for both loops. You can also stack multiple "X"s on-top of each other, with corresponding multiplicity of loops, to multiply the thrust.

    This system is, in-fact, very flexible. The alpha particles, instead of being deflected, could be used to energize Hydrogen propellant, making a hybrid NTR. Or a fraction could be used to power the particle accelerators themselves. You could temporarily flip the neutron beam back around to point into a compartment in the ship ("Reverse the polarity, Scotty!"), for purposes of catalyzing more Tritium from onboard Lithium stores (convenient because Tritium decays too quickly to comfortably pack it on long trips).

    From the back, you'd see at least one set of two pale pink lines crossing (the Deuterium and Tritium beams), a spreading red-orange line going back into the ship (the alpha particles), and a glow of the same color coming out as a column (the same, deflected back). The neutrons themselves are invisible, but if you happened to look directly into the beam you'd see god. Because you'd die instantly, you see.

    I took the liberty of hacking up a 3D model of a version with three "X"s. It's a very rudimentary model, but if an artist wants to use it as a jumping-off point for a better render, I can post the ".blend" file.

    (This fits into "Art", "Hard Sci-Fi Analysis", and "Ludicrous Ideas", but I'm putting it into the last category because there are too many uncertainties to really be confident it will work.)


    The big problem is that you will get a lot more scattering that fusion. This will cause the loss of deuterium and tritium particles from the beam (and consequently the loss of the energy of those particles) as well as degrading the beams until they are more of a spray than a collimated ray. This ends up wasting more energy than is generated by the fusion.

    Now since we are talking science fiction, you can posit wide angle collectors around the beam interaction point and efficient methods of reducing the beam emittance. One problem is that known methods of reducing beam emittance can also draw more energy.


    Luke Campbell, by scattering, I presume you mean that the D and T will scatter off each other, and often out of the beam, more-often than they'll fuse? I expect that could be mitigated by making the beams faster and letting them fly through free space for less time (artist's rendition expands the gap for cinematic clarity). Though, if too much D/T escape without fusing, that will indeed hurt the effective ISP, possibly to the point of impracticality.

    Note: given that our current efforts to make magnetically confined fusion don't break even on collected energy, I wouldn't be surprised if the paper's estimate of powering everything with 10% of the output doesn't pan out. The thing could be powered by an external reactor, instead. Although it would be convenient to collect energy from the drive, the point of using energetic reactions is to boost exhaust velocity (Ve).


    Using colliding beams for fusion is really, really complicated. The cross-section for fusion is very small, even in the best of conditions, the reaction volume is too low, being only the intersection of the beams.

    Assuming all the inefficiencies of supporting the beams, keeping the polarization, total mass of the systems required would far outstrip the mass of radiators of a conventional system.

    I still prefer the neutron moderated plasma-jet driven magneto inertial fusion.

    The idea is to use an hydrogen plasma shell formed by the collision of several plasma jets, the hydrogen is dense enough to moderate the neutrons. So the hydrogen jets, compress the fuel, absorb the neutron energy, shield the crew and work as propellant. The best of everything.

    Although a direction neutron source would be a very interesting experimental machine, I wonder how it compares with spallation neutron sources though.

    From a thread on Google Plus (2018)

    I. Introduction

    Nuclear fusion has long been considered an ideal form of propulsion for space travel. Fusion is roughly 4 times more energy dense than nuclear fission, which is, itself, roughly 2 × 106 times more energy dense than chemical fuels. This extreme energy density allows for rockets that can reach far higher speeds then any chemical rocket, and even exceed the propulsive capability of nuclear fission, while being able to operate with lower mass ratios. This, coupled with the high (approximately 4% of the speed of light) exhaust velocity of fusion-reaction products, puts interstellar travel within the reach of fusion-propelled vehicles as well as more near term uses such as interplanetary exploration and planetary defense.

         Fusion-propulsion suffers fromtwo primary complications though: the difficulty of igniting a self-sustaining fusion reaction and the large amount of ionizing radiation generated by the reaction, which requires a considerable mass of radiation shielding to protect against. Unfortunately, the fuel that is easiest to ignite (deuterium-tritium, or “DT") also produces most of its energy as ionizing radiation in the form of neutrons. Fuels that produce fewer neutrons are more difficult to ignite, requiring larger reactors and ignition power supplies, whose cost in mass is often more than enough to outweigh any reduction in shielding they may allow. This paper outlines how a well known nuclear physics technique, known as spin polarization, could potentially reduce the incident neutron radiation by ~20%, reduce fusion ignition requirements by ~45%, and increase the propulsive efficiency of the fusion rocket by more than 30% over a similar fusion rocket that does not use this technique. Spin polarization has long been know to affect the radiation emission of fusion reactions and more recently shown to lower the ignition requirements as well. Up until now though, it has never been considered for use in fusion rockets. The use of spin polarization would allow for lighter fusion rockets that suffer less radiation damage and require less circulating power for operation while also increasing propulsive efficiency.

         All nuclei possess an inherent angular momentum known as spin that plays a significant role in nuclear reactions, especially nuclear fusion. Spin polarization is the process of aligning the nuclear spin vectors of the fusion reactants prior to the reaction. For five-nucleon fusion reactions, notably DT and D3He (deuterium and helium-3), spin polarization serves to increase the cross section for fusion and force the reaction products to emit anisotropically. Increasing the reaction cross section lowers the requirements to reach fusion ignition, allowing the spacecraft to use less energy for ignition and requiring less total circulating power during operation. This will lower both the fusion reactor equipment mass and radiator mass. Additionally, the anisotropic emission of reaction products allows a substantial fraction (up to 80%) of the neutron radiation to be directed away from the spacecraft, lowering the craft’s shielding mass. For four-nucleon fusion reactions, notably DD (deuterium and deuterium), and those with greater than five nucleons, notably p11B (proton and boron-11), no clear benefit has been shown from spin polarization. Such reactions require more study before any comment can be made on their use.

         Several methods for producing spin-polarized fusion fuel have been considered and tested for the purposes of nuclear physics experiments and for producing beams of spin-polarized particles in particle colliders. For fusion reactors that require a constant stream of gaseous fusion fuel, various optical pumping techniques provide options for creating jets or beams of polarized fusion fuel. These techniques are quite technologically mature, but do suffer various polarization-loss mechanisms via contact with the walls of the fuel transport system. For fusion reactors that can operate with frozen fuel injection, spin polarization can be achieved via super chilling of the fuel and/or the application of a strong (>10 –T) magnetic field. This method allows for pellets of pre-polarized fuel to be created and stored before injection into the reactor. Neither method consumes a large amount of energy (of order 1-100 eV per atom) compared to the expected power output of a fusion reactor, thus, the additional power load on the spacecraft is small.

         A limiting factor on the utility of spin-polarized fuel is the depolarization rate in the fusion reactor. For rapidly pulsed fusion reactors with no significant magnetic fields, the rate of depolarization can be shown to be far slower than the expected reaction time. In long-pulse or steady-state fusion reactors, the recycling of fuel from the reactor walls can significantly deplete the population of polarized fuel. Additionally, the presence of an external magnetic field can quickly depolarize the fuel depending on the alignment of the spin polarization with the magnetic field. Reactors with complex magnetic topologies may not be suitable for use with spin-polarized fuel (science fiction authors: this means if a tramp freighter ship lets its fusion engine tuning get run down, the thrust will drop and the radiation from the engine will rise). The details of the fusion reactor itself are beyond the scope of this analysis, which seeks to investigate the bounds on the potential benefits of spin-polarization for fusion propulsion. The following sections will outline the positive impact spin polarization will have on fusion rockets that choose either DT or D3He fusion fuel.

