The increased use of high-power payload electronics in virtual all classes of spacecraft require generally larger thermal radiators as the spacecraft body can provide. The use of deployable radiators seems to be the next logical step to achieve the required increase in radiative area. Two technologies to design deployable radiators are presently known, which occupy the high and low end of the radiator performance scale: (1) Large deployable radiators based on loop-heat-pipes (LHP) are, due to the inherent technical complexity and high recurring cost, best suited for high-power telecoms. (2) Passive radiators based on high-conductive materials, such as pyrolytic graphite foils, are low-cost units. But due to the finite nature of the thermal conductivity in solids, these types of radiators have generally lower specific performance and may be typically employed in small sized spacecraft.
For the large number of medium sized spacecraft, we observe that a design methodology for deployable radiators is missing, which exhibit attractive recurring costs and would cover a thermal performances regime between the mentioned LHP and passive radiators. In order to fill this gap, we started the development of an innovative deployable radiator with integrated pulsating heat pipe (PHP), which would distribute the waste heat, coming via a flexible thermal link from the spacecraft body, over the radiator area.
The technology of pulsating heat pipes has been extensively studied during the last decades for many terrestrial and space applications. Although PHP use the phase change between the fluid states of liquid and vapor for heat transfer, they are much simpler as classical heat pipes or LHP, since a capillary structure for condensate return is not necessary. Fluid passages are small (1 to 2 mm in diameter), leading to thin, low-mass radiator plates. In addition, several (low-pressure) liquids can be selected as working fluid, which avoids design efforts for a high-pressure compartment as known from ammonia systems. Meanwhile it has been verified that performance of a LHP in horizontal position during ground tests is well comparable to in-orbit performance. An important task is to verify PHP operation with radiative cooling without adiabatic section, i.e., a cooling method with low heat flux density. Such an operational condition has not been investigated in the past.
We will present and discuss the development, design and testing of several demonstrator radiator units with embedded PHP having the following characteristics:
• Material: Graphite/Polymer compound
• Two plates with integrated fluid passages are adhesively bonded
• Plate dimension: 125 x 400 mm
• Heater area: 125 x 25 mm (each side)
• Cooling area 125 x 375 mm (each side)
• Number of fluid passages: 12 (6 turns)
• Plates with circular passages (2 mm diameter) and square cross section (2 x 2 mm)
Fluid: Acetone and Ethanol
• Heating by Kapton heater
• Cooling by radiation from both sides in a vacuum chamber against a shroud temperature of 200 K
• Tests of PHPs with different fill rates, inclination angles, and input power
We summarize the very successful test program, discuss open issues and the next development steps.
The project is co-financed by the European Union within the EDRF Program. Project ID: LURAFO3015A