"johnhare" writes:

Airbreathing is so desirable that substantial performance penalties
are going to be overlooked in order to incorporate them.

I very much question this claim --- especially for VTVL.

Air-breathing T/W ratios are so wimpy that they almost always force wings
and horizontal lift-off in the final analysis, since the engine cannot
lift the weight of the fully loaded vehicle. You can dream all you want
about air-breathing engines with a T/W ratio of "43 to 75," but I very much
doubt that you or _anyone_ will be shipping one any time soon !!!

Furthermore, you appear to have made the common false assumption that
air-breathing performance is independent of airspeed. In point of fact,
the effective I_sp of an air-breathing engine is roughly inversely
proportional to airspeed above roughly Mach 1, so that at Mach 6,
the effective I_sp of an air-breather is only a few times better
than a rocket burning the same fuel. And since you are also assuming
water _AND_ LOX injection, you must include these in your propellant input,
so that your effective I_sp is even further degraded. At this point,
your engine is starting to look more like a bad rocket than a
air-breather. And as Henry Spencer has pointed out many times
in this newsgroup, when a careful performance analysis is done,
one usually finds in the end that it is better to build a good rocket
that can double as a bad air-breather than an air-breather that can double
as a bad rocket.

Finally, since most of the propellant will still be consumed after
air-breathing has become useless, unless you go to two stages or otherwise
drop off your fancy air-breathing engines when you reach Mach 6 or so,
all that heavy turbomachinery becomes so much useless dead mass for most
of the trajectory to orbit.

In summary, I continue to remain unconvinced that air-breathing is even the
_least_ bit desirable for anything except possibly the first stage of TSTO.
Furthermore, the claim that air-breathers can achieve a T/W exceeding 40,
and will be useful for VTVL makes me fall down and roll on the floor,
laughing my head off...


-- Gordon D. Pusch

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