Home Up



Battle Of The X-Planes

As a multitude of programs vie for funding and attention, X-planes cross the final frontier of flight.


Illustration by Northrop Grumman

A secret air war is being waged in clear California skies and in dreary Pentagon briefing rooms. It is the battle of the X-planes -- some so secret they have never been photographed. The outcome of this quiet conflict will determine where and how the nation fights its future wars: in air or space, with humans or silicon chips in the cockpit.

All wars simmer before they erupt. The battle of the X-planes began in the early 1990s with advances in microelectronics, completion of the Global Positioning System (GPS) and the success of cruise missiles during Operation Desert Storm. Together, those developments convinced even the most conservative defense planners that it was time to change the technology of aerial combat. With this objective in mind, the Air Force Research Laboratory (AFRL) and the Defense Advanced Research Projects Agency (DARPA) began exploring new ways to fight from space. At the same time, AFRL and DARPA also embarked upon a program to alter close combat. The name of that program is Future Aircraft Technology Enhancements (FATE).

The Air Force has placed a cloak of
secrecy over two of its projects,
the X-41 Common Aerospace Vehicle
and the X-42 Pop-Up Upper Stage.

Trusting FATE

Anticipating that FATE would require a flying testbed, AFRL reserved the X-39 designation and began an aggressive research program. Experiments included developing damage-resistant composite airframes and a shape-changing wing to replace hinged surfaces. Drawing on advances in computer technology, work began on artificial intelligence systems that would allow an autonomous aircraft to make minute-by-minute flight-planning decisions to accommodate revised targeting plans and unexpected changes in the weather in the combat zone.

Although all of the major airframe manufacturers in business during the late 1990s were involved in some aspect of FATE, none has claimed credit for building a complete X-39. So much of the project is classified, it cannot be confirmed if the plane itself ever progressed beyond engineering studies and wind-tunnel tests. Andreas Parsch, a recognized expert on weapons designations, suggests that the aircraft that started the X-plane revolution is not a proper X-plane at all. He points out that although the X-39 designation was reserved by the Air Force, no formal written request to allocate X-39 to FATE was filed. "Therefore X-39 remained officially unassigned," he concludes.

Regardless of the official standing of the X-39, there is little question of the impact of the FATE project with which it is so closely linked. "FATE served as the catalyst for Unmanned Combat Air Vehicles," says aviation historian Jay Miller. These UCAVs, as they are now commonly known, will change the face of combat itself by allowing the United States to project force farther and with greater accuracy than field commanders ever imagined possible. But to fight with such precision would require precise intelligence about enemy positions and movements. To obtain this information, other X-planes would be created to extend the battleground into space.

Fighting From Space

In the days immediately after World War II, Allied intelligence made a remarkable discovery. Agents scouring German factories found plans for a piloted, winged rocket capable of reaching the United States. Space planes have fascinated aerospace designers ever since.

In the 1960s, the X-15 came closest to realizing this dream, as military pilots flew to altitudes above 50 miles to earn astronaut wings. The design of the space shuttle's large delta wing was a result of the Air Force's interest in flying single-pass orbital flights over the Soviet Union--the pilots would take off and land at Vandenberg Air Force Base in California. Launch facilities were built, but the idea proved impractical. And after the Challenger disaster the Air Force returned to relying upon expendable launch vehicles. Later, the Air Force considered using a modified version of NASA's X-33 single-stage-to-orbit space plane concept. NASA pulled the plug after more than a billion dollars of research and development. Simply put, the X-33 was too heavy to fly.

The Air Force would step back to the future with its X-40 Space Maneuver Vehicle (SMV). The unmanned 22-ft.-long, 2500-pound, graphite-epoxy and aluminum vehicle would ride into space atop a rocket, just like a conventional satellite. The small size of the X-40 meant it could remain on orbit for as long as a year.

Like the X-33, the X-40 was designed to launch on a few days' notice. By contrast, space shuttle flights and expendable rocket launches require weeks of preparation. Once on orbit, the X-40 could quickly perform missions as diverse as replacing low-orbiting satellites destroyed by space debris and keeping watch over terrorists on the far side of the planet. Its mission complete, the X-40 would re-enter the atmosphere where, guided by GPS and its autonomous landing system, it would safely touch down on a hard-surface runway.

