Surface tension confined liquid cryogen cooler

A cryogenic cooler is provided for use in craft such as launch, orbital and space vehicles subject to substantial vibration, changes in orientation and weightlessness. The cooler contains a small pore, large free volume, low density material to restrain a cryogen through surface tension effects during launch and zero-g operations and maintains instrumentation within the temperature range of 10.degree.-140.degree. K. The cooler operation is completely passive, with no inherent vibration or power requirements.

ORIGIN OF THE INVENTION 
The invention described herein was made by an employee of the United States 
Government and may be manufactured and used by or for the government for 
governmental purposes without the payment of the royalties thereon or 
therefor. 
FIELD OF THE INVENTION 
The present invention relates generally to coolers having containers or 
dewars for cryogens and more particularly to containers for cryogenic 
liquid coolers useful in applications involving the cooling of 
instrumention in space. 
BACKGROUND OF THE INVENTION 
The requirements of satellite and space probe borne super cooled 
instrumentation for cryogenic cooling fluids necessitate provision for 
containing the necessary cryogen supply during launch and for periods up 
to a year or more, thereafter. An example of such an instrument is an 
infrared sensor which requires cryogenic liquid cooling to obtain optimum 
infrared radiation sensitivity. Such on-orbit instrumentation must be 
maintained at temperatures of typically 10.degree.-140.degree. K. The use 
of cryogens avoids the power usage and complexity of a powered cooling 
system. 
Prior art orbital cryogenic systems required maintenance of the cryogen in 
the frozen state. If the cryogen is allowed to liquify, the vent port of 
the cooler can become blocked with liquid, resulting in the liquid being 
immediately pumped out to space, depleting of cryogen and introducing 
safety hazards at the vent exhaust. This condition can readily occur due 
to vibration during launch and weightlessness during orbit. Freezing of 
the cryogen requires cooling coils, coolant supply, and regulation 
equipment and instrumentation to assure operators that the cryogen is 
maintained in a frozen state, adding weight and complexity to the 
spacecraft. Since the frozen cryogen must be kept at a vapor pressuer 
below its triple point, generally below one atmosphere of pressure, a 
pumping system must be incorporated if the solid cryogen is to be 
maintained on the launch pad beyond the limited amount of time before heat 
leak of the system results in cryogen melting. These pumping systems are 
heavy, require power, add complexity to the system design and operation, 
decrease system reliability, and create safety problems. Without such 
pumping systems, the launch vehicle carrying the cryogenic device can 
remain on the launch pad for only a limited amount of time without 
servicing of the frozen cryogen dewar. Procedures to freeze and subcool 
the cryogen also add complexity and time to launch pad operations, which 
are normally time and safety critical. Another drawback of the frozen 
cryogen system is that its cryogen cannot be replenished in orbit or in 
space by service vehicles, a requirement if sensors are to remain useful 
beyond a relatively short time in space. The above-mentioned disadvantages 
and limitations of the prior art frozen cryogenic system could be overcome 
if a practical liquid cryogen system could be provided. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
cryogenic cooling system for cooling space based super cooled instruments 
wherein the requirement for maintaining the cryogen in the frozen state is 
eliminated. 
It is another object of the invention to provide an improved cryogenic 
dewar system capable of operating with a liquid cryogen supply while 
avoiding the chance of uncontrolled loss of liquid through the vent under 
launch and orbital conditions. 
It is a still further object of the invention to provide an improved 
cryogenic cooling system which can remain on the launch pad for extended 
periods of time without provision for cooling coils and pumping systems, 
or cooler servicing. 
