The Complete Book of Spaceflight: From Apollo 1 to Zero Gravity

with a two-wheel conventional landing gear at the nose.
Three X-15s were built and 199 missions flown, the last in
November 1968. 244, 285 (See table, “Comparison of the
X-1, X-2, and X-15.”)
Comparison of the X-1, X-2, and X-15
X-1 X-2 X-15
Length (m) 9.4 11.5 16.0
Wingspan (m) 8.5 9.8 6.7
Mass, fully
fueled (kg)
5,560 11,300 15,400
Thrust (N) 26,700 66,700 254,000
Maximum 1,540 3,369 7,297
speed
(km/hr)
(Mach 1.45) (Mach 3.20) (Mach 6.72)
Maximum 21,916 38,500 107,960
altitude (m) (67.08 miles)
X-20
See Dyna-Soar.
X-23
See PRIME.
X-24 489
X-24
An early manned lifting body used by the U.S. Air Force
and NASA to explore the low-speed flight characteristics
of a subscale model, the X-23A, tested in the PRIME
program. There were two configurations. The 7.5-m,
5,000-kg X-24A was carried under the wing of a B-52
bomber and released at an altitude of about 13,500 m.
The X-24A could then either glide back to Earth or use its
onboard rocket engine to accelerate to speeds of 1,600
km/s or climb up to 21,500 m to explore the characteristics
of the design under various flight conditions. A total
of 28 flights, including 10 glides, were flown from 1969
to 1971 before the design was changed to a delta-winged
X-15 Crew members secure the X-15 after a research flight, while the B-52 launch plane makes a low flyby. NASA