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Staged Combustion Q&A
by
strangequark
on 04 Sep, 2009 02:29
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Trying to get a clearer understanding on the challenges for staged combustion kerosene engines, and figured I'd toss this out there. I understand the reasons why oxidizer-rich cycles are desirable for a kerolox engine, but I'm curious why the preburner environment is seen as so challenging to choose materials for. I keep mentally comparing to jet engine combustors, which are also run at extremely oxidizer rich ratios, and at similar combustion temperatures. Is it that pure oxygen is that much more corrosive than air at high temp, and if so does anyone know how low the temperature would have to be to use comparable materials? Or is there some other factor at work that I'm not picking up on (likely)? I know this is kind of an obscure question, but I would greatly appreciate any insight that anyone can offer. Failing that, I would really like any pointers for good reading material on the subject.
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#1
by
sdsds
on 04 Sep, 2009 02:51
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Is it that pure oxygen is that much more corrosive than air at high temp[...]?
Yes. For example: if the partial pressure of oxygen is high enough, aluminum will burn. Vigorously.
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#2
by
Danny Dot
on 04 Sep, 2009 03:08
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Is it that pure oxygen is that much more corrosive than air at high temp[...]?
Yes. For example: if the partial pressure of oxygen is high enough, aluminum will burn. Vigorously.
But wouldn't the same be a problem for a hydrogen engine?
Danny Deger
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#3
by
Damon Hill
on 04 Sep, 2009 03:20
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A hydrogen engine typically runs fuel-rich; this is not to say that hot hydrogen doesn't have materials issues. Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
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#4
by
strangequark
on 04 Sep, 2009 04:16
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Yes. For example: if the partial pressure of oxygen is high enough, aluminum will burn. Vigorously.
That does make a fair bit of sense, I guess. So, would the overall static pressure be a factor too? For instance, would air at 5000psi be as bad as oxygen at 1000psi (assuming air is about 20% ox)? Increased pressure increases reaction rate for gas-gas reactions, but I don't know if it does for reactions occuring on solid surfaces too.
A hydrogen engine typically runs fuel-rich; this is not to say that hot hydrogen doesn't have materials issues. Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
Well, that and there's benefits to the power balances for the turbopumps when running oxidizer rich. AIAA has a great book called
Liquid Rocket Thrust Chambers that walks through it, and gives pretty good clarity as to why kerosene should be ox-rich and hydrogen should be fuel-rich. However, it has just about jack regarding the material sciences issues, and I've been really curious.
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#5
by
pm1823
on 04 Sep, 2009 04:32
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Failing that, I would really like any pointers for good reading material on the subject.
http://www.lpre.de/In russian, but it seems for me readable through Google translator.
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#6
by
Damon Hill
on 04 Sep, 2009 06:27
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To me the classic example of extreme is the Russian RD-253, a staged combustion engine that uses corrosive hydrazines and nitrogen tetroxide. I haven't found out much specific about the materials other than a long-standing mention of using zirconium as a lining in the hot sections. Other mention of oxidizer rich engines point out running with oxygen-compatible materials and limiting peak temperatures until it can run relatively fuel-rich in the combustion chamber. It all sounds like a very delicate and dynamic balancing act...
And that just a bit of foreign material getting into the hot sections and scratching the passivated surfaces can lead to a Bad Day At The Launch Pad.
Getting down to specifics seems to involve proprietary details the manufacturers are reluctant to talk about, but there must be a body of literature dealing with the general subject.
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#7
by
Propforce
on 04 Sep, 2009 15:08
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Yes. For example: if the partial pressure of oxygen is high enough, aluminum will burn. Vigorously.
That does make a fair bit of sense, I guess. So, would the overall static pressure be a factor too? For instance, would air at 5000psi be as bad as oxygen at 1000psi (assuming air is about 20% ox)? Increased pressure increases reaction rate for gas-gas reactions, but I don't know if it does for reactions occuring on solid surfaces too.
When you use the word 'static pressure', one assumes it's in the context of supersonic flow, e.g., P-static/ P-total relationship, but I assume you're referring to the partial pressure of O2 in Air?
When compare pure oxygen with air (rocket engine vs. airbreathing engine), one must consider the effect of
concentration of oxygen on materials. One way to understand this is to look at typical storage bottles for pure oxygen (such as in hospital/ transport trucks/ K-bottle, etc.) vs. those for air. The material of construction would be very different.
A hydrogen engine typically runs fuel-rich; this is not to say that hot hydrogen doesn't have materials issues. Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
Well, that and there's benefits to the power balances for the turbopumps when running oxidizer rich. AIAA has a great book called Liquid Rocket Thrust Chambers that walks through it, and gives pretty good clarity as to why kerosene should be ox-rich and hydrogen should be fuel-rich. However, it has just about jack regarding the material sciences issues, and I've been really curious.
You may try to google "oxidizer-rich material compatibility" and gain some insights. I might add that the U.S. and the Russians have very different approaches on how to handle oxygen-rich environment.
