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However, while the speed of the aircraft should be added to the muzzle velocity, what graham.d says is absolutely true and the rate at which the round slows down increases with the initial velocity of the round, so if the aircraft is traveling at high speed there is a definite risk of running into its own fire.
The Lockheed SR71 Blackbird from Kelly Johnson's 'Skunkworks' still claims the record, held for more than 30 years, for the fastest piloted air-breathing aircraft. Mach 3 plus. Approx 3,500 kph or a little under a km per second. This is seriously fast.If a stationary M61 cannon was fired at the tail of this passing SR71 the shells would impact the aircraft at a relative speed of approx 70 m/s. Or around 160 mph. (In the milliseconds of opportunity...)On the other hand, the cannon shell fired head on at this approaching aircraft would impact at a relative speed of approx 2,000 m/s, or about 7,200 kph, or about 4,500 mph. I won't go into the prospect of two SR71s, each armed with an M61, each at record speed approaching each other head on firing canons. What the heck...should opposing shells collide, they would impact each other at a relative speed of approx 4,000 m/s, or 14,400 kph, or 9,000 mph. There's a pretty good treatment of this extraordinary aircraft here:http://en.wikipedia.org/wiki/SR-71_BlackbirdWhich includes this:"A defensive feature of the aircraft was its high speed and operating altitude, whereby, if a surface-to-air missile launch were detected, standard evasive action was simply to accelerate..."
So if the SR-71 was flying parallel to the ground, this M61 cannon where mounted on foreward section the jet, velocity of the craft is at mach 3, a round is released firing foreward.From what you are saying that the round will be coming out of the barrel will be traveling 70 m/s with respect to the plane? Is this an accurate deduction?If it is a correct deduction, the bullet will only travel 8 meters in a forward arch before making its direct decent to the ground?
70 m/s; but not for long, as explained by graham.d.And of course, the SR71 was armed with nothing more offensive than cameras. Truly remarkable cameras, but that's another story...