Robert Heinlein’s 1947 SF novel Rocket Ship Galileo features a spaceship driven by an atomic pile that heats metal ingots (lead, IIRC, but the Wikipedia article says zinc) to boiling point for reaction mass. Today, that concept seems almost as fanciful and innocent as the ship being cobbled together by a lone backyard inventor and crewed by Boy Scouts who find Nazis on the Moon – but I can’t quite put my finger on why. Certainly, for environmental reasons, you would not want to introduce vaporized and possibly radioactive lead or zinc into Earth’s atmosphere, but it would be a harmless system for trans-atmospheric propulsion, wouldn’t it? And storing your reaction mass in metal-ingot form would save on space, compared to liquid oxygen or whatever, wouldn’t it? Yet no space program has ever used such a thing, AFAIK; every spaceship uses chemical propellants. What’s the basic flaw in the “atomic rocket” concept?
I would think one problem would be the lead fouling up tubes or whatever, as its a solid once it cools down. Easier to use something that cant do that?
Also found this:
http://www.fas.org/nuke/space/c02early.htm
"The most effective propellants, which produce the highest specific impulse, are those with the lowest molecular weight. Thus the ideal propellant for a nuclear rocket is mon-atomic hydrogen. "
So presumably lead is the exact opposite of that, as its so dense.
Otara
Brain
Take a look at http://adsabs.harvard.edu/abs/1992jpnt.confRW...B or just look-up NERVA. This has been tried. I didn’t get into all the reasons why it hasn’t been done but AFAIK, it is at least theoretically possible.
Testy
NERVA was powered by nuclear explosions, not a reactor. There have been studies showing that an atomic airplane is possible, but it wasn’t practical. An atomic spacecraft would certainly be possible, but we have no need of one just yet.
I believe you are thinking of Project ORION, which explored the feasibility of nuclear pulse propulsion. NERVA and associated programs used nuclear thermal propulsion, i.e. running a propellent stream through the reactor core to convert neutron radiation into heat and then releasing the propellent under pressure to obtain thrust. There is also the nuclear salt water rocket concept, in which radioactive salts are introduced directly into the propellent stream and the ensuing reactions occur in the rocket chamber and then exhaust. There are also other nuclear energy-based concepts like fractional fission rockets and the like, but all essential derive thrust by converting fission by-products into heat and thence into thrust. Theoretically you can obtain between 1-2.5 an order of magnitude more efficiency with nuclear thermal propulsion over chemical propellents, which puts it easily in the range of supporting interplanetary (but not interstellar) transit. Even at low thrust, if you can sustain thrust over an extended period, the efficiency is sufficient that you don’t end up with the propellent being a massive fraction of the overall vehicle mass.
The problems with nuclear powered rockets are essentially the same with any compact reactor; they’ll have a minimum size at which they can function, which limits complexity and amount of redundency of safety factors. They’ll also require shielding (assuming a living crew, or radiation-sensitive payload and avionics) and will have to function with minimal maintenance, which as any Navy nuc will tell you does not reflect the state of the art in the operation of current compact, portable reactors. Small reactors also tend to run on the edge of stability, especially if you are trying to extract high performance from them; the conceptual development of the nuclear thermal replacement for the Space Shuttle Main Engine ran into the problem that it simply couldn’t be man-rated, or indeed, given any guaranteed failsafe mode of operation. The Soviets played around with very compact sodium cooled reactors for satellite and space probe applications but have had numerous problems (including scattering one across northern Canada); the U.S. has stuck with Radioactive Thermoelectric Generators (RTG) in applications where solar power can’t provide sufficient energy.
As for the selection of propellent, in theory the low mass propellent will take less energy to accelerate to the same speed by a square factor to velocity. Effective exhaust velocity is king in terms of propulsion efficacy, although weight-specific impulse is what is typically used to compare different types of motors and is often the characteristic most cited for overall efficiency. Low atomic mass propellent also tends to expand faster and act thermodynamically more like an ideal gas, so you get more efficiency with a smaller nozzle bell, hence why liquid propellent chemical rockets are more efficient than solid propellent rockets. The highest specific impulse motors we have actually spray free hydrogen ions, albeit at almost immeasurably low thrust.
We’ll probably need some kind of nuclear propulsion before we can seriously talk about sending people to Mars or other planets. It’s beyond current capabilities today, but there are some concepts with limited prove-out which could provide that capability.
Stranger
I would think the biggest problem would be the weight of the radiation shielding around the reactor pile.
That, and the bureaucratic nightmare of OSHA, EPA, and DHS, who would all be reluctant to allow you access to a sufficient quantity of uranium to power the ship.
In two later books, Space Cadet and The Rolling Stones, monatomic hydrogen is the propellant of choice. Heinlein paid attention. 
