From HAYABUSA to AKATSUKI
Takeshi Oshima, Project Manager
Masatoshi Ebara, Systems Engineer
Two engineers at NEC have been the driving force behind the AKATSUKI project. They are a project manager in charge of all aspects of probe development, and a systems engineer responsible for overall planning and management of the satellite system. Oshima, who had been involved in the development of Asteroid Explorer Hayabusa as system manager, served for the first time as project manager for AKATSUKI. Oshima went on to select Masatoshi Ebara, who had just joined NEC.
- Both of you are quite young.
Oshima: Yes, particularly Ebara (laughs). You got selected in your first year at the company, right?
Ebara: Six month after joining the company. One day, Oshima just tapped my shoulder and said “OK, you'll be the system manager, I'm looking forward to working with you.”
Left: Takeshi Oshima, Project Manager, NEC Corporation
Right: Masatoshi Ebara, Systems Engineer, NEC Corporation
Oshima: To be exact, I serve as both project manager and system manager. The project manager oversees the entire project. In addition to technical issues regarding satellite development, the project manager is responsible for various aspects such as budget control, development progress, and staffing. On the other hand, the system manager handles all technological matters. The job of the system manager, who works under the project manager, is to supervise the overall satellite system, making sure that development goes smoothly. The system manager is in charge of coordination among the various subsystems and the management of resources such as weight and power. Ebara's post is that of “Acting System Manager” or “System Manager In-Charge”. I serve as backup and give advice, while he does the actual work. Even now, Ebara coordinates all the aspects related to AKATSUKI operations.
Ebara: Right now (mid-October 2010), operations last from 10 a.m. to about 5 p.m., and I'm always on my mobile phone. See, there it rings again! (he rose from his seat).
Oshima: The development of AKATSUKI began in 2003. In the beginning, I was acting as system manager, but then Hagino, who was the project manager for AKATSUKI, got busy, so from July 2007 I was appointed project manager in addition to system manager. So I went over to Ebara and tapped his shoulder (laughs).
Ebara: Actually, I was at JAXA's ISAS (Institute of Space and Astronautical Science) as a graduate student. My specialty was X-ray astronomy, and I was participating in the development of X-ray astronomy satellite Suzaku. Seeing various manufacturers actually work on making the satellite, I felt a strong attraction and felt like joining their side, and that's how I joined NEC.
- I see. So you were worthy of being the acting system manager, even just half a year after entering the company.
- AKATSUKI is a probe that will go out into the solar system on the heels of the asteroid probe Hayabusa, but which parts of Hayabusa's design were retained, and which parts were changed?
Oshima: Basically, the design elements of Hayabusa that were passed on are the heater controller, which controls the internal temperature of the probe and the data handling unit (DHU)*1, which handles data as the “brain” of the probe.
- *1 Data handling unit (DHU): This computer controls the overall operation of the satellite. All the commands (instructions) transmitted from Earth to the satellite and observation data such as the pictures transmitted from the satellite to Earth are processed by way of the DHU.
In order to give you an idea of the aspects that differ, I think that the easiest way would be to have you picture AKATSUKI's attitude in space. Almost everyone seems to imagine that AKATSUKI flies with its rounded and white antenna in the fore, or that it flies using its solar array paddles like the wings of an airplane.
- Yes, you may be right. And they probably imagine that Hayabusa flies by firing its ion engines and using its solar array paddles as wings.
Oshima: Yes, and in reality it's not like that. First, picture the solar system. The Sun's at the center, and Venus, Earth, and the other planets revolve around it. AKATSUKI's two solar array paddles are approximately perpendicular to the revolution plane of Earth (or the ecliptic plane).*
- *: AKATSUKI's two solar array paddles become perpendicular to the orbital plane.
- I see. So you could say that AKATSUKI flies with its solar array paddles raised like “sails”.
Oshima: With this attitude, the sunlight does not reach the side to which the solar array paddles are attached. Therefore, this was made the heat dissipation side for heat exhaust. As the remaining four sides may receive sunlight, they are covered with insulation material. The sensors for observing Venus are installed on one of these four sides. Because sunlight must not enter the field of view of the sensors on this side, the probe is operated so as to prevent this side from facing the Sun.
