NASA Investigation Board Report On The INITIAL FLIGHT ANOMALIES OF SKYLAB 1

Overall Description

Although fairly simple in concept, the meteoroid shield had to provide such a variety of functions that it was, in fact, a quite complicated device. It was, foremost, a very lightly built cylindrical structure 270 inches in diameter (in the deployed condition) by 265 inches long.

The general layout of the MS is illustrated in figure 4-1. The OWS, which it surrounds, is deleted in this figure for clarity. In brief, the MS is formed of a set of sixteen curved sheets of 2014 T6 aluminum panels, 0.025 inches thick, assembled at flanges and other fittings to form the cylinder shown. The forward and aft ends were reinforced with curved 7075 T6 angles.

Various special details were included in the assembly in order to hold it in place, deploy it in orbit, and provide access to the OWS interior during prelaunch activities. The principal means of holding the shield in place in orbit (and to a lesser extent during powered flight) was a set of tension straps under the main tunnel illustrated at position II in figure 4-1. These straps were bonded to the OWS wall and fitted with a hinge on each end to take the butterfly hinge that attaches to the adjacent MS panel as indicated in figure 4-2. These butterfly hinges were designed to rotate so as to lie against the sides of the main tunnel which enclosed the tension straps and various cable runs on the OWS.

Proceeding clockwise from the tension straps and butterfly hinges in figure 4-1, the next special feature is the auxiliary tunnel. This tunnel extends in an arch between panels of the thin meteoroid shield. The 28 titanium frames of this tunnel (fig. 4-3) provide a very springy section in the relatively rigid hoop provided by the rest of the shield. The auxiliary tunnel also encloses a smaller tunnel covering the wiring for the thruster attitude control system. Farther around, in position I, there are two curved rectangular smaller panels, included to provide access to the OWS.

Between positions I and IV, the two halves of the MS overlap and are joined by a series of 14 trunnion bolts and straps (shown in detail in 4-4). These trunnion bolts were used to adjust the tension with which the MS was held against the OWS. Adjusting the bolts in the trunnion assemblies was a major aspect in positioning and tightening the MS against the OWS (rigging).

In order to provide the extra 30 inches of perimeter required when the MS was deployed, a foldout panel assembly (released by ordnance) is included in the panel adjacent to the trunnions. The mode of operation of this foldout panel is indicated in figure 4-5. Detailed descriptions of the ordnance and its function are given later. The only remaining distinctive features of the MS are the panels located over the scientific airlock and wardroom window at position III. The MS is completed at the butterfly hinges and tension straps at position I.

Deployment Provisions

The deployment of the 265 inch long MS was accomplished by providing two folding panel sections on each side of a contained explosive pyrotechnic chain which extended axially for the full length of the MS except for short end reinforcements. These folding panels and a schematic version of the redundant pyrotechnics were previously presented in figure 4-5. Illustrations of the shield in the stowed, partially deployed, and deployed configurations are shown in figures 4-6, 4-7, and 4-8, respectively. A cross section through the ordnance and its schematic are shown in figure 4-9. When the ordnance strip is fired and separates the "fold-over" panel, the segments are released and the shield is deployed. After release of this folded panel, a number of swing arms are used to displace the shield away from the OWS wall and hold it there. A rotational force is applied to these swing arms by a total of sixteen torsion rods suitably spaced around the ends of the MS as shown in figures 4-10 and 4-11. When the MS is stowed for launch, there is a larger twist in the torsion rods than after deployment. Both stowed and deployed torque settings are tabulated in table IV-1. It can be noted in figure 4-7 how the links on one side of the ordnance chain swing in a direction opposite to those on the other side. The butterfly hinges on each side of the main tunnel permit the radial displacement of the shield at the location of the tension straps.

The MS should therefore be regarded as a very limp system, which depends on being stretched tight around the OWS to withstand the aerodynamic, vibratory, flutter and thrust loads at launch. After deployment, it needs very little strength to serve its primary objective as a meteoroid shield.

The Auxiliary Tunnel

The auxiliary tunnel, an assembly of which is shown in figures 4-12 and 4-13, extends from the forward skirt, down the full length of the MS shield, and below the MS by about 57 inches. Venting of this tunnel was provided through an outlet of 10 square inches under the corrugations of the tunnel cover at the aft end of the forward fairing as detailed in figure 4-14. The tunnel was intended to be sealed at the aft end by a rubber boot assembly shown in the photographs of figure 4-15 in both the stowed (A) and deployed (B) position, Note that the tunnel is displaced some 5 or 6 inches circumferentially upon deployment of the shield.

The main structural members of the auxiliary tunnel are titanium, arch shaped, frame springs. These frames provide the structural tie between two MS panels and provide both regulation of the preloading of the MS to the OWS and act as a flexible relief for diametrical changes resulting from thermal and pressure changes of the OWS.

