300 N. Washington St.
Suite B-100
Alexandria, VA 22314

GlobalSecurity.org In the News

Aerospace America November 2001

Explaining X-planes

by Ben Iannotta, contributing writer

In the decade following WW II, U.S. military officials initiated 14 experimental flight test programs as they aggressively sought to outperform the Soviet Union in aeronautics and rocketry.

Some of these efforts would have lasting impacts, others would not. Chuck Yeager piloted the X-1 rocket plane past the sound barrier in 1947. Engineers designed a nuclear-propelled airplane, called the X-6, which never flew. Two rockets, the X-11 and X-12, never flew either, but their designs inspired ICBMs and today's expendable Atlas satellite launchers. In the 1950s, the piloted X-13 and X-14 planes hovered over the tarmac and contributed to the design of the modern Harrier.

Those first years of the pioneering "X-vehicle" programs are widely viewed as the salad days of U.S. aerospace engineers. Yet in recent years, X-vehicle designations have been awarded at an even faster pace than they were in that early Cold War period. The Air Force, which manages such designations for NASA and the other services, has awarded 10 of the coveted X-numbers in just the last four years. This sudden proliferation of experimental programs and the reasons for it have sparked much discussion among analysts, engineers, and policymakers. One theory says the explosion is a logical response to the unique challenges facing modern engineers, who, after all, are trying to build remotely piloted combat planes, new types of airliners, and reusable spaceplanes. Another theory says Air Force managers are getting stiff pressure from program managers to award the designations. Still another says the X-vehicles are a way of keeping engineers sharp at a time when there are relatively few production contracts.

Air Force officials are mum in this debate. They insist that they are faithfully following DOD Directives 4120.15, "Designation and Naming Defense Military Aerospace Vehicles," and 4120.15L, "Model Designation of Military Aerospace Vehicles."

The quest for an X-number

The effort to gain an X-designation typically begins when a program manager makes a formal request to the Air Force Materiel Command Cataloguing and Standardization Center in Battle Creek, Mich. Numbers are awarded sequentially, although program managers have been known to ask for exceptions. "Take [the number] X-2001. Obviously that would be a very popular one. Well, we're at X-number 48. We just don't run up the list," says John Bauer, program manager for nomenclature at the Battle Creek office.

Bauer makes sure there are no outstanding issues with the request, then passes it on to Air Force headquarters at the Pentagon for approval. If it is approved, officials notify Bauer, who informs the program office. The process takes about 60 days.

For the most part, records show that the X-numbers are indeed awarded sequentially. However, a list of recent X-vehicles provided by Bauer does not include the X-39. Author Jay Miller, whose book The X-planes was re-released this year in a revised edition, reports that this designation appears to be reserved by the Air Force Research Laboratory in Albuquerque, N.M.

The most recent program to be issued an X-number is the blended wing body effort at NASA-Langley (see Aerospace America, April 2000, page 28). The X-48A will be a slow-speed flying wing that could someday evolve into a new type of airliner offering 20% greater fuel efficiency than existing planes while carrying up to 450 people. It will be a 14% subscale demonstrator with a 35-ft wingspan.

Explaining the boom

Sheila Widnall, who was secretary of the Air Force from 1993 to 1997, believes program managers are pushing for X-designations as a way to provide an incentive to engineers who otherwise might never get to see a new breed of aircraft fly.

"The fundamental issue is that today's young engineer may work on only one aircraft platform in their entire career. That makes it difficult to keep aircraft design teams fresh. Thus the push for X-designs," she says.

Although sensitive to the difficult times engineers face, Widnall, now a professor of aeronautics and astronautics at MIT, nevertheless cautions against giving planes X-designations for anything other than technological reasons. "I think projects have to be justified on their merits," she says.

One veteran NASA engineer says X-programs have become a way to keep engineering expertise alive at a time when the aerospace industry has contracted. "It is probably a reflection of where the whole aviation and aeronautics industry is going, because there are so few production airplanes," he says. "It's probably a practical approach."

Other independent analysts and government engineers have warned that the definition of what constitutes an experimental plane is slipping. Two officials cite the Joint Strike Fighter (JSF) program as the clearest example of this trend.

