Guy Norris/LOS ANGELES Graham Warwick/WASHINGTON DC
X is for experimental, and the USA's famous X-planes have played a key role in advancing aerospace technology over the more than 50 years since the Bell X-1 first flew. The nature of the X-series is changing, however. There is greater emphasis on rapid prototyping of subscale unmanned vehicles, and on government/industry cost sharing, in an effort to reduce the cost of demonstrating technology.
The X-series has it origins in the Research Aircraft Programme launched by NASA (then NACA) and the US military towards the end of World War II to explore transonic and supersonic flight. The series has scored some major successes, from the X-1 breaking the sound barrier in October 1947, to the X-15 reaching Mach 6.7 flight 20 years later. There have also been disappointments, from the underpowered X-3 supersonic jet to the cancelled X-30 hypersonic aerospaceplane.
There have also been anomalies, from the "X-19" designation used as a cover for Bell's secret competitor to the Lockheed U-2, to the X-32 and X-35 designations assigned to the competing Joint Strike Fighter concept demonstrators - not really research aircraft, but not true prototypes either. And there have been the oddities, from the X-25 gyro-glider intended to help downed aircrew escape capture, to theX-28 single-seat seaplane for river patrols.
Thanks to efforts to reduce the cost of designing, building and testing experimental aircraft, the X-series looks alive and well. Today there are no fewer than 12 active "X" programmes.
X-31
The X-31 was the first international "X" programme, originating as the joint US-German Enhanced Fighter Manoeuvrability demonstrator to explore close-in air combat at angles of attack (AoA) beyond the stall. Flight testing was completed in 1995, but efforts are under way to launch a US/German/Swedish programme to demonstrate tailless flight using the X-31.
The X-31 was designed jointly by Rockwell International and MBB under a programme involving the US and German Governments, the US Defense Advanced Research Projects Agency (DARPA), and the US Navy. The single-engined aircraft incorporates an advanced fly-by-wire flight control system, three-axis thrust vectoring, all-moving canards, fixed aft strakes and a composite double-delta wing.
The rudimentary thrust vectoring system consists of three carbonfibre paddles, mounted on the aft fuselage, which deflect the engine exhaust to provide pitch and yaw control. Normally "weathervaned" with the airflow, the canards are used for aerodynamic recovery from high AoA if the thrust vectoring system fails.
The first of two aircraft flew at Palmdale, California, on 11 October, 1990. The second aircraft followed on 19 January, 1991. During initial flight tests from Palmdale, the two aircraft used thrust vectoring to expand the post-stall envelope to 40í AoA. Operations moved to NASA Dryden, at nearby Edwards AFB, in February 1992, where the International Test Organisation expanded the envelope further.
In November 1992, the X-31 achieved controlled flight at 70° AoA and a controlled roll around the aircraft's velocity vector was accomplished at the same AoA. In April 1993, the X-31 achieved another milestone when the No2 aircraft executed a rapid, minimum radius, 180í turn using a post-stall manoeuvre. Flying well beyond the aerodynamic limits of any conventional aircraft, the "J-turn" or "Herbst manoeuvre" (after German post-stall proponent Wolfgang Herbst) proved the revolutionary potential of the X-31.
In subsequent years the X-31s engaged in air combat manoeuvres against F/A-18s. In 1993, the No2 aircraft was fitted with a helmet-mounted visual/audio display to provide better situational awareness during high AoA combat.
In 1994, the X-31's flight controls were reprogrammed to simulate tailless flight. The quasi-tailless tests, aimed at eventually removing the vertical tail altogether, were designed to demonstrate the drag, weight and radar cross-section reductions that could be achieved in future civil and military designs.
The last flight phase was near its ending in January 1995 when the No1 aircraft crashed just north of Edwards. The pilot ejected safely. By then, international interest in the tailless concept was growing, and after the surviving aircraft appeared at the 1995 Paris air show, theX-31 team conceived a new programme, VECTOR (Vectoring Extremely short take-off and landing Control and Tailless Operation Research), under which the vertical tail will be removed progressively until the X-31 becomes a completely tailless aircraft.
