Pradeep Bhandari

Mars Science Laboratory thermal control architecture

The Mars Science Laboratory (MSL 1 ) mission to land a large rover on Mars is being planned for Launch in 2009. As currently conceived, the rover would use a Multimission Radioisotope Thermoelectric Generator (MMRTG) to generate about 110 W of electrical power for use in the rover and the science payload. Usage of an MMRTG allows for a large amount of nearly constant electrical power to be generated day and night for all seasons (year around) and latitudes. This offers a large advantage over solar arrays. The MMRTG by its nature dissipates about 2000 W of waste heat. The basic architecture of the thermal system utilizes this waste heat on the surface of Mars to maintain the rovers temperatures within their limits under all conditions. In addition, during cruise, this waste heat needs to be dissipated safely to protect sensitive components in the spacecraft and the rover. Mechanically pumped fluid loops 2 are used to both harness the MMRTG heat during surface operations as well as reject it to space during cruise. This paper will describe the basic architecture of the thermal control system, the challenges and the methods used to overcome them by the use of an innovative architecture to maximize the use of heritage from past projects while meeting the requirements for the design.

Mars Pathfinder Active Heat Rejection System: Successful Flight Demonstration of a Mechanically Pumped Cooling Loop

One of the new technologies successfully demonstrated on the recent Mars Pathfinder mission was the active Heat Rejection System (HRS). This system consisted of a mechanicaily pumped cooling loop, which actively controlled the temperatures of the various parts of the spacecraft. A single phase Refrigerant 11 liquid was mechanically circulated through the lander and cruise electronics box heat exchangers. This liquid transferred the excess heat to an external radiator on the cruise stage. This is the first time in unmanned spacecraft history that an active heat rejection system of this type has been used on a long duration spacecraft mission. Pathfinder was launched in December 1996 and landed on the Martian surface on July 4, 1997. The system functioned flawlessly during the entire seven months of flight from Earth to Mars. A life test set up of the cooling loop was used to verify the life of the system. The life test system was run for over 14, 000 hours before complete examination of the components used in the life test was made. Some of the components used in the system were tested in the life test set up. The results from the life test loop indicate no major issues that would hinder the pumped loop operation for many more years.

Sizing and Dynamic Performance Prediction Tools for 20 K Hydrogen Sorption Cryocoolers

Two continuous operation 18 K/20 K sorption coolers are being developed by the Jet Propulsion Laboratory (JPL) as a NASA contribution to the European Space Agency (ESA) Planck mission that is currently planned for a 2007 launch. The individual sorption coolers will each be capable of providing a total of about 200 mW of cooling at 18 K and 1.4 Wat 20 K given passive radiative pre-cooling at 50 K. The hydrogen sorption coolers will directly cool the Low Frequency Instrument HEMT amplifiers to approximately 20 K and will also serve to intercept parasitics and pre-cool a RAL 4.5 K closed-cycle helium J-T cooler to 18 K for the separate High Frequency Instrument. To design the Planck sorption coolers a general sizing model and a detailed performance prediction model have been developed.

Mechanically Pumped Fluid Loop Technologies for Thermal Control of Future Mars Rovers

Mechanically pumped fluid loop has been the basis of thermal control architecture for the last two Mars lander and rover missions and is the key part of the MSL thermal architecture. Several MPFL technologies are being developed for the MSL rover include long-life pumps, thermal control valves, mechanical fittings for use with CFC-11 at elevated temperatures of approx.100 C. Over three years of life tests and chemical compatibility tests on these MPFL components show that MPFL technology is mature for use on MSL. The advances in MPFL technologies for MSL Rover will benefit any future MPFL applications on NASA s Moon, Mars and Beyond Program.

Mars Science Laboratory Rover System Thermal Test

On November 26, 2011, NASA launched a large (900 kg) rover as part of the Mars Science Laboratory (MSL) mission to Mars. The MSL rover is scheduled to land on Mars on August 5, 2012. Prior to launch, the Rover was successfully operated in simulated mission extreme environments during a 16-day long Rover System Thermal Test (STT). This paper describes the MSL Rover STT, test planning, test execution, test results, thermal model correlation and flight predictions. The rover was tested in the JPL 25-Foot Diameter Space Simulator Facility at the Jet Propulsion Laboratory (JPL). The Rover operated in simulated Cruise (vacuum) and Mars Surface environments (8 Torr nitrogen gas) with mission extreme hot and cold boundary conditions. A Xenon lamp solar simulator was used to impose simulated solar loads on the rover during a bounding hot case and during a simulated Mars diurnal test case. All thermal hardware was exercised and performed nominally. The Rover Heat Rejection System, a liquid-phase fluid loop used to transport heat in and out of the electronics boxes inside the rover chassis, performed better than predicted. Steady state and transient data were collected to allow correlation of analytical thermal models. These thermal models were subsequently used to predict rover thermal performance for the MSL Gale Crater landing site. Models predict that critical hardware temperatures will be maintained within allowable flight limits over the entire 669 Sol surface mission.

