cold- welding test environment - nasa · lab.) 1 feb. 1972 17 p cscl 13h ... and mrs. w. govett for...

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Technical Report 32-1547

Cold- Welding Test Environment

John T. Wang

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(NASA-CR-125603) COLD-WELDING TEST NENVIRONMENT J.T. Wang (Jet PropulsionLab.) 1 Feb. 1972 17 p CSCL 13H

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JET PROPULSION LABORATORY

CALIFORNIA INSTITUTE OF TECHNOLOGY

PASADENA, CALIFORNIA

February 1, 1972

Reproduced by

NATIONAL TECHNICALINFORMATION SERVICE

U S Deportment of CommerceSpringfield VA 22151

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https://ntrs.nasa.gov/search.jsp?R=19720016873 2018-06-14T22:48:24+00:00Z

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Technical Report 32-1547

Cold- Welding Test Environment

John T. Wang

JET PROPULSION LABORATORY

CALIFORNIA INSTITUTE OF TECHNOLOGY

PASADENA, CALIFORNIA

February 1, 1972

Prepared Under Contract No. NAS 7-100National Aeronautics and Space Administration

Preface

The work described in this report was performed by the Project EngineeringDivision of the Jet Propulsion Laboratory.

JPL TECHNICAL REPORT 32-1547 iii

Acknowledgment

Thanks are due Mr. R. L. Hammel of TRW for performing the experimental workreported herein, Messrs. J. Stephens, R. Bartera, and W. Owen for support during theMolsink test phase, Mr. D. T. Clift of AFRPL for providing the test equipments, Mr.T. Gindorf for his advice, Messrs. J. Schlue, D. Hess, and G. Ervin for theircomments, and Mrs. W. Govett for preparing the report manuscript.

JPL TECHNICAL REPORT 32-1547iv

Contents

I. Introduction. 1

II. Summary . . . . . . . . . . . . . . . . 1

III. Approach . . . . . . . . . . . . . . . . 2

IV. Friction Module . . . . . . . . . . . . . . . 3

V. Test Materials . 3

VI. Test Facilities . . . . . . . . . . . . . . . 3

A. TRW Oil-Diffusion-Pumped Vacuum Facility . 3

B. JPL Molsink Facility . 5

C. TRW Ion-Pumped Vacuum Facility. 7

VII. Test Results. . . . . . . . . . . . . . . . 7

VIII. Conclusions . . . . . . . . . . . . . . . . 8

IX. Recommendations . . . . . . . . . . . . . . 10

References . . . . . . . . . . . . . . . . . 11

Tables

1. Friction test materials . . . . . . . . . . . 3

2. Comparison of test environmental parameters. 8

3. Comparison of orbital, postlaunch UHV, molecular sink, and ion-pumpedvacuum results . 9

Figures

1. Cold-welding program approach . . . . . . . . . . . 4

2. Friction experiment, front cutaway view 4

3. Friction experiment, side cutaway view . 4

4. Friction experiment, top view 5

5. Oil-pumped ultrahigh vacuum system . . . . . . . . . 6

6. Molsink facility . 6

7. Ion-pumped friction test facility. 7

JPL TECHNICAL REPORT 32-1547 v

Abstract

Spacecraft designers concerned with the development of excessive friction orcold-welding in mechanisms with moving parts operating in space vacuum for anextended period of time should consider cold-weld qualification tests. To establishrequirements for effective test programs, a flight test was conducted and comparedwith ground test data from various facilities. Sixteen typical spacecraft materialcouples were mounted on an experimental research satellite (ERS-20), where amotor intermittently drove the spherical moving specimens across the faces of thefixed flat specimens in an oscillating motion. Friction coefficients were measuredover a period of 14-month orbital time. Surface-to-surface sliding was found to be thecontrolling factor of generating friction in a vacuum environment. Friction appearsto be independent of passive vacuum exposure time. Prelaunch and postlaunch testsidentical to the flight test were performed in a TRW oil-diffusion-pumped ultrahighvacuum (1.33 x 10- 6 N/m2 ) chamber. Only 50% of the resultant data agreed with theflight data owing to pump oil contamination. Subsequently, identical ground testswere run in the JPL Molsink, a unique ultrahigh vacuum facility, and a TRW ion-pumped vacuum chamber. The agreement (90%) between data from these tests andflight data established the adequacy of these test environments and facilities.

