XB-70 vs B-2

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?

"Felger Carbon" wrote
"Scott Ferrin" wrote

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?

The main reason is that Janes' doesn't know. And neither do we.

There's a consensous in the US that very high mach numbers have little
military utility. For all US fighters prior to the F-22, persistence at the
placard Mach number could be measured in (few) minutes. Operations research
from the Vietnam War showed (IIRC) that the number of combat minutes spent
at Mach numbers from 1.0 to 1.2 in the whole war were fewer than about 15
and_no_combat time was logged at Mach numbers over about M1.5. Movable
inlets like the F-4s dominated propulsion maintenance workload and so were
not judged to be worthwhile.

The F-22 designers were told that reduced signatures counted for_lots_more
than did maximum Mach number in afterburner. Maximum Mach number in mil-
power was also counted more heavily than AB Mach number. So the F-119 was
sized with enough airflow to meet the performance requirements in "dry"
operation and the inlet was tuned to operate efficiently in the speed range
from M1 to M2. Any performance above that level is fortuitous but wasn't
required. I suspect that if the designers could trade all performance above
M2 for greater persistence at M1.5, they would.

"Felger Carbon" wrote in message
k.net...
"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?


Maintenance monkeys??? Even I as an Ops type have a great respect for great
job maintenance MEN and WOMEN do under often difficult circumstances and
always long hours. I find your phrase highly insulting.

****ed,

Tex Houston