-
Clarification on Russian Engines
by
drbobguy
on 07 Feb, 2008 21:38
-
Hello all,
My understanding is that many Russian engines (NK-33, RD-170, RD-171, RD-180) run oxidizer-rich preburners. But I'm not sure on the question if the exhaust is also oxidizer-rich.
And is the purpose of running oxidizer-rich preburners only to prevent carbon deposition, or is there some kind of other tradeoff? I assume running oxidizer-rich in the combustion chamber/nozzle is because O2 is lighter weight than most hydrocarbon/oxygen products so you get higher ISP from the lighter molecules (e.g., why H2-LOX engines run fuel-rich).
Does anyone know specifically what metallurgical skills are necessary for the plumbing for the hot oxidizer-rich gas in between the preburner and combustion chamber?
Why was the US so reluctant to follow this path?
Sorry if these are naive questions. If someone could point me to literature on this I'd be very thankful.
I'm also keenly interested on the initial research in the Soviet Union on staged combustion.
-
#1
by
meiza
on 08 Feb, 2008 00:32
-
Actually kerosene rockets exhaust mainly CO and H2O from what I remember offhand - both are lighter than O2 although CO marginally. And you don't want heavy CO2! So run fuel rich.
-
#2
by
drbobguy
on 08 Feb, 2008 02:30
-
Russian rockets do not run fuel rich. It appears I was wrong and that you actually get MUCH higher chamber pressures (around double) by running oxidizer-rich as opposed to fuel-rich in the staged-combustion preburner, for LOX-hydrocarbon (Note I am NOT talking about the combuster/nozzle). For LOX-hydrogen, you're better off running fuel-rich like the SSME.
Does anyone have any good advice to a detailed history as to why the Soviets went for LOX-hydrocarbon and never LOX-H2 in first stages? (Even Zenit/Energiya was LOX-hydrocarbon).
-
#3
by
pm1823
on 08 Feb, 2008 02:55
-
LH2 is a too costly fuel for the first stage.
-
#4
by
drbobguy
on 08 Feb, 2008 03:00
-
The Soviet Union didn't care about costs. LH2 is cheap if you can just order a couple thousand people to make it. I mean, of course economics does come into play, but I suspect that's not the whole issue.
-
#5
by
Jim
on 08 Feb, 2008 03:04
-
drbobguy - 7/2/2008 5:38 PM
Sorry if these are naive questions. If someone could point me to literature on this I'd be very thankful.
I'm also keenly interested on the initial research in the Soviet Union on staged combustion.
Good luck.
Russians hold this close to their chest, they have a version of ITAR
-
#6
by
hop
on 08 Feb, 2008 03:30
-
-
#7
by
simonbp
on 08 Feb, 2008 04:58
-
The problem with (and beauty of) LH2 is that it burns so hot, but is stored so cold (20 K), requiring both high-temp metallurgy and cryogenic fuel pumps. The US had a step up because it spend a bunch of money to develop a hydrogen-powered P&W J-57 jet for the (canceled) successor to the A-12/SR-71. P&W took that technology/experience and applied to make first the RL-10, and then the J-2. The Russians decided instead to invest in improving the RP-1 rockets, and never really deployed LH2 until Energia...
Simon
-
#8
by
pm1823
on 08 Feb, 2008 11:58
-
Historically, from beginning Korolev planed to use LH2 on 2th and 3th stages of N-1, but had to adjust plans to KER-LOX with a lack of time and money to build engines for it and big LH2-plant on Baikonur, also he had a strong disagree with Glushko - which fight for UDMH+NT in all heavy rockets, because this type of engine\fuel is cost effective and also can be stored\used for ICBMs. When Korolev gone, POV of Glushko to 'stinky fuel' won. So, when I said "LH2 is a too costly fuel" this meant summary prise to have it on a first stage, not the prise of pound of LH2. Even now, in the new concept they have plans to use LCH4(liquid methane) on the first stages, but not LH2.
-
#9
by
pippin
on 08 Feb, 2008 13:42
-
drbobguy - 8/2/2008 4:30 AM
1. Russian rockets do not run fuel rich. It appears I was wrong and that you actually get MUCH higher chamber pressures (around double) by running oxidizer-rich as opposed to fuel-rich in the staged-combustion preburner, for LOX-hydrocarbon (Note I am NOT talking about the combuster/nozzle). For LOX-hydrogen, you're better off running fuel-rich like the SSME.
