|SpaceLiner animation, 2009. CC 3.0|
It is being reported that a futuristic vehicle named "SpaceLiner" that could fly at Mach 24 (24 times the speed of sound) and could carry 50 passengers from Europe to Australia in 90 minutes, would be available by 2050. The concept was initiated by German Aerospace Center in 2005 and it was supported by European Space Agency since 2009. The current concept involves launching the vehicle vertically like the space shuttles, separating the plane from the booster at an altitude of 80 kilometres and the plane glides towards the destination for landing at 25200 km/h.
Current technology is reliable enough for the rocket launching stage, where humans have launched countless space vehicles without any failures. However, the main challenges of this futuristic flight remain in the gliding stage. One of the most important aspect is the design of the plane which can affect the heat and shock-waves generated. Current conventional aircraft design doesn't allow hypersonic flight as the structure cannot support the extreme pressure and heat generated, about 576 times the atmospheric pressure and more than 6000°C of surface temperature. Apart from using smaller radius of curvature, new composite materials, and ceramic coating for the entire plane body, the plane should also adopt radically new designs like double ultra-wide fuselage that can reduce drag according to a research by MIT. At about Mach 2, the shockwaves formed are negligible to the aircraft body, but at Mach 24, the shock layer is very small that the shockwaves can interact with the aircraft and causes turbulence. The gases in the shock layer also becomes very hot due to the compression effect and will heat up the plane too, in addition to the heating by air resistance. At the shock layer, the air is so hot that nitrogen and oxygen molecules actually dissociate and the aircraft body is prone to react with these free radicals.
Apart from aircraft structural integrity, the testing and development process is also significantly more complex and difficult. A special hypersonic wind tunnel is required to gather performance data for the plane. For simulations at Mach 16, the air in the tunnel has to be preheated to about 8000°C so that when the air expands, the air will not cool to liquid state. The instruments used have to be high speed equipments so that the sufficient data can be collected before the tests end since the duration of the test is very short. The nature of the tests cause the power requirements of the wind tunnel to be very high. Besides that, the behavior of hypersonic airflow is very difficult to understand and analyse, especially for high angle of attack. The lack of understanding and theories in the hypersonic region make the development of futuristic planes like "SpaceLiner" extremely difficult, especially when the engineers have to consider the unknown effects of shockwaves at the body and the wings of the aircraft.