Author Topic: RS-68 - can the Isp be increased?  (Read 40061 times)

Offline yinzer

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Re: RS-68 - can the Isp be increased?
« Reply #40 on: 02/12/2007 08:52 am »
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Propforce - 10/10/2006  11:20 PM
To understand why, you must know the difference between a GG cycle and a SC cycle.  In addition to the pressure compression limit of its turbopumps, the GG cycle is inherent less efficient with its C* efficiency than both the SC cycle and the expander cycle because the MR is inherently higher in the main combustion chamber (MCC) of a GG cycle engine. All these compound to a less efficient engine, but less expensive to build than a SC cycle engine.  It's suitable for a throw-away (expendable) engine.

This makes no sense whatsoever.  The mixture ratio in the main combustion chamber can be set to whatever you want.  You can also run high chamber pressure gas generator cycles - the RS-2200 chamber was going to run at 2200 psia.

The problem with the gas generator cycle is that the propellant used gets dumped overboard (or at least not expanded nearly as much as the primary flow), which hurts your average Isp.  If the main chamber has an Isp of 450 sec, and 2% of the propellant goes through the gas generator and gets dumped overboard at zero Isp (very pessimistic), the Isp will go down 2% to 441 seconds.  The higher the chamber pressure, the more power the pumps have to generate, and the more propellant they need to run themselves, which hurts your Isp, which defeats the purpose of the higher chamber pressure.  The RS-2200 mentioned above used the gas generator exhaust to pressurize the base area, which got back the lost Isp and made the high chamber pressure worthwhile.
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Offline meiza

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Re: RS-68 - can the Isp be increased?
« Reply #41 on: 02/12/2007 12:52 pm »
Thanks yinzer, this is what I have always suspected but nobody has bothered to confirm, instead talking about "inherent differences in GG and SC cycles".

Offline rumble

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Re: RS-68 - can the Isp be increased?
« Reply #42 on: 02/12/2007 01:08 pm »
Wikipedia has some good stuff on this:
Staged combustion cycle:  http://en.wikipedia.org/wiki/Staged_combustion_cycle_%28rocket%29
Gas generator cycle:  http://en.wikipedia.org/wiki/Gas-generator_cycle_%28rocket%29
Expander cycle:  http://en.wikipedia.org/wiki/Expander_cycle_%28rocket%29

Looking at large engines, the F-1 used a gas generator cycle, with top-end isp=304sec.  The RD-171 uses a staged combustion cycle, with top-end isp=337sec.  (per astronautix.com, anyway)

Offline meiza

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Re: RS-68 - can the Isp be increased?
« Reply #43 on: 02/12/2007 01:40 pm »
Yes, I know all this. The original question was why couldn't they make a bigger gas generator and higher pressure pump for RS-68, and nobody gave any answer, instead just a lot of just beating around the bush.
The real answer is, you could do it but you'd dump so much propellant overboard from the GG that you'd hit diminishing returns pretty soon. There's of course the cavitation issue too, but that wasn't handled well either.

Offline TyMoore

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Re: RS-68 - can the Isp be increased?
« Reply #44 on: 02/12/2007 03:40 pm »
I think that what some of the others were getting at is that: sure, you could put a bigger, higher pressure pump on an RS-68. Then you'd have to redesign the injection manifold to handle the increased flow and pressure drop. And then you'd have to redesign the chamber to increase the (I think it's called the Critical Length?) the Volume of the combustion chamber slightly to handle the increased flow. And the nozzle throat would have to be changed (increased slightly.) Then you'd have to re-analyze the acoustics of the engine and redesign the injector baffles to suppress harmful resonances--build a few boilerplate engines and test them on the stand, maybe even blow a few up in the process. The short answer is that it would be a different engine--and I think that's what is trying to be avoided. What I understand about liquid propellant rocket engines, is that they are extremely fickle beasts. Once you get the design just so it works, it's better to not mess with it too much after that...

Atleast, that's my naive, non-professional take on it!

 ;)

Offline JIS

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Re: RS-68 - can the Isp be increased?
« Reply #45 on: 02/12/2007 04:47 pm »
Actually the Merlin and Vulcain did exactly the same what RS-68 r would do. Thrust is increased by substantial 10-20% but ISP by few percent. I think that Dr Stanley estimated an increase of ISP from 410s to 420s at maximum.

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Offline rumble

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Re: RS-68 - can the Isp be increased?
« Reply #46 on: 02/12/2007 05:09 pm »
The RS-68 is trying to do what the SSME does...  works well in a vacuum and at sea level.  

Now, if we weren't concerned about the engine being a good sea-level booster, It should be possible to make a good increase in vacuum isp by changing the area ratio and/or length of the nozzle, but sea-level isp would suffer.  On a vehicle using supplemental sea-level thrust (srb's...a la DIRECT or ARES-V), this may not be a killer, but for the Delta IV, decreasing the sea-level performance would be "bad."

