[AR] Re: SpaceX Single Stage to Orbit
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 28 May 2019 18:21:00 -0400 (EDT)
On Tue, 28 May 2019, Nels Anderson wrote:
... It seems perfectly reasonable that air breathers would have lower
gravity losses. What's not clear to me is that they 1) would be *much*
lower (in the context of a total delta-V budget of 9000+ m/s), and 2)
would not eaten up by larger drag losses.
Note, there are actually two separate issues here: airbreathing engines,
and wing lift. Winged rockets are a reasonable possibility, as witness
Pegasus and various unflown designs like RASV (the Shuttle doesn't count
because it made no attempt to exploit wing lift during ascent). Wingless
airbreathers are also possible in theory, although pretty useless in
practice because the T/W of airbreathing engines is so dismal.
One might think that, other things being equal, wing lift should show a
net gain even with typically-poor hypersonic L/D values, given that they
are still well in excess of 1. Unfortunately, wings incur not only a drag
penalty, but also a mass penalty, especially when they have to be
protected against high-speed heating. That makes the gain much more
arguable, and also rather harder to estimate. You might get some of that
mass back if it lets you reduce your engine thrust and hence engine mass;
you might get more back if you believe you want wings for reentry/landing
anyway. Maybe.
A more subtle issue with both wings and airbreathing engines is that they
tempt you to modify your ascent trajectory to stay in the atmosphere
longer, to exploit them more. That often seems to produce a death spiral
of escalating drag and heat and mass, in which the design never closes.
Henry
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