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How to Make Microgravity

Every time you drop something or knock an object off of a table then you produce a short period of microgravity. It is certain that you have experienced short periods of reduced gravity or even ‘full microgravity’ yourself. Producing 'research grade' microgravity is a bit more involved though.

Personal Microgravity Experiences

All you need to generate a little personal microgravity is to fall (preferably not too far). One of the easiest ways is in a lift.

 Hookes Law

If the lift is stationary or moving at a steady rate (situation 1) then the passenger will record their weight as exactly what it is on solid ground. In fact, so long as there are no windows, the passenger will not be able to tell that they are not on the ground.

If the acceleration of the lift is downwards (situation 2: either the start of a downwards trip or the end of an upwards trip) then the reaction between the passenger and the floor of the lift is reduced. As a result the passenger feels a little lighter but the effect is quite small.

If on the other hand the acceleration of the lift is upwards (situation 3: either the start of an upwards trip or the end of a downwards trip) then the reaction between the floor and the passenger is increased and the passenger feels heavier. The local effects of gravity have been increased.

If we disable all the safety devices and cut the cable situation 4) then the lift and everything in it will be in freefall. So long as air resistance does not become important as the lift falls then the passenger will experience full microgravity until the lift hits the ground (situation 5).

Another, slightly safer way of experiencing personal microgravity is to use a vertical drop slide, the like of which can be found in ‘more grown up’ soft play areas. These consist of a ledge that you can sit on (and look straight down), a high vertical wall that you fall past as you drop and a curved section at the bottom that gradually deflects your vertical fall into a safe horizontal velocity.

Another way of achieving personal microgravity might seem to be to use a freefall parachute jump from a plane or a balloon. A typical freefall jump is made from around 3800 m and the parachute is opened at an altitude of 750 m. The skydiver might experience around 60 s of ‘freefall’. If g is taken as 9.8 m s-1 comment on whether this is truly free fall. The sky diver will be traveling at around 185 km/h when they open their parachute. From a simple estimate calculate the maximum period of microgravity for the sky diver. What is the figure more likely to be?

Hookes LawDrop Towers

Whilst dropping through air is not a good way to produce microgravity dropping through a vacuum is. One of the simplest ways of achieving ‘research standard’ microgravity is to use a drop tower. Within Europe the largest of these is at ZARM (Zentrum für angewandte Raumfahrttechnologie und Mikrogravitation or The Centre for Applied Space Technology and Microgravity) in Bremen University, Germany. The tower is 146 m tall and encloses a totally free standing vacuum chamber, or ‘drop tube’ that has room for a 120 m drop. Experiments are mounted in cylindrical containers that are winched to the top of the drop tube. The air is pumped out of the tube until the pressure is just 10 Pa (the equivalent of an altitude of around 70 km) and when the experiment is ready to run the container is dropped. At the bottom of the tube the container falls into a ‘deceleration unit’ consisting of a 6 m deep container of polystyrene pellets. You can see a larger, higher resolution image of the ZARM drop tower by clicking on the small image on the right.

The drop tower is extremely useful to researchers because:

These factors make the drop tower an ideal platform for trying out new ideas in microgravity research or trialing experiments before moving on to longer term microgravity platforms. The disadvantages of drop towers are the relatively short periods of microgravity that can be generated, the size of experiment that can be accommodated (a cylinder 600 mm diameter and 1.7 m tall) and the deceleration at the end of the drop. Despite these restrictions a drop tower is the only platform that some experiments ever require.

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)? What restrictions might this method place on an experiment intended to make use of such an ‘extended drop’?

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 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?

Parabolic Flights

If you are flying you are always told by the cabin staff to leave your seat belt fastened ‘for your comfort and safety’. Certainly if the aircraft hits a patch of turbulence the sudden accelerations can leave you wishing that you hadn’t eaten the in flight food. This however is nothing compared to the ride experienced by researchers on parabolic flight aircraft (or ‘vomit comets’ as they are known to the press).

ESA’s parabolic flight platform is a specially modified Airbus A300. With up to 40 researchers and their experiments on board the ‘A300 Zero G’ takes off from Bordeaux-Mérignac airport and flies out over the Bay of Biscay. When there the pilots proceed to fly a number of parabolic hops. The parabolas mimic the path that a projectile would in the absence of air resistance. The result is that everything within the cabin; researchers, equipment and pilots experience weightlessness.

 Hookes Law

A parabolic manoeuvre starts at 6000 m. The pilot eases the nose of the aircraft up and opens the throttles to the engines. Almost immediately your apparent weight has increased by 80 % and you are feeling like lead on the floor, this is called ‘hypergravity’. Just twenty seconds later and you have climbed one and a half kilometres.

parabolic flightThe aircraft is currently nose up at 47°. The pilot throttles back to just enough thrust to overcome air resistance and your weight drops away. For the next twenty seconds you feel weightless and if you were to let go of the hand and foot holds you would drift around the cabin. There’s not time to do this though as in those few moments you have a lot of work to do (and nobody likes you floating into their experiment). In these twenty seconds you have climbed another kilometre and lost it again. The aircraft is now pointing 47° down (curiously nobody ever publishes this picture, it’s always the 47° nose up one that is released to the press). A voice calls a warning and then everything changes.

The pilot begins to pull back on the stick, raising the nose of the aircraft for the next twenty seconds you are back to hypergravity. Some macho individuals might even be seen doing press ups. You however are lying flat on the ground trying not to feel totally disoriented. The pressure eases off and you are back to flying straight and level at an altitude of 6 km. You have just flown your first parabola and you are probably feeling either elated or very sick. If you didn’t like the experience then the bad news is the there will be another thirty before you land (and then next one is in two minute’s time). In flight drinks and nibbles are not served but motion sickness medication is available to all that request it (preferably before takeoff). Just to make you feel a little more comfortable you should also know that the A300 Zero G is the largest plane in the world flying parabolas. It has never been rated as a ‘passenger aircraft’, every flight is considered a test flight and each of which carries five qualified test pilots, three of whom are needed to fly a parabola, each one controlling a separate axis of the aircraft, pitch, roll and yaw.

The quality of microgravity obtained on parabolic flights is not as good as in drop towers, being around only 10-2 g. However:

Safety precautions are however far more stringent as the lives of the researchers and the crew could be put at risk by a poorly designed experiment.

Is this data consistent with a parabola flown with the horizontal axis of the aircraft parallel to the velocity vector?

Assume that the velocity figures are correct. Calculate the maximum increase in altitude and the total length of the ground track over the period of one parabola (excluding the two hypergravity stages).

The aim of the initial hypergravity stage is to gain sufficient velocity and altitude so as to produce a lengthy period of microgravity. Why do the pilots not fly at a higher altitude and rely only on the downside edge of the parabola? This would remove the need for the first hypergravity stage. Alternately why do the pilots not increase the initial airspeed (810 km/h) so as to gain more vertical velocity and so fly longer parabolas?

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