"6_2_3_13_4.TXT" (14045 bytes) was created on 12-12-88 ORBITER/EXTERNAL TANK SEPARATION SYSTEM The orbiter/external tank separation system consists of the oxygen and hydrogen umbilical disconnects located at the lower left and right aft fuselage, one forward structural attach point just aft of the nose landing gear doors and two structural attach points located in the orbiter/external tank umbilical disconnect cavities. An umbilical retraction system retracts the orbiter umbilicals within the orbiter aft fuselage, and umbilical doors close over each of the umbilical cavities after separation. The 17-inch liquid oxygen and liquid hydrogen disconnects provide the propellant feed interface from the external tank to the orbiter main propulsion system and the three space shuttle main engines. The respective 17-inch disconnects also provide the capability for external tank fill and drain of oxygen and hydrogen through the orbiter main propulsion system and the T-0 umbilicals. The liquid hydrogen interface between the orbiter and the ground storage tank is provided by a T-0 umbilical located on the left side of the aft fuselage. The liquid oxygen interface between the orbiter and the ground storage tank is provided by a T-0 umbilical on the right side of the aft fuselage. 17-INCH DISCONNECT. Each mated pair of 17-inch disconnects contains two flapper valves, one on the orbiter side of the interface and one on the external tank side of the interface. Both valves in each disconnect pair are opened to permit propellant flow between the orbiter and the external tank. Before the separation of the external tank, both valves in each mated pair of disconnects are commanded closed by pneumatic (helium) pressure from the main propulsion system. The closure of both valves in each disconnect pair prevents propellant discharge from the external tank or orbiter at separation. Valve closure on the orbiter side of each disconnect also prevents contamination of the orbiter main propulsion system during landing and ground operations. Inadvertent closure of either valve in a 17-inch disconnect during space shuttle main engine thrusting would stop propellant flow from the external tank to all three main engines. Catastrophic failure of the main engines and external tank feed lines would result. To prevent inadvertent closure of the 17-inch disconnect valves during the main engine thrusting, a latch mechanism was added in the orbiter half of the disconnects. The latch mechanism provides a mechanical backup to the normal fluid-induced-open forces. The latch is mounted on a shaft in the flowstream so it overlaps both flappers and obstructs closure for any reason. In preparation for external tank separation, both valves in each 17-inch disconnect are commanded closed. Pneumatic (helium) pressure from the main propulsion system causes the latch actuator to rotate the latch shaft in each orbiter 17-inch disconnect 90 degrees, thus freeing the flapper valves to close as required for external tank separation. If the latch pneumatic actuator malfunctions, a backup mechanical separation capability is provided. When the orbiter umbilical initially moves away from the external tank umbilical, the mechanical latch disengages from the external tank flapper valve and permits the orbiter disconnect flapper to toggle the latch. This action permits both flappers to close. During ground mating of the external tank to the orbiter, the latch engagement mechanism in each 17-inch disconnect provides a go/no-go verification that flapper angle rigging is within stability limits. Misrigged flappers will prevent full engagement of latch. The angle of each flapper in each disconnect is still carefully rigged within specific tolerances to assure basic stability independently of the latch safety feature. EXTERNAL TANK SEPARATION SYSTEM. The external tank is separated from the orbiter at three structural attach points. Separation from the orbiter occurs before orbit insertion and is automatically controlled by the orbiter's general-purpose computers. External tank separation can be manually initiated by the flight crew using the same jettison circuits as the automatic sequence. Separation is controlled by the ET separation auto, man switch on panel C3 and the sep push button on panel C3. In the auto position, the onboard GPCs initiate separation. To manually initiate separation, the ET separation switch is positioned to man and the sep push button is depressed. The forward structural attachment consists of a shear bolt unit mounted in a spherical bearing. The bolt separates at a break area when two pressure cartridges are initiated. The pressure from one or both cartridges drives one of a pair of pistons to shear the bolt, with the second piston acting as a hole plugger to fill the cavity left by the sheared bolt. A centering mechanism rotates the unit from the displacement position to a centered position, aligning the bearing flush with the adjacent thermal protection system mold line. The aft structural attachment consists of two special bolts and pyrotechnically actuated frangible nuts that attach the external tank strut hemisphere to the orbiter's left- and right-side cavities. At separation the frangible nuts are split by a booster cartridge initiated by a detonator cartridge. The attach bolts are driven by the separation forces and a spring into a cavity in the tank strut. The frangible nut, cartridge fragments and hot gases are contained within a cover assembly, and a hole plugger isolates the fragments in the container. The aft separation involves right and left umbilical assemblies. Each assembly contains three dual-detonator frangible nut and bolt combinations that hold the orbiter and external tank umbilical plates together during mated flight. Each bolt has a retraction spring that, after release of the nut, retracts the bolt to the external tank side of the interface. On the orbiter side, each frangible nut and its detonators are enclosed in a debris container that captures nut fragments and hot gases generated by the operation of the detonators, either of which will fracture the nut. The right aft umbilical assembly consists of an electrical disconnect, the gaseous oxygen 2-inch pressurization disconnect used for pressurization of the external tank's oxygen tank and the 17-inch liquid oxygen disconnect. The left aft umbilical assembly consists of an electrical disconnect plate, the gaseous hydrogen 2-inch pressurization disconnect used for pressurization of the external tank's hydrogen tank, the 4-inch recirculation disconnect used during prelaunch to precondition the main engine and the 17-inch liquid hydrogen disconnect. After release of the three frangible nuts and bolts at each aft umbilical, three lateral support arms at each orbiter umbilical plate hold the plates in the lateral position when the external tank separates from the umbilical plates. Each 17-inch disconnect has been commanded closed. The orbiter