Augmented NTR Concept

[Excession]

While thinking about the LANTR concept (Lox-Augmented NTR), it occurred to me that I couldn't think of any particular reason why you would have to inject the oxygen downstream of the throat. You have an actual combustion chamber below the reactor core. And then I realized that you might even be able to pump the oxygen through the nuclear reactor as well, which would substantially increase the energy of the propellant even before it burned it burned in the combustion chamber. Both the fuel and oxidizer would pass through the reactor core and be heated to 3,000+ degrees, then combust and reach even higher temperatures before passing out through the nozzle (you would need some sort of film cooling to keep the nozzle from melting). I figure this would give you an NTR with the thrust of a LANTR without reducing the Isp, since the oxidizer would be injected upstream of the throat (and thus experience a higher area ratio) and would be heated by the NTR just like the fuel would be. It would even reduce issues with boiloff and increase the mass fraction attainable by nuclear stages, since you would have much of your propellant be denser, warmer oxygen.

So, I have some questions to help figure out if this is actually plausible:

Would combustion actually occur when the reactants are already at such high temperatures?

If it does, would combustion release as much energy as it does when it occurs at lower temperatures? What sort of temperatures could be attained, and how high would the resulting exhaust velocity be?

Could this work with more complex fuels like methane or ammonia, or would the high temperature break the molecules apart before they were even able to combust?

Is there some huge oversight I'm missing, or is this actually a decent concept?

[Vultur]

Will the reactor survive 3,000 degree oxygen exposure?

[cordwainer]

No combustion would occur before the oxidizer and propellant were heated unless you separate the streams somehow by dividing the reaction chamber. In which case some combustion might occur but only if the annulus where the streams meet provide sufficient pressure and stoichiometric fuel/oxidizer mixing(like the piston chamber of a combustion engine). Increase to Isp would actually be less than mixing in cryogenic oxygen downstream. In the first instance the gases would expand too quickly through the combustion chamber to gain much in the way of thermal expansion from the reactor.(less time spent in the reaction chamber exposed to the reactor) While in the latter case the increase of burning an already heated gas and oxidizer would not provide as much expansion and energy release as adding a cryogenic oxidizer to a stream of hot reactive gas like hydrogen. Also heating an oxidizer to the temperatures inside a nuclear reactor would have caustic effects on the materials inside your reactor and the walls of the reaction chamber. You would have to use some sophisticated materials to safe guard against oxidation. 

[ChrisWilson68]

It's pointless to try to do combustion in the chamber of a NTR.  The nuclear power source is already capable of heating the propellant as much as the designer chooses.  The limitation is the melting point of the walls of the chamber.  The point of LANTR is to throw LOX into the nozzle, after some expansion has taken place.  At that point, there's room to increase the temperature by having combustion happen there.

[john smith 19]

Will the reactor survive 3,000 degree oxygen exposure?

I'd guess that depends on what it's made of. Graphite seems to have been a popular choice and it would have burned to CO2. The pebble bed and pressed disk designs possibly.

But ChrisWilson68 is right. O2 is better left as an after burner and thrust augmentor for when the raw GH2 flow has bled off a chunk of it's heat as thrust.

OP's idea is just not very good. 

[Excession]

I'll try to make a sketch of the concept after class today, because I think some people are getting confused about what exactly I mean.

Will the reactor survive 3,000 degree oxygen exposure?

Given the various proposals for running water in NTRs, I would assume so; at that sort of temperature you can expect a substantial fraction of the water to decompose into hydrogen and oxygen. I imagine that would require some pretty fancy protective coatings on the fuel elements, though...

The limitation is the melting point of the walls of the chamber.

Although the temperature of a solid-core NTR is limited by the melting point of the fuel elements, because the reactor is the heat source for the fuel, the temperature of a chemical engine is not limited by the melting point of the materials because the chamber wall is not the heat source. You can cool it as much as you need to without any significant effect on the combustion occurring in the center of the chamber. The SpaceX Merlin, for example, has copper chamber walls with a melting point of only a third the temperature of the combusting kerolox fuel.

The question here is whether or not combustion would actually occur when the propellants have already been heated to such an extent, not whether you could obtain improved performance if it did.

[R7]

the temperature of a chemical engine is not limited by the melting point of the materials because the chamber wall is not the heat source. You can cool it as much as you need to without any significant effect on the combustion occurring in the center of the chamber.