Despite slow progress, VECTOR proponents are hopeful the programme will get under way at the US Navy's Patuxent River flight test centre by the end of 1999.
X-32
The X-32 is Boeing's Joint Strike Fighter (JSF) concept demonstrator aircraft. The JSF programme is aimed at replacing the US Air Force's F-16s and A-10s, the US Navy's A-6s, the US Marine Corps' AV-8Bs and F/A-18s and the UK Royal Navy's Sea Harriers with three variants of one highly common design.
The US Department of Defense awarded JSF concept demonstration contracts to Boeing and Lockheed Martin in November 1996. Each covers the construction of two demonstrators: one conventional take-off and landing (CTOL), the other short take-off and vertical landing (STOVL). Boeing's aircraft are designated X-32A (CTOL) and X-32B (STOVL).
The X-32 has a high-set delta wing, twin canted vertical tails, a deep fuselage and a prominent chin intake for its single Pratt & Whitney JSF119 engine, which is fitted with a two-dimensional thrust vectoring nozzle. The X-32A, representing the USAF/USNJSF, differs externally from the X-32B, representing the USMC/RN JSF, in having small wingtip extensions that increase payload/range.
The X-32B has a direct-lift STOVL propulsion system with twin retractable side-mounted thrust vectoring nozzles. The STOVL version has a translating inlet lip to improve low-speed operation. Ground tests of both the CTOL and STOVL engine variants are under way.
Construction of the X-32A began in November 1997 and final assembly is on schedule to begin at Palmdale in the first quarter of 1999. Assembly of the X-32B forebody started in October 1998 and is due to be completed in mid-1999. It will be shipped to Palmdale for final assembly by the end of the year. Both aircraft are set to fly in 2000, with one design to be selected in 2001 for the JSF engineering and manufacturing development phase.
X-33
Lockheed Martin Skunk Works signed a co-operative agreement with NASA in July 1996 to design, manufacture and fly the X-33 - a half-scale technology demonstrator for a single-stage-to-orbit reusable launch vehicle called the VentureStar. The vehicle is intended to reach orbit using its on-board rocket power, then glide back to earth for a conventional landing like the Space Shuttle.
Assembly of the X-33 is under way at Palmdale, with completion expected around June. The company then plans to transport the large, lifting-body shaped X-33 to a specially constructed launch site near Haystack Butte at the northern side of Edwards AFB. Suborbital test flights, initially to a USAF base in Utah 830km (450nm) away - are to begin in December 1999.
Critical elements of the design include the liquid oxygen tank and two liquid hydrogen tanks (the largest composite LH2 tanks ever made), which are part of the vehicle's structure. Another is the innovative XRS-2200 linear aerospike rocket engine being developed by Boeing's Rocketdyne division. Unlike in a conventional rocket, where expanding combustion gases are contained within a bell-shaped nozzle, the hot exhaust of the aerospike is expanded on an external ramp, and modulated byatmospheric pressure.
Problems producing the copper alloy plates that form the V-shaped ramp of the aerospike rocket have delayed the programme. Rocketdyne is confident of providing flightworthy engines to Lockheed Martin in time for the revised December 1999 launch target. In the meantime, the Skunk Works plans to conduct pre-flight tests on the airframe at its launch site, using simulated engine masses.
Fabrication by BFGoodrich of 1,309 inconel and titanium panels that form the metallic thermal protection system, as well as 17 blanket-insulated composite panels to cover the top aft area, is expected to be roughly 50% complete by the first quarter of 1999. January should see completion of the delivery of avionics and mechanical subsystems by risk-sharing supplier AlliedSignal. The five Aerojet Gencorp-made helium tanks for the reaction control system have already been delivered.
Contingent on successful tests of the X-33, Lockheed Martin plans to begin commercially funded construction of the first of two full-sized VentureStars in 2001.