    II. The Benefits of Spin-Polarized Fusion

         The favorable response of five-nucleon reactions to spin polarization makes two fuels of particular interest for a spin-polarized fusion rocket: DT and D3He. Previous beam-target experiments have verified the spin-polarized response of these fusion fuels in addition to a robust theoretical understanding of the thermonuclear burn properties. As such, all other fuels will be ignored for the remainder of this paper.

    A. Spin-Polarized DT Benefits

         DT fusion is currently themost-promising candidate for near-termnuclear fusion due to its extremely high reactivity and low ignition temperature compared to other fusion fuels [9, 10], and has arguably been the most studied fusion reaction for spin-polarization applications. It is well known that fully spin-polarizedDT fusion receives an increase in fusion cross section of 50%. This increase in cross section translates to an increase of reactivity of 32% at full polarization. The increase in reactivity provides three key benefits for the fusion reactor: increased fusion gain for the same ignition conditions, fewer DD secondary reactions, and lower ignition requirements. Spin polarization also causes the neutrons and alpha particles to be emitted anisotropically as seen in Figure 1.

    This anisotropic emission has important implications for shielding the spacecraft and for the momentum transfer between the fusion reaction and the spacecraft.

         The increased fusion gain provides a more-economic use of the expensive tritium fuel and, to a first-order approximation, provides a higher exhaust velocity from the fusion rocket. The fusion-gain increase factor in inertial systems scales with polarization fraction as shown in Eq. (1).

         Here, φ is the polarization fraction, ranging from 0 (unpolarized) to 1 (fully polarized). For the reaction of 100% polarized DT fusion fuel, the gain is expected to be 45% higher than an equivalent unpolarized case. This directly translates to more 45% more energy from the fuel using the same drivers. Given this increase in specific energy, one would expect – to first order – an increase of the exhaust velocity by 20%, since vex ∝ √2E. Fusion ignition schemes that rely on linear burn propagation may also be able to take advantage of the anisotropic alpha particle emission from spin polarized fusion and achieve far higher gains then in more conventional schemes.

         The lower ignition requirements on the fuel can directly translate to smaller, or less-complex fusion drivers. The benefits here are less clear since they are heavily dependent on the exact ignition scheme. Driver size reductions could vary anywhere from 20% to 66% . Assuming a laser-driven inertially-confined fusion reactor, the laser power reduction factor becomes:

         It can be safely assumed that the laser pulse length will not be significantly altered, so a reduction in laser power can also be taken to be a reduction in laser energy. If we additionally assume that laser mass scales with laser energy, the laser mass for a perfectly polarized DT reactor will be reduced by at least 20%. The drive laser is one of the largest contributors to radiator size, so the total mass reduction on the fusion rocket will most likely be even larger. Similar scalings would also most likely hold for other driver mechanisms, such as Z-pinch.

         Arguably the largest benefit from spin-polarized DT is the ability to direct the neutron radiation away from the spacecraft. Because 80% of the energy of DT fusion is released in the form of 14.1 MeV neutrons, this has significant implications for shielding mass requirements and radiator heat loads. There may also be the potential to use the directionality of neutron emission to extract more thrust from the fusion reaction via the placement of extra hydrogenous materials in the path of the neutron emission.

         When DT is fully spin polarized, the neutrons produced are emitted in a Gaussian distribution along the spin axis with a full width at half max of roughly 48.5°. There is, approximately, a 0.54% baseline percentage of neutrons emitted in all directions even when spin polarized, but this number is still far lower than in the unpolarized case which emits all fusion neutrons equally in all directions. Another component that lowers the neutron radiation flux is the reduction in DD secondary reactions. The DD fusion reaction has two branches, each of which has a 50% chance of occurring. One of these branches produces a 2.45 MeV neutron and is unaffected by spin polarization. Reducing the number of DD reactions that occur will lower the total number of neutrons emitted back toward the spacecraft. The reduction in DD yield, is shown as a function of polarization fraction φ and total yield YT in Eq. (3).

         To calculate the potential shielding reduction, we will begin by making the extremely conservative assumption that the spacecraft takes up a full 2π steradians behind the fusion reaction. This is equivalent to an infinitely large plate being used for converting fusion energy to thrust, rather than the much more common magnetic nozzle designs. We begin by noting that exactly 50%of the DD neutronswill impact the spacecraft in this case, meaning that spin polarized fuel will benefit from the reduction in DD yield. The impact of DT neutrons on the spacecraft will scale inversely to the polarization fraction, with a full 50% of the neutrons impacting in the unpolarized (φ = 0) case and only 0.27% of the neutrons impacting in the fully polarized (φ = 1) case. Eq. 4 describes the fraction of the total fusion yield that is absorbed as neutrons by the infinite plate representing the ship as a function of the total yield:

         At full polarization only 19.56% of the neutrons generated will impact the ship compared to a full 50% in an unpolarized case. A example of the change in neutron emission can be seen in Figure 2 for spacecraft similar to VISTA. The reduction in shielding mass can be roughly estimated using a linear absorption of the neutrons based on previous shielding estimates. A more detailed analysis would require Monte Carlo simulations that are beyond the scope of this work. To first order the reduction factor for shielding with polarization for DT can be modeled with:

         At maximum polarization, the shielding thickness can be reduced by by 6.8%, although the specifics of the radiation shielding can alter this significantly. Still, to first order, this can be assumed to be a linear reduction in shielding mass due to the reduction in thickness and no change in surface area. DT fusion also produces gamma rays and x rays that are unaffected by the spin polarization and will still need to be stopped by some thickness of radiation shielding that will make this approximation break down at some point. At low (MW class) fusion power levels the x-ray and gamma-ray shielding will most likely dominate in mass and the effect of spin polarization on shielding reduction will not be as stark.

         The other large benefit fromspin polarization of fusion fuel is the change inmomentumcoupling between the fusion fuel and the spacecraft. In a typical pulsed fusion rocket the charged alpha particles explode outwards isotropically, heating the remaining fuel and inert mass and causing all of that to expand outwards equally in all directions. A portion of this expanding plasma then impacts either a physical structure or is redirected by a magnetic field to produce thrust for the spacecraft. In the case of spin polarized DT fusion the alpha particles will be emitted in the opposite direction of the neutrons and with the same distribution, meaning that a cone of plasma will be launched towards the ship instead of the ship just subtending some portion of the isotropically expanding plasma as in the first case. This cone of plasma will allow for a better momentum exchange with the ship, resulting in both higher thrust and higher specific impulse via a higher effective exhaust velocity, similar in principle to the directional nuclear pulse units proposed for Project Orion.

         To model the gains in both thrust and specific impulse, we will utilize the same flat plate analysis as was used for the neutrons and will ignore potential electric and magnetic nozzle schemes due to their added complexity. The alpha emission will perfectly mirror the neutron emission shown in Figure 1, so the same analysis technique used for determining reduction in neutron shielding can be used. We then also integrate over the angle that the particle is presumed to impact on to the large plate, as a perfectly perpendicular impact will more effectively transfer forward momentum than an impact at a glancing angle. The final result can be seen in Eq. (6) with a maximum potential gain of 131.3% in both effective exhaust velocity and thrust.

         This equation provides the relative gain for specific impulse and thrust of any fusion rocket scheme as a function of polarization. Since no magnetic nozzles, or even shape optimized physical nozzles, were assumed to be in use, this can be seen as a conservative estimate of the gains from spin polarization. A more aggressive estimate can be made by assuming a perfect electric or magnetic nozzle that redirects all of the plasma at the ideal angle of reflection. In this case there is no need to integrate over angle and the maximum gains in thrust and specific impulse increase to ~160% instead of the ~130% from the flat plate analysis. This equation also does not take any gains from increased burn-up into account as that is fusion driver specific and not always going to apply, but increased burn-up will also provide for more thrust and a higher effective exhaust velocity for a given mass of fusion fuel.

