The thermal control design of the Robotic Lander Development Project’s new lander is intended to protect the lander’s key systems (such as batteries and electronics) during transit from the Earth to the moon – a trip that could take a few days to a few months depending on the chosen flight path – and during the up to six years of continuous operation on the lunar surface. A lander located near the lunar equator could be exposed to daytime surface temperatures near 400K (260F) and nighttime surface temperatures below 100K (-280F). To evaluate the feasibility of this thermal design, Marshall and APL engineers are conducting thermal analyses and tests of critical aspects of this design that are unique to the project’s mission concepts. Two of these activities are testing the solid rocket motor (SRM) insulation and testing the thermal approach and components used to maintain the operating temperature range for the spacecraft’s main electronics.
The solid rocket motor will help slow the lander as it approaches the moon. During the lander’s cruise to the moon, the SRM will be heated by the plumes from adjacent thrusters and be exposed to cold space during the voyage. The ultimate goal of the SRM insulation blanket testing is to ensure that the SRM is maintained within a temperature range of 40-100F, which is critical to the safety of the mission and the efficiency of the solid rocket motor. The hybrid insulation concept being evaluated combines standard, off-the-shelf, cost-effective Kapton-based multi-layer insulation, MLI, with new high-temperature insulation materials. The external high-temperature insulation materials, which can stand temperatures higher than 800C (1470F) protect the Kapton-based MLI, which has a melt temperature around 400C (750F) from the hot plume. The thermal subsystem team is also developing and testing external surface coatings with tailored optical properties for the high temperature portion of the insulation to reduce heat loss to space. The project began testing insulation materials at Oak Ridge National Laboratory in July 2010.
The Robotic Lunar Lander Development Project’s thermal subsystem team is also evaluating thermal control options for maintaining the appropriate temperature range for the warm electronics box (WEB), during the repeated and prolonged exposure to the lunar day and night thermal extremes. During the long (14 Earth days) lunar night, the electronics are insulated and isolated such that the waste heat from the continuously operating electronics is sufficient to make up for heat lost to the extremely cold environment. By doing so, very little extra electrical power from the batteries is needed, keeping the size of the batteries manageable and the electronics operable. During the lunar day, a variable heat transport link, employing a two-phase working fluid and capillary-driven flow, moves excess waste heat from the electronics to the radiator, which then rejects that heat to the environment via radiation. This variable link must transport heat efficiently during the lunar day but must shut off during the night to isolate the electronics from the radiator and must do so using as little electrical power as possible. The radiator is designed to accommodate the adverse views to the hot lunar regolith and the sun and the possible deposition of dust, which combine to create a difficult thermal environment in which to operate. To evaluate this overall thermal approach, engineers are conducting a number of studies including the following:
Comparative assessment of methods to insulate and isolate the electronics from the environment and the supporting lander structures that may be close to the ambient temperatures.
Testing of novel variable conductance heat pipes and loop heat pipes to establish the feasibility of each when exposed to the variable gravity and thermal extremes of these lunar missions.
Assessment of various radiator options that work in the challenging lunar environment.