X-34
Under development by Orbital Sciences as a technology demonstrator for future reusable launch vehicles (RLVs), the X-34 is also intended to be used as a platform for high-speed research and microgravity experiments. The air-launched, rocket-powered vehicle will be capable of autonomous operation at speeds up to M8 and altitudes up to 250,000ft (76,000m).
Under NASA contracts totalling $85 million, Orbital is building three X-34s - one test article and two flight vehicles - and will conduct 27 flight tests using its Lockheed L-1011 launch aircraft. The test article,A-1, will be delivered to NASA Dryden in January for ground vibration and captive carry testing. Flight testing is scheduled to begin in August.
Orbital says up to four unpowered drop tests are likely before the first powered flight, expected in December. Initial flights will be conducted within the airspace of White Sands Missile Range in New Mexico, the X-34 reaching speeds up to M2.5. Subsequent flights will expand the envelope to M8, 250,000ft and ranges up to 900km (500nm). Each vehicle is designed for a service life of 100 flights.
As a testbed for RLV technology, the X-34 incorporates an all-composite airframe, composite reusable fuel tanks, low-cost ceramic thermal protection systems and an autonomous guidance and landing capability based on an integrated differential global positioning/inertial navigation system. An integrated vehicle health monitoring system will enable rapid turnarounds - a major demonstration goal.
The X-34 will be powered by a 60,000lb-thrust (267kN) Fastrac liquid oxygen/kerosene engine under development by NASA's Marshal Space Flight Centre. The Fastrac is a simple engine with a single combined oxygen/kerosene turbopump. Originally envisaged as the forerunner of a commercial air-launched RLV, the X-34 is now viewed as the first in a series of X-designated "Pathfinder" demonstrators under NASA's Future-X programme to develop technologies for low-cost access to space.
X-35
Lockheed Martin's Joint Strike Fighter concept demonstrator is designated the X-35. Two aircraft are being built, the CTOL X-35A and the STOVL X-35B. Ground testing of the CTOL and STOVL propulsion systems was under way by the end of 1998. Flight testing of both variants is on schedule for 2000, leading to selection of the winning JSF design in 2001.
Lockheed Martin's JSF resembles a scaled-down F-22, with trapezoidal wing, conventional horizontal stabiliser and twin canted vertical tails. One unusual design feature is the diverterless side inlets for the single Pratt &Whitney JSF119 engine, designed to reduce radar cross-section. The X-35A will be flown first in USAF CTOL configuration, then converted to the carrier-capable (CV) version planned for the USN. This involves fitting the larger wing control surfaces, horizontal stabiliser and rudders required for a low-speed approach to the aircraft carrier.
The X-35B will have a STOVL propulsion system combining the basic JSF119 with a shaft-driven lift fan and three-bearing thrust vectoring main engine nozzle - both provided by Rolls-Royce. In STOVL mode, a dry clutch will engage the lift fan, which is mounted behind the cockpit and equipped with a D-shaped nozzle to vector the thrust fore and aft. The three-bearing nozzle will vector main engine thrust downwards, and from side to side. Some engine air will be diverted to roll ducts mounted in the wing, which will provide both lift and roll control.
Lockheed Martin Skunk Works has begun assembly of the X-35A demonstrator at Palmdale, beginning with delivery of the fibre-placed composite inlet duct produced by Alliant Techsystems. Lockheed Martin is working with Northrop Grumman and British Aerospace on the JSF development proposal that will be submitted in 2001 in competition with Boeing.
X-36
Built under a co-operative programme with NASA to demonstrate technology for an agile tailless fighter, Boeing's X-36 is now being used by the USAF to evaluate reconfigurable flight controls for tailless aircraft.
The X-36 research aircraft is a remotely piloted 28% scale model of a tailless fighter. The aircraft has no horizontal or vertical tails, flight control being provided by split ailerons and thrust vectoring. The 5.5m (18.2ft)-long, 575kg (1,270lb) vehicle is powered by a 700lb -thrust (3.1kN) Williams International F112 turbofan. The X-36 is flown from a ground-based cockpit equipped with a head-up display which superimposes flight data on the view from a camera on board the aircraft.
