Tandem, long-duration, ultra-high-altitude tethered balloon and its system characteristics

Abstract

In this paper, we propose a tandem tethered balloon scheme as a solution for increasing the operating altitudes of balloons to near-space altitudes of more than 20 km. By adding a middle balloon with a smaller volume than that of the top balloon in the middle weak wind zone, this scheme partially offsets the weight of the tether above the middle balloon, thereby greatly increasing the operating altitude of the tethered balloon. Another significant advantage of this scheme is that, when the working altitude exceeds 20 km, the opposite directions of the winds at high and low altitudes can be exploited to reduce the horizontal displacement of the top balloon, raising the safety factor and the wind-resistant capabilities of the tether. In this paper, a three-dimensional static model is established for the tandem ultra-high-altitude tethered balloon, and a static equilibrium governing equation of the tandem ultra-high-altitude tethered balloon is derived. In addition, a design method based on the principle of the tether safety factor has been proposed for the tandem tethered balloon. For the first time, a sensitivity analysis was conducted for the tether resistance coefficient, balloon resistance coefficient, and balloon lift coefficient for the design optimization and flight test stages. Finally, we analyzed the three-dimensional cross-sectional profile of the tether, the maximum tension of the tether, the length of the tether, and the variation of the angle between the ground end of the tether and the horizontal plane for a tandem tethered balloon system that remained airborne continuously for one month. This work confirms that the model is viable in complex and variable wind field environments and capable of long-duration flights.

Introduction

Near space has a wide range of application prospects for communication services, ground monitoring, aerial early warnings, and scientific research (Doliveira et al., 2016, Gonzalo et al., 2018). The United States, Japan, China, South Korea, and other countries have conducted investigations on prolonged airborne flight platforms for near-space development, including the launch of near-space airships (Bents, 2011, Xu et al., 2020), solar drones (Hwang et al., 2016, Zhu et al., 2014), long-duration super-pressure balloons (Cathey, 2008, Saito et al., 2014, Yoder et al., 2019), and ultra-high altitude tethered balloons (Izetunsalan and Unsalan, 2011, Chiba et al., 2015).

Traditionally tethered balloons as long-duration aerial platforms have been widely used for military early warning and atmospheric surveillance systems as well as ground-based observations. The representative altitudes achieved by traditional tethered balloons are those of the 71 M balloon of TCOM in the United States (Fig. 1) and the Jimu-1 balloon of the Chinese Academy of Sciences (CAS) (Fig. 2). The 71 M balloon can carry a payload of 1800 kg to an altitude of 4600 m and conduct continuous observations for up to 30 d (Jones and Schroder, 2001). The Jimu-1 tethered balloon of the Chinese Academy of Sciences (CAS) set a record in May 2019 for rising to an altitude of 7003 m, and the CAS is currently conducting research on the 9000-m-altitude Jimu-2 balloon. The concept of an ultra-high-altitude tethered balloon was proposed by the Advanced Research Projects Agency of the United States (Menke, 1967). The balloon floats in the stratosphere and is moored on the ground by a tether. Compared to airships and UAVs, ultra-high-altitude tethered balloons use very little energy and have the intrinsic advantages of having fixed locations and long-duration flight capabilities. They have provided a new avenue for developing the stratosphere.

In addition to conventional applications, such as relays, atmospheric observations, and radar early warnings, which use ultra-high-altitude tethered balloons, the Jet Propulsion Laboratory in the United States proposed that long-duration tethered balloons in the stratosphere at 22 km be used for small rocket launching to effectively reduce the fuel cost for the launch (Wilcox et al., 2011). Davidson et al. (Davidson et al., 2012) proposed the use of an ultra-high-altitude tethered balloon to inject particles into the upper atmosphere to mitigate the problem of global warming. Davidson also compared this method with other methods of transporting materials to the stratosphere and suggested that this method has cost and time advantages. At the 2013 symposium “Airships: A New Horizon for Science” hosted by the California Institute of Technology, stratospheric tethered balloons were discussed as a separate topic. This conference was centered around the idea that tethered stratospheric balloons have broad application prospects and research value (Miller et al., 2014).