    B. Spin Polarized D3He Benefits

         D3He fusion is the fusion reaction most commonly invoked when one wants to avoid neutron radiation. Although the primary reaction does not generate neutrons,DD and DT side reactionswill generate neutrons thatwill still irradiate the ship. As D3He is not much more reactive than DD, compared to DT’s roughly 100× higher reactivity, the number of DD side reactions will be far higher in a burning D3He plasma versus a pure DT plasma. The higher ignition requirements of D3He will also require a larger driver and/or larger fusion reactor. This can translate to larger shielding masses due to more surface area needing to be shielded on top of the higher mass drivers and associated radiators. Still, spin polarization offers the same benefit of a roughly 150% increase in cross section to D3He fusion as it does to DT fusion.

         Using the same logic as before, the reduction in driver mass and increase in gain and exhaust velocity will be the same as shown above. The same logic will also be used for the potential shielding reduction by assuming that half of all emitted DD and DT neutrons will impact the ship. Since there is no neutron collimation effect, the only reduction in radiation dose comes from the reduction in DD secondary reactions and DT tertiary reactions due to the increased burn-up of the primary fuel. The reduction in neutrons on the ship is shown in Eq. (7) and shielding reduction in Eq. (8).

         At full polarization, the neutron load of the ship is reduced by nearly 25%. However, this only translates to a roughly 5% reduction in shielding thickness due to the already thinner shielding compared to normal DT. As such, radiation shielding reduction is more of a secondary benefit compared to the primary benefit of reduced fusion-ignition requirements. Interestingly enough, at high enough spin polarization fractions DT fuel will actually out perform D3He in terms of neutron load onto the ship. This is due to the much larger DT cross section suppressing the DD side reactions more effectively than D3He. The fraction of fusion yield as neutrons emitted toward the ship versus the polarization fraction of the fuel for DT and D3He is plotted in Figure 3. D3He fusion will also benefit from the same increase in effective exhaust velocity and thrust as was outlined above for DT, and the equations will remain the same. Potentially even higher gains could be had via a magnetic nozzle that utilized the double jets of charged particles that spin polarized D3He will emit, but that is beyond the scope of this paper.

         Spin-polarization will also provide reductions in driver mass and lower ignition parameters for D3He fuel. These gains, along with the increased rocket efficiency, are a much larger motivation for using spin-polarized D3He fuel rather than the reduction in neutron radiation. For DT fuel the reduction in neutron radiation is more compelling and makes it competitive with D3He as a low neutron fusion fuel for space travel. This is counter to the common narrative that D3He is the superior fusion propulsion fuel due to the reduced neutron load on the spacecraft.

    III. Example of a Spin-Polarized Rocket

         To show the utility of spin-polarized fusion in future spacecraft, an example of the benefits will be applied to the well known VISTA fusion rocket design. VISTA was chosen because it is well documented and utilizes laser-driven inertial confinement DT fusion, which may be one of the best options for using spin-polarized fusion fuel. The rocket utilizes a magnetic nozzle and is built in a distinctive cone shape in an attempt to lower the area exposed to neutron radiation as shown in Figure 2.

         We start by assuming that VISTA adds a spin-polarization capability of negligible mass for a roughly 1835-ton dry mass spacecraft. For the sake of simplicity, the spin polarization will be assumed to be nearly 100% during the fusion burn. This reduces the neutron flux on the spacecraft by over an order of magnitude. There have been several VISTA radiation shielding designs with the primary differences between these design being the amount of neutron irradiation allowed on the magnetic nozzle coils and the use of extra frozen hydrogen remass as consumable neutron shielding. The final VISTA report listed 505 tons as the shield mass, so that is what will be used here.

         The shield consists of 137 tons of Li/LiH for neutron shielding, 360 tons of Pb for gamma rays and 8 tons of Be for x-ray protection. Spin polarization would only reduce the neutron shielding thickness as the primary DT gamma ray emission will not be affected. Some reduction in gamma shielding may be had due to fewer (n,γ) reactions being present on the ship, but that is beyond the scope of this paper. Using Eq. 5 the Li/LiH shielding mass is reduced to ~127.7 tons and the final shielding mass is ~495.7 tons. A secondary benefit to this is the reduction in neutron heating and associated heat rejection. Rather than the baseline ~7 GWof neutron heating that must be rejected from the shield, a spin polarized system has to reject only ~1.1 GW of neutron heating due to the much lower fluxes. This may translate to large reductions in radiator mass since the radiation shield is in contact with the cryogenic magnetic nozzle.

         The use of spin-polarization would also provide a reduction in laser driver mass from 150 tons to ~116 tons before radiator mass reduction is also taken into account. Exact reduction in radiator mass is difficult to estimate due to the complexities of both laser and magnetic nozzle cooling systems, although there will be potential for radiator reduction via the use of smaller drivers along with the aforementioned neutron heating reduction. The final VISTA dry mass will be ~1791.7 tons and is reduced by ~2.36% via spin-polarization before taking any radiator mass savings into account.

         VISTA assumes a very aggressive fusion gain of 1500 for the most optimistic case using laser fast ignition. Using spin polarization, it may prove easier to meet or exceed these fusion gains. Applying spin polarization to a VISTA-capable system may yield gains as high as 2175, allowing for nearly 11-GJ pulse energies which would increase the thrust and specific impulse by a factor of ~1.2. Conversely spin polarization could also be used to reach the original gain of 1500, but with less laser driver energy. This would directly translate to a reduction in both driver mass and radiator mass for the spacecraft.

         The superior momentum coupling of spin-polarized fuel will also provide effective exhaust velocity and thrust gains over the baseline VISTA rocket even if the pulse energies are kept the same. The nominal specific impulse and thrust of VISTA was estimated at 16,000 seconds and 240 kN respectively. A fully spin-polarized version has the potential to increase those values to >21,000 seconds and >315 kN before considering the extra gains from a properly designed magnetic nozzle. This would increase the jet efficiency from ~32% to ~77%and allow for less fuel to be used to perform the same proposed deep space missions, or for equivalent fuel loads to perform more aggressive missions that require shorter time frames or higher velocities.

         One potential concern of a spin-polarized DT rocket is the neutron radiation hazards to spacecraft behind the spin-polarized VISTA rocket. At maximum thrust power, an unpolarized VISTA spacecraft’s neutron radiation is dangerous to unshielded humans up to 36,000 km away. For a spin-polarized VISTA spacecraft, there are far fewer neutrons outside of the 48.5° FWHM Gaussian neutron emission zone, but the dose is much higher within that zone. Directly behind a spin-polarized VISTA spacecraft there will need to be a clear zone >40,000 km long to meet the same dose requirements as the 36,000 km spherical zone for unpolarized VISTA. During departure and braking burns, a spin-polarized VISTA rocket will need to take care to not irradiate other spacecraft beyond their limits.