Although he never did explain just how you keep hydrogen in a monatomic state.
beowulff
Like Stranger said, you’re thinking of an Orion. An Orion would be an amazing thing to watch from a (long!) distance. Think what it would look like on take-off. Setting off multiple groundburst nukes might be a tad hard on the environment though.
Testy
NASA To Boost Nuclear Space Science With Project Prometheus (2003):
NASA’s old Prometheus page:
http://exploration.nasa.gov/programs/prometheus/
Now links to the “Exploration Systems Mission Directorate”; which gets closest to covering atomic rocketships in its article on " Fission Surface Power Systems."
Well sure, but when elephant-like aliens invade and you’ve already had the Russians nuke Kansas and that didn’t suffice, what choice do you really have?
Gorsnak
An Orion commands serious respect! You could put your foot on just about anybody with one of those! 
Testy
At one point nuclear-thermal rockets seemed like such an improvement over chemical rockets that they were seriously considered for ICBM’s! However chemical rockets improved significantly since the days of ethyl alcohol fueled rockets like the V2 and the Redstone, and the performance gap narrowed. A nuclear-thermal system like NERVA might be some improvement over chemical rockets, but so far not enough to warrant further development. The problems are as follows: [ul][li]AFAIK, none of the proposed systems had a high enough thrust/weight ratio that they could take off from the Earth’s surface; so you would need convential first stage boosters and use the nuclear only for upper stages or orbital-transfer.[]As already mentioned, nuclear reactors don’t scale down very well. Nuclear isn’t that useful for anything smaller than a Saturn V scale payload. So unless you’re committed to a very ambitious manned presence in space, it isn’t going to be practical.[]All nuclear-thermal systems have the basic limitation that their exhaust can’t be hotter than the hottest temperature you can run a reactor without melting it. An ordinary hydrogen-oxygen flame is already at the limits of current materials technology, and there just isn’t much room for improvement given the finite list of materials with ultra-high melting points. Nuclear systems would offer improved performance only because they could exhaust a stream of hot hydrogen, with a lower molecular weight than H2O and therefore higher exhaust velocity for a given temperature.Against the above has to balanced the not inconsiderable weight of the reactor itself, plus the weight of shielding. Add in safety considerations, like having a a supercritical mass of pure fissionable material not explode on you, and the technical challenges are daunting. [/ul][/li]
BTW: I was curious where the two-month figure for a nuclear system to Mars comes from. If your trip time isn’t being dictated by a minimum energy Hohmann orbit, then what factors determine a two-month trip? Some performance feature of the system, or someone just pulled it out of a hat? Also, is Prometheus nuclear-thermal or nuclear-electric?
Here is a website that covers S-F type rocket design (including the atomic variety) in exhaustive detail.
Two months comes from the amount of speed you’re able to obtain, I’m sure. At it’s farthest distance from Earth, Mars is a two week journey from Earth if you can do a 1 G continuous acceleration. (Pluto is two months, IIRC, at that speed.)
Tuckerfan
Pulling 1 g all the way to Mars sounds like amazingly good performance. I had no idea you could do that with a NERVA or any other kind of engine.
Regards
Testy
AFAIK, you can’t. We don’t have anything at present which could generate the necessary amount of thrust without requiring gobs and gobs of chemical fuel. With a few zillion or so dollars, developing a nuclear engine that can do it is possble, but getting the money is the hard part.
You don’t need to have a tank of monatomic hydrogen. Low molecular weight is only important if you are accelerating your reaction mass by heating it up and letting it expand through a nozzle. And if you heat plain old diatomic hydrogen hot enough, it’ll break apart into monatomic hydrogen all on its own.
You’d need some kind of nuclear power source to supply the energy, and you’d have a bit of a problem with heat transfer and nozzle erosion since the temperatures involved are rather beyond what solid materials can withstand, but it works on paper at least!
OTOH a tank of monoatomic hydrogen, if you could stabilise such a thing, would form the basis of a kickass chemical rocket - the reaction energy from letting the hydrogen recombine into molecules in the reaction chamber gives an impressive specific impulse (over 2000 IIRC). But the exhaust would be plain old diatomic hydrogen, very hot and moving very fast.
Yes, but in the above-mentioned books, the fuel (what they’re pumping into the tanks) is specifically monatomic. There’s a scene in The Rolling Stones, for instance, where the protagonists’ ship is low on reaction mass, and a larger ship in the vicinity offers to transfer over some “single-H” from their reserves. Presumably, they’re not transferring the stuff over at temperatures high enough for dissociation.
By what criteria and what tradeoffs? You could get up to half the speed of light with gunpowder, provided you started out with the sun’s mass of propellent. My guess is they mean how fast a nuclear-thermal ship can get you to Mars using a single stage and a reasonable propellent/ dry weight ratio. But I don’t actually know.