The Liquid Droplet Radiator (LDR) has emerged as the most promising among a number of spacecraft radiator concepts, which seek to offer light-weight alternatives to conventional, primarily heat-pipe based, radiator technology. Relying solely on conventional radiator technology will in the future lead to an unacceptable mass penalty on the part of the radiator. In the LDR the solid surface of current radiator designs is replaced by a sheet-like spray of minute droplets, which radiate heat to space in between being ejected and collected external to the spacecraft. The droplet diameter is typically of the order of 100 μm.
Early studies in the US indicated that by employing the LDR the radiator mass could be reduced five to ten times for high-power applications stretching from 100 kW and upwards. In view of the great potential benefits of the LDR, ESA decided in 1989 to study the LDR from a European perspective, which meant focussing on power levels well below what had been conceived previously. Following an initial literature survey, a contract was awarded to Bertin et Cie of France in 1991 for an in depth study of the LDR for heat rejection in the range 1 to 100 kW. It included a review of future growth of heat-rejection requirements for European space activities and a trade-off between design options and selection of one design for a detailed concept investigation. The related Liquid Sheet Radiator (LSR) concept was also part of the study.
It appears that the most favourable design option for the reference mission, a 10 kW telecommunications satellite, as well as for the alternative, a 10 kW polar platform, would be a rectangular LDR with a mass considerably less than that of a comparable heat-pipe radiator. The design includes two droplet sheets (one out-going, one in-coming) and a structure, which consists of an unfurlable truss beam. Surrounding the droplet sheets, the truss beam is attached at one end to the exterior of the spacecraft via a yoke and a rotating drive to maintain grazing sun-incidence.
The droplet generator is modular, with a shear valve for turning it on and off and operates based on the cavity-disturbance mode. The droplet collector is of the linear type with an auxiliary film. A review of the technology status in Europe shows that all necessary competence for development of the LDR is available. Furthermore, critical technologies have been identified in order to help orienting a future development programme.