Angle of attack limits on air to air missiles

[totoro]

As we know, all planes have some sort of angle of attack limits. Some are software driven, others are not crossed due to visible effects such as buffeting, loss of control and loss of lift.

F16A for example has its instantaneous load limit set to 1G at mach 0,4; 2G at m 0,5; 4G at m 0,7; 6,5G at m 0,9 and so on. (30k feet, 24k lbs)

But what about missiles? Sparrow? Amraam? Sidewinder? Any missile, really. Are there any graphs showing their AoA limits or even better their G force limits as a function of speed? (at similar air density, meaning 30ish thousand feet?)

 

[riggerrob]

2 issues limit angle of attack:

1. Stalling. However sharply swept leading-edges ab develop vortex-lift even at high angles of attack.

2. Drag/Thrust. High angles of attack also develop high amounts of drag, so you need massive motors to keep the speed up.

 

[Avimimus]

The second issue is probably the most significant for air-to-air missiles. It is common to see design specs quoting '40 gees' or more... but the ability of a missile to sustain a turn without falling out of the air due to losing too much velocity is much less.

I sometimes wonder about the possibility of developing missiles which have lower absolute turn rates, but better sustained turn rates (especially as data links remove some of the sensor issues associated with targeting over long flight times, and more modern motors are better at sustaining energy levels throughout the flight).

 

[totoro]

It is common to see design specs quoting '40 gees' or more... but the ability of a missile to sustain a turn without falling out of the air due to losing too much velocity is much less.

But do we know at least some ballpark figures for any missile out there? So if 40g is marketing punchline for a missile - does that mean it can hit 40 g ONLY at the peak of its velocity? And after the rocket motor burns out, say 20 second after it burns out, the velocity gets halved - does it mean 40 g is unnatainable? How about 30 g? Do we have any graph that would connect G limits with missile's velocity? What would that curve of G limit look like, compared to similar curves for fighter planes?

 

[Bill Walker]

I recently had the chance to review raw and processed data from a program to develop turning charts for an SU-30. With both engines operating, anything over 6G led to a loss of airspeed, and 8+G led to a rapid loss of airspeed, even at optimum speed and altitude. This was acceptable for the SU-30, since the purpose of the manoeuvre is to point the sensors and weapons, not to catch up to and hit the target. High G manoeuvring of a coasting missile would quickly lead to a loss of airspeed, making it much harder to reach the target. I have to assume the 40G manoeuvres people discuss would only be of use for brief periods, under power. Still a brief period at 40G would be all that is needed. The missile doesn't need to do loops.

If I remembered the old equations and units correctly, 40G at Mach 4 equates to 0.365 rad/sec heading change, or a 90 degree turn in just over 4 seconds. The loss in airspeed over this time could be acceptable, if the engine is still running and the missile can resume longitudinal acceleration. At Mach 3 and 40G a 90 degree heading change would only take 3.2 seconds.

Equations can be found at http://aviation.stackexchange.com/questions/23162/for-light-ga-aircraft-is-there-a-standard-turn-and-what-does-that-mean/23164#23164

 

[stealthflanker]

As we know, all planes have some sort of angle of attack limits. Some are software driven, others are not crossed due to visible effects such as buffeting, loss of control and loss of lift.

F16A for example has its instantaneous load limit set to 1G at mach 0,4; 2G at m 0,5; 4G at m 0,7; 6,5G at m 0,9 and so on. (30k feet, 24k lbs)

But what about missiles? Sparrow? Amraam? Sidewinder? Any missile, really. Are there any graphs showing their AoA limits or even better their G force limits as a function of speed? (at similar air density, meaning 30ish thousand feet?)

AoA limit.. hmm Fleeman's tactial missile design book give figures on operational AoA for missile, unfortunately no speed figure given.

For missile though..i think structural consideration dominates the maximum G limit. basically the missile can pull as high as their structure allows. Another consideration is thermal load.

To know how much a missile can pull, one need to do the math itself. I tried doing it by making excel spreadsheet.. the problem is it need the missile's speed and AoA. Which might not be known or static throughout the flight.

 

[totoro]

amraam c5 performance brief, despite it being assembled from public sourced, seems lika a fair enough simulation.

it suggests missile maintaing 10 km altitude starts doing a sustained 30 g turn after its eighth second of flight (right as the rocket motor burns out). Two seconds later it's AoA changes from 15 degrees to 20 degrees. It still sustains a 30 g turn and two seconds after that (total flight time of 12 seconds) AoA reaches 31 degrees. Graph line ceases there though. did the missile lose all energy and enter a spin?

At the same time velocity graph shows 1200 m/s starting speed (as rocket motor burns out), goes to 960 m/s two seconds later, and drops to 700 m/s at 12 seconds of total flight time.

Now, less demanding turns (say 10 g turn) is shown as reaching little over 32 AoA but at that moment missile is going only 400 m/s. That's where the graph line ends.

Would that suggest that for some reason the missile doing 30 g turn somehow becomes uncontrollable at much higher speed, even though it's doing slightly less of AoA maneuver? How is that possible? I am assuming loss of lift is not the culprit here as the AoA is similar, and speed is much higher - there should be no way a 700m/s missile at 31 deg AoA has less lift than 400 m/s missile at 32 deg AoA.

But how come does controllability suffer with greater speed? What actually happens with that 30 g turning missile as it speeds drops below 700 m/s and its AoA skyrockets that is so fundementally different from a missile doing 10 g turn?

 

[Bill Walker]

In "normal" aircraft turning performance analysis there can be several limits to maximum G or turn rate:

Structural load limit
aerodynamic limits (lift surfaces will stall, even at Mach 4, at a high enough AoA. If you have multiple wings/fins and some stall and some don't you will have wild pitch rate changes.)
Drag limits (once drag increase exceeds excess thrust, high turn rates will decelerate your vehicle, this limits lift available to create G)