Weightlessness in free fall

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The 146m high ZARM drop tower in Bremen. Credit: DLR (CC-BY 3.0)
The 146m high ZARM drop tower in Bremen. Credit: DLR (CC-BY 3.0)

In Great Britain, drop towers were being used industrially by the 18th century. William Watt had thought of a new way of making good quality lead shot: He poured molten lead through a very fine sieve at the top of a tower. While the lead was falling, it became spherical and dried before it hit the ground.

Nowadays drop towers are used mostly for research. They are a relatively inexpensive way of carrying out experiments in weightlessness. During a free fall through high vacuum, the residual acceleration drops below a millionth of the Earth’s acceleration. The residual acceleration on the International Space Station (ISS), by comparison, can be up to one hundredth of the Earth’s acceleration.

One disadvantage of this method is the relatively short time available for experiments. The 146m high ZARM drop tower in Bremen delivers 4.75sec of weightlessness. This time can be doubled by catapulting the experiment capsule upwards, thereby setting it on a trajectory with 0 degrees horizontal deflection. There are other drop towers in the US or Japan that achieve only 1 or 2 sec of weightlessness. The 1000m long drop shaft “JAMIC” in Japan, which achieved 10 sec of weightlessness, has closed down for the time being.

What can we measure in this short space of time?

A lot of physical or chemical processes can be analysed in 3 to 9 seconds. Research in Bremen is focussed primarily on currents and combustion processes. But biological experiments can be carried out as well: Reactions in membranes take place within milliseconds and 4 seconds are even enough for the analysis of the swimming behaviour of paramecia (”slipper animals”). In fact, analysing biological systems in a drop tower has the advantage that the transition from 1g to 0g is abrupt. This means we avoid the aftereffects of increased acceleration, which we see e.g. in rocket or parabolic flights.

Drop towers are also used to test the practicability of apparatus or research questions that are being considered for long-term and therefore much more expensive campaigns. Many scientists use drop towers as a “stepping stone” to the ISS.