Cryocooler With Cold Compressor for Deep Space Applications

The unique built-in design features of the proposed mini pulse tube cryocooler avoid all thermal expansion issues enabling it to operate within a cold, 150 K environment. As such, the cooler addresses the need to prevent boiloff of cryogenic propellants on long duration remote missions. Due to its high heat capacity regenerator matrix, the cooler has a high efficiency and a small footprint, making its launch mass minimal. The coherent regenerator matrix configuration prevents movement and so prevents degradation over time. Due to its unique compressor being cold-tolerant down to 150 K the cooler can keep running even where the sun is so far away and dim that the energy captured by the spacecraft cannot keep the compressor warm. The compressor being designed to run cold and on minimal input power allows it to run at the inherently low solar intensity, which seriously depresses solar cell power generation. In all, our cryocooler innovation is an enabling technology for far-flung missions that have the need to preserve mission propellants in their liquefied state until needed to maneuver near a destination or a midway point. The design concept calls for: 1) Using regenerator materials from a recently developed class of high heat capacity rare earth alloys; 2) A compressor and coldhead design optimized for a low temperature heat sink; 3) Minimizing the known losses in the pulse tube proper. The cryocooler design will be possible due to the long standing cryocooler design and manufacture heritage of the team members. Phase I will verify the low temperature capabilities down to 150 K of an available mini compressor, and, will put forth the design of the crucial components of a complete 0.3 W at 35 K cryocooler targeted to run in a 150 K environment. Phase II will build and test a complete prototype cryocooler that is small in size and power consumption enabling long durations missions to planetary objects at remote locations within the solar system.

Pertinent NASA projects include missions to the outer planets and their moons, such as Europa, Titan, Neptune, and Triton. All of these missions need to carry considerable propellants for many years to be used in braking and/or orbital insertion maneuvers at the target location(s). High thrust propulsion (acceleration comparable to planetary surface gravitational acceleration) is required for these near planetary maneuvers. Significant benefit can be gained from improved mission performance, which is greatly impacted by propulsion system mass. Propellant is usually the predominant contributor to the mass of such chemical systems. Therefore, the largest potential system and mission performance gains are likely from improved specific impulse (ISP) by using cryogenic propellants. The challenges associated with cryogenic propellant use include the reduction of propellant boil-off so that the system is not penalized by the additional propellant mass required to accommodate typical boil-off. The availability of a low input power, low mass cryocooler will make this option more attractive and attainable. Madison CryoGroup's intent is to design the cryocooler for a low reject temperature, which makes it highly applicable to missions to remote locations and orbits that have a low level of solar input. Depending on the extent of radiation shielding applied, even earth orbiting systems may benefit from the innovation.

The design and construction process is targeted to produce a space-born cryocooler that will have a host of qualities in-demand for space missions. However, its small system size, efficient performance, long-life reliability, low input power and low vibration all make this cryocooler a high-value product that meets the demand of businesses aiming to deliver truly high-quality products for systems that require low failure rates. The proposed cooler is operable over a wide range of loads and temperatures. Several government agencies outside NASA (MDA, Air Force) and various commercial entities have interest in integrating small and efficient high-frequency cryocoolers into their equipment. For the proposed technology there are many potential business and civilian applications that require compact, reliable and efficient cryogenic cooling, such as: Cryopumps for semiconductor manufacturing, Superconducting magnets for MRI systems, SQUID magnetometers for heart and brain studies, HTS filters for the communication industry, Superconducting electronics and Liquefaction of industrial gases. Even for large system, in which one might apply distributed cooling loop schemes, our small cryocooler may be advantageous by installing multiple such discrete active coolers at strategic locations, when high reliability is the more pertinent feature.

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