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#8
by
Antares
on 04 Sep, 2009 17:33
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High pressure air is going to act somewhat differently than pure oxygen at the same partial pressure. This is because the N2 is going to act as both a chemical buffer and increase the thermal inertia of the system. Both retard reaction (heating) rates and maximum temperature.
I like Tom Flynn's book Cryogenic Engineering. It's generic to all cryo systems, but it's got lots of useful info and a chapter on safety.
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#9
by
pm1823
on 04 Sep, 2009 23:35
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And that just a bit of foreign material getting into the hot sections and scratching the passivated surfaces can lead to a Bad Day At The Launch Pad.
Actually, it must be bigger 0.16х0.16mm. (size of cell in filter before RD-170-derived turbopumps) and new ox-rich turbopumps have not only 'passivated surfaces' as NK-33, but thick inert-material inserts in the most dangerous places, so 'foreign particle' not a such big deal, as killing possible rotor-stator recontacts in transient start/stop modes.
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#10
by
mmeijeri
on 19 May, 2010 12:17
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Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
As an aside: I believe that is why the X-15 used ammonia.
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#11
by
tnphysics
on 19 May, 2010 23:20
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A hydrogen engine typically runs fuel-rich; this is not to say that hot hydrogen doesn't have materials issues. Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
Well, actually LCH
4/LOX could run with a 1:1 MR in the preburner (fuel-rich) without coking. For RP-1 (approximated as poly-CH
2), the minimum is 8:7. In general, one must burn all carbon to CO.
This is based on equilibrium and on the assumption that CO forms before H
2O. In some places in the preburner, soot might form.
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#12
by
gospacex
on 11 Jun, 2010 13:32
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A hydrogen engine typically runs fuel-rich; this is not to say that hot hydrogen doesn't have materials issues. Just about any other chemical propellant in a stage combustion design has to run oxidizer-rich to prevent coking and sooting of internal components.
Well, actually LCH4/LOX could run with a 1:1 MR in the preburner (fuel-rich) without coking. For RP-1 (approximated as poly-CH2), the minimum is 8:7. In general, one must burn all carbon to CO.
This is based on equilibrium and on the assumption that CO forms before H2O. In some places in the preburner, soot might form.
Can ethane or propane be burned stoichiometric or slightly fuel-rich without excessive soot formation?
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#13
by
Antares
on 12 Jun, 2010 03:55
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Suggest looking for a program called Stanjan. It's a freeware combustion solver. You can do your own calculations with it.
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#14
by
Propforce
on 13 Jun, 2010 13:45
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Suggest looking for a program called Stanjan. It's a freeware combustion solver. You can do your own calculations with it.
A *real* engineer does it by hand calculation ! ;-)
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#15
by
Antares
on 14 Jun, 2010 04:47
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Only to cleanse the mental palate or after an EMP
Hand-calcs are for math majors.
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#16
by
strangequark
on 07 Jul, 2010 14:29
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Suggest looking for a program called Stanjan. It's a freeware combustion solver. You can do your own calculations with it.
I've found CEA more user friendly.
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#17
by
TyMoore
on 10 Jul, 2010 15:41
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I believe the Russians used a very high chromium content Stainless Steel (.33 Cr or so) in their oxygen rich cycle engines...if it is passivated first by pickling in pure oxygen in a furnace under pressure, a relatively thick shell of chromium oxide forms at the surfaces exposed to the oxygen.
The chromium oxide layer prevents further oxidation of the internal components exposed to high pressure oxygen-rich gasses.
I believe the environment is too hot and too corrosive for more traditional passivization techniques such as gold plating which is used in the hydrogen rich preburners and turbines of the SSME. Infact, some of the strange discolorations inside the engine bells of fired engines must be a small amount of that gold plating from the high pressure turbopumps.
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#18
by
gin455res
on 11 Nov, 2010 10:15
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Has anyone ever built a full combustion rocket where the propellants are so fuel rich there is no oxidizer left after the 'pre'- burner? LOX/Methane might work well. Instead of trading mass fraction for delivery simplicity as in a pressure-fed, this would be trading isp for decent mass fraction, pump simplicity and pump robustness.
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#19
by
MP99
on 11 Nov, 2010 12:24
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Has anyone ever built a full combustion rocket where the propellants are so fuel rich there is no oxidizer left after the 'pre'- burner? LOX/Methane might work well. Instead of trading mass fraction for delivery simplicity as in a pressure-fed, this would be trading isp for decent mass fraction, pump simplicity and pump robustness.
I belive that "fuel rich" means exactly that - all the oxygen in the pre-burner is consumed, and the exhaust is a mix of combustion products & unburnt fuel. (I guess this means the input is richer than stoichiometric). I believe all US engines operate this way.
Oxidizer-rich would exhaust a mix of combustion products & unused oxidizer. RD-180 (Russian, obviously) operates this way.
cheers, Martin
Edit: "Russion"
Hand-calcs are for math majors.
