Problem

Continuing earlier work on heat pipes for fission power (further reading also available: Low-Cost Radiator for Fission Surface Power II) Advanced Cooling Technologies, Inc. (ACT) developed a series of titanium-water heat pipe radiators to remove the waste heat from Kilopower system convertors. The titanium-water heat pipes have an advanced fluid management design, enabling the heat pipes and the Kilopower system to:

1. Thermal Management: Transport waste heat in space and surface operations
2. Survive and startup smoothly after being exposed to a frozen condition during launching or afterward.

The Kilopower system is an affordable, small-scale nuclear fission power plant that is designed to produce 1 to 10 kW of electricity to support NASA future space transportation and planetary exploration missions (figure 1). The Kilopower system uses Stirling conversion to generate power. The thermal energy generated from a nuclear fission reactor is transferred to the Stirling convertor hot-end via a series of high temperature (>800°C) alkali metal heat pipes. Parts of the thermal energy will be converted into usable electricity. The remaining waste heat needs to be removed from the Stirling convertor cold-end and ultimately rejected to the space environment through radiators.

solution

ACT designed and fabricated multiple titanium-water heat pipes with radiator panels attached. Their thermal performance was validated through experimental measurement conducted both in ambient and a space-relevant environment (i.e. thermal vacuum chamber).  The titanium-water heat pipes, operating at 400K, must be able to function and survive under the following four conditions:

1. Operating in space without gravity forces.
2. Operating on a planetary surface with a reduced gravity force for working fluid return.
3. Testing on the ground to estimate space operation performance. To do so, the heat pipe evaporator should be slightly higher ( ~ 0.1 inch) than the condenser.
4. Survival and recovery from a frozen condition. During a launch period, the heat pipes will be orientated in extreme against gravity and the sink temperature could be lower than the freezing point of the working fluid. It is necessary to incorporate a special wick design to manage the working fluid within the heat pipe, which can (a) avoid liquid staying inside the condenser and bursting the pipe while freezing and (b) supply enough amount of working fluid to startup the heat pipe after being frozen.

ACT developed a series of titanium water heat pipes for the Kilopower system cooling, based on our earlier titanium/water heat pipe work.  As Figure 2 shows, the titanium water heat pipe has a C-shape evaporator to interface with the Stirling Convertor. Inside the evaporator, ACT inserted two types of screen mesh with different pore sizes which will enable the heat pipe to survive and recover from freezing. The rest of the pipe has an axial groove structure. Test results show that each titanium-water heat pipe is capable of transferring more than 400W of heat in the slightly adverse gravity inclination with a low thermal resistance at 0.01°C/W. The freeze/thaw test result (see figure 3) further demonstrates that the heat pipe can successfully recover from a frozen condition at -50°C to a normal space operation mode.

ACT also integrated aluminum flat sheets with the titanium water heat pipes through S-bonding, a cost-effective approach to join dissimilar metals. The titanium heat pipes with aluminum radiator panels are shown in Figure 4. Their thermal performance were tested in a space-simulated environment. As figure 5 shows, temperature distribution along the heat pipe with a radiator attached is very uniform, validating that the heat pipe radiators can effectively carry and reject the required waste heat in a space-relevant environment. ACT has delivered 7 titanium heat pipes to NASA Glenn Research Center for further performance validation.

ACT’s Titanium-water heat pipes have the following key advantages

1. Highly reliable – no pumps, no fans, and no compressors involved
2. Low mass – the weight of each Ti-water heat pipe with Al radiator is less than 0.75 kg.
3. Highly conductive – the overall thermal resistance of the heat pipe radiator is 0.02°C/W.
4. Operable in microgravity– it has been validated through ground testing that the heat pipe can transfer more than 350W of heat slightly against gravity.
5. Freeze/thaw tolerance – it has been demonstrated that the heat pipe can smoothly recover from a frozen state to normal operation.

The advanced titanium-water heat pipe technology can be further applied in high heat flux electronics cooling and various spacecraft thermal management system requiring low-mass, high-performance solutions. Learn more about ACT’s work in the development of water heat pipe technology, at the following links:

1. Earlier Fission Power Radiator work at ACT
2. Kuan-Lin Lee et al., Titanium water heat pipe for space fission power cooling” ANS Nuclear and Emerging Technologies for Space (NETS) 2018, Las Vegas NV (2018)
3. Copper-water heat pipes for cooling spacecraft electronics