2 Using drop towers to simulate microgravity

When objects are dropped from the top of drop towers, they achieve freefall as they drop. This briefly creates a microgravity environment (Figure 4).

Figure 4 The 140-metre drop tower in Bremen, Germany.

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This is a coloured photograph of the Bremen drop tower.

Figure 4 The 140-metre drop tower in Bremen, Germany.

Listen to Audio 1 which describes drop towers and rollercoasters. A transcript is also provided if you would prefer to read it.

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Transcript: Audio 1 drop towers and rollercoasters

WOMAN:

We have all experienced that momentary feeling of lightness when an elevator begins its downward motion. It is almost as if our weight had suddenly been reduced or, conceivably, that the pull of the Earth’s gravity had decreased for a moment. But imagine what it would be like if the lift cable had suddenly snapped and the lift, with you in it, had plummeted downward. Apart from stark terror, what else do you think you would experience during your fall? What would the physical experience of such a disaster be like?

Well, it would be just like jumping from a high tower. If your descent was unimpeded by the resistance of air, almost all sense of weight would vanish while you were falling. You would feel weightless, just as though you were an astronaut in outer space.

Not surprisingly, scientists who want to know how equipment will behave under the conditions found in spacecraft are keen to simulate the same conditions here on Earth. One way in which they can do this is by dropping their equipment from the top of a tower, or down a vertical shaft. There are a number of such research centres around the world where drop facilities of this kind are available. These are specialised facilities where steps are taken to avoid or overcome the effects of air resistance: simply dropping an object in the Earth’s atmosphere is not a satisfactory way of simulating the environment of outer space.

The figure above [Figure 4 here] shows the 140-metre drop tower in Bremen, Germany. The tower is airtight, so all air can be pumped out. Equipment under test is placed inside a specially constructed test vehicle and monitored by closed-circuit TV as it is catapulted up to the top and down again to the bottom of the tower. About 9.3 seconds of freefall can be achieved. During those few seconds, within the falling test vehicle, the effects of gravity are reduced to a tiny fraction of their usual value, a condition known as ‘microgravity’.

In the USA, at the John H. Glenn Research Center at Lewis Fields, NASA operates a 143-metre drop-shaft, as part of its Zero Gravity Research Facility. Microgravity investigations conducted at the research facility have concerned the spread of fire, the flow of fluids and the feasibility of space-based industrial processes that would be impossible under normal terrestrial conditions.

Another even longer drop-shaft is found in Japan. The Japan Microgravity Centre (JAMIC) has a 700-metre drop housed in a disused mine shaft. It would be impossible to evacuate the air from such a big shaft, so in this case the rocket-shaped test capsule is propelled down the shaft by gas-jets with a thrust that is designed to compensate for air resistance. Inside this capsule, there is a second capsule and the space between the capsules is a vacuum. The experiments are carried out in the inner capsule which, to a very good approximation, is in freefall. The two capsules decelerate during the final 200 metres of the fall.

By the time you finish this week you should be able to work out the duration of the fall in the JAMIC facility, and the highest speed attained by the capsule. You should also be able to work out the length of shaft that would be required to produce any given duration of microgravity.

If all this sounds a bit esoteric you might prefer to consider a different kind of drop-facility. The ‘Oblivion’ ride at the Alton Towers Adventure Park, UK, is described as ‘the world’s first vertical-drop roller-coaster’. It will not simulate the space environment, but it will produce a few seconds of terror from a simple application of linear motion.

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Audio 1 drop towers and rollercoasters

According to this New Scientist article [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] (Cross, 1990), up to 10 seconds of a microgravity environment can be replicated in the Japan Microgravity Centre (JAMIC) (Figure 5).

Figure 5 The Japan Microgravity Centre or JAMIC (Cross, 1990).

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This is a black and white drawing of the JAMIC drop tower where its total depth is shown as 720 m with a freefall zone of 500 m, braking zone of 200 m and an emergency braking zone of 20 m. The capsule has guide rails through this drop tower and is slowed down by an air damper and shock absorbing material.

Figure 5 The Japan Microgravity Centre or JAMIC (Cross, 1990).

To test your understanding of freefall and drop towers you should now complete Activity 2.

Next, you will carry out your own experiment to calculate the acceleration of gravity on Earth.