A 90 percent version, designated the X-40A, has been tested by the AFRL Military Spaceplane Technology Office at Kirtland Air Force Base in Albuquerque, N.M. Drop tests from a helicopter have demonstrated that its autonomous flight system works. A full-size X-40B space plane awaits funding.

Secret Space Planes

The X-40 SMV has two constraints. While it can carry satellites and weapons to low Earth orbits between 100 and 200 miles, it cannot place them on higher orbits or trajectories for re-entry. For these tasks the Air Force embarked upon the most secret of the current X-plane projects, the X-41 and X-42. Details of these two projects are sketchy. After they were quietly announced with a solicitation notice to the aerospace industry in January 1998, both the X-41 and X-42 quickly "went black." Neither official photos nor concept drawings of these two have been made public.

The original solicitation notice to prospective space plane contractors, however, does explain how these two X-planes are intended to work. "The X-41 is expected to be an experimental maneuvering re-entry vehicle capable of transporting a variety of payloads and dispensing them in the atmosphere," says X-plane historian Miller.

The X-42 would perform the opposite function, moving payloads weighing as much as 4000 pounds from low Earth orbits to orbits at altitudes of several thousand miles.

Mach-10 Bomber

The newest NASA-inspired X-plane to catch the eye of the Air Force is the X-43. Formerly known as the Hyper-X, it is heir apparent to the SR-71, the world's fastest air-breathing airplane. (The X-15 was a rocket plane that carried both fuel and oxidizer.) Thus far only a small proof-of-concept version, the X-43A, has been built. It begins its flight perched on the nose of a modified Orbital Sciences Pegasus rocket, which is slung below a modified B-52 bomber. As the B-52 reaches about 20,000 ft., the rocket drops away and ignites, propelling the X-43A to 95,000 ft. When the booster separates, the X-43A is traveling at several times the speed of sound, the speed necessary to jumpstart its revolutionary scramjet engine.

The purpose of the X-43A is to test the scramjet that will take it to hypersonic speeds (five times the speed of sound). A scramjet is a ramjet engine in which the airflow through the whole engine remains supersonic. This makes it possible for the aircraft to travel at speeds faster than Mach 20. Currently the SR-71 cruises slightly faster than Mach 3. The highest speed attained by NASA's rocket-powered X-15 was Mach 6.7. If all goes well, by the conclusion of its third test flight, the X-43A will reach Mach 10 and become the fastest airplane in the sky. In so doing, it will lay the technological groundwork for an aircraft that would be able to make the New York to Tokyo run in little more time than it now takes to fly from New York to Chicago ("Two Hours To Tokyo," July 2001, page 64). By 2024, military planners believe, the United States will need a new type of bomber that delivers Mach 10 speed and global range. Aside from space planes, it is the sort of performance that only a grown-up version of the X-43A can offer.

So far, things have not gone well for the X-43 program. A rocket engine malfunction during the first test forced launch safety officers to blow up the X-43A seconds into its first flight. Moments later, the unpiloted, 12-ft.-long research vehicle, which was manufactured by MicroCraft in Tullahoma, Tenn., crashed into the Pacific Ocean. The mishap was a setback, but doesn't spell the end for the program. NASA says that tests with its two remaining craft will resume in 2003.

Man vs Machine

In addition to the X-43, which could evolve into a hypersonic bomber, there are two other manned X-planes under development: the X-44 fighter and X-48 tanker/transport.

The X-44 is a tailless derivative of the F-22 Raptor, an air superiority fighter that recently went into limited production. "It is said to be based on an F-22 airframe, engines and systems," says weapons expert John Pike. There is, however, little resemblance to the F-22, as the tail assembly and wing-control surfaces have been removed. Instead, the X-44 will use vectoring nozzles that discharge controlled blasts of hot gases to control pitch and yaw. There is some disagreement among the aerospace experts as to the future of the X-44--it is rumored that the Air Force has ordered its cancellation.

The X-48 Blended Wing Body (BWB) would round out the future manned fleet. It is intended to be a support aircraft that refuels manned and unmanned aircraft, while carrying up to 40 troops and 23 conventional pallets of cargo. "The Blended Wing Body offers greater structural, aerodynamic and operating efficiencies than today's more conventional tube-and-wing design," says Pike. Together, these features mean greater range and room for expansion of the center body without altering the wings. "These capabilities make the BWB suitable as a long-range standoff weapons platform as well," says Pike. Northrop Grumman, which is working on the X-48, has one such concept, shown in the lead photo. NASA plans to test an 1800-pound, 35-ft.-long model of the X-48 in 2004.

