Spacecraft Thermal Control Handbook
Volume II: Cryogenics
Martin Donabedian
Chapter 1: Cryogenic Systems Overview
Although spacecraft with components that must be cooled to cryogenic temperatures of 2 to 150 K have been flown since the 1960s, recent years have seen a substantial growth in the emerging number of programs that include such spacecraft to service scientific, military, and weather observation missions. For example, the cooling of optics and detectors to reduce signal noise in infrared (IR) telescopes and the reduction of boil-off during long-term on-orbit storage of cryogenic fluids for propulsion systems and laser weapons are two of the principal applications of cryogenic cooling technologies for both today and the near future. A number of technologies can provide the cooling required for these and other applications; the choice depends on the desired temperature level, the amount of heat to be removed at that temperature, and the required operating life.
The graph in Fig. 1.1 gives an overview of which technologies are usually employed in each temperature/heat-load regime. (It assumes normal spacecraft mission durations on the order of one year or longer.) It was constructed by plotting data points from more than 60 systems that either have been fabricated and flown, have been tested, or have had a preliminary design proposed.
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Fig. 1.1. Cryogenic cooling methods. |
Radiators can be used, theoretically, down to about 60 K under ideal conditions. However, below about 100 K, their rejection capability falls dramatically because of the T4 nature of radiation heat transfer, and their overall feasibility is highly dependent on the spacecraft orbit, orientation, and attitude-control limitations. The lower radiating capability leads to a need for prohibitively large and heavy radiators, which may be too sensitive to environmental loads and heat leaks through support structures and insulation to be effective. Some programs plan on placing their spacecraft in orbits far from Earth or any other planet, and using large, highly insulated sun shields, to essentially eliminate environmental heating.
At higher temperatures, beginning at about 130 K, thermoelectric coolers are attractive for small cooling loads. For temperatures in the same range, but with higher total heat-rejection requirements, a combination of radiators and either heat pipes or pumped-liquid or pumped-gas loops may be needed to transport the heat and spread it out over a relatively large radiator.
The use of stored expendable systems provides a reliable and relatively simple method of cooling over a wide range of temperatures—from about 1.5 K (superfluid helium) to about 150 K (solid ammonia). These systems rely on the boiling or sublimation of a low-temperature fluid or solid to absorb a device's waste heat and reject it overboard in the vented gas.
As the heat-rejection requirement increases at low temperature (and in cases where a long mission duration is required, even at low heat rates), the weight of stored cryogen required becomes very large and active refrigerators become a more attractive option. Reliability, operating life, power, and consideration for vibration control are key issues. Several development programs offer the potential for use of these systems over a wide range of cooling requirements for extended durations.
What follows is a brief introduction to each of these principal cryogenic cooling technologies, which will be discussed in much greater detail in the remainder of the book.
Stored-Cryogen Cooling Systems
Stored-cryogen expendable-coolant systems use either cryogenic liquids in the subcritical or supercritical state, cryogenic solids, or high-pressure gas combined with a Joule-Thomson (JT) expansion valve system to provide cooling of spacecraft components. These technologies can provide cooling to temperatures ranging from near absolute zero to more than 300 K. The Infrared Astronomical Satellite (IRAS), which used stored liquid helium to cool an astronomical telescope, is shown in Fig. 1.2.
 Fig. 1.2. The IRAS satellite was cooled using stored liquid helium. |
In most cases, the required stored-cryogen cooling technology is well developed. The advantages of these systems are simplicity, reliability, relative economy, and negligible power requirements. The disadvantages are the systems' limited life, resulting from parasitic heat leakage, and the high weight and volume penalty for extended durations of use. Although high-pressure-gas storage systems with JT valves can overcome the long-storage limitation, the penalties associated with the storage of high-pressure gas and poor JT expansion efficiencies generally make system weights prohibitive as operating time increases. Stored-gas JT systems are thus useful only for special cases of short-term or intermittent cooling requirements.
Stored Liquid and Solid Cryogens
The primary limitations of the liquid-storage-type systems are the complex tank design (needed to minimize boil-off), the need for phase separation of subcritical fluids in the space environment, and the large weight and volume penalty for extended mission time.
An alternative to these systems is the use of stored cryogens in the solid state, which provides a higher heat content, higher density, and simpler system design than the liquid systems. The concept is schematically shown in Fig. 1.3. Limitations of solid systems include restrictions on detector mounting, specialized filling procedures, the need for a metal form or mesh to reduce temperature gradients as the solid dissipates, and more complex ground-hold considerations. The operating pressure of the cryogen must also be tightly controlled.
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Fig. 1.3. Use of a Joule-Thomson system to generate a liquid cryogen. |
High-Pressure-Gas Storage Systems
A second alternative to stored liquid cryogens is a JT cooler that utilizes the expansion of a high-pressure gas (i.e., one with pressure of 12 to 25 MPa, or even higher) through a JT valve, which results in cooling of the gas and, eventually, the formation of cryogenic liquid to be used as the coolant. The JT effect involves the ratio of temperature change to pressure change of an actual gas in the process of throttling or expansion (during a constant enthalpy process), without doing work or transferring heat.
