By Manuela Braun
"Ten seconds." Tension rises tangibly in the control room. "Five," states systems engineer Ingo Schwendtke. Everyone looking out of the windows of the shelter at the enormous pipes of the wind tunnel now raises their hands and sticks their fingers in their ears. "Zero." The systems engineer starts the trial. An bang reverberates through a darkened building at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) facility in Göttingen. Then air races down the 60 metre long wind tunnel at ten times the speed of sound and flows across the model of the sharp-edged SHEFEX II spacecraft. This elaborate trial lasts for just a few milliseconds.
"We have recorded the data," says test manager Alexander Wagner briefly and concisely, after everyone has taken their fingers back out of their ears. Relief can be seen clearly on the faces of the assembled scientists. In one thousandth of a second, 50 pressure sensors and 60 thermocouple elements on the stainless steel model of the SHEFEX 'Sharp Edged Flight Experiment' recorded measuring data while nitrogen streamed out of a small porous heat-resistant tile and across the metallic surface of the craft. This trial was the first of its kind. Until now, although the 70-centimetre model had been tested in wind tunnels, the active cooling system had not been tested under these flow conditions.
"It makes your coffee slop into the saucer"
In early 2011, the SHEFEX II flight experiment will blast into the skies above the Australian desert on board a rocket that will take it to an altitude of 200 kilometres. Then it will descend and re-enter the atmosphere. During this re-entry process, the spacecraft, measuring two metres in length, will experience temperatures of up to 1400 degrees Celsius. As nitrogen pours out of a protective, heat-resistant tile, it should spread across the projectile like a cooling film, and act as a buffer layer against the hot ambient air.
Throughout that entire morning, the tone inside the building at the DLR Institute of Aerodynamics and Flow Technology was one of concentrated excitement. "If everything isn’t perfectly co-ordinated, the system will tear itself apart," states systems engineer Ingo Schwendtke. Weighing in at over 480 kilograms - the weight of a small Smart car - the piston that is blasted down the tube by compressed air at speeds as high as 1000 kilometres an hour, compresses the propellant gas, a mixture of helium and argon, in much the same way as a giant bicycle pump. The recoil causes the entire system, weighing 280 tons, to jump backwards several centimetres, and an advanced warning signal is issued in the offices above the facility before the trial commences. "It really makes your coffee slop out of its cup," says test manager Alexander Wagner with a broad grin.
Airflow at 12 000 kilometres per hour
Mind you, the piston is by no means the only seriously robust component. A steel diaphragm is mounted between the compression tube and the shock tube. With a thickness of 1.6 centimetres, it gives way and bursts once a pressure of 1300 bar has been developed by the piston. "Just by way of comparison, a car tyre is filled to a pressure of about 2.5 bar. In other words, 1300 bar really is a lot," says Schwendtke. The systems engineer has the greatest of respect for the gargantuan power of this wind tunnel. This pressure is then transferred to the air in the thrust tube, blasting it across the model and its heat-resistant tiles at a speed of 12 000 kilometres per hour.
Wagner undoes the giant butterfly screws that secure the glass window on the test chamber. Just before the trial, technician Mario Jünemann carefully wiped all the dust off model and chamber with a cloth. Now a fine layer of dark dust has settled evenly across internal panels, glass windows and the model. This dust is produced during tests of this kind by the heat in the nozzle of the wind tunnel. However, it will not figure in the measuring results. For a few milliseconds, prevailing conditions inside this chamber closely approximated those that the hypersonic flight experiment SHEFEX II is likely to encounter at an altitude of 35 kilometres as it dives back towards Earth. The model performed well during this test. Even the piston, which the engineers remove from the compression tube with the help of a special robot, only has a slight blemish on one of its seals. "Wear," states Schwendtke unconcernedly. Given the forces that act on all components of this wind tunnel, it is scarcely surprising. The steel diaphragm has served its purpose and will be replaced at a later date. After one of these tests, the plate, with its centre now burst open, cannot be reused.
Now evaluation of the data starts
Attention moves to small viewing screen near the wind tunnel. Test manager Alexander Wagner had a camera recording the test through a window in the test chamber. The sharp edge of the model is clearly visible. Across it, lines and blurred veils flicker in black-white. "The computer simulations look virtually identical," says Tarik Barth, from the Braunschweig subsidiary of this DLR institute. Prior to the test, he ran some calculations on flow conditions around the cooling heat shield. Time and again, test manager Wagner runs through his short film. Then he points his finger at a darker area that progressively spreads down the edge of the model. "That's probably where the nitrogen is flowing through the heat-resistant tile." DLR researcher Hannah Böhrk from Stuttgart leans forward and stares intently. It was she who developed the cooling system for this spacecraft. If the measuring data around the heat-resistant tile with its gas shroud recorded a lower temperature than other areas, then the effectiveness of her cooling concept is confirmed. "The real work starts now with an evaluation of all the data," states Barth.
In this interview, DLR scientist Hannah Böhrk explains how active cooling functions on the SHEFEX II.