Technology : What goes up stays up a little longer

2019-02-27 05:19:07

By Jonathan Beard WHAT comes down can now first go up—to the joy of researchers in Bremen who can now explore the benefits of doing experiments in microgravity for twice as long as they could before. The drop tower at ZARM, the German Centre for Applied Space Technology and Microgravity, has installed a catapult that tosses experimental capsules to the top of the 145.5-metre tower, so that tests can be carried out under weightless conditions for the 10-second round trip, rather than just on the way down. “Most people are familiar with research carried out under weightlessness in the space shuttle, but drop towers provide a much cheaper place to do experiments in combustion, metallurgy and biology,” says Hansjoerg Dittus, a physicist at ZARM. The major drawback of such towers is time. While seedlings sprouting in orbit can be observed for hours or days, the Bremen tower provided just under 5 seconds, the time it took the 300-kilogramme capsule to fall 110 metres before coming to rest in a pit of polystyrene granules. The new catapult doubles this time. “The catapult accelerates over a distance of 7 metres and releases the capsule at a velocity of 46 metres per second. It ascends to the very top of the drop tube, then free-falls down to the braking bed,” explains Dittus. The vibrations created by the catapult are damped after about 100 milliseconds, after which the experiment takes place in an environment almost entirely free of vibration, with gravity a million times less than that normally experienced on Earth. A typical combustion experiment includes a combustion chamber, measuring instruments and computers to log the results, with everything packed into an aluminium cylinder 2 metres tall and 80 centimetres in diameter. “Normally, combustion is greatly affected by gravity-induced convection,” says Dittus. “But under microgravity, flames are spherical and the combustion process is much simpler.” The exaggerated G forces of the launch and landing have not proved to be a problem. “We have found that anything that can be shipped to Bremen by mail will survive the drop tower,” says Dittus. The catapult was planned when ZARM opened in 1990, but it became more urgent when Japan opened deeper microgravity facilities in former mine shafts. “The Japanese have a drop of 500 metres, but their capsules are going so fast that they need 200 metres braking distance,