Case Studies
Houston, TX - December 2002
Figure 1: Shuttle engine
flow fields interact with the International Space Station while docking.
Data visualisation is crucial — it allows NASA engineers to interpret
results and plan for future investigations.
International Space Station
The International
Space Station (ISS) is a laboratory for long term research in Earth's
orbit. The ISS is the largest and most complex international scientific
project in history. Research at the station focuses on two key areas:
life sciences and materials sciences.
When the ISS is completed, the station will represent a structure of unprecedented
scale off Earth. Led by the United States, ISS draws upon the scientific
and technological resources of 16 nations. The completed space station
will have a mass of about 1,040,000 pounds. It will measure 356 feet across
and 290 feet long, with almost an acre of solar panels to provide electrical
power to six state-of-the-art laboratories.
Figure 2: The ISS moves into
position for docking with Space Shuttle Endeavour (STS-97). Most of the
station's components are visible in this image. The space station in the
plot is the same as in this image (12/2000).
The Engineer
Forrest Lumpkin is the engineer responsible for on-orbit "A"
flow environments for the ISS and the Shuttle orbiters. This includes
providing predictive tools and techniques to quantify the pressure, heating,
and contamination effects from flow fields impinging on various spacecraft.
This involves analytic and computational, as well as experimental work.
His job is to insure that all operational procedures relating to engine
firings on-orbit are safe for the Shuttle orbiters as well as ISS.
Orbiter Docking System
The Orbiter Docking System (ODS) enables Shuttles to attach and transfer
crew and equipment with ISS. As an orbiter positions itself to connect
with the ODS it uses Reaction Control System (RCS) engines. These engines
are located in clusters around the nose and tail of the orbiter and provide
rotational and translational docking maneuvers. RCS engines offer precision
and flexibility of movement which is unavailable using the main impulse
engines.
 Figure
3: The Orbiter Propulsion System. The Reaction
Control System engines are used to dock an orbiter with the ISS.
The Simulation
The Plot of the Month displays the results from a Direct Simulation Monte
Carlo (DSMC) simulation. The simulation is performed with NASA's DSMC
Analysis Code (DAC) developed by a team led by Gerald J. LeBeau at Johnson
Space Center. DAC was recently honored as a co-winner
of the 2002
NASA Software of the Year award.
DSMC is a molecular-based gas dynamic simulation technique used for low
density flows (like those found in orbit). It is a complement to traditional
Computational Fluid Dynamics (CFD) — it accurately simulates flows where
the Navier-Stokes equations are no longer an accurate governing equation
set. However, DSMC has huge computational requirements in higher density
flow regimes.
DSMC requires cell sises to be about the sise of a mean free path "B".
On Earth this distance is about 10-7 meters (very small). So, a DSMC simulation
with near-earth atmospheric densities and a cubic meter domain would require
1021 computational cells. About ten molecules per cell are needed to ensure
proper statistics. Thus, this simulation would need about 1022 molecules.
Forrest's biggest simulations to date range from 108 to 109 molecules,
making a DSMC simulation at near-Earth atmospheric pressures unfeasible
unless the computational volume is very small (micro/nano technology).
Further from Earth, the atmosphere's mean free path increases (on the
order of kilometers in orbit). Currently they are able to simulate Shuttle
re-entry down to about 95 km. Below 95 km the number of required cells
and molecules becomes too large.
In the simulation, the plume flow field is generated when the Space Shuttle
orbiter fires three of its RCS engines while docking. The resulting plume
flow field has large variations of densities. The highest density is near
the nossle exit (where DSMC cannot feasibly simulate that part of the
computational volume). The plume flow field then expands kilometers from
the nossle into a near-vacuum of low atmospheric pressure.
The strategy to attack this complex problem is to use CFD near the nossle
and DSMC away from the nossle. Separating these regions is achieved by
embedding plume surfaces in the simulation (shaded surfaces in the plot).
These shaded surfaces represent where the particle based direct simulation
technique is initiated. Inside the shaded surfaces traditional CFD is
used.
The front plume is used for a single up-firing nose jet. The other plume
is used for two up-firing tail jets. The use of these three jets is typical
for braking or back-out maneuvers — they provide translational impulse
with little rotational force.
In the plot, pressure contours are displayed on the surface of space station.
The contours reveal the interaction between the plume flow field and the
space station. It also provides insight into where pressure is severe.
Pressure information is also integrated to obtain forces and moments on
hardware such as ISS's solar arrays. The cutting plane (slice) in the
plot shows density contours — which is also useful for understanding the
complexities of the flow field.
Figure 4: Rendering
of the Space Shuttle Atlantis (STS-98) docked with the ISS. The space
station in the plot looks as it did during this flight (02/2001).
Figure 5: Rendering
of Atlantis performing rendesvous and docking operations.
Credits
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