Author Topic: Staged Combustion Q&A  (Read 76468 times)

Offline Antares

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Re: Staged Combustion Q&A
« Reply #40 on: 11/14/2010 09:55 pm »
You'd have to ask a Russian about that.

Sure, aircraft engines are, but not rocket engines that I can think of.
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Offline mmeijeri

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Re: Staged Combustion Q&A
« Reply #41 on: 11/14/2010 09:59 pm »
Ah, I thought you were talking about aircraft engines.
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Offline tnphysics

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Re: Staged Combustion Q&A
« Reply #42 on: 11/15/2010 03:18 am »
Tungsten carbide can be sintered, so tungsten blades could be made by this process.

Offline madscientist197

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Re: Staged Combustion Q&A
« Reply #43 on: 11/15/2010 05:06 am »
The problem isn't the temperature at which a particular material melts per-se, the big question is what structural properties does the material have at a particular temperature (yes, I know that W is really hard at room temperature; I have no idea about really high temperatures)? A really good counter example is trying to make a turbine blade out of Niobium just because it has a high melting point.
John

Offline strangequark

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Re: Staged Combustion Q&A
« Reply #44 on: 11/15/2010 05:24 am »
The problem isn't the temperature at which a particular material melts per-se, the big question is what structural properties does the material have at a particular temperature (yes, I know that W is really hard at room temperature; I have no idea about really high temperatures)? A really good counter example is trying to make a turbine blade out of Niobium just because it has a high melting point.

Apparently tungsten has a pretty large drop-off in strength at around 1200C, due to a crystal structure change, and creep can be a big issue for polycrystalline tungsten.

Google Books Search.

Offline tnphysics

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Re: Staged Combustion Q&A
« Reply #45 on: 11/17/2010 11:58 pm »
The problem isn't the temperature at which a particular material melts per-se, the big question is what structural properties does the material have at a particular temperature (yes, I know that W is really hard at room temperature; I have no idea about really high temperatures)? A really good counter example is trying to make a turbine blade out of Niobium just because it has a high melting point.

Apparently tungsten has a pretty large drop-off in strength at around 1200C, due to a crystal structure change, and creep can be a big issue for polycrystalline tungsten.

Google Books Search.

According to the same book (no attack or disrespect intended-your statement is true for most alloys of W), non-sag (= creep resistant) W forms exist.

Offline strangequark

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Re: Staged Combustion Q&A
« Reply #46 on: 11/18/2010 12:04 am »
The problem isn't the temperature at which a particular material melts per-se, the big question is what structural properties does the material have at a particular temperature (yes, I know that W is really hard at room temperature; I have no idea about really high temperatures)? A really good counter example is trying to make a turbine blade out of Niobium just because it has a high melting point.

Apparently tungsten has a pretty large drop-off in strength at around 1200C, due to a crystal structure change, and creep can be a big issue for polycrystalline tungsten.

Google Books Search.

According to the same book (no attack or disrespect intended-your statement is true for most alloys of W), non-sag (= creep resistant) W forms exist.

None taken. But I also took it to imply that those are monocrystalline forms.

Offline tnphysics

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Re: Staged Combustion Q&A
« Reply #47 on: 11/18/2010 02:52 am »
They are long-grained forms.

Offline tnphysics

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Re: Staged Combustion Q&A
« Reply #48 on: 11/28/2010 07:29 pm »
Actually, tungsten would be catastrophically oxidized by H2O at that heat. Oops!

Offline randomly

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Re: Staged Combustion Q&A
« Reply #49 on: 12/24/2010 04:20 am »
Exactly what is it about oxygen rich staged combustion engines that make the Russian engines so efficient?

I understand the advantages of staged combustion, but what is the advantage of the oxygen rich variety?

Is it taking advantage of the phase change of LOX to GOX?

Offline Damon Hill

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Re: Staged Combustion Q&A
« Reply #50 on: 12/24/2010 05:03 am »
Exactly what is it about oxygen rich staged combustion engines that make the Russian engines so efficient?

I understand the advantages of staged combustion, but what is the advantage of the oxygen rich variety?

Is it taking advantage of the phase change of LOX to GOX?

Efficiency is due to being staged-combustion, which allows the entire mass of propellant to be used as high-velocity reaction mass.

Running oxygen rich is a necessity with kerosene fuel, to prevent coking of the turbopumps and ductwork/injectors in a staged combustion scheme.  If anything, I believe the main combustion chamber still runs slightly fuel-rich, because the average molecular weight of the exhaust is a bit lower and hence specific impulse is higher.
Also, to control the peak temperature of the turbopump components; running the mixture ratio at stoichiometric would melt those components.  The actively cooled main combustion chamber is better able to stand those temperatures.

Hydrogen simply approaches these issues from the opposite direction and has a big advantage by running fuel rich of lower molecular weight and thus higher exhaust velocity, equating to a higher specific impulse.
« Last Edit: 12/24/2010 05:14 am by Damon Hill »

Offline butters

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Re: Staged Combustion Q&A
« Reply #51 on: 12/24/2010 05:22 am »
Exactly what is it about oxygen rich staged combustion engines that make the Russian engines so efficient?

I understand the advantages of staged combustion, but what is the advantage of the oxygen rich variety?

