After delivery to NASA in January 1966, the HL-10 made its first flight on December 22, 1966, with research pilot Bruce Peterson in the cockpit. Although the XLR-11 rocket engine was installed, the first 11 drops from the B-52launch aircraft were unpowered glide flights to assess handling qualities, stability, and control. In the end, the HL-10 was judged to be the best handling of the three original heavy-weight lifting bodies. The HL-10 was flown 37 times during the lifting body research program and logged the highest altitude and fastest speed in the lifting body program. On February 18, 1970, Air Force test pilot Peter Hoag piloted the HL-10 to Mach 1.86. Nine days later, NASA pilot William H. "Bill" Dana flew the vehicle to, which became the highest altitude reached in the program. During a typical lifting body flight, the B-52—with the research vehicle attached to the pylon mount on the right wing between the fuselage and inboard engine pod—flew to a height of about and a launch speed of about. Moments after being dropped, the XLR-11 was lit by the pilot. Speed and altitude increased until the engine was shut down by choice or fuel exhaustion, depending upon the individual mission profile. The lifting bodies normally carried enough fuel for about 100 seconds of powered flight and routinely reached from to and speeds above Mach 1. Following engine shutdown, the pilot maneuvered the vehicle through a simulated return-from-space corridor into a pre-planned approach for a landing on one of the lakebed runways on Rogers Dry Lake at Edwards. A circular approach was used to lose altitude during the landing phase. On the final approach leg, the pilot increased his rate of descent to build up energy. At about altitude, a "flare out" maneuver dropped air speed to about for the landing. Unusual and valuable lessons were learned through the successful flight testing of the HL-10. During the early phases of the Space Shuttle development program, lifting bodies patterned on the HL-10 shape were one of three major types of proposals. These were later rejected as it proved difficult to fit cylindrical fuel tanks into the always-curving fuselage, and from then on most designs focused on more conventional delta wing craft.
HL-10 pilots
*John A. Manke — 10 flights, 7 powered flights
*William H. Dana — 9 flights, 8 powered flights
*Jerauld R. Gentry — 9 flights, 2 powered flights
*Peter C. Hoag — 8 flights, 7 powered flights
*Bruce Peterson — 1 flight, 0 powered flights
Unrealized space flight
According to the book "Wingless Flight", by project engineer R. Dale Reed, the HL-10 was considered to fly into space in the early to mid-1970s. Following the cancellation of the Apollo moon project, Reed realized that there would be substantial Apollo hardware left over, including several flight-rated command service modules and Saturn V rockets. The proposal was to add an ablative heat shield, reaction controls, and other additional subsystems needed for crewed spaceflight to the HL-10. The now space-rated vehicle would have then been launched in the space for the Lunar Module on a Saturn V launch vehicle with an Apollo CSM. Once in Earth orbit, it was planned that a robotic extraction arm would remove the HL-10 from the rocket's third stage and place it adjacent to the crewed Apollo CSM spacecraft. One of the astronauts would then spacewalk from the Apollo and board the lifting body to perform a pre-reentry check on its systems. It was planned that there would be two flights in this program. In the first, the lifting body pilot would return to the Apollo and send the HL-10 back to earth uncrewed. If this flight was successful, the second launch would be involve a piloted landing at Edwards AFB. Reportedly, Wernher von Braun was enthusiastic about the mission, offering to prepare two Saturn Vs and Apollo Command Service Modules. However, he was overridden by the Flight Research Center director, and nothing came of the proposal. Launching a Saturn V to low Earth orbit with a light payload would not be an efficient use of capability, and the Apollo program was ended mainly on cost grounds.
In the pilot movie, and an episode of The Six Million Dollar Man series, titled "The Deadly Replay", the HL-10 serial number 804 is identified as the aircraft flown by Col. Steve Austin when he crashed, leading to his transformation into a bionic man, and the HL-10 is also featured in this episode. Other episodes and Martin Caidin's original novel, Cyborg, contradict this, however, by identifying Austin's aircraft as a fictional cousin of the HL-10, the M3-F5. Further confusion is added by the fact that both the HL-10 and the M2-F2 are featured in the opening credits of the television show.
