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The Challenge of "Simulation"
Parachute Design Support for Manned Spacecraft

Kenji Takizawa
Associate Professor, Waseda Institute for Advanced Study

Space exploration and parachutes

To me, space meant NASA and the space shuttle. But last year, finally, the space shuttle was retired. Therefore, the only way to return from space now is by using parachutes.

Parachutes and fluid dynamics

I have never actually experienced using a parachute because I am afraid of heights. The parachutes that are known to a person like me are the fireworks parachutes that came in the sets of fireworks of my childhood days, which measured about 10 centimeters in diameter. I remember trying, but not quite managing, to catch the parachute as it fell to the earth, slowly swaying. The movement of the parachute and the behavior of the air around it actually depend on its size. The parachutes used in space exploration are so large that they cannot be compared with the fireworks parachutes. The parachutes to be used with the NASA Orion spacecraft, for example, measure about 80 ft in diameter. In other words, they are as large as a 25-meter swimming pool. One can surely imagine that it is quite a challenge just to open the parachute.

Numerical simulation of parachutes

The Orion spacecraft parachutes, which the Team for Advanced Flow Simulation and Modeling (T*AFSM) is currently working on, are ringsail parachutes. As shown in Figure 1, a ringsail parachute is constructed from rings and sails, resulting in a parachute canopy with hundreds of ring gaps and sail slits. The air that escapes through the gaps and slits stabilizes the parachute. If the gaps and slits are larger, however, the descent speed for the parachute increases. This would mean that to maintain a desired descent speed, a larger parachute would be needed. Parachutes are designed to meet the requirements of certain objectives, such as increased stability and decreased descent speed.

Figure 1 A slice of a ringsail parachute: upper portion (left) and lower portion (right)

To conduct a parachute drop test, we must first build a parachute and then take it up to a high altitude in the sky to drop it - this needs a very large-scale test. As you can imagine, the cost is high in terms of both money and time. To support these kinds of tests, we conduct numerical simulations.

There are a number of difficulties in numerical simulations of parachutes. The aerodynamics of the parachute depends on the canopy shape and the deformation of the canopy depends on the aerodynamics forces, and the two systems need to be solved in a coupled fashion. Such computations are called fluid-structure interaction (FSI) simulations. Professor Tayfun Tezduyar (Rice University, Houston) pioneered the modern computer modeling of parachutes. His team started 3D FSI modeling of parachutes in 1999 and spacecraft parachutes for NASA in 2006. The team from Waseda University is now an equal partner in the spacecraft parachute research and is playing a key role.

The numerical simulations that we conducted were first compared with the NASA drop tests under the same conditions. With these results, we were able to confirm that the computations matched well the results from the tests. With that, next we conducted numerical simulations of parachutes with different designs. For example, we compared parachutes with different canopy designs, like the parachutes shown in Figure 2. In addition, as shown in Figure 3, we made changes in the lengths of the cables known as suspension lines, which connect the canopy to the riser that connect to the spacecraft, and we performed computations. These changes had a significant impact on the dynamics of the parachutes. There were various results, including increased descent speed and gliding speed. We are repeating simulations such as these and refining the parachute designs to meet the objectives. The design choices will be confirmed with drop tests in the end, but numerical simulations help reduce the number of tests, and as a result, we can keep the cost down and shorten the development period as well.

Figure 2 Parachutes with different canopy designs that we computed

Figure 3 Parachutes with different suspension-line lengths that we computed

Spacecraft parachutes are typically used in clusters of two or three parachutes. This is because it becomes too difficult to open the parachutes as they become too large. That is why, for a heavy spacecraft, as shown in Figure 4, three parachutes are used together and the contact between the parachutes is a major computational challenge specific to FSI modeling of parachute clusters.

Figure 4 Computation of cluster parachutes that contact each other


In these ways, we are taking on the challenges of complex simulations every day, and the technologies we have developed are being used in various types of other simulations. For example, simulation of the FSI between blood flow and arterial dynamics can be accomplished with essentially the same technologies. Please visit the following Web site to see some of our numerical simulations: http://www.tafsm.org/~ktaki/.

Kenji Takizawa
Associate Professor, Waseda Institute for Advanced Study

Born in Ueda City, Nagano Prefecture.
Graduated from Ueda High School, Nagano Prefecture and the Department of Mechano-Aerospace Engineering, Tokyo Institute of Technology.
March 2005, Ph.D. Science, Department of Energy Sciences, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology.
After holding the posts: Researcher, National Maritime Research Institute, Japan (2005), Research Associate, Rice University, USA (2007), and Research Scientist, Rice University, USA (2009), he took up his current post in 2011.

Major publications include: CG Simulation by using CIP Scheme and Java [CIP-hou to Java niyoru CG Shimyureisyon], T. Yabe, Y. Ogata and K. Takizawa (Morikita Publishing Co., Ltd.); Computational Fluid–Structure Interaction: Methods and Applications, Y. Bazilevs, K. Takizawa and T. E. Tezduyar (Wiley-Blackwell).