Who remembers the Blackswift – the planned hypersonic successor to the legendary SR-71 Blackbird spy plane of Cold War fame? The Blackswift was intended to take off and land from a runway like a normal plane and achieve speeds of Mach 6 on scramjet propulsion (comfortably eclipsing its illustrious turbo/ramjet predecessor's Mach 3.5) – and carry out a barrel roll while doing so.
Unfortunately for lovers of spiffy hyperplanes, the Blackswift (aka Falcon HTV-3X, as seen above) never got off the drawing board. Its funding was cut by sceptical politicoes back in 2008, and since then the US military has had to content itself with the missile-style WaveRider testbed. The WaveRider will not develop into a reusable aircraft: it is released at height from a B-52 bomber, accelerates to Mach 4.5 using a rocket booster, and then lights up its scramjet to accelerate – its designers hope – to Mach 6. In its only live test so far, however, the WaveRider topped out at Mach 5 in what was described as a partial success.
From a military point of view, the idea of hypersonic scramjet missiles is mildly interesting, though somewhat marginal: after all, if a warhead really needs to get somewhere in a hurry, one can simply fire it out of the atmosphere on a multistage space rocket (aka Intercontinental Ballistic Missile) and so avoid all the problems of high speed in an atmosphere.
But in civilian life we might like to move from having to use expensive, throwaway multistage rockets every time we want to put something into space. We might also like our orbital launch vehicles, as well as being much more reusable than a rocket stack or a Space Shuttle package, to use atmospheric oxygen in place of some or all of the huge amounts of oxidiser that rockets must carry to burn their fuel with. Also, we'd like to avoid the huge expense and trouble involved in pointing heavy vehicles vertically up into the sky for launch – we'd like runway takeoff as well as runway landing.
That's why people found the Blackswift exciting: while it was never intended to reach the Mach-25-equivalent speeds needed to achieve orbit, it was going to be a big step forward from the SR-71, previously the benchmark for sustained, practical-ish high-speed air breathing flight from a runway. Like its illustrious forebear, the Blackswift was to burn relatively normal JP-7 jet fuel rather than impractical, dangerous, bulky hydrogen; like the Blackbird, it was to take off and land on a runway and make no use of throwaway booster rockets to get up to ignition speed*.
And that's why today is an exciting day, because news has arrived that Blackswift is not dead after all. The US Air Force, we learn from Aviation Week, has revived the aspiration for a reusable, hydrocarbon-fuelled runway hyperplane under the new name "High-Speed Reusable Flight Research Vehicle" (HSRFRV). Lovers of hyperplanes will need to be patient, however, as the USAF intends to take a cautious path toward building the new Blackswift; it is not expected to fly until 2021, following years of trials and tests with WaveRider-derived missile-style weapon carriers.
These early tests – which might lead to a hypersonic scramjet missile that could fit into the weapons bay of a B-2 stealth bomber – would prove the main new technology required to beat the SR-71 Blackbird: that is, hydrocarbon-burning scramjets. The Blackbird was propelled down the runway and up to high speed by two hefty afterburning turbojets which were mounted inside cunning nacelles fitted with a retractable spike. At high supersonic speed, these nacelles functioned as ramjets and the turbojets nested within them were superseded, effectively acting as fuel injectors for the ramjet combustion chambers.
But a regular ramjet, even when travelling supersonically itself, slows down the flow of air through its combustion chamber to subsonic speed in order to avoid blowing out the flame. As speed climbs through the low Mach numbers this causes unacceptable levels of drag to build up, which is why the SR-71 couldn't beat Mach 3.5 or so.