Thermal Control Subsystem

The function of the Thermal Control Subsystem (TCS) was to maintain all LRV components within specified temperature ranges during transit to and operation on the Moon. The TCS had to be engineered to function concurrently with the other subsystems of the LRV and to fit within an allocated weight limit of only 4.5 kg (10 lb). The LRV's electrical components could not be allowed to get too hot or too cold. This did not just apply to the LRV's operation on the Moon, but also had to be considered during launch to orbit, orbit of the Earth, trans-lunar flight, lunar orbit, descent to the lunar surface, and the period between the landing and the vehicle's deployment and utilization. Additionally, thermal control was required for the Space Support Equipment (SSE) which supported the LRV in the LM and allowed the LRV to be secured during transit and then deployed onto the Moon's surface. The mission profile that the LRV would be exposed to on the Moon would be during a 78-hour sunlit period of the lunar surface temperature cycle, which constituted the "lunar morning.'' This included solar elevation angles from 7 to 50 degrees, and lunar surface temperatures ranging from about 50 to 200 degrees Fahrenheit.

In addition to being able to operate on the Moon in /6 gravity in a hard vacuum environment, the potential problem of lunar dust effects on vehicle surfaces and components was an important factor that had to be quantified for this vital subsystem. For this reason, Earth-based tests of this potential problem area and how to minimize its effects had been conducted in 1967. From these test results, it was established that the presence of lunar dust on surfaces would significantly increase absorbed solar heat. Tests of a variety of methods resulted in the selection of a dust removal brush, which appeared to work well in these Earth-based tests. Additional tests of the LRV wheel and fender assembly with a lunar soil simulant were conducted in a reduced pressure chamber in the NASA KC-135 airplane (Vomit Comet), flying special loops to simulate the expected gravity. It was verified in

The completed LRV Qualification Unit sits on its support fixture at Boeing. The Qual.

Unit was required to thoroughly test and validate each system of the Lunar Roving

Vehicle. (NASA)

The completed LRV Qualification Unit sits on its support fixture at Boeing. The Qual.

Unit was required to thoroughly test and validate each system of the Lunar Roving

Vehicle. (NASA)

these tests that the fenders were a vital element in directing the trajectory of lunar dust stirred up by the wheels, and would protect LRV components from exposure to the dust.

Based on all of these requirements, a semi-passive TCS was implemented for the LRV. This included the use of passive thermal control techniques consisting of selected radiation surface finishes, heat sinks, flexible thermal straps, multi-layer insulation, and low thermal conductance component mounts. The electronic components with the tightest temperature limits were grouped together in an insulated compartment (with dust covers) over space radiators in the forward chassis area. The insulation comprised fifteen layers of thin sheets of perforated aluminized Mylar, with interstitial layers of Dacron net in between and Beta cloth for protection on the outside. The exposed LRV crew station and mobility subsystem components were assumed to be dust covered on the Moon.

The TCS approached complete autonomy within the specified mission parameters. It imposed only one constraint with regard to parking between traverses and required only one astronaut interface at the end of each driving traverse to initiate the automatically terminated cooling of the electronics by opening the dust covers over secondary surface space radiators. The original intent was for the TCS to be totally autonomous and not require any interaction with the astronauts. For the electronics grouped in the forward chassis area, an enclosed ammonia boiler, like those ultimately used on the Space Shuttle Orbiter, was considered but was deemed too heavy.

During operation, the batteries would be a great heat sink for their own internally generated heat, and an additional heat sink for some of the other electronic components. Flexible thermal straps were designed and tested to enable heat to be conducted from the Signal Processing Unit (SPU) to Battery 1 and from the Directional Gyro Unit (DGU) to Battery 2. The Drive Controller Electronics (DCE) had to be positioned too far from the batteries for effective thermal strapping. Passive thermal "heat pipes'', which are used extensively on present-day spacecraft, were not mature enough designs at the time of LRV development, so another heat storage and transfer method was needed.

