Author Topic: NTR lifter system  (Read 16927 times)

Offline Burninate

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Re: NTR lifter system
« Reply #40 on: 07/04/2014 08:05 am »
For an NTR lifter system from good ol' planet Earth propane or methane would offer better thrust and specific impulse than ammonia and less ullage than hydrogen. Problem would be that the carbon molecules might cause coking problems with prolonged use.
I had the same thought when I saw a table suggesting ~800s ISP for CH4-NTR, versus 1000s for H2-NTR, but this thread seems to be down on the idea;  Low temperature tolerance before dissociation, and the problem that any coking actually moderates neutrons.  I don't think I've seen any criticism detailed enough to conclusively rule it out for planetary exploration - might be the effect is negligible at this scale of usage.  It doesn't exist in a vacuum, however.  When we start to implement serious SEP systems, MAC, and solar-electric sails, inner solar system navigation and outer solar system flybys become much easier.  Outer solar system navigation and return missions still have a niche for NTR/NEP, but we're not really flying those very much in 2014.

For Mars & the belt - it helps, but SEP + methalox kick is pretty effective.
« Last Edit: 07/04/2014 08:08 am by Burninate »

Offline cordwainer

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Re: NTR lifter system
« Reply #41 on: 07/04/2014 08:09 pm »
I suppose you could inject other propellants or an oxidizer to increase dissociation and the coking issue might be an advantage for a recoverable or expendable type booster versus an SSTO vehicle. Having an expendable NTR booster could have advantages and the coking effect could be accelerated toward the end of it's flight regime and used to coat your fuel rods in a radiation absorbent material similar to coatings used in pellet bed reactors, thus allowing safer recovery of the fuel rods when the booster reenters the atmosphere.

Offline Hanelyp

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Re: NTR lifter system
« Reply #42 on: 07/04/2014 09:10 pm »
Quote
...propane or methane would offer better thrust and specific impulse than ammonia...
Ammonia, if heated enough, starts decomposing to H2 and N2, diatomic gases, with a fairly low average molecular weight for the mix.
If propane or methane decompose with heat they give soot and other products generally bad for engine performance.

Ammonia, molecular weight 17.
3 H2 + N2, average molecular weight 8.5.  Further advantage of diatomic gases.
Methane, molecular weight 16.
Methane, thermally decomposed, a mess.

I recall from Ignition! a mention that when running a carbon containing fuel you want at least enough oxygen to convert the carbon to carbon monoxide.  Otherwise you'd get soot or heavier carbon compounds.

Offline RanulfC

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Re: NTR lifter system
« Reply #43 on: 07/07/2014 05:52 pm »
"Coking" of any kind is bad in a reactor, as in a combustion chamber or tubing it blocks flow and causes hot-spots which WILL normally lead to melting and failure of the core material matrix. Because your diassociation is going to occur in the hottest part of the flow (middle of the reactor) you are not going to be able to "add" much in the way of oxidizer because of the impossiblity of having a fuel rod material matrix that can stand BOTH hot "fuel" and "oxidizer" at the same time.

Methane is pretty much out unless you find a way to "economically" (cost AND power wise) to break it down prior to injection into the reactor. (Then route the hydrogen to the reactor and the "carbon" somewhere else, not likely though)

Propane/Methane and LOX however for chemical power is quite feasible and effective, but they aren't good NTR propellants at all.

Burninate; We CAN if we wanted pretty much go anywhere in space inside the asteroid belt with pure chemical rockets with our present technology. It would be more difficult and expensive to do so, but there's no "technical" reason we can't be exploring Mars, Venus, Mercury and the nearer asteroids with the technology and propulsion systems we are using regularly today. We "choose" not to. None of our "exploration" programs have been designed or run with anything but short-range, specific goals as thier focus. This has not changed since the late 50s and early 60s on the run up to the Moon Landings.

We "choose" to continue to pursue this type of "program" to the exclusion of any other type because it allows those in "charge" to commit the minimum amount of support (be it financial, political or resources) to any "goal" and avoid spending or commiting large amounts of "support" (see previous) while avoiding any significant infrastructure or long-term goals that might require continued support that would require commitment to real space exploration. As long as this is the path that is "choosen" then this is the only viable "program" that will be supported. To get things going we NEED every propulsion system working together to give us the best "bang" for our "buck" on a comprehensive and wide ranging exploration program and follow ups. I"m not going to hold by breath however...

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline razorblade75

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Re: NTR lifter system
« Reply #44 on: 07/14/2014 02:41 pm »
it isn't "cooking" in lantr. the oxygen would be added after hydrogen has left the reactor.

Offline RanulfC

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Re: NTR lifter system
« Reply #45 on: 07/14/2014 05:08 pm »
it isn't "cooking" in lantr. the oxygen would be added after hydrogen has left the reactor.

LANTR adds the LOX into the nozzle section where it mixes with and burns with the hot-hydrogen from the reactor, yes. You don't want hot oxygen anywhere near going through your reactor :)

"Coking" on the other hand is what you get when you run hydrocarbons through a reactor where the "hydro" part gets highly heated and tends to seperate from the "carbon" parts which can and do become "deposites" on the inside of your reactor. Much like hot oxygen, this is a "bad" thing to have happen...

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

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