Everything you never knew you wanted to know about the Mercury Project
Cabin air inlet valve
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Description in December 1959 SEDR-104
The December 1959 edition of SEDR-104 included a diagram of the cabin air inlet valve which I have split into two separate diagrams. Fig 1 shows the physical appearance of the valve, and Fig 2 is a schematic diagram of its operation.
The cabin air inlet valve, Figures 1 and 2, provides ventilation and cooling for the suit circuit and cabin during capsule landing and post landing phases. It is a spring loaded close spoon type valve and is barometrically controlled. Prior to capsule launch the valve is manually latched closed so that the mechanism spring loaded detent pin rides on the large diameter of the aneroid plunger, and the valve arm is engaged by the spring loaded aneroid locking pin. During capsule launch the aneroid expands due to decreasing cabin pressure, and forces the aneroid plunger down. The valve mechanism detent pin then slips off the plunger large diameter onto the plunger small diameter.
During capsule landing phase, when the capsule descends to an altitude of approximately 20,000 +/- 3,000 feet, the aneroid retracts cabin pressure increases. Retraction of the aneroid moves the aneroid plunger upward, engaging the detent pin against the plunger larger diameter which in turn compresses the aneroid locking pin spring. This action releases the locking pin out of the valve arm and allows spring loaded valve to close. The valve arm is attached to valve shaft and moves with valve closing, thereby disengaging micro switches. Disengagement of micro-switches directs electrical power to stop cabin fan operation, close suit circuit shutoff valve which in turn opens the emergency oxygen rate valve. A manual control arm is provided to enable valve opening in the event valve failed to open at specified altitude. Actuation of the manual control arm contracts the locking pin and disengages locking pin from valve arm, allowing valve to close. In the event of a capsule low altitude abort, an explosive squib will force locking pin up to enable valve opening. Valve must be manually reset to close position. Opening of the valve enables suit Compressors to draw ambient air into suit circuit to provide suit circuit and cabin ventilation.
Changes after MR-2 Flight
During ascent on the MR-2 flight on on January 31, 1961 the cabin inlet valve unexpectedly opened, which prevented the environmental control system from keeping cabin pressure at the design level causeing activation of the emergency environmental system.
As a result of this problem the tolerances were changed in the detent mechanism.
Description in November 1961 SEDR-104
The description was changed slightly in the November 1961 edition of SEDR-104 and a updated schematic diagram was included. Changes in the description have been highlighted in italics.
The cabin air inlet valve, Figures 1 & 3, provides ventilation and cooling for the suit circuit and cabin during capsule landing and post landing phases. It is a spring loaded close spoon type valve and is barometrically controlled. Prior to capsule launch the valve is manually latched closed so that one mechanism spring loaded detent pin rides on the large diameter of the aneroid plunger (maximum allowable pull to set detent pin is five (5) pounds); and the valve arm is engaged by release link, which is engaged by the spring loaded aneroid locking pin. During capsule launch the aneroid expands due to decreasing cabin pressure, and forces the aneroid plunger down. The valve mechanism detent pin then slips off the plunger large diameter onto the plunger small diameter.
During capsule landing phase, when the capsule descends to an altitude of approximately 17,000 +/- 3,000 feet, the aneroid retracts as cabin pressure increases. Retraction of the aneroid moves the aneroid plunger upward, engaging the detent pin against the plunger larger diameter, which in turn compresses the aneroid locking pin spring. This action raises the locking pin from release link and allows spring loaded valve to close. The valve arm is attached to valve shaft and moves with closing, thereby disengaging micro switches. Disengagement of micro-switches directs electrical power to stop cabin fan operation and close the suit circuit shutoff valve which in turn opens the emergency oxygen rate valve. A manual control arm is provided to enable valve opening in the event valve failed to open at specified altitude. Actuation of the manual control arm mechanically, contracts the locking pin and disengages locking pin from valve arm, allowing valve to close. In the event of a capsule low altitude abort, an explosive squib will force the locking pin up to enable valve opening. Valve must be manually reset to close position. Opening of the valve enables suit compressor to draw ambient air into suit circuit to provide suit circuit and cabin ventilation. The cabin air outlet valve is basically of the same construction and functions in the same manner as the cabin air inlet valve.
Position and external form
The position of the valve changed between the early spacecraft designed for sub-orbital operation and the later vehicles. In spacecraft 5 and spacecraft 7 the valve was located at the right side of the forward pressure bulkhead, while in the later spacecraft it was moved to the top of the bulkhead.
For a discussion of the meaning of the term "orbital" in this diagram see the article Orbital vs earlier spacecraft
