New research aircraft are being announced at a rate that hasn't been seen since the post-World War II period, when the X-1 became the first in a distinguished lineage of craft designed predominantly for a single purpose: exploration of high-speed aerodynamics. During the late 1940s and throughout the 1950s, new shapes featuring swept wings, variable wings, tiny wings, and nearly no wings at all flew at speeds ranging from Mach 1 to more than Mach 6 and ventured out into the fringes of space (see "The X-Planes," Oct./Nov. 1993). Now a new generation of vehicles will take it from there.
While the majority of the first X-aircraft carried pilots, most of the next generation will be commanded remotely and flown by a combination of ground controllers, onboard computers, and autopilots. Aircraft with a hero strapped inside may have more sex appeal, but they are also heavier, more complicated, and more expensive. The highly focused research of the new craft, most of which are designed to prove various concepts, does not depend on onboard humans. And humans are increasingly unnecessary as more powerful avionics, navigation systems, and flight control computers are being matched up with lighter and more durable materials for reentry heat shields, as well as radical new methods for maneuvering without the use of traditional control surfaces such as elevators, ailerons, and rudders. This trend is being echoed in military aircraft design, which is also increasingly turning to remotely piloted vehicles.
Among the new class, there are two exceptions to the trend. One is the Boeing X-32, a prototype for a future airplane called the Joint Strike Fighter As the word "joint" suggests, this airplane is to be produced in different versions for different customers. There's a conventional version for the U.S. Air Force, a carrier version for the U.S. Navy, and a vertical-takeoff version for the U.S. Marine Corps and the United Kingdom's Royal Navy. Because these airplanes are intended to be relatively inexpensive to build and maintain, 90 percent of their parts will be common to all three versions. Beneath its nose, Boeing's prototype has a distinctive intake scoop that's vaguely reminiscent of the LTV A-7D Corsair II.
The other new X-plane that carries a real, live pilot is the Lockheed Martin X-35, which is competing with the X-32 for the Joint Strike Fighter contract. It has a trapezoidal wing instead of the Boeing design's delta shape, and distinctive twin intakes at the wing roots.
The Boeing-NASA X-36 is unique in being a remotely piloted research prototype for what could ultimately become a piloted fighter. The 28-percent-scale craft is powered by a Williams jet engine and is the only member of the new generation that has absolutely no vertical surfaces. The design is intended to prove that a shape can be both stealthy and highly agile. A video camera in the nose provides the pilot with a view from what would be the cockpit, and the airplane is flown from a station on the ground. The X-36 has completed its research flying and is currently being stored at NASA's Dryden Flight Research Center in California.
The X-32, X-35, and X-36 are all capable of supersonic speeds and intended for atmospheric flight. They are outnumbered by new X-craft designed to fly higher, faster, and out to Earth orbit and back.
The Lockheed Martin-NASA X-33 is a scaled-down demonstrator for a future Reusable Launch Vehicle, a craft aimed at reducing the cost to place payloads in orbit (see "Infrequent Fliers," Aug./Sept. 1999). It combines a wingless lifting body shape with a new type of engine called a linear aerospike, which is like a conventional rocket nozzle turned inside out; combustion takes place on its external surface instead of inside a bell-shaped nozzle. The X-33 takes off vertically, climbs to an altitude of 60 miles at speeds up to Mach 15, and lands on a runway like an airplane.
As an interim step, the NASA-Orbital Sciences X-34 will test various materials and concepts up to speeds of Mach 8 and altitudes of 250,000 feet. Powered by a rocket engine using kerosene and liquid oxygen, the X-34 will be dropped from a Lockheed L-1011 airliner that Orbital Sciences has transformed into a launcher for its Pegasus winged boosters (to which the X-34 bears a slight resemblance). It will return to land on a conventional runway and will be used to research techniques to reduce the time between flights of reusable launch vehicles.
A Pegasus booster will also be used as a carrier for the unusual X-43A series of NASA research vehicles, built by MicroCraft under a program called Hyper-X, for "hypersonic aerodynamics research." A B-52 will carry the booster-X-43A combination to launch altitude, then release it. The Pegasus will accelerate the X-43A to Mach 7 and Mach 10 for experiments in aerodynamics and scramjet propulsion ("scramjet" is shorthand for "supersonic combustion ramjet"). At the end of each flight, the craft will crash into the Pacific Ocean.
Another series of experimental flights will culminate in orbital research. The NASA X-38 is a craft designed to prove that a vehicle docked with the international space station can be used by the crew to return to Earth if necessary (see "Lifeboat," Aug./Sept. 1998). The X-38 is based on a wingless lifting body research vehicle, the X-24, which was conceived in the mid-1960s. Designed for a one-way trip, the X-38, which began drop tests last spring, will make its final test flights from orbit and land at very low speeds by deploying a huge parafoil, which has already proved successful in tests.
Its designation suggests that the Boeing-NASA X-37 is the X-38's predecessor, but the craft actually came into being only last August. Both it and the X-40A, which has already been dropped from a helicopter to glide to a successful autonomous runway landing, resemble scaled-down space shuttles. The two craft are aimed at developing technologies for an unpiloted orbiter that would be able to fly in space and return to land on its own. But the X-37, larger and more complex than the unpowered X-40A, has a rocket engine that uses jet fuel and hydrogen peroxide. Boeing says the X-37 could lead to a commercial launch vehicle that provides cheaper access to orbit.
The X-31, a delta-wing airplane with thrust-deflecting paddles and the ability to fly at extreme attitudes with its wings fully stalled (see "Stall Tactics," Apr./May 1991), was the last active member of the previous wave of test aircraft. If it gets further funding from the three partners backing it-Germany, Sweden, and the United States-it will embark on a second career. With most of its vertical fin and rudder removed, it will be used in a program called VECTOR, for Vectoring Extremely short takeoff and landing Control, and Tailless Operations Research, which will focus on aircraft using vectored thrust aboard carriers.
Expect the surge of new designs to continue. Other numbered slots for X-craft are reportedly allocated, and there's even a conceptual drawing of an X-44 in circulation (it's said to resemble a tail-less F-22).
The first generation of research craft were designed solely to conduct research in aerodynamics. The latest generation are combining high-speed research in aerodynamics and spaceflight with exploration of a realm that research aircraft have never probed before: economics. Reduced size and weight, smart avionics that replace the pilot, new low-cost materials that resist high-Mach heat-all signs point to a future in which performance remains the foremost goal, but affordability runs a close second.