"Flying Humvees" being designed by AAI and Lockheed Martin have made it through to the second phase of DARPA's Transformer (TX) program - but the sheer scale of the challenges in producing a fly-drive tactical vehicle is becoming clear.
Transformer is not simply a roadable aircraft - it is a four-seat vehicle that must be able to drive off-road, survive small-arms fire, and rapidly reconfigure into an aircraft that can take off and land vertically and be flown without pilot training.
Meeting these requirements is pushing the state of the art in lightweight materials structures, high power-to-weight engines and autonomous flight controls. But "there is covergence, we are seeing feasible designs," says DARPA Transformer program manager Stephen Waller, speaking at a recent AIAA conference.Concept: AAIConcept: Lockheed Martin and Piasecki Aircraft
*DARPA's vision for TX is a vehicle that can deliver a four-person squad from ship to shore, where it will be manually driven until the terrain becomes impassable, or IEDs too threatening, when it will transform within a minute into an aircraft, take off automatically, overfly the obstacle, land vertically and continue. The same vehicle could be used to insert and extract special forces, evacuate injured troops or - in automated mode - as an unmanned resupply truck.
That requires efficient propulsion for flight and ground travel, a way to vector thrust for VTOL while minimizing brownout, an off-road capability in a vehicle light enough for VTOL, and a way to stow any aerodynamic surfaces and propulsion systems not needed on the ground. In the face of these challenges, both AAI and Lockheed made sufficient progress with their very different designs to make it through to the second phase.
Concept: AAIAAI's TX (above) is a 7,500lb vehicle with an unpowered rotor for VTOL, a fold-out wing for cruise and a ducted fan for propulsion. A single 1,200shp Honeywell HTS900 turboshaft generates power to drive the four electric wheel motors, spins up the rotor for a "jump" take-off and drives the 56in-diameter ducted fan in forward flight. Ground speed is up to 80mph; flight speed range is 50-155kt; maximum altitude is 10,000ft.
Using Cartercopter's slowed rotor/compound technology, AAI's TX is essentially an autogyro with wings. In forward flight, lift transfers to the wing and the 50ft-diameter rotor slows until it is rotating solely to provide stability. Compared with a conventional helicopter, this reduces rotor drag. Retractable suspension pulls the wheels up into their wells to further reduce drag in flight. To land, the vehicle autorotates, the high-inertia, tip-weighed blades storing enough energy to enable a "zero-roll" touchdown. Concept: Lockheed Martin and Piasecki AircraftLockheed Martin's TX (above) is a 7,000lb vehicle with ducted fans that tilt from horizontal for VTOL and to vertical for forward flight. The 8.5ft outside-diameter fans are attached to a lifting-body center wing section mounted above the vehicle. This houses a pair of turboshaft engines that drive the fans and has a trailing-edge flap to increase lift at low speed. Flight speed is up to 130kt.
To convert from fly to drive, the outer panels of the 41.5ft-span wing fold inwards against the ducts and the complete wing/fan assembly rotates to lie along the length of the vehicle. A Pratt & Whitney EnduroCore heavy-fuel dual rotary engine then powers the four electric wheel motors. DARPA is aiming for a range - on the ground or in the air - of 250 miles on a tank of gas.
Phase 2 of the Transformer program is heading for preliminary design reviews at the end of the third quarter of fiscal 2012, after which DARPA will decide whether to select one team to proceed into Phase 3, which would culminate in prototype ground and flight demonstrations in mid-fiscal 2015. In addition to the objective "field vehicle" (FV) concepts shown above, the teams are designing prototype vehicles (PV). These would demonstrate the major features of their concepts, but would not be fully rugged and survivable tactical vehicles.Graphic: Lockheed MartinLockheed's PV is shown above. It would have mechanical two-wheel drive. AAI's PV would omit the composite door and floor armor to save weight, have a lower-power HTS900 engine and use off-the-shelf wheel motors and brakes. It would also be unmanned for flight, using off-the-shelf UAV flight-control hardware as the "vetronics". But in payload and performance, it would be close to AAI's field-vehicle design.
But the challenge of achieving ruggedness somewhere between that of an 8,200lb armored Humvee and a 1,500lb unarmored Cessna 172 is huge. "They have to be robust enough for on- and off-road use, which a lot of roadable aircraft are not good for," says Waller, adding: "There is enormous potential for SWAP [size, weight and power] growth in both designs." Cost is also a challenge, with DARPA aiming for around $1 million a copy compared with $400,000 for a Humvee and $4 million for light helicopter.
Concept: Lockheed Martin and Piasecki Aircraft
* UPDATE: I have been infomed Lockheed Martin's TX design is based on a configuration patented by Transformer team-member Piasecki Aircraft