AKA: RTV-G-4. Status: Retired 1950. First Launch: 1948-05-13. Last Launch: 1950-07-29. Number: 8 . Thrust: 267.00 kN (60,023 lbf). Gross mass: 12,862 kg (28,355 lb). Height: 17.25 m (56.61 ft). Diameter: 1.65 m (5.42 ft). Span: 3.56 m (11.69 ft). Apogee: 250 km (150 mi).
In February 1946, Malina at the Jet Propulsion Laboratory calculated that a two-stage research rocket, consisting of a captured German V-2 as first stage and JPL's small WAC Corporal as the second stage, could achieve Mach 9. In October the Army funded the project, code-named 'Bumper', under the umbrella of General Electric's Hermes program. Bumper would require solution of the problems of stage separation and rocket ignition at high altitude. The V-2 was only altered at the nose - rails were fitted into which the Wac Corporal's fins slotted. The WAC however was extensively modified, the resulting model being designated the Bumper Wac. Canted fins with 50% more area than the ground-launch WAC design were intended to impart the necessary stability for the high-altitude release atop the V-2. However the first tests showed the fins were ineffective, and spin rockets were added to stabilize the WAC at high altitude.
The rockets were staged by throttling back the V-2's engine when a predetermined speed was achieved. The V-2 then sent a signal to the Bumper Wac to ignite its engine. The WAC's thrust burned through a wire, signaling the V-2 to cut off its engine. The WAC could then slide out of its rails and begin its free flight.
In May 1948 the first Bumper was launched from White Sands with a dummy upper stage and a small solid-propellant charge to test stage separation. The first all-up flight was on 30 September 1948, but the Wac's engine exploded on ignition. The cause was found and corrected. On the fourth flight the V-2 failed. Finally the fifth flight, on 24 February 1949 was a success. The WAC reached a record altitude of 393 km
The V-2 failed again on the sixth flight, which ended high-altitude attempts. The last two Bumpers instead were fired horizontally once in the upper atmosphere to meet the original objective - aerodynamic data at hypersonic speeds. These long-range launches could not be made from land-locked New Mexico. The Army selected a launch site at Cape Canaveral in Florida, and the two Bumpers launched from there were the first use of the facility. The WAC failed on the first attempt, but on the last launch of the program, on 29 July 1950, the WAC was accelerated to 5260 kph.
Fully fueled round used and attained highest altitude to that date. In addition to testing equipment the flight was used to measure upper air temperatures. V-2 reached 102 km, 1170 m/s; WAC 400 km, 2300 m/s. Provided information on ion densities in the F-region of the ionosphere.
Bumper No. 8, a German V-2 with a 320 kg Army-JPL Wac Corporal, was fired from the Long-Range Proving Ground at Cape Canaveral at a very low angle of attack. The first-stage V-2 climbed 16 km before it exploded. The second-stage Corporal separated successfully, however, and traveled another 24 km. This was the first missile launch from Cape Canaveral.
Bumper No. 7 was the second missile launch from Cape Canaveral. This was to be a maximum range test of a two-stage vehicle, to study the problems in staging. The launch was delayed because of moisture in the vehicle. But when finally launched, the WAC achieved the highest sustained speed in the atmosphere to that date (Mach 9/2500 m/s) and 35.2 km altitude before impacting 305 km downrange.