Atomic Rockets

Introduction

RocketCat sez

Ken Burnside said it best.

Friends Don't Let Friends Use Reactionless Drives In Their Universes.

Yeah, I know that the blasted Tyranny of the Rocket Equation ruins science fiction writer's fun by making every gram count. But a Reactionless Drive is a solution that makes even worse problems. Kind of like removing lice by setting your hair on fire.

Sure you'll be giving Tyranny of the Rocket Equation concrete overshoes and dumping it into the ocean. But you will also be giving every space fleet, astromilitary, corrupt corporation, James Bond Villain and little Jimmy in his garage lab access to civilization-destroying relativistic weapons. Are you sure you wanna do that?

And besides, there's the iron-clad Law of Conservation of Momentum which says You Can't Do That. Sure, a future scientific breakthrough might let you have your way, but that's not the way to bet.

Mass ratios are the bane of atomic rocket designers. No matter how potent the drive is, you are going to have several kilograms of propellant for each kilogram of rocket. This puts severe limits on the sorts of missions a rocket can perform before the ever-hungry engine has to be fed again. Every gram counts due to the The Tyranny of the Rocket Equation

This is because all rockets utilize Newton's Third Law of action and reaction. You throw something backwards (the propellant) and in reaction the rocket moves forward. This is why rockets are called "reaction drives."

Naturally, the thought occurs that if you can figure out how to make a spacecraft move without using propellant, all the problems with mass ratio vanish. You'd have a "reactionless drive."

Which would be great, were it not for the unfortunate fact that it would violate the law of conservation of momentum.

Now, it is true that Newton's third law has some rare occasions where it does not apply (certain situations with magnetically coupled particles and gravitational forces acting between objects moving very rapidly), but the law of conservation of momentum is a genuine iron-clad rock-solid no-exception law. In a closed system the total quantity of momentum cannot change. It has been verified to within one part in 1e15, and no exception has ever been found.

Which means in a closed system, a reactionless drive is impossible, since it would change the total quantity of momentum.

(Note that it is possible to avoid that law with an open system, with something like a solar sail, a spacecraft launched by a mass driver based on an asteroid, pellet-stream propulsion, or a Starwisp. In these cases, the propulsion system is external to the spacecraft, so the system is open and the law does not apply.)

However, a little thing like violating a law of physics isn't going to stop the crack-pots. Face it, the second law of thermodynamics hasn't stopped all the people attempting to create perpetual motion machines of the first kind.


And even if you, the science fiction author, hand-waved one into existence for your SF novel, you've still got problems.

A working reactionless drive could turn a cheap solar power array and a brick into civilization destroying weapon of ubermassive destruction.

Burnside's Advice is Friends Don't Let Friends Use Reactionless Drives In Their Universes.

The trick is making a reactionless drive that doesn't give you the ability to shatter planets with the Naval equivalent of a rowboat (which would throw a big monkey wrench into the author's carefully crafted arrangement of combat spacecraft). Reactionless drives, with no fuel/propellant constraints, will give you Dirt Cheap Planet Crackers. If you have a reactionless drive, and stellar economics where most of the common tropes exist (privately owned tramp freighters), you also have gravitic drive missiles. Unfortunately avoiding Planet Crackers Done Real Cheap is almost impossible to justify on logical grounds, so SF author is faced with quite a daunting task.

Dean Drive

Oh, the Dean Machine, the Dean Machine,
You put it right in a submarine,
And it flies so high that it can't be seen —
The wonderful, wonderful Dean Machine!

Damon Knight

The fun started back in 1960 when the John W. Campbell (the father of the Golden Age of Science Fiction) decided to make some excitement by giving free publicity to Norman Dean and his infamous "Dean Drive". It allegedly could convert rotary motion into linear motion, i.e., it was a reactionless drive. U.S. Patent 2,886,976. "Just think," Campbell said, "stick one of these in a submarine and you have instant spaceship!"

Another common name for the Dean Drive is the "inertial drive."

Campbell was miffed that mainstream scientists were not even interested in looking at the drive. But in this case, the scientists were acting properly. Faced with the fact that the Dean Drive obviously violated the law of conservation of momentum, well, extraordinary claims require extraordinary proof. A box vibrating on a pan balance that makes the beam scale look like it had lost an ounce or two is not anywhere near convincing enough.

