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Enthusiast Joined: Oct 13, 2008 Posts: 20 Status: Offline

I've got a question for those who know well how guided missiles, esp. air-to-air work. Recently I've been very shocked to hear that a missile's rocket engine (say, Sidewinder's) doesn't work very long - actually it burns out soon after launch and then only "glides" towards the target.

So my questions are:

1) How long does a missile rocket engine really work?

2) The speed usually provided for various missiles in literature and catalogs must then be a top speed at the moment when engine burns out, right?

3) How useful is a missile at the edge of its range? I would assume that it must have very little kinetic energy and very little capability to maneuver even at, say, 70% of its max range.

4) What is the usual path of a missile flight? I used to think that it's a straight line towards the target (except for maneuvers if target changes course), but recently I've seen on some web forum a statement that the late F-14's Phoenix missile would use its engine power to climb very high (like 100k ft) and then glide towards the target.

(BTW, I wonder what happens if enemy is at its ceiling altitude and beyond the range of the missile's initial thrust phase - surely a missile can't keep an altitude very long, so that would give a missile probably like 20-40% of its nominal range, depending how long the engine works...)

If you have any propulsion knowledge on specific missile types & models (like AIM-9, AIM-120, etc.), then I'd be pleased to learn the differences too.

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Your asking for alot of information people shouldn't be talking about.

Enthusiast Joined: Oct 13, 2008 Posts: 20 Status: Offline

Come on, I'm not asking for precise data, I just want to understand how missiles work.

Flight profile depends on the missile used and the distance the target is engaged. Most long range a2a missiles fly a ballistic flight path; that is how the AIM-54 engaged targets. Short range missiles like the AIM-9 fly directly to the target. A missile really will not be fired at a target above the missiles engagement range. Almost any missile can outclimb a target and solid rocket fuel will always get you higher than a jet engine.

I kind of doubt you will find burn times, I am betting they are generalized or just plain classified.

Elite 1K Joined: May 26, 2005 Posts: 1368

1) It works until it runs out of fuel. Some motors have a high thrust "boost" section and a lower thrust "sustainer" section that burns for a longer period of time at a lower thrust output.

2) Those numbers are usually intentionally vague.

3) Pk falls off as range increases. Max range isn't as useful to know as max "no escape" envelope. You're not going to find that in public domain for current technology missiles.

4) Depends on the missile's guidance code. These kinds of variables can change from version to version of the same missile. Keep in mind though that higher altitude means less drag, and rocket motors don't breathe air.

Enthusiast Joined: Apr 07, 2008 Posts: 52 Status: Offline

Disclaimer: I am not military and do not work in the defense field. This is just what I have gathered from publicly-available books, video clips, etc. All answers are "in general" only.

Axure wrote:

1) How long does a missile rocket engine really work?

It depends... a few seconds for shorter-ranged missiles, up to maybe 15-20 for medium and long-range ones. Also, some motors will push out a lot of thrust for a short time, then burn out. Others will have a high initial thrust, then a lower "sustainer" thrust; you might see this on longer-ranged missiles. The exact thrust profile will be classified, but it is tailored for the application.

Quote:

2) The speed usually provided for various missiles in literature and catalogues must then be a top speed at the moment when engine burns out, right?

The speed provided in catalogs is virtually useless. It can vary widely depending on launch conditions. Launching from a high and fast aircraft will obviously give you more final speed than launching from low and slow. Target maneuvers can also affect the maximum speed reached--it's basic physics.

Maximum range will also vary--head-on, high closing speed shots at high altitude will give you a much longer range than tail chase shots down low and slow. Any maximum range figure quoted in a brocure will most likely be the former situation, if indeed it bears any relation to real numbers at all.

Quote:

3) How useful is a missile at the edge of its range? I would assume that it must have very little kinetic energy and very little capability to manouvre even at, say, 70% of its max range.

There's no way to really quantify this. Yes, the missile will eventually run out of energy and be unable to maneuver, but when and how that happens will be much different for each type of missile and the specific scenario.

Quote:

4) What is the usual path of a missile flight? I used to think that it's a straight line towards the target (except for manouvers if target changes course), but recently I've seen on some web forum a statement that the late F-14's Phoenix missile would use its engine power to climb very high (like 100k ft) and then glide towards the target.

Actually, most modern missiles try to lead their target using proportional navigation. They don't just point straight at the target's present position. Also, depending on the engagement range, medium- and long-ranged missiles will "loft" themselves like you describe to increase range--there's less drag up high, and coming down on the target from above trades potential energy for kinetic (giving you more terminal maneuverability).

Quote:

BTW, I wonder what happens if enemy is at its ceiling altitude and beyond the range of the missile's initial thrust phase - surely a missile can't keep an altitude very long, so that would give a missile probably like 20-40% of its nominal range, depending how long the engine works...

Again, there's no way to give a general answer for this, and any specific answers will most likely be classified.

Quote:

If you have any propulsion knowledge on specific missile types & models (like AIM-9, AIM-120, etc.), then I'd be pleased to learn the differences too.

Any such details (thrust profile, burn time) will most likely be classified for the AIM-120 and later AIM-9 versions. You might find some general information, but not specifics.

Enthusiast Joined: Oct 13, 2008 Posts: 20 Status: Offline

Thanks, guys, for those answers.

Quote:

I kind of doubt you will find burn times, I am betting they are generalized or just plain classified.

