Education and demonstration – Vehicle operator instruction or testing – Flight vehicle
Reexamination Certificate
2002-03-19
2004-04-13
Cheng, Joe H. (Department: 3713)
Education and demonstration
Vehicle operator instruction or testing
Flight vehicle
Reexamination Certificate
active
06719564
ABSTRACT:
BACKGROUND
In order to test the design and quality of spacecraft and their components, space simulation chambers are often employed to simulate the temperature and vacuum conditions of outer space. Although high vacuums and temperatures as low as 3° K are experienced in deep space, past history has established that vacuum pressures of less than 1 E
−5
Torr and temperatures less than 100° K are sufficient to provide the necessary testing conditions. Also, because spacecraft and their components are exposed to a broad range of temperatures in use, including 293° K at the earth's surface and the 3° K background temperature of deep space, testing at temperatures varying between about 293° K and 100° K or less is also desirable.
Space simulation chambers typically include a vacuum chamber containing a thermal shroud, or insulation layer, which defines a test cavity for receipt of the test component. A gaseous nitrogen cooling system is also typically provided for cooling the thermal shroud, and hence the test cavity, to the desired temperatures.
Currently-available space simulation chambers can be divided into two different types. In the first type, which is illustrated in
FIG. 1
, cooling of the test cavity is done solely with gaseous nitrogen. As shown in
FIG. 1
, space simulation chamber
10
includes gaseous nitrogen cooling system
11
for controlling the temperature of thermal shroud
12
located within a vacuum chamber
14
by heat exchange between the cooled recycled nitrogen gas and the thermal shroud. Gaseous nitrogen is circulated through cooling system
11
by blower
16
and heated or cooled to reach the desired temperature by either a heater
18
or a cooler
20
. In the particular embodiment shown, cooler
20
takes the form of a heat exchanger for heat exchange between the gaseous nitrogen in cooling system
11
and externally supplied liquid nitrogen. A pressure controller
22
can be used in conjunction with a gaseous nitrogen inlet valve
24
and a gaseous nitrogen release valve
26
in order to maintain a constant pressure in cooling system
11
. Alternatively, cooling system
11
can be operated at a constant density, constant volume flow, whereby the gaseous nitrogen pressure varies in proportion to the absolute temperature.
A problem with this system is that gaseous nitrogen cannot effectively operate as a coolant at temperatures below 100° K. As temperatures approach 100° K, gaseous nitrogen starts to condense, which makes the system virtually impossible to control. Therefore, such systems are typically operated at a lower limit of 110° K in order to maintain control. Moreover, because of the relatively low heat transfer properties of gaseous nitrogen, such systems must be operated with undesirably high volume flow rates, pressure drops and power consumption.
The second type of currently-available space simulation chamber is illustrated in
FIG. 2
, in which the same reference numbers are used as in FIG.
1
. As shown in
FIG. 2
, this space simulation chamber also includes vacuum chamber
14
, thermal shroud
12
and gaseous nitrogen cooling system
11
. In addition, this simulation chamber further includes liquid nitrogen cooling system
42
for supplying liquid nitrogen to thermal shroud
12
for heat exchange between the recycled liquid nitrogen coolant and the thermal shroud.
In the particular embodiment shown, liquid nitrogen system
42
includes a liquid nitrogen supply tank
44
, liquid nitrogen supply line
46
and gas vent
48
. Liquid supply line
50
is adapted to feed liquid nitrogen from supply tank
44
directly into the same coolant tubes in thermal shroud
12
which receive the gaseous nitrogen. Because controlling mixed gaseous/liquid nitrogen streams is difficult, suitable valving is provided to enable these coolants to be fed through thermal shroud
12
in an alternate fashion. This valving takes the form of a set of isolation valves
60
, as well as a purging system for the liquid nitrogen system including liquid nitrogen drain
52
, drain valve
54
, gaseous nitrogen purge line
56
, and purge valve
58
. As an alternative to this approach, liquid nitrogen can be supplied to and withdrawn from shroud
12
through separate coolant lines.
The space simulation chamber of
FIG. 2
can be operated in two different modes, a variable shroud temperature mode and a constant shroud temperature mode. In the variable shroud temperature mode, the temperature of thermal shroud
12
is varied to simulate the differing temperatures experienced in deep space in the same way as the space simulation chamber of FIG.
1
. However, because the space simulation chamber of
FIG. 2
includes a liquid nitrogen cooling system, the cooling capacity of this device is considerably greater than that of the
FIG. 1
system. As a result, the problems and constraints of the
FIG. 1
system due to its gaseous nitrogen only coolant system are largely eliminated in the
FIG. 2
design.
When the
FIG. 2
system is operated in a constant shroud temperature mode, thermal shroud
12
is kept at a constant, relatively low temperature and the temperature variations in deep space are simulated by intermittently heating the item to be tested with one or more infrared heaters
62
located in the test cavity of the device. This approach simplifies operations, because the shroud temperature is maintained constant. Moreover, although a considerably greater heat duty is generated when infrared heaters are used, this additional heat duty can be easily accommodated by the additional cooling capacity provided by the liquid nitrogen cooling system.
Although a combined liquid/gaseous nitrogen direct cooling system provides some significant advantages over a gaseous nitrogen only cooling system, additional problems and complications can arise. For example, switching between gaseous only and liquid only cooling modes can be complicated and time consuming. Furthermore, liquid nitrogen can cause damage to the test object in the event of a power failure. Moreover, due to the expense of the system and of the test objects, elaborate emergency procedures and/or equipment are often mandated for test facilities employing a liquid nitrogen coolant to cool the thermal shroud.
In light of these problems, it is desirable to create a space simulation chamber that can efficiently operate at temperatures below 110° K without the problems associated with cooling systems using liquid nitrogen to cool the thermal shroud.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been determined that the above advantages can be achieved by replacing the nitrogen-based cooling systems used for thermal shroud cooling with gaseous cooling systems based on helium or other analogous gases. In accordance with the present invention, it has been determined that, because of its higher heat capacity and lower condensation temperature, helium can achieve substantially the same cooling capacities as conventional cooling systems based on gaseous and liquid nitrogen combined, even though it remains in an essentially gaseous state. As a result, the inventive space simulation chamber can be operated in the same way as the
FIG. 2
device described above—i.e., in a constant shroud temperature mode with infrared or other heaters providing temperature variations inside the test cavity—without using the liquid nitrogen auxiliary cooling system previously required to make such systems work.
Accordingly, the present invention provides a new space simulation chamber comprising vacuum chamber, a thermal shroud in the vacuum chamber defining a test cavity therein, and a cooling system for directly cooling of the thermal shroud, wherein the coolant in the cooling system is helium gas or another gas having a specific heat of at least about 1500 J/kg-K and a condensation temperature of 77° K or less at one atmosphere pressure.
REFERENCES:
patent: 3566960 (1971-03-01), Stuart
patent: 3568874 (1971-03-01), Paine et al.
patent: 3710279 (1973-01-01), Ashkin
patent: 3825041 (1974-07-01), Cornog
patent: 4550979 (
McWilliams David A.
Than Yatming R.
Calfee Halter & Griswold LLP
Chart Inc.
Cheng Joe H.
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