Interest in the Dean Drive faded away as Dean refused to let anybody examine the gadget, with the notable exception of John W. Campbell and G. Harry Stine. At least without forking over some money first. Even (now) SF author Jerry Pournelle tried to get permission to examine the drive on behalf of the airplane company he was employed at the time, but was turned down.

After Dean died, Stine made a brief resurgence of interest in the 1980's, but it died too, and later so did Stine. A close examination of the patent reveals that the device is actually a complicated ratchet pulling itself along a metal tape, not a reactionless drive.

Physicist Milton Rothman notes that Dean Drive apologists wave their hands and talk about the strange relationship between force and changing acceleration as a justification for the drive, but all they are doing is revealing the depths of their ignorance about basic physics.

EmDrive

Yes, before you all email me, I have heard about Roger Shawyer's EmDrive. It too violates the law of conservation of momentum, and the inventor's experiments have not been replicated.

Since I wrote the above sentence back in 2010, there have been a couple of quote "replications" unquote. One was in China, the other at NASA.

My take is:

  1. Inventor Roger Shawyer's theoretical basis for his EmDrive appears to be total rubbish. It violates conservation of momentum, which would basically mean scrapping all of physics and starting over from scratch, yet still predicting the same results of every experiment in physics in the last few hundred years. This is because of the Correspondence Principle. Conservation of momentum is required and maintained in Maxwell's equations, Newtonian mechanics, special relativity, electrodynamics and quantum mechanics (and their combination, quantum electrodynamics).
  2. Shawyer's theoretical basis may have nothing to do with the equipment. That is, his basis may be rubbish but he accidentally stumbled onto an arrangement of equipment that actually does create anomalous thrust.
  3. It is a good rule of thumb to be skeptical of positive results when the measurements are at the limit of accuracy.
  4. The fact that three experiments by three different researchers have shown positive results is interesting. However, there are questions about the results.
  5. If the EmDrive actually works, it really and truly is a reactionless drive. Which means it is a weapon of mass destruction that would make the Dinosaur-killer asteroid look like a wet fire-cracker.

John Baez points out that the NASA experiment measured a force that was one thousandth as big as from the Chinese experiment (The incredible shrinking force! In 10 years the device will be using quantum gravity and producing even less force. ). And also that there were some serious problems with the experimental setup (which Mr. Baez goes into in detail).

Power Requirements

I had thought that one could hand-wave a reactionless drive but control it with some kind of limit on the damage. Specifically I thought that one could figure the kilowatt equivalent of the momentum change created by such a drive, and use that as the required power.

The experts at rec.arts.sf.science quickly educated me as to how naive I was.

The underlying problem is that breaking the law of conservation of momentum shatters the entire mathematical framework. The specific problem is that you will get different values for the kinetic energy expended depending upon the reference frame of the observer.

Isaac Kuo said:

There are basically two approaches you can use:

1. There is a special frame of reference. In this case, the "reactionless" drive is really pushing against an infinitely massive special frame of reference.

(ed note: which means you've just destroyed Einstein's Relativity, with all the collateral science damage that implies)

or

2. There is no special frame of reference. In this case, the only way to sort of preserve conservation of energy is to limit drive efficiency to that of a photon drive. This is not a very useful drive, though, since it has the same (low) performance as a photon drive.

(ed note: the photon drive, where one lousy Newton of thrust takes three hundred freaking megawatts!!)

Isaac Kuo

Dr. John Schilling said:

There is the complication that "energy of the thrust" is as meaningless a phrase as, e.g., "mass of the time".

Thrust is a force, not an energy. Force, multiplied by distance, gives an energy. A force of one pound, applied as an object moves over a distance of one foot, equates to (unsurprisingly) one foot-pound of energy. The same force, over a greater or lesser distance, comes to proportionately more or less energy.

In MKS, by the way, that would be one Newton of force over one meter of distance equals one Joule of energy. If we assume constant force and motion, we can extend that to one Newton of force applied constantly at a velocity of one meter per second, equals a power of one watt.