Under normal pressure and temperature conditions, a perfect gas would provide no cooling effect or temperature change for a throttling process. However, in an actual gas under conditions of high pressure and/or low temperature, molecular forces cause a change in internal energy when the gas expands. The change in internal energy during the expansion process results in cooling of the gas. The cooled, expanded gas is passed back over the incoming gas to cool itself. This results in recuperative cooling.
The process continues until a bath of liquid at the cooling temperature of the gas begins to form at the orifice. For certain gases, this effect occurs only below a specific inversion temperature. Helium (45 K), hydrogen (204 K), and neon (250 K), for example, require precooling to the indicated temperature before the JT expansion cooling effect occurs. Most other gases, such as nitrogen, argon, and air, have inversion points well above room temperature, and they require no special precooling.
The JT cooler (Fig. 1.3) consists of a finned tube in the form of a coil, an orifice and orifice cap, and an outer shield or coil. The finned tube's very small inside diameter provides the large ratio of surface area to volume necessary for effective heat exchange. For a fixed-orifice cryostat, the flow will vary with pressure. Thus only one pressure will provide just the desired refrigeration, as shown by Fig. 1.4. Traditional JT systems have suffered because of the inefficient matching of the pressure to the desired refrigeration. In recent years, however, self-regulating (or variable-orifice-size) cryostats have increased the capability for correct matching, or for compensation for changes in heat load or the tendency to gradually clog. These capabilities also allow for high flow during cooldown and nominal flow during steady-state operation.
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Fig. 1.4. Cooling capacity versus inlet pressure for an ideal Joule-Thomson cooler. |
The limiting life factor for long-life storage systems is the heat leak through the structural support members, plumbing, and electrical wires, and other miscellaneous heat shorts. To reduce these heat loads, retractable supports to be activated after launch have been proposed, but a flight-qualified design is yet to be built. For the near term, hybrid concepts utilizing a mechanical refrigerator to reduce the external shell temperature provide a practical alternative.
Cryogenic Radiators
In spaceborne applications, temperatures as low as about 60 K can be achieved by a suitably designed radiant cooler radiating into space. The low effective sink temperature of deep space provides an ideal environment for the passive radiant cooling of IR detectors and related devices to the temperatures indicated. This approach involves no moving parts, provides inherently long life, and requires no power.
The effective temperature of deep space is approximately 3 K. One or more detectors mounted to a suitably sized cold plate of high emissivity can radiate to this sink. The high vacuum of orbital altitudes eliminates the effect of convective heating in such an application; the cold plate, however, must be shielded (with a cone, for example) against heat from direct sunlight and, in the case of low-altitude orbits, against the heat inputs from thermal emission and reflected sunlight from the planet and its atmosphere (Fig. 1.5). Furthermore, the cold plate must be thermally shielded from the parent spacecraft. These considerations usually result in a passive cooler design that is tailored to a particular spacecraft system. A Raytheon Santa Barbara Research Center radiant cooler is shown in Fig. 1.6.
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Fig. 1.5. Conic shield is used to reflect environmental heat loads away from radiator. |
The type of orbit (e.g., near-polar or equatorial), orbit altitude, heat load, temperature, orientation of the spacecraft relative to a planet or the sun, and the location of the radiator all significantly influence radiator design. Ideally, the radiant-cooler patch (i.e., the detector mounting surface) is large enough so that thermal inputs (e.g., Joule heat, lead conduction, and radiative input through the optics) are small compared to the total power radiated by the patch at its equilibrium temperature. This permits flexibility in the optical and electrical design.
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Fig. 1.6. Raytheon radiant cooler. |
Cryogenic Refrigerators (Cryocoolers)
For applications requiring substantial cooling over an extended period, mechanical refrigerators, or cryocoolers, are usually the preferred design solution. The primary considerations that differentiate spacecraft-based refrigerators from those used on the ground or in aircraft are
- the need for high efficiency in order to minimize weight and power
- the need for reliable operation over extended periods without maintenance or repair
- (usually) the requirement to minimize vibration or other mechanical disturbances that could affect sensor pointing or violate jitter requirements
Practical refrigeration cycles for spaceborne use include the Stirling, reverse-Brayton, closed Joule-Thompson (JT), the pulse tube, and the chemisorption compression/JT expansion systems under development by JPL and others. Table 1.1 provides a historical summary of some cryocoolers that have been flown in space applications, including the host program, sponsor and manufacturer, cycle type, power required, and number of hours of successful operation.
Chapters 7 through 13 provide detailed discussions of the thermodynamic principles behind the various cryocooler cycles, the status and design details of specific coolers, and the many issues involved when a cryocooler is integrated into a spacecraft or instrument.
