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Old October 9 2012, 02:08 AM   #212
Location: Kentucky
Re: Envisioning the world of 2100

publiusr wrote: View Post
They still have ULA to worry about--and in the same way they went after Ares Constellation--they are going after Musk. The October 1 2012 issue of Aviation week has a cover story on Dream Chaser with loads of private spaceflight coverage. Sadly, there was a nasty little op-ed piece on page 10 called "FALCON 9 CALLED INTO QUESTION." I believe this was the same guy who also called RS-68 inefficient. He called Falcon aerodynamically unstable--which I don't buy--then fusses about thousands of pounds of unused kerosene due to the engines 2.2 mixture ration when 3.45 would be better. The fuel rich mixture allows for cheap engines.
Um, that's idiocy nested several levels deep. For one, the design mixture ratio doesn't have any effect on residual fuel because you don't load more than you need, and you design the tanks to hold what you'll load.

The F-1 and RS-27 on the Delta and Atlas used the same mixture ratio as the Merlin, about 2.2:1, which gives close to the optimal specific impulse for most pressure ratios but with a much lower nozzle exit tempertaure (for 1000 psi sea-level, 1900K at 2.2:1 vs. 2300K at 2.5:1 vs. 2500K at 2.7:1) and a lower chamber temperature (3550K at 2.2:1 vs 3700K at 2.7:1). I don't know that anyone has ever even tried running at a 3.45:1 mixture ratio because the specific impulse would be significantly lower (280 vs 300, for example) as would the exhaust velocity. Those are guestimates because I've never seen an RP-1 mixture chart that went past a mixture ratio of 3:1.

The RD-180 uses a mixture ratio of 2.7:1, but it makes the pre-burner difficult to make because it has to run in a high temperature oxidizing environment.
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