A Bullet in the Tail
Marty Bushnell
An interesting event happened to me one day when I was flight testing the (then developmental) F-15. We were experimenting with a modified Gatling type 20 mm cannon that operated at a higher hydraulic pressure to increase the rate of fire from 6000 rounds per minute to something like 8000. The gun barrels were also splayed a little to widen the bullet impact pattern. The theory was that the new pattern would have the bullet pattern density of the standard gun but a bigger impact area, increasing the probability of kill.
The test point was at level flight, high altitude, and something like mach 1.2. I fired a two-second burst with no apparent problem, then moved on to other test points. When I returned to the base, parked the plane, and shut down, the crew chief did his customary post flight inspection. He came up the ladder wide-eyed and showed me a 20mm round. He had dug it out of my right vertical stabilizer. I had shot myself in the tail.
Engineering analysis considered the fact that the F-15 gun line is elevated two degrees above the aircraft longitudinal axis, as opposed to the conventional mount in the fighters of the day of two degrees below the axis. The splayed barrels would effectively increase bullet path elevation even more. The thought was that perhaps I overtook the bullet as its trajectory arced back into my flight path. In the end, the most probable cause was concluded to be the likelihood that the bullet tumbled due to the high mach and it backed into me. I’m glad it didn’t back into the canopy or an engine. The damage was cosmetic.
I still am not sure whether the gun was meant to be fired at that speed.
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