"Martlets." The Mark II Martlets were cylindrical finned projectiles, about eight inches wide and five feet six inches long. They weighed 475 pounds. Inside the barrel of the space-cannon, a Martlet was surrounded by a precisely machined wooden casing known as a "sabot." The sabot soaked up combustive energy as the projectile flew up the space-cannon's sixteen-inch, 118-ft long barrel. As it cleared the barrel, the sabot split and the precisely streamlined Martlet was off at over a mile per second. Each shot produced a huge explosion and a plume of fire gushing hundreds of feet into the sky.

The Martlets were scientific research craft. They were designed to carry payloads of metallic chaff, chemical smoke, or meteorological balloons. They sported telemetry antennas for tracing the flight.

By the end of 1965, the HARP project had fired over a hundred such missiles over fifty miles high, into the ionosphere -- the airless fringes of space. In November 19, 1966, the US Army's Ballistics Research Lab, using a HARP gun designed by Bull, fired a 185-lb Martlet missile one hundred and eleven miles high. This was, and remains, a world altitude record for any fired projectile. Bull now entertained ambitious plans for a Martlet Mark IV, a rocket-assisted projectile that would ignite in flight and drive itself into actual orbit.

Ballistically speaking, space cannon offer distinct advantages over rockets. Rockets must lift, not only their own weight, but the weight of their fuel and oxidizer. Cannon "fuel," which is contained within the gunbarrel, offers far more explosive bang for the buck than rocket fuel. Cannon projectiles are very accurate, thanks to the fixed geometry of the gun-barrel. And cannon are far simpler and cheaper than rockets.

There are grave disadvantages, of course. First, the payload must be slender enough to fit into a gun-barrel. The most severe