One of the remaining three sides houses the rocket engine for orbital control used to insert the probe into the Venusian orbit. This side cannot be allowed to reach excessive temperatures either, so it is insulated as much as possible. The problem is that the side opposite the engine side faces up during assembly on the ground. The high-gain antenna for communicating with Earth is attached there.
- Right, on Hayabusa, this is the side with the parabolic antenna that is its design trademark.
Oshima: When we designed AKATSUKI, that antenna was a problem. Because AKATSUKI would travel to Venus, which is closer to the Sun, it would be exposed to strong sunlight. We planned to coat the parabolic antenna side with white paint to protect it, but found that the antenna would become a sunlight reflector.
- So that side is basically a concave mirror that concentrates light like a magnifying glass?
Oshima: That's right. The parabolic surface is just like a magnifying glass that concentrates light, which causes the feed system that emits radio waves and sticks out from the antenna surface to get hot. That's why we changed the traditional parabolic antenna to a newly developed flat antenna. Like Hayabusa, AKATSUKI was subject to strict weight limitations, and this flat antenna has seen its weight greatly reduced. At the same time, a single mechanism for fixing both the left and right solar array paddles when they fold down on the side with the antenna at launch time is provided. Using a system whereby the left and right paddles fold independently would have required two fixing mechanisms, so we saved weight to that extent.
The engineer who developed this flat antenna will later introduce it.
- Once the attitude was decided, the probe's design was determined by the side from which sunlight is received, right?
Oshima: While in the orbit of Venus, the probe will be exposed to strong light from the Sun as well as light reflected off Venus itself, so we had to design the probe so that it wouldn't overheat. For example, on Hayabusa, we used a solid state power amplifier (SSPA)*2 with 20 W output made with semiconductors for the communication system. SSPAs are highly reliable, but they give off considerable heat. So, for AKATSUKI, we used a traveling wave tube amplifier (TWTA)*3, which is a type of vacuum tube, instead of an SSPA. The output is actually the same, 20 W, but TWTAs are more efficient than SSPAs and give off less heat. Since the various systems on a satellite influence each other, simple comparison on a standalone basis is not appropriate, but the characteristics of the SSPA and TWTA systems can generally be summed up as shown in the following table.
- *2 Solid state power amplifier (SSPA): Communications in space use microwaves, which are radio waves that have a wavelength of 1 meter or less. For this reason, the communication systems on a satellite have equipment for amplifying microwaves. An SSPA amplifies microwaves by using electronic elements.
- *3 Traveling wave tube amplifier (TWTA): Unlike an SSPA, which is semiconductor-based, a TWTA is a microwave amplification device that uses a type of vacuum tube. It requires a higher operation voltage than an SSPA but amplifies microwaves more efficiently.
|Solid state power amplifier (SSPA)
20% to 30%
|Easily occurs under low vacuum conditions
20 W output
10 W output
|Traveling wave tube amplifier (TWTA)
30% to 40%
|Easily occurs under low vacuum conditions
20 W output
- * In the case of AKATSUKI, the output is double for the same amount of generated heat (10 W vs. 20 W), so TWTA is better suited for applications that involve the transmission of large amounts of information.
Oshima: TWTA requires a high voltage to operate. Application of a high voltage in a location with a low degree of vacuum will give rise to electrical discharge. Naturally we have implemented all kinds of measures to prevent discharge in the TWTA system, but since discharge occurrence would be a serious matter, we provided also a low-operating-voltage 10 W SSPA just to be safe.
Since the degree of vacuum inside the probe will not reach that of outer space and large amounts of data will not be transmitted for some time after the launch, the SSPA system is used during that time. The provision of two different types of amplifiers also raises overall reliability.
Ebara: We changed the design of those parts that gave us problems in Hayabusa to achieve higher reliability. For example, three reaction wheels for attitude control were used on Hayabusa, but two broke down. For AKATSUKI, we improved the structure of the reaction wheels and added one wheel, for a total of four. Based on lessons learned from Hayabusa, we adopted wheel operation procedures designed to reduce the likelihood of problems.
