The Almaz program was a highly secretive Soviet military space station program, begun in the early 1960s. Three crewed military reconnaissance stations were launched between 1973 and 1976: Salyut 2, Salyut 3 and Salyut 5. To cover the military nature of the program the three launched Almaz stations were designated as civilian Salyut space stations. Salyut 2 failed shortly after achieving orbit, but Salyut 3 and Salyut 5 both conducted successful manned testing. In addition to reconnaissance equipment, Almaz was equipped with a unique 23mm Rikhter (factory index 261P or 225P) rapid-fire cannon mounted on the forward belly of the station. This revolver cannon was modified from the tail-gun of the Tu-22 bomber and was capable of a theoretical rate of fire of 1800-2000 (up to 2600) rounds per minute. Each 168 gram (ammo 23-OFZ-D-R ) or 173 gram (ammo 23-OFZ-G-R) projectile flew at a speed of 850 m/s relative to the station. The cannon had a supply of 32 rounds and was tested at the end of the mission, when the station was operating in unmanned mode. To aim the cannon, which was on a fixed mounting, the entire station would be turned to face the threat. The Almaz series remain, to this day, the only known armed, crewed military spacecraft ever flown.