AMRAAM G load

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armedupdate

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Unread post21 Oct 2015, 06:40

I hear from many sources the AMRAAM has a g capablity around 30 Gs. Is this instantenous or sustained? And which variant? Seeing the newer AIM-120C-5s and AIM-120Ds have HOBs they should have 50 G like the French MICA BRAAM.

This is a quote from the author of Fighter Combat: Robert L Shaw.
the missile will require about five times the G capability of the target to complete a successful intercept.


So the AMRAAM max g load is probably when the missile is at its medium speed level,(~Mach 3). It will be too fast to manuver at Mach 4(max speed achieved at boost burnout) and too slow at Mach 0.5(close to when operational range is reached)

So if the AMRAAM boost till Mach 4 at around 9 seconds it should be the most maneuverable in the middle of that phase(I beleive Jane's praised the WVR capablities of the AMRAAM) but it lessens manuverablity as it goes to burnout. Then as it is in midcourse phase once it descends to Mach 3 it is the most maneuverable.

Another rough rule of thumb:-

The time it takes for a missile to lose 25% of its velocity after burn out at supersonic speeds.

Never @ > 100,000 m (~300,000 ft) ; in space
~150 seconds @ 24,000 m (~80,000 ft)
~70 seconds @ 18,000 m (~ 60,000 ft)
~25 seconds @ 12,000 m (~ 40,000 ft)
~10 seconds @ 6,000 ft (~20,000 ft)
~5 seconds @ Sea Level

So if its fired from 18,000 m alt. from an F-22 lets say the AMRAAM would have covered around 10 km in the boost phase before descending.

So by 70 seconds it can still save energy till it reached Mach 3 to 30 Gs where its most maneuverable to intercept 6 G plane. Since the average speed is around Mach 3.5 to reach that distance it is around 72 km.

So at 82 km the AMRAAM can still be extremely manuverable against a 6 G target.

How accurate is this prediction?
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vilters

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Unread post21 Oct 2015, 08:55

Missiles have rocket motors that burn a few seconds to get them up to speed.

At speed they are gliders.

How are you going to "sustain" a high "G" load on a glider?

More "G" is also more drag.
Missiles can do a single high "G" turn critically reducing energy and "G" capability, then they simply "drop" from the sky as a brik.
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Unread post21 Oct 2015, 09:06

Um the missile has fins for the glide...yes they can sustain a high G turn....how else will they hit a moving target?
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Unread post21 Oct 2015, 12:18

AMRAAM like any missile will likely be able to achieve maximum G forces when their rocket motor is still burning but are able to achieve quite high G forces mid-flight. For example VT-1 missile in Crotale NG SAM system can go up to 50 Gs but can still go 35 Gs 8 km away from launcher several seconds after the rocket motor has burned out. Speed then is something like 750 m/s when top speed is 1250 m/s. I think AMRAAM will be similar and can do max G turns when rocket motor is still burning or just burned out. It definitely can turn very hard longer when rocket motor is still burning. It also probably can do high G turns even at medium range depending on engagement geometry and how well it can maintain speed in that particular engagement. However missiles can't maintain such hard turns very long when gliding as they'd lose speed and energy in doing turns.

Every missile speed will vary a lot depending on altitude and nominally Mach 4 missile might go Mach 2.5 at low altitude and closer to Mach 5 if launched at very high speed and high altitude. The missile thrust has to overcome the drag which is significantly higher at low altitude. Of course lower altitude is better for maneuverability as control surfaces are more effective at lower altitudes. The same is true for target also though...
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Unread post21 Oct 2015, 13:21

Question, don't missiles have a separate motor charge for their boost phase and another one for their Terminal phase?

This makes sense since the hardest maneuvers will be done when the projectile is very close to it's target.
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Unread post21 Oct 2015, 13:43

zero-one wrote:Question, don't missiles have a separate motor charge for their boost phase and another one for their Terminal phase?

This makes sense since the hardest maneuvers will be done when the projectile is very close to it's target.


Some newest missiles do have such a rocket motors and they are called dual-pulse or multi-pulse rocket motors. They usually have two separate propellant charges which can be ignited separately. The second rocket motor can be ignited to sustain speed mid-flight or then used only for end-game. For example Most missiles have single-pulse rocket motor which burns only during acceleration phase and maybe some seconds after that to sustain speed. AFAIK, AMRAAM has single-pulse rocket motor as do most other air-to-air missiles. For example Standard SM-3 missile has dual-pulse rocket motors.
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Unread post21 Oct 2015, 13:44

zero-one wrote:Question, don't missiles have a separate motor charge for their boost phase and another one for their Terminal phase?

This makes sense since the hardest maneuvers will be done when the projectile is very close to it's target.

Answer, no. That is why the Meteor being a Ramjet is such a big deal.
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Unread post21 Oct 2015, 15:46

Since after burnout its decelerating so should it be negative Gs? And how does that relate to the G of the enemy airplane?

Also, I think at BVR, boost phase has very little turn since its only making slight adjustments. Its the terminal phase that counts.

Shouldn't be the least maneuverable the moment the rocket motor burns out since its going the max speeds? I don't expect it to turn well till it decelerates to around a medium level speed. This applies to aircraft as well. They pull their max Gs at a middile point of max speed and min speed. For example the F-104 chart.

