Author Topic: July 28 SpaceX presentations: Merlin 2, Falcon HLVs, Raptor, methane Merlin, etc  (Read 218201 times)

Offline Damon Hill

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If I read the flow diagram correctly, the Raptor is a staged combustion engine--it'd have to be to get such a high Isp even with a fully-expanded nozzle.  Thus, like the elegant RL10 and (most?) other expander cycle engines, it is a closed-cycle engine in which the entire propellant mass passes through the combustion chamber/throat/expansion nozzle to achieve the maximum possible exhaust velocity, rather than dumping the gas generator's working fluid out a low pressure duct.

Raptor is different from expander cycle engines in that its hot gas generator/turbopumps are apparently more powerful than an expander cycle's "warm gas" heat exchanger can be, and as such it may be the first vacuum-optimized staged-combustion hydro/lox engine.  Hence the very high Isp >and< high thrust.

There have been efforts to design very high Isp hydro/lox engines, but they had limited thrust apparently as a result of the limited energy that could be extracted from absorbing heat from the combustion chamber and exhaust nozzle.

I'm guessing Raptor is just another one of those optimized compromises; a higher Isp might have been possible via much higher SSME-type pressure and temps, but that gets difficult and expensive fast.  Here's hoping they can get that 470 sec performance figure.  It won't be any less of a design challenge.
« Last Edit: 07/31/2010 06:44 AM by Damon Hill »

Offline alexw

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I'm guessing Raptor is just another one of those optimized compromises; a higher Isp might have been possible via much higher SSME-type pressure and temps, but that gets difficult and expensive fast.  Here's hoping they can get that 470 sec performance figure.  It won't be any less of a design challenge.
     Staged-combustion in an upper stage (pure vacuum bell) seems quite rare, but the isp gain is impressive:
   NK-43:      346 sec / 395 klbs
   RD-0124:  359 sec / 66 klbs
   compare to something like RD-180/191/ at 338 isp (vac), or
   compare to Merlin Vacuum at 342 sec / 92klbs
   (most any other gas generator kerolox has vac isp in the 320's or so, no?)

    Are there any other staged-combustion upper stage engines? Hypergolic? Looking at LE5 (450sec) and HM7B (444sec), picking up 20-25 seconds by going gas generator -> staged combustion looks, uhh, not completely crazy ... but this is hardly a valid extrapolation process.
 
   They must have some reason for believing that it's easier to develop staged combustion than to develop a big expander cycle (RL-60, Vinci) and use multiple engines to get the thrust they need. (Although there's a reference somewhere arguing that ~60klbs need not in fact be an upper limit on expander motors). Sounds gutsy, regardless. If it was easy, wouldn't J-2X have gone that route?
    -Alex
   

Offline MP99

Also, anyone see how they're planning on Raptor having a 470s vacuum Isp? That's a *very* tall claim, which I'm likely to call Incorrect on unless they've got some very good numbers or hotfire tests to back it up. SSME is 453s vacuum, by comparison.

I agree that 470s is a tall claim, but remember that the SSME is a ground-lit engine, which generally means the vacuum Isp is less than an equivalent air- or vacuum-lit engine. It still gets good vacuum Isp only because its chamber pressure is so high. If you put a big bell on the SSME, you'd get somewhat better Isp than 453s (perhaps around 470s), but it wouldn't work at sea level anymore.

At 250:1, Raptor is planned to have well over 3x the expansion ratio of SSME.

cheers, Martin

Offline rklaehn

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Repost from another thread:

Here is the presentation as a pdf, so everyone can read it (converted with open office).

The conversion is not perfect, but better than nothing if you don't want to install a pptx viewer.
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Offline madscientist197

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Points:
* They're claiming that Merlin 2 will have a T/W of 150! The highest T/W of any engine (that I aware of) is the NK33 with ~136. The F1 was only 94.
* Three Merlin 2s at 70% throttle for engine out capability.
* WTF is with the dead sea scrolls reference?
« Last Edit: 07/31/2010 09:22 AM by madscientist197 »
John

Offline Nascent Ascent

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It's nice to have something to look forward to... for a change.
“Why should we send people into space when we have kids in the U.S. that can’t read”. - Barack Obama

Offline rklaehn

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It's also interesting that they're sticking with gas generator cycle for the Merlin 2 instead of staged combustion.

Not surprised one bit about this. A simpler engine cycle plus it solves the problem of having a separate roll control system when it's used as drop in replacement for F9 first stage (look at the diagram in the 2nd post).

I am not sure if they are planning to use the gas generator exhaust for roll control. It looks like they plan to feed it into the engine bell like the f1 did. They could have two different bell designs for the 1 engine and the 3 engine config, but that would probably not be a good idea.
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Offline rklaehn

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* WTF is with the dead sea scrolls reference?

It's a joke. Black water (=oil) shall elevate thy children (=mankind) to the heavens (=space). But I don't get the other part. Probably because I am not familiar enough with ancient hebrew units of currency and measurement :-)
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Offline MP99

It's a joke. Black water (=oil) shall elevate thy children (=mankind) to the heavens (=space). But I don't get the other part. Probably because I am not familiar enough with ancient hebrew units of currency and measurement :-)

Quote
nor shalt thou burn rocks

Rock = coal = solid fuel = SRB's.

cheers, Martin
« Last Edit: 07/31/2010 09:54 AM by MP99 »

Offline marsavian

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The greatest sin of the budget debate will be choosing a no-competition rocket.  Crushing the ambition and capabilities of such unique companies (like ULA and SpaceX) will be a real sin.  But the demand for "job creation" is stronger than just about anything else.

