GRAHAM WARWICK / WASHINGTON DC
US industry is pushing for government funding of more X-vehicle experimental programmes in a drive to maintain its technological edge in aerospace
From the X-1 to the X-50, over a span of 56 years, the USA's famous series of experimental vehicles has contributed uniquely, if not always successfully, to the advancement of aerospace.
The pace with which X-programmes are launched has picked up in recent years, with an emphasis on demonstrating technology for unmanned air vehicles and reusable launch vehicles.
US industry would like to see the pace increase further, as flight demonstrators help to attract new talent, as well as to develop the skills and retain the services of experienced engineers.
Despite a high mortality rate among modern X-vehicle programmes - for technical and budgetary reasons - several are active, and more are likely to emerge from programmes such as NASA's Space Launch Initiative.
This review of currently and recently active programmes illustrates that the USA's X-vehicle culture remains vibrant - and the envy of other industries.
X-31 - EXTREME VECTORING
Flight testing of the Boeing/EADS X-31 - the first international X-plane - has resumed under the VECTOR programme to demonstrate extremely short take-off and landing (ESTOL) capability. Funded by the US Navy and the German defence ministry, the $53 million programme involves the use of thrust vectoring and high angle-of-attack (AoA) approaches to reduce landing speeds by up to 30%.
Two X-31s were built by Rockwell (now Boeing) and MBB (now EADS) for the US/German Enhanced Fighter Manoeuvrability programme, and completed 580 flights between October 1990 and June 1995, demonstrating the tactical utility of post-stall manoeuvring using three-axis thrust vectoring. Powered by a single General Electric F404 and fitted with thrust-deflecting paddles, the canard/double-delta aircraft was flown to post-stall AoAs up to 70í and outmanoeuvred Boeing F/A-18s in simulated air combat. Later flights simulated tailless operation. The No1 aircraft crashed in January 1995 at Edwards. The pilot ejected safely.
The follow-on VECTOR (Vectoring ESTOL Control Tailless Operation Research) programme began in March 2000, the surviving X-31 returning to flight status at Patuxent River in February 2001. The aircraft was then modified with new flight control software, flush air-data system, autothrottle, triplex air-data and inertial-navigation/global-positioning systems, belly camera and associated cockpit display, and high-integrity beacon landing system. Flying resumed in May.
ESTOL testing is planned to begin by November, involving approaches to a "virtual" runway at 5,000ft (1,525m) altitude. Approaches will be flown at AoAs up to 35í with the gear up and 25í with the gear down, to check out the GPS-based precision landing system, which uses ground-based "pseudolites". A total of 25 flights are planned. ESTOL "to the ground" testing is set to begin in December, with 20 flights planned. Approaches will be flown hands off at AoAs up to 25í.
The X-31's normal approach angle of attack is 12í, resulting in a landing speed of 160kt (295km/h). At an AoA of 25í, landing speed will be reduced to around 115kt. The aircraft will automatically pitch up to the required AoA, fly the approach and "de-rotate" just before touchdown.
Plans for extremely short take-offs and tailless/reduced-tail flights have been cut for budgetary reasons. EADS's primary interest in the VECTOR programme is in demonstrating its advanced air-data system. This promises to provide more accurate information, particularly at high angle-of-attack and low speed. The low-observable system has 12 flush pressure ports located radially around the X-31's modified nose cone.
X-32 and X-35 - FIGHTERFORERUNNERS
Boeing and Lockheed Martin were awarded Joint Strike Fighter (JSF) concept demonstration contracts in 1996. Each built two aircraft - designated X-32 and X-35 - to demonstrate commonality between the three JSF variants, carrier approach handling qualities, and short take-off and landing (STOVL) performance.
Boeing's X-32A was flown 66 times between September 2000 and February 2001 to demonstrate both the conventional take-off and landing and aircraft carrier variants of Boeing's JSF design. The X-32B STOVL demonstrator was flown between March and July 2001, completing 78 flights.
The delta-wing, twin-tail X-32 was powered by a single Pratt & Whitney F119-614, with a direct-lift STOVL propulsion system in the X-32B. In STOVL mode, the two-dimensional thrust-vectoring cruise nozzle was closed, redirecting engine thrust to two swivelling lift nozzles.