JPL TECHNICAL REPORT 32-1547vi

Cold-Welding Test Environment

I. IntroductionSpacecraft mechanisms with moving mechanical parts

are frequently required to operate in the high vacuum ofspace for an extended period of time. A significant concernof a designer is the possibility of metal-to-metal cold-welding or significant increase in friction in areas whereone surface slides or rolls over another. Lubrication isusually applied to avoid excessive friction, which precedescold-welding. Naturally formed oxides tend to reducefriction and serve as lubricants. It has been found thatsurfaces stay lubricated in passive vacuum environmentexcept where abrasion occurs due to surface rubbing. Insome cases, lubrication cannot be used for fear of contami-nating optical surfaces, which further increases the poten-tial of cold-welding.

Although numerous studies have been done, the mecha-nism of cold-welding is still not well understood. A numberof factors can affect the behavior of surfaces in vacuum.Load, degree of contamination, temperature, materialproperties, lubrication, and the molecular flux environmentare important parameters. The lack of quantitative under-standing of the interrelationship of these parameters makesit difficult to predict the friction behavior of a spacecraftmechanism operating in space. Consequently, groundtesting to qualify spacecraft hardware with a potential

cold-welding problem becomes a valuable means of gainingconfidence in design adequacy.

II. SummaryA cold-welding program was initiated in 1969 to deter-

mine the proper test environment for qualifying spacecraftmechanisms. The specific objectives were:

(1) Determine if the JPL Molsink, a unique vacuumfacility, can simulate space sufficiently well forperforming cold-welding tests.

(2) Once the adequacy of Molsink is established, deter-mine if the degree of sophistication characterizedby the Molsink is necessary for adequate cold-welding tests; that is, identify the minimum environ-mental test requirements.

To achieve objective (1), a 10-day ground test wasperformed in the Molsink (Ref. 1) to provide a directcomparison with the in-space friction data (Refs. 2-5)previously acquired by the Air Force Rocket PropulsionLaboratory (AFRPL) Experimental Research Satellite No.20 (ERS-20). The ground test used the same test materialcouples and equipments. The flight data were acquiredover a 14-month period. Prelaunch test data were taken in

JPL TECHNICAL REPORT 32-1547 1

a TRW oil-diffusion-pumped ultrahigh vacuum chamber(1.33 x 10- 6 N/m2 ). Additional postlaunch ground testswere conducted in a TRW oil-pumped chamber whichincluded tests to evaluate the effects of ultraviolet radia-tion, inert gases, oxygen, water vapor, laboratory air, andcontrolled oil vapor contamination. Only 50% of the grounddata agreed with the flight data. The Molsink adequacy wasestablished by the good agreement (90%) between theMolsink and flight data.

Objective (2) was to determine if the degree of sophisti-cation characterized by the Molsink was required. This wasa very important question because Molsink is a uniqueresearch facility, costly to construct and operate. It isimpractical to perform spacecraft mechanism testing inlarge quantities in this facility. Therefore, if a less sophisti-cated class of facilities could provide an adequate testenvironment, many of the problems of uniqueness, availa-bility of facilities, and high cost could be eliminated forprojects. Facilities considered appropriate for evaluationincluded chambers equivalent to the JPL facility in termsof cleanliness, but with slightly varied environments, e.g., afacility without a moltrap (a cryogenic inner chamberwithin the Molsink), higher sink temperature, differentpressure levels, etc. While several vacuum chambers wereunder consideration, a cold-welding test on an electriccontactor (Ref. 6) performed by AFRPL strongly indicatedthat an ion-pumped facility might be adequate. To verifyits ability, a test (Ref. 7) was performed in a TRW ion-pumped vacuum chamber. The results showed that thistype of facility could provide an adequate test environ-ment. The facility was electronically pumped. The residualgases were ionized and then adsorbed by the chamber wall.Adsorption was enhanced by titanium sublimation pump-ing. Rough pumping was accomplished by cryogenicsorption pumps. Consequently, the danger of pump oilcontamination was totally eliminated. Ion-pumped vacuumfacilities can be found in most aerospace organizations.Thus, this research finding concludes that adequate cold-welding test environments exist wherever ion-pumped (nooil system) facilities are found.