No, propellant flow rates set by chosen chamber pressure and O/F. You cannot use arbitrary amount of fuel to cool the chamber as much as you need because it's got to go into the chamber, affecting prementioned parameters. Amount of cooling heat sink is therefore limited, so are heat transfer characteristics of chamber materials.

Did some BOTE analysis with RPA. If hydrolox engine uses gaseous propellants heated to 3000K then chamber temperatures goes way above 4000K, depending on chosen Pc and O/F. For 50 bar O/F 1 gives 4250K, O/F 6 gives 4540. Isps quite nice 962s/608s. Tried 95 bar and the numbers were 1/4511K/999s and 6/4802K/630s. Raising Pc above that resulted errors, I guess the program decided that it's getting too close to plasma to give realistic results. For comparison SSME chamber temperature is ~3600K.

Gotta to remember that at these temperatures radiative heating becomes substantial, and it increase with fourth power of temperature. So for instance going from 3600K to 4540K seems like "only" 26% increase but radiative heating increases (1.26)⁴ = ~2.5 times.

[cordwainer]

At 4500K your reactor elements would start to melt and you would end up with a liquid core NTR.

[Excession]

No, propellant flow rates set by chosen chamber pressure and O/F. You cannot use arbitrary amount of fuel to cool the chamber as much as you need because it's got to go into the chamber, affecting prementioned parameters. Amount of cooling heat sink is therefore limited, so are heat transfer characteristics of chamber materials.

Yes, obviously the amount of cooling will be dependent on what your engine cycle lets you get away with. The point was, however, that the performance of a chemical engine is not directly related to the temperature of the chamber wall in the same way that a NTR's performance is to the temperature of the fuel elements.

I imagine that something with this sort of temperature would make heavy use of film cooling on top of the regenerative cooling, which should hopefully insulate the chamber from the worst of it.

Did some BOTE analysis with RPA. If hydrolox engine uses gaseous propellants heated to 3000K then chamber temperatures goes way above 4000K, depending on chosen Pc and O/F. For 50 bar O/F 1 gives 4250K, O/F 6 gives 4540. Isps quite nice 962s/608s. Tried 95 bar and the numbers were 1/4511K/999s and 6/4802K/630s. Raising Pc above that resulted errors, I guess the program decided that it's getting too close to plasma to give realistic results. For comparison SSME chamber temperature is ~3600K.

Wait, RPA lets you set the temperature of the propellants before they enter the chamber? How do you do that? I don't recall seeing an option for that when I've used it in the past.

Very interesting numbers...

At 4500K your reactor elements would start to melt and you would end up with a liquid core NTR.

Yes, which is why you would only combine the propellants after passing them through the reactor. I whipped up this image to show what I mean...

[R7]

I imagine that something with this sort of temperature would make heavy use of film cooling on top of the regenerative cooling, which should hopefully insulate the chamber from the worst of it.

Some fancy 3D printed transpiration cooling might work, perhaps combined with higher temp material than copper. I fear the radiative heating is biggest challenge to deal with.

Wait, RPA lets you set the temperature of the propellants before they enter the chamber? How do you do that? I don't recall seeing an option for that when I've used it in the past.

First when adding propellant have the "complete list of components and products of reaction" option selected, gives you the red options in component list (gaseous H2, O2). When the components are added to the Propellant specification window double-click on the component row at "Temperature" or "Pressure" column to enter an value. Can also edit the preferred unit and mass ratios (for triprop and above combinations) the same way.

Yes, which is why you would only combine the propellants after passing them through the reactor. I whipped up this image to show what I mean...

Nice work!

[lkm]

Does anyone know what the effect would be of running Deuterium augmented with  a Lithium-6 compound through an NTR to try and generate some fusion in the reaction mass increasing T/W?

[R7]

Err .. 'devices' utilizing lithium-deuteride fusion use a fission bomb primary. Rocket chamber temperatures are several orders of magnitude too low to ignite any fission fusion

but back to augmented NTR; to facilitate stable combustion it would need a proper injector, no? How would that be cooled, usually the job of cold propellant.

[Stormbringer]

incidentally Project rho (a Hard Sci-Fi site) has compiled a table of both fictional and proposed and real world propulsion systems and the various NTRs, thier statistics, and diagrams of how they work and differ from each other are in there.

http://www.projectrho.com/public_html/rocket/enginelist.php

[lkm]

Err .. 'devices' utilizing lithium-deuteride fusion use a fission bomb primary. Rocket chamber temperatures are several orders of magnitude too low to ignite any fission

I assume you meant fusion, because there's certainly fission going on in an NTR.
If you put lithium-6 in a reactor you breed Tritium through neutron activation, my question was really would this reaction impart a useful amount of extra energy to the reaction mass and would the  high energy Tritium produce any fusion with Deuterium before it thermalized and if so would it produce a useful amount of it, assuming you replaced the hydrogen in an NTR with Deuterium and then added an appropriate ratio of, say, lithium borohydride.

