XPost: rec.arts.sf.written, sci.space.policy, rec.arts.sf.movies
From: fjmccall@gmail.com
Jeff Findley wrote on Mon, 28 May 2018
10:40:55 -0400:
>In article ,
>fjmccall@gmail.com says...
>>
>> "Robert Clark" wrote on Mon, 28 May
>> 2018 07:19:51 -0400:
>>
>> >
>> >With altitude compensation, allowing even first stage nozzles to achieve
the
>> >highest possible vacuum Isp, and using also lightweight structures such as
>> >composite tanks, the payload fraction of an SSTO can be comparable to that
>> >of multi-stage rockets, ca. 3%.
>> >
>>
>> Sorry, but that doesn't follow. If you can build an SSTO using those
>> technologies you can also build a multi-stage rocket using the same
>> technologies. The multi-stage rocket will always do better.
>
>True for lightweight structures. Not so true for altitude compensating
>engines (e.g. aerospike engines and the like).
>
Why not? What prevents the use of such engines on a multi-stage
vehicle once you have them for an SSTO?
>
>For a two stage to orbit vehicle, it's just a lot easier to optimize
>the first stage engines for operation in the atmosphere and optimize the
>upper stage engines for operation in vacuum. This is exactly what
>SpaceX has done with Falcon 9's Merlin engines. The vacuum version of
>Merlin has a different (longer) nozzle design.
>
'Easier'. In other words, not a thing in the world prevents using
altitude compensating nozzles on the first stage other than 'harder'
and 'more expensive'. And you'd get a similar sort of advantage doing
that as you would get on an SSTO, just to a slightly smaller degree
because the first stage doesn't go quite as high.
--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
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