Tests have established the adequacy of an ion-pumpedvacuum (1.33 x 10-6 N/m2 ) for clean test items such as thefriction module. The friction module does not produce oily,long chain hydrocarbons nor does it outgas excessively tomake it difficult to maintain partial pressure at 1.33 x 10- 6

N/m2 or less at all times during test. Potential spacecrafttest candidates such as science instruments are as clean asthe friction module. Other spacecraft parts and testequipment in vacuum, including cables, potting com-pounds, etc., are required to be non-oil-producing and withlow outgassing rate to avoid contaminating optics. In some

cases, spacecraft parts are preconditioned. However, it isconceivable that some test items may outgas sufficiently orsmall leaks may develop in the vacuum system during test,causing the partial pressure to exceed 1.33 x 10-6 N/m2

and thus rendering the test environment inadequate.Consequently, tests may have to be aborted. It is thereforeadvisable to determine the minimum acceptable testenvironment for all test items.

A test was performed in an ion-pumped vacuum whichwas modified by the controlled introduction of six gases:argon, carbon dioxide, methane, nitrogen, oxygen, andwater, the common residual gas species in a vacuumsystem. Sliding friction data were taken under partial gaspressure ranging from 1.33 x 10-3 to 1.33 x 10- 5 N/m2 ofeach of the test gases. The data were compared to previousorbital, Molsink, and ion-pumped friction data. In two outof 16 material pairs (Co x Be, Co x WC), the maximumfriction force attained appears controlled by the gasintroduction. Selective influence of particular gas introduc-tions was noted for two additional pairs (WC x A120 3, Cox Ag). Other comparative data suggest the quantities andspecies used had little controlling influence on the fric-tional behavior of the remaining pairs.

It has been observed during tests that surface-to-surfacesliding is the controlling factor of generating friction in avacuum environment. Friction appears to be independentof passive vacuum exposure time. Therefore, a cold-welding test can be accelerated by using the mechanical'sliding mission requirement of the mechanism as theprimary control test parameter in simulation tests.

III. ApproachThrough literature search and consultation with experi-

menters in the field, it was found that AFRPL has per-formed several cold-welding tests, including a flight test. Byvirtue of common objectives, a JPL-AFRPL joint researchprogram was set up to determine the adequacy of testenvironment for cold-welding. JPL thus gained access tothe AFRPL extensive flight and test background.

The approach of the program was to perform a series oftests to provide direct comparisons between ground testdata and flight data. The flight data were secured over aone-year orbital time on satellite ERS 20. Active runs weremade intermittently while the two friction modules pro-duced reciprocating sliding motion between 16 pairs offrictional surfaces, each of a different material combina-tion. The 10-day ground tests were designed to duplicatethe flight experiment, except that the passive dwell-time

JPL TECHNICAL REPORT 32-15472

between active runs was scaled down. The reduction ofpassive dwell-time was aimed at developing a valid methodof accelerating a cold-welding test.

Table 1. Friction test materials

Class Rider Flat

Because of the poor correlation between oil-pumpedvacuum data and flight data acquired by TRW for AFRPL,the JPL Molsink was used for the next cold-welding test. Asa result of the Molsink test success, a much less sophisti-cated ion-pumped vacuum facility was used to determinewhether it can also provide an adequate test environment.Subsequently, a test was performed in a modified vacuumenvironment to determine how sensitive the ion-pumpedvacuum environment was to residual gases caused byoutgassing. The cold-welding program approach is shownin Fig. 1.

1 440C SS

1 Cobalt (Co)

1 Cobalt (Co)

1 Aluminum (Al)

2 Tungsten carbide (WC)

2 440C SS

2 Cobalt (Co)

Boron nitride (NB)

Cobalt (Co)

Beryllium (Be)

Beryllium (Be)

Gold plate (Au/52100)

Burnished tungstendiselenide(WSe 2 /17-4PH)

Silver plate (Ag/17-4PH)

2 440C SS

IV. Friction ModuleEach friction module (Figs. 2-4) provides the mechanical

structure to test eight specimen pairs. A self-contained dcdrive motor and cam follower simultaneously drives eightmoving specimens, which are individually mounted on theend of the compound flexure arms, across the faces of eightfixed specimens in an oscillating motion. Individual straingage sensors for each test pair allow measuring a real-timeanalog force signal of both the drag and normal forces.Initial normal forces are set by adjustment of the individualmoving sample holders at the end of their flexure arm.Adjustment of the fixed specimen alignment with respect tothe moving arm is provided by adjustment nuts at theoutboard edge of the fixed specimen support webs.