Is it taking advantage of the phase change of LOX to GOX?

As I understand it, the rationale for the oxidizer-rich staged-combustion in the Russian kerolox engines is that a fuel-rich preburner would produce soot when using hydrocarbon fuels which may cause problems with downstream valves and injectors.

Fuel-rich preburners are generally preferable for hydrolox staged-combustion. Note that the hydrolox staged-combustion engines for Energia/Buran used fuel-rich preburners.

I don't think that anyone has produced an oxidizer-rich hydrolox powerhead, unless we count the American Integrated Powerhead Demonstrator, which used both fuel-rich and oxidizer-rich preburners in a hydrolox full-flow staged-combustion cycle.

What makes the Russian kerolox engines special by comparison is that the U.S. has never flown a staged-combustion kerolox engine of any kind. The closest America has ever come is RS-84, which was also an oxidizer-rich design.

Offline mmeijeri

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Re: Staged Combustion Q&A
« Reply #52 on: 12/24/2010 02:33 pm »
Isn't another reason for using an oxidiser-rich preburner that you have more power to drive your pumps, which allows higher chamber pressures and thus high thrust at lift-off without sacrificing too much Isp?
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Offline Seer

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Re: Staged Combustion Q&A
« Reply #53 on: 01/19/2011 08:12 pm »
Does anyone know why the rd-180 only has a thrust/weight ratio of 80? The nk-33 has T/W of 136, with half the chamber pressure, in comparison.

Offline tnphysics

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Re: Staged Combustion Q&A
« Reply #54 on: 01/19/2011 08:22 pm »
Exactly what is it about oxygen rich staged combustion engines that make the Russian engines so efficient?

I understand the advantages of staged combustion, but what is the advantage of the oxygen rich variety?

Is it taking advantage of the phase change of LOX to GOX?

Efficiency is due to being staged-combustion, which allows the entire mass of propellant to be used as high-velocity reaction mass.

Running oxygen rich is a necessity with kerosene fuel, to prevent coking of the turbopumps and ductwork/injectors in a staged combustion scheme.  If anything, I believe the main combustion chamber still runs slightly fuel-rich, because the average molecular weight of the exhaust is a bit lower and hence specific impulse is higher.
Also, to control the peak temperature of the turbopump components; running the mixture ratio at stoichiometric would melt those components.  The actively cooled main combustion chamber is better able to stand those temperatures.

Hydrogen simply approaches these issues from the opposite direction and has a big advantage by running fuel rich of lower molecular weight and thus higher exhaust velocity, equating to a higher specific impulse.

A pet peeve: the reason fuel-rich is more efficient is because the smaller molecules release their energy more efficiently at the limited expansion ratios practical.

The decrease in mean molecular weight is more than canceled out by the decrease in temperature.

Also, running fuel-rich makes it FAR easier to avoid burning up the MCC-both by lowering the temp and by making the mixture reducing, thereby preventing oxidation of the walls.

Offline mmeijeri

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Re: Staged Combustion Q&A
« Reply #55 on: 01/19/2011 08:24 pm »
Would coking still be a problem with pure dodecane instead of RP-1, say produced through a Gas To Liquid process?
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Offline Downix

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Re: Staged Combustion Q&A
« Reply #56 on: 01/19/2011 08:38 pm »
Does anyone know why the rd-180 only has a thrust/weight ratio of 80? The nk-33 has T/W of 136, with half the chamber pressure, in comparison.
The RD-180 is a cut down version of a much larger engine, and still has many of the larger-engine components involved, in order to reduce the cost.  This makes it heavier as a result. 

In addition, the NK company normally makes jet turbines.  Their turbopump system reflects this, single shaft.  It came at a cost of rotational physics getting in the way, but if you counter-mount them, the engines cut each other out.  Or if your control system knows how to compensate for the spin.  The reduction in components enables further weight savings. 
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Offline Seer

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Re: Staged Combustion Q&A
« Reply #57 on: 01/19/2011 08:48 pm »
ok, so could the nk-33 raise its chamber pressure to the rd-180's level and have even higher T/W?

A second unrelated question: is the chamber temperature lower for kerosene/lox engines than hydrolox engines assuming the same pressure?

Offline Downix

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Re: Staged Combustion Q&A
« Reply #58 on: 01/19/2011 09:53 pm »
ok, so could the nk-33 raise its chamber pressure to the rd-180's level and have even higher T/W?

A second unrelated question: is the chamber temperature lower for kerosene/lox engines than hydrolox engines assuming the same pressure?
That is part of what Aerojet is doing.  The AJ-26 has higher thrust than the NK-33 on the N-1, which means, yes, better T/W.  But it cannot go as high as the RD-180 without failure.  IIRC, it can handle a 40% boost in pressure at most, but it fatigues the system faster.

And I think the temperature for hydrogen is lower.
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Offline Antares

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Re: Staged Combustion Q&A
« Reply #59 on: 01/21/2011 06:35 pm »
IMEO, thrust to weight of an engine is a dumb metric. It has no bearing on the final goal of getting something to a position and velocity in space. Weight of the whole vehicle and thrust to weight of the whole vehicle are what matter. The parochial metrics of an engine guy aren't for systems thinkers.
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

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