Space borne Cooling Systems

Space borne Cooling Systems

1. Some spacecraft payloads require cooling to low temperatures.
2. The most common types of cooled instruments include IR-sensor focal planes and optics, as well as Low-noise amplifiers for RF receivers.
3. Several devices are available for cooling such applications, including radiators, stored-cryogen cooling systems, and refrigerators.
4. The Defense Support Program (DSP)satellite uses a system of radiators to cool the optics and focal plane.

Parts in DSP sensor –

1. Thermoelectric coolers
2. Helium circulators
3. Phase Change Materials (PCM) canisters
4. Forward facing radiators (FFR)
5. TCS electronics unit and power supply

Concept -

• Helium circulator transports focal plane/PCM heat to nonsolar illuminated forward-facing radiator (FFR)

• Circulator turned off when FFR solar illuminated-focal plane heat melts PCM

• PCM is refrozen when circulator resumed refrozen when circulator resumed

Working:-

1. The optical elements (mirrors) and the telescope enclosure and baffles are cooled passively by covering the telescope enclosure with low-absorptance, high-emittance quartz mirrors.
2. Cooling the optics and enclosure reduces the amount of IR radiation emitted from those surfaces.
3. Without this cooling, the sensors at the focal plane would not be able to see their targets over the IR "noise" created by the telescope itself.
4. The focal-plane assembly is connected to a phase-change-material(PCM) heat sink and a passive radiator by a pumped-helium loop.
5. The operating principle of this system is the transporting of heat from the focal plane and PCM to the radiator by means of a pumped-helium loop during the half of the orbit when the sun does not shine on the radiator. During the other half-orbit, solar illumination heats the radiator to temperatures well above those of the focal plane.
6. To avoid a focal plane temperature rise, the helium circulation is shut off, effectively decoupling the radiators, and the heat loads from the focal plane are stored in the PCM.
7. When the sun moves behind the vehicle, the circulator is turned back on to reject the focal-plane heat and the excess heat stored in the PCM.
8. Minimizing heat leaks into the forward-facing radiator by the use of MLI and low-conductance supports on the back side is critical to achieving low-temperature performance.
9. Even small heat leaks into the radiator during the shadowed halforbit can raise its temperature considerably from 173 K. (Because of the T 4 nature of radiation-heat transfer, only one-fifth as much heat is needed to raise radiator equilibrium temperatures one degree at 173 K than at room temperature.)
10. For lower-temperature radiators the sensitivity is even greater; for example, the sensitivity is greater by a factor of 50 at 80 K than at room temperature. For this reason, low-temperature radiators are extremely sensitive to heat loads from the environment or heat leaks from the spacecraft.

Devices requiring cooling to very low temperatures and having limited lifetime requirements (less than 1 or 2 years) usually employ stored-cryogen cooling systems. Designs for such devices use a cryogenic fluid or solid stored in a debar as a heat sink to absorb waste heat from the device and maintain it at a low temperature. An example of such a system is the Infrared Astronomical Satellite (IRAS). The cryogen in this case is 70 kg of helium stored at 1.85 K in a tank that is wrapped around the satellite's telescope assembly.

Spacecraft Cryogenic Cooling

The HgCdTe detector arrays are required to be cooled to 80K during operation. A number of methods for producing cryogenic temperatures

in spacecraft have been used in the past and were considered for CIRS(composite infrared spectrometer).

1. Stored Cryogens

The simplest way of attaining cryogenic temperatures on a spacecraft is to carry a dewar of a cryogenic material in either liquid or solid form.The detectors to be cooled would then be mounted on the wall of the dewar to use the cryogen as a heat sink. Suitable cryogens could be for example solid argon or liquid helium. The temperature at which the cryogen boils or sublimes is varied by throttling the exhaust valve to vary the pressure above the cryogen.

       This method of cooling was used successfully by the CLAES instrument on the UARS spacecraft. Stored cryogen coolers are

unsuitable for use on the Cassini mission because of the extremely long mission duration - CLAES was a heavy, earth orbiting instrument which still had only an 18 month lifetime so making a stored cryogen system

which would weigh only 2.5kg (the mass of the CIRS cooler) and which would have sufficient life time for the Cassini mission would be

impractical.

2. Mechanical Coolers

A number of mechanical coolers have been flown to produce 80K temperatures on space borne instruments for example the ISAMS instrument on UARS used two Stirling cycle coolers producing approximately 800mW of cooling at 80K. They consumed approximately SOW of electrical power and weighed over 4kg. This level of cooling is considerably more than the 200mW total cooling power needed for the CIRS 80K stage.

           The mechanical coolers currently available commercially have an operating lifetime which is too short for use on Cassini, and would create vibration problems on CIRS. A large amount of development work would therefore be required to reduce the power requirement and increase the operating life time to levels usable in the outer solar system.

3. Radiative Coolers

Radiative coolers are simple passive cooling devices. The principle of operation is that a high emissivity radiator is directed towards space which has an effective temperature of approximately 4K and an emissivity of unity.

For deep space missions radiative coolers have a number of advantages over the other types of coolers discussed for cooling detectors:-

• They require no electrical power to provide cooling. This is especially important in the outer solar system where solar arrays are not usable so the electrical power must be provided by thermal generators (RTG's).
• There are no moving parts, giving an effectively unlimited lifetime. The main limitation is the degradation of the surfaces due to contamination or radiation.
• Their mass is significantly lower than an equivalent stored cryogen cooling system.