Re: SpaceX - Why Not RS-27A?


John Schilling wrote:
> In article <1128621496.931108.236030@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Ed Kyle
> says...
>
> >Russell Wallace wrote:
> >> Ed Kyle wrote:
> >> > McDonnell Douglas (pre-Boeing) chose RS-68 because,
> >> > in the wake of Rocketdyne's STME development work,
> >> > that was the engine most ready to be developed at the
> >> > time. This decision was also made during the 1990s,
> >> > when hydrogen booster engine R&D was all the rage -
> >> > back before they gave Russian Glushko (Energomash)
> >> > engineers passes to get through the Redstone Arsenal
> >> > security checkpoints, into Marshall Space Flight
> >> > Center where they mounted their RD-180 in the big
> >> > Saturn V test stand and blew away the hydrogen myth
> >> > right in front of the red-faced keepers of U.S.
> >> > propulsion knowledge.
>
> >> I'm curious, what was the hydrogen myth that they blew away, and how did
> >> they do so? (I assume RD-180 is a kerosene engine that outperformed some
> >> hydrogen engine in some important figure of merit?)
>
> >The myth was that hydrogen engines could provide the
> >most efficient booster propulsion. RD-180 was the
> >first staged-combustion kerosene booster engine tested
> >at Marshall. It proved to be 15% more efficient than
> >U.S. kerosene engines at sea level, and 11% more
> >efficient in vacuum,
>
> Be careful, here. Stating that the RD-180 has 11-15%
> greater efficiency than US-built *kerosene* engines, does
> nothing to demolish the "myth of hydrogen". First off,
> you need to establish what you mean by "efficiency".

I am using specific impulse as an indication of engine
efficiency. And I should have added a sentence that
read something like, "the hydrogen myth was based on
a comparison of staged combustion cycle hydrogen engine
technology (SSME for example) with existing U.S.
gas generator based kerosene engine performance".

> It
> is not a well and singularly defined term. Then, using
> whatever definition of efficiency you chose, you have to
> compare the RD-180 with *hydrogen* engines, like say the
> RS-68.

Compare the Delta 4 and Atlas 5 core stages. The kerosene
Atlas 5 core stage produces more ideal delta-v than the
hydrogen Delta 4 stage, even when the two are normalized
to gross liftoff weight. The comparison is fair because
the two launch vehicles are designed to use upper stages
with similar propellant loadings.

- Ed Kyle

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