Ammonia and Propylene Loop Heat Pipes with Thermal Control Valves - Thermal/Vacuum and Freeze/Thaw Testing

Abstract

It is often desirable to partially or completely shut down a Loop Heat Pipe (LHP), for example, to maintain the temperature of electronics connected to the LHP on a satellite during an eclipse. The standard way to control the LHP is to apply electric power to heat the compensation chamber, reducing the pressure differential across the system and decreasing LHP flow. The amount of electrical power to shut down an LHP during an eclipse on orbit is generally reasonable due to the short duration in the cold environment. On the other hand, for LHPs on lunar and Martian landers and rovers, the electrical power requirements can be excessive, since the Lunar night lasts for 14 days. For example, the Anchor Node Mission for the International Lunar Network (ILN) has a Warm Electronics Box (WEB) and a battery, both of which must be maintained in a fairly narrow temperature range using a variable thermal conductance link. During the lunar day, heat must be transferred from the WEB to a radiator as efficiently as possible. During the night, heat transfer from the WEB must be minimized to keep the electronics and batteries warm with minimal power, especially with a very low (100 K) heat sink. A mini-LHP has the highest Technology Readiness Level, but requires electrical power to shut-down during the 14-day lunar night, with a significant penalty in battery mass: 1 watt of electrical power translates into 5kg of battery mass. Ammonia and propylene LHPs with Thermal Control Valves (TCVs) were developed to provide passive variable thermal conductance without electrical power. The TCV routes vapor to the condenser, or bypasses the condenser and routes the vapor back to the compensation chamber, depending upon the environmental temperature conditions. Thermal vacuum testing of both LHPs with thermal control valves demonstrated the ability of the TCV to passively maintain a warm evaporator during roughly 24 hours of operation at a 0W power input and a -60°C sink. For lunar applications, the sink temperature during the lunar night could reach as low as -223°C. It is possible for ammonia to freeze, potentially causing structural damage as the ammonia melts and expands. Freeze/thaw testing of a vertical condenser on an ammonia LHP with TCV was performed that showed negliglbe change in condenser dimensions after 9 freeze/thaw cycles.