From: henry@spsystems.net
In article ,
Jon Watts wrote:
>What I find confusing here is part of what I thought I understood
>about the design trade-offs in the SSME. I thought the the SSMEs high
>chamber pressure was chosen to allow a high expansion ratio [at] sea
>level...
That was a large part of it. (The remainder was fitting three high-thrust
high-expansion engines into the very cramped space available at the rear
of the orbiter, which called for high pressures to keep the engines
compact.)
> and that a high expansion ratio at sea level was needed to
>improve sea level Isp.
No, the point of a high expansion ratio is high *vacuum* Isp. The SSME
does most of its work in vacuum or near-vacuum. Improving Isp on that end
is far more important, especially since the SRBs are doing most of the
work at sea level. You want the SSMEs to be able to *run* at sea level,
so they can be started before launch, but how well they perform there is
of little importance.
The SSME was originally going to have a telescoping nozzle extension --
deployed at altitude -- to increase its vacuum expansion ratio still
further. The switch to a fixed nozzle saved money and complexity, but did
hurt performance a little.
>However from these data the RS-68 achieves a
>(slightly) higher sea level Isp despite having less than half the
>chamber pressure and less than one third the expansion ratio.
This is partly because the RS-68 doesn't have SRBs to carry it up to
altitude, so its sea-level performance matters more, and the design
optimization was shifted somewhat towards better atmospheric operation.
--
No, the devil isn't in the details. | Henry Spencer
The devil is in the *assumptions*. | henry@spsystems.net
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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