Two X-36s were built under a $20 million co-operative agreement between Boeing (then McDonnell Douglas) and NASA, which shared the costs roughly equally. The programme also demonstrated rapid prototyping, with the first vehicle rolled out just 28 months after the contract was awarded. Boeing says the X-36 was designed, produced and tested in about half the time and at one-tenth the cost of a full-scale manned vehicle.
The X-36 first flew on 17 May, 1997, and 31 flights were completed by 12 November, using just one of the vehicles. The second was completed, but never flown. The X-36 achieved a maximum angle of attack of 40í and demonstrated fighter agility with thrust vectoring on and off. Boeing says that, above 18í AoA, theX-36's roll performance exceeded that of any conventional fighter.
Flight testing resumed in December 1998 under the USAF Research Laboratory's RESTORE programme. The aircraft has been equipped with "neural net" control laws, enabling the flight control computer to detect and compensate automatically for any flight control failures. Five to 10 flights were planned by year end, during which failures were to be injected into the X-36's single-channel digital flight control system to validate the operation of the RESTORE algorithms.
X-37
The designation X-37 is reserved for NASA's Future-X programme to demonstrate technologies for low-cost access to space, but it is not yet clear whether it will be applied to the Advanced Technology Vehicle (ATV) to be built by Boeing as a Future-X Pathfinder.
NASA's Future-X programme is intended as a continuous series of flight demonstrations to validate technologies beyond those incorporated in the X-33 and X-34. The programme envisages both Pathfinder narrow-focus demonstrators and more complex Trailblazer integrated technology demonstrators. Possible Trailblazer X-vehicle candidates include an upgraded X-33.
The first two Pathfinders are the X-34 and Boeing's ATV, which is intended to demonstrate technologies for a reusable craft capable of autonomous "orbital-to-landing" operations. The unpiloted ATV will be built and flight tested under a $150 million, 50:50 cost-sharing agreement between NASA and Boeing. Drop tests, from NASA's B-52, are expected to begin in mid-2001, leading to a Space Shuttle flight in late 2001/early 2002.
The ATV is externally almost identical to Boeing's X-40 Space Manoeuvre Vehicle demonstrator. The vehicle will be deployed by robot arm from the Shuttle payload bay to conduct autonomous on-orbit operations, then re-enter and glide to a automatic runway landing. The ATV will reach speeds of M25 on re-entry and will be the first X-vehicle to be flown in both orbital and re-entry environments.
The reusable ATV will have an all-composite airframe with durable thermal-protection system, storable non-toxic liquid propellants, and an integrated vehicle health monitoring system to enable rapid turnarounds. Modularity will allow future technologies to be tested.
X-38
The X-38 is a technology demonstrator for a Crew Return Vehicle (CRV), or lifeboat, for the International Space Station (ISS). The CRV could also form the basis for a vehicle that would be launched on the Ariane 5 booster.
The X-38 resembles the X-24A lifting body, and will glide back from orbit after jettisoning a de-orbit engine module. On entering the outer atmosphere, the X-38 crew will deploy a steerable parafoil parachute for the final descent to landing. The vehicle, designed for seven occupants, will then be steered automatically to a precise landing on skids, rather than wheels. The crew will have the option to switch to back-up manual systems, to control the orientation in orbit, pick a de-orbit site, and steer the parafoil.
Studies of the X-38 concept began at NASA's Johnson Space Centre (JSC) in 1995 and resulted in a contract being awarded to California-based Scaled Composites in 1996 for the construction of three full-scale atmospheric test vehicles. Drop tests of the first X-38, Vehicle 131, from the agency's B-52 began at NASA Dryden in March 1998. All did not go according to plan - the parafoil twisted and ripped during deployment, and tests were suspended.
