NASA's X-Fleet

If not for the X-planes, America might never have reached space. It was in the first X-plane, X-1, that Chuck Yeager flew faster than the speed of sound in 1947. More than 100 X-plane variations have followed, each one furthering our understanding of spacecraft design. Today, there are several new X-planes being developed. We will take a look at three of them:

• The X-37, which will test many space plane technologies, including re-entry capabilities.
• The X-34, a suborbital vehicle that will test technologies to reduce cost, time and personnel for space launches.
• The X-33, a reusable launch vehicle (RLV) that is a prototype for a space shuttle replacement.

Photo courtesy NASA The X-37 will test re-entry capabilities of the experimental space plane materials.

Of the three X-planes mentioned above, the X-37 is the newest and fastest. Unlike other space planes, the X-37 will not be launched under its own power. It is designed to be launched into space on a secondary vehicle. The unmanned, programmable plane will ride onboard the space shuttle as a secondary payload. Once in orbit, the X-37 will be deployed from the shuttle's cargo bay. It will then remain in orbit for 21 days, performing numerous experiments before returning to Earth and landing like an airplane.

In 1998, NASA selected Boeing to design the X-37, and a year later an agreement was reached to develop the new space plane. The X-37 is the only one of the three space planes that is designed to be an orbital plane, and to travel at Mach 25 speed, which means it is capable of traveling at about 17,500 miles per hour (28,163 km/h). The goal of the X-37 project is to test RLV technologies in harsh space environments, and demonstrate about 40 advanced airframe, propulsion and operations technologies. A major focus of the X-37 project is to improve the thermal protection systems that keep spacecraft from burning up during re-entry. NASA has said that the first flight tests for the X-37 will begin in early 2002, and it could go into orbit on the space shuttle later that year.

The X-37 looks a little bit like a miniature model of the space shuttle. It is 27.5 feet (8.38 meters) long, which is shorter than an average school bus and only about half the length of the current space shuttle payload bay. At 6 tons, the X-37 is extremely light for a NASA spacecraft, weighing the equivalent of about three sport utility vehicles. It has a wingspan of only 15 feet (4.57 m) and has its own experiment bay, which measures 7 X 4 feet (2.13 X 1.21 m). The vehicle will be propelled by the AR-2/3 rocket engine, which has been used since the 1950s and can produce more than 7,000 pounds of thrust. The AR-2/3 uses JP-10 jet fuel, a type of kerosene, and hydrogen peroxide as propellants.

Like the X-37, the X-34 space plane is testing new technologies to build future spacecraft that will lower mission costs. However, while the X-37 is still about a year and a half from getting off the ground, testing for the X-34 is already under way.

Photo courtesy NASA An artist's concept of the X-34, a test vehicle for future generations of reusable launch vehicles.

In June 1999, NASA strapped the unmanned X-34 to the underbelly of an L-1011 carrier aircraft for a "captive carry" flight, in which the X-34 remained attached to the L-1011 for the duration of the flight. During the test flight, scientists were able to analyze several functions of the X-34, including the release of rocket propellant into the engine and electrical connections between the X-34 and the L-1011. Later, the X-34 will be dropped from the L-1011 at an altitude of 40,000 feet, and glide unpowered down to a runway landing.

With the look of a new age Concorde jet, the suborbital X-34 will be able to travel at Mach 8, which is 5,600 mph (9,012 kph). Larger than the X-37, the X-34 is 58 feet (17.6 m) long and has a wingspan of 28 feet (8.53 m). Eventually, the X-34 space plane will be powered by a Fastrac rocket engine, a cheaper engine than previous engines used by NASA. The Fastrac is built mostly from off-the-shelf components, and has fewer parts than other rocket engines. The Fastrac rocket engine operates with a single turbopump, which is composed of only two pumps -- one for kerosene and one for liquid oxygen. The engine's gas generator cycles a small amount of kerosene and oxygen to provide gas to drive the turbine, and then exhausts the spent fuel.

Probably the most ambitious of NASA's space planes, and the most expensive, is the X-33. It also happens to be the space plane that is the farthest along in development. In the next section, we'll talk about a space plane based on the X-33 design that may one day replace the space shuttle.