Lockheed Martin's X-35A completed 27 flights during October and November 2000 to demonstrate the conventional take-off and landing version of the company's JSF design. It was then converted to the short take-off and landing variant, making 39 more flights as the STOVL X-35B between June and August 2001. The carrier variant was represented by the X-35C second demonstrator, which was flown 73 times between December 2000 and March 2001.
Resembling a scaled-down F-22, the X-35 was powered by a Pratt & Whitney F119-611, with a shaft-driven lift fan mounted behind the cockpit in the X-35B. In STOVL mode, the cruise nozzle was swivelled downward, the lift fan engaged and engine bleed air ducted to roll posts in the wing. In September 2001, Lockheed Martin was selected to develop the F-35 JSF based on the X-35 demonstrator.
X-33 and X-34 - terminated testbeds
Lockheed Martin signed a co-operative agreement with NASA in 1999 to design, build and fly the X-33, a half-scale technology demonstrator for its VentureStar single-stage-to-orbit reusable launch vehicle. Suborbital tests were planned with the unmanned X-33, but technical problems led to delays and cost overruns and in March 2001 the programme was cancelled after NASA had spent more than $900 million and industry over $350 million.
The lifting-body X-33 was designed to take off vertically powered by a Boeing Rocketdyne XRS-2200 linear aerospike rocket engine, then glide back to a horizontal runway landing - all autonomously. The programme came unstuck in November 1999 when the X-33's composite liquid-hydrogen fuel tank failed during ground testing, delaying a first flight from 2000 to 2003.
Like the X-33, Orbital Sciences' X-34 reusable launch vehicle (RLV) technology demonstrator fell victim to cost overruns and a change in direction at NASA. The programme was cancelled in 2001 after three of the unmanned air-launched testbeds had been built and three captive-carry flights conducted under Orbital's Lockheed L-1011 TriStar carrier aircraft.
Orbital was awarded a contract in 1996 to develop the X-34 as a low-cost testbed for RLV technologies including all-composite airframe and autonomous landing system, but costs had risen to $205 million and powered flights slipped to 2002 when the programme was cancelled.
In 2000, NASA launched its five-year, $4.8 billion Space Launch Initiative (SLI) to develop technology for a second-generation RLV to replace the Space Shuttle. But the agency decided the benefits to be derived from flight testing the X-33 and X-34 did not warrant the use of SLI funding to take the programmes to completion.
X-36 - TURNING TAILLESS
Boeing (originally McDonnell Douglas) built two X-36 tailless agility research aircraft for NASA - 28%-scale remotely piloted models of a proposed low-observable fighter. The unmanned aircraft completed 33 flights between May 1997 and December 1998. A canard configuration with lambda wing, using spilt ailerons and thrust vectoring for directional control, the X-36 demonstrated greater agility than current generation fighters despite having no vertical or horizontal tail. The last two flights were conducted for the US Air Force Research Laboratory to demonstrate the ability of neural-net flight control software to compensate for the damage or failure of control effectors on a tailless fighter. X-36 hardware and software, avionics, flight controls and Williams International F112 engine, have been reused for Boeing's X-50 canard rotor/wing demonstrator.
X-37 - reusable spaceplane
Final assembly of the X-37 reusable spaceplane is under way at Boeing Phantom Works under a $173 million cost-sharing agreement with NASA signed in July 1999. Unpowered atmospheric drop tests of the unmanned vehicle are scheduled for early 2004, from NASA's Boeing B-52.
The original plan for orbital flight tests has been shelved, but Boeing has proposed an on-orbit demonstration of the X-37 under NASA's Space Launch Initiative programme to develop technology for a second-generation reusable launch vehicle. The company's SLI concepts feature an X-37-derived upper stage.
The X-37 is designed to be carried into space on an expendable launch vehicle, to operate on orbit for up to 21 days before re-entering at up to Mach 25 and returning to a conventional runway landing - all autonomously. The vehicle has an all-composite airframe, durable thermal-protection system, opening experiment bay and Boeing Rocketdyne AR-2/3 on-orbit propulsion system.
The delta/twin-tail reusable spaceplane is scaled up from the X-40A space manoeuvre vehicle, which was built by Boeing for the USAir Force. Atmospheric drop tests of the unpowered X-40 have demonstrated the autonomous runway acquisition and landing technology planned for the X-37.