- I understand that there were strict weight restrictions, but the booster rocket was changed from M-V to H-IIA*4 during AKATSUKI's development. Did using a larger rocket result in relaxed weight restrictions?
- *4 Change from M-V to H-IIA rocket: The development of AKATSUKI (PLANET-C) began based on the assumption that it would be launched using an M-V rocket. However, in July 2006, the Ministry of Education, Culture, Sports, Science and Technology decided to abandon the M-V rocket due to its high cost. As a result, AKATSUKI was launched using an H-IIA rocket instead of the originally planned M-V9 rocket.
Oshima: When the probe reaches Venus, it will enter an orbit around Venus using propulsion from the onboard maneuver engine (see figure below). Therefore, if the probe weight at launch is increased, the engine size must also be increased or the probe will not be able to enter the Venusian orbit. When we made the change, however, design was already at quite an advanced stage and using a larger engine would have been impossible, so we continued design under the assumption of an M-V launch.
- Even so, vibration and other conditions differ between M-V and H-IIA. Meaning different testing standards for the probe to be launched.
Oshima: Before the actual flight model, we usually create test models called Structural Thermal Models (STM)*5. Next, we run tests for the vibration levels of HIIA and the environmental levels of the Venusian orbit to see if the design is appropriate. The tests of the flight model were conducted at the ISAS (Institute of Space and Astronautical Science)'s testing facilities in Sagamihara. Shipping the flight model all the way to Tsukuba would have been too much trouble, so we decided to try to do in Sagamihara all that could be done there.
- *5 Mechanical test model (MTM) and thermal test model (TTM): When developing a satellite or probe, engineers first create test models that have the same structure and thermal conditions as the actual unit, and then subject them to vibration tests and other ground tests using a vacuum chamber (an enclosure within which a partial vacuum is created for testing purpose) to check whether the satellite or probe operates as designed. These models are called the mechanical test model (MTM) and thermal test model (TTM).
Even so, the launch analysis showed that the probe was too light and excessive vibration would be generated. In order to reduce the vibration, we would have to combine AKATSUKI with another heavy item, and IKAROS was selected for that purpose. The sibling relationship between AKATSUKI and IKAROS began at that time.
- How did the assembly of AKATSUKI go? I've heard that Hayabusa's complicated structure made its assembly difficult.
Oshima: Well, it wasn't easy. Hayabusa's side panels open downward like covers, but AKATSUKI has at its center a tube-shaped component called a thrust tube that contains the engine and fuel tank. As a result, the sides and the top panel with the antennas form a block that is removed as a whole.
The electronics being installed on the side panels, the wiring is routed from the top panel to the side panels, and in order to reduce weight, an integrated assembly that uses as few connectors as possible was used. So, during assembly, we had to proceed with utmost care to avoid the wiring getting pinched in the hinges between the top panel and the side panels.
Ebara: A young group of engineers mostly in their 30's was in charge of AKATSUKI's assembly and testing. Women were among the participants. Since Nishine, a real veteran who had also been in charge of Hayabusa's assembly, was part of that group, we were not worried about the assembly.
- Just like for Hayabusa, AKATSUKI was designed based on the certainty that “Nishine will be able to put it all together.”
Ebara: The testing team having a tall member with long arms, some parts were designed knowing that he would be able to handle them.
Oshima: Yes, that's right. I mentioned that AKATSUKI was configured so that the top and side panels would be laid on from the top, and we positioned connectors at the top knowing that he's be able to reach them thanks to his tallness.
- Do you have any comments about your experiences as project manager and system manager?
Oshima: I felt that my job as system manager for Hayabusa went smoothly, but I encountered many difficulties as project manager for AKATSUKI. I realized that interpersonal communication is indeed a big job… I became keenly aware that Hagino, the project manager for Hayabusa, had really taken care of all of us without us realizing it. Basically, the key to successfully managing a project is for the persons in charge of the various parts to cover for each other whenever possible.
