Heavy duty: US Army backs tiltrotor as future battlefield airlifter

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Setting the stage for a budget showdown, the US Army is backing an advanced tiltrotor as the solution to its looming need for a heavylift battlefield transport

After decades of false starts and dead ends, the US armed services' search for an all-new theatre airlifter is entering a critical phase, with a crucial twist. The latest combat vehicles are rapidly outgrowing their Boeing CH-47 Chinooks and Lockheed Martin C-130 Hercules, and the US Army and US Air Force are backing different approaches to moving armoured forces around the battlefield.

With its rotary-wing background, the army favours a vertical take-off and landing solution, while the air force, with its focus on fixed-wing, is following a short take-off and landing track. But budget constraints could force the Department of Defense to choose between the two approaches.

tr65-190

Since cancellation of the US Army's Heavy Lift Helicopter in 1974 and the US Air Force's Advanced Medium STOL Transport in 1979, the services have relied on improvements to the Chinook and Hercules to meet their in-theatre transport needs. But fielding of the army's new Future Combat Systems (FCS) is set to begin in 2015, and the need for a more-capable battlefield airlifter is becoming urgent.

Both the army and air force are working towards X-plane technology demonstrations by the middle of next decade and hope this will lead to the development and fielding of new intra-theatre transports after 2020. But these plans look likely to spark a budget battle.

Later this year, both the army and USAF will approach the DoD's Joint Requirements Oversight Council for approval to proceed to the next stages in their respective Joint Heavy Lift (JHL) and Advanced Joint Air Combat System (AJACS) projects. Joint-service approval is essential, but industry expects each to oppose the other's programme because they will compete for funding.

A DoD directive that new programmes must have competitive demonstrators to reduce development risk looks set to push up costs. The army and air force both want to fly demonstrators by 2015, and the need to fund competing JHL and AJACS X-planes is certain to increase the stakes in any budget battle.

joint heavy lift one design

Garnering support from other services is critical, and is influencing requirements. The army, for example, favours a tiltrotor over a helicopter because its speed and altitude capability makes the JHL more suitable for roles other than manoeuvring medium-weight armour. "Among other missions are long-range special operations, distributed sustainment and shipborne seabasing," says JHL programme manager Bruce Tenney.

The USAF, meanwhile, is adapting its requirements to the army's bulkier payloads while keeping in mind that AJACS is intended to replace the C-130 in all the air force roles it performs, including combat rescue, special operations, gunship and tanker.

Whether operated by the US Army or the USAF, a new heavylift transport with either VTOL or "super STOL" capability is seen as essential to enable the army's concept of mounted vertical manoeuvre - moving forces by air to strike directly at enemy positions anywhere in the war theatre. US commanders also want the ability to deploy forces directly from the sea if access is denied.

 

Only option

But the increasing size and weight of US armoured vehicles - the operational Stryker and the FCS vehicles under development - effectively rule out using even the latest C-130J tactical transport, leaving the Boeing C-17 strategic airlifter as the only available option.

The US Army began looking again at the need for a heavylift rotorcraft in 1999. By 2004, and after several changes of acronym, the requirement had morphed into the DoD-backed Joint Heavy Lift project. In 2005, the army's Aviation Applied Technology Directorate awarded five JHL concept design and analysis (CDA) contracts.

Completed in May 2007, these studies looked at five VTOL concepts: the Bell Boeing Quad Tiltrotor (QTR), Boeing Advanced Tandem Rotor Helicopter (ATRH), Karem Aircraft Optimum Speed Tilt Rotor (OSTR) and Sikorsky X2 Crane and X2 High Speed Lifter. Government teams also studied single main rotor, tiltrotor and compound helicopter designs.

qtr

JHL requirements for the CDA phase included the ability to self-deploy 3,900km (2,100nm) without refuelling and, in theatre, to carry one FCS-class vehicle 460km with a vertical take-off and landing. The required radius increased to 925km with a rolling take-off. The FCS vehicle weight assumed for the baseline JHL was 20t, but study "excursions" looked at payloads as high as 26t.

