Re: SSME vs. RS-68 performance


Brian Perry wrote:
> As a point of interest, this over-expansion is one of the reasons we see
> the "mach cones" in the SSME plumes early in flight...a complex series
> of shock waves and expansion fans brings the flow back to ambient
> pressure, and the cones are a visible artifact of the process.
>

You are correct that the Mach diamonds (or "barrel shocks") visible in
the SSME exhaust is due to the flow being overexpanded.

However, the converse is *not* always true: just because you see Mach
diamonds, it does not imply that the nozzle flow is overexpanded.
UNDERexpanded nozzle flows also generate a similar pattern of shock
waves in the exhaust. This can bee seen in the exhaust of jet fighters
with afterburner on, etc.
--
Andrew J. Higgins Mechanical Engineering Dept.
Associate Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/

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