It was John Glenn’s birthday this week and at 95 the former Senator from Ohio is the last of the original seven US astronauts recruited by NASA for the agency’s first manned space program, Project Mercury.
Glenn later made a run at presidential politics and flew on Space Shuttle mission STS-95 as a mission specialist, but he is most famous for piloting the first US orbital mission, Mercury-Atlas 6, in February 1962.
Hollywood has had a field day with the Mercury astronauts, notable in the film adaptation of Tom Wolfe’s The Right Stuff, but as an engineering challenge, the whole program was remarkable.
Like a design and test challenge? Start with a spacecraft that has to contain life support, tracking, communications, emergency escape and attitude control with as much redundancy as possible and do it with a slide rule.
Since many experts at the time felt that zero-G and the space environment would incapacitate an astronaut due to everything from psychological stress to eyeballs that wouldn’t focus, automatic systems were necessary for critical functions.
And there was no on-board computer. Power came from batteries, running through temperamental and hot-running inverters. Attitude control, critical for the re-entry phase, was through mechanical gyros, primitive optical horizon scanners and the seat of the pilot’s pants.
Control rockets? Forget it. Mercury capsules were oriented by a monopropellant reaction control system that used platinum catalysts to decompose hydrogen peroxide into jets of steam. If the electrical systems failed, a manual proportional system mechanically opened valves to fire the jets.
Getting home meant not only orienting the capsule with the retro rockets pointed in the right direction, but firing them at the exact second needed to bring the capsule down where recovery ships were waiting. This meant that timing was everything, kept with electromechanical sequencers used with two on-board clocks to keep the mission in synch.
Glenn’s first words at launch were, “Roger. The clock is operating. We’re underway.”
And the spacecraft was small. Boosters at that time were designed to carry nuclear weapons and had little throw weight to spare, so weight was critical. This meant a very cramped capsule and weight-saving measures everywhere from explosive bolts on the pilot hatch to a phenolic resin ablative heat shield that sacrificially burned to carry away the blazing heat of re-entry.
The spacecraft was designed and built by McDonnell Aircraft Corporation (now part of Boeing) and used exotic metals like beryllium and Rene 41 riveted and welded together by hand, without automation.
Propelling Colonel Glenn into orbit was another masterpiece of engineering, with the General Dynamics/Convair Atlas booster. Weight was critical here too and one gifted engineer, Karel Bossart, had a provocative idea: why not replace the rocket’s airframe with a thin-walled, pressurized structure like a party balloon?
The Atlas missile was eventually highly successful with its thin stainless steel sheet structure held up by nitrogen gas, but at the time the Mercury Project was introduced, Atlas was more famous for a string of failures that resulted in massive explosions that destroyed the vehicle and sometimes the launch pad too.
Reliability had to be engineered concurrently with light weight, but if it all went wrong, Glenn and the other six Mercury astronauts had to rely on a tractor rocket that would yank the complete capsule assembly off the exploding booster, then automatically deploy the parachutes and rescue equipment.
Since no human’s reaction time could trigger this system fast enough in a catastrophe, an Abort Sensing and Implementation System was added to do it automatically. The system was actually tested in flight unexpectedly when an unmanned Mercury-Atlas vehicle exploded.
As you test and instrumentation engineers know, if you add enough sensors to a system, they become a source of new failure modes themselves. This happened to Glenn during his Mercury Atlas 6 flight, when a limit switch intended to signal the deployment of his capsule’s landing impact attenuation bag erroneously triggered an alarm in Mission Control. If true, the heat shield would have been waving like a flag a yard behind the capsule, a fatal problem during re-entry.
The workaround was to re-enter the atmosphere without jettisoning the spent re-entry retro rocket pack, which was strapped to the heat shield. In theory, the straps would hold the shield in place until air loads took over.
The warning signal was erroneous, but the new re-entry mode required Glenn to override the automatic sequencers and perform critical milestone events manually between retrofire and atmospheric entry. He followed the impromptu procedures radioed from Mercury Control and the rest, as they say, is history.
Of course it all ended well, but an important lesson for engineers who work in mission or safety-critical system building is rarely understood: The end user, whether an astronaut or a barista at Starbuck’s has to have faith that their system is robust, reliable and safe. As important as it is for a system to be good, the user has to know it’s good.
Design and build to boost reliability and confidence too.