2 440C SS

3 Tungsten carbide (WC)

3 440C SS

3 Tungsten carbide

3 440C SS

3 Cobalt (Co)

4 440C SS

Burnished naturalmolybdenum disulfide(MoS2 /17-4PH)

Burnished syntheticmolybdenum disulfide(MoS2 /17-4PH)

Tungsten carbide (WC)


Aluminum oxide

(bulk A12 0 3)

Boron nitride (BN)


15% glass-filled Teflon(PTFE)

V. Test Materials 4 440C SSPolyimide (Vespel SP21)

The primary criterion applied in the selection of the testmaterial couples for the ERS 20 flight test was to provide across section of material combinations whose friction mightbe responsive to variations in the test environment. Fourgeneral classes were used:

(1) Metals and alloys.

(2) Thin film lubricants.

(3) Inorganic compounds.

(4) Polymers.

The 16 material couples are listed and defined in Table 1.

VI. Test Facilities

A. TRW Oil-Diffusion-Pumped Vacuum Facility

The test vacuum facility consists of a double-wallultrahigh vacuum chamber. The outer guard vacuumemploys conventional O-ring seals. The 0.226-m3 innerultrahigh vacuum chamber utilizes copper shear seals.When the inner chamber is clean, dry, and empty, it iscapable of operation at 1.33 x 10- s N/m2 measured withnude ion gages mounted within the chamber itself. Theinner chamber and main trap regions have bakeout heatersmounted on the guard vacuum side. Likewise, tracedcopper cooling lines and cryogenic reservoirs are located

JPL TECHNICAL REPORT 32-1547 3

LITERATURE OSCILLATING1/ RIDER

EXPERIMENTERS SPECIMEN -

LUBRICATION EXPERTS

USE OF FLIGHT TEST MODULES

J NACCESS TO EXTENSIVE FLIGHT AND TEST BACKGROUND

DOES FACILITY PROVIDE ADEQUATE TESTENVIRONMENT?

J<NT /MODIFIED MOLSINK ENVIRONMENT

OIL-PUMPED ULTRAHIGH VACUUM ENVIRONMENT

ION-PUMPED VACUUM ENVIRONMENT

DOES CHAMBER PROVIDE ADEQUATE TEST9ST J ENVIRONMENT?

J HOW SENSITIVE IS THE ION-PUMPED VACUUM SYSTEMU TO RESIDUAL GASES CAUSED BY OUTGASSING, ETC?

Fig. 1. Cold-welding program approach Fig. 2

FIXED B OSCILLATINGSPECIMEN / -RIDER

SPECIMEN

BELLOWS

2. Friction experiment, front cutaway view

Fig. 3. Friction experiment, side cutaway view


FIXED SPECIMENSUPPORT PLATEADJUSTMENT

-FIXED SPECIMENS

Fig. 4. Friction experiment, top view

on these surfaces. The design allows for differential bakeoutup to 400°C and cooldown to liquid nitrogen temperatures.

The pumping system for this facility consists of twoseries oil-diffusion pumps which are extensively trapped.Figure 5 schematically shows the pumping system for theinner chamber. The trapping system consists of the dual-chevron trap design over the primary diffusion pump andthe LN 2 flood trap of the right-angle elbow design. Thediffusion pump oil used in the facility is DC 704.

B. JPL Molsink Facility

The Molsink facility at JPL (Fig. 6) was designed toprovide a close simulation of the vacuum and cold sink ofspace for surface effects testing. Cryopumping is accom-plished by a 2.44-m-diameter spherical molecular trap

(moltrap) of wedge-fin configuration which was constructedof 0.041-cm-thick aluminum sheets that are spot-welded tothe aluminum tubes. The tubes are cooled to below 14°Kby helium supplied by a 1000-W helium refrigerator. Theangles of the fins are such that their projections are tangentto a 25.4-cm sphere at the center of the moltrap, aconfiguration that has been shown to provide an order ofmagnitude capture improvement over a smooth-wall cham-ber when the test item gas load is from within the 25.4-cmspherical volume. It has been computed that only 4 ofevery 10,000 molecules of 02, N2 , Ar, CO, and CO2 emittedfrom a 25.4-cm spherical 300°K test item would restrike it.

Chemical pumping is accomplished by an electron-beamtitanium sublimator mounted on the inner door of thechamber and used to coat nearly the entire moltrap atintervals during a test. This greatly increases the adsorptionrate for H2 and 02.


