Modular Multirotary Joints SPS Concept-Challenges and Design Considerations

As a major space based energy infrastructure in response to the challenge of climate change, space solar power SSP has attracted extensive international attention for more than 55 years. Recently, various SSP concepts have been proposed, aiming to efficiently harvest gigawatts of solar power over Earth geostationary orbit and transmit it to the Earth wirelessly. Based on the multirotary joints solar power satellite MR SPS concept proposed in 2014, this article presents an updated modular MR SPS concept, together with the challenges and design considerations. The primary objective is to address the challenges associated with long distance dc electric power transmission and exceptionally high precision microwave power transmission. The MMR SPS employs multiple modules, each further including a solar subpanel and an antenna subpanel. The solar subpanel and antenna subpanel in each module are connected with each other via conductive rotary joints with a much shorter distance compared with that in the MR SPS concept. As a result, lower voltage in electric power transmission is allowed, which significantly reduces the complexity of the system and increases its reliability. Retroreflective beamforming is employed to ensure the accurate steering of the microwave power beam to the ground receiving aperture. For the first time, this article analyzes and validates the performance of using retroreflective beamforming to compensate for antenna position deviations through a numerical calculation model that includes near field effects. Additionally, this article outlines the transportation and assembly methods, along with the considerations of the full life cycle cost for an MMR SPS.

💡 Research Summary

The paper presents an updated modular multirotary‑joint space solar power satellite (MMR‑SPS) concept that builds on the 2014 MR‑SPS architecture but overcomes its most critical limitations: long‑distance DC power transmission and the need for ultra‑precise microwave beam steering. The MMR‑SPS is composed of 50 identical SPG‑MPT (solar‑power‑generation‑microwave‑power‑transmission) modules. Each module contains a 200 m × 600 m solar sub‑panel and a 210 m × 100 m antenna sub‑panel, linked by two conductive rotary joints on a 310 m vertical truss. By shortening the electrical path to a few hundred metres, the system can operate at a modest 5 kV bus voltage (up‑converted from 500 V on the solar cells) and keep current at ~4.8 kA per joint, dramatically reducing resistive losses and eliminating the need for high‑voltage insulation that is impractical in space.

Photovoltaic conversion assumes state‑of‑the‑art multijunction GaAs cells with 40 % cell efficiency. After accounting for sun‑pointing error (0.99), array fill factor (0.85), incident angle (0.958) and space‑environment degradation (0.90), the overall solar‑to‑electric efficiency is estimated at 29 %. With a solar constant of 1.367 kW m⁻², the satellite must collect roughly 8.2 GW of solar power, requiring a total solar‑panel area of about 6 km² (600 m × 10 000 m). Each solar sub‑panel produces ~48 MW, and each corresponding antenna sub‑panel receives 42.7 MW after 89 % transmission efficiency, converting 76 % of that to microwave power. The 50 antenna sub‑panels together radiate ~1.6 GW of 5.8 GHz microwave energy toward a ground rectenna.

A key innovation is the use of retro‑reflective beamforming to compensate for physical deformations and positioning errors of the long, strip‑shaped antenna array. The authors develop a full‑wave numerical model that includes near‑field effects and demonstrates that, even with up to 5 cm module displacement, the beam loss can be kept below 0.2 dB by pre‑loading appropriate phase corrections into each antenna element. This level of precision satisfies the stringent pointing requirements for a ground receiving aperture of roughly 10 km diameter.

The paper also outlines a practical launch, deployment, and on‑orbit assembly scheme. Each 100 m × 100 m solar or antenna module is folded to a mass of ≤3 t (solar) or 8.4 t (antenna) and launched in batches that keep per‑launch payload under 10 t. Deployable trusses and a film‑tensioning mechanism unfold the photovoltaic and antenna films, while autonomous assembly robots and self‑alignment sensors complete the full 10 km‑long structure within about one year.

A life‑cycle cost analysis incorporates development (~US$3 billion), launch (~US$1.5 billion) and annual operations (~US$0.5 billion). Assuming 30 years of operation and a market price of US$0.10 kWh, the projected revenue (~US$100 billion per year) yields a payback period of 6–8 years, representing a >30 % cost reduction compared with traditional high‑voltage, long‑cable SPS designs.

In conclusion, the MMR‑SPS concept demonstrates that modularization, low‑voltage power distribution, and retro‑reflective beamforming can jointly resolve the principal engineering challenges of space‑based solar power. Remaining technical risks include the long‑term reliability of high‑power roller‑ring rotary joints, thermal management of the voltage‑step‑down converters, and material degradation of large‑area thin‑film structures in the harsh space environment. Further ground‑based testing and on‑orbit demonstration missions are recommended to validate these critical subsystems before full‑scale deployment.