Is the future of air travel SUPERSONIC? Scientists are developing transport that travels at super-high speeds without the boom Engineers are studying the physics of flight in a bid to reduce sonic boom U.S. scientists are testing different shaped planes in wind tunnels While Japanese aeronauts are dropping models from high-altitude balloons Commercial plane builders are also experimenting with Mach cutoff This is when jets control sonic boom by controlling speeds and altitudes By Graham Warwick, Science Uncovered It's more than a decade since Concorde was retired from service yet the dream of travelling faster than the speed of sound, and getting from London to New York in under four hours, has not gone away. No company will build a supersonic passenger plane unless it is allowed to fly supersonically over land - something that was off-limits for Concorde. So, aeronautical engineers are now looking closely at the physics of aircraft flight to see if they can find a way to reduce the sonic boom of future aircraft - to bring supersonic back without the bang, or at least with one that's significantly quieter. The article 'Supersonic: Back with a Bang' appears in issue three of Science Uncovered, on sale now Aeronautical engineers are looking at the physics of aircraft flight to see if they can find a way to reduce the sonic boom of future planes, illustration pictured. Work has already begun on developing aircraft with quieter booms, thanks to tests on radically different shapes being carried out in wind tunnels and in the air In a conventional supersonic aircraft, shockwaves from the nose, cockpit, inlets, wings and other features coalesce as they move through the atmosphere into strong shocks emanating from the nose and tail. These are known as bow and tail shocks, respectively. As these shockwaves pass over someone on the ground, air pressure rises sharply, declines, then rises rapidly again - it's this that produces the classic "double-bang" sonic boom. Reshaping the aircraft to produce a longer, more slender shape that slips through the air is the best way to generate shockwaves of lower, more equal strength that attenuate as they pass through the atmosphere and do not coalesce into such strong bow and tail shocks. Stretching the nose to break the bow shock into a series of weaker shockwaves is particularly effective. This lowers and spreads that initial pressure peak and softens the first bang of the sonic boom. Work has already begun on developing aircraft with quieter booms, with tests on radically different shapes being carried out in wind tunnels and in the air. The U.S. Defense Advanced Research Projects Agency (DARPA) started the Quiet Supersonic Platform programme in 2000. This led to the Shaped Sonic Boom Demonstrator - a Northrop F-5 with a dramatically reshaped forward fuselage. In 2003, the Shaped Sonic Boom Demonstrator proved an aircraft's shockwave can be altered to produce a less intense boom. Gulfstream Aerospace already builds a $65 million business jet that flies at Mach 0.925 - just shy of the speed of sound - and has long said that its elite customers are willing to pay more for an aircraft that can fly at higher speeds. In 2006-7, Nasa and Gulfstream flew an extendable nose boom, called the Quiet Spike, on board a Boeing F-15. The retractable 24ft-long, carbon fibre-composite spike was flown at speeds of up to Mach 1.8. It extended while the craft was in flight and shaped the sonic boom of the supersonic business jet by breaking up the bow shock into three smaller shocks. As well as a modified nose, a highly swept wing has also been found to reduce the sonic boom, as does a long wing root - the part of the wing that meets the fuselage. The ultimate goal with all of this reshaping is a smooth sine-wave boom signature, like an "S" on its side, which would transform the jarring double bang into an almost-inaudible soft thump. In a supersonic aircraft, shockwaves from the nose, cockpit, inlets, wings and other features coalesce as they move through the atmosphere. As these shockwaves pass over someone on the ground, air pressure rises sharply, declines, then rises rapidly again. This produces the 'double-bang' sonic boom The U.S. Defense Advanced Research Projects Agency started the Quiet Supersonic Platform programme in 2000. This led to the Shaped Sonic Boom Demonstrator, pictured. It is a Northrop F-5 with a dramatically reshaped forward fuselage. In 2003 it proved shockwaves can be altered to produce a less intense boom It's a difficult task, as it is not only the nose and the tail of the aircraft that require attention. The complex shock systems generated by supersonic inlets and engine nozzles must be taken into account, too. Supersonic: Back without a Bang appears in issue three of Science Uncovered, pictured, on sale now Sonic boom noise is measured in perceived decibel level (PLdB). Concorde's boom was a window-rattling 105 PLdB. Researchers believe 75 PLdB would be an acceptable level for unrestricted supersonic flight over land, but Nasa is aiming more ambitiously for 70 PLdB or lower. This looks achievable for a small supersonic business jet, because boom is proportional to aircraft weight, but is a much greater challenge for a heavier airliner. Nasa's also been looking at larger aircraft - setting itself a target of 85 PLdB for a supersonic aircraft it thinks could conceivably be built by 2020. Cruising at speeds of Mach 1.6-1.8, this aircraft would be slightly smaller and slower than Concorde, but significantly quieter and more efficient. In wind-tunnel tests, designs from both Boeing and Lockheed Martin - funded by Nasa - which would carry between 30 and 80 passengers, achieved boom levels as low