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F-82 Twin Mustang gunpod

Michel Van

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i found link to Italian blog
with picture of a Gunpod for F-82

http://sobchak.wordpress.com/2008/03/31/gunslingers-f-82-twin-mustang-gunpod/

after page the pod supply 8 Browning M2 (each with 440 shots, caliber 50)
6 gun in wing 2 under the pod (make in total 14 guns in F-82)
 

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Weasel Pilot

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14 !! .50 caliber BMG...now THAT's an attention step! Wonder how much the aircraft slowed as these were being fired? If memory serves, we lost about 5 kt per second when firing the GPU-5 pods (30mm, same round as A-10's GAU-8) off our F-4Es.
 

Lauge

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Weasel Pilot said:
Wonder how much the aircraft slowed as these were being fired? If memory serves, we lost about 5 kt per second when firing the GPU-5 pods (30mm, same round as A-10's GAU-8) off our F-4Es.

No contest! A (probably over-) simplified, back-of-the-envelope calculation shows that:

Force = the time derivative of the impulse.

Impulse = mass X velocity

so

Force (recoil): mass X velocity X rate-of-fire

This ignores the recoil produced by the propellant gasses leaving the barrel together with the projectile (but I said it was simplified ;)).

So, for the GAU-13 in the GPU-5 pod:

Projectile mass: 15,1 oz (for the API) = 430 g = 0,430 kg
Muzzle velocity: 3.600 ft/sec = 1.030 m/s
rate of fire: 2.400 rds/min = 40 rds/sec

Recoil: 0,430 X 1.030 X 40 = 17.716 N

For 14 Browning M2 .50 cal's:

Projectile mass: 622,5 grains (for the M8 API) = 40,3 g = 0,0403 kg
Muzzle velocity: 3.050 ft/sec = 930 m/s
Rate of fire (per gun): 750 rds/min = 12,5 rds/sec
Rate of fire (combined): 14 X 12,5 = 175 rds/sec

Recoil: 0,0403 X 930 X 175 = 6.558 N

or roughly 1/3 that produced by the GAU-13.

The source for the above data is Wikipedia and Janes Infantry Weapons, 2009 edition.

The above calculation, as stated, ignores the recoil force produced by the propellant gasses (both the extra mass being ejected, as well as the propulsive "rocket" effect of the gas leaving the muzzle), and it tells you little about the "felt" recoil, since the mass of the gun itself is ignored. However, since 14 Browning M2's mass about 530 kg (38 kg pr. gun) and the GAU-13 comes in at 151 kg, "felt" recoil of the GAU-13 (1/3 the mass and 3 times the generated recoil force) should be significantly more than that of the 14 M2's.

There's probably a couple of 30mm-size holes in my calculations, so corrections are welcome (provided they are suitably polite ::) ).

Regards & all,

Thomas L. Nielsen
Denmark
 

r16

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ı remember reading the A-10 was getting about 4500 kg of drag through gun . It fired faster at 4200 rpm (?) but are the guns same , do they have the same velocity out of the barrel ?
 

Lauge

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r16 said:
....do they have the same velocity out of the barrel ?

As I read it, the difference between the GAU-13 (gunpod) and the GAU-8 (in the A10) was the number of barrels. 4 in the GAU-13 vs. 7 in the GAU-8. Muzzle velocity should therefore be the same for the two weapons.

PS, then a drag force of 4.500 kg corresponds to about 45.000 N. Adjusting the calculation above for increased rate of fire (4.200 rds/min instead of 2.400 rds/min) gives a recoil force of 31.000 N. The difference up to the 45.000 N would be due to the over-simplified nature of the calculation, such as ignoring the contribution from the propellant gases*.

Regards & all,

Thomas L. Nielsen
Denmark

*) Actually, I seem to remember reading somewhere (don't as me for a source) that roughly 1/3 of the recoil force from a firearm comes from the ejected propellant gases. Adjusting for this brings us right up to the 45.000 N...... B)
 

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