mvpilot172 t1_j6idy94 wrote on January 30, 2023 at 3:37 PM

Reply to comment by Creepy_Toe2680 in NASA tested new propulsion tech that could unlock new deep space travel possibilities by Creepy_Toe2680

It stills needs liquid fuel though doesn’t it? Just does not need an oxidizer so it saves that portion of liquid fuel.

cjameshuff t1_j6iswvx wrote on January 30, 2023 at 5:13 PM

Yes, that part is completely wrong. Nuclear rockets still use propellant. Nuclear thermal rockets use about half as much by mass as the best chemical rockets, but they only get their peak performance with LH2, which takes up about 5 times as much volume for the same amount of mass. A nuclear thermal spacecraft will be a big pile of propellant tanks (likely drop tanks so you don't have to carry empty tank mass around) strapped together with a nuclear rocket engine at the back and a small payload tacked onto the front.

The "45 days" claim appears to be in reference to the "wave rotor" stuff that's been getting massively overhyped. Basically, as described, they propose sticking a widget between the nuclear reactor and the nozzle that somehow doubles the specific impulse while halving the flow rate.

This means doubling the power output of the reactor. Since the power output of the reactor is already limited by the need to keep it from melting, and the reactor is cooled by the propellant flow which you've just cut in half, it's not clear how this doesn't result in the reactor, well, melting. Also, even if it worked, doubling the specific impulse isn't nearly enough of a gain to allow a 45 day trip to Mars.

They then throw in nuclear-electric propulsion, which requires heat exchange loops, many megawatts of electrical generation capacity, giant radiator arrays, and arrays of ion thrusters. They assume all this can be done "with minimal addition of dry mass", and this is how they double the performance again to get their 4000 s number. However, it doesn't actually appear to have anything to do with the wave rotor.

NASA's giving one guy $12500 to look at it. It's not taking anyone to Mars any time soon.

Worldofbirdman t1_j6kxwsf wrote on January 31, 2023 at 1:33 AM

I'm sure they could figure out a way to use the temperature of outside the space craft for cooling. As soon as I read your comment I did a quick look and it's -455f or something similar. I guess an issue could be heat exchange from a vacuum to whatever the cooling system is, but that's above my brain grade.

Edit: temperature I'm referring to is the vacuum of space.

Phoenica t1_j6l207z wrote on January 31, 2023 at 2:03 AM

> I guess an issue could be heat exchange from a vacuum to whatever the cooling system is, but that's above my brain grade.

That's sort of the whole problem though. Whatever particles are around in a near-vacuum might be very cold, but there are also very very few of them. There just isn't anything to exchange the heat to. A vacuum is an insulator, that's how Thermos bottles work for example.

andrew_calcs t1_j6lbekx wrote on January 31, 2023 at 3:14 AM

The problem with space being cold is that it’s also empty. You know how a windy day at 40 degrees out feels much colder than when it’s 20 degrees out but with no wind? Take that to its logical extreme. Things do cool down in space, but not by convection or conduction so it’s very slow.

-Prophet_01- t1_j6lliz1 wrote on January 31, 2023 at 4:39 AM

Heat exchange in vacuum largely works with a different principle and is much, much less efficient. Without other molecules to transfer heat to, we're left with black body radiation. Lower efficiency means bigger radiators. It's basically trying to cool down by giving off infrared light.

Something that could be done with a small coolant loop through a river or a glorified AC on earth, requires large sail-like structures in space (sails because it maximizes the surface to throw out that thermal radiation).

Karcinogene t1_j6klrbl wrote on January 31, 2023 at 12:04 AM

The thermal nuclear engine has propellant, technically not fuel. It's not combusted, just ejected backwards at high speed. It's gets used up, but it's not a source of energy.

Jobotics t1_j6ls3fu wrote on January 31, 2023 at 5:43 AM

The detonation engine still uses both fuel and oxidizer. It isn't nuclear. The nuclear engine was just mentioned at the end as another engine being worked on.