Unmanned Combat

Judging from the X-plane lineup, unmanned aircraft have an even more promising future in combat. The X-45, X-46, X-47 and X-50 are all intended to be pilotless aircraft.

The X-45A, the most mature of these designs, successfully completed its first flight on May 22, 2002, at NASA's Dryden Flight Research Center in Edwards, Calif. The flight lasted only 14 minutes, but the Air Force, which is jointly sponsoring the project with DARPA, immediately hailed it as a breakthrough. "This flight represents a significant jump in our quest to mature the technologies and systems required to integrate UCAVs into the future Air Force," says Col. Michael Leahy, DARPA's UCAV program manager.

The most critical test is ahead. To be effective in combat, UCAVs must prove their ability to adapt to air combat tactics. An important part of the X-45 program is seeing how well these unmanned aircraft operate in coordinated groups. In combat, groups of three or four X-45s would hunt in packs. Tests with the X-45A are scheduled to begin in 2003. By 2006, X-45B models--larger than the X-45As--will demonstrate their ability to operate alongside manned aircraft. "In combat, UCAVs will perform extremely hazardous missions, such as the suppression of enemy air defenses," says Leahy.

Because they operate off carrier flight decks, naval aircraft have to meet different technical requirements than land-based planes. Envisioning a need to equip the Navy with a remote fighting capability, DARPA initiated a pair of Navy UCAV programs. The X-46 UCAV-N designation was assigned to Boeing, the X-47 UCAV-N designation was given to Northrop Grumman. Only one will be put into production.

Although it has acknowledged working on a Navy version of a UCAV, Boeing has kept its X-46 UCAV-N activities under wraps. By contrast, Northrop Grumman has showcased its work on the X-47 UCAV-N, which the company calls Pegasus. This aircraft should not be confused with the Orbital Sciences rocket, also named Pegasus, used to launch the X-43A.

On Feb. 23, 2003, the X-47A UCAV-N achieved a significant milestone with a 12-minute autonomously controlled flight at the Naval Air Warfare Center in China Lake, Calif. Engineers tested low-speed handling and navigation, then attempted a landing. This part of the test was especially critical to the Navy. Using naval GPS equipment as the primary navigation source, the X-47A successfully simulated a carrier flight deck landing. "Regular unmanned flight operations aboard a flight deck at sea have never been attempted, and Pegasus addressed some of those key concerns today," said Gary W. Ervin, Northrop Grumman Integrated Systems sector vice president for Air Combat Systems.

A look inside the nearly 28 x 28-ft. diamond-shape X-47A airframe reveals extensive use of lightweight composite materials. An advanced autonomous flight control system performs the second-by-second adjustments in control surfaces that eliminate the need for a tail assembly. The powerplant is a Pratt & Whitney JT15D-5C engine that produces 3200 pounds of thrust.

Eliminating Runways

Although the Air Force and Navy are leading current X-plane development, the Army and Marines are poised to be the chief beneficiary of the X-plane's most advanced flying wonder. The aircraft's designation, X-50, tells it all. X-plane numbers are assigned in sequence. In the normal scheme of things, the Boeing Dragonfly should have been the X-49. Boeing disagreed. The Dragonfly, it said, would be the first true canard rotor-wing, a perfect 50/50 balance between a rotary and fixed-wing aircraft, thus meriting the designation of X-50. And so the X-49 designation was skipped.

"An operational canard rotor-wing UCAV would be able to take off and land in confined areas without a launch or recovery system, rapidly transition to and from a fixed-wing mode, and fly at speeds in excess of 375 knots," explains Pike. This high speed is possible because the rotor can be stopped in flight, thus reducing drag. An unmanned version could take off and land in confined areas, such as small landing pads on ships. The X-50A is currently at the Boeing Phantom Works facility in Mesa, Ariz., where it is undergoing tests in the hover pit in preparation for its first flight in 2003.

Since World War II, developments in military aircraft have quickly found their way into civil aviation. If you have flown in a Boeing 707, you have traveled in a plane originally created as an Air Force tanker, the KC-135. There is talk of a passenger-carrying X-50 derivative, capable of operating without a runway or pilot. It could be just the ticket for flying into the 21st century.


Copyright 2010 Tim Stouse
Last modified: December 10, 2010
All 3rd Party Copyrights are acknowledged.
Material reproduced here is for educational and research purposes only.