NASA will revisit an engineering technique which could solve many of the problems associated with hypersonic flight, according to reports. The research might offer genuinely re-usable space launch vehicles, among other benefits.
At present, the only method humanity has to accelerate things up to the Mach 25+ speeds required to reach orbit is the use of multiple rocket stages. As a result, launch costs are high and only a few things are thought worth putting into space: satellites, space probes, and nuclear warheads, mostly.
If access to space is ever to become easier something new will be needed. It would be useful, for instance, to build aircraft which operated like jets, generating most of their thrust by scooping air at the front and throwing it out at the back faster - rather than having to carry all their reaction mass like a rocket.
Unfortunately, as aircraft accelerate through the lower Mach numbers it becomes very hard to slow down the incoming air to subsonic speed so that normal fuel will burn in a combustion chamber. One approach is to burn hydrogen fuel in a supersonic airflow - the so-called "scramjet", or supersonic combustion ramjet. Scramjet test platforms have got up to around Mach 10; but they have their problems.
An alternative - or possibly complementary - technology is the use of Magnetohydrodyamic (MHD) effects. The idea here is to extract energy in the form of electricity from hypersonic airflows entering the intakes. This could slow down the stream through the engines to a manageable level. The electricity could then be passed to the back of the aircraft and used to accelerate the exhaust gases, boosting thrust; or it might be used to power onboard systems.
Russian researchers were said to have done concept studies for such an aircraft in the early '90s, under the name "AJAX". One of the problems of the MHD-ramjet idea is that the incoming air needs to be ionised so its passage through the inlet generator produces work. It wasn't really clear what method the Russian weapons labs favoured here.
Now Flight International reports that NASA is taking a fresh look at MHD ramjet/scramjet technology, with the idea of using it in a "two-stage-to-orbit" reusable launcher configuration. NASA is supposedly thinking of a specialist jet engine setup not unlike that which drove the famous SR-71 "Blackbird" spyplane of Cold War days.
The Blackbird's conventional (but brutal) J-58 turbojets could keep burning up to Mach 3+ because of their special intakes, which slowed the intake air down for them using a retracting central spike. Now NASA hypersonics boffin Dr Isaiah Blankson reckons that MHD energy-suckers in the intakes could do even better, letting a turbine engine run at up to Mach 7. This would permit the reusable first stage of the future NASA launcher to take off from a runway and get its piggyback orbiter well up into scramjet-type flight regimes, all using just one set of engines.
"We would put a device ahead of the engine to ionise the flow, extract the energy then put it back in the combustor," Blankson told Flight.
"We need a very efficient way of ionising the flow field. If we can increase ionisation efficiency by 40 per cent we can be very competitive."
Reportedly, Blankson says extracting 30 to 40 per cent of the inflow energy would cut its speed by 50 to 75 per cent. That sounds counterintuitive, as kinetic energy is proportional to the square of velocity, but presumably a man with his background knows what he's on about. Potentially, a Mach 7 flow would slow to Mach 3 downstream of the MHD, and then a Blackbird type setup could handle it.
Blankson has a team working on the idea at NASA's Glenn Research Centre in Ohio, with various studies planned over the next two years. The hope is that the latest advances in high-voltage pulsed power systems might make feasible what hasn't been to date.
One does note, of course, that there would be other uses for standstill-to-Mach-7 airframes which ran on fairly ordinary fuel and potentially had huge amounts of spare electrical power. Blankson has worked with the military before, as it happens.
If his team does well, America might get hypersonic missiles, fighters or bombers - perhaps armed with directed-energy rayguns of some kind - well before it gets a reusable space launcher.
Which is either depressing or cool, depending on your viewpoint. ®