Wanting to build up Ebara, I'd entrust him with all kinds of things, like “Thanks for taking care of that.” That way, even when I wasn't around, he'd be able to manage things, and, if somebody talked to him, that information would go to me.
Ebara: I got built up alright! (laughs). Communication is indeed important. It felt like I always was in meetings with Oshima. At the company, I tried not avoid using the phone as much as possible. I'd go directly to the floor of the person in charge of a particular subsystem and talk face-to-face. Even discussions that would be difficult over the phone go smoothly when you talk face-to-face as your mood will then change. Even with parties outside the company, I'd go directly to meet them when things got stuck.
Oshima: In my case, rather than go meet people, I'd frequently hang out on their floor, and once in a while have a meeting with the person in charge.
- JAXA's Professor Ichiro Kawaguchi, the Hayabusa project manager, said that Hayabusa would usher in the age of solar system exploration, but what do you two think is coming next?
Ebara: Having had the opportunity to be involved with AKATSUKI, which is an interior planet*6 probe, next I'd like to go on to exterior planets*7. Hayabusa's successor, for example. And more than anything, I'd like to serve as an actual system manager the next time, instead of acting system manager (laughs).
- *6 Interior planets: Planets in the solar system whose orbits lie inside that of Earth are called interior planets. This term specifically refers to Mercury and Venus.
- *7 Exterior planets: Planets in the solar system whose orbits lie beyond that of Earth are called exterior planets. This term specifically refers to Mars, Jupiter, Saturn, Uranus, and Neptune.
Oshima: I think that you are up to it. If anything comes up, you can always ask me (laughs). As far as I am concerned, the first thing is to successfully insert AKATSUKI in an orbit around Venus. We weren't able to make Mars Explorer NOZOMI orbit the planet. Having myself participated in the development of NOZOMI, successful insertion this time would be sweet revenge.
I'm currently involved in studies of Hayabusa's successor and would like us to go to Jupiter next. To do so, I want to use the solar powered sail that was considered following IKAROS. That would be an enormously ambitious project, combining a giant solar sail measuring 50 meters in diameter and thin solar sails, and would enable us to further boost the power of the ion engines that were used on Hayabusa and get to Jupiter in just a few years.
Oshima built Ebara up, and Ebara repaid that trust. The result, AKATSUKI, is now well on its way to Venus. Next will come return trips to other asteroids and expeditions to Mars and Jupiter, always farther and farther on. Indeed, the dawn of a new age of solar system exploration is in sight.
Oshima recently set up an astronomical telescope on his verandah and has taken to observing Jupiter on weekends. “I can see its belts pretty clearly. Jupiter is the ultimate target for us in Japan.”
JAXA's ISAS is already studying missions to Jupiter and the Trojan group of asteroids*8 using solar powered sails. Oshima may be able to participate in Jupiter's exploration as an active member. And Ebara, lower in years, can look forward to going even farther.
Someday, a probe Ebara is working on may travel to a planet even farther out than Oshima's “ultimate target,” Jupiter. This would mark the beginning of the golden age of space exploration.
- *8 Trojan asteroids: In general, when a small celestial body orbits a large one, there is a gravitationally stable space in the range from 60° in front of to 60° behind the smaller celestial body. Therefore, as a result of the positional relationship between the Sun and Jupiter, there is a large concentration of asteroids in the range from 60° in front of to 60° behind Jupiter. These asteroids are called the Trojan asteroids.
Researched and written by Shinya Matsuura, October 13, 2010
Engineering Manager, Space Systems Division, NEC Corporation
Joined the company in 1990. After he developed on-board computers, he was the system manager in charge of the technologies of the overall design of MUSES-C (Hayabusa).
Since July 2003, he has been the system manager in charge of the overall design of the system for PLANET-C (AKATSUKI)
Since July 2007, he has been the AKATSUKI project manager.
Space Systems Division, NEC Corporation
Joined the company in 2006. Participated in the development of X-ray astronomy satellite Suzaku at ISAS of JAXA while still in graduate school.
Since November 2006, he has been in charge of coordinating all aspects related to probe systems as systems engineer for PLANET-C (AKATSUKI).