GRAHAM WARWICK / WASHINGTON DC
The USA's Quiet Supersonic Platform programme is showing that a long-range, low boom future strike aircraft is feasible - Part 1
Later this year, a Northrop F-5 will streak along the supersonic corridor at NASA Dryden in California at Mach 1.5 and 30,000ft (9,000m), creating a classic sonic boom. Minutes later, a second, modified F-5 will follow at the same speed and altitude, but with a quite different, hopefully quieter, sonic boom signature.
If the September/October tests succeed in demonstrating that sonic boom signature can be controlled by aircraft shaping, one of the key technologies behind the USA's Quiet Supersonic Platform (QSP) programme will have been validated. The next step, industry hopes, will be an X-plane to prove that a long-range, low-boom supersonic aircraft is feasible. It will look very different to today's supersonic aircraft.
"We have changed the face of supersonics," says Dr Richard Wlezien, US Defense Advanced Research Projects Agency (DARPA) QSP programme manager. "We have simultaneously looked at long range and low boom, and found they are not mutually exclusive." This could pave the way for a future US Air Force long-range, high-speed strike aircraft.
One year into the two-year QSP effort, Wlezien says substantial progress has been made towards meeting the programme's challenging goals. "We asked contractors to do something which had never been done before: design a vehicle that is low boom and efficient," he says. "The conclusion is we can do low boom and efficient simultaneously. That's a real eye-opener."
DARPA set a sonic boom mitigation goal of a 0.14millibar (0.3lb/ft2) initial shock overpressure. "That's an order of magnitude below what we can do with existing aircraft," says Wlezien. In parallel, performance goals set stringent demands on overall lift-to-drag ratio (L/D), thrust specific fuel consumption and payload fraction for the QSP. The baseline QSP is a 45,000kg (100,000lb) gross-weight aircraft, with M2.4 cruise and 11,000km (6,000nm) range. "The 20% payload fraction is the toughest goal. That's half the payload of the [Boeing B-52] in an aircraft that weighs a whole lot less," says Wlezien.
Taken individually, the goals are a stretch; taken as a set in an integrated vehicle they are a tremendous challenge, he says. "Contractors have told us this is the toughest programme they have ever worked, and we are surprised they have come up with ways to get there."
Low boom design
Design for low boom and efficiency results in a long, thin aircraft with lift distributed along its length, low wave drag, and highly integrated propulsion. Boeing's design has thin, unswept natural laminar flow wings fore and aft and a swivelling main wing which is deployed for take-off and landing and stowed along the top of the fuselage for cruise. Lockheed Martin's QSP has a slender, sinuous fuselage, highly contoured swept wing with underslung engines, and a V-tail.
Integration has been a key aspect of the programme. "QSP required us to work not just low boom, but low boom and efficiency. That has been the gem of the programme," says Charles Boccadoro, Northrop Grumman QSP programme manager. The company, along with Boeing and Lockheed Martin, completed system studies under Phase 1 of the programme, each defining their preferred QSP concept and identifying the technologies needed to produce a real vehicle.
Airframers looked at a range of technologies investigated under other QSP contracts. The results of these studies are being released this week, at the American Institute of Aeronautics and Astronautics annual aerosciences symposium in Reno, Nevada. Flight International will report from Reno next week.
Wlezien says supersonic laminar flow emerged from the studies as the biggest contributor to high L/D. Laminar flow reduces drag on the long, low-lift designs. "The question is how to integrate laminar flow into a real vehicle," says Wlezien. Airframers are looking at both natural laminar flow, using the thin, unswept wing design developed by Reno Aeronautical, and laminar flow control using distributed surface roughness, a technique developed by Arizona State University.
Natural laminar flow looks promising, but incurs a boom penalty because of the short lifting length of the unswept wing. Distributed roughness, meanwhile, works with swept wings and looks well suited to low-boom designs.
"We are using distributed roughness to enable laminar flow," says Boccadoro. "We also like natural laminar flow for certain surfaces, but not the principal lifting surfaces. Unswept primary surfaces are not compatible with low-lift, low-boom designs - the lift is too abrupt."
Propulsion integration promises substantial benefits, says Wlezien. Northrop Grumman's QSP has top-mounted, mixed-compression inlets. "Matched inlets result in less spill drag, and less external compression and expansion fields to deal with, compared to pure external compression inlets," says Boccadoro.
Empty weight is critical with the 25-30m (80-100ft)-long QSP. "Aircraft structure is usually very dense, but the QSP is a slender, low-density vehicle. A lot of the volume is not used," says Wlezien. The structural concepts developed "are more like spacecraft design". Boccadoro says advanced composites will be essential.
Exotic methods of reducing sonic boom are being investigated, including off-body energy addition to create a virtual bow shock that has the effect of virtually extending aircraft length and reducing boom strength. "It's like putting on a very long nose," says Wlezien. "The challenge is in the energetics."
Another active method of boom reduction being investigated is the thermal keel, a linear ramjet which has the effect of virtually lengthening the aircraft and spreading out the initial pressure rise, reducing boom loudness. "Airframers are looking at whether they can integrate the technology," says Wlezien.
But such solutions may not be needed. "A key finding of our studies was that the QSP goals could be achieved without active or exotic boom reduction technologies, which are too immature," says Boccadoro.
Making history
Northrop Grumman hopes to prove aircraft shaping is sufficient by flying the F-5 Sonic Boom Demonstrator (SBD). "We are on the verge of making aviation history," says Boccadoro. "We have never done a low-boom supersonic flight before."
Low boom is not about reducing shock waves, he says, but about changing the sonic boom signature by shaping the aircraft to stop shock-waves coalescing into a classic "N-wave". The F-5SBD will have a reshaped forward-fuselage to modify pressure distribution and produce a non-coalesced boom with a "flat-top" signature.
"A flat-top signature is a substantial manipulation of the pressure wave." says Boccadoro. Lengthening and blunting the F-5's nose will strengthen the bow shock, while reshaping the lower fuselage will control the pressure rise behind the shock, managing the compression and expansion waves to prevent coalescence.
While the goal of the test is to prove the pressure wave can be manipulated into a shape that persists to the ground, it will result in a quieter boom. "We expect the F-5 SBD to generate a flat-top wave with a substantial reduction in the magnitude of the initial overpressure as a result of shaping," says Boccadoro.
If the F-5 tests succeed, the next step could be a low-boom, efficient supersonic demonstrator, although nothing has been funded beyond QSP Phase 2. "The goals look reasonably attainable. In Phase 2, the contractors must make a compelling case the vehicle is doable," says Wlezien.
Source: Flight International
