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 day, the thermal link must transfer heat from the WEB electronics to the radiator as efficiently as possible, to minimize the radiator size. On the other hand, the thermal link must be as ineffective as possible during the Lunar night to keep the electronics and battery warm with minimal power, even with the very low temperature (100 K) heat sink.
A trade study was conducted that examined five different variable thermal links: 1. Pumped Loop, 2. Thermal Switch, 3. Variable Conductance Heat Pipe (VCHP), 4. Loop Heat Pipe (LHP), and 5. LHP with bypass valve. The table compares the potential thermal links.
Figure 1. Anchor Node with WEB Located in Middle.
Table 1. Comparison of Potential Thermal Links.
|Technology Attributes||Mechanical Heat Switch||VCHP||Mini Loop Heat Pipe||Mechanically Pumped Coolant Loop|
|Heat Transfer Capacity Range, W||1 to 20||1 to over 100||10 to over 100||25 to over 500|
|Configuration Flexibility||Inflexible, needs to be located close to the heat sink||Flexible||Very flexible, can easily transfer heat over large distances, over a meter||Very Flexible, can transfer heat over very long distances|
|Heat Collection Flexibility (at source)||Constrained to small foot print||Constrained to small foot print||Constrained to small foot print||No constraint on foot print|
|Heat Rejection Flexibility (at sink)||Constrained to small foot print||Constrained to small foot print||No constraint on foot print||No constraint on foot print|
|Typical mass, kg||0.10 to 0.12||0.3 to 0.5||0.3 to 0.5||4 to 20|
|Conductance, W/K On||0.4 to 0.5||20||10 to 15||5 to 10|
|Conductance, W/K Off||0.02 to 0.025||0.01 to 0.04||0.01 to 0.03||0.03 to 0.05|
|Electric Power, W||None||1-2 for tight thermal control||1 for “off condition” 5 for start up (a few min.)||3 to 10 for “on condition” (including electronics)|
|Heritage||Excellent (test on Mars)||Excellent for grooved wicks||Excellent for Space||Excellent for Space|
Figure 2. ILN Loop Heat Pipe Thermal Switch Concept with Bypass Valve.
Mechanically pumped loops were dropped from further consideration, since they are not passive, and require power, which is limited on the Anchor Node. Thermal Switches were dropped due to their low thermal conductance.
The variable thermal link could be:
- Loop Heat Pipe (LHP)
- Loop Heat Pipe (LHP) with bypass valve
- Variable Conductance Heat Pipe (VCHP) with internal reservoir
The table compares LHP and VCHP variable thermal links. Either the LHP or the VCHP could be used as the variable thermal link. Both have similar “on” and “off” thermal conductances, both have flown in space, and both have similar masses.
Table 3. Comparison of Loop Heat Pipe and Variable Conductance Heat Pipe Variable Links
|Mass||0.3 kg||0.45 kg|
|Shutdown Power||0 to 2 W||0 W|
|Start-Up Heater||5 W||0 W|
|Conductance – On||~ 15 W/K||~20 W/K|
|Conductance – Off||0.01 W/K||0.04 W/K (all Al)0.008 (5 in. SS)|
|TRL Level – Conventional||9||9|
|Radiator Coverage by Condenser||Full||Partial, unless Raised|
|May require Radiator CCHPs||No||Yes|
A mini- Loop Heat Pipe (LHP) has the highest TRL level. However, supplying 1 W of power through the 14-day Lunar night requires roughly 5 kg of mass. A mini-LHP with a bypass valve requires more development and validation but eliminates the electrical shut-down power. A VCHP with a hybrid wick requires the most development. The benefit is that it would be much less expensive to fabricate than a loop heat pipe. ACT is currently working with NASA Marshall to develop the mini-LHP with bypass valve, and the VCHP.