DarkenedOne":16bdg36k said:
So there are a number of things you are missing here.
First of all, ISP has absolutely nothing to do with mass of the engine itself. It depends on the exhaust velocity of the propellant. The higher the exhaust velocity the more ISP you get regardless of the engine size.
Secondly even if the nuclear powered rocket engine was 10 times are large as its chemical counterpart it would still be significantly better for the high delta-v transfers. Trips to the moon or Mars and back will require significant velocity changes. Higher ISP rockets are going to be useful for these missions.
Thirdly in space who cares about radiation. It already has tonnes of radiation. Thus who cares about radioactive exhaust, or radiation from the reactor. The reactor could just run open core. The only concern is for the crew, which can be solved by doing things like placing the fuel tank between the reactor and the crew compartment just like in regular rockets. Then again the crew has to be protect from the substantial amount of space radiation as well.
NTR are not 10 times better than chemical. More like twice as efficient in terms of ISP but slightly less thrust than chemical.
I know that ISP has nothing to do with the mass of the rocket but two other things do. Thrust to weight ratio and dry mass.
A NTR could in theory take an equal mass to the moon as a chemical one but with about half the propellant. However when you use the rocket equation you have to count the dry mass of the rocket . This is where the performance hit occurs. The mass of shielding and the mass of the reactor are much higher than the mass of the fuel tanks and engines of the chemical rocket.
In the real world this means that the nuclear rocket needs more propellant to account for this mass than a chemical one. This is what I mean by performance hit and this is why lightweight nuclear reactors and shielding is important for both NTR and NEP.
A similar thing happens in the case of chemical rockets. LOX/Hydrogen is in theory gives better performance than LOX/Kerosene and Hypergolics. In reality LoX/Hydrogen is better but not by as much as you would expect by ISP alone. The density of Kerosene and of hypergolic means that a tank that hold an equal amount(in terms of energy) is smaller and masses less than the one needed for hydrogen. Additionally the mass of the insulation needed to keep the Hydrogen cold counts against it.
Thrust to weight ratio is another thing to consider. In the case of NTR it is similar but less than a chemical rocket which causes greater gravity losses than a chemical one. Again requiring a bit more propellant.
In terms of lunar travel, a NTR would take 4 days to make the trip as opposed to a chemical one that would take 3 days due to lower thrust. While not horrific the edge would go to chemical. NTR could be better for large cargo. When it comes to in space travel with people the faster trip time is usually better.
In terms of mars travel an NTR makes more sense. It could get there a month or two faster than a chemical rocket because it could accelerate for a longer period of time.
The other hassle is the radioactive exhaust. While there are trajectories that can be used to prevent radioactive exhaust coming back to earth the fact that you need to worry about stuff like this is an hassle. Not counting the expense of a nuclear reactor and so on.
Honestly when it comes to mars chemical, NTR, and electric propulsion are all valid ways to get there. I prefer NEP because it offers hope that you can learn to reuse the spacecraft although I do know that after such a trip the reactor might need to be replaced.