Or maybe they want something derived from a vehicle that already works and is minimum risk, cost and schedule to develop but of course others can see what best fits their world view. So why is SpaceX developing HLVs beyond Saturn V when we are told incessantly by the NuSpace crowd they are not necessary ? Also why would the DoD develop a 1.7mlbf RP-1 engine now if SpaceX are already doing this and NASA won't be doing one now ?
« Last Edit: 07/31/2010 11:17 AM by marsavian »

Offline mrhuggy

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Instead of transporting big 6-10m stages from Hawthorn to Cape. How about building the the tanks at the Cape, engines and avionics at Hawthorn and doing the final assembly there.

Imagine the jobs that would be created and the happy politicians.
Chris Hugman
mr.huggy.net

Offline rklaehn

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The greatest sin of the budget debate will be choosing a no-competition rocket.  Crushing the ambition and capabilities of such unique companies (like ULA and SpaceX) will be a real sin.  But the demand for "job creation" is stronger than just about anything else.

Or maybe they want something derived from a vehicle that already works and is minimum risk, cost and schedule to develop

You mean the altas V? Ares V is a completely new launch vehicle where the biggest commonality with shuttle is the color scheme.
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Offline ugordan

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I am not sure if they are planning to use the gas generator exhaust for roll control. It looks like they plan to feed it into the engine bell like the f1 did.

Hmmm, looks like you're right, judging also from the engine flow diagram.

I found their comparison of a RS-84 vs. a GG engine performance mildly shocking. Does the same performance in the end really sound plausible?
« Last Edit: 07/31/2010 12:10 PM by ugordan »

Offline ugordan

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So why is SpaceX developing HLVs beyond Saturn V when we are told incessantly by the NuSpace crowd they are not necessary ? Also why would the DoD develop a 1.7mlbf RP-1 engine now if SpaceX are already doing this and NASA won't be doing one now ?

A proposed evolution plan and actually developing are two very different things. Which part of the presentations indicated to you they actually are developing any of this?

Offline marsavian

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The greatest sin of the budget debate will be choosing a no-competition rocket.  Crushing the ambition and capabilities of such unique companies (like ULA and SpaceX) will be a real sin.  But the demand for "job creation" is stronger than just about anything else.

Or maybe they want something derived from a vehicle that already works and is minimum risk, cost and schedule to develop

You mean the altas V? Ares V is a completely new launch vehicle where the biggest commonality with shuttle is the color scheme.

A SD-HLV will be cheaper and quicker to the 75-130 tons requirement than any Atlas V derivative.

Offline marsavian

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So why is SpaceX developing HLVs beyond Saturn V when we are told incessantly by the NuSpace crowd they are not necessary ? Also why would the DoD develop a 1.7mlbf RP-1 engine now if SpaceX are already doing this and NASA won't be doing one now ?

A proposed evolution plan and actually developing are two very different things. Which part of the presentations indicated to you they actually are developing any of this?

SpaceX are not going to stop at Falcon 9 (Heavy) if they can help it. They would not be spending development time and money now doing research if that was their intention. On page 3 of the Propulsion presentation there is the title 'Near-term Propulsion Needs'. Clearly the case for BEO exploration/exploitation with only 30mT vehicles has not convinced Elon Musk ;).
« Last Edit: 07/31/2010 01:49 PM by marsavian »

Offline mike robel

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So just what makes Saturn V class launch vehicles sustainable in these peoples eyes while everyone else says they are not and that we could never have sustained them in the first place?

Offline rklaehn

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A SD-HLV will be cheaper and quicker to the 75-130 tons requirement than any Atlas V derivative.

If NASA was actually willing to build a genuinely shuttle-derived vehicle like DIRECT, it might be quicker (though probably not cheaper). But there is little evidence that MSFC will not just continue building Ares I/V if given the chance. I think that MSFC is institutionally incapable of building a working launch vehicle, let alone a cheap one.

But this is getting too off-topic for this thread.
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Offline ugordan

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SpaceX are not going to stop at Falcon 9 (Heavy) if they can help it.

The available money and reality will do that for them. A Merlin 2 is projected to take more than the company has spent in total up until now and cost $50M a piece and it would be a replacement for the 9 Merlin 1Ds. That's almost the price of the entire F9 booster right there. Despite these dreams of heavy lift, F9 will be their commercial revenue generator for quite some time to come. So what's the business incentive for making that vehicle more expensive?
« Last Edit: 07/31/2010 01:58 PM by ugordan »

Offline rklaehn

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So just what makes Saturn V class launch vehicles sustainable in these peoples eyes while everyone else says they are not and that we could never have sustained them in the first place?

What is so difficult about this? Once EELV class launch vehicles no longer have sufficient capacity to satisfy demand, you build a bigger one. But not before.

Currently all existing EELV infrastructure is underutilized. It would be insane to build a saturn V class launch vehicle before maxing out the existing capacity. And that is exactly what NASA proposed to do with constellation.
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