A7)- a3 77

MODIFIED APOLLO CRYOGENIC OXYGEN TANK DESIGN

Kermit Van LeuvenProject Engineer

Beech Aircraft Corporation

Boulder, Colorado

ABSTRACT

Assessment of the Apollo 13 mission indicated that some design changesbe incorporated into Apollo cryogenic oxygen storage tanks. Thesechanges broadly fit into three categories. They were: deletion of thefluid equilibration motors and redesign of heater assembly, material

changes for internal tank wiring and density sensor, and the addition

of a heater assembly temperature sensor. Development of a cryogenicoxygen tank incorporating these changes is presented.

PRELIMINARY EVALUATION

Analysis of required design changes (reference Figure 1 which illus-trates the original tank design), indicated that the heater assembly

would be the key element and that the primary development problem wasto incorporate metal sheathed wiring. A solution to this problemdictated the following general criteria for sheathed wiring:

1. Small diameter to:

(a) mimimize conducted heat leak,

(b) keep the containing conduit diameter small to allowinstallation in the existing pressure vessel neckopening and provide low conducted heat leak in shortconduit lengths, and

(c) provide maximum flexibility since the sheathed wireswould have to be pulled through the containing conduitat installation.

14

https://ntrs.nasa.gov/search.jsp?R=19720016137 2020-01-26T03:03:21+00:00Z

2. Good handling resistance.

3. Completely compatible with silver solder brazing temperatures

(13250F).

4. LOX-GOX compatibility for all materials.

5. Appropriate electrical characteristics such as current carryingcapacity with acceptable self-heating, insulation propertiesand capacitance.

These criteria were then used to screen available metal sheathed wiringand resulted in the selection of a sheathed wire produced by the

Rosemount Engineering Company of Minneapolis. The wire produced by

Rosemount had shown superior characteristics in the areas of flexibility, handling tolerance, and internal materials. An additionalbonus was realized with this wire selection in that heater elementscould be fabricated with integral cold leads inside a continuousmetallic sheath. This capability eliminated the need for heaterlead terminations on the heater assembly.

Construction details of the selected metal sheathed wiring areillustrated in Figure 2. Figure 2 also shows a cross section ofthe sheathed wire. The outer sheath material is 321 stainlesssteel. Nominal finished outside diameter is 0.059 inch with 0.010inch wall thickness. The insulation material is crushed and com-pacted Refrasil which is fused silicon dioxide or quartz. TheRefrasil is applied as a woven braid of fused quartz fibers and iscrushed and compacted when the metallic sheath is drawn. The con-ductor materials are 0.0158 inch O.D. Nichrome V for heater elementsand 0.020 inch O.D. nickel-clad copper for cold leads. Figure 2illustrates the manufacturing process used to fabricate the sheathedwire and shows how cold leads and heater elements are joined so thatthey can be enclosed in a single continuous sheath. Figure 2 alsoillustrates how a completed heater element is terminated withhermetically sealed headers.

CONCEPT DEVELOPMENT AND SELECTION

The selection of a specific metal sheathed wire and preliminarycompilations of its design application parameters allowed detailed

15

engineering development of the required design changes. This in turnled to the comprehensive hardware development and design verificationtesting program presented in Table I. The resulting new design isillustrated in Figures 3 and 4.

In addition to the items shown in Table I, a development program toeliminate teflon materials from the tank density sensing probe waspursued. Two development density sensors were fabricated usinginsulating materials other than teflon. The materials were fusedquartz and Alsimag ceramic. Subsequent testing of these two probesindicated that both materials were feasible; however, considerablefurther work would be required to perfect them to production designstatus. The main problem areas were: moisture sensitivity andparticle generation.

DESIGN AND PRODUCTION DEVELOPMENT

BAC encountered four significant manufacturing process developmentproblems during the fabrication of these initial tanks. They were:

1. Perfection of tooling and braz-ing techniques for installationof Rosemount heater elements on the heater assembly supporttube. These spirally wrapped elements were difficult to keepin place and simultaneously allow a smooth continuous brazewith uniform temperature distribution. The problem was resolvedby more sophisticated retaining tools and the incorporation of apreheater into the brazing tool.

