Re: VTVL?

In article <i6hri.21144$j7.379657@xxxxxxxxxxxxxx>,
Russell Wallace <russell.no.spam@xxxxxxxxx> wrote:

If the first stage has to carry jet engines, wings and landing gear
anyway, why not use them for takeoff and climb to whatever height and
speed you can get to on jet engines?

Because jet engines have a small thrust to weight ratio compared to
rockets. At takeoff the vehicle is full of fuel, so the 1st stage
weighs roughly ten times what it does when landing. The wings and jet
engines required for horizontally launching such a heavy thing are far
larger than those needed to land the same 1st stage when it is empty.
Much bigger wings mean much larger aerodynamic stresses during ascent.

Note also that if an orbiter launches in one orientation and lands in
another it needs internal structure to withstand stress along multiple
axes. That makes it heavier than one which only moves along one axis.
Heavier is not so bad because that just requires making the 1st stage
larger, and fuel is cheap; but worse, it also makes the orbiter denser,
meaning that its re-entry heating and terminal speed also increase.


How would the vertical reentry and landing work? No wings on the
orbiter, so does it just drop in free fall, upright orientation, and
turn the engines on at the last moment or what?

Exactly. The orbiter is mostly empty tanks when re-entering and
landing, so both the peak heating and free-fall speed are not too bad
(easily less than 100 meters per second). The notion of starting the
engines just before landing gives some people the willies, but that's
just because of the unfamiliarity of the idea. Remember that every
commercial aircraft has systems which have to work perfectly in the last
minute to avoid crashing. Similarly, because these launch vehicles are
reuseable, the design will have been tested hundreds of times before
entering service, and each flight article will have been tested several
times at least to prove that there were no manufacturing errors.

There might also be useful applications for operating either stage by
itself, as a sub-orbital vehicle.
.

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