The tunnel also serves to protect the thrust attitude control system cables located in a small channel shaped cover permanently attached to the OWS and shown in figure 4-13. A segmented and corrugated outer skin form an aerodynamic fairing for the complete system and seals between forward and aft fairings.

Thermal Control

Although the primary purpose of the meteoroid shield is that of providing protection of the OWS from meteoroids, it also plays a significant role in the thermal control system. Much of the overall thermal design was accomplished passively by. painting the outer surfaces of the MS black except for a large white cross-shaped pattern on the earth side during flight. The entire surface of the OWS wall was covered with gold foil. The overall choice of finishes biased the thermal design toward the cold side, it being easier to vernier control by heating rather than cooling.

Friction Between MS and OWS Wall

To provide a uniform tension throughout the MS upon assembly and rigging for flight, and to permit transfer of the trunnion bolt tension into the frames of the auxiliary tunnel, it was necessary to minimize friction between the MS and the extemal surface of the OWS. This was accomplished by applying a teflon coating to the entire inner surface of the MS assembly. Special care was also taken to assure that all fastening rivets be either flush with or below the teflon surface of the MS. In addition to considerations of friction, the elimination of rivet head protrusions was important in not damaging the rather delicate gold surface used to provide the proper emissivity of the outer OWS wall surfaces as mentioned above. This was a vapor deposited gold surface applied to a kapton backing and bonded to the outer workshop wall with an adhesive. A typical cross section through the entire workshop wall members is shown in figure 4-16.

Panel Details

The 16 panels comprising the meteoroid shield were formed of 0.025 inch thick aluminum stock fitted with doublers and angles to permit their assembly. A typical detail of the longitudinal joints between the sixteen panels is shown in figure 4-17. In each of these panel joints, 96 holes of 1/8-inch diameter were drilled to vent any air trapped under the MS skin. In detail B of figure 4-17 is shown the special panel joint required next to the SAS-1 wing because of the unavailability of sufficiently wide panel stock for the panel under SAS-1. It was a strap" of metal of this special joint that became embedded in the SAS-1 cover and prevented automatic deployment of SAS-1 in orbit. It is, perhaps, of passing interest to note the longer length of exposed bolts in this particular joint.

Around the top of the panels is located an angle and a neoprene rubber rain or weather seal as shown in figure 4-18. This seal was not intended to be an aerodynamic seal and could not be expected to accommodate significant relative deflections between the OWS and MS surfaces. To provide meteoroid protection at the two ends of the MS, small strips of thin stainless steel "fingers" were squeezed down between the OWS and the MS when stowed. These fingers, deployed, are visible in the photograph of figure 4-10. The thrust load of the

MS, which weighs some 1200 pounds, is transferred to the forward flange of the aft skirt through a group of twelve thrust blocks as shown in figure 4-19. Figures 4-20 and 4-21 depict the.MS as laid out flat to identify the relative locations of the various panels, openings, joints and other features of the complete assembly.

    Figure 4-1. - Meteoroid shield.   

    Figure 4-2. - Butterfly hinges which connect meteoroid shield to straps running under main tunnel.   

    Figure 4-3. - Photograph of titanium frame springs in auxiliary tunnel.   

    Figure 4-4. - Trunnion strap assembly as used in rigging   

    Figure 4-5. - Meteoroid shield deployment ordnance and foldout panels.   

    Figure 4-6. - Meteoroid shield in its stowed or rigged condition for launch.   

    Figure 4-7. - Meteoroid shield partially deployed.   

    Figure 4-8. - Meteoroid shield deployed for orbit.   

    Figure 4-9. - . Ordnance schematic and cross section view for meteoroid shield release.   

    Figure 4-10. - Photograph showing typical swing link and latch detail.   

    Figure 4-11. - Drawing of typical swing link and torsion rod assembly.   

    Figure 4-12. - Assembly view of auxiliary tunnel.   

    Figure 4-13. - Wiring tunnel for TACS running inside auxiliary tunnel.   

    Figure 4-14. - Views showing vent area provision for auxiliary tunnel.   

    Figure 4-15. - Photographs of auxiliary tunnel boot. (a) STOWED POSITION.   

    Figure 4-15. - Photographs of auxiliary tunnel boot. (b) DEPLOYED POSITION.   

    Figure 4-16. - Typical cross section through members of the orbital workshop wall.   

    Figure 4-17. - Longitudinal joint detail of MS.   

    Figure 4-18. - Rain seal at typical top end of MS flange.   

    Figure 4-19. - Thrust block detail (one of twelve).   

    Figure 4-20. - Meteoroid shield laid flat.   

    Figure 4-21. - Meteoroid shield laid flat.   

TABLE IV-1. - OWS METEOROID SHIELD SWINGLINK SETTINGS AND MEASUREMENTS