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

490 X-33
configuration and renamed the X-24B. In this form, a further
12 glides and 24 powered flights were made. The
X-24 program was concluded in 1975.
X-33
An almost half-scale prototype of the VentureStar
reusable single-stage-to-orbit launch vehicle proposed by
Lockheed Martin. The X-33 program was begun by
NASA in 1996 but ran into development problems with
the lightweight composite materials from which the vehicle’s
propellant tanks were intended to be made. Budget
constraints finally led NASA to abandon the X-33 and
related X-34 programs in 2001.
X-37/X-40A
The X-40A is a 90%-scale model of the X-37 Space
Maneuver Vehicle, intended to fly into space inside the
Space Shuttle payload bay to deliver satellites and perform
reconnaissance missions and other tasks. At the end
of its mission, it can return to the Shuttle or land on its
own. The X-40A has conducted several drop tests from a
helicopter. The X-37 will be about 8.5 m long and should
be capable of making at least 20 flights and landings.
X-43A An artist’s conception of the X-43A in hypersonic flight. NASA
X-38
A prototype of a proposed ACRV (Assured Crew Return
Vehicle) for the International Space Station. The sevenseat
X-38, with a design based on that of the X-24 from
the 1960s, was being developed at Johnson Space Center
and Dryden Flight Research Center and had made several
test flights. However, work on the program was stopped
in 2001 following NASA budget cuts.
X-43A
A subscale prototype that forms part of NASA’s Hyper-X
program to develop a commercial hypersonic suborbital
aircraft. The X-43A scramjet is designed to fly at speeds
between Mach 7 and Mach 10, easily beating the world’s
previous fastest air-breathing plane, the Mach-3 SR 71
Blackbird. The X 43-A is launched aboard a Pegasus from
a modified B-52 bomber. The first flight, on June 4,
2001, ended in failure when the Pegasus launch vehicle
veered out of control and had to be destroyed just minutes
before its passenger was due to fire its scramjet
engine. NASA has other X-43As with which it will
resume flights when the problem with the Pegasus has
been resolved.