The question is, velocity relative to what?

If it is a rocket, the relevant velocity is that of the rocket's own exhaust relative to the rocket itself. For an "intertialess thruster", the answer isn't clear and the power or energy associated with a given thrust will change widely depending on what reference frame you use to measure your velocity.

Which is one facet of the reason "inertialess thrusters" seem to be physically nonsensical. However, if you really need one for some SFnal purpose, you could try either

  1. the one universally invariant velocity in real physics. That being the velocity of light, giving you a figure of three hundred megawatts of power per Newton of thrust. A tad high for most purposes, I think, and functionally equivalent to saying your thruster is a photon drive or a (nearly-)massless-neutrino drive or a Dark Energy Rocket or whatever.
  2. the velocity of the spacecraft relative to some absolute reference frame. Either a cosmic absolute, or a local absolute tied e.g. to the nearest massive body or bodies in whatever manner is most convenient to the story. This is functionally equivalent to the old aetheric theories, and you can mine those for ideas.
Dr. John Schilling

Why doesn't this reference frame problem occur with an ordinary rocket? Isaac explains:

Because an ordinary rocket has a "reaction". The amount of kinetic energy added to the rocket by a rocket thrust depends upon what frame of reference you look at it. Indeed, there are plenty of frames of reference where the rocket thrust subtracts kinetic energy from the rocket! So you can't meaningfully talk about THE amount of kinetic energy added to the rocket. However, you CAN meaningfully talk about how much kinetic energy the rocket adds to the system because kinetic energy is also added to (or subtracted from) the rocket exhaust. No matter what frame of reference you use, the total amount of kinetic energy in the rocket plus the exhaust is increased by the same amount.

Isaac Kuo

In an ordinary rocket, both the kinetic energy of the rocket and the kinetic energy of the exhaust will change. Different observers will disagree about the absolute change of each, but will agree about the net change in kinetic energy, and so energy conservation can be enforced.

Example: A hundred-kilogram satellite ejects one gram of nitrogen through a cold-gas thruster at a velocity, relative to the spacecraft, of one hundred meters per second.

An observer at rest relative to the initial position of the spacecraft will see it accelerate to 0.001 meters per second, increasing its kinetic energy by 0.05 millijoules. The exhaust will be observed to accelerate to 99.9995 meters per second, with resulting kinetic energy of 4.99995 Joules. The total kinetic energy increase, provided by the expanding gas, comes to 5 Joules.

An observer zipping along in the opposite direction at 1,000,000 meters per second, will see both the spacecraft and the propellant as having had an initial velocity of 1,000,000 meters per second, and an initial kinetic energy of 50,000,000,000,000 Joules and 500,000,000 Joules, respectively. The spacecraft accelerates to 1,000,000.001 meters per second, giving it a new kinetic energy of 50,000,000,100,000 Joules - a gain of 100,000 Joules. Far cry from the .05 millijoules the stationary observer had thought the spacecraft acquired.

But the moving observer will have seen the slug of exhaust gas decelerate from 1,000,000 m/s to 999,900.0005 meters per second, with a new kinetic energy of 499,900,005 Joules. That's a loss of 99,995 Joules. So the net change in energy is, spacecraft +100,000.0, exhaust -99,995.0, or plus 5.0 Joules. Both observers agree on conservation of energy. And, for that matter, momentum.

If there were only the spacecraft involved, they'd be arguing about the missing hundred kilojoules.

Dr. John Schilling

If you're talking about a "true" reactionless drive, where energy is converted directly into momentum (or angular momentum), then there are lots of complications. Consider for instance that kinetic energy goes as the square of the speed:

K = (1/2) m v^2

So the power P you need to accelerate is dK/dt:

dK/dt = (1/2) m [2 v dv/dt] = m v a

As you can see, the power is a function of not only the acceleration that you want (which seems obvious), but also the speed at which you're currently traveling. The snag there is that your current speed is frame dependent. Consider that you're already doing your job and accelerating along nicely. At that point you pass someone who is already coasting at nearly the same speed you are. He sees you using much less power! Who's right?