How does G change with deceleration? For example an airplane like the F-16 can go 9 Gs probably around Mach 0.5-0.8? However it can never pull as much at max speeds of Mach 2, once it decelerates with no aditional burn, how does the deceleration change the G capablity with each moment loss of speed? Once it reaches Mach 0.5 again can it pull the same number of Gs?(in this case negative G?)
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Unread post21 Oct 2015, 16:44

armedupdate wrote:How does G change with deceleration? For example an airplane like the F-16 can go 9 Gs probably around Mach 0.5-0.8? However it can never pull as much at max speeds of Mach 2, once it decelerates with no aditional burn, how does the deceleration change the G capablity with each moment loss of speed? Once it reaches Mach 0.5 again can it pull the same number of Gs?(in this case negative G?)


At sea level an F-16 can pull 9G from ~M0.6 all the way to top end - it can also pull 9G at M2.0 at certain altitudes.

In this context 9G has to be positive because usually its the direction of the force on the human body (head down) - e.g. for arguments sake a pilot cant withstand more than 9 positive G or -3 negative G (usually pitch in controlled flight ) for various reasons - but can withstand more lateral G for example.

A missile AFAIK has no such concerns so is probably just described in terms of G - so if you can get some aero charts for a an example missile you may expect it to be able to pull max G above a certain low airspeed all the way depending on altitude/attitude/AOA etc.
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Unread post21 Oct 2015, 17:07

well first off, G is a byproduct of turn rate and speed, so you can pull a massive amount of G at extremely high speeds but the doesn't necessarily mean you're turning very tight. If the max 30G turn of the Slammer is achieved at such high speeds such as Mach 3, then the turn rate may not be extremely tight. Not sure about my Math, it could be around 15 degrees\second, Sprts may be able to come up with more accurate estimates.

And how long can the AMRAAM sustain that 30G turn is also another story. Since we now know (through Sprts and Hornetfinn) that the AMRAAM uses a single pulse motor, then in a BVR shot, it's basically a supersonic dart.

I can see why Gums held the "bat turn" in such high regard as it would be really effective once you knew where the incoming missile is, do a bat turn and force the missile into a climbing turn and it will loose energy very quickly, giving you a good chance to survive the BVR engagement.
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Unread post21 Oct 2015, 17:42

i have always thought AIM-120 have boost and sustain motor :? , with only boost it would be rather shitty at BVR
if we know thrust/drag, Cdo... etc of a missile, i suppose we can even measures it's turn rate at different speed
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Unread post21 Oct 2015, 17:59

zero-one wrote: Not sure about my Math, it could be around 15 degrees\second, Sprts may be able to come up with more accurate estimates.

above 36,000 feet, 30G, 3.0M, instant turn rate is ~19 deg/s with a radius of 8,730ft. Now the question is can an AMRAAM generate enough lift to do that? An AIM-120B has an empty weight of about 223lb (fuel burned) so 30G is a load of 6690lb (or slg-ft/s^2). The dynamic pressure at 3.0M at 36,000 ft is ~3,000 slg/ft-s^2, this means the missile needs a lifting area of ~2.23 ft^2. This would be the Cl*Aref. Those little fins cant have more than ~0.17ft^2 of area each, say four of them are pulling for a total area of ~0.68ft^2 (based on my own estimation, miniZAP gives 0.697 which may account better for the non-triangular rear fins). This means the combined fins and body would need a Clmax of 3.2 using the miniZAP fin area. If this were true than an AIM-120B would have a "stall speed" of 171 knots at sea level. As such I am skeptical that the Clmax would be so high. The much higher Reynolds numbers of M3 flight could certainly increase the Clmax but I remain skeptical of a value of 3.2. I would need to see more data on missile lift.
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Unread post21 Oct 2015, 18:20

Data I have for big-finned Sidewinders indicates a Clmax of 1.3 and a Lifting Area of 1.8 ft^2
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Unread post21 Oct 2015, 19:32

I don't think missiles have to climb much since they are basically dropping on the targets. The trajectory is probably fired over the fighter and then using the fits to glide down since you want the missile in an area will it can still catch up to tail chase targets.

How to measure turn rate by looking at Gs, speed and atlitude?

Also don't missiles turn the hardest at their medium speed levels rather than their top speeds?
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Unread post21 Oct 2015, 20:15

armedupdate wrote:How to measure turn rate by looking at Gs, speed and atlitude?


turn rate in degrees of turn per second is (G*32.2*57.3)/(V) where G is the G load, 32.2 is the correction from G to acceleration due to gravity, 57.3 is the correction for degrees per radian, and V is the speed in feet per second.

armedupdate wrote:Also don't missiles turn the hardest at their medium speed levels rather than their top speeds?


Let's qualify these terms. Let's call low speed a speed at which the missile is too slow to hit maximum G. Corner (Medium) speed will be the speed around which the missile is first able to reach maximum G. High speed will be everything between medium and top speeds.

At low speeds there is not enough airflow over the fins to turn at maximum G so the fins will be at maximum deflection. Interestingly, in this speed region the missile will still have near its tightest available turn (smallest turn radius). speed loss due to turning will be large here.

At medium speeds the missile finally has enough airflow to hit maximum G with the fins at maximum deflection. This speed will give the highest G, fastest turn rate, and tightest turn radius. Speed loss due to turning will still be large so you will quickly end up at low speed.

At high speeds the missile is structurally limited to maximum G and the fins will NOT be at maximum deflection. A maximum G turn at these speeds results in slower rates and larger radii as speed increases. Speed loss due to turning will be smaller as the lift coefficient used for turning is smaller as speed increases

So at and above Corner Speed, the missile can turn at maximum G if needed. At corner speed everything about the turn is at it's best, but as the missile will quickly be much slower it would hopefully already be near the target by this point.
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