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A: The most important factor to be considered when we decide the flight profile is the changes in the outside temperature and inside gas temperature to determine their impact on buoyancy due to expansion and compression.
It is obvious that the gas in the airship compresses as it cools down and expands as it warms up. One of the reasons for cooling down is the decrease in the outside temperature due to ascending. The faster the airship goes up, the bigger the difference between the inside and the outside becomes, and that causes more impact on buoyancy.
For ascending, the air inside the airship or the ballonet (airbag), is vented, thus the inside temperature decreases due to adiabatic expansion, and that impacts buoyancy. In addition, the effect of wind on the airship significantly reduces heat. These factors are deeply related to condensation and evaporation on the surface of the airship. We also have to incorporate airspeed.
Heating by the Sun is another factor, and its impact changes depending on the weather, the strength of the Sun's rays, and the duration of exposure to the Sun.
For descending, the opposite phenomena occur, such as increasing ambient temperature and insulation compression.
The airship is exposed to such temperature change every moment both inside and outside, thus the buoyancy changes complicatedly. Maintaining the stable airship attitude and altitude by assuming the changing factors appropriately requires very sophisticated technical knowledge.
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The Stratosphere Platform Project was launched with high expectations as one of the Millennium Projects led by the Cabinet Office. It has now advanced to a level to accumulate knowledge for element technologies and operation using two kinds of test craft.
In August, we carried out an experiment on another test airship for a stratosphere flight at Hitachi Port (in Ibaraki Prefecture). With this huge test airship that is almost 47 meters long, our challenge is to verify if we can make such an airship reach the Stratosphere. The test airship is also for the purpose of verifying the appropriateness of materials, from the film material through to the structural design. As the aircraft is huge, it may feel the effects of outside forces like wind that can bend the aircraft body in some situations. Despite these doubts, we were able to fly the test airship up to an altitude of 16 km.
The test held in the town of Taiki is an experiment for an airship to stay at a predetermined point in the sky. For a radio relay station and surveillance from the sky, it is necessary to have an aircraft that can remain in a specific point in the sky. Even though the stratosphere at an altitude of 20 km is calm, some wind exists. In order to reach there, power and control are required. We are therefore conducting tests to overcome many factors such as the appropriateness of our flight profile for outside temperatures and buoyancy, whether the thrust power is sufficient, and if the handling operation on the ground is reasonable.
This project is not a large-scale project by JAXA standards; however, due to its possible diversified use and superiority, the media, including magazines, expressed high expectations, and are treating the project as a hot issue. Research and development on airships similar to our "Stratosphere Platform Project" has been underway in many countries mainly for security reasons (surveillance.)
Airships used to be a vehicle for travelers represented by "The Zeppelin". Now, they seem to be mainly used for advertising and sightseeing tours. But the future of airships will be to act as a reliable platform for high-speed, large-volume radio communications.
In order to meet these high expectations, we would like to achieve test goals that lead to future usages. From early summer to winter this year, our tests have reached a critical phase. We will keep you informed of our achievements.
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