The solution is that you either need to play by the rules of the game and use reaction drives (even if it's just reaction momentum, like a photon drive), or posit a special frame in violation of special relativity. With the special frame, now there's a "correct" frame where all the kinetic energy calculations are "official" and everyone agrees on them.

Erik Max Francis

Submarines In Space

Several times in science fiction, a reactionless drive is invented. And then the scientist gets the bright idea that if they mount the drive inside a submarine they will have Instant Spaceship.

In reality this would not work very well. A submarine is build to resist stronger pressure outside pressing in, not stronger pressure inside pressing out. And if the submarine is nuclear powered, you had better attach some kind of heat radiator. Nuclear submarines get rid of heat by sucking in cold ocean water and spewing out hot heat sink water. This won't work in space, there isn't any ocean. Not to mention the fact that a sub nuclear reactor's coolant system requires gravity to work.

This trope seems to have been invented by John W. Campbell jr., in an article he wrote about the Dean Drive in 1960. Other novels that use this theme include The Daleth Effect by Harry Harrison (1969), Gilpin's Space by Reginald Bretnor (1983), Salvage and Destroy by Edward Llewellyn (1984), and Vorpal Blade by John Ringo (2007). There is a mention of an "inertial drive" (another name for a Dean Drive) in Randall Garrett's Anything You Can Do but there it is used as a way to make recon drones float in the air.


This also seems to have influenced a certain Matt Jeffries, designer of the original Starship Enterprise, Klingon Battle Cruiser, and related works. A couple of his designs feature a "sail" or "conning tower" which are common to submarines. Perhaps he read Campbell's Dean Drive article and was inspired. If the first few starships were actually refitted submarines, maybe purpose-built starships would retain the conning tower for tradition.


The first Matt Jeffries design with a conning tower was the Botany Bay aka DY-100 from the Star Trek episode "Space Seed." It was later re-used as Automated Ore Freighter Woden in "The Ultimate Computer".


Around 1967, the AMT plastic model company wanted to cash in on Star Trek mania. They wanted to make a line of plastic model starship kits, but of their own design. So they hired Matt Jeffries to make a starship, the Galactic Cruiser Leif Ericson. Again it had the signature submarine conning tower. Unfortunately the kit was a financial disappointment, and further starships in the line were cancelled. The kit was re-issued in 2011 due to demand from those who had the original kit when they were young.

In the early 1970's, when Larry Niven and Jerry Pournelle were writing the classic The Mote in God's Eye, they used the Leif model as the inspiration for the INSS MacArthur.


Around 1975 Matt Jefferies was hired by George Pal to work on a TV series based on THE WAR OF THE WORLDS. As you can see the Hyperspace Carrier Pegasus is an outgrowth of the Leif Ericson. Note that instead of two side engines, the Pegasus has four, two on each side. For the TV series, Jefferies actually had the Pegasus upside down in relation to the Leif Ericson, in order to make the connection less obvious. The TV series was never picked up, alas. But this is a facinating glimpse of what might have been.


Occasionally in later science fiction illustrations one again finds the submarine conning tower.

Analog Dean Drive Article

A modern nuclear-powered submarine needs only relatively minor adaptations to make an ideal spaceship; it has everything it needs, save for the space drive.

The Dean drive requires a rotary shaft drive; our nuclear submarines turn nuclear energy into heat, produce steam, drive a turbine, and generate electric power. Electric power is perfect for running the Dean drive.

The modern submarines are — we have learned from past sad experience — equipped with lifting eyes so that, in event of accidental collision, quick salvage is possible. Pontoons can be towed in place, sunk beside the ship, and hitched to the built-in lifting eyes, and the ship refloated. The eyes are, of course, designed into the ship so that the structure can be lifted by those eyes without structural damage to the hull.

Dean drive units could be attached directly to the existent eyes. (ed note: you can see this in the image. The two bands around the submarine's waist hold the Dean Drive units. This also means the ship's direction of motion is in the direction the conning tower is pointed, which would make sense.)

The pressure hull of modern submarines is designed to resist at least 600 feet of water pressure; its actual thickness is a piece of classified data, of course, but we can guesstimate it must be at least 4 inches thick. After the second Bikini bomb test, the old submarine Skate was still in pretty fair condition; the light-metal streamlining hull looked like the remains of an airliner crash, but the pressure hull was perfectly intact. Stout stuff, a sub’s pressure hull.