X-38 - LIFEBOAT beached
NASA has stopped work on the X-38, a prototype emergency crew-rescue vehicle (CRV) for the International Space Station (ISS), after new administrator Sean O'Keefe concluded the one-way mission was too narrow. As a result, industry is working on designs for a more capable vehicle able to carry crews to and from space.
The seven-occupant CRV was to be carried into orbit by the Space Shuttle, and remain attached to the ISS until needed as a lifeboat. The vehicle was then to glide back from orbit, jettisoning its deorbit propulsion stage during re-entry and deploying a steerable parafoil parachute for the final descent to a skid landing.
NASA, with involvement from the European Space Agency, designed the X-38 in-house using the lifting-body configuration of the USAir Force's Martin X-24A, flown in the mid-1960s. Two 80%-scale atmospheric test vehicles were built by Scaled Composites and eight drop tests from NASA's B-52 were conducted between March 1998 and December 2001.
On the final flight, the unmanned, unpowered X-38 was released at 45,000ft and flew autonomously, reaching transonic speed, before being slowed by a drogue parachute and deploying the remotely controlled steerable parachute. A third, orbital test, X-38 was nearing completion when the axe fell. NASA plans to complete the vehicle for instructional use.
X-39 - FATE UNKNOWN
The designation X-39 is unassigned, but was reserved for use by the USAir Force Research Laboratory and may have been intended for subscale unmanned demonstrators planned under the Future Aircraft Technology Enhancement (FATE) programme to evaluate technologies for future fighters. Funding was transferred to the X-45 unmanned combat air vehicle technology demonstrator.
X-40 - DROPPING IN
Begun as a 90%-scale prototype of the USAir Force's proposed Space Manoeuvre Vehicle (SMV), the unmanned X-40A has served as an 80%-scale precursor for NASA's X-37 reusable spaceplane. The X-40, which lacked the X-37's thermal protection and on-orbit propulsion, was drop tested to demonstrate low-speed flight dynamics and autonomous landing.
A single drop test was conducted under the SMV programme in August 1998, from a Sikorsky UH-60 helicopter at Holloman AFB, New Mexico. This was followed by seven drop tests at NASA Dryden, from a Boeing CH-47 Chinook, in support of the X-37 programme. The X-40 was released at 15,000ft, autonomously acquired the runway and landed conventionally.
X-41 and X-42 - IN The black
The classified X-41 is "an experimental manoeuvrable re-entry vehicle carrying a variety of payloads through a suborbital trajectory, then re-entering and dispersing the payload in the atmosphere", says the USAir Force. Status is not known.
The X-41 is a technology demonstrator for the USAF's proposed CommonAero Vehicle (CAV), a conventionally armed manoeuvrable re-entry vehicle that could be deployed by a ballistic missile, aircraft or spaceplane. Potential payloads include a 450kg (990lb) penetrator warhead, four small-diameter bombs or six mini-missiles.
Also classified, the X-42 is "an experimental expendable liquid rocket motor upper stage designed to boost 2,000-4,000lb payloads into orbit", says the US Air Force. The status of this programme to demonstrate technology for launch vehicle "pop-up" upper stages is not known.
X-43 - HYPERSONIC TRAILBLAZER
The X-43 is the centrepiece of revitalised US research into hypersonics, with four versions planned. The X-43A is the first in NASA's Hyper-X series of unmanned hypersonic flight demonstrators.
The first flight in June 2001 ended prematurely when the air-launched X-43A's Pegasus booster went out of control. A second attempt at a Mach 7 flight is planned for May or June 2003. If successful, the X-43 will become of the world's fastest aircraft, breaking the record held for over 30 years by the rocket-powered X-15.
NASA's Hyper-X hypersonic experimental vehicle effort is the scaled-back successor to the National Aero-Space Plane (NASP) programme, which was cancelled in 1994. The X-43 is a small-scale model of the X-30 piloted single-stage-to-orbit demonstrator planned under the overambitious NASP programme.
The X-43A is intended to demonstrate an airframe-integrated supersonic-combustion ramjet (scramjet) at M7 and M10. Working with Boeing, MicroCraft has built three of the 3.7m-long expendable vehicles, which are powered by an uncooled, hydrogen-fuelled, dual-mode ramjet/ scramjet produced by GASL.
