There has long been a tantalising gap between the world of astronaut-style space travel and subsonic airliners – apart from the brief period of Concorde and its Russian counterpart, the Tu-144. With the speed of sound as a benchmark – known as Mach 1, around 660mph at a typical airliner cruise height – these beautiful craft forged a new frontier in passenger travel as they cruised along at just over Mach 2.
There remains a dream to banish the 'red-eye' forever by being able to whisk passengers on key routes like New York-London in a single hour. For this, a new generation of airliners are needed capable of hitting speeds approaching Mach 5 - or more. And a host of key players, from aircraft makers like Boeing to NASA, are at work to turn dream into reality.
While companies like Virgin Galactic aim to take folk beyond Earth’s atmosphere, Germany's main aerospace research centre DLR is planning a so-called SpaceLiner that will zoom into the upper atmosphere about 50 miles up with the aid of a detachable re-usable rocket booster, before gliding back to its destination on Earth at speeds of between Mach 15-20. High speed would be complemented by low emissions, thanks to a planned mix of hydrogen rocket fuel and liquid oxygen feeding an engine whose main waste product would be harmless water vapour.
DLR's Martin Sippel is heading up the project, and has a vision of a fleet of SpaceLiners based on new light, super-strong materials, advanced engine cooling technologies, plus heat shielding to safeguard against the intense heat of hypersonic flight. As with Virgin Galactic, though, seats won't be cheap! Bank on a few hundred thousand bucks at today's prices. And be patient, as Sippel envisages first boarding in the 2040s for those keen to do London to Sydney in 90 minutes.
If you aren't keen strapping into something closer to an Apollo rocket than a Boeing 777, the other main avenue of research is into planes powered by either a ramjet or its faster cousin, the scramjet (supersonic combustion ramjet). As these fly through the air at supersonic speeds, they use the flow of high-speed air through the engine to provide oxidizing oomph for burning fuel, reducing the amount of propellant a scramjet airliner would need to carry.
Ramjets differ from scramjets by letting air slow to subsonic speed before combustion, while scramjets keep the air at supersonic speeds. On the plus side, scramjets can easily pass the Mach 5 mark where supersonic becomes hypersonic. (SEE BOX). On the downside, they are considerably harder to make work.
Though the first scramjet tests took place way back in the 1950s it wasn't until the late 1990s that successful controlled prototypes were being tested by Russia and the US, achieving speeds of up to Mach 7. In the last few years, US prototypes have reached Mach 10. Some claim speeds of Mach 24 are achievable – though these would be for scramjet missiles rather than passenger planes! And so far, hypersonic scramjet test flights have been short affairs - Boeing's X-51A holds the current duration record of 210 seconds at hypersonic speed. You can see the 2013 record-breaking test here.
Scramjets can easily pass the Mach 5 mark where supersonic becomes hypersonic
The X-51A is a key stepping stone toward possible hypersonic airliners, featuring a scramjet integrated with an airframe. Weight was balanced by lift and control surfaces trimmed and controlled by the vehicle. “It was a true flying machine,” says Boeing's Chief Scientist of Hypersonics, Kevin Bowcutt.
The 'wings' of the X-51A were small due to the small size of the vehicle but these would be scaled up for any hypersonic airliner based on it, with the current estimate being for craft carrying between 50-100 passengers. Boeing's research on hypersonic engines is complemented by its creation of futuristic aerospace materials, including a strong but lightweight HRL micro-lattice metal that is over 99% air, and with future investment, could be used for many elements of a hypersonic airliner.
Kevin Bowcutt sums up the state of affairs for scramjet airliners neatly. “The primary challenges for flight at Mach 5 and beyond include developing much larger scramjet engines, combined-cycle (multi-mode) engines to provide power from takeoff to high speed, more durable high-temperature materials, and robust safety systems. Technical progress is being made in each of these technology areas.”
Bowcutt believes, however, that scramjets are an unnecessary push for wannabe supersonic passengers. “The world is small enough that travelling faster than Mach 5 is unnecessary. And Mach 5 can be done with a combination of turbine and ramjet engines.”