Therefore, fusible mass "wax tanks'' were used to store the excess heat generated in the DCE (7.7 kg of wax) and the SPU (4.9 kg of wax) during operation. The added advantage of these wax tanks was that they acted as "thermal dampers'', maintaining the DCE and SPU at constant temperatures while the wax was being melted. The wax would then be solidified for re-use when the dust covers were opened at the end of driving on each EVA by exposing the thermal radiators on top of the wax tanks. It was planned to have the dust covers automatically close using bimetallic spring actuators when battery temperatures reached a safe 45 degrees Fahrenheit (+ 5 deg. F.)

The job of ensuring that the TCS was up to the task fell to the engineers in the Propulsion Division of the Astronautics Laboratory at the Marshall Space Flight Center (MSFC). There were hundreds of engineers working on the LRV at NASA and its subcontractors, scattered across the United States. A fortunate few were recent college graduates who had the opportunity to work on one of the most challenging and exciting programs they would encounter in their careers. One of those young engineers was Ronald A. Creel. Ron had served as a cooperative education student in college and came to work full-time at MSFC shortly after the LRV program was launched in 1969. He was immediately assigned to the design and testing for thermal control of the LRV Mobility Subsystem. This involved working with the A.C. Delco Electronics Division of GM, which was in charge of development of the Mobility Subsystem. Creel was involved with the thermal vacuum testing of the LRV brakes, fluid damper, and steering system in the fall of 1970.

This was followed by computer simulation modeling and thermal vacuum testing of a %-scale mobility subsystem at the Boeing facility in Kent, Washington. Creel related, "We almost didn't get the LRV mobility system tested due to the overheating and failure of a ground test motor which was used to power the treadmill with obstacles, over which the LRV system was driven. We had to work extra long hours one weekend in order to rig up a coolant system using copper tubing and good old water. With the test system repaired, the mobility system test proceeded very well. The test technicians had a healthy skepticism about this young thermal engineer and his computer performance predictions for motor temperatures.

(from left) John Young, Gene Cernan, Fred Haise, Charlie Duke, Tony England, Gordon Fullerton, and Don Peterson pose with the LRV qualification test unit at the Marshall Space Flight Center. 1 November 1971

As we approached the maximum expected test operation period one night, the technicians skeptically asked me what the maximum drive motor temperature would be while we were out to dinner. I answered that my prediction was a maximum temperature of 254 degrees F. When we returned from dinner, the technicians verified that the maximum temperature had actually been 255 degrees F. They were not so skeptical about the young thermal engineer after that.''

Hugh Campbell, who was LRV Lead Thermal Engineer, felt confident in putting Creel to work on additional modeling and thermal vacuum testing of the LRV forward chassis. This would include the batteries, Signal Processing Unit (SPU), Drive Control Electronics (DCE), Directional Gyro Unit (DGU) - indeed anything electrical having to do with the LRV. In addition, human factors involving the surface temperatures that the astronauts would come into contact with were another critical issue. This was the "time-temperature" constraint for all surfaces which might come into contact with the astronauts or their extra-vehicular mobility units (suits and backpacks).

These and other factors went into the preparation of the software thermal models used to help both in the thermal design of LRV electrical and other subsystems and also in verifying thermal performance for all expected storage and operating environments. Correlating these thermal models with the test data was very important and allowed these "clean" test models to be subsequently altered to match the expected operation on the Moon and to generate realistic temperature predictions. LRV surface optical properties (solar absorption and infrared emission) were regularly measured in order to adjust both the computer thermal models that took into account these factors and the internally-generated and externally-applied heat loads, to verify expected performance.

"A primary concern,'' according to Creel, "was to have the LRV thermal control system be responsive to the variations of driving and operation of the LRV's on the Moon and the need to fully support the astronauts during all nominal and contingency operations. We refined the thermal computer models for thermal control system verification and mission planning based on correlation with thermal vacuum test results. This included the full-up Qualification Test Unit tested in the vacuum chamber in Kent, Washington. There were dynamometers on each wheel and solar simulations at a Sun angle of sixty degrees, which exceeded the expected level for planned Moon missions. My thermal modeling for mission support was ultimately rewarded with receipt of the astronaut's 'Silver Snoopy' award. This was for simplifying a complex and cumbersome LRV thermal model into a much more responsive and useful thermal model for mission support.''

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