[Nuclear Rocket]Idea to increase Isp and thrust of nuclear rocket

[FireStorm000]

I had an interesting idea for increasing the Specific Impulse and Thrust of a nuclear rocket engine; it's probably not practical - best case scenario 20% increase in exhaust velocity, with proportional increase in thrust, at the cost of 1.5x to 2x weight increase, and likely much lower - but I still thought the concept was interesting enough to see what people here think. Even if it wouldn't work, I think I'd learn a great deal from knowing why.

From my reading, the performance of a nuclear thermal rocket is strongly tied to it's operating temperature. It seems that making a nuclear reaction produce more power is trivial, it's getting rid of the heat you make that causes all of the problems.

My idea was that instead of passing the reaction mass through the reactor once, you pass it through twice. Rockets already do this to an extent, cooling the rocket with a small amount of cryogenic fluids and passing the resulting hot gas to power the turbopump. In short, instead of just cooling the engine with some of the gas, we pass all of the fluid through the reactor, up to a turbine, and back down to the reactor to be reheated and ejected. Instead of the turbine just powering a pump though, it now powers a large generator as well. We can than use this electrical power to heat the reaction mass further after it passes through the reactor the second time.

The only part that seems iffy to me is electrically heating the gas after the second pass. The other stuff would likely have practical issues, but seems theoretically sound to me.

As far as the amount of power one could potentially add vs a normal nuclear thermal rocket, I think it's capped at 50%, and adding in other inefficiencies would lower that further. I arrived that that by saying that all of the power that the reactor produces needs to be cooled by the incoming fuel, be extracted by the turbine or show up in the exhaust after the second pass, and creating a relation with Carnot's theorem to determine the temperature after the turbine and before the second pass. Not totally sure my equation is valid though.

Any thoughts? Is this theoretically sound, or am I missing something?

 

[K^2]

The limiting factor is temperature of the propellant. You can't really heat it any more, or stuff will start melting. If you can come up with a way to heat the propellant further without melting whatever does the heating, that would allow for an improvement. In your fix, you are simply moving problem from reactor itself to whatever the electrical heater is going to be. And thus, the total mass is increased without actually increasing the I_SP.

 

[FireStorm000]

The limiting factor is temperature of the propellant. You can't really heat it any more, or stuff will start melting. If you can come up with a way to heat the propellant further without melting whatever does the heating, that would allow for an improvement. <snip>

My thought on that front was to use something akin to the RF heating that VASIMR uses, just at a higher power, so that none of the electronics would need to come in contact with the hot exhaust. I'm not clear if there are any materials that could insulate against gas at 4-5000K. The gas would be (very) weakly ionized at that temperature, so the magnetic confinement shielding used in VASIMR probably wouldn't be an option.

 

[FireStorm000]

Hmm. Highest melting point I could find in a brief search was 3800K for carbon, which is only about 200 degrees more than the melting point of tungsten. Still, it should be possible to heat the gas to a higher temperature than the reactor core can take. I was only assuming 2750K as my max core temp, based on some reading about the expected temp and Isp for the next gen nuclear rocket NASA is (or maybe was, not sure) planning for their new lift vehicle.

 

[K^2]

Problem is that almost everything starts to evaporate by these temperatures. That includes tungsten and carbon. The core itself you can run molten. That introduces a whole bunch of new challenges, but you can do it.

As for RF heating, you first have to turn your propellant into plasma. Which might be a really cool thing to do, but that limits the mass flow dramatically cutting your thrust. And at that point, you might as well just use a VASMIR or similar. Which, again, for a long flight a nuclear reactor with VASMIR might be just what you want. But the main advantage of the NTR is that you get double the ISP at still considerable thrust to weight ratio. You are not going to get that with any electromagnetic component in actual propulsion. (I might eat these words some day, but that's the state of tech for now.)

 

[FireStorm000]

It's one of those ideas that sounded great at first, but the further I got through working it out and removing simplifications, the less practical it got. Since averting the heat issue and keeping a solid core was the primary objective, I'm not sure we've ruled it out, but I do acknowledge the complexity increase alone is prohibitive. Regardless, it's still a fascinating concept to me, so I'll keep playing devil's advocate if that's alright.

As for the RF heating, I'm thinking of what they use to turn the argon into plasma in VASIMR in the first place; they refer to Helicon Waves(Which I haven't the foggiest idea about) as being the first part, and then doing the rest of the work with cyclotron resonance. So either those Helicon Waves can heat something that's only barely a plasma, or the article simply glossed over how they make the argon a plasma in the first place. Unfortunately, anything but the most basic plasma dynamics concepts are a bit over my head, but on the other hand, if it works in one place, it should work under equivalent conditions elsewhere. The catch with that first phase/stage is that it only works in the presence of a strong axial magnetic field apparently.

Speaking of magnetic fields, I also saw another thread here arguing about whether or not fire qualifies as a plasma; while the slim consensus appeared to be no, they agreed that fire at as little as 1500K was ionized enough to respond to electric and magnetic fields. That does create interesting possibilities for magnetic confinement being useful, more-so than I originally thought. It really depends on how hard it is to create a cold plasma from the 2500K-3000K Hydrogen leaving the reactor.

My rough calculation was that getting 25MJ_electric for each Kg of mass passing through the reactor is reasonable, based on turbine, generator, and Carnot efficiency, but I'm going to re-check that shortly. I calculate a mass flow rate of 0.355g/s on the VASIMR 200kW module, or 562.96MJ/kg. On the one hand, the power the reactor would put out is massively lower per unit mass, but on the other, we aren't going for 5000s Isp. Adding our 25MJ/kg of electric power to the 62MJ/kg thermal after leaving the nuclear reactor, and assuming a 90% efficient conversion of that energy to useful kinetic energy, I get 12,514m/s exhaust velocity. Without the electric booster, and at the same efficiency, I get 10,564m/s without the VASIMR style booster, for an 18.5% improvement. That 90% efficiency is likely much higher than reality though, and the efficency is likely different for the thermal part vs the electric.