TCOM analyzed the feasibility and parameter sensitivity of ultra-high-altitude tethered balloons in the 1990s (Euler et al., 1995: 1612., Badesha et al., 1996). Grant (Grant et al., 1996) used the Newton–Raphson iterative method to solve the two-dimensional dynamics equations of an established high-altitude tethered balloon and analyzed the static and dynamic lift-off processes of the system. Badesha and Bunn (2002) established a two-dimensional dynamic simulation model for stratospheric tethered balloons and a two-dimensional wind field model, and they analyzed the responses of the balloon and tether under horizontal and vertical two-way airflow disturbances caused by a thunderstorm. Akita (2012) proposed a sea-anchored stratospheric tethered balloon that was easy to lift and recover and studied its feasibility.

Unlike conventional aerial vehicles, the performance of an ultra-high-altitude tethered balloon cannot be assessed using a single wind speed indicator, as it will be affected by the entire vertical profile of the wind field simultaneously. Based on the target operating locations and times of ultra-high altitude tethered balloons, none of the references above specified the vertical profile of the wind field at the location of the balloon and tether in advance, and the maximum wind speeds in the strong wind zone of the above studies were no greater than 31 m/s. In the second part of this paper, we analyze the wind field in July at the Siziwangqi Scientific Balloon Station in Inner Mongolia. July is the month with the lowest wind speed. The analysis shows that the wind speed in the strong wind zone reached 55 m/s. This poses a great challenge to the implementation of ultra-high-altitude tethered balloons, because as the wind speed increases, the horizontal offset distance of the balloon increases, the length of the tether increases, and the balloon will bear a greater weight. Eventually, the buoyancy of the balloon will not be sufficient to bear the weight of the tether, and simply increasing the balloon buoyancy will cause a direct increase in the tension in the tether and reduce the tether safety factor. This will be analyzed in detail in the third section of this paper. In the ultra-high-altitude tethered balloon study of Chiba et al. (2017), the maximum wind speed in the strong wind zone reached nearly 50 m/s, but the diameter of the tether reached 50 mm, and the weight of the tether alone exceeded 60 t. At present, the maximum balloon payload of NASA scientific balloons is only 6.35 t (Kubara, 1974). Thus, this scheme has far exceeded the current technology level. Costello et al. (2012) proposed the use of tethers with streamlined cross-sectional shapes to reduce the drag force on the tether, thereby reducing the horizontal displacement of the balloon. However, the manufacturing difficulty of streamlined tethers and how to keep the streamlined shape of the tether aligned with the wind direction at all altitudes are the main problems to overcome.

During the winter–summer seasonal alternation, there are latitudinal wind transition layers and weak wind speed zones in the stratosphere (Belmont et al., 1975). By adding a middle balloon that is small in comparison with the top balloon in the middle weak wind zone, we can partially offset the weight of the tether above the middle balloon and thereby substantially increase the operating altitude of the tethered balloon. The analysis presented in this article showed that, while the tandem tethered balloon can increase the flight altitude, it can also take advantage of the wind field characteristic that the wind directions at high and low altitudes are opposite during the winter–summer seasonal alternation, which reduces the horizontal displacement of the balloon and the tether. The tether safety factor did not decrease but actually improved.

In this paper, we propose the scheme of a tandem, long-duration, ultra-high-altitude tethered balloon and systematically analyze the advantages, parameter sensitivity, and flight stability of the tandem tethered balloon starting from static modeling analysis and design method research. The paper is organized as follows. In Section 2, we analyze the wind field and present the structural components of the ultra-high-altitude tethered balloon. In Section 3, a three-dimensional static model is established for the ultra-high-altitude tethered balloon. The static equilibrium equation is derived, and a comparison is made between the tandem tethered balloon and the single balloon in terms of the improvements to the safety factor of the tether and the wind-resistant capabilities. In Section 4, the design flow of the tandem tethered balloon is presented. A systematic analysis of the sensitivity to the tether diameter, the aerodynamic resistance coefficient of the tether, the aerodynamic resistance coefficient of the balloon, the aerodynamic lift coefficient, and the variation of the long-duration flight parameters of the tandem tethered balloon are presented in Section 5. The final part of the paper provides conclusions.