umbilical plates are retracted inside the orbiter aft fuselage approximately 2.5 inches by three hydraulic actuators and locked to permit closure of the umbilical doors in the bottom of the aft fuselage. Hydraulic system 1 source pressure is supplied to one actuator at each umbilical, hydraulic system 2 source pressure is supplied to the second actuator at each umbilical, and hydraulic system 3 source pressure is supplied to a third actuator at each umbilical. The retraction of each umbilical disconnects the external tank and orbiter electrical umbilical in the first 0.5 of an inch of travel and releases any fluids trapped between the 17-inch disconnect flappers. ORBITER UMBILICAL DOORS. An electromechanical actuation system on each umbilical door closes the left and right umbilical cavities after the external tank is jettisoned and the umbilical plates retracted inside the orbiter's aft fuselage. Each umbilical door is approximately 50 inches square. The doors are held in the full-open position by two centerline latches, one forward and one aft. They are opened before the mating of the orbiter to the external tank in the Vehicle Assembly Building. The orbiter umbilical doors normally are controlled by the flight crew with switches on panel R2. In return-to-launch-site aborts, the doors are controlled automatically. The ET umbilical door mode switch on panel R2 positioned to GPC enables automatic control of the doors. The GPC/man position enables manual flight crew control of the doors. The ET umbilical door centerline latch switch on panel R2 positioned to gnd permits ground control of the door centerline latches during ground turnaround operations. The stow position, enables flight crew manual control of the door centerline latches. The talkback indicator above the switch indicates sto when the door centerline latches are stowed, which permits closure of the doors, and barberpole when the latches are latched or the doors are in transit. The ET umbilical door left and right latch, off, release switches on panel R2 are used by the flight crew to unlatch the corresponding centerline latches during normal operations. Positioning the respective switch to release provides electrical power to redundant ac reversible motors which operate an electromechanical actuator for each centerline latch that causes the latch to rotate and retract the latch blade flush with the reusable thermal protection system mold line. It takes approximately six seconds for the latches to complete their motion. The talkback indicator above the respective switch indicates rel when the corresponding latches are released. The latch position of each switch is used during ground turnaround operations to latch the respective door open, and the talkback indicator indicates lat when the latches have latched the doors in the open position. The talkback indicators indicate barberpole when the latches are in transit. The off position of the switches removes power from the motors, which stops the latches. The ET umbilical door left and right , open , off , latch switches on panel R2 normally are used by the flight crew to close the umbilical doors. Positioning the switches to close provides electrical power to redundant ac reversible motors, which position the doors closed through a system of bellcranks and push rods. It takes approximately 24 seconds for the doors to close; and when they are within 2 inches of the closed position, ready-to-latch indicators activate the door uplatch system. Three uplatch hooks for each door engage three corresponding rollers near the outboard edge of the door and lock the door in preparation for entry. The motors are automatically turned off. The talkback indicator above the respective switch indicates cl when two of the three ready-to-latch switches for that door have sensed door closure. The open position of the switches is used during ground turnaround operations to open the doors. The talkback indicator indicates op when the doors are open and barberpole when they are in transit. The off position removes power from the motors, which stops the doors' movement. The ET umbilical door switch on panel R2 positioned to GPC provides a backup method of releasing the centerline latches and closing the umbilical doors through guidance, navigation and control software through cathode ray tube display item entry during an RTLS abort. The operation of the centerline latches and closing of the umbilical doors are completely automated after external tank separation when the ET umbilical door switch on panel R2 is positioned to GPC . Two seconds after external tank separation, the centerline latches release the doors and the latches are stowed. The ET umbilical door centerline latch talkback indicator indicates sto when the centerline latches complete their motion eight seconds after external tank separation. The left and right umbilical doors are closed, and the ET umbilical door left and right talkback indicates cl 32 seconds after separation. The left and right umbilical door latches latch the doors closed, and the ET umbilical door left and right talkback indicates lat 38 seconds after separation. Each umbilical door is covered with reusable thermal protection system in addition to an aerothermal barrier that required approximately 6 psi to compress to seal the door with adjacent thermal protection system tiles. A closeout curtain is installed at each of the orbiter/external tank umbilicals. After external tank separation, the residual liquid oxygen in the main propulsion system is dumped through the three space shuttle main engines and the residual liquid hydrogen is dumped overboard. The umbilical curtain prevents hazardous gases (gaseous oxygen and hydrogen) from entering the orbiter aft fuselage through the umbilical openings before the umbilical doors are closed. The curtain also acts as a seal during the ascent phase of the mission to permit the aft fuselage to vent through the orbiter purge and vent system, thereby protecting the orbiter aft bulkhead at station Xo 1307. The curtain is designed to operate in range of minus 200 F to plus 250 F. The umbilical doors are opened when the orbiter has stopped at the end of landing rollout. Various parameters are monitored and displayed on the flight deck control panel and CRT and transmitted by telemetry. Contractors for the separation system include Hoover Electric, Los Angeles, Calif. (external tank umbilical centerline latch and actuator; umbilical door actuator and umbilical door latch actuator); U.S. Bearing, Chatsworth, Calif. (external tank/orbiter spherical bearing); Bertea Corp., Irvine, Calif. (umbilical retractor actuator); Space Ordnance Systems Division, Trans Technology Corp., Saugus, Calif. (orbiter/external tank separation bolt/cartridge detonator assembly, 0.75-inch frangible nut orbiter/external tank umbilical separation and 2.5-inch frangible nut/pyro components in orbiter/external tank aft attach separation system).