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#60
by
gospacex
on 21 Jan, 2011 20:45
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IMEO, thrust to weight of an engine is a dumb metric.
It would be stupid to compare engines by T/W without looking at the fuel, then yes. For example, all LH engines will look "worse" than RP-1 ones.
If you do consider the kind of fuel used, it is a completely valid metric, because weight of the engine eats into payload.
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#61
by
MP99
on 21 Jan, 2011 22:00
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IMEO, thrust to weight of an engine is a dumb metric.
It would be stupid to compare engines by T/W without looking at the fuel, then yes. For example, all LH engines will look "worse" than RP-1 ones.
If you do consider the kind of fuel used, it is a completely valid metric, because weight of the engine eats into payload.
If Isp & thrust (ie gravity losses) are the same, then that would be true for an upper stage engine. However, I'd suspect that the additional mass would often be associated with improved performance (Isp or Thrust).
Even if Isp & thrust are the same for a 1st stage engine, mass of the 1st stage (and therefore of the engine) is certainly less than 1:1 with payload to orbit. I suspect a small additional engine mass balanced by a small improvement to Isp or thrust may deliver better performance overall. Especially with a 1st stage that stages relatively early (eg Kerolox 1st stage with high thrust Hydrolox 2nd stage).
cheers, Martin
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#62
by
Antares
on 22 Jan, 2011 01:24
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Dry mass is dry mass. Within a stage I don't care if it's tank, engine or computer. It all counts the same.
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#63
by
Propforce
on 25 Jan, 2011 17:14
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Dry mass is dry mass. Within a stage I don't care if it's tank, engine or computer. It all counts the same.
OH SACRILEGE !!
Now listen here <pulling up pants>, don't you realize that engine is KING in the launch vehicle business? The rest of vehicle is just a flying engine test stand, and payload is just an excuse so someone will pay us to fly engines.
How dare you to relate dead structures & electrons to a LIVING, BREATHING MACHINE !
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#64
by
john smith 19
on 18 Sep, 2011 18:44
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I'll note that if you have an Oxidizer and a Fuel rich pre-burner each can drive a matching turbopump with *no* concerns about a seal failure let the drive gases mix with the pump contents.
This is not the case with the SSME where there are 3 seals between the turbine and the HPOTP and 2 chambers with separate venting and monitoring.
During testing it was found the planned volume of purge helium in these inter seal cavities had to be raised nearly 5x to ensure no hazardous mixing.
Going to dual fuel rich/ox rich pre-burners would eliminate a *major* failure mode.
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#65
by
baldusi
on 21 Sep, 2011 18:31
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I'll note that if you have an Oxidizer and a Fuel rich pre-burner each can drive a matching turbopump with *no* concerns about a seal failure let the drive gases mix with the pump contents.
This is not the case with the SSME where there are 3 seals between the turbine and the HPOTP and 2 chambers with separate venting and monitoring.
During testing it was found the planned volume of purge helium in these inter seal cavities had to be raised nearly 5x to ensure no hazardous mixing.
Going to dual fuel rich/ox rich pre-burners would eliminate a *major* failure mode.
And double some other risks. It's all trade offs by trying to estimate probabilities.
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#66
by
strangequark
on 21 Sep, 2011 18:50
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Going to dual fuel rich/ox rich pre-burners would eliminate a *major* failure mode.
And then you have the entire ox-rich problem to deal with. Engineering rule no. 1: TANSTAAFL.
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#67
by
john smith 19
on 24 Oct, 2011 22:12
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Going to dual fuel rich/ox rich pre-burners would eliminate a *major* failure mode.
And then you have the entire ox-rich problem to deal with. Engineering rule no. 1: TANSTAAFL.
True. Such is life.
But note this.
The seal issue is a *dynamic* problem. Excessive leakage through the seals, insufficient purge pressure are 2 outcomes (with *many* possible root causes leading to them). It all has to work on the vehicle, on the day (and on the SSME *every* day) in a heavy and complex way.
Going to an Ox rich pre-burner driven LOX turbo pump designs out these issues but demands addressing material science issues that have not been properly addressed in the US for *decades*, despite having been successfully solved in the FSU decades ago.
Regarding T/W I'll note H2/O2 engines have the worst T/W of *any* rocket engine not explicitly designed for launch vehicle use.
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#68
by
baldusi
on 21 Nov, 2011 12:07
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I understand that staged combustion give better thermal efficiency to an engine. But I was wondering, if full stage combustion is so clear cut. Or is just an issue of easier turbine sealing and simplified injector design? I mean the injector because I'm assuming it would be an only gas injector against a mixed gas/liquid (SC) or liquid/liquid(GG) injector.
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#69
by
DMeader
on 21 Nov, 2011 23:02
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Full staged as in both the fuel side AND the oxidizer side? I thought oxidizer-rich hot gas was a bad thing... leads to burned engine components, fires, etc.
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#70
by
baldusi
on 22 Nov, 2011 14:25
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Isn't the Integrated Powerhead Demonstrator and the RD-270 that?
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#71
by
DMeader
on 22 Nov, 2011 15:14
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Isn't the Integrated Powerhead Demonstrator and the RD-270 that?
Isn't the RD-270 late sixties-early seventies technology, and I can't find anything less than a couple of years old on the IPD. Were either successful?