I don't know this...only assuming:  The SSME is more tuned for vacuum operations, and the RS-68 is more tuned for sea-level operations.  But while I assume that's the case, since the SSME is a staged combustion engine, its sea-level isp is still equivalent to the RS-68 sea-level isp--365sec.  It's in a vacuum where the difference is REALLY obvious (407sec vs. 452sec)

Offline meiza

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Re: RS-68 - can the Isp be increased?
« Reply #47 on: 02/12/2007 06:17 pm »
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TyMoore - 12/2/2007  4:40 PM

I think that what some of the others were getting at is that: sure, you could put a bigger, higher pressure pump on an RS-68. Then you'd have to redesign the injection manifold to handle the increased flow and pressure drop. And then you'd have to redesign the chamber to increase the (I think it's called the Critical Length?) the Volume of the combustion chamber slightly to handle the increased flow. And the nozzle throat would have to be changed (increased slightly.) Then you'd have to re-analyze the acoustics of the engine and redesign the injector baffles to suppress harmful resonances--build a few boilerplate engines and test them on the stand, maybe even blow a few up in the process. The short answer is that it would be a different engine--and I think that's what is trying to be avoided. What I understand about liquid propellant rocket engines, is that they are extremely fickle beasts. Once you get the design just so it works, it's better to not mess with it too much after that...

Atleast, that's my naive, non-professional take on it!

 ;)

No. The total flow wouldn't increase. Just the pressure. Keep the engine the same size to avoid making a new engine.

Offline TyMoore

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Re: RS-68 - can the Isp be increased?
« Reply #48 on: 02/13/2007 04:08 am »
...well, how else are you going to increase the chamber pressure? I suppose you can make the injector orfices slightly smaller, and squeeze the volume of the chamber slightly, and also decrease the area of the throat. It's still going to be a different engine--but I'll admit it ought to be closer to the original  than going the other way...[I don't have my Rocket Propulsion Elements, 4th edition infront of me either!]


Offline meiza

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Re: RS-68 - can the Isp be increased?
« Reply #49 on: 02/13/2007 01:23 pm »
Yeah, you are right, many changes would be needed. I don't know how drastic these would be.

Offline bombay

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Re: RS-68 - can the Isp be increased?
« Reply #50 on: 02/14/2007 12:08 am »
For my own edification, what is the theoretical ISP maximum for the RS-68 chemical propellant combinations?
Or in other words, at what point do engine modifications become useless because the combustion affects have peaked?

Offline yinzer

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Re: RS-68 - can the Isp be increased?
« Reply #51 on: 02/14/2007 12:32 am »
The RL10-B2 gets 460-some seconds, and one could imagine an even bigger nozzle.
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Offline Antares

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Re: RS-68 - can the Isp be increased?
« Reply #52 on: 02/14/2007 03:47 am »
RS68 turbopump performance changes will be minimal.
Changing the chamber changes wall integrity.  SSME opened the throat and stopped wall cracks (fuel leaked through them into the chamber).
There's no way a regen nozzle would increase Isp 30-40sec on a GG cycle engine.  Too much flow lost in the GG.
A lot of the changes talked about here result in different engines for Ares V and Delta IV.
Nozzles are sized for a certain altitude depending on chamber pressure.  Any bigger and it will shake apart or collapse.
« Last Edit: 05/27/2010 08:08 pm by Antares »
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.

Offline meiza

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Re: RS-68 - can the Isp be increased?
« Reply #53 on: 02/14/2007 01:32 pm »
Btw, there's some presentations on L2 that are somewhat related to this, about J-2X. :)

Offline publiusr

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Re: RS-68 - can the Isp be increased?
« Reply #54 on: 02/18/2007 09:14 pm »
According to Space News, RS-68 upgrades with have both Air Force and NASA support. Very good.

Offline Jim

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Re: RS-68 - can the Isp be increased?
« Reply #55 on: 02/18/2007 09:30 pm »
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publiusr - 18/2/2007  5:14 PM

According to Space News, RS-68 upgrades with have both Air Force and NASA support. Very good.

They are not the end all source.  A bit of the sky is falling

Offline publiusr

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Re: RS-68 - can the Isp be increased?
« Reply #56 on: 02/18/2007 09:33 pm »
And which bit would that be?

Space News has good people working for it. Are you just angry that I contradicted your pronouncement that Space Based Radar was for ground targets?  And no that note--I see that NRO has at least offered real money for it.

Offline Jim

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Re: RS-68 - can the Isp be increased?
« Reply #57 on: 02/18/2007 10:33 pm »
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publiusr - 18/2/2007  5:33 PM

And which bit would that be?

Space News has good people working for it.

Not quite, It is the National Equirer for the space program

Offline Antares

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Re: RS-68 - can the Isp be increased?
« Reply #58 on: 02/21/2007 04:31 am »
Sorry, Jim.  Space News is right in this case.  And for space, they're probably the best news source (not saying much), way better than AvWeek.

Edit: DUH!.  NSF and its readers are the best news source for space!
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.

Offline Magnus_Redin

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Re: RS-68 - can the Isp be increased?
« Reply #59 on: 02/24/2007 06:59 pm »
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TyMoore - 13/2/2007  6:08 AM

...well, how else are you going to increase the chamber pressure? I suppose you can make the injector orfices slightly smaller, and squeeze the volume of the chamber slightly, and also decrease the area of the throat. It's still going to be a different engine--but I'll admit it ought to be closer to the original  than going the other way...[I don't have my Rocket Propulsion Elements, 4th edition infront of me either!]


Would not slightly smaller injector orifices only increase the preassure loss thru them?
And how would chamber volume affect preassure?
But smaller throat makes sense.
The big change ought to be higher pump preassure.

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