In article ,
BOB URZ writes:


Scott Ferrin wrote:

You could make a case for a Sam being able to knock out any airplane. So why
build any at all?

Assuming B70's were built and upgraded umpteen times by now,
what do you think the results would be with a re-engine of the six pack with a
modern supercruise engine such as a F119?

Bob

Actually I'd be surprised if the F119 powered one wasn't *slower*.
The J93 was designed for Mach 3 and high altitudes. The F119 is not.

I realize that airflow and inlet geometry are critical for a high
mach plane, but what would be different in the guts of the engine?
Did the XB-70 burn a non standard fuel like the SR71?
(I'll over-simplify a bit here - its too late to throw numbers around.)
The biggest issue is the pressure ratio of the compressor. Basically,
there's 2 ways to get a lot of thrust out of a jet engine. You can
compress a lot of air moderately, or a smaller amount of air a lot.
Then you add heat up to the point that the materiels in the turbine
section can still pretty much hang together, and take soem of that
energy out as you turn the turbine/compressor comination.
Then, if you really want to go fast, you add more heat, until you
either can't pump fuel in any faster, or the tailpipe starts to melt.

Engines with higher compression ratios tend to be more fuel efficient.
The problem is, when you're compressing the air, you're heating it
up. The more you compress, the hotter it gets. If the engine's not
moving, it's no big deal. But as the engine is moving, the inlets
supply air compressed by ramming the air into the inlets. The faster
you go, teh more compressed, and hotter, it gets. When the
compressed, hot air comes out of the compressor, into the combustors,
it's then really hot. With a highpressure ratio compressor, that
means that you can't burn much fuel before you'll exceed the turbine's
temperature limits, and you may even exceed the working temperature of
the compressor. A low pressure ratio engine is less efficent by
itself, but at high speeds, taken in combination with the inlet
system, it's more efficient, and develops more thrust. Oh, yeah, one
more thing - as the turbine drives the compressor, it extracts energy
from the hot gas. The higher the compression, the more energy gets
extracted. A lower pressure engine at high speed has more heat energy
after the turbine, going into teh tailpipe, and so requires less fuel
to be added by the afterburner to reach its maximum temperature.
High pressure engens tend to be most efficient around Mach 0.9, and
low pressure engine are most efficient at a much higher speed -
typically in the range of Mach 1.5 or so. The speed at which you
start losing thrust becasue you can't burn enough fuel in the
combustors is also higher. A high pressure engine is still much more
efficent at its best speed, however. With the afterburner operating,
the thrust curves look about the same, increasing as airspeed
increases until the pumps can't feed any more fuel. So - Military
Power cruise speeds will be higher for the low pressure engine, but
the dash speed would be the same no matter what. (Modulo materials
limits in the compressor - the J79 in the F-104 or F-4 is limited to a
maximum Ram Temperature or 100 deg C at the compressor face - that
usually occurs somewhere around Mach 2 at altitude).

Basically, in the 1950s, when the B-70 was being designed, the ideal
high supersonic engine was to have been a large, single-spool turbojet
with a compression ration of about 7-9:1, and a Mass Flow of around
300#(mass)/sec. That pretty much suns up the J93, the DH Gyron, the
Orenda Iroquis, and the MiG-25's powerplants. The greater thrust
from the main gas generator means tht you don't have to be dumping as
much fuel into the Afterburner, so supersonic endurance and range are
improved. Note that the ultimate expression of a supersonic het
engine is the Ramjet, where the whole turbojet section is viewed as a
liability and is chicked out, leaving just an inlet and an
afterburner.

If you're going to build an airplane that will actually be spending
most of its time cruising around Mach 0.9, but you want to have a high
dash speed for short periods, you're much better off going with a high
pressure engine.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster