A relatively simple system using a preloaded pneumatic piston latch with a pyrotechnic valve release was developed. The system was the only candidate that met all the imposed requirements utilizing reliable state-of-art components. The development of the latch system from its first use on Mariner '69 Mars Flyby Spacecraft through its most recent use on the Voyager Spacecraft that will fly to Jupiter and Saturn is reviewed

Kievit, J. C.

English

NASA Technical Reports Server

PNEUMATIC PRELOADED SCANNING 
SCIENCE LAUNCH LATCH SYSTEM* 
By J. C. Kievit 
NASA Jet Propulsion Laboratory 
ABSTRACT 
Sophisticated spacecraft science payloads mounted to scanning or pointable 
platforms required preloaded platform launch latches with a low shock release. 
Brainstorming and trade studies conducted by JPL and Northrop Aerospace 
resulted in a relatively simple system using a preloaded pneumatic piston latch 
with a pyrotechnic valve release. The system was the only candidate that met 
all the imposed requirements utilizing reliable state-of-the-art components. 
This paper traces the development of the latch system from its first use on 
JPL's Mariner '69 Mars Flyby Spacecraft through its most recent use on the 
Voyager Spacecraft that will fly to Jupiter and Saturn. 
INTRODUCTION 
A pneumatic preloading latch system has successfully met the need for a 
high-reliability, low-shock launch latch for science instrument scanning plat- 
forms on JPL spacecraft for all missions since Mariner '69. The increasing 
size, weight and complexity of scanning payloads on interolanetary spacecraft 
demands special attention to launch load transfer to stationary structures. The 
load transfer must consider the control of instrument launch environment and 
releases the platform on command after the boost phase of the mission. Snecial 
problems were preloading of joints, control of nvro shock, cleanliness of 
release, reliability and testing of the system..Studies conducted during the 
first design of the pneumatic latch showed non-preloaded joints, rapid release 
of strain energy and close coupled structure borne pyro shock were the contri- 
butors of environmental conditions that needed to be eliminated in a science 
latch. Several devices, 
and pinpullers, 
such as pyre actuated release nuts, exolosive bolts 
failed to meet at least one of the criteria established for an 
acceptable system. A gas pressurized preloading cylinder type of latch met all 
launch load requirements. The release event using a pyrotechnically actuated 
valve to bleed off the nitrogen gas resulted in a soft release with low shock 
levels at the instruments. 
*This paper presents the results of one phase of research carried out at the 
Jet Propulsion Laboratory, California Institute of Technology, under Contract 
No. NAS 7-100, sponsored by the National Aeronautics and Space Administration. 
181 
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This paper presents the history and evolution of the pneumatic latch 
system. Some of the testing and flight application information are also 
presented. Several test problems and pieces of information relative to 
challenges of the design are also discussed. 
BACKGROUND INFORMATION 
Scanning science platforms in excess of 50kg have been flown on several 
JPL spacecraft. The normal boost vehicle induced environment on such a platform 
is usually in excess of the load carrying capability of the support bearings or 
structure needed for other phases of the mission. A structural mount in par- 
allel to cruise load path is required for launch. The device used to latch 
the platform to the structure must meet a long list of constraints and desirable 
traits to be selected for use. The specific requirement used in selection of a 
pneumatic latch were: 
1. ability to carry the launch loads to the latch points; 
2. latch joints must be linear through the load range; 
3. release should be slow to minimize shock from stored energy; 
4. electrical power for latch release should conform to pyro-type pulse; 
5. contamination and release effects on spacecraft should be minimized; 
6. device should have a relatively long shelf life; 
7. maintenance should be simple for successive uses; 
8. latch up and monitoring of preload should be simple and continuous; 
9. reliability and failure modes should be understood; 
10. flight telemetry should be built-in and simple; 
11. weight and cost should be compatible with aerospace technology. 
Initial studies and a later reevaluation of available release deivces 
considered fourdevicesas prime candidates for the job. They were: 1) pyro 
release nuts; 2) pyre pinpullers; 3) ball detent locks (Quantic Industries 
type); and 4) preloading pneumatic latch devices. All of these devices are 
used in todays spacecraft for various functions and a strong case can be made 
to use each one as a platform latch. Trade studies determined that the 
pneumatic latch was able to meet all requirements and was definitely superior 
to other latches in two major areas. First, the pneumatic latch gave the best 
controlled release with little or no pyro shock effects. Second, the pneumatic 
latch had better reuse capability. The only drawback seemed to be in the area 
of weight required to make a personnel safe pressure vessel for the preloading 
cylinder. In sunmary, the pneumatic latch was chosen and has been used in 
various configurations on scan platforms on all JPL interplanetary spacecraft 
since 1969. 
DETAIL DESCRIPTION 
Stress related details of the design of the pressure vessel and latch 
comoonents is all state-of-the-art technology and will not be discussed except 
182 
in relation to philosophy of the configuration. The first oneumatic scan 
platform latch designed at JPL contained a single acting piston and return 
spring, a torsion spring pivoted tee-bar latch, interconnecting plumbing, pyre 
release valve, pressure transducer and various load and tie-down hardware. The 
cross-section of a typical %riner spacecraft cylinder and the pneumatic 
schematic are shown in Figure 1. 
This unit was used satisfactorily on Mariner '69, '71 and '73. System 