2. Maintenance of dry sheathed leads throughout the manufacturingcycle. The sheathed wiring readily wicked air moisture in anyprocess which heated and cooled the leads in an unsealed condi-tion. Moisture in the leads would result in their insulationresistance and dielectric strength being less than specificationrequirements. This problem was resolved by minimizing theoccurrence and duration of leads being in an unsealed conditionand when leads were required to be unsealed all work was donein dry boxes or under hot dry blanket purge conditions.

3. Brazing problems associated with installation of transitionspline pins between sheathed wire hermetic headers and theoriginal design Apollo main electrical connector. The problems

were primarily associated with small clearances between pins inthe main electrical connector which made torch brazing verydifficult and resulted in multiple reheat of some connector pins.This problem was eventually resolved by a new electrical connectordesign. The original and new design connectors are contrasted inFigure 5.

4. A problem occurred with the heater element platinum braze jointduring component acceptance testing of heater assemblies. Theproblem was continuity failures in the platinum braze jointsor close to the joint in the nickel-clad copper lead. Investi-gation of the problem traced the cause to inadequate wetting ofthe Nichrome V heater element wire during the joint braze opera-tion, manual control of centering during the joint braze operation,and an extremely severe annealing operation following drawing ofthe wire sheath. All of these problems were resolved by changingthe joint braze and annealing processes. The investigation alsoshowed that the existing component acceptance tests would havescreened out any heater assemblies with potentially defectiveheater elements. However, both BAC and Rosemount inspection andcomponent acceptance tests were made more stringent as a resultof this problem.

DESIGN QUALIFICATION

Qualification of the new design oxygen tank commenced in mid-October.Table II indicates the items qualified, the tests conducted and abrief synopsis of the test results. One failure was encountered duringvibration qualification testing. The bulk fluid temperature sensor MIcable separated where it entered to the sensor housing. The failureresulted in severe degradation of the first qualification tank(XTA0033) vacuum when sorbed gases in the magnesium oxide insulationwere released into the tank vacuum annulus. This failure occurredprior to running the last axis of random design proof vibration.Investigation of this failure included re-evaluation of the vibrationtest levels and exposure times. Results of this re-evaluationindicated that the test requirements were unnecessarily severeespecially in the area of exposure time. The test levels wereredefined and a second qualification tank, XTA0037, was subsequentlyexposed to the new test requirements without incident.

17

Successful qualification of the new design oxygen tank was completedjust a matter of days before launch of Apollo 14. In fact, the missionsimulation test setup being used for the last qualification test of thetanks was recycled and used to "fly" a parallel mission with the tanksystem in Apollo 14.

Performance of the new design oxygen tanks was completely satisfactory.The performance ratings of the original design oxygen tank and the newdesign oxygen tank are compared in Table III.

18

-Wire conldlit

PROBE ASSEMBLY

tI I

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Figure 1

ORIGINAL BLOCK II OXYGEN TANK

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Temperature

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CONDUCTORHEATER ELEMENT - NICHROME AND NICKEL CLAD COPPERTEMP SENSOR - GOLD - 3% PLATINUM

QUANTITY - NICKEL CLAD COPPERStO 2 - INSULATOR

SIEATH: .010" THICKCONDUCTOR MATERIAL: TYPE 321 STAINLESS STEEL

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OXYGEN TANK ASSEMBLY

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TABLE III

PERFORMANCE RATINGS

30

I T E M BLOCK II DESIGN NEW DESIGN

Tank Weight (pounds) 80.85 79.7

Heater Watt Density 2.8 2.1(watts/sq. in.)

Maximum Heater Temperature(12.5% density & after 1 hr.) 580.0 490.0(Degree F)

Maximum Flow (lb/hr) 0.790 0.825(measured LO-19)

Electrical Conduit Heat Leak 1.93 2.29(BTU/hr)

Fluid Temperature Sensor Heat Leak 0.0 1.53(BTU/hr)

Total Tank Heat Leak 28.05 29.26(BTU/hr)