    IV. Conclusion

         As outlined in this paper, spin-polarized fusion has the potential to provide noticeable (> 2% before accounting for radiator savings) reduction in the dry mass of fusion rockets via shielding and driver mass reductions, while also lowering radiation damage and heating concerns. The use of spin polarization also provides higher fusion burn-ups (up to 45%), resulting in potentially higher exhaust velocity and more efficient use of costly fusion fuel. Spin polarization also provides for more efficient momentum coupling to the spacecraft, which results in ~130% greater effective exhaust velocity and thrust. Recent advancements in spin polarization for nuclear physics experiments have shown that the technology is rapidly maturing and has potential to be implemented on near-term fusion rockets. The advantages of spin polarized fusion are primarily seen by DT fueled fusion rockets, but rockets utilizing D3He may also choose to use spin-polarized fuel to lower the prodigious ignition requirements of this advanced fuel. Additional fusion burn-up and neutron gains may also be had by using a cylindrical fusion ignition scheme that takes better advantage of the anisotropic alpha particle emission from spin polarized fuel. Various Z-pinch schemes seem best suited for this sort of technique. Spin-polarized fusion seems to be a straight forward method of increasing the performance of many fusion rocket designs and should be considered when designing future spacecraft.

    Constant Acceleration Equations

    Conventional rocket engines are so weak that their burn times are generally measured in minutes. They spend lots of time coasting. Torchships on the other hand can burn for hours or days, up to burning for the entire duration of the trip.

    Constant Acceleration Or Death

    As a side note, since these equations assume a constant acceleration, the rocket's thrust will have to be continually reduced as the expended propellant continually reduces the spacecraft mass.

    Remember that acceleration is equal to rocket thrust divided by spacecraft mass. If the mass goes down the acceleration will go up. To keep the acceleration the same you'll have to manually reduce the thrust. You'd want to do this anyway or the rising acceleration will do a Solomon Epstein on the crew and squish them like bugs.

    Constant-acceleration Delta-V

    To calculate the deltaV resulting from accelerating at a fixed rate for a certain period of time:

    plainVanillaDeltaV = A * t


    • plainVanillaDeltaV = deltaV result (m/s)
    • A = acceleration (m/s2)
    • t = duration of acceleration (seconds)

    Obviously if you want to calculate duration or acceleration level:

    t = plainVanillaDeltaV / A

    A = plainVanillaDeltaV / t


    (ed note: Gabriel GAB Fonseca derived equations to calculate the total distance traveled by a constantly accelerating rocket taking into account the change in mass as the propellant is expended. Understand that the Distance is how far from the starting point the spacecraft will be when the tanks go dry or the burn ends. After that the spacecraft will continue to coast at whatever its plainVanillaDeltaV is, gradually increasing the distance traveled.)

    (ed note: What follows is Mr. Fonseca's explanation)

    To calculate the distance travelled by a rocket, one needs to start somewhere. That somewhere should, under ideal circumstances, be an equation that our aspiring rocketeer actually knows. In my case, that equation would be the formula for the acceleration of a rocket as a function of time, as seen bellow:

    Eq.1: a(t) = (mDot * Ve) / (M - mDot * t)


    a(t) = acceleration of a rocket as a function of time
    mDot = propellant mass flow
    Ve = engine exhaust velocity
    M = rocket mass when full
    t = burn duration

    Fun fact to those that don't speak Calculeese: Velocity is nothing more than the derivative of distance by time. The version we all know and love from high school, V = d/t, is actually just the formula for the average speed of an object. If you want to know the instantaneous speed of an object, that is, it's speed at an exact given moment, you have to do it Newton style, so we go and take that derivative and find V. (Calculus looks harder than it actually is, but we don't really need to go into all the nitty-gritty details — for now just take my word for it.)

    But wait, there's more! Do you know what happens if we take the derivate of V as a function of time? Why, we get the object's acceleration! I'll be darned, so it turns out that the acceleration is the double derivative of distance by time!

    You might be asking, "sure, that's all well and good, but how does this help us?". Why, we now have a relation that connects distance and acceleration: d'' = a (Those apostrophes mean we're taking the double derivative of d or distance). So now we've got ourselves a happy little equation we can use to find out the formula for distance as a function of time! We just have solve the equation we just found backwards, using the inverse operation of the derivative.

    "But what is the inverse operation of the derivate?", you ask? Well, that would be the integral, that... No, wait, don't run! Don't worry, you won't be doing any of this heavy duty calculus yourself, I promise you, ok?

    So, where were we? Oh, yes: To find out distance as a function of time, we'll have to integrate the acceleration as a function of time, twice. To aid me in demonstrating these equations, we'll have with us a little test rocket with known parameters — our rocket will have:

    Mass when Full (M) = 5000 Kg
    Mass when Empty (Me) = 1000 Kg
    Engine's Mass Flow (mDot) = 40 Kg/s
    Exhaust Velocity (Ve) = 9320 m/s

    With these, we can fill in the equation for Acceleration as a function of time — after one last thing, that is: We need to know how long our rocket takes to burn completely dry. This is rather simple; we just subtract from the rocket's full mass it's mass when completely empty, to find out how much fuel it has (5000Kg - 1000Kg = 4000 Kg). Knowing that every second 40 Kilograms are drained from our rocket, we can find out how many second it takes to drain all our fuel and propellant (4000/40 = 100). We now know that the total burn time of our test rocket is 100 seconds.

    So, plugging our values into the formula, we have:

    Eq.1a: a(t) = (40 * 9320)/(5000 - 40t)

    To find out what the rocket's acceleration was during a specific moment of it's burn, just plonk it's value in seconds after burn start on the little t to find out. If we graph this equation, with the x axis being time (in seconds) and the y axis being acceleration (in m/s2):

    All good so far, but now we need to integrate. Before we do that however, first I'll tell you a little secret: There are actually two ways of integrating something. There's the definite integral, where you have an upper and a lower limit to your function, and you have the indefinite integral, where the function is not limited and it just keeps going (There’s actually more to it than this, but the exact definition won’t be that important to our purposes). So, if we take the definite integral with our lower limit being the start of our burn (0 seconds) and the upper limit as the burn's end (100 seconds), after all the numbers have been properly crunched, we have the following result:

    Result 1a: 9320 * ln(5) = approximately 15000

    If you don't know how to integrate, but want to follow along, don't worry — you can integrate using Wolfram, the free computational engine. Just type in

    definite integral of (372800/(5000-40x)) lower limit 0 and upper limit 100

    and presto! Out comes our solution.

    "Wait a second… this looks oddly familiar". Well, it should! That's none other than Konstantin Tsiolkovsky's Rocket Equation, and if you're interested in spacecraft at all, you know this formula by heart! Our outputted formula has an exhaust velocity (9320) multiplied by the natural logarithm of a rocket's mass ratio (5), just like the rocket equation! It turns out that the math we just did is exactly what Tsiolkovsky did on the early days of the 20th century. We have calculated our test rocket's ΔV the hard way.

    If we are stumbling into rocket equations, we must surely be walking in the right direction! So now, if we take the indefinite integral of the formula without our test values inputed, we should get…

    Result 1b: -Ve * ln(M - mDot * t)

    If you want to do this on Wolfram, we will have to do a bit of mathematical hocus-pocus. Wolfy can't understand the notation (variable names) we are using, so we will have to

    write M as a
    write mDot as b
    write Ve as c
    write t as x

    Putting it all together, the Wolfram unreadable (mDot * Ve)/(M - mDot * t) becomes the Wolfram readable:

    indefinite integral of (c*b)/(a - bx)

    "Wait, this isn't the rocket equation"!

    (ed note: because ΔV = -Ve * ln(M - mDot * t) does not look quite like ΔV = Ve * ln(M / Me) )

    Well, actually, it is the rocket equation — it's just an alternate form of writing it! No, seriously, I'm telling you it is. How, you ask? Well, uhm… Look, just take my word for it, ok?

