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 (same type used in the Bell X-1) was installed, the first 11 drops from the B-52 launch 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 (M2-F2/F3, HL-10, X-24A).