Hypersonic tests aim at space
German engineers have recently completed shock tunnel testing of a novel SCRamjet engine which might someday allow better access to space.
With the shuttle programme mothballed and Concorde but a distant memory, it could be argued that we are entering a lull period in aerospace innovation.
That’s not to say all development has ceased; but it will mostly likely look rather different from the bloated budgets of bygone days.
Indeed, Virgin Galactic is due take its first paid-up customers to the edge of space this year, while Oxford-based Reaction Engines is in the early stages of developing Skylon – an unpiloted, reusable single-stage-to-orbit spacecraft capable of delivering payloads.
A similar project that has received considerably less attention is the Australian-led international SCRAMSPACE (Scramjet-based Access-to-Space Systems).
While not quite as ambitious in scope as Skylon, SCRAMSPACE I is certainly further down the line in terms of technology readiness, with wind tunnel tests successfully completed in Germany last month and a full test flight penned for March 2013.
The 1.8m-long axisymmetric prototype spacecraft looks, for all intents and purposes, much like a regular rocket. But it actually aims to overcome many of the drawbacks of existing space access systems with its supersonic combustion ramjet (SCRamjet) air-breathing engines.
Unlike a conventional jet engine, with its rotating compressor blades, a SCRamjet has no moving parts and channels airflow at supersonic speed through its intakes, mixing it with hydrogen fuel for combustion, ultimately creating rear thrust.
For this reason SCRamjets alone cannot be used to propel any craft from a standstill, but must use primary boosters to first reach supersonic speeds.
Nevertheless, by using oxygen from the air they can forgo the need to carry it as fuel for least part of the spacebound journey. And there’s the small matter of being able to reach theoretical speeds of up to Mach 16 (19,600 km/h).
A team at Queensland University achieved the first ever flight demonstration of supersonic combustion in a SCRamjet engine in 2002 with the HyShot craft, although without any discernable thrust.
NASA and Boeing have since achieved SCRamjet hypersonic cruising with their X-51 Waverider, although space access is not the goal in mind for this project.
In the meantime HyShot evolved to become SCRAMSPACE I – an international project led by Queensland. The German Aerospace Centre (DLR) has been working with the Australians since the early 2000s and continues to take a central role in development.
In November last year, Queensland shipped over its latest SCRamjet engine for testing at DLR’s unique 62-metre-long High Enthalpy Shock (HEG) wind tunnel in Gottingen.
The tests addressed the complex aerothermodynamic process taking place in the engine and were overseen by overall project lead Prof Russell Boyce of Queensland and Dr Klaus Hannemann of DLR.
‘A conventional wind tunnel – which the automobile or aircraft industry uses – is a closed circuit wind tunnel where you have a fan that accelerates the air and then a nozzle and it goes round and round – basically you can run it forever,’ Dr Hannemann told The Engineer.
‘Ours is completely different. We compress air with a free flying piston to very high pressure and temperature, then we rupture a steel diaphragm a couple of millimetres thick and the high pressure gas expands into the shock tube and generates a shock wave.’
Ultimately it creates a ‘reservoir’ of air travelling at up to 22,000 km/h, but only for the briefest of milliseconds.
‘We’re talking about reservoir temperatures of 10,000 Kelvin. If you were to operate it for minutes the tunnel would be gone,’ said Hannemann.
‘But the test time is long enough to allow a settled, steady flow through the engine, so we can inject the hydrogen and we can measure the development of the pressure and find out what’s really going on.’
Indeed the team was able to gather a comprehensive dataset from fast acting pressure gauges mounted in the walls, thermocouples for assessing heat transfer and OH-chemiluminescence imaging to characterise the flame as it develops.
The tests proceeded largely as planned and corresponded very well to prior computational fluid dynamics (CFD) models.
They were also able to test out different experimental regimes such as upstream injection where the hydrogen fuel is injected in the intake rather than the combustor, in order to have more time for the fuel to mix with the air. It is hoped this may circumvent a phenomenon called ’unstart’, which plagued NASA’s most recent tests last year of its Waverider, where the transition from rocket boost to the SCRamjet fails.
‘There are still some key questions, like how do I inject the fuel, how do I get it mixed and burned completely if possible, in a very short distance – and remember it’s supersonic of course so the time that the particles of fuel and air are in the combustor is not long , microseconds,’ Hannemann said.
‘You could just make the engine longer, but then the drag increases with increasing Mach number and net thrust will not be possible.’
SCRAMSPACE I is scheduled for launch in the Australian desert at the Woomera Test Range in March 2013 with the help of DLR’s mobile rocket base (MObile RAketen Basis; MORABA). The spacecraft will be transported to an altitude of 340 kilometres by two rocket stages. After leaving the atmosphere, the scramjet will separate from the launcher and control rudders will stabilise it for the return journey.
During the return flight, the vehicle will be accelerated to Mach 8 (9900 km/h). The part of the experiment important to the scientists takes place at an altitude of between 27 and 32 km when the nose cone will be ejected to expose the scramjet intake the engine will ignite and a wide range of instruments will analyse the combustion.
It will crash land in the Australian desert but the critical data for the researchers will, hopefully, have already been transmitted to the ground through a radio link.