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Vulcan M61 rotary cannon


marka
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My usual interest is in WW2 sims, but I have a question about rotary canons, such as the M61 Vulcan. The barrels rotate at high speed, and, at different points in the cycle, a round is chambered, fired, and extracted. Clearly, the barrel doesn't pause in the firing position. Doesn't that give a significant sideways velocity to the round as it leaves the barrel? How far does the barrel travel between the time the round is fired, and when it leaves the muzzle? Is that sideways velocity just compensated for in the aiming?

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I've seen vulcans fired in ground mode (the old towed Vulcan AA systems the Army used to have) and I have seen them fired from AC-130s. The targeting systems would adjust the lead electronically. For both systems, either the target or the platform are moving at a high rate of speed. I wouldnt think there is any side spin to worry about. The cannon are all electronically controlled and as I am sure you know that can be calibrated down to the nth of a millisecond. So to the eye that gun is turning at a high rate of speed, but as long as its not faster than the bullet or shell traveling down the barrel I dont think it would be a problem.

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There is a sequence at approx. 3 min into this video

where you can see that the barrel is almost stationary compared to the bullet velocity.

Best regards,

Thomm

Interesting vid. Thank you. The vid shows, quite clearly, that, for the minigun, the barrel actually *stops* at the firing position. I would think that the barrel assembly on the M61 would have too much inertia to stop as each round was fired, but I don't know that for a fact. My assumption is that the barrels rotate at constant velocity, and the bullets are fired as the barrels move.

I did some calculations. See if I made an error:

The rate of fire for the M61 is 6000 rounds per minute (lower rates can be selected), which is 100 rounds per second, for all 6 barrels, or 16.7 rounds per barrel per second. If a barrel fires once per revolution, that translates directly to 16.7 revolutions per second for the barrel assembly.

The length of the gun is stated to be 1.875 meters. From photographs, the barrel length seems to be about 80% of the gun length, which gives a barrel length of 1.50 meters.

The muzzle velocity is just over 1,000 meters/sec. Assuming that the round acceleration is constant down the length of the barrel, the average velocity of the round traversing the length of the barrel would be 500 m/s. 1.5 m barrel length / 500 m/s average velocity means that it would take the round about 0.003 sec to travel the length of the barrel. At 16.7 revolutions/sec, the barrel would rotate 0.05 revolutions, or 18 degrees, during that time.

It seems to me (my favorite phrase!) that 18 degrees is not a huge amount of travel, but it is significant, and will cause the bullet to travel sideways as well as forward when it is fired. That degree of lateral velocity could probably be compensated by angling the gun slightly. IOW, the path of the bullet will not be exactly along the axis of the gun.

I could imagine staring straight down the barrel(s) of an M61, when someone pulls the trigger, and being pleasantly surprised when all the rounds hit the wall next to you. I wouldn't volunteer to try it out, though!

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The vid shows, quite clearly, that, for the minigun, the barrel actually *stops* at the firing position.

Ha, I almost fell into the same trap!!

But watch it again, it is deceiving: The replay speed is just slowed down much more when the bullet is fired! You can tell it from the speed of expansion of the gases. Obviously the breaks between the barrels being fired would be too long if they used "bullet time" all throughout the video!

Best regards,

Thomm

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From http://en.wikipedia.org/wiki/Talk%3AGatling_gun:

The physics is correct, there is a cross-velocity to the projectile that results from the spinning cluster... it is, however, very predictable and very consistent. It is partly compensated for by having a slight cant to the barrels (they aren't actually parallel to each other or to the centerline of the gun) and the remainder and is easily compensated for in the fire control calculations.

Best regards,

Thomm

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Ha, I almost fell into the same trap!!

But watch it again, it is deceiving: The replay speed is just slowed down much more when the bullet is fired! You can tell it from the speed of expansion of the gases. Obviously the breaks between the barrels being fired would be too long if they used "bullet time" all throughout the video!

Best regards,

Thomm

Hmmm....I'm not so sure....

I can see that the playback slows way down as the bullet exits the muzzle, but the barrel appears to be completely stationary as the bullet flies forward. Doing the same calculation as before:

M134: barrel length = 0.559 m

muzzle velocity = 853 m/s

number of barrels = 6

rate of fire = 4000 rpm

(According to wikipedia, it is rated at 6,000 rpm, but is usually fired at 4,000 for improved reliability).

4000 rpm (6 barrels) = 667 rpm per barrel = 11.1 rounds/sec/barrel = 11.1 rev per sec angular velocity of barrel assembly.

muzzle velocity = 853 m/s -> 426 m/s avg velocity in barrel

barrel length = .559 m / 426 m/s = 0.00131 sec in barrel

0.00131 sec * 11.1 rev per sec = 0.014 rev * 360 deg/rev = 5 degrees displacement while round travels the length of the barrel.

During the slo-mo firing footage at 3:00, the bullet is seen travelling at least twice the length of the barrel after it leaves the muzzle, and the barrel does not appear to rotate at all, when it should have moved about 5 degrees in that time. There is another, less slowed-down shot, at about 3:55, where the barrels appear to keep rotating when the round is fired.

It seems that it would be unnecessarily complicated to devise a system of stopping the barrel for 1.3 ms as each round is fired, when the lateral acceleration caused by a continuously rotating barrel could be compensated for by just biasing the alignment of the barrel. But, I'm not an engineer.

Any other thoughts?

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