Variable geometry intakes

How relevant is Mach 2+ performance these days - how relevant was it at
all
?

It was never tactically relevant.


I can imagine high speed being useful when intercepting the odd Foxbat or
two, but otherwise - how often would you require such high speeds ?


An F-4 could theoretically reach launch parameters for a mach 3, 70,000'+
target doing about 1.4 at 36,000. Biggest problem was controlling target
aspect in the horizontal. More speed would have helped some there.

Speed can be useful in minimizing raid penetration and increasing AA missile
LARs. It's also very useful when leaving hostile territory, albeit that's
typically at mid altitudes where Q vice mach is the controlling factor.

R / John

This is the same pedantic question that I've heard for twenty-nine years.

These velocities are a consequence of meeting specific excess power
requirements (P_s). A positive P_s allows an aircraft to accelerate (gain
velocity), sustain G, or climb in altitude, or any of these three. P_s
does
not come free. P_s is computed as:

(Thrust - Drag) * Velocity / Weight

A natural consequence of a fighter's design is speed. The design is a
result
of tradeoffs. The fact that fighters rarely exercise their supersonic
capabilities is not relevent. By reducing the thrust of the engines to
limit
the aircraft speed to M-1.5 or M-1.0, the aircraft's performance is other
realms is sharply limited as well.

Since high speed is a natural consequence of a fighter's design, the USAF
and
USN have taken advantage of it.

Implying that very high speed is a free benefit of high thrust. The F-16
has more thrust but is slower than the F-104, but it's more capable in many
ways. It's not a function of reducing thrust, but rather a function of
optimizing the design for mission-related functions. Mach 2 speed isn't one
of those functions and has been deemphasized (ala F-14B versus F-14A). At
the other extreme, the (highly specialized) SR-71 is a legitimate 3.2 cruise
airplane, yet is severely Q limited.

Probably the best example is the F-18, which has excellent performance
subsonic but rapidly runs into a brick wall above the number (highly
configuration dependent). IMO, too much high speed performance was
sacrificed (high indicated airspeeds are illusive as well), or more
correctly the drag was never really designed out of the F-17, its prototype.
Despite its shortcomings, its a pretty capable airplane, even if it's slower
than many earlier jets with half the thrust.

R / John

On Sun, 11 Apr 2004 06:11:25 -0500, "John Carrier"
wrote:

How relevant is Mach 2+ performance these days - how relevant was it at
all

It was never tactically relevant.

Agreed. But, high speed was a big advertising/PR point during the
Century Series days. I flew two aircraft that were definitely Mach 2
capable, but in 23 years of tactical aviation never went M-2 once.

The parallel might be the horsepower of your sporty car--while the car
might be capable of 155+ MPH, it really won't be done by 99.99% of all
owners. The corollary benefit of good acceleration between 30-75 MPH
is what most users will take advantage of.


I can imagine high speed being useful when intercepting the odd Foxbat or
two, but otherwise - how often would you require such high speeds ?


An F-4 could theoretically reach launch parameters for a mach 3, 70,000'+
target doing about 1.4 at 36,000. Biggest problem was controlling target
aspect in the horizontal. More speed would have helped some there.

When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up. At the high closing speed the interval
between R-max and R-min was brief and waiting to pull until within
range meant the target would be past you before you could fire. Any
angular displacement horizontally would drastically compound the
problem. Virtually impossible to pull enough lead.

Speed can be useful in minimizing raid penetration and increasing AA missile
LARs. It's also very useful when leaving hostile territory, albeit that's
typically at mid altitudes where Q vice mach is the controlling factor.

Once again, you're spot on. Speed in knots is clearly life. Speed in
Mach is propaganda.


Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
Smithsonian Institution Press
ISBN #1-58834-103-8

When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up.

How true. I've run simulations where target aspect just got away (and it
wasn't much to begin with). And even with horizontal geometry solved being
a bit late for the pitch one would never get the nose up fast enough to
center the dot. With Sparrow, it was imperative the missile get off in
medium altitude autopilot gain and with a lead-collision geometry wired at
launch. Any major inflight guidance corrections by the missile would drive
PsubK from fair to non-existent.

R / John

When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up.

