SST: A Half-Baked Idea

Date:         11 Apr 98 02:16:18 
From:         westin*nospam@graphics.cornell.edu (Stephen H. Westin)
Organization: Program of Computer Graphics -- Cornell University
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I know that Concorde has a MGTOW in the range of a 767 (408,000lb.,
according to BA's Web site), though carrying far fewer passengers a
shorter maximum range. I also know that nearly half of that weight is
fuel, not only to maintain a Mach 2 cruise, but just to get the plane
off the ground.

Much of the fuel is burned in the takeoff run, both because the
engines are running inefficiently and because the wing doesn't work
that well at low speeds: it has to get going a lot faster than most
airliners just to get off the ground. I read, in fact, that an aborted
takeoff in Concorde has to dissipate as much energy as one in a 747;
mass is lower, but V1 is higher.

These inefficiencies, of course, are due to compromises between
high-speed performance and low-speed performance. Those pure turbojets
are relatively efficient at Mach 2 and 60,000 feet, but at low speed
and altitude, with afterburners engaged, the specific thrust figures
are pretty low. And the supersonic delta wing is pretty draggy at low
speeds.

I was wondering about using a second aircraft to avoid these
compromises; say, strap an SST to the back of a 747, Shuttle-style,
for takeoff. After attaining a decent airspeed and altitude, the SST
would start engines and unlatch from the carrier plane. Think of it as
a multi-stage rocket: it's not necessary to carry the extra fuel tanks
and big engines all the way to orbit, and upper stages can be
optimized for their high-altitude environment. The big wing and
high-bypass engines of the 747 would work better up to, say, 350
knots, where a specialized high-speed wing and engines would take
over. Perhaps the carrier plane could even be unmanned, and return to
land automatically. Of course, the SST would still land on its own,
but I don't think that uses a lot of fuel anyway.

I realize that this would complicate things a lot, and there would be
big questions of economy, reliability, and safety. But I wonder:

o How much could this extend the range of an SST, as its fuel tanks
  would still be fairly full at the start of cruise? Could Concorde
  become a trans-Pacific aircraft this way, for example?

o Could this actually result in a net fuel savings, despite the
  extra weight and drag of the carrier plane?

o Would the 747 be big enough for the job, or would something bigger
  have to be built? This might be a job for an expanded GE90, for
  example.

o The Concorde/747 combo might be a basis for concrete estimates, but
  as Concorde weighs over twice what the Shuttle Orbiter does, it
  probably wouldn't work. What optimizations could be made with a
  purpose-built 2-plane combo, based on current technology?

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.