    (If you really want a proof, derive this formula and the traditional form of the rocket equation by time, they will both yield the same answer — the formula we just integrated)

    So now we have the formula for the speed of a rocket as a function of time! We are halfway there! Our new equation now looks like this:

    Eq.2: V(t) = Ev * ln(M/(M - mDot * t))

    If we plug in our test rocket's values and plot it's graph, we have the following, with the x axis as time (in seconds) and the y axis as speed (in m/s):

    Now, to integrate this formula! Before we proceed to the last step however, I want to confess something to you: I had to do this bit twice. The first time, the humble potato that now writes to you tried to integrate the formula with the values for the test rocket plonked in, yielding a monstrous equation which actually didn't really work at all. I guess that what I'm trying to say is, basically: Kids, always check your math. It's only through our mistakes that we learn how to do it right.

    Anyway, now that that is out of the way, we can finally be about integrating our second equation. I warn you though, even if you do know calculus, this operation is not for the faint hearted — I have no shame in admitting that I had to rely on the computer to crunch the numbers for me on this one. If we input our equation into Wolfram:

    indefinite integral of c*ln(a/(a - bx))

    we finally get our result; what we've all been waiting for!

    Eq.3: d(t) = Ve * ((t - M/mDot) * ln(M/(M - mDot * t)) + t)

    Plugging our little test rocket's numbers in it, we get:

    9320 * ((100 - 5000/40) * ln(5000/(5000 - 40* 100)) + 100) =
    9320 * (-25 * ln(5) + 100) =
    9320 * (-40.236 + 100) =
    9320 * 59.764 =
    557,000.966 meters

    So the distance travelled by our test rocket in a full burn is ~ 557,001 meters! (557 Kilometres, or ~ 348 Miles, or 0.1858% of a light-second)

    If we graph this equation, with the x axis as time (in seconds) and the y axis as distance from the starting point (in meters):

    The magic is that this equation will work for any burn, as long as we don't start goofing around and plugging "t"s higher than the time to drain our tanks fully dry.

    So now, a worked example: I want to calculate the distance travelled by my test rocket when it burns to half it's ΔV reserve — so we need to know how much mass our rocket has after expending half of it's ΔV. Knowing our test rocket's full ΔV is ~15,000m/s:

    Eq.4a: 7499.98 = 9320 * ln(Mc/1000)

    Where Mc the current mass, that is, what we want to find. So, isolating Mc:

    Eq.4b: Mc = e(7499.98 / 9320) * 1000 = 2236.068 Kg

    Now that we know our current mass, we know that we have expended (5000 - 2236.098) 2763.932 Kg of fuel and propellant. If we divide this by our engine's mass flow, we get the time it took to expend this amount, which is (2763.932/40) 69.0983 seconds. If we plonk down this as our t, we get:

    9320 * ((69.0983 - 5000/40) * ln(5000/(5000 - 40 * 69.0983) + 69.0983) =
    9320 * ((-55.9017) * ln(2.236) + 69.0983) =
    9320 * 24.113 =
    224,734.5 meters from the starting point

    If we want to know the distance travelled when our test rocket, starting from rest, has half of it's ΔV remaining up to when it has a quarter of it left, however, what we do is:

    Eq.5a: 3749.99 = 9320 * ln(Md/1000)

    Where Md is the mass when we have 1/4 of our ΔV. Isolating Md:

    Eq.5b: Md = e(3749.99 / 9320) *1000 = 1495.349 Kg

    Which means that the rocket expended 5000 - 1495.349 = 3504.651 Kg of fuel and propellant, which takes 3504.651/40 = 87.6162 seconds. Knowing that to have only half of it's ΔV left our rocket must have already burned for 69.0983 seconds, we can find out the burn time to go from half to a quarter ΔV subtracting: (87.6162 - 69.0983 = 18.5179). So, plugging our mass at the start of the burn (when we have half our ΔV left in the tanks) and our burn time to the formula:

    9320 * ((18.5179 - 2236.068/40) * ln(2236.068/(2236.068 - 40 * 18.5179)) + 18.5179) =
    9320 * ((-37.3838) * ln(1.4953) + 18.5179) =
    32409.3 meters from the starting point

    But that result is for if our rocket was originally at rest at the start of the burn — so what if we were already moving at those 7500 m/s our half-empty ΔV reserve seems to imply? Simple, we just add in our starting speed times our burn's time!

    Eq.3d: 7500 * 18.5179 + 32409.3 = 171,293.5 meters travelled from the starting point

    As such, if we want to write the formula so that it can handle any situation, we have:

    Eq.6: Vo * t + Ve * ((t - Mc/mDot) * ln(Mc/(Mc - mDot) + t) = total distance travelled during prograde burn

    Well, for now that would be all; still, I'm sure that it is possible to re-write this formula to give you time as a function of distance — but that is something for another day!

    Brachistochrone Equations

    What exactly is a Brachistochrone anyway?

    A Hohmann orbit is the maximum transit time / minimum deltaV mission. Weak spacecraft use this because they do not have a lot of deltaV. All current space probes use Hohmann because currently there ain't no such thing as a strong propulsion system.

    A "Brachistochrone" is a minimum transit time / maximum deltaV mission. Torchships use this because they have lots of deltaV to spare.

    With a constant-acceleration mission, you aim your rocket at the destination and burn at a constant acceleration. The trouble is that almost everybody wants to come to a stop at their destination, nobody want to go streaking past it at twenty kilometers per second.

    Rocket Engine 101: you point your rocket exhaust in the exact opposite direction your ship is traveling in order to speed up (accelerate), you point the exhaust in the exact same direction in order to slow down (decelerate). So for a brachistochrone you accelerate constantly to the midpoint, flip over (Heinlein calls this a "skew flip", The Expanse calls it a "flip-and-burn"), and decelerate to the destination. You come to a halt at the destination and everybody is happy.

    Weaker torchships will accelerate up to a certain velocity, coast for a while, then flip and decelerate to rest.

    Brachistochrone missions are not only of shorter mission time, but they also are not constrained by launch windows the way Hohmann are. You can launch any time you like.

    You Can't Stop-on-a-Dime

    It is very important to note that it takes exactly the same amount of time to slow from a speed X to speed zero as it took to accelerate from speed zero to speed X. There is no way to jam on the brakes for a stop-on-a-dime halt. Other than "lithobraking" aka "crashing into a planet".

    People who played the ancient boardgame Triplanetary or the new game Voidstriker discovered this the hard way. They would spend five turns accelerating to a blinding speed, find out to their horror that it would take five more turns to slow down to a stop. A lot of novice players had their spaceships go streaking off the edge of the map or smacking into Mars fast enough to make a crater before they learned this.

    This is why a Brachistochrone accelerates to the mid-way point then decelerates the rest of the trip. The idea is to come to a complete stop at your destination, which means taking as much time to brake as you took to accelerate.

    In the first episode of The Expanse, the good ship Canterbury has been doing a leisurely burn accelerating from Saturn's rings to Ceres. Suddenly they have to come to a screeching halt in order to help a ship in distress called the Scopuli. In order to shed all their delta V they have to do a "flip-and-burn", flipping the ship over so the thrust opposes their vector. The "burn" part is where they put everybody into accelerations couches and pump them full of anti-acceleration drugs so they don't die under the multiple-gee thrust. Again if they decelerate with the same leisurly burn they accelerated with, it will take exactly the same time, ending up with them overshooting the Scopuli. By doing a violent deceleration they do not overshoot, but they do injury to the crew and severe damage to the ship.


    As RocketCat pointed out, torchships are unobtainium. Therefore, it doesn't mean the math no longer applies. You just need different equations.

    Please note these are nice simple equations, but only hold true if the spacecraft is not traveling at significant fractions of the speed of light (below 0.14 c). If it is, then you have to use the huge ugly relativistic equations with big nasty pointed teeth and covered in dangerous hyperbolic trigometric functions.

