Drop Towers and Tubes
In a practical sense, microgravity can be achieved with a number
of technologies, each depending upon the act of free fall. Drop towers and
drop tubes are high-tech versions of the elevator analogy presented in a
previous section. The large version of these facilities is essentially a
hole in the ground.
| Drop towers accommodate large experiment packages,
generally using a drop shield to contain the package and isolate the
experiment from aerodynamic drag during free fall in the open environment.
NASA's Glenn Research Center in Cleveland,
Ohio has a 145-meter drop tower facility that begins on the surface
and descends into Earth like a mine shaft. The test section of the
facility is 6.1 meters in diameter and 132 meters deep. Beneath
the test section is a catch basin filled with polystyrene beads.
The 132-meter drop creates a microgravity environment for a period
of 5.2 seconds.
To begin a drop experiment, the experiment
apparatus is placed in either a cylindrical or rectangular test
vehicle that can carry experiment loads of up to 450 kilograms.
The vehicle is suspended from a cap that encloses the upper end
of the facility. Air is pumped out of the facility until a vacuum
of 10 -2 torr is achieved. (Atmospheric pressure is 760 torr.) By
doing so, the acceleration effects caused by aerodynamic drag on
the vehicle are reduced to less than 10 -5 g. During the drop, cameras
within the vehicle record the action and data is telemetered to
||A smaller facility for microgravity research
is located at the NASA Marshall Space Flight Center in Huntsville,
Alabama. It is a 100-meter-high, 25.4-centimeter-diameter evacuated
drop tube that can achieve microgravity for periods of as long as
4.5 seconds. The upper end of the tube is fitted with a stainless
steel bell jar. For solidification experiments, an electron bombardment
or an electromagnetic levitator furnace is mounted inside the jar
to melt the test samples. After the sample melts, drops are formed
and fall through the tube to a detachable catch fixture at the bottom
of the tube (Figure 2).
Additional drop facilities
of different sizes and for different purposes are located at the
NASA Field Centers and in other countries. A 490-meter-deep vertical
mine shaft in Japan has been converted to a drop facility that can
achieve a 10 -5 g environment for up to 11.7 seconds.
Airplanes can achieve low-gravity for periods of about
25 seconds or longer. The NASA Johnson Space Center in Houston, Texas operates
a KC-135 aircraft for astronaut training and conducting experiments. The
plane is a commercial-sized transport jet (Boeing 707) with most of its
passenger seats removed. The walls are padded for protection of the people
inside. Although airplanes cannot achieve microgravity conditions of as
high quality as those produced in drop towers and drop tubes (since engine
vibrations and air buffeting transmit throughout the vehicle), they do offer
an important advantage over drop facilities- experimenters can ride along
with their experiments.
|| A typical flight lasts 2 to 3 hours and carries experiments
and crewmembers to a beginning altitude about 7 km above sea level.
The plane climbs rapidly at a 45-degree angle (pull up), traces a
parabola (push-over), and then descends at a 45-degree angle (pull
out) (Figure 4). During the pull up and pull out segments, crew and
experiments experience between 2g and 2.5g. During the parabola, at
altitudes ranging from 7.3 to 10.4 kilometers, net acceleration drops
as low as 10 -3 g. On a typical flight, 40 parabolic trajectories
are flown. The gut-wrenching sensations have earned the aircraft the
nickname of "vomit comet." NASA also operates a Learjet for low-gravity
research out of the NASA Glenn Research Center. Flying on a trajectory
similar to the one followed by the KC-135, the Learjet provides a
low-acceleration environment of 5x10 -2 g to 75x10 -2
g for up to 20 seconds.|
| Small rockets provide a third technology
for creating microgravity. A sounding rocket follows a suborbital
trajectory and can produce several minutes of free fall. The period
of free fall exists during its coast, after burn out, and before entering
the atmosphere. Acceleration levels are usually at or below 10 -5
g. NASA has employed many different sounding rockets for microgravity
experiments. The most comprehensive series of launches used SPAR (Space
Processing Application Rocket) rockets for fluid physics, capillarity,
liquid diffusion, containerless processing, and electrolysis experiments
from 1975 to 1981. The SPAR could lift 300 kg payloads into freefall
parabolic trajectories lasting four to six minutes (Figures 5, 6).
