## 1 Introduction: Previous Research on Momentum-Transfer Tethers  
After 56 years of space exploration, the space launch industry remains reliant only on chemical rockets. This paper will investigate if any merit exists in implementing an alternative launch assist technology with functionality of existing upper stages--momentum transfer tethers. For this purpose, Section 1 reviews the literature on the tether-assisted propulsion. Section 2 quantifies how specifics of the LEO (low-earth orbit) environment--a relatively dense atmosphere with a small density scale--along with micrometeoroids will affect operations of the tethers. Section 3 discusses the demerit of implementing tip masses or counterweights on momentum-transfer tether. Section 4 reviews the literature on the proper calculation of the tether tensile strength. Section 5 reviews existing solutions for the payload capture using tethers, and an original solution, based on a mid-air satellite retrieval system is proposed. In Section 6, four example orbital launch missions with momentum-transfer tethers are designed and their parameters analyzed. Section 7 concludes on the areas of development most critical for future implementation of the momentum-transfer tethers as launch assistance means. In 2003, [PERSON] [1] stated that the necessity to create a sustainable (high launch-rate) infrastructure was historically the delay to both launch-assist catapults and tether-assisted propulsion. The concept of an earth-based catapult typically has multi-GW electrical power requirements, due the necessity of launching crafts heavy enough to pierce the Earth's atmosphere. In 2013, [PERSON], with the StarTram team [2], promoted a design with peak electrical power of 104 GW, and that power cannot be scaled down to any reasonable (10-100 MW) range. However, momentum-transfer tethers operate at near-vacuum and can be implemented even with currently meager energy technology. However, how high must the launch rate be? Do the recent advances in ion propulsion and high-strength polymers improve the merit of momentum transfer tethers? Are rotating tethers stable? Let's try to answer these questions, starting with a review of already completed research related to space tethers. Experiments were performed in space to master tether deployment. The work of [PERSON], in 2001 [3], stresses the importance of fast (\(>\)1 m/s) deployment to avoid problems with built-in stresses and electric charges. The longest tether stably deployed to date has reached a length of 31.7 km, albeit unintentionally [4]. According to Aerospace America Magazine [5], in 2010, the near-term target was to achieve reliable deployment and operation of 1 km tethers, although other projects [6] succeeded with non-librating deployment of 20 km-long tethers in 1993. In 2007, [PERSON] [7] described in detail a tether brake used to control deployment of the 31.7 km-long tether in the SpaceMail experiment [4]. Therefore, the author concludes that current technology is sufficient to deploy 10-20 km-long tethers in the LEO environment. Although significant research has been conducted by the scientific community on hanging or librating tethers for momentum transfer, in 1888, [PERSON] [8] proved that optimal orbital transfer was achieved using a tether with constant tension during the entire transfer process. Therefore, all hanging and librating tether configurations were marked as less efficient, in terms of momentum transfer, for a given tether mass. Among many rotating momentum-transfer tether configurations promoted on the Tether Unlimited site, the LEO bolo (rotating tether) [9,10] deserves special attention, as it is scalable to low tip speed. The orbital geometry of the LEO bolo is shown in Figure 1. The tether of the LEO bolo is also reusable; although, in 2010, [PERSON] [11] proposed using a system with two rotating tethers for orbital capture--one disposable and one reusable. However, non-reusable tethers must be orders-of-magnitude stronger than existing ropes to be mass-effective as means of momentum transfer.