Analysis showed that the drogue 'chute had been distorted by higher than anticipated wake effects behind the vehicle. The parafoil was redesigned and heavier rigging lines added. A subscale version of the new parafoil was tested successfully at the US Army's Yuma Proving Grounds in Arizona at the end of July 1998. Further tests of a larger parafoil were conducted from a C-130 in September. Some problems remained, however, and the planned restart of drop tests at Dryden slipped from October 1998 to January 1999.
The second X-38, Vehicle 132, was delivered to Dryden in September 1998 and is scheduled to fly in February. V132 has the full lifting-body flight control system, which allows the vehicle to fly autonomously before parafoil deployment. The space flight vehicle, V201, is close to completion at JSC in Houston, Texas.
NASA plans to conduct atmospheric tests of the first two X-38s until at least June 2000. The first unpiloted space flight test is set for November 2000, when V201 will be carried into orbit by the Space Shuttle. The first of a planned fleet of four operational CRVs is due to enter service on the ISS in 2003.
X-39
The designation X-39 is unassigned, but has been reserved for use by the USAF Research Laboratory. The designation may have been intended for subscale unmanned demonstrators planned under the Future Aircraft Technology Enhancements (FATE) programme to evaluate technologies for future fighters, but funding for these was transferred to the DARPA/USAF Unmanned Combat Air Vehicle demonstration, which involves similar airframe technologies.
The FATE programme continues to look at technologies such as active aeroelastic wing, self-adaptive flight controls, advanced compact inlets, robust composite sandwich structures and photonic vehicle management systems.
X-40
Under the USAir Force's Space Manoeuvre Vehicle (SMV) programme, Boeing built theX-40A for drop testing to validate the autonomous approach and landing system. A successful drop test was conducted at Holloman AFB, New Mexico, in August 1998.
The SMV is a small spacecraft that could function as a reusable satellite bus or upper stage, with a payload capability of about 550kg (1,200lb). The vehicle could be deployed by the Shuttle or from a suborbital launch vehicle such as the X-33 or Boeing's DC-X. The SMV would then conduct autonomous in-orbit operations, releasing its payload or returning it to earth.
The X-40A, a 90% scale model of the SMV, was dropped from an H-60 helicopter to validate the differential GPS automatic landing guidance system. Plans now call for drop tests from NASA's B-52, beginning in the fourth quarter of 1999. These will also pave the way for drop and flight tests of the NASA/Boeing ATV.
X-41 and X-42
Classified programmes. The X-41 involves "-an experimental manoeuvrable re-entry vehicle carrying a variety of payloads through a suborbital trajectory, and re-entering and dispersing the payload in the atmosphere", says the USAF. The X-42 is "-an experimental expendable liquid rocket motor upper stage designed to boost 2,000-4,000lb [900 1,800km] payloads into orbit".
X-43
Following cancellation of the X-30 piloted single-stage-to-orbit demonstrator for the National Aero-Space Plane in 1994, NASA evaluated several more modest projects before beginning the $170 million Hyper-X hypersonic experimental vehicle programme. Three flights of small-scale test vehicles are planned, two at M7 and one at M10, to demonstrate the operation of an airframe-integrated supersonic-combustion ramjet (scramjet).
The 3.7m (12ft)-long vehicles, designated X-43s, are being built by Tullahoma, Tennessee-based MicroCraft, and the 760mm (30in)-long scramjet engines are being produced by GASL of Ronkonkoma, New York. The first engine was delivered to NASA Langley in August 1998, for use in high-speed ground tests. An identical engine will be mated to its flight vehicle in February and delivered to NASA Dryden for the first flight, due in early 2000.
Flights will be conducted over the Western Test Range, off the Californian coast. The X-43 will be boosted to the test speed and altitude by a modified Orbital Sciences Pegasus air-launched booster. Launched from a B-52, the booster will accelerate the 1,000kg X-43 to M7 or M10 at altitudes up to 100,000ft, where the X-43 will separate from the Pegasus and fly under its own power. The engine will operate for only 7s on the first flight, but this will be enough to collect measurements to validate windtunnel and computational data, says NASA.
Source: Flight International