The foregoing and other objects are accomplished by providing a cooler, 
according to the present invention, containing a high surface area, low 
density open cell material such as ceramic or carbon "sponge" 
substantially throughout the contained volume therein. When the dewar 
according to the present invention is filled with cryogenic fluid, the 
cryogenic is acquired by the sponge material and held in place due to the 
surface tension properties of the cryogen. This technique has been used in 
cryogenically frozen biomedical specimen shipping containers for 
terrestrial use; however, these shipping containers do not use zero 
gravity effects to favorably orient the cryogen, they do not directly use 
the cooling power of the liquid phase of the cryogen for precise 
temperature control, and they do not use a cold finger to allow cooling of 
remote instrumentation which is not actually situated with the dewar. All 
of these capabilities are original and critical to the operation of this 
invention. In the present invention, the liquid cryogen is kept away from 
the vent while the dewar is undergoing launch or zero-gravity operations 
and is forced to make good thermal contact with an internal cold finger 
inside the dewar. The "sponge" filled dewar according to the present 
invention overcomes the above-mentioned disadvantages of the prior art 
system in an inherently simple, reliable, and inexpensive device which 
will result in reduced costs, enhanced reliability and safety, and fewer 
ground servicing requirements for the launch vehicle. The inventive cooler 
system will provide serviceability on-orbit, which will be a wholly new 
capability for space borne cryogenic systems. This serviceability is 
extremely important to planned long duration on-orbit facilities. On the 
launch pad, the liquid cooler system can be replenished, a capability that 
is advantageous where long pre-launch delays are common, such as for the 
manned space shuttle.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the figure, which illustrates a surface tension contained 
liquid cryogen cooler 10 according to this invention. Cooler 10 is formed 
by a generally cylindrical inner tank 12 having upper wall 14, sidewall, 
16, and lower wall 18 defining an interior space 20. Outer vacuum shell 22 
is located exterior to and generally conformal to inner tank 12 and is 
made up of outer wall 24 and contains insulation layer 26. Insulation 
layer 26 is located in intimate contace with inner tank 12 and is spaced 
inward from outer wall 24 to form vacuum void 28. Inner tank 12 is secured 
within vacuum shell 22 by means of a low thermal conductivity inner tank 
support system 30, shown here as a strap, but which may also be struts, 
truss, or beam. Mounting rings 31 attached to the outer vacuum shell 22 
allow the cooler 10 to be attached to the spacecraft containing the 
instrumentation to be cooled. 
Cold finger 32 is mounted along the center axis of inner tank 12. Cold 
finger 32 is generally cylindrical in shape and extends from outside 
cooler 10 through inner tank 12 into and along the axis of cylindrical 
vacuum sleeve jacket 33. Cold finger 32 is effective to transfer heat from 
exterior sources, such as instrument detectors, along its length to the 
liquid cryogen 34 contained within the porous sponge 36. The liquid 
cryogen 34 is shown surrounding the cold finger 32 in a typical zero-g 
orientation. 
Sponge 36 is located and extends substantially throughout interior space 20 
of inner tank 12 and is effective to maintain the contained liquid cryogen 
in a fixed position relative to the cooler 10 through the liquid's surface 
tension properties. 
Vent tube 38 is generally cylindrical and extends in fluid communication 
with vent void space 40 in sponge 36 located within the upper portion of 
interior space 20 through upper wall 14 of inner tank 12 and through outer 
vacuum shell 22 to the atmosphere and is effective to vent vaporized 
cryogen from cooler 10. A pressure regulator 42 is positioned within vent 
tube 38 to maintain system pressure at a desired level to maintain the 
cryogen in the liquid state, i.e., above its triple point, when vented to 
open space. Pressure regulator 42 may be an absolute type for maintaining 
a precise operating temperature, or a check valve, where variation in 
operating temperature is acceptable. Fill tube 44 is generally cylindrical 
and extends in fluid communication with fill flow relief space 46 through 
upper wall 14 of inner tank 12 and through outer vacuum shell 22 to allow 
filling inner tank 12 of cooler 10 with cryogenic fluid. A cryogenic fluid 
coupler 47 is included to allow repeated servicing of cooler 10. 
In operation, heat is transferred from a satellite mounted sensor through 
cold finger 32 to the liquid cryogen maintained within cooler 10. 
Vaporized cryogen resulting from this heat transfer is vented through vent 
tube 38 to the atmosphere. 
Sponge 36 is a high surface area, low density open cell material which is 
preferably rigid and is capable of acquiring and holding in place liquid 
cryogen, due to the high surface tension of the cryogen with the sponge. A 
preferable sponge material is a micropore ceramic, composed of silicon, 
with free volume of 95% or greater, such as that designated as H.T.P.-6, 
which is available from Lockheed Missile and Space Company, Sunnyvale, CA. 
This material is more widely known for its use as thermal protection tile 
on the space shuttle. While the ceramic sponge material is remarkably 
durable, it is composed of brittle microscopic fibers which could be a 
source of particulate contaminates in the vent gas. A variety of steps can 
be taken to avoid vent gas contamination. That shown is a conventional 
filter 48 on the vent line 38 upstream of pressure regulator 42. 
Tank 12, shell 22 and other structural features can be made of suitable 
metal, glass, composite or ceramic materials as is known in the art. In 
the embodiment shown, inner tank 16 and vacuum shell 24 is made of 
aluminum, support system 30 is a series of fiberglass support straps, and 
cold finger 32 is copper. Insulation layer 26 can be of any desired 
insulating material and may be disposed in single or multiple layers. 
Outer vacuum shell 22 is so disposed and configured within cooler 10 as to 
maintain a vacuum and thus further minimize heat load on inner tank 12 
from the environment. 
It will be understood by those skilled in the art that the embodiment shown 
and described is only exemplary and that various modifications can be made 
in the practice of the invention within the scope of the appended claims.