Boeing's JSF variants bear the designations X-32A-B; Lockheed Martin's variants are labeled X-35A-C. The planes are competing against each other for a lucrative engineering and manufacturing development contract to build a new fighter for the USAF, Navy, Marines, and British Royal Navy.

Peter Merlin, a California-based aeronautics historian, says that in earlier times these planes would have been called prototypes. A NASA engineer agrees, saying the JSF planes to do not appear to be pushing the flight envelope enough to be considered X-planes. But JSF officials argue that these aircraft are technology demonstrators rather than production prototypes and thus deserve the X-designation.

History shows that experimental fighter planes have typically tested fundamentally new aerodynamic approaches. For instance, in the late 1980s and the early 1990s, NASA's X-29A jet tested an innovative forward-swept wing, an approach that still has not made its way onto production aircraft.

Merlin and other critics remain concerned about what they call a loosening of the historical X-plane criteria. "[Air Force managers] seem to be handing out designations willy-nilly to things that in my mind shouldn't be X-planes," says Merlin. He is one of a small group of "aerospace archaeologists" who scour the western deserts in search of charred X-plane remains.

Among the simplest and most provocative of theories to explain the X-plane boom is the one espoused by analyst John Pike, who operates an Internet site called Global-security.org. "I would cynically attribute it to a lack of adult supervision," Pike says.

Not everyone is convinced that any of these factors explains the X-plane boom. Officials in this camp argue that the spate of new designations is mostly a result of engineers attempting to cross legitimate technological hurdles.

"There are lots of factors involved here, but I feel the most important is the growing awareness that we are at a point with many technologies where we must go to flight to get answers," says NASA's Vince Rausch, program manager for the X-43A hypersonic research program at NASA-Langley.

Rausch sees no evidence that Air Force managers are losing discipline when it comes to X-plane designations. "If you look at the history of X-vehicles, you will see some very interesting ones that don't fit the mold over the years," he says.

Rausch did not provide examples, but he could have been referring to the tiny, one-man X-28A Osprey seaplane tested by the Navy in the early 1970s, or to the X-26A, a gangly sailplane with a prop that was supposed to fly silently over enemy territory with surveillance equipment.

Rausch also does not believe program managers have changed over the years, or that they are "lobbying any harder now than in the past." He agrees there is a trend toward dividing up huge aerospace challenges into focused, manageable experiments, a move he believes has great merit.

Rausch worked for years on the National Aerospace Plane Program (NASP), a now-defunct project that was a joint effort by the Air Force and NASA to develop a hypersonic plane that was supposed to revolutionize global air travel and spaceflight. NASP had called for construction of the X-30A, a complicated test vehicle aimed at proving the key elements of the aerospace plane's design.

NASP and the X-30A were canceled, but the goal of hypersonic flight lives on in a dramatically scaled-back form through the NASA X-43A Hyper-X program. The X-43A will focus almost exclusively on testing supersonic combustion ramjet technology, or scramjet for short. Unlike a rocket engine, the scramjet will not carry a huge tank of oxidizer with it; rather, it will rely on oxygen in the atmosphere.

Recent X-vehicles

The arrival of the Bush administration has sparked a fresh look at today's experimental vehicle programs. Of the most recent vehicles given the designation, five are unpiloted craft that will enable engineers to study the challenges of operating a reusable space vehicle.

The largest and most expensive of these, the NASA-Lockheed Martin X-33, was supposed to test the design of a plane that would fly into space without shedding rocket stages. NASA withdrew from the program in March because of technical problems and cost overruns. The project had never recovered from the explosion of a propellant tank on a test stand in 1999. Pike and other analysts had expected DOD to adopt the troubled program, but so far defense officials have reportedly decided not to do so.

NASA also did not include the delayed X-34 test bed in the Space Launch Initiative. DOD has made no move to take over that program either. NASA's initiative instead favors smaller scale reusable rocket efforts that divide the spaceflight challenges into discrete experiments.

