Nuclear ground-orbit vehicle
 

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? Is using water as reaction mass going to be significantly more environmentally harmful than using hydrogen (water is so much cheaper and easier to use it's usually worth the loss of delta-V)? Is it possible to build a nuclear ramjet that does not release radioactive material into the atmosphere (Project Pluto from the 1960s sounds like something you really do not want to release into the atmosphere)?

Re: Nuclear ground-orbit vehicle
 

Reasonably safe for crew and passengers, less so for everyone else. There isn't an easy way to keep nuclear drives from spewing radioactive debris everywhere. There maybe some tricks to reduce damage, but it's probably still a major hazard if you use it regularly.

Re: Nuclear ground-orbit vehicle
 

Maybe not.

Back in the 60s they deliberately blew up an operating NERVA prototype on an outside testing ground just to see how bad it would be. Ah, the unregulated days of yesteryear.

The results don't seem to have been particularly bad. Probably quite a bit like the hypothetical terrorist "dirty bombs". Scary but not actually any worse than equivalent high explosive (or in this case, having a big fast-moving thing fall on you).

Re: Nuclear ground-orbit vehicle
 

Using it once or twice isn't gonna hurt things to badly, especially compared to the assorted nuclear testing done historically.

If you need it for emergency earth-to-orbit vehicles, fine. But if you regularly use it to launch satellites, etc, my impression was it will add up to a serious problem.

Re: Nuclear ground-orbit vehicle
 

The actual problem for NERVA rockets and ground to orbit was sucky power-to-weight ratio even though they had superior fuel efficiency.

Maybe 2, maybe 3x better fuel use for somewhere between 10 and 30x as much engine weight (greater cost too of course). That was with hydrogen too. Water ameliorated the power-to-weight problems while giving away the fuel efficiency. Heat water with a nuclear reactor or create it by combusting H2 and O2 and efficiency is the same at the same temperature.

The real attraction of NERVAs occurs when you can run them for long times but don't have to achieve high Gs. Some of the prototypes were run for 30 minutes to an hour. The Shuttle's main engines with their 8 minute burn times are pretty much unique (and consequently complex and expensive). 2 minutes is more common for liquid fuel rockets.

So 3 Gs for 8 minutes favors chemical rockets. Once you're in space 48 minutes at 0.5 Gs gives you the same Delta-V with 1/6th as much NERVA engine.

It's not obvious that the fuel can't have a sealed barrier between the it and the water/hydrogen reaction mass. Bombarding water/hydrogen with neutrons and gamma rays doesn't create problematic materials. If the actual reactor materials stay inside the engine there are no real pollution problems.

Now, nuclear salt water rockets are an all-around stupid idea but the NERVA type aren't so bad.

Re: Nuclear ground-orbit vehicle
 

I was under the impression that only the Nuclear Salt-Water rocket and the Orion drive had appreciable fall out. The major concerns for all the NTRs was that one of them would blow up and the reactor contents would spread everywhere.

Re: Nuclear ground-orbit vehicle
 

Unless I'm mistaken, an NTR uses a fission reaction to produce heat, then uses said heat to increase the temperature of the fuel before it is spewed out as exhaust. Unless you are running the fluid directly over the reactor's fuel rods, it shouldn't pick up any radioactive material. The fuel itself may end up irradiated, but irradiated doesn't necessarily mean radioactive (it can in some cases, but I think those generally make rather unstable radioactive elements that decay themselves away rapidly).

EDIT: ninja'ed by Fred

Re: Nuclear ground-orbit vehicle
 

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.

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.

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.

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.

Re: Nuclear ground-orbit vehicle
 

According to Aerospace Projects Review V2 N1, the engine designers were never able to prevent the engine elements from eroding off dust-to-sand-size particles, so the Pluto would have left a "noticeable" radioactive trail.

Re: Nuclear ground-orbit vehicle
 

Using the rules in Spaceships, a nuclear thermal engine using water as reaction mass is less efficient than a standard chemical rocket, having less thrust and giving the same delta-V. It can be designed as a ramrocket however, which lowers the reaction mass required to reach orbit. The real gain for this type of rocket however, is fuel costs. If one can successfully build a single stage (or with the Spaceships rules effectively single stage, using a small upper stage that never separates) rocket of this type, reaction mass costs are going to be vastly different. The fuel/remass for an HEDM rocket, which can easily be single staged, costs 6000$ per short ton. Ordinary rocket fuel costs 800$ per short ton (and can be made into an effectively single stage design). Water costs 20$ per short ton. Fuel cost per passenger in similar designs go from several thousand dollars to a couple of hundred, and fuel costs are always going to be the majority cost to get anything into space. This is what is attractive about the design.