    Travel Distance

    First figure the distance between the two planets, say Mars and Terra.

    The "superior" planet is the one farthest from the Sun, and the "inferior" planet is nearest. The distance from the Sun and the superior planet is Ds and the distance between the Sun and the inferior is Di. No "church lady" jokes please.

    Obviously the maximum distance between the planets is when they are on the opposite sides of the Sun, the distance being Ds + Di. And of course the minimum is when they are on the same side, distance being Ds - Di. Upon reflection you will discover that the average distance between the planets is Ds. (when averaging, Di cancels out.)

    (maximumDistance + minimumDistance) / 2 = average distance
    ((Ds + Di) + (Ds - Di)) / 2 = average distance
    ((Ds) + (Ds)) / 2 = average distance
    2 * Ds / 2 = average distance
    Ds = average distance

    So either just use Ds or randomly choose a distance between the max and min.

    If you want to actually calculate the distance between two planets on a given date, be my guest but I'm not qualified to explain how. Do a web search for a software "orrery".

    TRANSIT TIME given distance and desired acceleration

    If you know the desired acceleration of your spacecraft (generally one g or 9.81 m/s2) and wish to calculate the transit time, the Brachistochrone equation is

    T = 2 * sqrt[ D/A ]

    (ed note: pay attention, it is D DIVIDED by A)


    • T = transit time (seconds)
    • D = distance (meters)
    • A = acceleration (m/s2)
    • sqrt[x] = square root of x

    Remember that

    • AU * 1.49e11 = meters
    • 1 g of acceleration = 9.81 m/s2
    • one-tenth g of acceleration = 0.981 m/s2
    • one one-hundredth g of acceleration = 0.0981 m/s2

    Divide time in seconds by

    • 3600 for hours
    • 86400 for days
    • 2592000 for (30 day) months
    • 31536000 for years

    Timothy Charters worked out the following equation. It is the above transit time equation for weaker spacecraft that have to coast during the midpoint

    T = ((D - (A * t^2)) / (A * t)) + (2*t)


    • T = transit time (seconds)
    • D = distance (meters)
    • A = acceleration (m/s2)
    • t = duration of acceleration phase (seconds), just the acceleration phase only, NOT the acceleration+deceleration phase.

    Note that the coast duration time is of course = T - (2*t)

    ACCELERATION given distance and desired transit time

    If you know the desired transit time and wish to calculate the required acceleration, the equation is

    A = (4 * D) / T2

    Keep in mind that prolonged periods of acceleration a greater than one g is very bad for the crew's health.

    Yes, it is supposed to be 2 * sqrt[ D/A ], NOT sqrt[ 2 * D/A ]

    Don't be confused. You might think that the Brachistochrone equation should be T = sqrt[ 2 * D/A ] instead of T = 2 * sqrt[ D/A ], since your physics textbook states that D = 0.5 * A * T^2. The confusion is because the D in the physics book refers to the mid-way distance, not the total distance.

    This changes the physics book equation from

    D = 0.5 * A * t^2


    D * 0.5 = 0.5 * A * t^2

    Solving for t gives us t = sqrt(D/A) where t is the time to the mid-way distance. Since it takes an equal amount of time to slow down, the total trip time T is twice that or T = 2 * sqrt( D/A ). Which is the Brachistochrone equation given above.



    Basic Brachistochrone Delta-V

    Now, just how brawny a rocket are we talking about? Take the distance and acceleration from above and plug it into the following equation:

    transitDeltaV = 2 * sqrt[ D * A ]

    (ed note: pay attention, it is D MULTIPLIED by A)


    • transitDeltaV = transit deltaV required (m/s)

    Detailed Delta-V

    The rocket on the Brachistochrone trip will also have to match orbital velocity with the target planet. In Hohmann orbits, this was included in the total.

    orbitalVelocity = sqrt[ (G * M) / R ]


    • orbitalVelocity = planet's orbital velocity (m/s)
    • G = 0.00000000006673 (Gravitational constant)
    • M = mass of primary (kg), for the Sun: 1.989e30
    • R = distance between planet and primary (meters) (semi-major axis or orbital radius)

    If you are talking about missions between planets in the solar system, the equation becomes

    orbitalVelocity = sqrt[1.33e20 / R ]

    Figure the orbital velocity of the start planet and destination planet, subtract the smaller from the larger, and the result is the matchOrbitDeltaV

    matchOrbitDeltaV = sqrt[1.33e20 / Di ] - sqrt[1.33e20 / Ds ]

    If the rocket lifts off and/or lands, that takes deltaV as well.

    liftoffDeltaV = sqrt[ (G * Pm) / Pr ]


    • liftoffDeltaV = deltaV to lift off or land on a planet (m/s)
    • G = 0.00000000006673
    • Pm = planet's mass (kg)
    • Pr = planet's radius (m)

    The total mission deltaV is therefore:

    totalDeltaV = sqrt(liftoffDeltaV2 + transitDeltaV2) + sqrt(matchOrbitDeltaV2 + landDeltaV2)

    Do a bit of calculation and you will see how such performance is outrageously beyond the capability of any drive system in the table I gave you.

    If you want to cheat, you can look up some of the missions in Jon Roger's Mission Table.


    For some ballpark estimates, you can use my handy-dandy Transit Time Nomogram. Be warned, this only does torchship Brachistochrone trajectories, it cannot calculate Hohmann transfers or anything else.

    A nomogram is an obsolete mathematical calculation device related to a slide rule. It is a set of scales printed on a sheet of paper, and read with the help of a ruler or straight-edge. While obsolete, it does have some advantages when trying to visualize a range of solutions. Print out the nomogram, grab a ruler, and follow my example. You can also purchase an 11" x 17" poster of this nomogram at . Standard disclaimer: I constructed this nomogram but I am not a rocket scientist. There may be errors. Use at your own risk.

    Let's say that our spacecraft is 1.5 ktons (1.5 kilo-tons or 1500 metric tons). It has a single Gas-Core Nuclear Thermal Rocket engine (NTR-GAS MAX) and has a (totally ridiculous) mass ratio of 20. The equation for figuring a spacecraft's total DeltaV is Δv = Ve * ln[R]. On your pocket calculator, 98,000 * ln[20] = 98,000 * 2.9957 = 300,000 m/s = 300 km/s. Ideally this should be on the transit nomogram, but the blasted thing was getting crowded enough as it is. This calculation is on a separate nomogram found here.

    The mission is to travel a distance of 0.4 AU (about the distance between the Sun and the planet Mercury). Using a constant boost brachistochrone trajectory, how long will the ship take to travel that distance?

    Examine the nomogram. On the Ship Mass scale, locate the 1.5 kton tick mark. On the Engine Type scale, locate the NTR-GAS MAX tick mark. Lay a straight-edge on the 1.5 kton and NTR-GAS MAX tick marks and examine where the edge crosses the Acceleration scale. Congratulations, you've just calculated the ship's maximum acceleration:2 meters per second per second (m/s2).

    For your convenience, the acceleration scale is also labeled with the minimum lift off values for various planets.

    So we know our ship has a maximum acceleration of 2 m/s2 and a maximum DeltaV of 300 km/s. As long as we stay under both of those limits we will be fine.

    On the Acceleration scale, locate the 2 m/s2 tick mark. On the Destination Distance scale, locate the 0.4 AU tick mark. Lay a straight-edge on the two tick marks and examine where it intersects the Transit time scale. It says that the trip will take just a bit under four days.

    But wait! Check where the edge crosses the Total DeltaV scale. Uh oh, it says almost 750 km/s, and our ship can only do 300 km/s before its propellant tanks run dry. Our ship cannot do this trajectory.

