Toyota Motor Corporation abides by the “Five Main Principles of Toyoda,” a written statement of the teaching of Sakichi Toyoda, one of which is to “Always be studious and creative, striving to stay ahead of the times.” At our Frontier Research Center, we envisage social issues that Japan will face in the future, and in 2018, launched the “Mothership Project”^{*1 *2}, for the future energy security of Japan, which is heavily reliant on imported resources. The project conducts research to fly a large kite in the atmosphere’s tropopause region, to utilize the westerly jet stream to generate energy. But there are many challenges to overcome in order to fulfill this research, such as producing a kite that is large and light, and finding a way to keep it tethered while airborne. This article introduces the base fabric of the Mothership Project’s kite, replete with unique technologies.

Project Overview

From the perspective of achieving carbon neutrality, we need to switch to using renewable energy. Among the various renewable energy sources available, we have turned our attention to the westerly jet stream, in Japan’s upper atmosphere, which is said to have the highest average wind speed in the world (image 1). In the Mothership Project, we aim to fly a large kite approximately 10 kilometers above the ground in the westerly jet stream, using high-strength, highly elastic fiber (high-performance fiber) to tether it to a land-based electric generator to produce power (left picture, image 2). Furthermore, as shown in the right picture in image 2, neighboring kites could be connected with wire, creating a “sky crane” system to aerially transport cargo between kites. Furthermore, we also anticipate utilization of the high altitude as an aerial platform, for example, as an above-cloud relay station transmitting satellite optical communications to the ground.

The Constant Struggle of What Began as In-house R&D on a Voluntary Theme

The Mothership Project began in 2018 with five members. One of the key elements in technology needed to establish the platform was making the kite component as light as possible. Being a kite, it could not be a rigid construction like an airplane, but instead, should be exceptionally light and flexible, and we therefore opted for an inflatable structure. This structure has an inflatable tube with an exterior membrane made of light but strong textile as its frame, while the pressure of the air inside the tube is maintained by the inner membrane, like a bicycle tire tube, to maintain rigidity (image 3). In order to develop the design, construction and assessment technology, we first began building a smaller prototype, measuring approximately seven square meters. For the base fabric for the exterior membrane, we purchased composite fabric made from laminated super fibers, impregnated with resin and cured into sheets (image 4), advancing the research through trial and error.

Flight trials were an enormous struggle, but by leveraging aeronautical engineering, materials mechanics and control engineering, we achieved our initial goal of flying the kite 1,000 meters above the ground in May 2020 (video 1). We will report on the flight trial in a separate report at a later date.

Decision to Develop New Base Fabric

Through several flight trials we came to realize that, in order to improve the kite’s airworthiness, we needed to make the kite with base fabric that was lighter, with a higher load capacity to withstand strong winds, and less susceptible to stretching of the stitching (a phenomenon where gaps between fibers widen around the stitching: image 5). The conventional base fabric used for the smaller kite had variations in fiber layer density, as shown in image 4, and was therefore unable to achieve the targeted load capacity. A further problem was that, when stitched sections were placed under heavy load, stretching occurred, which weakened the kite’s frame.

In the textile industry, each process is handled by a different company. We therefore had to contact and consult with the companies collaborating on development of the new base fabric: the yarn and textile manufacturers, and the resin processor. The companies we consulted all possess advanced technologies, but were unaccustomed to research and development, and were initially hesitant towards development of new base fabric. But after several discussions, they came to understand our sentiment towards the Mothership Project and finally agreed to be part of the development. In June 2021, during the COVID-19 pandemic, four companies: Toyobo Co., Ltd.^*3 (Toyobo), Sakai Sangyo Co., Ltd.^*4 (Sakai), Toyo Cloth Co., Ltd.^*5 (Toyo Cloth) and Toyota, held an online launch rally, marking the start of development of the new base fabric.

Overcoming Challenges and Issues

In order to make a lighter base fabric with a higher load capacity, we asked Toyobo to develop a new fiber based on their top-class light and strong high-performance fibers But due to a characteristic of the fiber where its strength declined at high temperatures, it presented a problem for resin processing, because normal resins that cure at high temperatures could not be used. In response, Toyo Cloth developed a process using solvent that allows the resin to cure at low temperatures. They also proposed surface treatment of the high-performance fiber to increase the adhesion between fiber and resin.
Also, in order to prevent stretching when sewing, they decided to weave the cloth. Strict quality control is essential during the cloth manufacturing process to prevent fibers from breaking or becoming tangled. At Sakai, they produced high quality textiles based on years of craftsmanship, including uniformly arranging the over-3000 warp (longitudinal) threads one-by-one by hand to avoid them breaking.
As a result, we were able to produce a new base fabric that, compared with the conventional fabric, was 31.5% lighter, with tensile strength approximately 1.3 times greater for the base fabric itself, and roughly 3 times greater for the stitching (images 6 and 7).

Future Plans

We intend to construct a kite with the new base fabric (image 8) and to hold a 24-hour continuous flight trial in inland Fukushima^*6 in March 2022. By using the new base fabric, we can make the kite lighter and strengthen the stitching, whereby we hope to significantly improve its durability and aerial stability in strong winds.
We continue in our efforts with our hopes pinned on one day seeing the realization of the Mothership Project.

Author: Eiji Itakura
Eiji Itakura was a core team member during type certification and production certification missions when Toyota was involved in aircraft engine development. He was later involved in future mobility planning and research. His dream is the realization of the Mothership Project. He also believes that the Mothership Project may be able to assist in disasters. Please contact him for further information.