I am not seeking precise numbers. I just wanted to know whether the rocket engine burns out really quickly, like in the first 5-15 seconds, or does it run for a significant time (like, say, 30% of max range), or does it run practically till the end (which almost never happens, as I've learned only recently - unless traget is very close, of course).

Quote:

It works until it runs out of fuel.

That's so smart, I wouldn't figure that out myself. Wink

Seriously though, my question, obviously, was exactly that - for how long does the fuel last?

My current assumption is that only very short range missiles (Sidewinder class) would be propelled for most or the whole flight, while anything beyond that, including an AMRAAM, would need to amass kinetic (speed) and potential (alt) energy until the engine dies after, like, 30-60 seconds (?), and then just glide towards the target.

EDIT:
gtg947h, ok, you've posted while I was writing this answer. But your post seems to confirm, more or less, my latest assumption... So now, I guess, I know how it works.

I always thought the more interesting question was how the missile actually moves its flight control services. Not like there is room for hydraulics or a hamster with a yoke. Granted, you might already know, but still pretty darn interesting considering how quickly they actuate Laughing

Enthusiast Joined: Oct 13, 2008 Posts: 20 Status: Offline

I have no clue, but I would guess in the smallest machines (like AIM-9) it could be an electric motor linked almost directly to the control surface - probably with only one gear between them.
Or, if not a big angular granularity is need (say, only 3 or 5 positions) it could even be a purely eletromagnetic "switch," which would act in a snap.

But these are pure guesses. Smile

BTW, when will they start setting up rear-pointed IR missiles in fighters to shake off an enemy off one's tail? Wink

BTW2, it would be cool to put a small amount of rocket fuel in a missile for the terminal phase when it actually engages a target - to give it an edge over a manouvering enemy, when the rocket has lost most of its energy. (But they probably already do that, we just don't know. Wink

Otherwise I would have to apply to Raytheon to teach them how to do their job Wink

)

Forum Veteran Joined: Jul 01, 2007 Posts: 547

The US won't put rearward facing missiles for two reasons. The first is that the missle would have to overcome the aircraft's velocity in the oppossite direction just to get to zero airspeed and the second, and most important reason, is that no one gets behind us.

Enthusiast Joined: Apr 11, 2008 Posts: 82 Status: Offline

They move the control surfaces with devices that burn a type of pyrotechnic* that creates gas pressure as it burns creating a kind of compressd gas. So you could say they are pneumatically controlled. Thats why CAP-9s are 1.4 beacuse that Gas Grain Generator is in the Seeker/Guidance/Control section and the head is live. However there are missiles (ie. AIM-7s) that are controlled by hydraulics but as a general rule our munitions are controlled by some form of compressed gas.

As far as burn times and rocket motor specific that goes from classified to top secret but you can look at you tube missile shot videos and come up with a guess.

I.Y.A.A.Y.A.S.

I thought I would add to this a little bit since I used to be the lead for A/A missiles on the F-16.

The AIM-9 uses what we call "proportional navigation guidance". When the missile first leaves the aircraft it follows a ballistic path until it clears the aircraft, so there is actually a delay before the missile executes any steering commands. Now the simple explanation is that it causes the missile to fly to a point in space to intercept the target rather then simply chase the target. The missiles flight continually changes until the target is destroyed by contact or proximity or aerodynamic drag slows it down and the projected collision point becomes further ahead of the target. The more detailed explanation is that it causes the angular separation between the missile gyro line of sight and the missiles airframe to increase so that when you reach the gimbal limit of the seeker head the gyro losses lock on the target and guidance is then terminated.

As far as the motor, the missile keeps accelerating until the motor burns out (Not going to put the time in here) and then basically glides to intercept the target.

As far as control surfaces movement, there is actually what we call a thermal battery inside the missile which enables servos in the missile to move the control surfaces and maneuver the missile.

Cheers,

ViperDude

Enthusiast Joined: Jun 05, 2008 Posts: 30 Status: Offline

Some (vague) points in random order:

AA rocket motors burn only a short time and produce meaningfull thrust for even shorter time. Burn times above by gtg947h are in the right ballpark Only the ramjets just entering service have all-flight burn times.

AA missile range is not only affected by closure rates but the altitude! Missile range graphs opens like x^2 graph when altitude increaces. Typical quoted AA missile range is achieved well above 50k ft (against non-maneuvering target).

Because drag decreses at higher altitudes a lob flight profile (missile climbs at the starts of its flight and flies most of its way at low density air with less drag and then dives to its target) achieves the longest range. A lob trajectory can only be used with range information and inertial navigation system (or missile tracking from FCS radar in case of SAMs) on the missille. That's why older missiles and most heat seakers fly (modified) proportial filght paths which leads to shorter ranges. Modern medium range missiles (eg AMRAAM) use very sophisticated and adaptive flight path algorithms.

Target relative altitude to shooter doesn't make a huge difference to AA missile range. Higher absolute target altitude usually increases missile range.

There is no any good single figure for max range for any missile - several graphs are needed. Public data from different missiles is not comparative.

Public range figures tend to be much higher than actual launch distances on practical scenarios.

Because rocket motor burns a very short time and produces a lot of thrust, drag is a small factor when calculating for max missile speed. For an example if a missile achieves M2.0 when launched from a hovering helicopter it will achieve almost M4.0 when launched from a fighter flying at M2.0 at the same altitude.

Pressure recervoirs, pressure produced by burning gunpowder and electric motors are the most used ways to power control surfaces of missiles.

Questions?

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