    The key is to remember that 2 m/s2 is the ship's maximum acceleration, nothing is preventing us from throttling down the engines a bit to lower the DeltaV cost. This is where a nomogram is superior to a calculator, in that you can visualize a range of solutions.

    Pivot the straight-edge on the 0.4 AU tick mark (meaning, stick an imaginary pin into the 0.4 AU mark and rotate the straight-edge around it). Pivot it until it crosses the 300 km/s tick on the Total DeltaV scale. Now you can read the other mission values: 0.4 m/s2 acceleration and a trip time of a bit over a week. Since this mission has parameters that are under both the DeltaV and Acceleration limits of our ship, the ship can perform this mission (we will assume that the ship has enough life-support to keep the crew alive for a week or so).

    Of course, if you want to have some spare DeltaV left in your propellant tanks at the mission destination, you don't have to use it all just getting there. For instance, you can pivot around the 250 km/s DeltaV tick mark to find a good mission. You will arrive at the destination with 300 - 250 = 50 km/s still in your tanks.


    Which reminded me that I had not worked out how long it would take to get home on a one-gee boost, if it turned out that I could not arrange automatic piloting at eight gees. I was stymied on getting out of the cell, I hadn't even nibbled at what I would do if I did get out (correction: when I got out), but I could work ballistics.

    I didn't need books. I've met people, even in this day and age, who can't tell a star from a planet and who think of astronomical distances simply as "big." They remind me of those primitives who have just four numbers: one, two, three, and "many." But any tenderfoot Scout knows the basic facts and a fellow bitten by the space bug (such as myself) usually knows a number of figures.

    "Mother very thoughtfully made a jelly sandwich under no protest." Could you forget that after saying it a few times? Okay, lay it out so:

    AASTEROIDS(assorted prices, unimportant)

    The "prices" are distances from the Sun in astronomical units. An A.U. is the mean distance of Earth from Sun, 93,000,000 miles. It is easier to remember one figure that everybody knows and some little figures than it is to remember figures in millions and billions. I use dollar signs because a figure has more flavor if I think of it as money — which Dad considers deplorable. Some way you must remember them, or you don't know your own neighborhood.

    Now we come to a joker. The list says that Pluto's distance is thirty-nine and a half times Earth's distance. But Pluto and Mercury have very eccentric orbits and Pluto's is a dilly; its distance varies almost two billion miles, more than the distance from the Sun to Uranus. Pluto creeps to the orbit of Neptune and a hair inside, then swings way out and stays there a couple of centuries — it makes only four round trips in a thousand years.

    But I had seen that article about how Pluto was coming into its "summer." So I knew it was close to the orbit of Neptune now, and would be for the rest of my life-my life expectancy in Centerville; I didn't look like a preferred risk here. That gave an easy figure — 30 astronomical units.

    Acceleration problems are simple s=1/2 at2; distance equals half the acceleration times the square of elapsed time. If astrogation were that simple any sophomore could pilot a rocket ship — the complications come from gravitational fields and the fact that everything moves fourteen directions at once. But I could disregard gravitational fields and planetary motions; at the speeds a wormface ship makes neither factor matters until you are very close. I wanted a rough answer.

    I missed my slipstick. Dad says that anyone who can't use a slide rule is a cultural illiterate and should not be allowed to vote. Mine is a beauty — a K&E 20" Log-log Duplex Decitrig. Dad surprised me with it after I mastered a ten-inch polyphase. We ate potato soup that week — but Dad says you should always budget luxuries first. I knew where it was. Home on my desk.

    No matter. I had figures, formula, pencil and paper.

    First a check problem. Fats had said "Pluto," "five days," and "eight gravities."

    It's a two-piece problem; accelerate for half time (and half distance); do a skew-flip and decelerate the other half time (and distance). You can't use the whole distance in the equation, as "time" appears as a square — it's a parabolic. Was Pluto in opposition? Or quadrature? Or conjunction? Nobody looks at Pluto — so why remember where it is on the ecliptic? Oh, well, the average distance was 30 A.U.s — that would give a close-enough answer. Half that distance, in feet, is: 1/2 × 30 × 93,000,000 × 5280. Eight gravities is: 8 × 32.2 ft./sec./sec. — speed increases by 258 feet per second every second up to skew-flip and decreases just as fast thereafter.

    So — 1/2 × 30 × 93,000,000 × 5280 = 1/2 × 8 × 32.2 x t2 — and you wind up with the time for half the trip, in seconds. Double that for full trip. Divide by 3600 to get hours; divide by 24 and you have days. On a slide rule such a problem takes forty seconds, most of it to get your decimal point correct. It's as easy as computing sales tax.

    It took me at least an hour and almost as long to prove it, using a different sequence — and a third time, because the answers didn't match (I had forgotten to multiply by 5280, and had "miles" on one side and "feet" on the other — a no-good way to do arithmetic) — then a fourth time because my confidence was shaken. I tell you, the slide rule is the greatest invention since girls.

    But I got a proved answer. Five and a half days. I was on Pluto.

    (Ed note: I learned it as


    from Miss Pickerell on the Moon, probably the first science fiction story I ever read at the tender age of 8. Not counting the Fireball XL5 Little Golden Book. She named her little black cat "Pumpkins" after the mnemonic for Pluto. Cat bears a suspicious resemblance to RocketCat.)

    In Slide Rule terminology: K&E is Keuffel & Esser, noted manufacturer of quality slide rules. 20 inches is twice the size and accuracy of a standard slide rule. Log-log means the rule possesses expanded logarithmic scales. Duplex means there are scales on both sides of the rule and the cursor is double sided. Decitrig means the rule possesses decimal trigometric scales.)

    From HAVE SPACE SUIT - WILL TRAVEL by Robert A. Heinlein, 1958

    Thanks to Charles Martin for this analysis:

    Sky Lift Derivation

    In Heinlein's short story "Sky Lift", the torchship on an emergency run to Pluto colony does 3.5 g for nine days and 15 hours. 3.5 g is approximately 35 m/s2 and 9d15h is 831,600 seconds. 35 m/s2 * 831,600 s = 29,100,000 m/s total deltaV.

    Assume a mass ratio of 4. Most of Heinlein's ships had a mass ratio of 3, 4 is reasonable for an emergency trip.

    Ve = Δv / ln[R] so 29,100,000 / 1.39 = 21,000,000 m/s exhaust velocity or seven percent of the speed of light.

    A glance at the engine table show that this is way up there, second only to the maximum possible Antimatter Beam-Core propulsion, and twice the maximum of Inertial Confinement Fusion. If Heinlein's torchship can manage a Ve of ten percent lightspeed it can get away with a mass ratio of 3.

    Charles Martin

         “How high, sir?”
         Berrio hesitated. “Three and one-half gravities.”
         Three and a half g’s! That wasn’t a boost — that was a pullout. Joe heard the surgeon protest, “I’m sorry, sir, but three gravities is all I can approve.”
         Berrio frowned. “Legally, it’s up to the captain. But three hundred lives depend on it.”