as 79 PLdB, beating the target. At this level, the sonic boom would be more of a thump than a loud bang. Nasa is setting the bar even higher - it thinks 70 PLdB is achievable with more refinement. Studies are still taking place to try to reduce the volume level even further. Supersonic could be superseded by something even faster. Mach 2.5 is about the speed limit for gas-turbine engines. Any faster and the temperature and pressure of air entering the engine is too high for the turbo machinery inside. To fly at hypersonic speed - Mach 5 and above - requires a different type of engine. A supersonic-combustion ramjet, or scramjet, has no moving parts. Instead of the rotating compressor and turbine in a jet engine, air is compressed and expanded by complex systems of shockwaves under the front of the aircraft, inside the inlet and under the fuselage at the rear. Scramjets have been under development for decades, but a breakthrough came in May 2013, when the U.S. Air Force Research Laboratory's Boeing X-51A WaveRider flew for 240 seconds over the Pacific on scramjet power, reaching Mach 5.1 and running until its fuel was exhausted. The next step is to build a high-speed cruise missile, able to strike distant targets in minutes, not hours. Lockheed Martin's Skunk Works - builder of the Mach 3.5 SR-71 Blackbird spyplane - has unveiled plans to develop a successor, dubbed the SR-72 (pictured above). Designed for reconnaissance and strike missions, the SR-72 would combine turbojet and ramjet/scramjet engines to enable the aircraft to take off from a runway, accelerate to a Mach 6 cruise, and then return to a conventional runway landing. If it can secure funding from the U.S. Defense Department, Lockheed Martin believes a prototype could be flying as soon as 2023 and the SR-72 could enter service by 2030, potentially paving the way for commercial applications of scramjet technology. However, any design that's created must be practical. While the tests point to a long, slender aircraft design in order to produce a publicly acceptable sonic boom, the aircraft must still fit in its gate at the airport and be capable of taxiing without hitting anything. Extensive reshaping of airports around the world would again reduce the viability of any aircraft. 'The long, skinny fuselage is not a practical answer,' said Nasa's supersonic programme manager Peter Coen. In our pursuit of boom reductions, we examine the whole, three-dimensional shape of the vehicle, including the engine configuration. Even then, we keep in mind that the airliner has to meet all of the other requirements which are part of good design practice. In 2006, Nasa and Gulfstream Aerospace flew an extendable nose boom, called the Quiet Spike, pictured. Its shape was able to modify the sonic boom of the supersonic jet by breaking up the bow shock into three smaller shocks The next logical step would be a low-boom X-plane flight demonstrator, to collect the data that shows that booms of 70-80 PLdB are acceptable. Japan is the only other nation with a serious interest in supersonic aircraft, and the Japan Aerospace Exploration Agency (JAXA) is conducting its own research into low-boom technology. In 2011, in Sweden, JAXA dropped two dart-like shapes from a balloon at an altitude of 30km. Reaching Mach 1.8 on descent, one model produced a normal N-wave. The longer model produced a shaped low-boom signature. Called D-SEND, these tests were planned to culminate in the flight of an unmanned supersonic glider, which would be released from the balloon at 30km altitude and dive at Mach 1.3 over an array of sound-measuring blimps. In 2011, Japan's Aerospace Exploration Agency dropped two dart-like shapes from a balloon at an altitude of 30km, pictured. Both reached Mach 1.8 and while one model produced a normal wave, the longer model produced a shaped low-boom signature Meanwhile, there is a way to fly at low supersonic speed over land without any boom, which exploits a phenomenon called Mach cutoff. The speed of sound increases with temperature, causing the shockwave to be refracted as it enters warmer air closer to the ground. Below a threshold Mach number - about 1.2 for an aircraft flying above 35,000ft - the shockwave will not reach the ground and no boom will be heard. It's a pragmatic solution to a truculent - and expensive - problem. U.S. company Aerion, which is designing an aircraft that would use ultra-smooth, drag-reducing airflow to fly efficiently, both supersonically and subsonically, is looking at Mach cutoff as a way to get a supersonic business jet off the ground without the need for the complex shaping of a low-boom design. Aerion has yet to persuade any manufacturer to build its jet. It's more than a decade since Concorde, pictured, was retired from service. No company will build a supersonic passenger plane unless it is allowed to fly supersonically over land - something that was off-limits for Concorde Nasa, meanwhile, continues to build its understanding of sonic booms and improve its design tools. Recent flight tests have looked at the phenomenon of focused sonic booms. These are intense "superbooms" created when supersonic aircraft accelerate, decelerate or manoeuvre, but are heard at only one place on the ground. If that location can be predicted accurately, flights can be planned to place the superboom where it causes minimum disturbance - over water or empty desert. There are a lot of hurdles to overcome - the biggest of which is building a flight demonstrator. But the trend is clear - quiet and efficient supersonic transport is within reach if industry is prepared to take the risk. Supersonic: Back with a Bang appears in issue three of Science Uncovered, on sale now.