The vehicle is attached to a modified OrbitalSciences Pegasus booster and air-launched from NASA's B-52. The Pegasus boosts the X-43A to the test speed and altitude, where the vehicle separates to fly autonomously and the scramjet ignites to burn for 5-7s. The X-43A then descends to its destruction in the Pacific.
The failed first flight was planned to demonstrate scramjet operation at M7, the highest speed possible in ground test facilities. The second test will be a repeat attempt at a M7 flight, while a third flight at M10 is planned for 2004. An M10 flight would provide the first data with which to benchmark hypersonic computational fluid-dynamics models.
The next version will be the X-43C. Scheduled to fly in 2008, this vehicle will be powered by the HyTech hydrocarbon-fuelled dual-mode scramjet under development by Pratt & Whitney for the USAir Force. The fuel-cooled, flight-weight HyTech engine is expected to accelerate the 4.9m long X-43C from M5 to M7 after separation from the air-launched Pegasus. Ground testing of the engine began at GASL in July.
There are two powerplant options for the planned but unfunded X-43B, a scaled-up version of the X-43A intended to demonstrate a reusable combined-cycle propulsion system for future air-breathing launch vehicles. NASA is developing both rocket-based and turbine-based engine options for the X-43B, which is planned to fly in 2010.
The rocket-based combined cycle (RBCC) engine is being developed by RBC3 - a consortium of Aerojet, Boeing Rocketdyne and Pratt & Whitney - under NASA's Integrated Systems Test of an Airbreathing Rocket (ISTAR)programme. The RBCC is capable of powering the X-43B from zero airspeed to M8.
The turbine-based combined cycle (TBCC) powerplant option is being pursued under NASA's Revolutionary Turbine Accelerator (RTA) programme. The high-Mach turbine engine could power the X-43B from zero airspeed to M4-5, where the HyTech dual-mode scramjet would take over and accelerate the vehicle to M8.
GeneralElectric has been awarded a contract to build the ground-test RTA (Flight International, 30 July-5 August). Rolls-Royce and Williams are competing to build the smaller flight-test engine, four of which would power the X-43B, sharing inlets and nozzles with the scramjets.
NASA plans to choose between the ISTAR RBCC and RTATBCC options after ground tests of both engines in 2006-07. The X-43B will be released from the B-52 at M0.8 to power itself to M7 before gliding back to a runway landing. The reusable vehicle will have a 25-flight design life.
A potential follow-on vehicle, the X-43D, is being studied by NASA. A direct development of the X-34A, this would be powered by a cooled hydrogen-fuelled dual-mode scramjet, which would run for 10s and accelerate the vehicle to M15.
X-44 - DIRECTION REQUIRED
The designation X-44 MANTA(Multi-Axis No-Tail Aircraft) is attached to a NASA/USAF proposal, currently dormant, to modify a Lockheed Martin/Boeing F-22 prototype to demonstrate flight without any aerodynamic controls, using multi-axis thrust vectoring. The horizontal and vertical tails would be removed and a delta wing fitted. Pitch/yaw-vectoring nozzles would provide all flight control.
A feasibility study has been conducted. The X-44, if it proceeds, would be a follow-on to the thrust-vectoring X-31 and tailless X-36, as well as the USAF/NASA ACTIVE (Advanced Control Technology for Integrated Vehicles) programme involving a Boeing F-15 modified with multi-axis thrust vectoring.
Lockheed Martin has proposed the FB-22, a derivative of the F-22 with delta wing and multi-axis thrust vectoring, to the USAir Force as a long-range strike aircraft.
X-45 - UNMANNED BOMBER
Flight testing of the X-45 unmanned combat air vehicle (UCAV) began in May at NASA Dryden. Boeing Phantom Works has built two X-45A technology demonstrators for the joint US Defense Advanced Research Projects Agency (DARPA) and USAir Force programme, and is designing a larger X-45B "fieldable prototype" UCAV.
The DARPA/USAF programme aims to demonstrate the technical feasibility and military utility of using unmanned air vehicles to perform suppression of enemy air defences (SEAD) and strike missions autonomously. Testing of the two X-45As will culminate in 2004 with an end-to-end demonstration involving multi-vehicle control, co-ordinated flight, inter-vehicle communications, dynamic retasking, co-operative targeting and weapon release.