Evaluation of Hydride Compressor Elements for the Planck Sorption Cryocooler

Hydrogen sorption cryocoolers are being developed for the European Space Agency Planck mission to provide nominal 19 K cooling to instruments for measuring the temperature anisotropy ofthe cosmic microwave background with extreme sensitivity and resolution. The behavior of the metal hydride sorbent beds used in the compressor dominates both the performance and reliability of these sorption cryocoolers. The compressor elements have been designed to minimize their input power requirements and to enhance durability during extended temperature cycling while in operation. The Lanthanum-Nickel-Tin alloy LaNi4.78Sn0.22 in the sorbent beds circulates and compresses the hydrogen refrigerant gas while the ZrNi alloy is used to provide variable pressure in the gas-gap heat switches for each compressor element. Characterization tests have been performed on the compressor elements built for an Engineering Bread Board (EBB) cooler to evaluate the behavior of both the sorbent bed and gas-gap switches under conditions simulating flight operation. These results provide a basis for predicting EBB cooler performance and to identify any design deficiencies prior to fabrication of the flight compressor elements. In addition, experiments were done on compressor elements that had been operated up to several thousand cycles to assess degradation in the sorbent hydride and reduction in the effectiveness of the gas gap switches in reducing parasitic heat losses

From Concept-to-Flight: An Active Active Fluid Loop Based Thermal Control System for Mars Science Laboratory Rover

The Mars Science Laboratory (MSL) rover, Curiosity, which was launched on November 26, 2011, incorporates a novel active thermal control system to keep the sensitive electronics and science instruments at safe operating and survival temperatures. While the diurnal temperature variations on the Mars surface range from -120 C to +30 C, the sensitive equipment are kept within -40 C to +50 C. The active thermal control system is based on a single-phase mechanically pumped fluid loop (MPFL) system which removes or recovers excess waste heat and manages it to maintain the sensitive equipment inside the rover at safe temperatures. This paper will describe the entire process of developing this active thermal control system for the MSL rover from concept to flight implementation. The development of the rover thermal control system during its architecture, design, fabrication, integration, testing, and launch is described.

Initial test performance of a closed-cycle continuous hydrogen sorption cooler, the Planck sorption breadboard cooler

The Jet Propulsion Laboratory (JPL) is developing a continuous hydrogen sorption cryocooler for the ESA Planck mission, which will measure the anisotropy in the cosmic microwave background. The sorption cooler is the only active cooling for one of the instruments and it is the first of a chain of three coolers for the other instrument on Planck. The cooler has been designed to provide a cooling capacity of 1.1 W at a temperature below 20 K with a temperature stability requirement of 100 mK over a compressor cycle (667 s). The performance of these coolers depends on many operating parameters (such as the temperatures of pre-cooling thermals shield and the warm radiator and their fluctuations) and compliance can only be assessed through a detailed testing of the whole cooler and its interfaces. A breadboard sorption cooler (EBB) is undergoing testing to verify the flight cooler design performance in terms of input power, cooling power, cold end temperature and cold end temperature fluctuations, heat load on the pre-cooling stages, and heat flow to the warm radiator. We present initial test data compared to predictions based on previously performed component tests.

CFD analysis for assessing the effect of wind on the thermal control of the Mars Science Laboratory Curiosity Rover

The challenging range of landing sites for which the Mars Science Laboratory Rover was designed, requires a rover thermal management system that is capable of keeping temperatures controlled across a wide variety of environmental conditions. On the Martian surface where temperatures can be as cold as -123C and as warm as 38C, the rover relies upon a Mechanically Pumped Fluid Loop (MPFL) Rover Heat Rejection System (RHRS) and external radiators to maintain the temperature of sensitive electronics and science instruments within a -40C to 50C range. The RHRS harnesses some of the waste heat generated from the rover power source, known as the Multi Mission Radioisotope Thermoelectric Generator (MMRTG), for use as survival heat for the rover during cold conditions. The MMRTG produces 110 W of electrical power while generating waste heat equivalent to approximately 2000 W. Heat exchanger plates (hot plates) positioned close to the MMRTG pick up this survival heat from it by radiative heat transfer. Winds on Mars can be as fast as 15 m/s for extended periods. They can lead to significant heat loss from the MMRTG and the hot plates due to convective heat pick up from these surfaces. Estimation of this convective heat loss cannot be accurately and adequately achieved by simple textbook based calculations because of the very complicated flow fields around these surfaces, which are a function of wind direction and speed. Accurate calculations necessitated the employment of sophisticated Computational Fluid Dynamics (CFD) computer codes. This paper describes the methodology and results of these CFD calculations. Additionally, these results are compared to simple textbook based calculations that served as benchmarks and sanity checks for them. And finally, the overall RHRS system performance predictions will be shared to show how these results affected the overall rover thermal performance.

Design of Accumulators and Liquid/Gas Charging of Single Phase Mechanically Pumped Fluid Loop Heat Rejection Systems

For single phase mechanically pumped fluid loops used for thermal control of spacecraft, a gas charged accumulator is typically used to modulate pressures within the loop. This is needed to accommodate changes in the working fluid volume due to changes in the operating temperatures as the spacecraft encounters varying thermal environments during its mission. Overall, the three key requirements on the accumulator to maintain an appropriate pressure range throughout the mission are: accommodation of the volume change of the fluid due to temperature changes, avoidance of pump cavitation and prevention of boiling in the liquid. The sizing and design of such an accumulator requires very careful and accurate accounting of temperature distribution within each element of the working fluid for the entire range of conditions expected, accurate knowledge of volume of each fluid element, assessment of corresponding pressures needed to avoid boiling in the liquid, as well as the pressures needed to avoid cavitation in the pump. The appropriate liquid and accumulator strokes required to accommodate the liquid volume change, as well as the appropriate gas volumes, require proper sizing to ensure that the correct pressure range is maintained during the mission. Additionally, a very careful assessment of the process for charging both the gas side and the liquid side of the accumulator is required to properly position the bellows and pressurize the system to a level commensurate with requirements. To achieve the accurate sizing of the accumulator and the charging of the system, sophisticated EXCEL based spreadsheets were developed to rapidly come up with an accumulator design and the corresponding charging parameters. These spreadsheets have proven to be computationally fast and accurate tools for this purpose. This paper will describe the entire process of designing and charging the system, using a case study of the Mars Science Laboratory (MSL) fluid loops, which is en route to Mars for an August 2012 landing.