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NASA Funds Designs for a Nuclear Thermal Propulsion Rocket (space.com)
"Dangerous radiation. Overstuffed pantries. Cabin fever. NASA could sidestep many of the impediments to a Mars mission if they could just get there faster," writes Space.com, which reports NASA is now exploring an alternative to chemical rockets.
In August, NASA announced an $18.8-million-dollar contract with nuclear company BWXT to design fuel and a reactor suitable for nuclear thermal propulsion (NTP), a rocket technology that could jumpstart a new era of space exploration. "The strengths with NTP are the ability to do the very fast round trip [to Mars], the ability to abort even if you're 2 to 3 months into the missions, the overall architectural robustness, and also the growth potential to even more advanced systems," Michael Houts, principal investigator for the NTP project at NASA's Marshall Space Flight Center, told Space.com. NTP rockets would pull all that off by offering about twice the bang for the buck that chemical rockets do... "Nuclear thermal propulsion can enable you to get to Mars faster, on the order of twice as fast," said Vishal Patel, a researcher involved in subcontract work for BWXT at the Ultra Safe Nuclear Corp. in Los Alamos, New Mexico. "We're looking at nice 3- to 4-month transit times."
You heat water or hydrogen (probably hydrogen,since it gives you a higher Isp at lower temps). And squirt it out the back...
"I do not agree with what you say, but I will defend to the death your right to say it"
The point of the "nuclear" is you heat up the exhaust hotter than you can get by just burning fuel and oxidizer. Exhaust velocity (the Isp part of the rocket equation) goes like the square root of energy released per unit of fuel burned. That sets a point of diminishing returns on chemical rockets. You can get go faster if inject more energy into the exhaust, but not if that energy comes from more fuel you have to carry with you and burn. Nuclear gets you orders of magnitude more energy density that you can dump into the exhaust with effectively no penalty other than the weight of the reactor, which grows much more slowly than an equivalent weight of chemical fuel would. That said, ain't no one building anything to go into space for under 30 million. This is more like research. I'd be surprised if any metal gets cut for anything other than individual component bench testing. TRL 1 type stuff.
More like crib from NERVA, and per a friend of mine who's father was one of these, in the 1980s someone had the bright idea of gathering the surviving team members and getting an infodump from them. Click on the Project Timberwind link, that was likely how it happened, the timing is right.
Almost correct. The temperatures are not actually hotter than a chemical rocket, but you can use pure hydrogen as fuel. Since hydrogen molecules are lighter than typcal exhaust gasses (water, CO2 etc), at the same temperature they are moving faster. That means you need less mass for the same velocity change in the rocket, or you can go faster on the same fuel.
The best chemical fuels are around 4500 M/s exhaust velocity. Storable chemicals are more like 3000 M/s. Nuclear thermal rockets get to around 10,000M/s So in principal you can go 2X as fast with the same fuel to mass ratio.
There are lots of caveats. The reactor is heavy. The radiation shielding is heavy - these both mean that you need a very large spacecraft before you have a net win in performance.
You probably don't want to turn one of these on before you are in orbit due to the potential problems with an accident (and the thrust to weight is pretty small anyway).
An additional problem is that its difficult to store hydrogen for long periods of time - you would need a complex and heavy refrigeration system. Or you can just use the nuclear rocket for leaving earth, and conventional storable chemicals for arrival.
Its a reasonable idea but with a lot of engineering tradeoffs that need to be considered. Its .... rocket science.
If you are going with a high specific impulse and also greater-than-micro thrust propulsion system, you will need some kind of thermodynamic cycle to generate the required electric power, and that cycle will need to reject heat. Furthermore, the heat rejection for the cold side of that cycle into vacuum involves Stefan-Boltzmann T^4 limited radiators -- the "radiator" in your aging apartment building benefits from convection of air that is not on option in space.
Even a photovoltaic cell is subject to the Carnot limit on efficiency. The solar cell has the advantage that the hot side is surface-of-the-Sun hot in terms of the radiation spectrum of the impinging light whereas you have large surface area of the panels to radiate from the cold side. However clumsy and bulky solar panels are, you will need something almost as clumsy and bulky for radiators for a nuclear energy cycle to generate electricity venturing farther out from the Sun.
Is Discovery a nuclear-electric craft? In the 2001 A Space Odyssey genre of science fiction, you still get to wave your hands a lot even though it was meant to portray a plausible near-term future rather than warp drives and Star Trek transporters. Early concepts of Discovery had large space radiators making it dragonfly-like in appearance, but that wasn't "cool" so it ended up with this thin spine with the habitat at one end and presumably the nuclear power plant way at the other end. I never did figure out what those "pods" or "bunkers" were along the spine -- too small for cryogenic propellant storage, too small for proper Stefan-Boltzman fourth-power-of-surface-temperature radiators.
There are crazy concepts for more effective space radiators involving spraying water or pellets to get enormous surface area and then somehow recapturing the water or solid pellets so you don't end up losing them. Discovery didn't seem to depict that system.
And then there is nuclear thermal, but those are much lower specific impulse, not that much better than chemical rockets, especially when you consider the bulk of liquid hydrogen tanks and the weight of the nuclear reactor. Your "radiator" (Carnot-cycle cold side) is to blast H2 molecules out your rocket nozzles, a lot of H2 molecules. We have come full circle from the NERVA project of the 60's to VASIMIR or whatever kind of much higher impulse nuclear or solar-electric propulsion back to nuclear thermal, again?