NICMOS Cooling System

The HOST mission on board Space Shuttle Discovery, STS-95, has demonstrated the zero-gravity operation of the reverse turbo-Brayton cycle cryocooler in the NCS. As a result of this successful demonstration, this system will be used to cool the NICMOS instrument after SM3B (2000).

These cryogenic coolers allow longer operational lifetimes than is presently possible with expendable cryogenic systems. It is expected that with the NCS installed on HST, NICMOS's operational life will be extended by at least five years beyond SM3B. The NCS works using three fluid loops. The first loop is the circulator loop. When installed in the HST, gas will be circulated in this loop between the cooling system and the inside of the NICMOS cryostat. This carries heat away from the cryostat and keeps the detectors at their operating temperature (73 Kelvin or -200 C). For the HOST mission, the NICMOS cryostat was replaced by a simulator that contains an exact replica of the plumbing and interfaces in the NICMOS. This device is called the NICMOS Cooling Loop Simulator (NCLS).

NICMOS Cryocooler

The second loop is the primary cooling loop. It contains a compressor, a turboalternator, and two heat exchangers. This loop implements a reverse-Brayton thermodynamic cycle, and provides the cooling power for the entire system. It is the heart of the NICMOS cryocooler.

In generating this cooling power, a significant amount of heat is also generated (up to 500 Watts). This heat is carried away from the primary cooling loop by the third loop in the NCS, called the Capillary Pumped Loop. This loop connects the main heat generating component, the compressor, with the external radiator, which radiates the heat into space. The heat is carried by evaporating ammonia on the hot end, and recondensing it at the cold end of the CPL.NICMOS Radiator

The Electronics Support Module (ESM) controls the major functions of the NCS. It contains an 8051 microprocessor, which implements the control laws for cooler functions including compressor, turboalternator and circulator speed. It also controls the CPL reservoir temperatures, regulating the quantity of heat transported to the radiator by the CPL. In the background, it collects and monitors critical NCS telemetry as well as general housekeeping telemetry, and relays commands to the NCS subsystems.