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#72
by
Antares
on 25 Nov, 2011 18:55
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The RD-170 and NK-15 families are both oxidizer rich preburners. The ducts just have to be made out of metals that can take supercritical GOx at that temperature. Western technology has only gotten there in the last 15 years and still about TRL 6.
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#73
by
strangequark
on 19 Apr, 2012 00:12
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I understand that staged combustion give better thermal efficiency to an engine. But I was wondering, if full stage combustion is so clear cut. Or is just an issue of easier turbine sealing and simplified injector design? I mean the injector because I'm assuming it would be an only gas injector against a mixed gas/liquid (SC) or liquid/liquid(GG) injector.
I know I'm kind of resurrecting an old thread (and one I started in my younger days), but I found this question while doing a google search on IPD from
another question you asked, baldusi.
Full flow staged combustion is very clear cut from a performance standpoint. The power available to your turbines is linearly proportional to the mass flow you can put through them. Then, the pressure rise your pumps give to your propellant is linearly proportional to power.
So, full flow-->lots of mass flow through turbines-->lots of pumping power-->higher pump output pressure-->higher chamber pressure
For a first stage engine, higher chamber pressure means you can have a larger expansion ratio, while still avoiding flow separation from the nozzle. It also means a smaller throat for the thrust level, which means your nozzle (and engine as a whole) can be smaller for the same expansion ratio.
Lots and lots of performance benefits aside from the turbine seals and injection. However, you are also talking about an engine that may eat you alive in development costs. I think I remember Jon Goff saying that anything over 1500 psi just isn't worth it
.
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#74
by
Roga
on 26 Mar, 2013 05:43
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I understand that staged combustion give better thermal efficiency to an engine. But I was wondering, if full stage combustion is so clear cut. Or is just an issue of easier turbine sealing and simplified injector design? I mean the injector because I'm assuming it would be an only gas injector against a mixed gas/liquid (SC) or liquid/liquid(GG) injector.
Full flow staged combustion is very clear cut from a performance standpoint. The power available to your turbines is linearly proportional to the mass flow you can put through them. Then, the pressure rise your pumps give to your propellant is linearly proportional to power.
So, full flow-->lots of mass flow through turbines-->lots of pumping power-->higher pump output pressure-->higher chamber pressure
For a first stage engine, higher chamber pressure means you can have a larger expansion ratio, while still avoiding flow separation from the nozzle. It also means a smaller throat for the thrust level, which means your nozzle (and engine as a whole) can be smaller for the same expansion ratio.
Most of the benefit is that in a staged combustion engine all your propellant is exhausted at the full chamber pressure. In a gas generator cycle, you can't just dump the hot exhaust into the chamber because its pressure has dropped over the turbine, and you can't dump it upstream of the pumps or you'll flash-boil your propellants in the feed lines. You have to dump it overboard at some small fraction of main chamber pressure.
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#75
by
Hyperion5
on 26 Mar, 2013 22:58
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I understand that staged combustion give better thermal efficiency to an engine. But I was wondering, if full stage combustion is so clear cut. Or is just an issue of easier turbine sealing and simplified injector design? I mean the injector because I'm assuming it would be an only gas injector against a mixed gas/liquid (SC) or liquid/liquid(GG) injector.
I know I'm kind of resurrecting an old thread (and one I started in my younger days), but I found this question while doing a google search on IPD from another question you asked, baldusi.
Your "younger days"? Strangequark, you're too young to be talking like that. It's like when my 33-year old English professor started going on a rant about "when I was your age" back when I was in college.
Full flow staged combustion is very clear cut from a performance standpoint. The power available to your turbines is linearly proportional to the mass flow you can put through them. Then, the pressure rise your pumps give to your propellant is linearly proportional to power.
So, full flow-->lots of mass flow through turbines-->lots of pumping power-->higher pump output pressure-->higher chamber pressure
For a first stage engine, higher chamber pressure means you can have a larger expansion ratio, while still avoiding flow separation from the nozzle. It also means a smaller throat for the thrust level, which means your nozzle (and engine as a whole) can be smaller for the same expansion ratio.
Lots and lots of performance benefits aside from the turbine seals and injection. However, you are also talking about an engine that may eat you alive in development costs. I think I remember Jon Goff saying that anything over 1500 psi just isn't worth it .
I'm curious, but does anyone know how many different forms of staged combustion exist or have been thought up? I can think of at least three off the top of my head.
Twin-preburners (one oxygen rich, one fuel-rich)
Examples: RD-170 family
Single pre-burner & turbopump (runs oxygen-rich or fuel-rich)
NK-33, RD-162, RD-0120
Full-flow staged combustion
Examples??
Does the SSME count as a unique staged combustion cycle due to its resonance chambers?
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#76
by
Jim
on 26 Mar, 2013 23:09
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SSME is two fuel rich preburners
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#77
by
MP99
on 27 Mar, 2013 06:46
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SSME is two fuel rich preburners
Four?
cheers, Martin
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#78
by
Galactic Penguin SST
on 27 Mar, 2013 06:58
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SSME is two fuel rich preburners
Four?
cheers, Martin
Two fuel rich + two LOX rich?
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#79
by
MP99
on 27 Mar, 2013 08:27
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SSME is two fuel rich preburners
Four?
cheers, Martin
Two fuel rich + two LOX rich?
No, now I look again Jim was right.
Two fuel rich burners, with the LP turbo-pumps driven from the pumped output of the associated HP pumps (so two fuel-rich burners, each driving two pumps).
cheers, Martin
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