The small solar-power sail demonstrator IKAROS launched in May 2010 has achieved all of its technological goals and is still cruising in the solar system. Its total flight distance as of June 2013 was about 3 billion km. Accelerated by solar radiation pressure, its velocity increment reached 400 m/sec over the three-year flight.
In this article, I look back at the research, development, and operation of solar-sail technology from the standpoint of my specialty, astrodynamics.
Research: Realization of Centrifugal-Deployment Technology
In view of my research background, I wanted to realize a solar-sail flight mission in outer space. First, I had to figure out how to make and deploy a solar sail. What awaited me at that time was a profound world of material, structural, and dynamic sciences.
Regarding deployment, from the start our research focused on using only centrifugal force generated by the rotation of the entire sail. Most other solar-sail researchers in the world use extendable sail trusses. In contrast, the centrifugal deployment system has merits such as lightweight structure and great flexibility in design and sail size. In addition, our choice of a different system from others is based on our strategic concept to bring originality to our research.
Our sail research emphasized understanding and control of deployment behavior. The sail must be very thin and large, so we inevitably face hard-to-predict wrinkling and folding. Fig. 1 shows the transition profile of the strain energy (in this article, referred to as internal energyE stored in the wrinkles and folds of the sail at the centrifugal deployment event. Pattern A in Fig. 1 shows ideal deployment, where the internal energy monotonously decreases by the effect of centrifugal force. In this case, the sail surely and fully deploys with the minimum energy. Reality is not that simple, however. Other cases such as B and C in Fig.1 can occur easily. Case B is a fold pattern where a minimal point occurs during deployment, while case C shows an abnormality during dynamic deployment where exchange between kinetic energy and internal energy is too great. How can we realize an ideal deployment such as Case A? This was the question that we had to answer.
Very thin film flutters lightly with faint airflow in the atmosphere and hangs immediately and loosely in a gravity environment. The enemies of our experiment were air and gravity. In order to eliminate these two effects and verify sail deployment, we conducted numerous experiments in the first five years. Our experiments include deploying the sail: by revolving it in a vacuum chamber, at high-altitude using a balloon, and in a revolving closed container. A critical step in our experiments was the successful centrifugal deployment of a 10m-class sail in space using the sounding rocket S-310-34 in 2004. In 2007, we also successfully deployed a 20m-class sail using a scientific balloon.
Feedback from these experimental results improved our numerical-simulation technology. Consequently, we established an environment where we could design large sail, which was too large to experiment, relying only on calculation.
The sail-folding method adopted for IKAROS was initially called the Tsuda fold.ELater, various ideas from project members were incorporated and finally it was named square-type sail.ESince its fold lines are all straight, this method has many advantages such as easy manufacturability and a good fit for centrifugal deployment. Its most popular feature is that it requires no cutting. One of the beauties of Japanese origami (paper folding) is that it allows us to create various shapes without using scissors. Our folding method is entirely appropriate for Japanese solar sail technology. Thus, Japanese aesthetics also contributed to the selection of the sail-deployment method.
After the inauguration of the IKAROS project, a number of folding methods including the square-type sail were proposed. We founded two research groups, sail structure groupEand sail material group,Ewith respective experts from inside ISAS and outside. They evaluated and examined various methods in detail from various aspects. One candidate method showed a remarkable mathematical beauty far exceeding the square film. In order to select a single method, we scrutinized according to hundreds of evaluation criteria and had intensive discussions. It is my unforgettable memory (bad dream?) that we had discussions all night through almost every week with experts joining the groups.