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Frequently Asked Questions

What is it for?

The drop tower is a research device used to simulate the weightlessness of outer space. We say that during a test, the experiment experiences 0 g ("zero g").

Why do you need to drop things?

The absence of gravity changes the way some materials, especially fluids, behave. Scientifically, it is interesting to be able to simulate the conditions in space and use the brief "free fall" period to increase our understanding of physics.

As a practical (i.e. engineering) matter, understanding how fluids behave in space is very important for fuel and life support systems on satellites, space stations, etc.

If an object is falling, isn't gravity pulling it down?
How can that be zero g?

During a drop, the payload falls freely to the ground. The pull of gravity is not opposed by any forces (like the force on the bottom of your feet when you stand on the ground). Therefore, while an object it is falling freely, the absence of any forces to oppose the effect of gravity allow objects to experience no net effect due to the gravity of earth.

So "zero g" does not mean that gravity has been eliminated. Rather, by removing the reaction forces from the surroundings, dropping an object allows the effect of gravity to be neutralized. When the payload is sitting motionless on a table (say, before it loaded into the drop tower), the pull of gravity is opposed by the table, and that introduces forces throughout the object. Removing the table -- by allowing the payload to drop -- removes the reaction force, and therefore removes any response of the system to its own weight. 

What is the drag shield, and why do you need it?

The drag shield is a fairing -- an aerodynamic wrapper -- that surrounds the payload. The drag shield is not connected to the payload. During a drop the payload and drag shield fall independently, with the payload completely enclosing the payload.

During a drop, the drag shield experiences the resistance of the air -- the air drag. Because it is inside the drag shield where there is essentially no air motion, the payload is falling slightly faster than the drag shield. In other words, because the drag shield experiences air friction, it is not completely free-falling, and therefore is not a good model of weightlessness.

Although the drag shield is moving slower because of air friction, the payload never catches up, because the drag shield is given a carefully designed "head start"

So, the purpose of the drag shield is to remove air friction, i.e. "drag" as a force on the payload. The drag shield eliminates the effect of drag on the payload, which allows the payload to experience an extremely low (net) effect of gravity.

How do you stop the payload at the bottom?

The payload (and drag shield) are stopped with the electromagnetic force produced by powerful magnets near the bottom of the drop tower.

Aluminum fins protrude from the side of the drag shield. At the bottom of the tower, very powerful, C-shaped magnets are arranged in a vertical channel just wide enough for the fins on the drag shield. As it drops, the drag shield is guided by cables that keep the fins aligned with a narrow gap in the magnets in the braking section.

When the electrically conducting fins move through the powerful magnetic field, an electrical field is induced in the fins. That field creates currents (eddy currents) that induce their own magnetic field. The eddy current exists in response to the field, but it is resisted by the magnetic field that it creates. That resistance creates a reaction force, which slows the drag shield.

The magnets were chosen to create the eddy current and its reaction force to slow the drag shield to a near stop. No external power is needed. After it passes, and is slowed by, the magnetic brake, the drag shield (and the payload inside it) harmlessly plop onto a small pad at the bottom of the tower.

Eddy current brakes are used on amusement park rides and some trains.

In an electric or hybrid electric vehicle with regenerative braking, the eddy currents are used to store electricity in the battery. Thus, electrical power is consumed when the vehicle is moving forward (overcoming friction or accelerating), and some of the kinetic energy is recovered during braking.

What are you measuring during a typical experiment?

The primary focus of research in the DDT is studying the way that fluids react to the combination of zero gravity and complex container shapes. The experiments measure the shape of the interface between liquids and gases and how geometric features of the container control the shape of the interface.

The measurements allow engineers to test mathematical models of the way that liquids conform to the shapes of contains in both static and dynamic conditions. Ultimately the mathematical models are used to design piping systems, storage tanks, and other fluid-handling equipment for use in space.

Could I ride on a drop?

No. The payload is not designed to be safe for humans. The deceleration during braking at the bottom is severe and would be very distressful, if not harmful, to a human riding in the payload.

What does it sound like?

Expect to hear a whooshing sound, similar to an electric car going by at 55 MPH.

Are there other drop towers like this?

Yes and no. We use an approach similar to that used by NASA, but ours is in Portland and is highly accessible to both users and onlookers. There are significantly more complex towers with up to 10 second free fall times in China, Germany, Japan, and the USA.

This drop tower does have several unique features, in addition to being the only research level drop tower in a public space. The two most notable innovations are the braking system and the release mechanism is unique.

Who paid for this?

The funding for the drop tower came from private individuals and corporations. In addition to cash donations, several companies made in-kind contributions of materials and labor.

What do I need to study to do this kind of research?

Most of the research is done by engineers and applied mathematicians. If you are thinking about going to college, look at programs in mechanical engineering or aerospace engineering. If you are already in college (say in an engineering program), pay especial attention to your courses in fluid mechanics, differential equations, and experimental methods.

How long have you been studying fluids in microgravity?

The lead researcher, Dr. Mark Weislogel, has been working for or with NASA for twenty years. Members of the research team include senior scientists with PhDs and years of experience, graduate students working on Masters Degrees and PhDs, and undergraduates in engineering (from freshmen to senior year students).

What would happen if I dropped something solid, like a brick, or a baseball? Could I measure a change in the behavior of those materials?

During the drop, the solid object would experience no reaction force against the bottom of the payload container. If a brick had feelings, it would feel like it is floating.

However, the molecules of a solid object stay in a fixed position relative to each other -- that is what makes a solid solid. (We're ignoring lattice vibrations responsible for thermal energy storage any sound and compression waves traveling through the solid.)

The drop tower is designed to study the motion of liquid when the effect of gravity is nullified during free fall. Unlike a solid, the molecules of a liquid can (relatively easily) move relative to each other. During free fall, the reaction force due to the fluid weight is removed, and other forces, notably the effect of surface tension, act on a fluid and cause it to reconfigure. That response -- the reconfiguration of matter due to the neutralization of the net effect of gravity -- is much weaker and not as interesting in solids.