High Temperature Radiators for Electric Propulsion Systems

The VASIMR propulsion system uses a high temperature Loop Heat Pipe (LHP) radiator to reject heat from the helicon section. The current baseline radiator uses titanium/water LHPs, however, deployable radiator and trace heating features are required to keep the water in the condenser from freezing when the radiator is turned off. The proposed project will develop high temperature toluene LHP radiators that will minimize the freezing problem, since the freezing temperature of toluene is roughly 100oC lower than water. Preliminary calculations on the toluene LHP radiator showed that a graded wick is required, with the pore size decreasing from the center to the surface of the wick. One goal of the project is to develop a graded alumina wick that reduces wick mass, back conduction, and pressure drop, enabling toluene as the working fluid. The ceramic wicks will also have near net shape fabrication, eliminating much of the current machining which adds costs to the LHP wicks. Optimizing the radial variation in porosity and permeability reduces the transport line sizes of toluene LHPs, significantly improving their mass and performance. The ceramic wicks can also be used in conventional LHPs, potentially reducing the LHP wick mass, pressure drop, and back conduction by 50 to 90% when compared with conventional nickel wicks. Phase I will demonstrate the feasibility of fabricating these wicks, demonstrate the wicks' machine-ability, conduct life tests, and evaluate the benefits of a graded wick versus a conventional wick with uniform properties. In Phase II, ACT will fabricate and test toluene LHP radiators with graded ceramic wicks to fully demonstrate their performances. We expect the technology to reach TRL 6 at the end of Phase II.

LHPs with graded ceramic wicks can also be used in commercial and military satellites. Another application is cooling of aircraft components, for both manned aircraft and UAVs. ACT has already identified actuator cooling as one area where graded wick LHPs can be very beneficial. Heat must be removed from the electrically powered actuators, which are located in the aircraft wing. A second area is cooling the FADEC (Full Authority Digital Engine Control) box on both military and commercial aircraft. A third application is for planar LHPs used to cool electronics.

The immediate NASA application is high temperature toluene LHP radiators for the VASIMR propulsion system. These radiators are used to remove heat from the Helicon section of the VASIMR rocket and reject the heat at around 200oC. While water LHPs have been baselined for the first generation of VASIMR rockets, the water must be prevented from freezing in the LHP condenser. The freezing point of toluene is roughly 100oC lower than water, minimizing the freezing problem. Wicks with optimally graded pore radius and permeability properties in the radial direction are required to enable the use of toluene as the working fluid. The ceramic wicks developed on the project will be able to meet this requirement while also reducing mass and back conduction. These wicks can also be used to reduce the mass and improve the performance of conventional ammonia LHPs. NASA is currently examining space power systems for spacecraft and the lunar surface. ACT plans to use the ceramic wicks developed on this program in future LHPs that it manufactures for NASA spacecraft as well as DOD and commercial satellites.

This is early/mid-stage (TRL 6) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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