Other required JHL attributes included landing gear that would stay within the strength limits of ship decks and permit vertical take-off and landing on unprepared airfields and sloping surfaces. Hover downwash could not exceed DoD rotorcraft standards and the cargo-handling system had to enable the self-loading and discharging of combat-ready payloads.

Briefing industry on the CDA results in mid-2007, Tenney unveiled a "One-JHL" design called the High Efficiency Tilt Rotor (HETR). Developed by using insights gained from various studies, the HETR was conceived as a single, common representation of the JHL to support various modelling, simulation and analysis activities.

"HETR's purpose is to represent what JHL is," says Tenney. "It is not intended to be the only viable solution. Other configurations that can meet the ultimate set of requirements for the competitive technology-demonstration programme will be in the mix." But HETR underlines the army preference for a tiltrotor.

The AATD has dropped from consideration the slower JHL concepts - Boeing's tandem-rotor ATRH and Sikorsky's coaxial-rotor X2 Crane. With a 240kt (445km/h) cruise, Sikorsky's X2 High Speed Lifter is within the required speed range, but Tenney is concerned that its performance will be marginal when scaled up to carry the increasingly bulky FCS vehicles.

After the first round of JHL studies, Bell Boeing's QTR is still in contention alongside Karem's twin-rotor OSTR, while Sikorsky is dusting off its work on the Variable-Diameter Tilt Rotor (VDTR) to stay in the game. "QTR is still in the mix, as is VDTR," says Tenney.

Unveiling the HETR concept concerned some competitors because it closely resembles Karem's OSTR. But it also draws on NASA's Heavy Lift Rotorcraft programme, which produced a design for a 120-seat tiltrotor similar in size and shape to HETR. "NASA is part of the JHL team," says Tenney. "Also we have moved closer to the commercial profile NASA is looking at, so you get a similar result."

Key characteristics of the HETR include a low-drag airframe, lightweight structure and systems, and a high-efficiency proprotor. Features include a high aspect-ratio wing for reduced download and induced drag aerodynamic shaping for reduced separation and interference drag and active control of stability, vibration and load alleviation to reduce drag and weight and increase reliability.

Like NASA's HLR - and Karem's OSTR - the HETR has a variable-speed propulsion system. This allows the proprotors to operate at higher speed in vertical flight for hover performance and lower speed in forward flight for cruise efficiency. It requires a turboshaft engine with wide operating range a multi-speed transmission with high power-to-weight ratio and a lightweight rotor with a wide tip-speed range.

Karem believes an advanced tiltrotor eliminates the need to compromise between vertical lift and efficient transport. The company was formed in 2000 by Abe Karem, creator of the Predator unmanned aircraft. Karem pioneered the optimum-speed rotor on the A160 Hummingbird unmanned helicopter, but in 2004 sold the design to Boeing to focus on applying the technology to tiltrotors.

Karem is the dark horse in the JHL race and makes ambitious claims for its baseline design, the TR65-190, including a speed of 335-370kt, cruise altitude of 35,000-40,000ft (9,100-12,200m) and fuel efficiency matching the best fixed-wing transports. The aircraft has two 19.8m (65ft)-diameter four-blade rotors and a wing span of 40.8m, including tip sections that tilt with the nacelles to reduce download.

Projected operating empty weight is 28,100kg (61,950lb) - a figure that has Karem's competitors raising their eyebrows (the similarly sized C-130J weighs 34,300kg empty). Karem points to its track record of designing lightweight, low-drag airframes. Design gross weight is 50,400kg. At this weight, disc loading is 81.5kg/m² (16.7lb/ft²) - similar to that of Sikorsky's CH-53E, the company says.

Karem says the TR65 "performed capably" on the missions analysed under the CDA study, providing the required 460km radius with 20t payload on the design high/hot VTOL mission. Projected radius exceeded 1,200km on a sea-base VTOL mission, and the self-deployment range carrying 120 troops and extra fuel was 10,600km.

