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Mach 5 Airliner Operations Face Huge Challenges

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Will hypersonic airliners be too hot to handle . . . literally? The issues involved in ground handling of a Mach 5-plus transport still simmering after its intercontinental hypersonic hop are among the unique challenges being considered as researchers address the potential operation of future high-speed airliners.

While most hypersonic transport projects have focused on the basic design and aerodynamic, propulsion, structures and systems technologies required, the operational aspects are equally challenging and hard to predict, say developers.

For clues on how to tackle some of these issues, hypersonic researchers are turning to previous high-speed operations such as the Anglo-French Concorde Mach 2 airliner or Mach 3.2 Lockheed SR-71 reconnaissance aircraft. However, speaking at the American Institute of Aeronautics and Astronautics Hypersonics and Spaceplanes conference in Glasgow, Lockheed Martin Advanced Development Programs Manager Rob Vermeland says that while these lessons may be useful to some degree, the operational challenges of a hypersonic transport versus a supersonic or subsonic aircraft represent “a whole different animal.”

 Future high-speed airliner projects like that being considered by the European-Japanese Hikari group have outlined a notional target for turnaround time of around 48 hr. Yet for the SR-71, even with the full attention of a dedicated ground crew, high-tempo operations were simply not feasible, says Vermeland. “If we had one sitting in the hangar here and the crew chief was told there was a mission planned right now, then 19 hr. later it would be safely ready to take off.” 


Then there is the question of maintainability. “The SR‑71 often would not come back ‘Code One,’ or flight ready. More often it was Code Two, which meant things had to be fixed, or worse Code Three, which meant major things had to be fixed. It might return with delaminated panels, rivets that had popped out in flight or even broken inlet parts. The average time to get the plane back and ready to fly was a week. If something broke it could be a month,” says Vermeland.


 “So if the high-speed goal is a 48-hr. turnaround, it will take a lot of effort to achieve. Airlines today try to turn an aircraft around so it flies nearly all the time. These aircraft will be sitting on the ground for around 100 times longer than they’ll be flying, so think of the economics of that.” The answer, he suggests, will be the development of intelligent, embedded prognostic, data-tracking and health-monitoring systems linked to an efficient logistics process, so that spare parts are ready and waiting when an aircraft arrives.


 Safety considerations should guide the design from the start, Vermeland says. The SR-71 was notoriously prone to suffering inlet “unstarts” at high speed, which were “a violent event for the pilot. It slapped them against the canopy. What will happen to our passengers if there’s an event that maybe an Air Force pilot is OK with, but your average businessman is not going to want to happen?” Passengers will also expect the comfortable cabin of today’s subsonic aircraft despite the extreme conditions outside. “Don’t imagine our passengers will have pressurized suits. What happens if we lose cabin pressure in these vehicles? What happens when you have a failure, and if you can’t afford to have a failure, what safety systems do you need to prevent that?”


 Researchers expect future hypersonic vehicles will need to work within today’s air transport infrastructure. But given the unusual requirements of high-speed aircraft for everything from fueling and ground handling to long runways and high-radius turns on taxiways, Vermeland asks if it may be better to develop dedicated “point-to-point” hypersonic airports. “Maybe it is worth the expense of designing a vehicle for that type of operation and not the other way around. There’s the whole infrastructure to consider for hydrogen fuel, and what about ground handling? Today’s cabin attendants and ground staff open the door on arrival. They push it in and there’s lots of touching to a part where there may be high temperatures. Do they put on heat-resistant suits or fire-retardant gloves?”


 Hideyuki Taguchi, green engine research leader at the Japan Aerospace Exploration Agency, says current civil aviation certification standards are simply “not suitable for such an advanced aircraft.” The regulations governing forward visibility for the crew, for example, may be difficult if not impossible to meet with current study configurations that have both pilots and passengers shielded inside the vehicle between liquid-hydrogen fuel tanks. “This cannot be certified by the FAA because the crew cannot see directly out of the front of the aircraft,” he says.


 Hypersonic vehicles will need to navigate carefully to avoid exposing populated areas to sonic booms, says Sergey Chernyshev, executive director of TsAGI, the Russian central aerohydrodynamic institute. “The carpet of exposure to [a] sonic boom will be larger because of the vehicle’s much higher altitude. However, for vehicles we have studied, [under the European-funded Hexafly-Int high-speed vehicle project]” with a 360-ton takeoff weight, the maximum amplitude of the shockwave is only around 60-plus Pascals [1.25 psf] overpressure, so not so high as expected.” This is approximately the same as a boom generated by the space shuttle following reentry at Mach 1.5 and 60,000 ft. altitude but less intense than the 90 Pascals produced by the Concorde at Mach 2 and 52,000 ft.

 “To deal with the issue, we must think of the shape and configuration of the aircraft, the flight profile and atmospheric conditions. Typically, we calculate the sonic boom on shockwave density from straight flight, but if we have maneuvers or g factors while accelerating, this gives us increasing density because of focusing of shockwaves from different trajectories,” he says.


 As well as sonic boom concerns, the potential impact of emissions will be a major factor in both the vehicle’s design and how it is operated, says Sebastien Defoort, a research engineer at French aerospace research center Onera. But the severity will vary depending on cruise altitude, speed, fuel type and a deeper understanding of the potential climatic effects of water vapor in the stratosphere. “We will have to make this green from the start. There are some strong public concerns that need to be taken into account,” he says.


 “A conventional airliner flying at an altitude of 10 km will emit, as an order of magnitude, 300 tons of carbon dioxide and 140 tons of water vapor. A Mach 5 aircraft will fly at 25 km and, if it is hydrogen-fueled, will emit a lot more water vapor but no carbon dioxide. But if we fly at Mach 8, we will fly a lot higher and deposit a lot more water vapor in the upper atmosphere,” Defoort adds. A small fleet of hypersonic vehicles could add 15 megatons of water vapor a month to the atmosphere, most of it in the northern hemisphere.


“We have to be careful, as these figures have yet to be consolidated, but the data seem to show that water vapor has a very large effect on climate and temperature change. Nitrous oxide emissions have an effect, but are secondary for hydrogen fuel.” More detailed analysis shows there are additional effects of the chemistry of the atmosphere that can reduce the residence time at higher altitudes. The bottom line, says Defoort, is “it may be better to fly at Mach 8 and higher altitudes.” In addition, he says, “there are special trajectories that may mitigate the emissions effects, for example, flying over the poles will help because residence times of water vapor can be lower.” 





Edited by MigBuster
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