How Space Planes Will Work

Replacing the Space Shuttle
Nearly two years behind schedule, NASA still plans to complete the X-33 space plane.
Nearly two years behind schedule, NASA still plans to complete the X-33 space plane.
Photo courtesy NASA

A new era in space travel began on the morning of April 12, 1981, when the first space shuttle, the Columbia, flew into orbit. Since then, the space shuttle has remained NASA's primary launch vehicle for conducting research and deploying satellites and other spacecraft into space. The space shuttle has also allowed astronauts to construct the International Space Station.

However, despite the shuttle's many accomplishments, the fact remains that it is extremely expensive to launch into space. Each pound of payload in the shuttle's bay costs $10,000 to launch. According to NASA, each of the space shuttle's two solid rocket boosters carry about 1 million pounds (453,592 kg) of solid propellant. The large external tanks hold another 500,000 gallons of supercold liquid oxygen and liquid hydrogen. These two liquids are mixed and burned to form the fuel for the shuttle's three main rocket engines. The cost of this huge amount of propellant, and of recovering and replacing the solid rocket boosters for every mission, is extremely expensive. NASA's solution to the problem is the X-33.

The X-33 is a prototype for a unique single-stage-to-orbit vehicle. Its wedge-like shape is unlike any spacecraft that has preceded it. At its base, the X-33 is 77 feet (23.5 m) wide, and the vehicle is 69 feet (21 m) long. The purpose of this design is to allow the spacecraft to hold all of the needed propellant onboard the ship, thus eliminating the need for solid rocket boosters. By eliminating the boosters and the main fuel tank, NASA will trim much of the liftoff weight that makes space shuttle missions so expensive. Launch costs for the X-33, or a derivative of the X-33, are expected to be only a tenth of the cost of launching the space shuttle.

There have been problems with the X-33 project, which began in 1996. At present, it is nearly two years behind schedule, and the costs are far exceeding expectations. NASA and Lockheed Martin have already spent more than $1 billion on the X-33, and it is still only three-quarters finished. In November 1999, tests on the graphite fiber composite fuel tanks failed, which had NASA scientists scrambling to design a new tank out of traditional aluminum material. Despite these setbacks, NASA said that it is going ahead with building the X-33, and now expects to have a functioning vehicle ready for suborbital flight in 2003.

NASA said that the Aerospike rocket engine is more efficient than the conventional Bell rocket engine.
Photo courtesy NASA

Two uniquely designed engines will propel the spacecraft. The X-33 will be the first space plane to use Linear Aerospike engines. The shape of the engines is a better fit for the wedge-shaped space plane than the conventional bell nozzle rocket engines, according to NASA. In contrast to the nozzle of the Bell rocket engines, the Aerospike nozzle is V-shaped, called a ramp. The hot gases are shot from the chambers along the outside of the ramp's surface. These new engines will propel the X-33 to speeds up to Mach 13 (9,100 mph / 14,645 kph).

The ultimate goal of the X-33 project is to produce a commercial aircraft called the VentureStar, which would be the successor to the space shuttle. The VentureStar will be about twice the size of the X-33 prototype, and will use the same type of engines and the same construction materials. However, it will be able to achieve Mach 25, which is the necessary speed to maintain Earth orbit. Not only would the VentureStar be used to put payloads into space, but it could also be used as a space tourism vehicle. The success or failure of the X-33 will determine whether the VentureStar becomes the vehicle that enables public access to space.

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