Thermal Regulation System of Millimetron Space Observatory
According to the requirements, the thermal control system must ensure the cooling of 10-m space telescope down to 4K and cooling of on-board instruments. That will be implemented by a combination of passive (sunshields) and active cooling (mechanical cryo coolers).
Critical points to achieve the requirements:
- The telescope will be located around L2 point (Sun-Earth system), such orbit has the best environment for radiation cooling
- Maximize the effectiveness of radiation cooling
- Accommodation study to avoid warm elements
- Minimization of heat flow from warm to cold parts
- Cooling via set of temperature stages
- Design will be based on space qualified mechanical cryo coolers
Implementation of this principle affects almost every aspect of the space observatory design to a greater or lesser degree, therefore thermal design drives structural design.
The observatory is equipped with large deployable sunshields to block the light and heat from the Sun, Earth, and Moon. Millimetron orbital position the Earth-Sun Lagrange point L2 keeps all three bodies on the same side of the spacecraft at all times, that maintains a constant thermal environment for the telescope. The sunshield system passively cools the telescope to the temperature level of 30-50K. Each sunshield consists of 2 layers of pretension two-sided aluminum-plated polyimide thin-films that are supported and 12 deployable spokes. The nearest to the primary mirror cryoshield will be connected to the 20K level of the mechanical coolers and cooled down to 20 K. It will have a different structure comparing to the sunshields.
Set of sunshields
Cryoshield design (active cooling system)
Radiative heat exchange and temperature levels through the sunshields and on-board active cooling system
A key moment for the thermal design of the Millimetron is to reduce the heat load for the stages of active cooling system operation. The dominant factor behind the heat load was the heat conduction through the basic structure. Two solutions were implemented: use the materials with low thermal conductivity and reduce the ratio of the cross-sectional area-to-length in truss elements. However, finding a solution was challenging since the main truss assembly must have sufficient stiffness and strength to support the payload module and its mass exceeding 3000 kg during the launch. Following launch, the primary mirror basic structure is required only to apply the loads associated with observatory operation: L2 orbit insertion, repointing, and station keeping. To reduce the thermal conductance of the basic structure, we have assumed that launch loads associated with the telescope and instruments will be applied via some detachable interface between cryogenic and warm parts of the basic structure.
Structural design of basic structure and the space between cold and warm module
A combination between active and passive cooling should be carefully designed by taking into account an appropriate margin of cooling capacity in each temperature level, because the active mechanical cooling is very expensive and complicated. Minimization of active mechanical cooling reduces cost and risks of mission.
Preliminary heat flow map (calculated)
Budget of the heat loads on the temperature levels
|Level||Radiation Exchange,W||Structure Conduction, W||Cable networkConduction*,W||Heat dissipation of instruments, W||ΣQ, W|
Specification of space cryo coolers
Pulse Tube Cooler PT15K
4K-class Joule Thomson cooler
1K-class Joule Thomson cooler
|TRL||TRL5/6 (planned in 2019)||TRL8||TRL5|
|Cooling power|| 800mW at 20K
5W at 100K
|40mW at 4.5K (EOL)||
10mW at 1.7K (EOL)
|Input power||300 W||90 W at EOL||75 W at EOL|
|Mass||21 kg||15 kg||28 kg|
|Life time||-||> 3 years (5years as a goal)||>5 years|