19 February 2010

At the time of Project Mercury, the Atlas as a launch system had a reliability rating of 90%, while acceptable for a weapons system, was unsuitable for a manned rocket launch vehicle. As a result, a whole series of initiatives under the umbrella of what was called the Pilot Safety Program was put into place to improve not just the reliability of the Atlas rocket for Project Mercury, but also to improve its safety. The most visible of the resulting efforts as the escape tower on the Mercury spacecraft that could pull it away from the Atlas in case of a malfunction.

But from the production floor at Convair-San Diego to the launch pad at Cape Canaveral in Florida, steps were taken with the Atlas rocket wherever possible to provide what would later be called a "man-rating" for the vehicle. As every possible malfunction couldn't be prevented or accounted for without unreasonable cost, Convair created the ASIS system- Abort Sensing and Implementation System. In short, ASIS was a collection of sensors placed at key points and components of the Atlas rocket. A malfunction detected by any of the ASIS sensors could trigger the escape tower that would pull the Mercury spacecraft clear of the Atlas. It was estimated that 1/3 of the cost of the Mercury-Atlas vehicle was invested in ASIS. In addition, redundancy had to built into ASIS to prevent any malfunctions of the safety system itself.

Part of ASIS was a key ability for the astronaut to trigger the abort should there be a failure of any sort that didn't trigger the system. There was a switch on the instrument panel of the Mercury spacecraft that if flipped by the astronaut, would trigger the rockets of the escape tower and pull the spacecraft away from the booster. The engineers nicknamed it the "Chicken Switch" and Convair management was adamant that it not be referred to as such and in particular with the Mercury astronauts as they feared what it would do to their morale. On their first visit to the Atlas plant in San Diego, the first question from the astronauts was "So what's this Chicken Switch we've been hearing about?"

Source: Atlas- The Ultimate Weapon by Those Who Built It by Chuck Walker with Joel Powell. Apogee Books, 2005, p232-234.


  1. I worked on Mercury at McDonnell in St. Louis from April of 1959. The escape tower was a part of the design at that time and it was used in the suborbital flights as well as the Atlas orbital flights. It has been a long time to remember the details but the tower was intended for lower altitude escapes and was jettisoned by a small nozzle in the middle of the three larger ones.

    Bob Axsom
    MAC badge number 86305

  2. Thanks for reading and commenting, Bob. One facet of the US space program that has always fascinated me is how different aerospace companies' designs were integrated into space vehicles- McDonnell's Mercury spacecraft mated to Convair's Atlas ICBM. Apparently the first test launches (unmanned), the weight of the adapter and the Mercury spacecraft caused the forward part of the Atlas to buckle. It was eventually solved by using a slightly thicker metal in the forward part of the balloon structure tank.