Boston
NASA placed a heavy bet on new technology last week, when it picked Lockheed
Martin to build the X-33鈥攁 half-scale version of the space shuttle鈥檚
proposed successor. Lockheed Martin鈥檚 proposal 鈥減ushes the technology most,
which is in NASA鈥檚 and the nation鈥檚 interest鈥�, according to Edward
Crowley, head
of the aeronautics and astronautics department at the Massachusetts
Institute of
Technology and a member of the evaluation panel that picked the design.
By the end of 1999, when the X-33 contract expires, NASA will have spent
$941 million on the experimental vehicle, which Lockheed calls the
Venture Star. Lockheed will have invested $220 million. The company won
NASA approval in the face of competing designs by McDonnell Douglas Aerospace
and Rockwell International.
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The X-33 will be uncrewed. It is designed to reach altitudes of 80
kilometres
and speeds of up to Mach 15. At least 15 test flights are scheduled for
the nine
months beginning in March 1999. If the tests are successful, the next step will
be to build a full-scale, reusable single-stage to orbit vehicle (SSO).
SSO rockets need to carry nine times their own weight in fuel, so it is
crucial to keep the weight of the vehicle itself to a minimum. With this in
mind, all the X-33 proposals were designed to be built from graphite-based
composites rather than the aluminium used for the shuttle.
Venture Star鈥檚 鈥渓ifting body鈥� design is like an arrowhead with two small
fins. It does not need wings, which add extra weight. 鈥淚t has no wasted
aerodynamic area or parasitic wing weight,鈥� says Steve Cook, NASA deputy
programme manager.
The large triangular body also stays cooler during re-entry, allowing
Lockheed to fit metallic heat shields instead of the ceramic tiles used in the
shuttle and the two competing X-33 designs. The shuttle鈥檚 ceramic tiles are not
very durable and must be waterproofed between flights. NASA spends 17 000
working hours inspecting and repairing them after each shuttle mission.
Lockheed鈥檚 metallic shield will be built up from square panels, made from a
honeycomb of inconel superalloy or titanium, which are both used in aircraft
engine exhaust ducts. The outer layer of the panels will form the skin of the
whole craft. The fuel tanks get extra insulation from a ceramic fibre
blanket.
Lockheed will power its X-33 with 鈥渁erospike鈥� engines (鈥淔light of the
aerospike鈥�, 快猫短视频, 6 July). These require
higher-temperature
materials for their nozzles than conventional engines, but are more powerful.
They are also less sensitive to changes in pressure as the vehicle rises into
orbit.
Perhaps the biggest advances that have been made since the shuttle was
designed are in computers and electronics. Unlike the shuttle, which is
flown by
a pilot after re-entry, the X-33 will fly and land itself. The full-scale
version will carry astronauts, but Cook says they will be passengers
rather than
pilots.
Some of NASA鈥檚 goals鈥攕uch as putting payloads in orbit for
$1000
a pound鈥攁re reminiscent of early targets for the shuttle. For example, a
report released in 1973 predicted that the shuttle could put payloads in orbit
for about $640 per pound. Today the cost is $10 000.
