Contributed by Forrest E. Lumpkin
Aerospace Engineer, Aeroscience & Flight Mechanics Division, NASA Johnson Space Center
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.
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 related to engine firings on-orbit are safe for the Shuttle orbiters and 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.
This plot 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) – accurately simulating flows where Navier-Stokes equations are no longer an accurate governing equation set. But, DSMC has huge computational requirements in higher density flow regimes.
DSMC requires cell sizes to be about the size 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 nozzle exit (where DSMC cannot feasibly simulate that part of the computational volume). The plume flow field then expands kilometers from the nozzle into a near-vacuum of low atmospheric pressure.
The strategy to attack this complex problem is to use CFD near the nozzle and DSMC away from the nozzle. 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.