Here's the quote describing the propulsion system:
All this space below carries the working fluid. The reaction mass. It's water heated to dry steam by the atomic pile and expelled through this jet.I don't know how to read that as anything other than a steam-powered spaceship. Was that really how Heinlein thought an atomic rocket engine would work? share
I don't know how to read that as anything other than a steam-powered spaceship. Was that really how Heinlein thought an atomic rocket engine would work?
jclarke's answer is correct. Here's some amplification:
The term "reaction mass" can refer to anything which can be accelerated and then "thrown away" from the ship. In theory, you could take a pile of gravel into space and slingshot single pebbles away from your ship to propel it. In practice, you couldn't repeat that often enough to get into orbit, so a system which can accelerate a great mass in a short time is needed. That means most propellants are liquids or solids which react to produce heat and thus expand. That expansion is directed by a nozzle, which guarantees it will leave the engine in the desired direction. The only nuclear rocket the US ever tried used hydrogen gas as the reaction mass, and the exhaust was ionized hydrogen atoms. That's more efficient than turning water into steam. jclarke is also correct in pointing out that modern engines fueled by liquid hydrogen and liquid oxygen produce superheated steam as their reaction mass. Some very long duration deep-space probes use "ion thrusters" which electrically accelerate single atoms as their reaction mass. Not much thrust, but they can run for years at a time.
OK, well first off I'm surprised that anything like a steam-powered spaceship is ever a good idea, and I accept that you guys are right about that. That said, I don't consider the burning of Hydrogen and Oxygen as steam-power, even though the reaction mass is individual water molecules which can be called "steam". When I think of steam power, I think of heating liquid water until it boils and then venting the steam in the desired direction. So for example, if some other liquid were boiled for power, I'd consider that steam-power too even though it's not water steam. Energy from the burning of Hydrogen or any other substance is what I'd call "chemical power", regardless of what ends up being thrown from the spaceship. So for me I guess it's just a matter of definition.
shareRead Rocket Ship Galileo for a more detailed explanation about how the nuclear-powered rocket engine was supposed to work. Heinlein discusses the pros and cons of using different types of material for reaction mass. In the book, IIRC, they use lead.
But yeah, a nuclear rocket like this is quite simple, massively powerful...and utterly impractical for use on as an Earth-orbit lift vehicle, because whatever you use as reaction mass ends up spewed out the nozzle as highly radioactive waste. But such an engine would make for a fantastic constant-boost interplanetary exploration vessel. Want to go to Mars in days instead of years? Easy. We could build such a ship with current technology, and for not much money.
We could build it on the ground for "not much money". But since we have to build it in space it would cost a few hundred billion probably. About the same as the ISS.
I'm not sure the radioactive reaction mass is as big of a problem as you imply though. You're not using a huge amount of reaction mass; it would be pretty sparse, would disperse quickly, and wouldn't add much to the overall background radiation of the environment.
The biggest fear with ground-launched nuclear rockets would be accidents rather than radioactive exhaust. A failure like the explosion of the Columbia would mean both the nuclear pile and all the irradiated metal being spread over a massive area. You could mitigate the risk a little by launching over water, but given the current anti-nuclear attitude of the general public I'd say no amount of risk-mitigation would be enough to convince them to support such a plan.