tests performed during the development program proved that the performance was 
exceptional. Leakage of nitrogen gas past the viton piston seals was so small 
that it could not be measured in a 30-day pressure monitoring test. Two items 
of interest showed up in the test program that caused further development effort.. 
First, the restrictor used to release the gas to control the unlatch rate would 
have to be dual function; one,trap debris from blow-by of the pyro valve piston 
without clogging;two, control flow. This was accomplished by using a sintered 
porous plug of 25-50 micron filter material in the line in front of the 5-10 
micron plug used to produce a controlled pressure drop. This approach was 
successfully tested and has been used in all the systems to date. The second 
item of interest was the pressure fill valve, shown as part of the pyrotechnic 
valve in Figure 2, did not seal reliably if torqued to specification on succes- 
sive uses. To cure the problem, the ball valve seat was lapped to renew the 
sealing surface and an O-ring sealed cap was added to the fill port. These 
measures combined with a procedure to reduce the torque on the ball valve any 
time it was closed except during final prelaunch pressurization eliminated the 
problem. One peculiarity that showed up in flight was a delayed tee-bar 
withdrawal after bleeddown. The flight telemetry data provided by an event 
switch on the rotating tee-bar did not trip for a few minutes after the pressure 
bleed-off. Testing at JPL to check this phenomenon showed the cause to be long- 
time cold flow of the O-ring seals into the machining imperfections of the 
mating surface. The breakout force was not overcome immediately by the piston 
return spring resulting in a delayed release of up to two mintues. No corrective 
action was taken to change the design once the situation was understood but the 
event time was increased to allow sufficient time for operation. 
For the Viking Program, a complete redesign was undertaken resulting in 
the cross-section and schematic shown in Figure 3. The items of special note 
were the double acting piston design that eliminated the time delay phenomenon 
noted above and weight reduced components that used light metal alloy parts in 
place of all steel construction. A further departure from previous designs was 
the use of elastomer sealed bolted connections between the tubing and the 
cylinders instead of an all welded design. This reduced the cost of parts and 
made system cleaning and maintenance much easier. The seal in the joint was a 
special viton face seal washer "Gaskoseal" provided by Parker Seal Company used 
as shown in Figure 4. It should be noted that although seven of these seals 
were used in the system, no leakage problems were ever encountered. The double 
acting piston approach provided extra margin for unlatch because the differen- 
tial area of the piston developed considerable unlatching force as pressure 
approached equalization on both sides of the piston. A typical blow down event 
is shown in Figure 5 where the reduced data indicated a 200-pound (890 N) 
additional force to move the piston. In the event of a piston bind, the 
pressure would tend to increase on the large area side of the piston and raise 
the total unlatching force to approximately 800 pounds (3560 N). 
183 
I , . . . . -_ ---- 
The evolutionary process of the scan platform latch was ongoing for the 
Voyager spacecraft. In this case a swingout boom mounted scan platform was 
used that required an axialwithdrawalof a collet latch instead of the 
previously used rotating tee bar. This change was made because the spring 
deployed boom could introduce forces that prevented a gap from developing be- 
tween the tee bar and its mating part preventing its movement. The system made 
use of all the previous developments. The Voyager latch cross-section is shown 
in Figure 6. Development of thissystemwas uneventful and modifications of the 
operation were minimal. An item of interest for the mechanism desimer is the 
collet made from titanium with some interesting,manufacturing innovations. It 
was manufactured as a turned part, the nine finger slots were maue by a single 
electro discharge machine pass and it was shotpeened on the inside to open the 
fingers ofthe &let to allow easy stud withdrawal. The Voyager spacecraft 
launched in 1977 unlatched its platform according to plan and all platform 
mounted science worked successfully. 
SAFETY RJX4TED PROBLIXS 
As ment ioned earlier in the paper, the methods of calculating stresses in 
pressure vessels is well known but complications arise when pressurized units 
are used around personnel. One of the big problems of weight reduction of the 
pneumatic latch is that it is often under full pressure when the spacecraft is 
being worked on. Safety requirements are therefore enforced on the design. The 
latches shown for Viking and Voyager in this paper are admittedly heavy since 
they met ASME boiler code requirements, i.e., Proof Test to twice working 
pressure with burst capability of four times working pressure. This seems an 
undue penalty but if ignored the operational approvals to use the equipment may 
be a greater problem than the extra weight. No specialized equipment is re- 
quired to support a pneumatic latch since JPL fills from standard "K" bottles 
of clean nitrogen. 
CONCLUDING RIXARKS 
The penumatic latch concepts presented in this paper give the mechanism 
designer a versatile tool for use as a launch latch in space applications. The 
concepts involved contain no statistical qualification requirements or shock 
tests as is often the case with direct acting pyre devices. Careful design 
builds in reliability and functional margin that assures successful operation. 
184 
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188 
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Figure 5. Viking Orbiter System - Struc/Sep Review Scan 
Latch Blow Down Cycle 
189 
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Figure 6. Voyager Scan Platform Latch 
190 


Pneumatic preloaded scanning science launch latch system

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