Increasing thrust in a Nuclear Engine by increasing the propellant Flow rate

[FreeThinker]

All designs I read about Nuclear Engines are about maximizing the Isp of the engines. However, high Isp has the disadvantage of lower thrust, which is not good if you want to escape the gravity-well of a planet of large mun. Therefore sometimes you want to lower the Isp. One simple but effective solution to this problem, might be to simply increase the flow rate. By increasing the flow rate, the propellant should eventually have less time to heat up, resulting in a lower exit temperature and therefore a lower Isp.

Now the question is, is this assumption correct?

Edited July 25, 2015 by FreeThinker

[heng]

a) you do not want to use high ISP engines to escape the gravity well. that's what SRBs are for.

yes, higher flow rate means less ISP. no, lower ISP does not causally correlate to higher thrust. you would not gain any thrust by this method.

"specialization" is the keyword. one engine to lift off, high thrust, low ISP, another engine for orbital/deep space with low thrust, high ISP.

I guess your approach would make sense if - and only if - your goal would be to have as few engines as possible. having only one multipurpose engine might save weight... but I do not think we have one...

[K^2]

no, lower ISP does not causally correlate to higher thrust. you would not gain any thrust by this method.

You absolutely would, because energy is quadratic on exhaust velocity, while thrust and I_SP are merely linear. If I quadrupple the flow rate, with the same heat transfer, I expect halved exhaust velocity, meaning halved I_SP, but double the thrust. (Half velocity * Quadrupple rate = Double thrust.)

[windows_x_seven]

I think that there's a project nuclear engine that also uses oxidizer. It sprays a jet of oxidizer on the nozzle, increasing thrust.

[Thomassino]

I think that there's a project nuclear engine that also uses oxidizer. It sprays a jet of oxidizer on the nozzle, increasing thrust.

But ISP isn't very high and it overheats very quickly

[peadar1987]

The best way would be to have a more powerful reactor.

Or else add more engines. The real-life NERVA was actually comparable in thrust to chemical engines IIRC, the thrust was only nerfed in KSP for gameplay balance reasons.

[K^2]

The best way would be to have a more powerful reactor.

Which is heavier, and the whole point here is to get a better TWR.

If for some insane reason, you could not implement staging and had to use NTR for everything, increasing flow really works. Trouble is, you still won't have good TWR by the time you've plunged way bellow I_SP of conventional rockets. So at that point, you're way better off to simply use a conventional stage to get the NTR to orbit. Once you're in orbit, TWR of an NTR is quite sufficient.

[AngelLestat]

My doubt always was if it would be possible to have a 1 stage nuclear rocket able to launch from venus clouds to orbit, first using the atmosphere as proppelent then changing to on board proppelent.

The orbital speed is lower, and the gravity is also a bit lower. Can be enoght?

The problem to use this on earth is the radiactive pollution that we might add and possible crash on the surface.

[Thomassino]

My doubt always was if it would be possible to have a 1 stage nuclear rocket able to launch from venus clouds to orbit, first using the atmosphere as proppelent then changing to on board proppelent.

The orbital speed is lower, and the gravity is also a bit lower. Can be enoght?

The problem to use this on earth is the radiactive pollution that we might add and possible crash on the surface.

ISP at sea level would be miniscule, so the thrust. This would happen to any chemical engine at Venus, so you'd be better to use propellers or baloons, then at upper atmosphere "light" up NERVA on external cycle, then on internal cycle (with possible ISRU on surface or on the way up to have propellant)

[TheDarkStar]

ISP at sea level would be miniscule, so the thrust. This would happen to any chemical engine at Venus, so you'd be better to use propellers or baloons, then at upper atmosphere "light" up NERVA on external cycle, then on internal cycle (with possible ISRU on surface or on the way up to have propellant)

What about a nuclear jet engine for getting out of the thickest part of the atmosphere?

[MinimumSky5]

ISP at sea level would be miniscule, so the thrust. This would happen to any chemical engine at Venus, so you'd be better to use propellers or baloons, then at upper atmosphere "light" up NERVA on external cycle, then on internal cycle (with possible ISRU on surface or on the way up to have propellant)

That was what AngelLestat was suggesting, he was launching from the cloudtops, not the surface. Pollution from a NERVA, as I understand it, would be minimal, as the nuclear fuel stays inside the engine and isn't exhausted.