How true. I've run simulations where target aspect just got away (and it
wasn't much to begin with). And even with horizontal geometry solved being
a bit late for the pitch one would never get the nose up fast enough to
center the dot. With Sparrow, it was imperative the missile get off in
medium altitude autopilot gain and with a lead-collision geometry wired at
launch. Any major inflight guidance corrections by the missile would drive
PsubK from fair to non-existent.

R / John





Getting scrambled off air defense alert in Korea (Kunsan and Taegu), and
sometimes nothern Japan (Misawa) in the early 70s against high speed targets
was not all that unusual. The tracks were at 50k'+, south or eastbound
headings, usually near the NE corner of S Korea by the DMZ. Presumably
Foxbats. The tracks were usually M 1.5+, always out of N Korea and were, in
all likelihood, just probing of our tactics and reaction times, or else their
own quick reaction alerts protecting Bears transiting the Sea of Japan. GCI put
us on a max performance TO and burner climb to 25k', directly head on with the
track We continued the climb to 35k', started the pull at M 1.2 in a shallow
dive, took the Judy as we started the pull.. The bogeys never came near the
DMZ and almost always broke off just after we started the snapup maneuver.. I
am a little hazy on the details of the intercept geometry but I recall the
setups looked do-able up until we/they broke it off off. Seemed to happen once
a month or so.

Generally on AD alert out of Kunsan you had a good probability of getting
scrambled so it was interesting duty. You didn't often get put against a
potentially hostile track, though.. I suspect we got scrambled to protect
intell assets (EC-121, 130, or Navy ship) operating in the Sea of Japan or the
Yellow Sea as we usually got vectored to an orbit. Usual load was 4 x Aim-7 and
4 x Aim-9 though we carried the gun if the mission was predesignated to protect
drones and or destroy them if the controller lost the link.

We practiced snapup attacks with F-4 targets but at much lower altitudes. An
F-4 target could harely fly at 45k'.with three bags at mil power. I don't
recall those being a particlar challenge.so long as you got head-on early and
only had to snap 10k' to fire. I guess if you train for it several times a
week you got the hang of it..

(SteveM8597) wrote in message ...
When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up.

Prior to transferring to TAC and the F4 I had gone to ADC's
Interceptor Weapons School in the F102A. I also had executed quite a
few passes against a M2.0 B58 - right down the throat. Once in the F4
program I and a couple Gibs explored snapups against a Foxbat using
the F4 simulator. We found that a flyup rather than a snapup was more
successful. In addition we used super-elevation, keeping the steering
dot below dead center of the scope, so that the missile would
approximate a zero-G trajectory. The real-life problem would be
getting into a 180 intercept and M2+ far enough out in front to climb
on up to launch position. Here is where a fan of converging
interceptors would maximize Pk. (Doctrine would be to launch
everything - all the missiles - radars and all-aspect IRs.)
Walt BJ

John Carrier wrote:

When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up.

How true. I've run simulations where target aspect just got away (and it
wasn't much to begin with). And even with horizontal geometry solved being
a bit late for the pitch one would never get the nose up fast enough to
center the dot. With Sparrow, it was imperative the missile get off in
medium altitude autopilot gain and with a lead-collision geometry wired at
launch.

It would still SWAB (Switch-after-boost), though.
With later missiles, your chances actually improved firing
from level (just at/inside Rmax); no snap-up equals no need
to recenter the dot; you're already there.

Any major inflight guidance corrections by the missile would drive
PsubK from fair to non-existent.

Agreed. Our simulator (WCS maintenance; APM-307) flew all
profiles with the aircraft tricked into thinking it was
flying at 44,000 ft, though. It was the one variable we
couldn't change without modifying the hardware...

SNIP

We found that a flyup rather than a snapup was more
successful. In addition we used super-elevation, keeping the steering
dot below dead center of the scope, so that the missile would
approximate a zero-G trajectory. The real-life problem would be
getting into a 180 intercept and M2+ far enough out in front to climb
on up to launch position. Here is where a fan of converging
interceptors would maximize Pk. (Doctrine would be to launch
everything - all the missiles - radars and all-aspect IRs.)
Walt BJ

Works as long as launch altitude is below AIM-7 threshold for autopilot high
gain.

R / John