Nuclear thermal rocket + nuclear pumped laser

[vda]

New post from armchair rocket engineer department.

A bit of generic engine theory as I understand it, please correct me whereever I am wrong.

What is the problem with more efficient rocket engines? At the first (very naive) glance "more efficient" rocket engines are those with higher Isp. But in practice there is a problem. In order to increase Isp by x2 you need to increase _kinetic_ energy_ of exhaust by x4, because p=mv, but E=(mv^2)/2. Why is this a problem? Required power levels grow quadratically and with very high Isp's are becoming truely astronomical.

What is the problem with _that_, one might ask? Aren't we getting much better fuel efficiency instead? The problem: with those power levels you *have to* be very efficient (I mean, like 99,99...%) in converting energy to thrust, or more precisely, in NOT converting it to heat. That's exactly why the rocket with "best possible" Isp, the photonic rocket, is unlikely to ever work. It needs godawful amounts of power, and if you accidentally convert 0.000..001% of it to heat, the rocket will instantly vaporize.

Thus any high-Isp + high- or moderate-thrust engine inevitably has to employ some advanced means of using more power for thrust and less power for waste heat.

Back to topic of "Nuclear thermal rocket + nuclear pumped laser".

I was thinking about improving nuclear thermal engines. They said to have Isp of ~900 (with solid core reactor, the only one tested so far). How to improve that? What about making core reactor to also generate laser beam ("nuclear pumped laser"), and use that for additional heating of exhaust?

["nuclear pumped laser" picture: http://www.ippe.obninsk.ru/podr/tpl/device/lael_a.html ]

ASCII picture of the engine (copy-paste into e.g. Notepad and use fixed font to see it clearly):

--------------\------/....
-||||||||||||..\----/.....
-|||||||||||||..----...***
-||||||||||||||......*****
-##########=====**********
-##########=====**********
-||||||||||||||......*****
-|||||||||||||..----...***
-||||||||||||../----\.....
--------------/------\....

---, /, \ - chamber, nozzle walls
### - nuclear reactor core - heats gas and also generates laser beam
=== - laser beam
... - chemically different gas is injected into the chamber, one which is effectively absorbing laser's wavelength ("opaque gas")
*** - laser beam passes thru the engine throat, meets opaque gas and heats it up.
||| - the rest of the reactor chamber is filled with gas which is used as lasing medium.

proportion of ||| and ... are not to scale - to be determined by "real" engineers if they ever consider building the thing.

What's the idea? The idea is to take a "usual" nuclear thermal design, increase reactor power, convert some % of power to laser beam, and convert beam's energy to more heat in a place where we don't risk melting down the reactor - in the throat. Note that a optically thick layer of "opaque gas" is protecting throat's walls from extreme overheat.

Is it viable at all? In particular, which gases can be used for that?

[Marcus]

I can't reach the link, so maybe I'm missing something, but here's my take:

I don't understand how you intend to generate a laser with a reactor core. You need complicated and delicate hardware to generate a laser beam, and placing that hardware in a reactor core in a region of high radiation flux is not reall feasable. Not only that, but you need a system to power the laser, which means energy conversion hardware. You could use your NTR fuel as a heat rejection system for your energy conversion hardware, but even so, it's still not really a good idea.

The real problem is, you're adding energy conversion cycles which are inherantly inefficient. You might have 98% efficiency heating gas with your laser, but your energy conversion hardware is probably not going to be better than 50% efficient. Now, I guess you could try to capture that inefficiency by pumping "waste" heat from that system back into your working fluid, but I have a feeling the whole system is just going to be either: 1) Too heavy. 2) Too complex. and 3) Too unreliable to work.

The best way to boost NTR Isp is to come up with better materials that can withstand higher temperatures in the "combustion chamber" and nozzle. Or come up with some form of magnetic containment system for the fuel. Once you go critical in a reactor, that bugger is going to get HOT. The problem now is how to get that heat from the nuclear fuel to the working fluid without melting your hardware. The reason you can't do better than about 1000s Isp is simply due to the fact that we don't have any materials that can handle higher temperatures that could give you better Isp. There's no real need to come up with complicated schemes to pump more heat into the working fluid through a series of energy-conversion steps. Better to just go straight to the source.

[George CA]

Link works for me, and it's very interesting, thanks.

[vda]

Marcus - 5/3/2007  8:24 PM
I don't understand how you intend to generate a laser with a reactor core.

Fission fragments and/or neutrons are exciting lasing medium by colliding with atoms of the medium. I believe this is demonstrated to work (how efficiently it can be made to work - that's another, very important question).

You need complicated and delicate hardware to generate a laser beam, and placing that hardware in a reactor core in a region of high radiation flux is not reall feasable. Not only that, but you need a system to power the laser, which means energy conversion hardware.

That's the point. There is no additional hardware. Reactor is _the hardware_ and high radiation flux is _the source_ of energy here.

The real problem is, you're adding energy conversion cycles which are inherantly inefficient. You might have 98% efficiency heating gas with your laser, but your energy conversion hardware is probably not going to be better than 50% efficient.

It's exactly to opposite - a fraction of power is _not_ used for reactor heating now. All energy of fission fragments/neutrons spent for creation of population inversion and then a laser beam is *not* going thru nuclear -> thermal -> kinetic/electric/whatever cycle. All this energy flies away from the core as light, and is converted to heat and then to kinetic energy of exhaust _not_ in the reactor core, but in the throat.

(It's sort of obvious, but - of course it's not intended to run reactor colder, but to run it with higher power level at the same temperature as comparable "pure" NTR, and gain higher Isp)

[Marcus]

Sounds very interesting. I'll go do some research on the "nuclear pumped laser" and then maybe I can offer a more educated opinion.

[Olga]

Hi to all. Iinterested in what's going on in Russia concerning "laser to deflect asteroids"? Then follow http://rufund.ru/Docs/laser/Doclad_laser_ENG.zip

[Suzy]

Is that document actually written by Sergei Krikalyov?!

[Olga]

It is written by him in Russian. The Russian version is presented on the same site.