F-16.net

Enthusiast Joined: Jan 10, 2007 Posts: 51 Status: Offline

Guys

Does anyone know whether A-A missiles (such as the AIM-120, AIM-9, R-77 etc) can actually turn at all above 40,000ft with those tiny fins?

F-16.net Sponsor

Enthusiast Joined: Sep 02, 2006 Posts: 31 Status: Offline

If not, they would be useless. These tiny fins are more than enough at high speed.

Elite 3K Joined: May 10, 2004 Posts: 3098 Status: Offline

Yep, just remember that these missiles are probably inherently either neutrally dynamically stable or slightly unstable as the center of pressure looks to be close to the center of gravity. Combine an easy to turn missile (neutral or negative stability) with Mach many speed and it's all over but the kill Twisted Evil

Elite 1K Joined: Aug 19, 2004 Posts: 1092

If they're going fast enough, they can turn very well. If they run out of energy, they turn very poorly.

Elite 3K Joined: May 10, 2004 Posts: 3098 Status: Offline

Raptor_One wrote:

If they're going fast enough, they can turn very well. If they run out of energy, they turn very poorly.

Unless they have thrust vectoring like some of the new ones Wink

Yeah dfs at angels 40 are pretty common. Don´t you think the engines would have burned out in the typical long range BVR fight that happens up there?

In addition I´d guess that the body itself creates a fairly large fraction of the lift

Elite 1K Joined: Aug 19, 2004 Posts: 1092

hansundfranz wrote:

Yeah dfs at angels 40 are pretty common. Don´t you think the engines would have burned out in the typical long range BVR fight that happens up there?

In addition I´d guess that the body itself creates a fairly large fraction of the lift

The missile fins aren't for generating lift (well... not really). They're for stabilization purposes and for generating pitch/yaw/roll moments. Missile fins don't generate much total lift compared to the missile body unless you're talking about body angles of attack very close to zero. In that case, the missile body (which is usually symmetric) won't generate any lift. In addition, the missile fins are also symmetric so they won't generate any lift (unless they're deflected up/down) when the missile body is at zero AoA. Missiles just don't work very well when they're slow. Slow for a missile means anywhere close to being subsonic I think. Smile

Also, thrust vectoring isn't for low-speed/low-energy maneuverability when it comes to missiles. A missile with a rocket motor still burning has plenty of energy and will get to very high speeds in no time. Thrust vectoring is needed for control outside the range of the missile's conventional aerodynamic control surfaces. At really high AoA, the fins are pretty useless (except for grid-fin control surfaces which are cool, but more draggy than planar fins) so you need control power from vectored exhaust. Of course, thrust vectoring is only useful when you actually have thrust. Very Happy

Once you run out of thrust, you better have speed and (hopefully) altitude.

Enthusiast Joined: Jan 10, 2007 Posts: 51 Status: Offline

Thanks for the replies guys - it just seemed logical that in the thinner air they wouldn't turn as well - and maybe should be easier to outmaneuver.

Elite 1K Joined: Jul 22, 2005 Posts: 1088 Status: Offline

flighthawk wrote:

Thanks for the replies guys - it just seemed logical that in the thinner air they wouldn't turn as well - and maybe should be easier to outmaneuver.

It couldn't possibly be any other way all else being equal. If you have more lifting area you're going to turn better at a given speed. Pretty much all of your missiles that are designed for high altitude kills have either large fins or very high speed (at the higher speeds body lift is sufficient but then your turning radius is going to go up for a given amount of Gs pulled so. . .).

Elite 1K Joined: Aug 19, 2004 Posts: 1092

The thing about high altitude is that whatever you're trying to kill faces the same challenges in terms of maneuverability as the missile itself. Assuming we're talking about some sort of aircraft and not a ballistic missile warhead, the aircraft won't maneuver as well due to the low density just like the missile. Missile an aircraft obey the same laws of physics at high or low altitude.

Enthusiast Joined: Jan 10, 2007 Posts: 51 Status: Offline

Raptor_One wrote:

The thing about high altitude is that whatever you're trying to kill faces the same challenges in terms of maneuverability as the missile itself. Assuming we're talking about some sort of aircraft and not a ballistic missile warhead, the aircraft won't maneuver as well due to the low density just like the missile. Missile an aircraft obey the same laws of physics at high or low altitude.

Thanks Raptor - I was basing the assumption on that bombers with larger wing areas are apparently more agile than smaller winged fighters higher up due to thinner air etc - so was thinking that a larger winged craft would still be able to turn better.

so much for that assumption Confused

Forum Veteran Joined: Dec 14, 2006 Posts: 505

flighthawk wrote:

I was basing the assumption on that bombers with larger wing areas are apparently more agile than smaller winged fighters higher up due to thinner air etc - so was thinking that a larger winged craft would still be able to turn better.

I don't know if what you were thinking applies to the missiles.

