Highways Into Space: A first-hand account of the beginnings of the human space program by Glynn S. Lunney Page A

spacecraft to a safe separation distance. By the time that the escape tower was jettisoned at about two minutes and thirty seconds, the vehicle was out of the zone of primary concern to the range safety officer. Then, escape was like a normal separation except that the crew action to separate also initiated an engine cutoff command to the Atlas engine. These were the onboard systems to separate the spacecraft.
    Next came the techniques to decide to initiate an abort. Some vehicle failure modes, such as a hardover control signal or engine positioning system, could result in very rapid loss of control and vehicle breakup, especially during the first sixty to ninety seconds when the ship can be experiencing high dynamic pressures and resultant loads. Because of these possibilities, an automatic sensing and implementation system (ASIS) was designed to protect against this type of failure that exceeds the human ability to react quickly enough. This launch vehicle system would then trigger an abort sequence by the spacecraft. As in the destruct system, loss of electrical continuity between the spacecraft and Atlas would also trigger an abort. Operator intervention to initiate an abort was also available to the crew and to the Flight Director.
    Unless we had the luxury of selecting a time to initiate an abort, the spacecraft would land anywhere in the Atlantic based on the trajectory conditions at the time. There was some possibility of landing point control for an abort in the last thirty seconds or so of Atlas flight. After separation at these velocities and altitude, the time of retrofire could be varied from a minimum of thirty seconds to turn around to blunt end forward to a maximum of four minutes and still have coverage of the retrofire sequence from Bermuda. The responsibility for selecting the fire time was assigned to the position of Retro Controller, consistent with his on-orbit responsibility for the calculation and orchestration of the retro fire maneuver. These two positions worked in very close coordination for the return to earth planning, with the FIDO assuring the best quality navigation solution for the retro calculation.
    We spent a lot of time analyzing what combination of velocity and flight path angle would constitute an acceptable orbit. We finally ended up with a boundary of velocity and flight path angles that would provide the spacecraft with enough energy for a safe orbit. On the plot of velocity on the horizontal and flight path angle on the vertical, the go-no-go line looked like a slightly curved bow, as if the line representing a zero flight path angle was an arrow in the bow pointing towards a lower velocity than nominal. The boundary was about one hundred feet-per-second below the nominal cutoff velocity to assure energy for at least one safe orbit before the drag would cause the spacecraft to re-enter. This margin was a very small percentage (less than half of a percent of the required velocity of about twenty five thousand feet-per-second) and the evaluation required to assure an adequate margin could include the judgment of the operator since the random deviations of measured velocity could be a significant portion of the one hundred feet-per-second margin.
    I did not appreciate it enough at the time, but the support of John Mayer was very important and gratifying to me in terms of the confidence he had in my ability to perform this job. For the position of Flight Dynamics Officer (FIDO) in the Mercury Control Center at the Cape, Chris selected Tec Roberts. This choice had been made sometime in 1960. Tec had arrived at STG in April of ‘59 with the AVRO contingent and within one year had established himself as one of Chris Kraft’s key managers and advisors. Although Tec did not have a detailed grounding in orbital mechanics, his common sense and good judgment made him the logical choice for this job. His performance in this role set the standard for the FIDO position and it remains so over four