And very fine stuff indeed as protection against the average meteor; the light streamlining hull would stop the micrometeors, of course.

Not even 4 feet of steel would stop primary cosmic rays, of course… but those inches of armor steel would have considerable damping effect on the Van Allen radiation belt effects.

The nuclear subs have already been tested with full crews for 30 continuous days out of contact with Earth’s atmosphere; their air-recycling equipment is already in place, and functions perfectly. What difference if the ‘out of contact’ situation involves submersion in water, instead of out in space?

The modern nuclear submarine is, in fact, a fully competent space-vehicle, lacking only the Dean drive.

With the Dean drive, the ship, if it can lift off the Earth at all, can generate a one-G vertical acceleration. Since that acceleration is being generated by engines capable of continuous operation for months — if not years — at a time, the acceleration can simply be maintained for the entire run; there would be no period of free-fall for the ship or crew. Therefore the present ship structure, equipment, and auxiliary designs would be entirely satisfactory. Also, a sub has various plumbing devices with built-in locks so the equipment can be used under conditions where the external pressure is widely different from the internal.

In flight, the ship would simply lift out of the sea, rise vertically, maintaining a constant 1000 cm/sec drive. Halfway to Mars, it would loop its course, and decelerate the rest of the way at the same rate. To the passengers, and to the equipment on board, there would be no free-flight problems.

There is one factor that has to be taken in to account, however; the exhaust steam from the turbine has to be recondensed and returned to the boiler. In the sea, seawater is used to cool the condenser; in space, the cold vacuum would do the job.

The tough part would be the first 100 miles up from the Earth; ice could be used.

As a crash program, this could have been done — if work started when Dean first applied for his patent — in 15 months. The application went in in July 1956; 15 months later would have been October 1957.

Under the acceleration conditions described above, a ship can make the trip from Earth to mars, when Mars is closest, in less than three days. And even when Mars is at its farthest possible point, on the far side of the Sun, the trip would only take 5 days.

It would have been nice if, in response to Sputnik I, the US had been able to release full photographic evidence of Mars Base I.

from "The Space Drive Problem" by John W. Campbell, Jr in Analog Magazine June 1960

The Daleth Effect

Analog December 1969. Illustration for Harry Harrison's "In Our Hands, The Stars", which was later expanded into the novel The Daleth Effect.

Harry Harrison wrote an amusing but cautionary tale called The Daleth Effect. In the novel, an Israeli scientist discover the principle for a reactionless drive. Naturally the first real test is the Submarine Spacecraft trick.

He returns to his native Denmark to develop it. He wishes to develop the idea without it falling into the hands of the military, since it also has potential as a weapon. Good luck with that.

Denmark keeps it a secret until they feel obligated to use the technology in public to rescue some cosmonauts stranded on Luna. Any fool could have told the Danes that no good deed goes unpunished.

Naturally the US, Soviet Union, and other powerful nations will stop at nothing to lay their hands on this technology. The race is on! They try all sorts of tactics to pressure the Danes but to no avail. They look on with helpless rage as the Danes establish a Lunar base and make a large ship for a visit to Mars.

Like absolute idiots the Danes invite foreign dignitaries to ride on the Mars trip. Naturally pretty much 100% of the dignitaries turn out to be secret agents. Hilarity ensues. And then the novel has a most ironic and satisfying ending.

Gilpin's Space

Eccentric but brilliant scientist Saul Gilpin invents a magic hyperspace faster-than-light propulsion system / antigravity surface-to-orbit gadget which can be cobbled together from parts available from your local hardware store. He mounts it on a submarine and has instant starship. Then he and the submarine depart for parts unknown.

This makes the totalitarian government very unhappy. They want to use this technology, they do not want citizens getting their hands on it. Makes it far to easy to escape the totalitarian state. Then they find out that Gilpin has mailed blueprints of the gadget to quite a few people. Hilarity ensues.

