XPost: rec.arts.sf.written, sci.space.policy, rec.arts.sf.movies
From: jfindley@cinci.nospam.rr.com
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).
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.
Because of the nature of TSTOs, no one has yet to put serious effort
into actually flying altitude compensating engines on an orbital launch
vehicle. This is truly an area where NASA should have funded a
technology demonstrator. X-33 doesn't count because it included
*several* other bleeding edge technology features so the linear
aerospike engine never actually flew so we don't have actual flight data
on it.
Jeff
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