2. Does anyone have any good advice to a detailed history as to why the Soviets went for LOX-hydrocarbon and never LOX-H2 in first stages? (Even Zenit/Energiya was LOX-hydrocarbon).
1. For staged combustion engines it's easy: if you run the preburner fuel rich you get gaseous kerosene which will cause massive coking in your turbines and ruin them right away, so you have to run oxy-rich. This in turn is much more challenging from a materials POV (hot oxygen will burn up almost everything), so you avoid it on LH2, where you don't have the coking issue.
2. Kerolox generally is not a bad idea for first stages (as you can see on Atlas and Saturn 5) because it has a much higher density than LH2 so your first stage can get much smaller, albeit heavier, but that doesn't matter much for a first stage. It does matter for upper stages so LH2 is the better choice here.
-
#10
by
wingod
on 08 Feb, 2008 15:44
-
drbobguy - 7/2/2008 9:30 PM
Russian rockets do not run fuel rich. It appears I was wrong and that you actually get MUCH higher chamber pressures (around double) by running oxidizer-rich as opposed to fuel-rich in the staged-combustion preburner, for LOX-hydrocarbon (Note I am NOT talking about the combuster/nozzle). For LOX-hydrogen, you're better off running fuel-rich like the SSME.
Does anyone have any good advice to a detailed history as to why the Soviets went for LOX-hydrocarbon and never LOX-H2 in first stages? (Even Zenit/Energiya was LOX-hydrocarbon).
It results in a more efficient launch vehicle to have a LOX/RP first stage. The booster is smaller in proportion to the overall system due to the density of the RP fuel. The russians and the Germans and the Lockheed people understand this.
-
#11
by
drbobguy
on 08 Feb, 2008 18:39
-
It results in a more efficient launch vehicle to have a LOX/RP first stage. The booster is smaller in proportion to the overall system due to the density of the RP fuel. The russians and the Germans and the Lockheed people understand this.
I understand there is a tradeoff in density of LH2 (and hence tank/structure volume and weight) and the higher ISP with LH2/LOX due to lighter molecules at higher speeds exiting the nozzle.
1. For staged combustion engines it's easy: if you run the preburner fuel rich you get gaseous kerosene which will cause massive coking in your turbines and ruin them right away, so you have to run oxy-rich. This in turn is much more challenging from a materials POV (hot oxygen will burn up almost everything), so you avoid it on LH2, where you don't have the coking issue.
Actually from what I read last night in
Liquid Rocket Thrust Chambers: Aspects of Modeling, Analysis, and Design this is only part of the reason (coking). The main reason is that with hydrocarbons, you can get much higher chamber pressure by running the precombustion chamber oxidizer-rich. You have to use fuel for cooling, so if you run the pre-burner fuel rich you have the problem of not being able to put much fuel through the preburner.
Maybe my understanding is wrong and someone can clarify.
From the textbook:
For oxygen and kerosene propellants, the energy release of the oxidizer-rich approach is over 400% greater than for the fuel-rich approach, within the same turbine temperature limits. The important fator, however, is the potential for this energy release to be converted into useful turbine work, which is discussed next. pg. 631
Therefore, considering equal system temperatures and peak system pressures, the oxygen-rich cycle provides a chamber pressure that is 87% higher. In practice, this ratio could be even higher because the selection of 20% of the kerosene to cool the fuel-rich cycle main chamber was very aggressive. pg. 632
But for hydrogen, you want to run fuel-rich:
The combined effect of improved energy release and improved turbine work potential more than offsets a moderate increase in required pumpwork, and results in a fuel-rich preference for staged combustion cycles if hydrogen is the fuel. pg. 632
These are completely theoretical thermodynamic and mechanical arguments that ignore materials effects (hydrogen embrittling metal or oxygen eating away at metal or coking).
I do have one last question for someone knowledgable.... in the book cited above, the calculation assumes that fuel sent to cool the chamber can't be used for the preburner. Is there a reason this is true? Why can't fuel be sent to cool the chamber/nozzle and then run through the preburner?
-
#12
by
pippin
on 08 Feb, 2008 18:48
-
Well, they are not completely ignoring them since they assume the same turbine temperature. But you're right, that's a strong point.
-
#13
by
drbobguy
on 08 Feb, 2008 18:54
-
pippin - 8/2/2008 10:48 PM
Well, they are not completely ignoring them since they assume the same turbine temperature. But you're right, that's a strong point.
