Thread: AIM 9 and AIM 120 warhead

I am reading how AIM-9 or AIM-120 air to air missile works and I can't realy understand how the detonation mechanism and warhead works.


For AIM-9 they say it uses four laser diodes and four photo diodes located behind control fins that are mounted perpendicular to the flight direction. When rocket is parralel with the target plane the laser light from diodes bounce of the plane's body back to the photo diodes. At that time high explosives in the warhead is detonated and energy of the explosion propels alternating welded rods forming a destruction ring. They call it annular blast-frag or continuous-rod warhead.


Since laser diodes/photo diodes are mounter perpendicular to the flight direction that means that heat seaker must not directly steer the missile to hit the target plane but instead it has to steer it parallel with the target plane in order this rod ring can have any effect.


With AIM-120 the detonation mechanism is proximity radar. Those missiles uses high explosive blast-fragmentation warhead and I suspect radar steers the missile directly to the target plane and when close enough the high explosive detonates thus propeling the shrapnels in the direction of flight, just like below animation shows.





That makes sanse but AIM-9 does not. Can someome explains.

Most missiles don't (and indeed aren't intended/ expected to) actually hit the target¹. It's far easier to get a missile "somewhere in the area" than it is to get them to (infallibly) impact the target. Given this then, bearing in mind that missiles are usually launched at targets that are approaching or retreating the missile's path will be, as you noted, parallel to the target's?
Thus the "sideways-looking" fusing system will detect that target as the missile "flies past" and detonate the warhead.
There are, however, 3 further considerations:
1) for any impact/ approach angle other than purely 90 degrees there will be a reflection (and even then there could be a reflection from excrescences on the target aircraft depending on impact location),
2) it's possible that the "look angle" of the lasers isn't confined solely to 90 degrees from the missile's axis (i.e. there's a built-in allowance for fairly large graze angles²),
3) the laser fusing is, so far as I can find, an additional system (usually combined with IR) (i.e. fusing isn't reliant on only the lasers).

1 Missiles that are designed to hit are fairly rare and thus stand out: examples being the BAC Rapier and Shorts/ Thales Starstreak.
2 Apparently it's not confined to 90 degrees: Target proximity is continuously being measured by a laser, transmitting a coded signal under a specific angle with the missile longitudinal axis.

following the flight path of the target will get you behind the target so even if a laser diode fuse has let's say 60 degree angle and is mounted on the sideways of the missile will not see target. It's the same as lion with sideways located eyes - it could not hunt antilopes.

If the missile is approaching the target with opossite direction then againg sideways laser fuse won't work.
Same will be in case missile will get the target from above or bellow at the right angle.

In any scenario I don't see how sideways mounted diodes will work.

On howstuffwork it is written that primary and only fuse system on AIM9 are 4 laser diodes and 4 photo diodes, mounted behind canars.


I still can't find any usefull animation/explanation regarding continuous-rod warhead and how they destroy target.

and if you watch pac3 missile animation video https://www.youtube.com/watch?v=xU9C2iLm764 you'll see that it uses head mounted radar (not sideways) and that is uses shrapnels that are acelerated forward with the direction of flight and since they are somewhat angled it covers greater area so there is no need to hit the target but just to get close enough.

Originally Posted by svizoman

following the flight path of the target will get you behind the target so even if a laser diode fuse has let's say 60 degree angle and is mounted on the sideways of the missile will not see target.

Not true:
1) missiles usually have an "offset", i.e. actual "aim point" is not the centre of the heat signature but at a distance from it (ideally to take out the pilot).
2) IF the missile does actually home in on - and hit - the centre of the heat signature then it has physically hit the target: the OTHER fuse will initiate detonation, or even the laser because of excrescences (for example if the missile goes down the jet pipe then reflections from the inside of the pipe will trigger detonation).

It's the same as lion with sideways located eyes - it could not hunt antilopes.

I'll repeat myself for clarity: Most missiles don't (and indeed aren't intended/ expected to) actually hit the target. Lions "are".

If the missile is approaching the target with opossite direction then againg sideways laser fuse won't work.

This is false. The missile (usually) does not impact the target. Therefore it will fly past. Thus a sideways-looking sensor will see the target and trigger detonation.

Same will be in case missile will get the target from above or bellow at the right angle.

Same reply.

In any scenario I don't see how sideways mounted diodes will work.

And yet they do... That should give you a clue that your perception of intercept geometry is flawed.

On howstuffwork it is written that primary and only fuse system on AIM9 are 4 laser diodes and 4 photo diodes, mounted behind canars.

I can't find anything on How Stuff Works that states that the primary and only fuse system is laser.
(Apart from the fact that How Stuff Works is a "beginner's guide" rather than a solid source).
But a laser cannot possibly be the primary and only system since it wasn't even fitted until the -9L version of Sidewinder. I wonder how all the previous versions worked?
I DID find this on How Stuff Works: The control system recognizes that the missile is right next to the target and triggers the warhead. See those words "next to"?
As for what fusing systems ARE fitted try this one:

