for a few seconds, which seemed like "two hours" to the M-21 crew. A second launch took place on 27 April 1966; the D-21 reached its operational altitude of and speed of over , though it was lost due to a hydraulic pump failure after a flight of over . The USAF's interest in the program continued and more D-21s were ordered after the second launch. A third flight took place on 16 June with the D-21 flying through its complete flight profile, though its camera hatch was not released due to an electronics failure. The fourth and final launch from an M-21 on 30 July ended in disaster. Unlike the three previous launches this one was performed straight and level, not in an outside loop to assist in the separation of the drone from the aircraft. The D-21 suffered engine problems and struck the M-21's tail after separation, leading to the destruction of both aircraft. The two crew ejected and landed at sea. The pilot, Bill Park, survived, but the Launch Control Officer, Ray Torrick, drowned. Following the accident, Johnson suggested launching the D-21 from the Boeing B-52 Stratofortress bomber and adding a solid rocket booster to get it up to speed. The drone was modified by adding attachment points on its spine to mate with the carrying pylon on the B-52 and its belly attachment points were adapted to accommodate the rocket booster necessary to increase its speed and allow its ramjet to operate. Its vertical stabilizer was increased in size by approximately 20%. The modified drone version was designated D-21B (there was no D-21A). Two B-52Hs were modified to carry a pair of drones each by means of two large underwing pylons that replaced the smaller pylons used for the AGM-28 Hound Dog cruise missiles. The tail gunner's and electronic warfare officer's stations were replaced with two launch control stations. Command and telemetry systems were added, and high-speed cameras were installed to track the drones as they separated from the pylons. The launch control officer on the B-52H could communicate with the D-21B and could make it self-destruct. The solid-propellant rocket booster was both larger and heavier than the drone; it was long and weighed . It had a folding tail fin on the bottom to stabilize it while the rocket was firing. The booster had a burn time of 87 seconds and a thrust of .