I don't think you are wrong in saying that coking is a problem if run fuel-rich. I assume the combustion products of RP-1 and LOX in a fuel-rich equilibrates with much more carbon soot (since it's not clean burning). It's just that in this case, you want to run oxidizer-rich both for the coking reason, and because theoretical efficiency is much higher running oxidizer-rich.
Although like I said, this seems to depend on the assumption that fuel sent for reactive cooling cannot be used in the preburner. I don't know why that necessarily the case, but I am quite ignorant on these matters.
To me it is just very interesting that the rocket design approach in the Soviet Union was completely different than in the US, and that this problem of oxidizer-rich hydrocarbon staged combustion engines was deemed to be intractable in the US, whereas the Soviets made it work.
One more question: does staged-combustion become impractical past a certain point? I mean, the pressures are so high that there are limits to tubing diameters, etc. I'd imagine an engine the size of the F-1 could not be made staged as opposed to GG, is that correct?
-
#14
by
edkyle99
on 08 Feb, 2008 18:59
-
wingod - 8/2/2008 10:44 AM
It results in a more efficient launch vehicle to have a LOX/RP first stage. The booster is smaller in proportion to the overall system due to the density of the RP fuel. The russians and the Germans and the Lockheed people understand this.
NASA had this all figured out and successfully demonstrated in 1963-64, 45 years ago. The next-closest thing to a high thrust kerosene first stage is a solid propellant first stage. Thus Ares I.
BTW, the checkerboard tracking pattern on the SA-5 S-IV interstage is the inspiration for the ID image that accompanies my messages. The pattern also appeared on Corporal and Redstone missiles, among others.
- Ed Kyle
-
#15
by
drbobguy
on 08 Feb, 2008 19:47
-
Ed, did NASA have a staged LOX/RP-1 technology demonstrator? Or are you talking about the F-1?
-
#16
by
meiza
on 08 Feb, 2008 23:33
-
edkyle99 - 8/2/2008 7:59 PM
wingod - 8/2/2008 10:44 AM
It results in a more efficient launch vehicle to have a LOX/RP first stage. The booster is smaller in proportion to the overall system due to the density of the RP fuel. The russians and the Germans and the Lockheed people understand this.
NASA had this all figured out and successfully demonstrated in 1963-64, 45 years ago.
I think Saturn I fits under the umbrella of "the Germans" in wingod's comment.
The next-closest thing to a high thrust kerosene first stage is a solid propellant first stage. Thus Ares I.
BTW, the checkerboard tracking pattern on the SA-5 S-IV interstage is the inspiration for the ID image that accompanies my messages. The pattern also appeared on Corporal and Redstone missiles, among others.
- Ed Kyle
-
#17
by
meiza
on 08 Feb, 2008 23:34
-
drbobguy - 8/2/2008 8:47 PM
Ed, did NASA have a staged LOX/RP-1 technology demonstrator? Or are you talking about the F-1?
He meant kerosene for first stage and hydrogen for upper stages. Which the Saturn rockets had...
-
#18
by
edkyle99
on 09 Feb, 2008 00:11
-
meiza - 8/2/2008 6:34 PM
drbobguy - 8/2/2008 8:47 PM
Ed, did NASA have a staged LOX/RP-1 technology demonstrator? Or are you talking about the F-1?
He meant kerosene for first stage and hydrogen for upper stages. Which the Saturn rockets had...
That's right.
Going further, here is the reason that NASA was flying hydrogen upper stages as early as 1963.
http://en.wikipedia.org/wiki/Abe_Silversteinhttp://en.wikipedia.org/wiki/Silverstein_Committeehttp://www.hq.nasa.gov/office/pao/History/SP-4404/ch10-4.htmThis is also the reason that the Soviets did not fly hydrogen stages until the 1980s. The USSR did not have an Abe Silverstein. Even von Braun was averse to the idea of rapid development of hydrogen upper stages. He initially supported a Saturn B, rather than Silverstein's Saturn C. Saturn B would have used a big kerosene second stage. Silverstein's argument for rapid hydrogen development convinced von Braun.
Meanwhile, in the USSR, Glushko resisted hydrogen development to protect his storable propellant methods. Korolev focused on kerosene. Etc. No Abe Silverstein, no hydrogen.
- Ed Kyle
-
#19
by
edkyle99
on 09 Feb, 2008 01:08
-
Upon further consideration, I think that it would have been more correct to say that Glushko opposed hydrogen engine development based on his engineering beliefs.
- Ed Kyle
