Thermal switch maintains temperature of planetary lander, within definite range, by transferring heat. Switch produces relatively large stroke and force, uses minimum electrical power, is lightweight, is vapor pressure actuated, and withstands sterilization temperatures without damage

Anderson, A. J.

Friedell, M. V.

English

NASA Technical Reports Server

August 1974	 B74-10092 
NASA TECH BRIEF 
S	 Langley Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Heat-Transfer Thermal Switch 
A thermal switch was devised to maintain the 
temperature of a planetary lander, within a definite 
range, by transferring heat. It was necessary that the 
device produce a relatively large stroke and force, use a 
minimum of electrical power, and be light in weight. 
This transfer switch meets these requirements and 
withstands sterilization temperatures on the order of 
419 K (295° F) without damage. The device is vapor 
pressure actuated and consists of two major sub-
assemblies (see illustration). The contactor is the heat-
transfer subassembly and contains a heated platen, a 
soft contact seat, flexible-foil copper conductors, a base 
block, and linkage. The actuator subassembly is the 
temperature sensor and produces the force to open and 
close the contact seats.
The thermal switch is powered by a vapor pressure 
actuator in which Freon is used as the pressure agent. 
For a given temperature, the vapor pressure of Freon is 
constant and thus permits the actuator bellows to 
stroke without pressure loss due to volume increase. A 
linkage mechanism transmits actuator motion to a 
movable thermal contact (shown closed) which is 
connected by flexible conductors to the base. The base 
is fastened to the component to be heated. The movable 
thermal contact closes on a continuously heated platen 
whenever the actuator temperature drops below an 
established level. For slow decreases in temperature, the 
thermal switch responds by closing to near contact 
(convection heating), through light contact, to full 
pressure contact. 
RF Shield 
	
\	 Heated	 Contactor 
	
Contact Support \
	
Platen	 Seal 
Assembly	 \	 \ Contact Seat 
Charging Tube
Carrier Assembly 
Base	 Elevating 
Dust Seal	
Block	 Link
Bel	
Copper 
WIcrank F
onductor 
Clevis Bushing Hole for
	
;Ch	
Connecting Rod	 Locking Pin 
Primary Chamber	 Assembly	 During amber
Assembly 
	
Secondary	 Bellows Assembly 
Chamber
Heat-Transfer Thermal Switch 
. (continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United Slates 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000092 2020-03-17T14:05:36+00:00Z
The component, the temperature of which is to be 
controlled, is attached to the actuator subassembly by 
bolts through footpads that are integral with the actuator 
body. This provides good heat transfer and the highest 
practical response to temperature changes. The actuator 
body is a stainless steel pressure chamber which is 
divided into two sections by metallic bellows. The 
bellows serve as flexible barriers to retain the Freon 
and are sized to provide the spring force to close the 
thermal contacts. One end of each of the bellows is 
welded to a header on a shaft, which transmits the 
bellows motion to the thermal contact seat through a 
connecting rod, a bellcrank, and an elevating link. 
Attached to the movable thermal contact seat are 
flexible copper-foil thermal conductors that convey heat 
to the base. These flexible conductors are made from 
200 leaves of copper 0.003 cm (0.001 in.) thick which 
are assembled into a cross shape. They are diffusion 
bonded where they intersect at the middle and at the 
outer ends of the four arms. This provides each arm 
with a flexible unbonded center section. The ends of the 
arms are attached to the base block after being set in 
conductive silicone grease, to provide minimum contact 
resistance. 
The intersection of the cross shape is part of the 
movable contact seat. It is made soft to provide good 
thermal contact even with contaminants such as dust. 
The soft seat is a composite made by casting a thin layer 
of tin 5.08 cm (2 in.) square on the copper foil, 
applying thermally conductive silicone grease, and cover-
ing it with gold-nickel foil. The seat will accept a piece 
of wire 0.1588 cm (0.0625 in.) long by 0.0254 cm 
(0.0100 in.) in diameter without reducing thermal 
conductance.
The thermal conductance data from the switch, 
which is designed to operate on the planet Mars, is 
as follows:	 E 
Measured Conductance 
[J/hr °C (Btu/hr °F)] 
1. Switch open 
a. Mars surface, 15 torr 
CO 2 pressure 133	 (0.07) 
b. Vacuum, 2x10 6 torr 379 (0.02) 
2. Switch closed 
a. Mars surface, 2 torr 
CO 2 pressure 9485 (5.0) 
b. Vacuum, 2x10 6 torr 8537 (4.5)
This thermal switch is designed to be fully closed at 
278 K (40° F) and fully open at not more than 289 K 
(60° F), at atmospheric pressures from 6.67 to 20 torr. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B74-10092 
Patent status: 
NASA has decided not to apply for a patent. 
Source: M. V. Friedell and A. J. Anderson of

Martin Marietta Corp.
under contract to
Langley Research Center 
(LAR-1 1232) 
B74-10092
	 Category 06,07 • 


Heat-transfer thermal switch

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