[R7]

Yes, meant fusion, thanks for the correction.

IANANP but doubt there would be any meaningful energy except from the Li6 fission. If the resulting tritium would immediately continue to do D-T fission in substantial quantities wouldn't nuclear power plants utilize this instead of pondering how to contain >100 million kelvin plasma?

[lkm]

Yes, meant fusion, thanks for the correction.

IANANP but doubt there would be any meaningful energy except from the Li6 fission. If the resulting tritium would immediately continue to do D-T fission in substantial quantities wouldn't nuclear power plants utilize this instead of pondering how to contain >100 million kelvin plasma?

Well there seems to be very little work on hybrid fission-fusion designs or power reactors using hydrogen as coolant so it might not be a natural thought to have, beyond that fusion power reactors require to be able to generate more power than they put in after having been recovered through some thermodynamic cycle while rockets just need to convert power to thrust. Think of the fission reactor as the input power source and the fusion reaction as a more efficient means to impart that power to the reaction mass, not to generate extra power.
It could be, though you're right, probably not, that it creates sufficient fusion to be useful for thrust but useless for power generation. However I really have no idea of the answer either, hence asking the question.
That still leaves the question though of whether using  a lithium based reaction mass would be a good idea, lithium borohydride is actually a pretty good rocket fuel apparently.

[Hanelyp]

Lithium in NTR propellant, exposed to heavy neutron flux, is an interesting proposition.

n0 + Li6 -> T3 + He4 + 4.784 MeV
a lot of energy, but consumes a neutron.

n0 + Li7 -> T3 + He4 + n0 - 2.467 MeV
reduces neutron energy.

[TyMoore]

"...And then I realized that you might even be able to pump the oxygen through the nuclear reactor as well, which would substantially increase the energy of the propellant even before it burned it burned in the combustion chamber..."

Sending oxidizer through the core invites oxidation disaster: the materials used to 'contain' the oxygen will likely oxidize when heated to nearly 4000 degrees...and at the high pressures expected..failure will be the likeliest outcome.

In a fission LANTR, oxygen is injected downstream of the throat in the cooled section of the de Levaal nozzle.

I don't think fast fission of Li6 would be practical as an augmented power source for NTR. Too complicated and added weight and complexity would make it undesirable.

[lkm]

I don't think fast fission of Li6 would be practical as an augmented power source for NTR. Too complicated and added weight and complexity would make it undesirable.

Quite possibly, but the purpose of the suggestion isn't so much to augment power production but to improve heat transference and so reduce the weight for a given engine thrust. There are  two limitations for NTR design, the thermal limit of the reactor and the rate of heat transference from the fuel to the reaction mass, the former is addressed through material choice and the latter by fuel geometry. The Lithium suggestion was an attempt to provide a clever solution to the latter problem, by moving some of the energy production into the reaction mass where it can thermalize quickly with the rest of the gas the overall heat transfer rate should improve allowing for a smaller, lighter, engine. That's the theory anyway. How complicated it is depends on the difficulty of using a lithium based reaction mass.

[RanulfC]

To the OP, note though there have been "proposals" to use water in an atomic rocket it has never been actually PRACTICAL to use water as a reaction mass. It disassociates at far to low a temperature to be practical. The reactor element coating CAN be designed to withstand hot oxygen corrosion but it severly limits your fuel and cladding choices.

Injecting oxygen into your reactor won't work because (as shown in your illustration) two different reactor elements would be required (one for the oxygen and one for the hydrogen) which would not in all likelyhood be either chemically or nuclear compatable. (Different "fuels" different fission rates and all that implies) The exhaust hydrogen stream from any typical NERVA reactor is already in excess of the auto-ignition temperature sufficent to allow the LANTR to work. Heating the oxygen doesn't help the reaction.

Several of the MITEE design reactors were VERY high thermal output so as to further disassociate the H2 into H1 in the reactor which would then recombine in the exhaust for even higher exhaust energy. None were ever built to my knowledge but the theoretical temperatures were high enough the possibilty was there.

Randy