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Astronomical News - September 1998


From:   Ron Baalke
Sent:   Monday, August 31, 1998 8:04 PM
To:     astro@lists.mindspring.com
Subject:   [ASTRO] The NICMOS Cryocooler
[From July 1998 issue of Space Telescope Science Institute Newsletter.]
The NICMOS Cryocooler
Editor's note: The following has been adapted from the HST Newsletter issued
by the HST Project Office at Goddard Space Flight Center, with permission.
A high-tech cooler for HST will join John Glenn and the rest of the STS-95
crew during their nine-day mission this October.  The cooler is an experiment
as part of HOST, the HST Orbital Systems Test.  If this test is successful,
the cooler will be installed on HST during the third servicing mission as a
means of extending the life of NICMOS.  The solid nitrogen cryogen now in
NICMOS is expected to run out by mid-November of this year.
The device is a reverse turbo-Brayton cycle cooler.  It is powered by a
compressor with a tiny turbine running at up to 300,000 RPM.  Robotic
electron discharge milling (EDM) machines are used to micro-machine the
aerodynamic portions of the cooler in order to produce well-balanced
components that produce no detectable vibration.  The system also uses a
miniature cryogenic circulator to remove heat from the NICMOS dewar.
This device is capable of high cooling capacity (> 10W at 60K), extremely
low vibration, and high reliability (> 15 years with proven gas bearings).
It provides first-stage cooling for advanced cryogenic systems, and it serves
as a direct replacement for liquid or solid nitrogen-based systems.  The
cooler is also rated as safe for Shuttle and Space Station operations.  The
producer is Creare, Inc., of Hanover NH.
STS-95 will be the first flight of such a cooler.  During the nine-day
mission, engineers will evaluate the cooler's performance, and, if it is
successful, will install it on HST.
The NICMOS Cooling System (NCS) has three cooling loops.  The first is the
circular loop which interfaces directly to the NICMOS dewar.  During the HOST
mission, the circular loop will connect to a controllable thermal load to
simulate the NICMOS dewar.  This NICMOS Cooling Loop Simulator (NCLS)
consists of a simulated NICMOS dewar aft dome and has exactly the same parts,
materials, design, and interface representation as the cooling loop in NICMOS.
The second loop is the Primary Cooling Loop which contains a turbo compressor,
a turbo-alternator, and heat exchangers to the circulator loop and capillary
pumped loop.  The third loop is this Capillary Pumped Loop which contains a
heat exchanger at the primary cooling loop interface to transport the heat to
an external radiator.
Two electronics units are needed as well.  The NICMOS Cryocooler and
Electronics Support Module runs the compressor/alternator loop and the
circular loop.  The Aft Shroud Cooler Controller provides temperature,
pressure, and power monitoring, as well as control of the capillary pumped
During the HOST mission, the design of the NICMOS Cryocooler will be validated
by cooling the NCLS to 70 K with short-term stability at the cooling loop
interface of 0.1 K.
The NICMOS Cryocooler is now undergoing intensive laboratory testing.  An
initial test verified that the device can, in fact, produce 11 W of cooling
at 60 K.  Further tests will mate the cooler to the simulator, and verify
that the cooler works as expected with a NICMOS surrogate.
If the NICMOS Cryocooler is successful, it can at least double the useful
lifetime of this powerful infrared instrument.
[NOTE: An image showing a similar cryocooler is available at the Creare
website, http://www.creare.com/cryogenics.html]



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