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e of pi
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« Reply #435 on: 06/21/2012 03:25 PM » |
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I have a question about how rocket engine chamber pressure improvments relate to potential ISp and thrust improvements. Looking at it, both the compressible and incompressible forms of Bernoulli's princible seem to suggest that exhaust velocity should rise in proportion to the square root of the pressure increase (i.e. if the pressure ratio is 1.5, then the exhaust velocity and ISp should see a ratio of 1.22).
However, looking at some instances of this in the real world, like the Merlin 1C-1D which is supposed to go from 6.77 MPa to 9.7 MPa, which is a ratio of 1.43. My understanding is that this would be an increase of 19% in ISp, but the increase is more like 2%: 304 to 310s vacuum. I see something similar looking at Astronautix numbers for the change from H-1 to RS-27, so clearly I'm not fully accounting for something critical. Is this that I've mis-understood or mis-applied the theory, or are there real world complications that the theory isn't accounting for?
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Antares
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« Reply #436 on: 06/22/2012 03:35 AM » |
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Isp is a weak but non-zero function of chamber pressure. Combustion temperature is a stronger contributor to Isp. I'll see if I can find the equations at work tomorrow and edit this.
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kevin-rf
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« Reply #437 on: 06/22/2012 01:10 PM » |
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(i.e. the Boeing 787's fuel efficiency is mostly due to structure).
On the 787, a big chunk of those efficiency improvements are the engines. It is the engines under performing that have caused the efficiency headaches, not the plane being overweight. The composite structure just reduced mass, allowing higher fuel loads and/or larger payloads. Just look at the 747-8's and the jump it took with the same engines. (btw. This discussion can easily spin out of control, since the 787 was really optimized for long Pacific routes and doesn't offer as great a leap forward (compared to the 767) on shorter routes. Though to be fair, ANA's only complaint to date is the window tint is not dark enough). Of course, like everyone else I can't wait to see the 787-9 with it's laminar tail :-)
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sdsds
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« Reply #438 on: 07/02/2012 12:37 AM » |
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Recently staged-combustion methane has received considerable attention, but for upper stage use how would an expander cycle methane engine compare? Expander cycle works for RL10, arguably the best existing upper stage engine. Is there some property of methane (vs. hydrogen) that makes it less suited for an expander cycle?
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strangequark
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« Reply #439 on: 07/02/2012 05:07 AM » |
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Recently staged-combustion methane has received considerable attention, but for upper stage use how would an expander cycle methane engine compare? Expander cycle works for RL10, arguably the best existing upper stage engine. Is there some property of methane (vs. hydrogen) that makes it less suited for an expander cycle?
An expander will have a lower T/W. The available power is limited, which means expanders have lower chamber pressures (this is part of why they are largely relegated to upper stage use). Methane has a lower specific heat than hydrogen, which makes it less effective at heat transfer. However, it's still been done. Admittedly, I haven't run through the calcs, but I think you'd have to run at a lower pressure, or maybe a faster flow (higher pressure drop) in the nozzle/chamber regen section. The benefit of expander is that it's simple, and the turbine drive gases are as gentle as a kitten on the turbomachinery (RL-10 isn't far from room temperature hydrogen). The downside is you will have a much larger engine for the same thrust level. Now, deviating on that topic somewhat, there has been some work done on a combined expander/staged cycle. The idea is that you still get a substantial amount of your energy from the regen cooling of the nozzle and chamber, but then you dump a little bit of oxidizer into the flow to bump up the temperature to maybe 400-500 Kelvin (As opposed to 900-1200K).
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baldusi
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« Reply #440 on: 07/02/2012 11:41 AM » |
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The Japanese have their bleed expander cycle, which is replacing the GG of the open cycle with the expander, but still dumping the gases after they've passed through the turbine, in an open cycle. Apparently they can take it to something like 1,500kN. I guess it won't have good T/W, then?
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strangequark
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« Reply #441 on: 07/02/2012 04:13 PM » |
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The Japanese have their bleed expander cycle, which is replacing the GG of the open cycle with the expander, but still dumping the gases after they've passed through the turbine, in an open cycle. Apparently they can take it to something like 1,500kN. I guess it won't have good T/W, then?
Do you have a reference for that? I know about the LE-5B, but that's RL-10 class. I have heard about expander bleeds hypothetically going to higher thrust (which makes sense, you can have a much higher pressure ratio across the turbine), but I'm not aware of any high thrust production engines that use expander bleed. As for T/W, LE-5B is about 50, which is in line with RL-10.
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baldusi
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« Reply #442 on: 07/02/2012 04:32 PM » |
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Can't really remember where did I found this papers. I think it was Mitsubishi Heavy Industries Annual Technology discussion, or something like that. Please bear in mind that this is open cycle expander, not close cycle.
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gin455res
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« Reply #443 on: 07/29/2012 08:32 PM » |
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How much of an engine's throttlability is due to nozzle constraints and how much is due to the pump?
And can multi-chambered engines with a single pump, stop the flow to individual chambers to increase throttlability (assuming a pump has a significantly wider power range than a single nozzle can except without the flow separating)?
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Robotbeat
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« Reply #444 on: 07/30/2012 07:40 PM » |
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It's usually the injector design and the chamber design that affect throttleability. The nozzle doesn't have much to do with it.
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MP99
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« Reply #445 on: 07/30/2012 09:09 PM » |
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Not over-expansion?
cheers, Martin
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Fequalsma
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« Reply #446 on: 08/01/2012 01:37 AM » |
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Here's a talk by Al Miller, Boeing director for 787 Technology Integration http://www.uwtv.org/video/player.aspx?mediaid=16215102Most of the 787 performance improvement is due to the powerplant, much more than the structures.
From a systems level too, you might be better off investing your money in improved structures to reduce vehicle inert mass (i.e. the Boeing 787's fuel efficiency is mostly due to structure).
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Robotbeat
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« Reply #447 on: 08/03/2012 05:06 PM » |
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Not over-expansion?
cheers, Martin
Ah, true.
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baldusi
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« Reply #448 on: 10/23/2012 09:44 PM » |
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If the problem of using cooling on the turbo blades is that they suffer too much stress to tolerate the channels. As the size growths, does the law of surface to mass makes it possible to use channels? Or are the effect of strength non scalability bigger?
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Antares
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« Reply #449 on: 10/24/2012 12:52 AM » |
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Resonant frequencies in the turbopumps can also affect throttleability.
Nozzle expansion plays very little into throttleability. You want the engine at full blast anywhere that would be a problem.
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