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Technical Paper

Thermal Performance of a Miniature Variable Conductance Loop Heat Pipe

Loop Heat Pipes (LHP) of different designs are currently used in aerospace applications worldwide. Historically, LHPs were considered primarily as candidates for high power, high adverse elevation and high heat flux applications such as deployable radiators for large satellites, thermal bus devices, high heat flux payloads, etc. A new look at the LHP technology was presented in 1998 (Ref. 1), and the miniature LHP concept was introduced to the industry. Because miniature LHPs are frequently serving as “thermal shunts” for payloads and instruments, controllability issues played an important role from the very beginning of their development. For instance, the electrical power that is available for thermal control of Mars rovers on the Martian surface is limited. Because of this limitation, the thermal control systems for the new generation of Mars rovers were required to be absolutely passive.
Technical Paper

Testing of a Loop Heat Pipe with Two Evaporators and Two Condensers

Most existing loop heat pipes (LHPs) consist of one single evaporator and one single condenser. LHPs with multiple evaporators are very desirable for cooling multiple heat sources or a heat source with large thermal footprints. Extending the current LHP technology to include multiple evaporators and multiple condensers faces some challenges, including operating temperature stability, adaptability of loop operation to rapid power and sink temperature transients, and sizing of the compensation chambers (CCs). This paper describes an overview of an extensive testing program for an LHP with two evaporators and two condensers. Tests performed include start-up, power cycle, sink temperature cycle, CC temperature cycle, and capillary limit. Test results showed that the loop could be started successfully in most cases, and the operating temperature was a function of the total heat load, heat load distribution between the two evaporators, condenser sink temperature and ambient temperature.
Technical Paper

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

Future planetary science missions planned for Mars are expected to be more complex and thermally challenging than any of the previous missions. For future rovers, the operational parameters such as landing site latitudes, mission life, distance traversed, and rover thermal energy to be managed will be significantly higher (two to five times) than the previous missions. It is a very challenging problem to provide an effective thermal control for the future rovers using traditional passive thermal control technologies. Recent investigations at the Jet Propulsion Laboratory (JPL) have shown that mechanical pump based fluid loops provide a robust and effective thermal control system needed for these future rovers. Mechanical pump based fluid loop (MPFL) technologies are currently being developed at JPL for use on such rovers. These fluid loops are planned for use during spacecraft cruise from earth to Mars and also on the Martian surface operations.
Technical Paper

Integrated Pump Assembly - An Active Cooling System for Mars Pathfinder Thermal Control

The Mars Pathfinder spacecraft which will be launched in December 1996 features an active cooling system for controlling the temperature of the spacecraft. This will be the first time that such a mechanical pump cooling system is used on an interplanetary or long duration flight (over two weeks) in space. The major element of the cooling system is the Integrated Pump Assembly (IPA). It uses centrifugal pumps to circulate liquid freon to transfer heat from spacecraft electronics to an external radiator. The IPA consists of redundant pumps, motor control electronics, thermal control valves, check valves, and an accumulator. The design and flight implementation of this pump assembly were accomplished in less than two years. This paper describes the design, fabrication, assembly, and testing of the IPA.
Technical Paper

High Temperature Mechanically Pumped Fluid Loop for Space Applications –Working Fluid Selection

Mechanically pumped single-phase fluid loops are well suited for transporting and rejecting large amounts of waste heat from spacecraft electronics and power supplies. While past implementations of such loops on spacecraft have used moderate operating temperatures (less than 60ºC), higher operating temperatures would allow equivalent heat loads to be rejected by smaller and less massive radiators. A high temperature (100 to 150ºC) mechanically pumped fluid loop is currently being investigated at the Jet Propulsion Laboratory (JPL) for use on future Mars missions. This paper details the trade study used to select the high temperature working fluid for the system and the initial development testing of loop components.
Journal Article

Design Description and Initial Characterization Testing of an Active Heat Rejection Radiator with Digital Turn-Down Capability

NASA's proposed lunar lander, Altair, will be exposed to vastly different external temperatures following launch till its final destination on the moon. In addition, the heat rejection is lowest at the lowest environmental temperatures (0.5 kW @ 4K) and highest at the highest environmental temperature (4.5 kW @ 215K). This places a severe demand on the radiator design to handle these extreme turn-down requirements. A radiator with digital turn-down capability is currently under study at JPL as a robust means to meet the heat rejection demands and provide freeze protection while minimizing mass and power consumption. Turndown is achieved by independent control of flow branches with isolating latch valves and a gear pump to evacuate the isolated branches. A bench-top test was conducted to characterize the digital radiator concept. Testing focused on the demonstration of proper valve sequencing to achieve turn-down and recharge of flow legs.
Technical Paper

An Experimental Study of the Operating Temperature in a Loop Heat Pipe with Two Evaporators and Two Condensers

This paper presents a comprehensive experimental study of the loop operating temperature in a loop heat pipe (LHP) which has two parallel evaporators and two parallel condensers. In a single evaporator LHP, it is well known that the loop operating temperature is a function of the heat load, the sink temperature and the ambient temperature. The present study focuses on the stability of the loop operating temperature and parameters that affects the loop operation. Tests results show that the loop operating temperature is a function of the total system heat load, sink temperature, ambient temperature, and heat load distribution between the two evaporators. Under most conditions, only one compensation chamber (CC) contains two-phase fluid and controls the loop operating temperature, and the other CC is completely filled with liquid. Moreover, as the test condition changes, control of the loop operating temperature often shifted from one CC to another.
Technical Paper

Active Control of the Operating Temperature in a Loop Heat Pipe with Two Evaporators and Two Condensers

This paper describes a test program on active control of the operating temperature in a loop heat pipe (LHP) with two evaporators and two condensers. Test results shoe that when the CCs were not actively controlled, the loop operating temperature was a function of the total heat load, heat load distribution among evaporators, condenser temperature and ambient temperature. Because of the many variables involved, the operating temperature also showed more hystereses than an LHP with a single evaporator. Tight operating temperature control can be achieved by controlling the compensation chambers (CCs) at a desired set point temperature. Temperature control was achieved by maintaining one or both CCs at the desired set point through cold biasing and external heating. Tests performed included start-up, power cycle, sink temperature cycle, CC temperature cycle, and capillary limit.