AIM-9 LATE SUBTYPE COMPARISON TABLE

Subtype	AIM-9J	AIM-9L	AIM-9M	AIM-9P-4/5	AIM-9R
Service	USAF	Joint	Joint	USAF	USN
Seeker Design Features
Origin	AIM-9E	AIM-9H	AIM-9L	AIM-9J/N	AIM-9M
Detector	PbS	InSb	InSb	InSb	Focal Plane Array
Cooling	Peltier	Argon	Argon	Argon	-
Dome Window	MgF2	MgF2	MgF2	MgF2	Glass
Reticle Speed [Hz]	100	125	125	100	Focal Plane Array
Modulation	AM	FM	FM	FM	Focal Plane Array
Track Rate [deg/s]	16.5	classified	classified	>16.5	classified
Electronics	hybrid	solid state	solid state	solid state	solid state
Warhead	blast/fragmention	Annular BF	Annular BF	Annular BF	Annular BF
Fuse	Passive-IR	IR/Laser	IR/Laser	IR/Laser	IR/Laser
Powerplant Specifications
Manufacturer	Hercules/Aerojet	Hercules/Bermite	MTI/Hercules	Hercules/Aerojet	MTI/Hercules
Type	Mk.17	Mk.36 Mod.7,8	Mk.36 Mod.9	SR.116	Mk.36 Mod.9
Launcher	Aero-III	Common	Common	Common	Common
Missile Dimensions[ft]
Length	10.0	9.5	9.5	10.0	9.5
Span	1.9	2.1	2.1	1.9	2.1
Weight[lb]	170.0	191.0	191.0	190.0	191.0

(From here). This is an indication that at no point was a laser fusing system the only method (nor is does indicate that such a system was the primary one).

I still can't find any usefull animation/explanation regarding continuous-rod warhead and how they destroy target.

So the diagrams here don't make sense to you? Or the written description here? Or the numerous diagrams here?

Note: adding content to a post AFTER it's been replied to isn't a good idea. However...

Originally Posted by svizoman

]and if you watch pac3 missile animation video https://www.youtube.com/watch?v=xU9C2iLm764 you'll see that it uses head mounted radar (not sideways) and that is uses shrapnels that are acelerated forward with the direction of flight and since they are somewhat angled it covers greater area so there is no need to hit the target but just to get close enough.

You're having comprehension problems.
1) PAC-3 is a hit-to-kill missile (i.e. it's NOT the same as AIM-9).
2) that forward-looking radar is NOT a proximity fuse (as shown in the video YOU used as a source). The video quite clearly states that the forward-looking radar is used for terminal GUIDANCE to achieve a direct hit (ergo your "just get close enough" is also false).

[QUOTE=Dywyddyr;604652]
missiles usually have an "offset"

The word offset, now things in my head have a way more sense.
I just couldn't understand sideways mounted fuses or continuous-rod warhead.

Now I see missile that are guided with offset compared to the center of the target, will eventualy get perpendicular to the target at any given scenario, weather they follow path of the target, heading towards the target or simply coming from above or below. But if this is how missile operates than detection and detonation must really be quick process. Few miliseconds to late and shrapnels will miss the target.

Now also this rod forming ring (continuous-rod warhead) has way more sense to me. I read on the wiki that they weld rods in alternating maner and since they are made from some sort of soft metal they simply twist thus forming a ring. This ring is losing momentum from the point of detonation and when the circle extend to a full diameter this rods has no more momentum left. I gues welds must be stronger than the rigidity of the rods themselves in order to twist before weld brakes?

Shame I can't find any video actualy showing in slow motion how this type of warhead cuts trough target body.

Originally Posted by Dywyddyr

Note: adding content to a post AFTER it's been replied to isn't a good idea. However...

Originally Posted by svizoman

]and if you watch pac3 missile animation video https://www.youtube.com/watch?v=xU9C2iLm764 you'll see that it uses head mounted radar (not sideways) and that is uses shrapnels that are acelerated forward with the direction of flight and since they are somewhat angled it covers greater area so there is no need to hit the target but just to get close enough.

You're having comprehension problems.
1) PAC-3 is a hit-to-kill missile (i.e. it's NOT the same as AIM-9).
2) that forward-looking radar is NOT a proximity fuse (as shown in the video YOU used as a source). The video quite clearly states that the forward-looking radar is used for terminal GUIDANCE to achieve a direct hit (ergo your "just get close enough" is also false).

Sorry for my mistake, don't be mad.
I am really thankful you are kind enough helping me understand things. I started by reading the article of sidewinder on the howstuffworks and it just fascinated me how many ingenious ideas they use on a single missile. If you just look at the rollerons for stabilisation or simplicity of laser diode fuses or smartness of a annular blast-frag.

Originally Posted by svizoman

The word offset, now things in my head have a way more sense.
I just couldn't understand sideways mounted fuses or continuous-rod warhead.

This is in "modern" missiles: early versions didn't have that offset.
BUT: achieving a direct hit is difficult (extremely so, that's why most missiles aren't hit-to-kill). In the design stages for missiles a primary question is "What is the expected miss distance?". Once that's known then the warhead size can be established (i.e. if it's known that a typical miss distance would be 20 feet then the warhead MUST be of such a size to be effective against targets at that distance).
The less accurate the missile the larger warhead is needed and conversely¹, the more accurate the smaller the warhead. But accuracy costs money (more than warheads²).

1 E.g. the unguided (but deadly) AIR-2 Genie with a nuclear warhead. (Guidance at that time was either very complicated³ or very inaccurate. Or both).
2 Within limits of course: a very large warhead requires a very large missile and a very large motor - making the overall system more expensive.
3 As in "too complicated to fit into an aircraft".

Edit:

I gues welds must be stronger than the rigidity of the rods themselves in order to twist before weld brakes?

It's not at all unusual for welds to be stronger than the base material on which they're used.