Quiet, Efficient Supersonics Within Sight?
2:28 PM on Mar 29, 2012
NASA is claiming a breakthrough in quiet supersonic aircraft, with successful windtunnel tests of designs that combine low sonic boom and low cruise drag - characteristics once thought to be mutually exclusive.
Commenting on the completion of boom and performance tests by Boeing and Lockheed Martin, NASA Supersonic Fixed Wing project manager Peter Coen says: “This is a breakthrough. It’s the first time we have taken a design representative of a small supersonic airliner and shown we can change the configuration in a way that is compatible with high efficiency and have a sonic signature than is not a boom.”
The trick is in the shaping of the airframe to tailor the shockwaves so that they produce a sinusoidal pressure signature at the ground, rather than the abrupt N-wave shape of a traditional "double-bang" sonic boom. Instead of powerful bow and aft shockwaves, the aim is to generate lots of smaller shocks along the airframe that attenuate more quickly as they travel through the atmosphere. Boeing's model (above) shows the result of all that subtle shaping.
Boeing's design for a 202ft-long, 30-passenger supersonic airliner acheived a boom level of 81PLdB at Mach 1.8 in NASA Ames' 9 x 7ft tunnel. Lockheed's design for a 230ft-long, 81-passenger trijet (below) achieved 79PLdB at Mach 1.6. NASA's goal is 85PLdB. "That's 25dB less than Concorde and 20dB less than the best we achieved under HSR [NASA's High Speed Research program]," Coen says.
Concept: Lockheed Martin
Both concepts for a 2025-timeframe small supersonic airliner achieved the cruise lift-to-drag ratio required to meet their design range of 4,000nm. "We've broken the low-boom/low-drag paradox, where you could get one, not both," he says. "They achieved low boom with a good level of supersonic lift-to-drag."
But we can do better, it seems. Longer term, NASA is targeting a boom level of 65PLdB, and 70PLdB is widely regarded as the threshold for public acceptance of unrestricted supersonic flight over land. Both the Boeing and Lockheed designs are at the high end of what would be acceptable. “We’ve learned that shaping technology will improve, and we'll probably be able to reduce the boom further,” Coen says.
So Boeing and Lockheed will further refine the shaping of their designs in Phase 2, for which NASA has two goals: to reduce boom levels across the full 60-mile-wide ground "carpet", and not just under the aircraft's track, and to measure the effects of engine inlet and nozzle shock systems on the boom signature. Refined models will go back in the tunnel in October/November.
CFD graphic: Lockheed Martin
Getting the aft-fuselage shockwave shaping right is tricky. The 3D geometry optimizer used "makes some pretty fine adjustments to the aft end of the aircraft," says Coen. "Shock position is pretty important, and even small shocks from the nozzle flow could have an effect." The models tested this year had only representative flow-through nacelles. but the new ones will have accurately modelled inlets and nozzles, with their shock systems.
The Skunk Works' John Morgenstern says Lockheed's design achieved a full-carpet low boom averaging 79PLdB "with a cruise L/D impact of less than 10%". He is aiming for a boom of less than 78PLdB, and ideally less than 73PLdB, with the refined design, which will also introduce natural laminar flow to further reduce drag. Boeing Research & Technology's Todd Magee says his team will evaluate ramp and conical inlets and take another look at how the upper-mounted engines on their design are integrated with the body.
All this success is increasing pressure on NASA to find the funds - from somewhere - for a low-boom supersonic flight demonstrator. The agency has been working with Gulfstream on design of the X-54A Low Boom Experimental Vehicle (LBEV - below), but does not have the budget unless it cuts other programs or persuades Congress or its government and industry partners to put in money.
For now, NASA uses an F-18 to fly a dive maneuver that creates a shaped boom at a certain point on the ground. But to assess the public acceptance of shaped booms, it needs to get away from the boom-inured populace around Edwards AFB. “Ultimately we would like to do a flight demonstration of low boom in steady level flight, as a way to look at community acceptance,” says Coen.
awt, NASA, supersonics