Going for the simpler ramjet approach rather than scramjet could bring things considerably closer too. “The ramjet tech is far more doable,” argues Matthew Dickinson, who lectures in mechanical engineering at London's UCLA. “Looking at some of the hyper jet tech displayed in the past few years, the engine developments are just over the horizon.”
Supersonic v. Hypersonic
Breaking Mach 5
Important changes occur when a craft approaches Mach 5 – a dividing line between supersonic and hypersonic speeds. The difference is at the level of the very air itself. As a craft travels faster, the air around it heat up – often to temperatures of several thousand degrees which change it to a different physical state.
Last year, Airbus were actually granted a US patent on designs for a ramjet airliner instantly dubbed 'Concorde 2'. The firm envision a craft using three separate propulsion systems: a normal jet engine will be used for take-off; a rocket booster to push the plane speed as it climbs to around 18 miles up; then a ramjet kicking in to take the plane to a cruising speed of Mach 4.5.
This year, meanwhile, Virgin have signed options to buy 10 supersonic airliners based on a prototype currently being built by pilot and former Amazon executive Blake Scholl. Scholl's new jet – christened Boom – is aiming to fly between London and New York in 3.5 hours, and he claims it will beat rival planes to market because it will use existing technology rather than new technology requiring approval by regulators.
Announcing the new prototype in March 2016, Scholl also emphasised the price advantages of his planned new airliner. “We are talking about the first supersonic jet people can afford to fly,” he said, citing a planned ticket price of $5000 return. The first Boom prototype is slated to have its first test flights at the end of 2017, with commercial flights then on the horizon within a few years.
BEATING THE BOOM
A key problem that plagued Concorde – and which needs to be addressed by all future supersonic airliners – is the sonic boom created by planes as they pass through the sound barrier. This led to Concorde being banned from flying at supersonic speeds anywhere over land, due to the noise experienced at ground level.
However, a host of futuristic aircraft designs – nicknamed X-planes (experimental planes) – are coming off drawing boards which promise to greatly minimise sonic booms. Novel shapes are a common factor in what NASA has dubbed N+2 airliners - second generation beyond current technology – including needle-like noses plus wings ranging from amazing curves to vast V-shapes. Novel engine placements are another factor, with one futuristic Boeing design proposing two top-mounted engines rather than any under the wing.
Back at Germany's DLR, Martin Sippel argues his SpaceLiner will always out-perform ramjets or scramjets when it comes to the sonic boom issue. “Rocket propulsion allows flying at much higher altitudes because the atmospheric air is not used. For this reason, the sonic boom issues on the ground are less severe than for high-speed vehicles flying at lower altitudes.”
Sippel also reels off other advantages he sees for the SpaceLiner concept. “Rocket propulsion is an established technology with all associated advantages and disadvantages well known. It offers a reduction in development times for potential implementation of a hypersonic transport,” he argues. “In contrast, the most advanced flight experiments for SCRAM propulsion were only a fraction of the size actually needed for a passenger transport. Additionally, they were operating for less than 1/10th of the required flight times.”
But then the Wright Brothers first flight only lasted 59 seconds, and look what that has led to...
FLYING HIGH, FLYING FAST
Experts also believe hypersonic airliners wouldn't prove too challenging to either pilots or passengers - even if the experience may be very different to a present-day airliner. For example, the heat of airflow caused by hypersonic flight would make normal sized plane windows a potential weak point in the fuselage, so future fast fliers will make do with either very small or no windows. Outside views could, however, be beamed onto a screen or provided via VR systems.
Regarding piloting, a high degree of automation would be built in to any scramjet airliner – as it is for present-day planes. For the SpaceLiner, Sippel cites past experience gained via the Space Shuttle and Soviet Buran spaceplane.
And how about the passenger experience? “Passengers will experience no more than 2.5g peak acceleration, and only during the initial start phase - perfectly within a reasonable range for untrained passengers,” says Sippel.
Speaking in January this year, Jaiwon Shin - associate administrator for NASA’s Aeronautics Research Mission Directorate - had this to say: “We're at the right place, at the right time, with the right technologies”. So, while we may still have a wait, the dream of an unforgettable very fast flying rollercoaster ride to anywhere on Earth definitely looks like it might become reality.