"Gordon D. Pusch" wrote in message
...
"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.

It is somewhat irritating that you snipped a single sentence from
the paragraph to make it seem that my position is far from where
it really is. I pasted the paragraph in below to show the difference.

There are three performance curves on the graph. The low one is
what many people shoot for. Airbreathing is so desirable that
substantial performance penalties are going to be overlooked in
order to incorporate them. The middle curve is where performances
just match. The most difficult one is where not including airbreathers
must be justified in terms of simplicity or cost. I am interested in the
middle curve.

This is my position. I am looking for useful answers to what airbreathers
have to achieve to match rockets. The many people that write papers
and advocate airbreathing at any cost should be keelhauled.

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 !!!

You should have noted that my post suggested that this would be the required
performance to match rockets, not that the 43-75 was feasable in the
forseeable planning horizon.

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.

Are you responding to my post or someone elses? I am comparing
performance usefullness of a subsonic airbreathing system. Supersonic
requires fairly heavy (by comparison) intakes.

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.

This is definately in response to someone else.

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...

My orriginal question stands unaddressed, What performance is required
of an air breathing engine in order to match an all rocket LV performance.
I questioned whether the numbers I have derived for a requirement are
accurate enough for reasonable decisions to be made.
You should note that I also said that T/W of 25 was in reach maybe.

While you are rolling on the floor laughing, care to make a small wager
on the capabilities of a concept demonstrater? Since you are so certain,
you should be willing to offer really good odds.



-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'