XB-70 vs B-2

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.

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 suppose my point was expanded range or payload using the
F119. Who knows, maybe there is a orphan XB70 made out
of spare parts laying around groom lake somewhere.
Might be an interesting platform to test some of the new high
tech aero spike or pulse engines.

Bob



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On Sat, 20 Dec 2003 15:19:54 -0600, BOB URZ
wrote:



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?

It didn't use special fuel (although they wanted to use a fuel with
boron added at one point). I *think* part of the reason for the
differences would be related to the compression ratio of the engines
and that they were straight turbojets rather than turbofans. The
XB-70, Blackbirds, and Mig-25 all used low pressure engines I *think*
because at high speed they compressed the air so much to get something
to work with that by the time it got to the engines they couldn't take
the temperature of compressing a ton more. ISTR the J93's compression
ratio being around 9 to 1 whereas something like the F100-129 is up
around 32-1 or 36-1. Then again they though the could get the
Crusader III up to 2.9 with a J75 so who knows. All of this is just
my opinion from what I've read over the years so hopefully someone
will weigh in who knows a lot about it.

Scott Ferrin wrote:

It didn't use special fuel (although they wanted to use a fuel with
boron added at one point). I *think* part of the reason for the
differences would be related to the compression ratio of the engines
and that they were straight turbojets rather than turbofans. The
XB-70, Blackbirds, and Mig-25 all used low pressure engines I *think*
because at high speed they compressed the air so much to get something
to work with that by the time it got to the engines they couldn't take
the temperature of compressing a ton more. ISTR the J93's compression
ratio being around 9 to 1 whereas something like the F100-129 is up
around 32-1 or 36-1. Then again they though the could get the
Crusader III up to 2.9 with a J75 so who knows. All of this is just
my opinion from what I've read over the years so hopefully someone
will weigh in who knows a lot about it.

IIRC, the XB-70 engine had bypass ducting similar to the
engines on the SR-71.
--
--Matthew Saroff

I'm not an actor, but I play one on TV.
Check http://www.pobox.com/~msaroff, including The Bad Hair Web Page

"BOB URZ" wrote in message
...


Scott Ferrin wrote:

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?

Hey, it's Xmas. Mebbe I can make another mistake.

For a supersonic aircraft, the purpose of the inlet geometry is to
reduce the supersonic airflow at atmospheric pressure to subsonic
airflow at super-atmospheric pressure. ;-)

This means there's more oxygen to burn more fuel, thus getting more
power. It also runs the engine hotter. The faster the supersonic
aircraft goes, the hotter the engine can run.

This raises the following critical question: how long an engine life
do you want? I understand the Mig-31 Foxhound is _capable_ of
astonishingly high speeds, as it has demonstrated on at least one
occasion in the mideast. It generally doesn't, because an immediate
engine overhaul/replacement is then needed.

The F-119 engined F-22 has fixed inlets and is not especially fast.
The engine, therefore, does _not_ have to be made of the *very*
expensive highest-temperature alloys. My question is, how long would
the F-119 last in a mach3 aircraft like the B-70?

The F-119 engined F-22 has fixed inlets and is not especially fast.

What makes you think fixed inlets make a difference?


The engine, therefore, does _not_ have to be made of the *very*
expensive highest-temperature alloys.


The F119 is made of MUCH better alloys than the J93. I think the
temperature problem is handled by the fact that the J93 compresses the
air much less than an F119. So does the J58 and the engine the Mig-25
uses.


My question is, how long would
the F-119 last in a mach3 aircraft like the B-70?

"Scott Ferrin" wrote in message
...

The F-119 engined F-22 has fixed inlets and is not especially fast.

What makes you think fixed inlets make a difference?

At high mach, the air bounces several successive shock waves off the
inlet, in the process of dropping the airspeed and increasing the air
pressure.

Airplanes such as the F4 Phantom, the Tomcat, F-15, Mig-25 and Mig-31
(among others) have variable inlets specifically so the inlet can be
tuned to the speed for most efficient operation. The SR-71 had a
spike arrangement in front of the engine that performed as a variable
inlet.

Airplanes such as the F-16 and F-22 use fixed inlets, and have lower
top speeds (according to Janes). Fixed inlets are used either to
reduce costs, or (in the case of the F-22) to improve stealth
caracteristics. Variable inlets aren't very stealthy.

On Sun, 21 Dec 2003 22:39:43 GMT, "Felger Carbon"
wrote:

"Scott Ferrin" wrote in message
.. .

The F-119 engined F-22 has fixed inlets and is not especially fast.

What makes you think fixed inlets make a difference?

At high mach, the air bounces several successive shock waves off the
inlet, in the process of dropping the airspeed and increasing the air
pressure.

Airplanes such as the F4 Phantom, the Tomcat, F-15, Mig-25 and Mig-31
(among others) have variable inlets specifically so the inlet can be
tuned to the speed for most efficient operation. The SR-71 had a
spike arrangement in front of the engine that performed as a variable
inlet.

Airplanes such as the F-16 and F-22 use fixed inlets, and have lower
top speeds (according to Janes). Fixed inlets are used either to
reduce costs, or (in the case of the F-22) to improve stealth
caracteristics. Variable inlets aren't very stealthy.



Fixed inlets have been discussed to death here. Basically it comes
down to the speed they were designed for. A fixed inlet can be
optimized for high speed.

"Scott Ferrin" wrote in message
...

Fixed inlets have been discussed to death here. Basically it comes
down to the speed they were designed for. A fixed inlet can be
optimized for high speed.

Since variable inlets are not needed for any purpose, why do so many
jet fighters use them? To give the maintenance monkeys something
further to do?

Why, according to Janes', does the ancient-history F-4 have a higher
top speed than the future-generation F-22?

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