OSTR is a "big step in VTOL/STOL fuel efficiency", the company claims, projecting an improvement in tonne-kilometres per kilogram of fuel of almost 300% over the Bell Boeing V-22 tiltrotor - and more than 150% over the C-130J - putting the TR65-190 into a class similar to the Boeing 737-800 airliner.

The company also makes aggressive claims for the aircraft's cost - projecting a $55 million flyaway price based on a cost per kilogramme of empty weight similar to fixed-wing military transports. As with the weight projections, these claims are greeted with scepticism by Karem's established competitors, but they explain the army's enthusiasm for the concept.

Critical concern

Cost is a critical concern, given the V-22's hefty price. Karem's estimate aside, the industry consensus is that the JHL will cost more than the C-130J at $60-70 million, but less than the C-17 at about $190 million. "We do not anticipate it will be more expensive than the [Airbus Military ] A400M," says Tenney.

Mention of the A400M is significant as the European airlifter is closest in size to the aircraft the army wants. The AATD is updating its "model specification" for the JHL to incorporate the CDA results and accommodate the ever-bulkier Stryker and FCS vehicles, and will specify a cargo box close to the A400M's.

While the Stryker is driving width, the FCS infantry combat vehicle - now exceeding 27t - is driving the payload requirement. "We need a bigger box and the A400M is very close to the new baseline," says Tenney.

Bell Boeing has already investigated a bigger fuselage under the CDA studies. The baseline 20t-payload JHL QTR has a C-130 cross-section, 15m-diameter rotors and a 16.4m-long cargo box. The large-fuselage "Baseline 2A" design has a 26t payload, A400M cross-section, 16.5m rotors and an 18.3m cargo box. With rotor tip clearance driving fuselage length, the cargo box is longer than required for JHL, but adds only about 2,300kg to the empty weight, says Bell Boeing.

Tenney says contractors should get the revised model specification by the end of January to allow them to update their designs. The army has secured about $40 million in funding for fiscal years 2008 and 2009 to conduct risk-reduction work in preparation for a potential five-year, $2.5 billion technology-demonstration programme beginning in FY2010.

As well as funding Bell Boeing, Karem and Sikorsky to refine their concepts to meet the new baseline, AATD plans to award contracts to study specific aspects of the JHL. These include cargo-handling systems enabling rapid loading and unloading. "There are questions we need answers to before we proceed into a technology demonstration," says Tenney.

New engine

Vital questions include whether the rotors must fold for shipborne operation, and whether two or four engines are needed for one-engine-inoperative performance. A twin-engined JHL would need 20,000shp (15,000kW) -class powerplants, while a four-engined aircraft like QTR would still need turboshafts twice the size of any US engine now flying. "We will need a new engine," says Tenney.

Aiming for first flights by 2015, the AATD hopes to build competitive "near full-scale" X-plane demonstrators - "three-quarters scale or bigger, and within the capability of existing engines", he says, although they could be converted from available turbofans.

JHL seems ambitious, but is gaining credibility. Tenney says the AATD has secured "significant high-level DoD support" for the risk-reduction effort in preparation for a technology demonstration. Funding is coming from the US Army, Navy, Marine Corps and other sources - not the USAF - and the head of US Special Operations Command has joined those calling for an X-plane programme, he says.

"We are convinced JHL is technically feasible, and there are multiple configurations that are potentially viable still in the mix," says Tenney. "We believe it is very feasible to produce an aircraft with the payload, range and speed characteristics we are looking for. We need to go out and demonstrate it."

But the army has yet to secure funding for a technology demonstration in the FY2010-15 budget cycle. If the demonstration slips, Tenney is concerned the urgency of the requirement will force the services to look elsewhere for a new battlefield airlifter. And that could be a bigger C-130 derivative, a proposed STOL C-17B - or the A400M.