Will the next-generation atmospheric-entry vehicle be the shiitake mushroom-type?
In reports on the sounding rocket S-310-41 in the May and September editions of ISAS News, we introduced an experiment of new flying vehicle that deployed an aerodynamic decelerator like umbrella, entered the atmosphere and quickly slowed down. In this article, we would like to outline the story of our research up to the experiment.
Entry to the atmosphere is an unavoidable passage when we return astronauts or payloads from outer space or land explorers on atmospheric planets such as Mars. The most difficult hurdle to entry is aerodynamic heating. This is caused by atmospheric molecules colliding with the surface of vehicles flying at high speed and losing their kinetic energy in conversion to heat.
Countermeasures are needed to prevent the burning of valuable spacecraft caused by aerodynamic heating. Until now, research and development on this issue aimed at "heat resistance." The tiles covering the Space Shuttle are an example of this approach. We had doubts about this approach, however, and looked for another way to avoid aerodynamic heating. The answer was easy. Since the cause of aerodynamic heating is atmospheric molecules colliding with the vehicle, the natural answer is to reduce their number. In other words, we should limit high-speed flight to the high-altitude region where atmospheric density is low. Thus, we required an aerodynamic decelerator that works well in high altitude so that vehicles could almost complete deceleration before reaching the dense low-altitude atmosphere. The strength of air resistance required by the aerodynamic decelerator is proportional to atmospheric density, however, so the aerodynamic decelerator air brake must be enlarged. If we simply enlarge, for example, the asteroid explorer HAYABUSA's capsule, the weight increases and, worse, its size becomes a problem during launch. So we came up with the idea of an aerodynamic decelerator like umbrella made of cloth that is unfolded before entry to the atmosphere. This is the so-called deployment-type, flexible-membrane-structure aeroshell.
We hoped to realize the aeroshell applicable for actual flight. In the research to create new vehicle, only computer simulation or experiment in laboratory is insufficient. Actual flight demonstration is required to convince people. Thus, we started the research and development focusing on the flight demonstration about ten years ago. Fig. 1 is the history of the experimental vehicles developed by us in the past. The rightmost one is the vehicle that underwent the flight demonstration onboard a sounding rocket in the summer of 2012. From its color and shape, it was given the nickname "shiitake mushroom-type" by a visiting reporter.
Balloon experiment that marked a turning point in development
The trigger to initiate full-scale research and development of the deployment-type, flexible- membrane-structure aeroshell toward actual application was our presentation of the concept at a symposium on space-flight dynamics in 2002 (shown far left in Fig. 1). At the time, we were conducting basic research such as behavior of membrane material in a supersonic airflow. Fortunately, we were advised that we should conduct a balloon experiment as the first step. Our presentation to eagerly hope for flight test might give a strong impression. Thus, we could begin the flight experiment next year.
With the cooperation of the Scientific Balloon Center (currently the Research and Operation Office for Scientific Ballooning) and the involvement of students of ISAS and other universities in addition to our laboratories' students, in just eight months we completed the process of planning the flight experiment, designing and manufacturing the experimental model, and performing various confirmation tests. We built the experimental vehicle to meet the ballooning-day deadline. In the summer in 2003, however, we were unable to perform the flight experiment at Sanriku Balloon Center because the vehicle failed to release from the balloon gondola. We were crestfallen to see the vehicle returning on the gondola to the Center. We took it for a valuable lesson, however. During the second attempt in 2004, we refined the experimental vehicle and succeeded in stably dropping the vehicle equipped with flexible aeroshell from 40km altitude. This success of the balloon experiment was a turning point in our development of the flexible aeroshell, considerably accelerating further research and development.