Nuclear ground-orbit vehicle

[malloyd]

Originally Posted by raniE

I'm thinking about TL9 space travel and wondering about nuclear thermal reaction engines. Looking at GURPS Spaceships, an NTR using water as reaction mass is one of the cheapest ways of getting stuff into orbit. How dangerous, for the environment, crew, passengers and humans in general, would such a vehicle be to operate, assuming that there are no accidents in which reactor material is ejected from the spacecraft?

Assuming that, almost perfectly safe. The only hazard from fission powerplants or engines for spacecraft anybody not from the lunatic fringe of the anti-nuke movement worries about much comes from the situation that violates that assumption. What happens to the fissionables if the ship blows up? Or burns up on reentry? This wouldn't be as bad as say Chernobyl - that involved tons of fuel escape (anywhere from 5-200 tons depending on who you believe), and nobody is going to design an engine that heavy - but wouldn't be good.

Shielding the crew and passengers from a nuclear engine is fairly trivial, no more difficult than shielding them from a power reactor, and easier than shielding them from cosmic rays. It adds weight to the engine, but the GURPS statistics have usually been selected from designs that do that quite adequately

Shielding the surrounding environment from the operating engine adds more weight, which is usually considered unnecessary, given that a few hundred meters of air works as well and nobody is supposed to be close to the back side of an operating rocket anyway. It can mean you need a certain amount of movable shielding on the ground brought up to the engine before debarking, and some environmentalists are going to scream about animals that happen to fly too close or run across the runway.

Radioactive material in the exhaust is fairly small even for direct contact engines with solid fuels (Isps in the 600-1200 second range), you don't after all want the fuel to be soluble in your reaction mass, and entirely avoidable by removing the direct contact (again this increases engine weight, for a heat transfer loop). Designs with much higher Isps (2500-4000 seconds) like gas core or nuclear salt water designs unavoidably have more radioisotopes in the exhaust, simply because those temperatures *require* direct gas phase contact with the no longer solid fission fuel.

Is using water as reaction mass going to be significantly more environmentally harmful than using hydrogen

No. I suppose theoretically you can get a little more neutron activation of the oxygen and impurities, and if you have a direct contact stuff is more soluble in water, but while this might be measurable, its going to be trivial.

Is it possible to build a nuclear ramjet that does not release radioactive material into the atmosphere

Sure, you need that same heat transfer loop, which makes it heavier, for not really very much gain, given how little isotopes are in the exhaust in the first place.

(Project Pluto from the 1960s sounds like something you really do not want to release into the atmosphere)?

AFAIK, the Pluto nuclear ramjet involved no significant release of radioisotopes in the exhaust. What it did have is essentially no shielding, so everything nearby was exposed to a considerable dose of gamma rays, and a smaller neutron flux. It'd have been reasonably safe to use it in the atmosphere on a cruise missile you didn't expect to come back, it just wasn't safe to get close to it, and produced some neutron activation of the aircraft.