As the experiment is in freefall it will be uniformly accelerated so we can use the standard equations of motion:
But as the capsule is dropped the initial velocity will be zero. So:
In fact up to 9.48 seconds of microgravity are available for some experiments. How can tall would the tower need to be to achieve this in a ‘straight drop’ and how can it be achieved in such a short tower (look at the tower diagram for a hint)?
For a straight drop of 9.48 s:
This is four times the height of the actual tower. The near ten second drop can be achieved by launching the experiment capsule vertically at the beginning of the experiment using the catapult that is positioned beneath the main tower.
What restrictions might this method place on an experiment intended to make use of such an ‘extended drop’?
To use a catapult the experiment must survive an initial, high acceleration. Whilst any experiment will have to survive the deceleration of the polystyrene bead tank they only have to do that after the experiment has been completed. An initially high acceleration may well disrupt an experiment before it has a chance to run!
The catapult accelerates the capsule to 48 m s-1 in 0.28 s. The average acceleration can be calculated by:
This is of course an average acceleration but there is no guarantee that the acceleration is uniform. If it is not then the peak acceleration will be even higher. Either way this is a very high acceleration and so any experiment hoping to take advantage of it must be very rugged.
What is the velocity of the experiment capsule at the end of the microgravity period and what is the deceleration of the capsule when it enters the polystyrene pellets?
The drop itself is uniformly accelerated and so we can again use the basic equations of motion. For the velocity at the end of freefall:
Assuming uniform deceleration in the bead bed:
In practice the deceleration does not use the entire bead bed depth and so the true average deceleration is closer to 25 g. Also the actual deceleration (sample graph shown below) is not uniform and so the peak accelerations are around 50 g. In practice experimenters are advised to design their experiments to withstand decelerations of 100 g. Shock absorbers are not advised as at these accelerations they tend to make matters worse by storing some of the kinetic energy and then releasing it again producing even more accelerations. The best advice is to bolt everything down tightly so as not to allow any movement. Think for a moment, could you design a piece of precision experimental equipment that could withstand accelerations of 100 g?
The ‘drop tube’ which is the vacuum chamber that the experiments take place in is not attached to the concrete tower that surrounds it. Why should this be?
Have you ever stood on top of a tall tower on a windy day? If it is tall enough and not surrounded by skyscrapers then the top of the tower will rock back and forth in a rather disturbing way. The ZARM tower is tall (146 m) very thin and the only tall building for some considerable distance. If the drop tube itself were to flex like this then the experiment capsule could end up bouncing off the sides and so drops could only take place on calm days. Also the flexing could lead to damage to the tube even if a an experiment were not taking place. The free standing arrangement means that the outer tower protects the inner drop tube from the elements and experiments can continue regardless of the weather.