NASA will use the new X-37 reusable rocket, and data from the recently tested X-40A vehicle, to study the challenges of operating an unpiloted reusable spaceplane in orbit and then returning it safely to Earth. Boeing engineers originally designed the X-40A to meet an Air Force call for a space maneuver vehicle that could deploy payloads in orbit or shadow enemy satellites for up to a year. NASA has borrowed the X-40A to gather data that will contribute to the design of the X-37 landing system.

Earlier this year, technicians at NASA-Dryden dropped the unpiloted X-40A seven times from a CH-47 Chinook helicopter at altitudes of up to 15,000 ft.

The X-37 is not designed to reach space on its own. Engineers plan to launch it atop an expendable rocket in 2004. They had originally hoped to deploy it from the space shuttle cargo bay, but that plan was abandoned because of the shuttle's crowded launch manifest. Once in orbit, the unpiloted X-37 will open its payload bay doors and deploy its power-generating solar arrays.

This year NASA has also conducted drop tests of another experimental vehicle, the X-38. It is designed to clear the way for development of a crew rescue vehicle, or lifeboat, for the space station. In an emergency, the rescue vehicle would undock from the station and glide home with the station's seven-member crew. NASA engineers carried out a seventh drop test of the X-38 from a B-52 carrier plane over Dryden in July. At the direction of the White House, however, the agency abandoned plans for an operational vehicle earlier this year. The unpiloted X-38 uses a lifting body design adapted from the X-24A, a piloted plane that flew in the late 1960s and early 1970s.

Air Force records also list another reusable X-vehicle program, the X-41A; however, little information on the vehicle is available. The Air Force designation office describes it as "an experimental maneuvering reentry vehicle which carries a variety of payloads through a suborbital trajectory, reenters the Earth's atmosphere, and safely dispenses its payload in the atmosphere."

Most of these X-vehicles will study the problem of access to space in terms of the return trip to Earth. The X-43A is the only X-program that is poised to actively address the problem of getting into space. Engineers plan to boost the X-43A to Mach 7 using a Pegasus carrier rocket. The X-43A will separate from the booster and briefly fire its supersonic-combustion ramjet before descending into the Pacific 10 minutes later.

The first X-43A vehicle was destroyed in June when its Pegasus carrier rocket flew out of control. That leaves two additional vehicles built by Micro Craft of Tullahoma, Tenn. The next test flight is on hold, however, pending the results of an investigation into the Pegasus failure.

Two of the most recent X-vehicles have tested designs for the new U.S.-British JSF, a single airframe with variants that will offer supersonic flight, stealth, and the ability to land on aircraft carriers or hover over remote airfields. Lockheed Martin and Boeing finished their flight test programs earlier this summer in a dramatic fly-off.

Lockheed Martin flew the X-35A-C between October 2000 and August of this year. Boeing flew its JSF planes, the X-32A and B, between September 2000 and July. The Air Force was expected to announce the winner of the fly-off in October.


Three of the most recent X-vehicles are precursors to operational unmanned combat air vehicles (UCAVs). Someday, squadrons of these remotely piloted planes could attack targets that would normally be struck by preprogrammed cruise missiles or piloted warplanes.

The most visible of the experimental UCAVs is the X-45A, built by Boeing for the Air Force. The company unveiled the plane in September 2000 (see Aerospace America, March 2001, page 28). The aircraft is similar in appearance to an earlier unpiloted X-plane, the X-36. Eventually, the X-45A will be joined in flight by another experimental plane, the X-45B, to demonstrate coordinated attacks.

Industry and government engineers are also working on UCAVs that will be flown off aircraft carriers. The X-46A is the most mysterious of these. "I am unable to detect the existence of an X-46," Pike says. Air Force records say only that the X-46A is built by Boeing for the Navy and that the program "includes surveillance and operations from aircraft carriers."

The Air Force describes the X-47A exactly the same way, except to say that it was built by Northrop Grumman for the Navy. The plane earned its X-designation in July, shortly before construction of the vehicle was completed, Northrop said in a press statement. A full-scale model of the plane has been displayed at the Naval Air Museum at Patuxent River, Md.

Copyright 2001 American Institute of Aeronautics and Astronautics, Inc.