Scientific and Technical Aerospace Reports Volume 38 July 28, 2000

Pressure gradients, i.e. pressure head, and buoyancy driven convection can be greatly decreased in experimental systems by
acceleration of the laboratory reference frame at a rate consistent with the acceleration due to gravity. This may be done in a number
of ways, the best known of which is the use of orbiting spacecraft. Other techniques include the use of aircraft following an
appropriate parabolic trajectory or drop towers. The result is an experimental condition in which fluids experience virtually no
outside forces relative to the laboratory reference frame. Such conditions are appropriate for the study of processes with diffusion
dominated heat and/or mass Under, the study of phase transitions in the absence of pressure gradients, the study of solutal or thermal
capillary convection, or containerless processes. Ways of achieving these conditions and complexities that arise in performing
experiments in this environment are discussed.
Author
Microgravity; Acceleration; Pressure Gradients; Buoyancy
20000067655 NASA Marshall Space Flight Center, Huntsville, AL USA
A Technique for Rapidly Deploying a Concentration Gradient with Applications to Microgravity
Leslie, Fred, NASA Marshall Space Flight Center, USA; Ramachandran, Narayanan, Universities Space Research Association,
USA; [2000]; 1p; In English; No Copyright; Avail: Issuing Activity; Abstract Only
The latter half of the last century has seen rapid advancements in semiconductor crystal growth powered by the demand for
high performance electronics in myriad applications. The reduced gravity environment of space has also been used for crystal
growth tests, especially in instances where terrestrial growth has largely been unsuccessful. While reduced gravity crystal growth
affords some control of the gravity parameter, many crystals grown in space, to date, have structural flaws believed to result from
convective motions during the growth phase. The character of these instabilities is not well understood but is associated with thermal
and solutal density variations near the solidification interface in the presence of residual gravity and g-jitter. In order to study
these instabilities in a separate, controlled space experiment, a concentration gradient would first have to be artificially established
in a timely manner as an initial condition. This is generally difficult to accomplish in a microgravity environment because the
momentum of the fluid injected into a test cell tends to swirl around and mix in the absence of a restoring force. The use of magnetic
fields to control the motion and position of liquids has received growing interest in recent times. The possibility of using the force
exerted by a non-uniform magnetic field on a ferrofluid to not only achieve fluid manipulation but also to actively control fluid
motion makes it an attractive candidate for space applications. This paper describes a technique for quickly establishing a linear
or exponential fluid concentration gradient using a magnetic field in place of gravity to stabilize the deployment. Also discussed
is a photometric technique for measuring the concentration profile using light attenuation. Results of the ground-based experiments
indicate that the concentration distribution is within 3% of the predicted value. Although any range of concentations can
be realized, photometric constraints are discussed which impose some limitations on measurements.
Author
Deployment; Gradients; Microgravity; Nonuniform Magnetic Fields; Concentration (Composition)
20000067686 Alabama Univ., MAE Dept., Huntsville, AL USA
Frequency-Weighting Filter Selection, for H2 Control of Microgravity Isolation Systems: A Consideration of the ”Implicit
Frequency Weighting” Problem
Hampton, Roy David, Alabama Univ., USA; Whorton, Mark S., NASA Marshall Space Flight Center, USA; [1999]; 18p; In
English
Contract(s)/Grant(s): NAG8-1598; RTOP 398-96-02
Report No.(s): IMTC-9184; No Copyright; Avail: CASI; A03, Hardcopy; A01, Microfiche
Many space-science experiments need an active isolation system to provide them with the requisite microgravity environment.
The isolation systems planned for use with the International Space Station (ISS) have been appropriately modeled using
relative position, relative velocity, and acceleration states. In theory, frequency-weighting design filters can be applied to these
state-space models, in order to develop optimal H2 or mixed-norm controllers with desired stability and performance characteristics.
In practice, however, since there is a kinematic relationship among the various states, any frequency weighting applied to
one state will implicitly weight other states. These implicit frequency-weighting effects must be considered, for intelligent frequency-weighting
filter assignment. This paper suggests a rational approach to the assignment of frequency-weighting design
filters, in the presence of the kinematic coupling among states that exists in the microgravity vibration isolation problem.
Author
Hydrogen; Microgravity; Vibration Isolators; Bandpass Filters; Frequencies; Spaceborne Experiments; Weighting Functions;
H-2 Control
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