         Kleuger said, “Doctor, let’s see that curve.” The surgeon slid a paper across the desk; Kleuger moved it so that Joe could see it. “Here’s the scoop, Appleby—”
         A curve started high, dropped very slowly, made a sudden “knee” and dropped rapidly. The surgeon put his finger on the “knee.” “Here,” he said soberly, “is where the donors are suffering from loss of blood as much as the patients. After that it’s hopeless, without a new source of blood.”
         “How did you get this curve?” Joe asked.
         “It’s the empirical equation of Larkin’s disease applied to two hundred eighty-nine people.”
         Appleby noted vertical lines each marked with an acceleration and a time. Far to the right was one marked: “1 g—18 days” That was the standard trip; it would arrive after the epidemic had burned out. Two gravities cut it to twelve days seventeen hours; even so, half the colony would be dead. Three g’s was better but still bad. He could see why the Commodore wanted them to risk three-and-a-half kicks; that line touched the “knee,” at nine days fifteen hours. That way they could save almost everybody, but, oh, brother!
         The time advantage dropped off by inverse squares. Eighteen days required one gravity, so nine days took four, while four-and-a-half days required a fantastic sixteen gravities. But someone had drawn a line at “16 g—4.5 days.” “Hey! This plot must be for a robot-torch — that’s the ticket! Is there one available?”
         Berrio said gently, “Yes. But what are its chances?”

         Joe shut up. Even between the inner planets robots often went astray. In four-billion-odd miles the chance that one could hit close enough to be caught by radio control was slim. “We’ll try,” Berrio promised. “If it succeeds, I’ll call you at once.” He looked at Kleuger. “Captain, time is short. I must have your decision.”
         Kleuger turned to the surgeon. “Doctor, why not another half gravity? I recall a report on a chimpanzee who was centrifuged at high g for an amazingly long time.”
         “A chimpanzee is not a man.”
         Joe blurted out, “How much did this chimp stand, Surgeon?”
         “Three and a quarter gravities for twenty-seven days.”
         “He did? What shape was he in when the test ended?”
         “He wasn’t,” the doctor grunted.

         The ship was built for high boost; controls were over the pilots’ tanks, where they could be fingered without lifting a hand. The flight surgeon and an assistant fitted Kleuger into one tank while two medical technicians arranged Joe in his. One of them asked, “Underwear smooth? No wrinkles?”
         “I guess.”
         “I’ll check.” He did so, then arranged fittings necessary to a man who must remain in one position for days. “The nipple left of your mouth is water; the two on your right are glucose and bouillon.”
         “No solids?”
         The surgeon turned in the air and answered, “You don’t need any, you won’t want any, and you mustn’t have any. And be careful in swallowing.”
         “I’ve boosted before.”
         “Sure, sure. But be careful.”
         Each tank was like an oversized bathtub filled with a liquid denser than water. The top was covered by a rubbery sheet, gasketed at the edges; during boost each man would float with the sheet conforming to his body. The Salamander being still in free orbit, everything was weightless and the sheet now served to keep the fluid from floating out. The attendants centered Appleby against the sheet and fastened him with sticky tape, then placed his own acceleration collar, tailored to him, behind his head.

         The room had no ports and needed none. The area in front of Joe’s face was filled with screens, instruments, radar, and data displays; near his forehead was his eyepiece for the coelostat. A light blinked green as the passenger tube broke its anchors; Kleuger caught Joe’s eye in a mirror mounted opposite them. “Report, Mister.”
         “Minus seven’ minutes oh four. Tracking. Torch warm and idle. Green for light-off.”
         “Stand by while I check orientation.” Kleuger’s eyes disappeared into his coelostat eyepiece.

         When the counter flashed the last thirty seconds he forgot his foregone leave. The lust to travel possessed him. To go, no matter where, anywhere go! He smiled as the torch lit off.
         Then weight hit him.
         At three and one-half gravities he weighed six hundred and thirty pounds. It felt as if a load of sand had landed on him, squeezing his chest, making him helpless, forcing his head against his collar. He strove to relax, to let the supporting liquid hold him together. It was all right to tighten up for a pullout, but for a long boost one must relax. He breathed shallowly and slowly; the air was pure oxygen, little lung action was needed. But he labored just to breathe. He could feel his heart struggling to pump blood grown heavy through squeezed vessels. This is awful! he admitted. I’m not sure I can take it. He had once had four g for nine minutes but he had forgotten how bad it was.

         Joe then found that he had forgotten, while working, his unbearable weight. It felt worse than ever. His neck ached and he suspected that there was a wrinkle under his left calf. He wiggled in the tank to smooth it, but it made it worse.

         He tried to rest — as if a man could when buried under sandbags.
         His bones ached and the wrinkle became a nagging nuisance. The pain in his neck got worse; apparently he had wrenched it at light-off. He turned his head, but there were just two positions — bad and worse. Closing his eyes, he attempted to sleep. Ten minutes later he was wider awake than ever, his mind on three things, the lump in his neck, the irritation under his leg, and the squeezing weight.
         Look, bud, he told himself, this is a long boost. Take it easy, or adrenalin exhaustion will get you. As the book says, “The ideal pilot is relaxed and unworried. Sanguine in temperament, he never borrows trouble.” Why, you chair-warming so-and-so! Were you at three and a half g’s when you wrote that twaddle?

         The integrating accelerograph displayed elapsed time, velocity, and distance, in dead-reckoning for empty space. Under these windows were three more which showed the same by the precomputed tape controlling the torch; by comparing, Joe could tell how results matched predictions. The torch had been lit off for less than seven hours, speed was nearly two million miles per hour and they were over six million miles out. A third display corrected these figures for the Sun’s field, but Joe ignored this; near Earth’s orbit the Sun pulls only one two-thousandth of a gravity — a gnat’s whisker, allowed for in precomputation. Joe merely noted that tape and D.R. agreed; he wanted an outside check.

         His ribs hurt, each breath carried the stab of pleurisy. His hands and feet felt “pins-and-needles” from scanty circulation. He wiggled them, which produced crawling sensations and wearied him. So he held still and watched the speed soar. It increased seventy-seven miles per hour every second, more than a quarter million miles per hour every hour. For once he envied rocketship pilots; they took forever to get anywhere but they got there in comfort.
         Without the torch, men would never have ventured much past Mars. e = mc2, mass is energy, and a pound of sand equals fifteen billion horsepower-hours. An atomic rocketship uses but a fraction of one percent of that energy, whereas the new torchers used better than eighty percent. The conversion chamber of a torch was a tiny sun; particles expelled from it approached the speed of light.

         “Oh, there’s one thing I don’t understand, uh, what I don’t understand is, uh, this: why do I have to go, uh, to the geriatrics clinic at Luna City? That’s for old people, uh? That’s what I’ve always understood — the way I understand it. Sir?”
         The surgeon cut in, “I told you, Joe. They have the very best physiotherapy. We got special permission for you.”
         Joe looked perplexed. “Is that right, sir? I feel funny, going to an old folks’, uh, hospital?”
         “That’s right, son.”
         Joe grinned sheepishly. “Okay, sir, uh, if you say so.”
         They started to leave. “Doctor — stay a moment. Messenger, help Mr. Appleby.”
         “Joe, can you make it?”
         “Uh, sure! My legs are lots better — see?” He went out, leaning on the messenger.

         Berrio said, “Doctor, tell me straight: will Joe get well?”
         “No, sir.”
         “Will he get better?’
         “Some, perhaps. Lunar gravity makes it easy to get the most out of what a man has left.”
         “But will his mind clear up?”
         The doctor hesitated. “It’s this way, sir. Heavy acceleration is a speeded-up aging process. Tissues break down, capillaries rupture, the heart does many times its proper work. And there is hypoxia, from failure to deliver enough oxygen to the brain.”
         The Commodore struck his desk an angry blow. The surgeon said gently, “Don’t take it so hard, sir.”
         “Damn it, man — think of the way he was. Just a kid, all bounce and vinegar — now look at him! He’s an old man — senile.”
         “Look at it this way,” urged the surgeon, “you expended one man, but you saved two hundred and seventy.”

    From SKY LIFT by Robert Heinlein (1953)

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