Boom Time for Cluster II Spacecraft.
6 August 1999
We are all familiar with normal gravity which holds us to the ground andpulls us backto Earth when we try to jump or fly. However, as images of space missionsshow,people and objects become 'weightless' once they enter orbit. Over theyears,spacecraft engineers have been obliged to develop various ways of copyingsuchunusual conditions.Astronauts use highly complex, expensive equipment, involving water tanks and parabolic flights in large aircraft, to simulate zero gravity. For less well funded unmanned missions, engineers are often obliged to resort to more ingenious, cheaper, techniques. Such is the case for the second Cluster II spacecraft, (FM7), which has just completed three days of boom tests at the facilities of IABG near Munich.
Studying magnetic and electric fields in space is not easy. In order to obtain accurate measurements, the scientific instruments have to be placed as far away from the body of the satellite as possible. In the case of the Cluster II spacecraft, this means that they are located at the end of two 5-metre-long booms. One boom carries the Flux Gate Magnetometer (FGM) experiment, while the other carries a second magnetometer experiment known as STAFF.
Unfortunately, booms of this length will not fit inside a rocket fairing, so they have to be folded during launch and designed to open out once the spacecraft reaches orbit. But even then, the engineers' worries may not be dispelled. What if the booms get stuck or damaged instead of deploying properly?
The obvious answer is to test them. But reproducing the weightless conditions of outer space is far from easy. Using an idea developed by Dornier Satellitensysteme of Friedrichshafen, Germany, the Cluster II team have developed their own innovative technique for testing boom deployment in a 'weightless' environment. All they need are two balloons filled with helium gas and a toy aeroplane motor.
Each boom on a Cluster spacecraft is divided into two sections joined by hinges. The effects of gravity on the boom are overcome by tying strings between the three-metre-diameter rubber balloons and the two end sections of the folded boom.
The next problem to solve is how to deploy the folded booms. In space this would be done by firing small explosive charges to open the clamps holding them in place, then rely on centrifugal forces created by the spinning spacecraft to do the rest.
On the ground, in a clean integration room, another method is needed. The ingenious solution is to use a small propeller, about 15 cm in length, which is driven by a model aircraft engine. Although a petrol engine may be used, an even simpler version powered by a small electric motor is quite adequate.
The tiny motor, which is attached to the boom's central section between the two balloons, is operated for less than a minute, but this is long enough to provide the momentum to open the lightweight boom. Once the procedure is completed and the clearances are checked, the entire process is repeated for the second boom.
"It's a perfect, low-cost, low-tech answer to a difficult engineering problem", commented Cluster II Assembly, Integration and Verification Manager, Hans Bachmann.