OSCILLATINGRIDERSPECIMEN -

5

FI _ ~LNFLOODEDELBOW

ULT RAHGH VACUUMCHAMBER

L <<<<<<A4NC LN CHEVRON GUARD

MECHANICAL 20 CHEVRON / \ VACUUMPUMP

MECHANICAL

BACKING DIFFUSION DIFFUSIONPUMP PUMP

Fig. 5. Oil-pumped ultrahigh vacuum system

CRYOSORPTIONPUMP

INNER AND WINDOW

TAILED DEWAR

OUTER CHAMBER

GUARD VACUDIFFUSIONCRYOGENIC QUARTZ

CRYSTAL MICROBALANCE

CRYOGENIC MASSSPECTROMETER

Fig. 6. Molsink facility


COMPRESSOR

6

C. TRW Ion-Pumped Vacuum Facility

The test facility employed in this study was chosen asrepresentative of ion-pumped ultrahigh vacuum systemscommonly found throughout aerospace organizations. Inparticular, a room-temperature chamber wall provided themajor view factor to the experimental apparatus and testsurfaces. The facility and the test hardware are shown inFig. 7. The facility consists of a double-ended horizontaltest chamber 46 cm in diameter by 76 cm in length. Thepumping system consists of a 400-liter/s ion pump and atitanium sublimation pump (TSP). Roughing the chamberfrom ambient pressure is accomplished by three sorptionpumps. A sorption pump contains an extended surfaceporous structure which is cooled to liquid nitrogen temper-ature to condense the gas. These pumps are periodicallyrejuvenated by bakeout at elevated temperature. As can beseen in Fig. 7, numerous flanges on the test chamber permitthe mounting of instrumentation and electrical and me-chanical feedthroughs. This facility has been operatedempty to less than 1.33 X 10- 9 N/m2 .

Table 2 presents a comparison of test environmentalparameters.

GAS INLET MANIFOLD

VII. Test ResultsFriction represents a surface-sensitive property which

may be influenced by variations in the environment of thefacility employed. Comparison of the results for eachmaterial combination under each of the test conditionsprovides a basis for judging the adequacy of simulatedenvironments. The orbital friction results further providereference information for comparisons with other simu-lated test conditions.

The test results are presented in terms of coefficient offriction. The coefficient of friction Mk is defined as themeasured sliding force divided by the applied normal forcebetween the sliding surfaces:

F F

normal n

The overall experimental accuracy for calculating afriction coefficient is estimated as ±10%.

wE 777-C1

FRICTION MODULE -

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Fig. 7. Ion-pumped friction test facility


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Table 2. Comparison of test environmental parameters

Test environmental TRW oil-pumped Molsink TRW ion-pumped Modified TRW ion-parameters facility facility facility pumped facility

1. Wall configuration Smooth Wedge fin Smooth Smooth

2. Wall temperature, °C 23 -259 23 23

3. Diffusion oil Some None None Nonecontamination

4. Partial pressure, 1.33 X 10-6 1.33 X 10-6 to 1.33 X 10-6 1.33 X 10- 3 toN/m2 1.33 X 10 - 9 1.33 X 10-6

5. Test volume 0.23 m3 2.44-m-diam 46 X 76-cm 46 X 76-cmspherical trap cylindrical cylindrical

6. Titanium None Yes Yes Yessublimation pumping

7. Residual gas Long chain He, Ne N2 A, CO2 , CH4 ,species hydrocarbons, N2 , H2 0, 02

oil

Summarized in Table 3 are typical coefficient of frictionvalues for each test combination. Included in this summaryare data taken in an oil-pumped ultrahigh vacuum. Com-parison of this with the orbital data showed generalagreement between seven combinations and disagreementof eight combinations; no definitive tests of one combina-tion occurred.

By contrast, a comparison of molecular sink test datashows twelve combinations agree. The early wearout ofsynthetic MoS 2 in the flight test was the result of anontypical sample. For three material combinations, nodirect comparisons could be made. The lack of orbital datafor one of these was due to the release of a shear pin duringthe flight test. Two combinations were not tested in themolecular sink tests because of inoperative strain gages.

The ion-pumped friction tests show good agreementbetween 14 of the 16 materials pairs. The lack of correla-tion in the synthetic MoS 2 test is attributable to the lack ofsignificant test data in the flight test. The result is compara-ble to previous ground tests suggesting an abnormality inthe flight test sample. The early wearout of the silver-coated sample in the ion-pumped test did not occur in anyof the previous test series. Prior to wearout, the frictionvalue appeared nominal.

