Q: What was the most difficult aspect of developing Hayabusa for you?
Okudaira: For me, that would be the temperature environment. Because Hayabusa had to land on an asteroid exposed to the Sun, we designed it so that the unit can be cooled down easily. And, when the temperature fell, a heater was used to heat up the unit. However, due to launch delays, the target asteroid was changed twice during development.
Q: The target asteroid was Nereus first, then changed to 1989ML, and finally to Itokawa, right?
Okudaira: That's right. However, when the target asteroid changed, the distance to the Sun on touchdown and the temperature environment changed as well, and so the design had to be revised each time. Of course, this didn't apply just to the temperature environment. Other basic conditions, which have an influence on the design, sometimes changed and became unclear, making revisions very difficult.
Mechanical System Engineer,
NEC Engineering
Shouji: It was definitely tough to change the design when the target asteroid changed. When Itokawa became the target, the orbit changed considerably, so we had to change the oscillation direction of the medium-gain antenna by 90 degrees. However, there was not enough oscillation space on the side on which the antenna had been attached, so the antenna had to be attached in parallel with the direction of travel. Because the internal equipment layout had already been fixed, there was no room to run wiring to the antenna, and, as a last resort, we ran wiring under the high-gain antenna and over the outside of the probe, and then covered the wiring with yellow Kapton insulation material.
Q: Was doing something like that really okay?
Shouji: It was okay design-wise, but it affected Mr. Nishine’s job of assembling the probe.
Nishine: I'll talk about that in detail a little later. Basically, the effect on probe assembly was large enough to almost make the design impossible.
Okudaira: Not only did the conditions change, but sometimes we didn't even know the conditions required for the design. The design of the sampler horn, for example, depended on where the probe would land. If the probe landed on flat sand, it would have been okay for the sampler horn to be short, but, if the probe landed at a rough, rocky location, the sampler horn had to be tall enough to prevent the solar array paddle and equipment from bumping into rocks. Unfortunately, you would have to actually go to the asteroid to know what the ground is like. Therefore, we ended up having discussions with science professors to determine the probable conditions, but it was hard work. The discussions took a really long time.
Shouji: Regarding the difficulty of developing Hayabusa, I remember being under extraordinary pressure to reduce the probe weight. If I reported even a tiny increase in the weight at a design meeting, the issue was scrutinized, and resulted in long discussions about where to reduce the weight by an equal amount. Continuously finding ways to reduce the weight posed a formidable challenge to the mechanical system engineers.
Oshima: Regarding Hayabusa's weight, it was clear from the beginning that, had we performed our calculations based on the maximum conceivable weight, designing the probe would not have been possible. Therefore, our policy was to control the weight by using a standard weight we wanted to keep Hayabusa under for our calculations, to which we added a margin, but we soon used up the margin and it was extremely difficult to avoid exceeding the maximum weight.
Q: How did you go about solving all these problems?
Oshima: Whenever there was a problem, I brought it back to the company for discussion. Once we determined the direction we wanted to go in, we went and discussed the problem with JAXA. I really can't stress the importance of considering the total optimization of the project enough. We system engineers attended every meeting about equipment to keep track of the Hayabusa project as a whole. However, for this probe, total optimization involved more than just the efforts of system engineers. It was clear to everyone that many difficult conditions, including the overall weight constraints, stood in the way of success, and the active cooperation of every engineer in charge of a sub-component was indispensable for creating such a demanding overall design. It's probably the case that, because the difficult problems facing the system were made clear to everyone, the sub-component engineers were able to work in one direction towards overall optimization.
Shouji: I think reducing the weight of the probe structure was one of the most effective ways to reduce the total weight. The structure accounted for a large proportion of the probe weight, so reducing this weight was extremely effective.
When I worked on reducing the weight, I bought and read nonfiction books about the development of the Zero fighter. The Zero fighter was a combat plane whose weight had to be reduced as much as possible. When I got stuck on the design, I just shook my head and reminded myself that my situation wasn't nearly as tough as that faced by the Zero fighter designers.
The end result of my efforts, from the truly tiny weight reductions to the more significant ones, was a close to 20 percent reduction in the original weight of the probe. Not too bad, if I say so myself. (Laughs)
(Handling) Technician,
NEC TOSHIBA Space Systems
Nishine: As I mentioned earlier, once the design was refined, it was left to us to assemble the probe.
Shouji: That's because we trust you guys to do a good job.
Nishine: I guess it's our fate to work together on small satellites under difficult weight conditions. I was in charge of handling, which is incorporating finished parts into the probe. My work included not only assembly but also repeatedly attaching and removing parts each time various tests were performed. For Hayabusa, a team of several people including me was set up and put in charge of handling.
