Everything you never knew you wanted to know about the Mercury Project
Stabilization control system
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This article is organized like the Stabilization Control System section of the SEDR-104 Mercury familiarization manual except that the descriptions of the variations for the test capsules have been incorporated into the relevant sections.
The Mercury stabilization control systems were quite complex, and the nomenclature used is confusing. I've just re-read the chapter on this topic in SEDR-104, motivated by a desire to analyse the propellant consumption problems during MA-7 on the 44th anniversary of that flight, and I think that I understand it a little bit better.
Perhaps one reason that the system was complex was that the idea of providing the astronaut with any form of control was an afterthought. The control stick and it's means of controlling the system were grafted on to the original automatic system.
There were two separate RCS systems called Automatic and Manual, each had its own H2O2 propellant tank and set of asociated thrusters, these systems operated independently, there were no plumbing connections between teh two. The thrusters were operated by a combination of solenoids in both systems, The manual system also had proportional manual valves which could be used in lieu of the solenoids. The automatic system had two sets of thrusters a low power set (1 lbf thrust) and a high power set (24 lbf for pitch and yaw, and 6 lbf for roll).
So far, I've described the "engine" part of the attitude control systems. There were a variety of "control" parts.
The ASCS - automatic stabilization control system, was in effect a combination analog computer/transistorized relay control system which could turn the thrusters in the automatic rcs system on and off. It operated solely on inputs from instrumentation (a sequencer, accelerometer, gyros, and horizon sensors).
There were also three ways in which the pilot's stick could be used to control the attitude control systems.
The RSC - rate stabilization control system, was a separate analog/transistorized relay controller with it's own gyros. It took analog inputs from the pilot's control stick as commands to change the rate of roll, pitch, and yaw change. It controlled the thrusters in the manual reaction control system via solenoid valves.
The astronaut could also directly control the manual rcs thrusters in manual proportional mode, which used thruster control valves mechanically linked to the control stick via pushrods and bell-cranks.
Fly-by-wire control is described in the manual as a mode of the ASCS, but it really provided direct control over the thrusters in the automatic reaction control system. This is one confusing aspect about the naming of the automatic reaction control system.
Although the Mercury spacecraft was said to have four distinct attitude control modes, automatic, fly-by-wire, rate-stick, and direct; in actuality the system was quite complex and required the coordinated setting of several controls to select these "modes." There were a total of six controls to set the mode, a three position switch (Norm-Aux-FBW), a two position switch (Auto/Rate Comd), and four T-handles which mechanically operated valves in the propellant plumbing. Three of these enabled firing the automatic rcs system pitch, yaw, and roll thrusters. A fourth, labeled MANUAL routed propellant in the manual rcs system either to the solenoids or to the manual proportional valves.
This gives a total of 3x2x2x2x2 or 48 different settings of the mode controls, or 12 if we lump the three pitch, yaw, and roll T-handles together and treat them as an auto rcs on-off control.
With some of these settings it was possible to have both the automatic rcs and manual rcs thrusters firing. This is what happened by accident several times on the MA-7 flight. It had also happened to John Glenn. After MA-7 the control systems were reworked to make it harder to do this, but despite this, Wally Schirra also fell prey to the problem.
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