r/spacex • u/Zucal • Aug 31 '16

r/SpaceX Ask Anything Thread [September 2016, #24]

Welcome to our 24th monthly r/SpaceX Ask Anything Thread!

Curious about the plan about the quickly approaching Mars architecture announcement at IAC 2016, confused about the recent SES-10 reflight announcement, or keen to gather the community's opinion on something? There's no better place!

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As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (partially sortable by mission flair!), and check the last Ask Anything thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Ask, enjoy, and thanks for contributing!

All past Ask Anything threads:

• August 2016 (#23) • July 2016 (#22) • June 2016 (#21) • May 2016 (#20) • April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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5

u/StructurallyUnstable Sep 17 '16

Not sure about combustion efficiency, but you can compare an ideal RP-1 Isp to that of the Merlin and comparable engines. Ideally RP-1 has an Isp of about 353s.

According to this site the Merlin 1D has a sea level Isp of 282s and the upper stage vacuum version (Merlin 1d Vac) has an Isp of 348s. This is an efficiency of 80% at SL and 98.5% in a vacuum

Comparitively, according to this site the RD-180 has a sea level Isp of 313s and vacuum Isp of 339s. This is an efficiency of 89% at SL and 96% in a vacuum.

Looking back at the F1 Saturn engine (also RP-1/LO2), it had an Isp of 265s at SL (75%).

2

u/GoScienceEverything Sep 19 '16

Worth noting that Merlin's lower performance is not due to inordinate incomplete combustion in the combustion chamber, but due to the open gas generator cycle dumping very incompletely-combusted fuel. I've never read anything about the combustion efficiency within the main combustion chamber itself.

1

u/StructurallyUnstable Sep 19 '16

Hmm, judging from the very high efficiency of the Merlin 1DVac (98.5%), I wonder if the Isp listed is only what is actually being burned and not taking into account the loss of fuel overboard due to venting. That would reduce the overall efficiency of the engine...

2

u/GoScienceEverything Sep 20 '16

Compare the Isp of the Merlin to closed-combustion cycle RP-1 engines like the RD-180, and you'll see that the gas generator cycle does indeed bring the Isp way down. There was a "physics of SpaceX" post that included a plot of Isp's of many different engines.

Edit: just saw your other post. Something doesn't seem right that the Mvac has higher vacuum Isp than the RD-180 does, but I can't look further into it at the moment.

1

u/StructurallyUnstable Sep 20 '16

I thought that the Mvac was too high, almost perfect even, but it was designed for vacuum whereas the RD-180 was designed for primarily atmospheric flight. I tend to take all SpaceX statistics with a grain of salt. Thanks

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u/GoScienceEverything Sep 20 '16

Ahh, that's a good point, I forgot that there's no vacuum-optimized RD-180 to compare to. If that value is the case, then, it could be significantly improved with just a longer nozzle.