The 8m long, 10.3m span tailless X-45A has a 3,600kg empty weight and is powered by a Honeywell F124 with yaw thrust vectoring. The X-45B, more representative of the planned A-45 operational UCAV, is 9.8m long, with a 14.3m span and 6,350kg empty weight. The larger vehicle is powered by a General Electric F404.
The X-45Bhas fully integrated avionics, two functional weapon bays, low-observable features and provisions for the full sensor suite, satellite communications and aerial refuelling. Three vehicles are planned, with the first scheduled to fly in late 2004. They will be used for increasingly complex multivehicle demonstrations culminating in joint operations with manned aircraft as part of a strike force.
X-46 - CARRIER CAPABLE
The designation X-46A has been assigned to Boeing's design for a naval unmanned combat air vehicle (UCAV-N). The US Navy and Defense Advanced Research Projects Agency plan to award at least one contract to demonstrate an unmanned vehicle able to operate from aircraft carriers and perform surveillance, suppression of enemy air defences and deep strike missions.
Both Boeing and Northrop Grumman (see X-47) are working under UCAV-N risk-reduction contracts awarded in May. Boeing's proposal draws heavily on its X-45 UCAV for the USAir Force. The tailless, flying-wing vehicle would be roughly 10.4m long, with a 13.4m span, and be able to carry 1,200kg of munitions up to 1,200km or perform surveillance missions lasting up to 12h.
X-47 - UCAV ON DECK
Northrop Grumman has built the X-47A Pegasus as a private venture to support the design of a naval unmanned combat air vehicle. The kite-shaped experimental unmanned vehicle is being prepared for flight testing at China Lake, California, and is intended to demonstrate aero-dynamic qualities suitable for autonomous operations from an aircraft carrier.
The tailless X-47A is 8.5m long, with a wing span of just under 8.5m, and is powered by a Pratt & Whitney Canada JT15D turbofan. A total of 15h of flight testing is planned, involving up to 40 landings. The vehicle is a precursor to the larger X-47B UCAV-N technology demonstrator.
Northrop Grumman was awarded a UCAV-N risk-reduction contract in May, as was Boeing, and is awaiting release of a request for proposals to build the full-scale demonstrator. This has been delayed while DARPA and the US Navy debate whether to award one contract or two, to maintain competition for the UCAV-N development programme scheduled to begin in 2007.
X-50 - HALF AND HALF
Boeing plans to begin flight testing its X-50A Dragonfly Canard/Rotor Wing (CRW) technology demonstrator in September at Yuma, Arizona. CRW is a vertical take-off and landing concept that aims to combine the hover efficiency of a helicopter with the cruise speed of a fixed-wing aircraft.
The manufacturer and US Defense Advanced Research Projects Agency are sharing the $21 million cost of the CRW demonstration equally. Boeing Phantom Works has built two subscale, unmanned X-50As reusing avionics, flight controls and Williams F112 engines from the X-36 tailless fighter demonstrators.
CRW differs from previous stopped-rotor designs in that lift during conversion between rotary- and fixed-wing flight is provided by a forward canard and aft horizontal stabiliser. This allows the rotor to be unloaded, slowed, stopped and locked perpendicular to the fuselage. In helicopter mode, exhaust from the turbofan engine is ducted to tipjets on the two-blade reaction-drive rotor. During conversion to fixed-wing mode, engine exhaust is diverted progressively away from the rotor/wing to an aft cruise nozzle.
An operational manned or unmanned CRW is expected to be capable of cruise speeds up to 375kt and altitudes approaching 35,000ft. The X-50A is not designed to go beyond 150kt as testing will focus on conversion between rotary- and fixed-wing mode. Eleven flights are planned. The X-50A will take off vertically and operate as a helicopter at speeds up to 60kt. Above 60kt, the canard and stabiliser will begin to generate lift and by 120kt will support the vehicle's weight. Conversion to fixed-wing flight will take place at 130kt.
The CRW is a candidate configuration for the DARPA/US Army Unmanned Combat Armed Rotorcraft programme, which is likely to result in the next in the illustrious series X-vehicle series.