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| 1ST STAGE/2ND STAGE INTERFACE
| N WALKER
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View of the first and second stage interface, without the interstage. The actual Saturn flight vehicle had a 11,000 pound interstage ring structure to connect the two stages. The interstage was tall enough to house the five 2nd stage engines showing in the photo. At the end of 1st stage burn, the spent stage was separated just below the 2nd stage engines. To have separated any higher would have risked damage or destruction of the engines during the separation process. Eight retro motors mounted aft on the departing 1st stage helped to acheive clean separation. Startup of the five J2 2nd stage engines was aided by the firing of eight ullage motors mounted on the interstage. The purpose of this was to settle the 2nd stage propellents for trouble free feed into the turbo pumps. After 30 seconds of 2nd Stage burn, The interstage was separated just above the engines and below the tankage. This got rid of unneeded weight and also avoided overheating equipment mounted in the 2nd stage aft section. The total separation process was called "dual plane separation". The main load across the separation interfaces was compressive so the joints were designed with butted compression fittings and only individual .025 inch thick aluminum straps (218 of them) provided tensile strength. This enabled the relatively small linear shaped charge (LSC) device to cut the straps at the appointed time. The LSC was small in cross section but over 100 feet in length to reach around the 33 foot diameter of the separation plane. For the dual plane separation two LSC assemblies were needed.
The missing interstage is reputed to have ended up as a consession shack.
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