The Martlet 3A was an diameter, gun-fired rocket projectile that theoretically was able to reach 500 km altitude. As HARP's first attempt at a low-cost sabotted rocket system, the projectile was built with fiberglass or aluminum bodies. A standard 6-inch rocket was bonded to an aluminum case, and the rocket nozzle was supported by a pusher plate, which would impart the acceleration thrust to the missile through the aluminum wall casing. Fiberglass limited acceleration to 3600 g (corresponding to a velocity of at rocket ignition). The Martlet 3A's original objective was to carry a 40-lbs payload to an altitude of 500 km, which theoretically was feasible if the system could be launched at full gun pressures. The rocket motors' solid propellant deformed during firing and the design was never successful, despite several test firings.