Although airplanes, drop facilities, and small rockets can
be used to establish a microgravity environment, all of these laboratories
share a common problem. After a few seconds or minutes of lowg, Earth
gets in the way and the free fall stops. In spite of this limitation,
much can be learned about fluid dynamics and mixing, liquid-gas surface
interactions, and crystallization and macromolecular structure. But to
conduct longer term experiments (days, weeks, months, and years), it is
necessary to travel into space and orbit Earth. Having more time available
for experiments means that slower processes and more subtle effects can
To see how it is possible to establish microgravity conditions
for long periods of time, it is first necessary to understand what keeps
a spacecraft in orbit. Ask any group of students or adults what keeps
satellites and Space Shuttles in orbit and you will probably get a variety
of answers. Two common answers are: "The rocket engines keep firing to
hold it up." and "There is no gravity in space."
Although the first answer is theoretically possible, the
path followed by the spacecraft would technically not be an orbit. Other
than the altitude involved and the specific means of exerting an upward
force, there would be little difference between a spacecraft with its
engines constantly firing and an airplane flying around the world. In
the case of the satellite, it would just not be possible to provide it
with enough fuel to maintain its altitude for more than a few minutes.
The second answer is also wrong. In a previous section,
we discussed that Isaac Newton proved that the circular paths of the planets
through space was due to gravity's presence, not its absence.
Newton demonstrated how additional cannonballs would travel
farther from the mountain if the cannon were loaded with more black powder
each time it was fired. With each shot, the path would lengthen and soon,
the cannonballs would disappear over the horizon. Eventually, if a cannonball
were fired with enough energy, it would fall entirely around Earth and come
back to its starting point. The cannonball would begin to orbit Earth. Provided
no force other than gravity interfered with the cannonball's motion, it
would continue circling Earth in that orbit.
| Newton expanded on his conclusions about gravity and
hypothesized how an artificial satellite could be made to orbit Earth.
He envisioned a very tall mountain extending above Earth's atmosphere
so that friction with the air would not be a factor. He then imagined
a cannon at the top of that mountain firing cannonballs parallel to
the ground. As each cannonball was fired, it was acted upon by two
forces. One force, the explosion of the black powder, propelled the
cannonball straight outward. If no other force were to act on the
cannon ball, the shot would travel in a straight line and at a constant
velocity. But Newton knew that a second force would act on the cannonball:
the presence of gravity would cause the path of the cannonball to
bend into an arc ending at Earth's surface (Figure 7).
This is how the Space Shuttle stays in orbit. It is
launched in a trajectory that arcs above Earth so that the orbiter is
traveling at the right speed to keep it falling while maintaining a constant
altitude above the surface. For example, if the Shuttle climbs to a 320-kilometer-high
orbit, it must travel at a speed of about 27,740 kilometers per hour to
achieve a stable orbit. At that speed and altitude, the Shuttle's falling
path will be parallel to the curvature of Earth. Because the Space Shuttle
is freefalling around Earth and upper atmospheric friction is extremely
low, a microgravity environment is established.
Orbiting spacecraft provide ideal laboratories for microgravity
research. As on airplanes, scientists can fly with the experiments that
are on the spacecraft. Because the experiments are tended, they do not
have to be fully automatic in operation. A malfunction in an experiment
conducted with a drop tower or small rocket means a loss of data or complete
failure. In orbiting spacecraft, crewmembers can make repairs so that
there is little or no loss of data. They can also make on-orbit modifications
in experiments to gather more diverse data.
| Perhaps the greatest advantage of orbiting spacecraft
for microgravity research is the amount of time during which microgravity
conditions can be achieved. Experiments lasting for more than two
weeks are possible with the Space Shuttle. When the International
Space Station becomes operational, the time available for experiments
will stretch to months. The International Space Station will provide
a manned microgravity laboratory facility unrivaled by any on Earth