I have heard some claims that big wing bombers could outturn some of the fighters of their days at high altitude.

I think in some cases it might be true. So your logic may not be entirely unfounded.

Quote:

Few fighters of the day could reach the normal operating altitude of the B-36, and at altitude the giant bomber with its lower wing loading was said to be able to out-turn the faster fighters. (Others - including former fighter pilots - challenge this statement.)

http://www.bigpedia.com/encyclopedia/B-36

Elite 1K Joined: Jul 22, 2005 Posts: 1088 Status: Offline

Raptor_One wrote:

The thing about high altitude is that whatever you're trying to kill faces the same challenges in terms of maneuverability as the missile itself. Assuming we're talking about some sort of aircraft and not a ballistic missile warhead, the aircraft won't maneuver as well due to the low density just like the missile. Missile an aircraft obey the same laws of physics at high or low altitude.

Yes they do and a bomber with a wing is going to have a lot more area to use to turn with than the missile. Just as say a Nike Hercules is going to be able to turn better at Mach 3 and 100,000 feet than a Patriot at Mach 3 and 100,000 feet.

Elite 1K Joined: Jul 22, 2005 Posts: 1088 Status: Offline

flighthawk wrote:

Raptor_One wrote:

The thing about high altitude is that whatever you're trying to kill faces the same challenges in terms of maneuverability as the missile itself. Assuming we're talking about some sort of aircraft and not a ballistic missile warhead, the aircraft won't maneuver as well due to the low density just like the missile. Missile an aircraft obey the same laws of physics at high or low altitude.

Thanks Raptor - I was basing the assumption on that bombers with larger wing areas are apparently more agile than smaller winged fighters higher up due to thinner air etc - so was thinking that a larger winged craft would still be able to turn better.

so much for that assumption Confused

Depends on the bomber and the fighter. It's well known that a Vulcan could out turn a Lightning at hight altitude. There is no rule that says "if it's an aircraft it can't out turn a missile" or "if it's a bomber it can't out turn a fighter". Nature doesn't know or care what kind of vehicle it is. It comes down to power, speed, altitude and lift/drag. What you call it makes no difference whatsoever.

Elite 1K Joined: Aug 19, 2004 Posts: 1092

All I was trying to say is that an aircraft will lose maneuverability as it gains altitude. The higher you go, the lower the air density gets. In addition, air breathing engines produce less thrust the higher up they go for the same basic reason (decreased air density and the resulting decrease in mass flow rate through the engine). Rocket motors are less prone to atmospheric effects since they carry their own oxidizer. They are only affected by the under- or over-expansion of exhaust gases exiting the nozzle due to changes in atmospheric pressure. Think about the way the shuttle's solid rocket boosters' exhaust plumes look just before they run out of fuel. That's over-expanded flow and is an off-design condition if I remember correctly. As far as I know, this is the only significant atmospheric effect on a rocket motor's thrust output.

I think I got a little bit off track there. The simple point I was trying to get across is that any aircraft is less maneuverable at high altitude vs. low altitude. The same is likely the case for a missile too, unless the missile is somehow more maneuverable at speeds it can only reach at high altitudes (i.e. Mach 3-4+). I doubt that is the case though. Missiles simply perform better in terms of range and energy potential at high altitude because there's less drag, but I doubt they can turn faster/tighter up high than down low. Anyway, it's a well known fact that A-A missiles are much more likely to hit a high-flying target than a low-flying target... especially when launched from high altitude and high speed (F-22 supercruising for example). This is because a missile (when fired within reasonable parameters) has more of an energy-maneuverability advantage on a high-flying aircraft than vice-versa. At low altitude, the situation is somewhat reversed (for a fighter at least). The fighter aircraft can light its afterburner(s) and maintain Mach 1+ for much longer than the missile. A missile will decelerate rapidly after its rocket motor burns out when at low altitude. In addition, it won't reach its blistering top speed which occurs at high altitude. It might do Mach 2 at low altitude while the rocket motor burns for a few short seconds and then it's decelerating quite rapidly. If it has to maneuver after rocket motor burnout down low, induced drag will sap its energy even more.

Another thing one has to consider in the performance equation is weight. A missile doesn't weigh very much and has a huge T/W ratio. Even after rocket motor burnout, it can pull 20G turns given enough kinetic energy. And it pulls these high G turns in an incredibly tight turning radius. Also, some missiles have big enough blast radii that successfully outmaneuvering them won't always ensure a great enough miss distance to escape the expanding rod warhead nastiness or whatever they're using in A-A missile warheads these days. Some missiles simply aren't meant to be outmaneuvered. It's too dangerous. If this is the case, a pilot must always keep far enough away from an enemy missile threat to ensure he can successfully drag an enemy's shot out to the point where it's no longer a viable threat.

F-16.net

Forum: F-16 Armament & Stores

Air to Air Missile performance at high altitude