Salvage and Destroy

An ancient alien interstellar empire is worried about the large US and Soviet submarine fleets. Once Earth discovered anti-grav and FTL drives, the warlike unstable Earthlings would have a ready-made fleet of combat starships. This could turn into a nasty problem.

(ed note: aliens on Earth are covertly observing a US submarine)

“She’s an attack sub.” Joshua altered course as the black hull came sliding toward us out of the dawn mists. He gave one blast on the horn. “Mark, can you read the number on her sail?”

“SSN-767.” Mark put down his binoculars and took Jane’s Fighting Ships from the book rack in the wheelhouse. “USS Muskelunge. Four thousand six hundred tons dived. One hundred and ten meters overall. Six torpedo tubes plus subsurface attack missiles. Pressurized water-cooled reactors feeding two steam turbines. Speed dived-fifty knots plus. Complement—one hundred and ten.” He closed the book. “She’s among the most powerful warships in the Cluster—now the Ult fleets are laid up.”


“Fit that sub with inertial drive and she’d be ready for space!” said Joshua. “And there’s over four hundred like her at sea.”

An instant space fleet!” remarked someone on the foredeck.

“They couldn’t make a vortex passage. They couldn’t get out of this starfield. They wouldn’t have anybody to fight.”

“They’d find somebody. Or settle for fighting each other!”


Add inertial drive and Earth would have a space fleet! However unlikely, the idea was chilling. The men and women with me would gladly crew a human fleet, apparently blind to the outcome of such madness, as the Terrans were blind to the imminent effects of their own folly.

From nuclear submarine to inertial spaceship—an immense leap. Yet that hunter-killer exemplified a leap of the like magnitude. From sail to atomics in a hundred years! If the Terrans survived the next hundred would they leap into the dark? Into the Cluster?

I shook myself. Not even their present exponential advance would take them to vortex transits within a century. But within centuries? Up to the stars or down to hell?

from Salvage and Destroy by by Edward Llewellyn (1984)

Vorpal Blade

Shortly after they'd stopped the invasion, the Adar had given him another strange device. On first tests, it had appeared to be the world's most powerful nuclear hand grenade. Any electrical power sent to it, so much as a spark of static, and, well, there was a boom. A really big boom. "There should have been an earth shattering Ka-Boom!" boom. Putting three-phase on it had, in fact, erased a solar system.

The Adar didn't know what it was supposed to do but Weaver had basically guessed that it was, in fact, some sort of Faster-Than-Light drive. It took nearly a year of tinkering, and two more planets, to figure out that it was, in fact, such a drive. It had taken another year to create the first prototype starship.

By then, Weaver had switched sides in the ongoing sales war, leaving the Beltway and taking a direct commission in the Navy, which was the lead service in developing the world's first spaceship. He'd pointed out even before switching sides that the Navy just made more sense. The President wanted a presence off-world as fast as possible. They'd picked up enough intel in the brief war to know that the Dreen had some sort of FTL as well. Finding out where the Dreen were, whether they were headed to Earth through normal space, was a high priority. The only way to make a spaceship, fast, was to convert something. The obvious choice had been one of the many ballistic missile submarines that were being decommissioned.

So Weaver, while continuing to consult on engineering issues, was now the astrogation officer of the Naval Construction Contract 4144. Despite a couple of shakedown cruises around the solar system, the Top Secret boat had yet to be named. The 4144 had all the beauty and problems of any prototype. Most of the equipment was human, much of it original to the former SSBN Nebraska. Other bits were Adar or Human-Adar manufacture. The fact that it worked at all was amazing.


"How fast are the missiles? I mean, space is big, right, so they have to be fast?" Miller continued peering out the window, on a submarine, in front of him. The window seemed to be harder to get used to than the fact that he was standing inside humanity's first starship. A freakin' window on a submarine, he thought.

"The propulsion system is a mix of Adar tech and human. The thing is basically designed around the old nuclear thermal rocket concept but uses a small quarkium reactor instead of a fission reactor. No radiators needed and we use a dense Adar coolant for propellant instead of LOX or hydrogen or water. The Adar stuff gives us waaaay better m-dot. Using an Adar material for the nozzle we were able to get over eight thousand seconds of specific impulse out of it."

from Vorpal Blade by John Ringo (2007)