[AngelLestat]

yeah someone post in the science section a boing patent about a nuclear jet engine, but not sure if can allow a change in the proppelent.. (air to onboard tank, similar to skylon but without oxidant)

yeah I imagine that the radiation on the atmosphere would be minimal, but if never was try it in earth must be due the danger of crash on land or the thrust/weight ratio is not enoght.

[Thomassino]

What about a nuclear jet engine for getting out of the thickest part of the atmosphere?

Something like thermal turbojet? Yes, it possibly can be doable, but I am worried about TWR. It would have to be some kind of venus spaceplane. Now when I am thinking about it, something like NERVA-aerospike hybrid would have been the best.

That was what AngelLestat was suggesting, he was launching from the cloudtops, not the surface. Pollution from a NERVA, as I understand it, would be minimal, as the nuclear fuel stays inside the engine and isn't exhausted.

There is NASA proposal called Project Havoc, which proposes manned floating airship in altitude of about 50km (there is more or less equal atmospheric pressure to earth's - around 100kPa). On orbit return would have been thermal turbojets very useful but they are very heavy and with low TWR. Original proposal counts with SRB as far as I know.

[SomeGuy12]

Regarding a bigger reactor : increasing reactor power output seems like the easiest solution. I mean, we know there's basically no limit - you can keep adjusting the reactor geometry to produce a more energetic fission reaction until the reactor itself glows white and explodes with a flash. You don't have to have the various safety measures that civilian power reactors have - you could make your nuclear engine run just a hair short of exploding like a fizzled fission bomb.

Since you can relatively easily increase power output, you'd compensate for the increased power by increasing the propellant flow rate. More thrust at the same ISP, reactor design is just more dangerous. Now, one problem with this is what happens when the burn is over? How do you stop the reactor from getting so hot it sends out a cloud of fuel vapour?

[K^2]

Pollution from a NERVA, as I understand it, would be minimal, as the nuclear fuel stays inside the engine and isn't exhausted.

That's true if your fuel is hydrogen. But with random atmospheric gas, you are likely to be producing secondary radiation. Think radioactive dust after a nuclear explosion. That dust didn't come from the bomb, but from irradiated environment. Many perfectly stable and safe isotopes have capability of capturing neutrons from a radioactive source and becoming radioactive themselves. So if you just scoop up atmo and run it through an NTR engine, you are going to be leaving a radioactive plume. Hard to tell if it would be significant, but the risk is definitely there.

Now, Russians are currently working on a variation of NTR which does not use direct heating. They have a reactor that generates power, and an electric heater in the heat exchanger. So the propellant never gets close to the radiation source. That engine only exists on paper at the moment, but this is the sort of setup you can safely use in an atmosphere.

Something like thermal turbojet? Yes, it possibly can be doable, but I am worried about TWR. It would have to be some kind of venus spaceplane. Now when I am thinking about it, something like NERVA-aerospike hybrid would have been the best.

You might have a bypass section in your turbine, similar to a typical turbofan, which would supplement TWR. You'd want to be able to disable bypass and go to a ramjet mode at some point, though. In fact, a thermal version of SR-71 Blackbird jets is probably your best bet.

Edited July 25, 2015 by K^2

[SomeGuy12]

Now, Russians are currently working on a variation of NTR which does not use direct heating. They have a reactor that generates power, and an electric heater in the heat exchanger. So the propellant never gets close to the radiation source. That engine only exists on paper at the moment, but this is the sort of setup you can safely use in an atmosphere.

Does this even pass a paper test? That is, would you have a high enough power:weight ratio to even leave the ground, assuming reasonable, informed estimates for the mass of the reactor + shielding + heat engine + electric heat elements + airframe?

What do the Russians plan to do if it crashes...it would have about as much lethally radioactive material in the core as a nuclear submarine's reactor. Crashes can spread that stuff around really finely. Reactor fuel is so hot that the lethal dose takes mere seconds..