With exception of the differences noted in the followingparagraphs, none of the modified ion-pumped conditions

produced a systematic difference in friction for any of theother test couples. Comparative behavior in orbital,molecular sink, or ion-pumped vacuum tests is generallygood.

In two instances (Co x Be and Co x WC) the introduc-tion of modifying gases into the ion-pumped vacuumappears to limit the development of the maximum frictionforce; however, neither couple shows a unique sensitivityto any of the particular modifying gases used. This suggestseither physisorbed or impurity chemisorbed gases areinvolved in limiting the shear force development.

Two couples showed a unique decrease in friction whenexposed to one or more specific modifying gases. The WCX A120 3 shows a sensitivity to water vapor - a resultpreviously found in Ref. 2. The Co x Ag following wearoutshows a friction decrease when exposed to H2 0, 02, or N2.The specific influence noted is likely associated with theformation of nitrides and oxides of silver.

VIII. Conclusions1. Both the ion-pumped vacuum facility and the Molsink

are capable of producing a cold-welding vacuum environ-ment equivalent to space.

2. Modified ion-pumped vacuum environment is accepta-ble for testing many material combinations. The addition of


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trace amounts of a cross section of active gases includingoxygen and light hydrocarbons suggests that for manymaterial systems these species do not alter vacuum fric-tional behavior significantly. However, as previously noted,two material couples showed sensitivity to all gases andtwo others were sensitive to specific gases. This indicatesthat residual gases must be maintained at a very low levelto avoid excessive molecular flux impinging upon testitems.

3. Oil-pumped vacuum environment is not acceptablefor cold-welding test even though some lubricated systemsappeared insensitive to pump oil contamination. Thepresence of long chain hydrocarbons in pump oil is theprimary factor in reducing unlubricated boundary layersliding friction forces and invalidating the test results.

4. Accumulated mechanical sliding (mileage) was foundto be the dominant factor in the development of frictionbetween uncontaminated engineering surfaces. Passivedwell (nonsliding) had little if any effect on materialcouples tested.

5. No evidence of friction dependence on orbital altitudewas found.

6. No universal conclusions from these tests regardingthe influence of hardness, solubility, or crystal structure canbe made. Generally, those materials having a high shearstrength exhibit high coefficients of boundary layer friction.

IX. Recommendations1. The cold-welding test technology and test criterion

developed under this research program can be directlyapplied to all space missions.

2. The Molsink or an ion-pumped vacuum should be usedfor performing cold-welding tests.

3. Oil-pumped vacuum environment should not be used.

4. A cold-welding test should be accelerated by using themechanical sliding mission requirement of the mechanismas the primary control test parameter in simulation tests.The rate of sliding should be the same as that required bythe mission or higher.

5. Spacecraft assemblies with mechanical moving partswhich are required to operate in space vacuum for anextended period of time should be cold-weld tested except:

(a) Those assemblies which have been flight-proven onprevious spacecraft and retain basically unchangedmechanical design.

(b) Those assemblies which are hermetically sealed.

(c) Those assemblies which use special self-lubricatingmaterials.


References

1. Hammel, R. L., Molecular Sink Vacuum Friction Test, AFRPL-TR-69-208. AirForce Rocket Propulsion Laboratory, Edwards, Calif., Nov. 1969.

2. Hammel, R. L., In-Space Friction Tests, AFRPL-TR-69-207. Air Force RocketPropulsion Laboratory, Edwards, Calif., Nov. 1969.

3. Hammel, R. L., Horn, H. J., and Sliff, H. T., Environmental Research Satellitesfor In-Space Friction Experiments - System Description Document,Document 05319-6016-R0-00. TRW Systems, Redondo Beach, Calif., July 1967.

4. Environmental Research Satellite No. 20 - Operations Plan, Document No.X-513-67-128. NASA/GSFC, Greenbelt, Md, March 1967.

5. Hammel, R. L., AFRPL Ground Test Friction Experiment OperationsManual, Document 12644-6001-T0-00. TRW Systems, Redondo Beach, Calif.,June 1969.

6. Sheldon, D. B., Coldwelding Research Final Report, AFRPL-TR-209. AirForce Rocket Propulsion Laboratory, Edwards, Calif., Jan. 1970.

7. Hammel, R. L., Ion Pumped Vacuum Friction Test, AFRPL-TR-70-120. AirForce Rocket Propulsion Laboratory, Edwards, Calif., Nov. 1970.

JPL TECHNICAL REPORT 32-1547NASA - JPL - Coml., L.A., Calif.

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