A: There is no "bony framework". It is categorized as a "soft type airship" that maintains its shape from the tension of its outer layer (film) maintained by the pressure difference between inside and outside. The key technology is the specific strength of the film material. We make full use of Vectran, a fabric that became famous for its use as an airbag for the NASA Mars probe. It is a light and strong "super fabric" that can withstand the long journey through space. The outer layer of the aircraft is a sandwich structure in which Vectran and a resin film called "EVAL" are sandwiched by urethane.
The inside of the airship is filled with helium gas whose molecules are very small. Therefore, the outer film is required to be very fine at the molecular level in order to contain the helium inside.
The thickness of the film is about 0.2 mm (about the same size as a postcard), but it is strong enough to bear 70 kgf of tension even if it is cut into a 1-cm wide rectangular piece.
A: Gas is left in the airship, because it would be flattened if gas was completely vented out as no bony framework supports the aircraft. Therefore, once we fill it with gas, we seldom vent it out. The inside and outside pressure difference on the ground is only 4/1,000, but that is good enough to maintain the shape of the airship. I believe that the Tokyo Dome is also maintained at the same pressure difference.
A: It weighs a total of 6.4 tons, and the film accounts for the half of this. Propulsion and other mechanisms are installed by an attachment that looks like a "knitted stomach band". At the time of an experiment, the weight is set to around 100 kg through the buoyancy of the helium gas controlling the amount of gas. Flight control is tested by flying the aircraft up and down and making turns using two sets of propellers.
A: The range from 10 km to 50 km in altitude is called the "Stratosphere", and the area at an altitude of 20 km is relatively calm.
On the other hand, in the troposphere, where we live, convection currents occur that let warm air go up and cool air come down. Clouds and rain are the phenomena in our world. In the stratosphere, temperature is consistent, and the air is "in a stratum". Therefore, there is neither convection currents nor cloud nor rain. Wind is also very calm. The pressure is 1/20 and air density is 1/15 of that on Earth.
A: Yes, it is almost always done in the early morning hours with the calm of dawn because the airship feels the greatest impact from wind on the ground. So, the experiment crews do not wake up early but work overnight from the previous night to prepare for the test. They work very hard.