Answers Your Questions
| From: Andrew Nguyen, of Los
Angeles, Calif. |
To: Wayne Hale, flight director
Question: What is meant when the public affairs officer says "guidance is converging" in the shuttle launch videos?
Answer: During first stage -- liftoff to SRB separation -- the shuttle guidance software is operating "open loop." In other words it is essentially following a set of pre-planned actions based more on its velocity than directly on the actual vehicle performance. At SRB separation, the shuttle software transitions from first stage (major mode 102) to second stage (major mode 103). Unlike first stage guidance, second stage guidance is "closed loop." The guidance software attempts to calculate how to fly from where it is to where it wants to go, which is the projected main engine cutoff (MECO) point at a specific altitude, velocity, flight path angle and orbital plane. The software must cycle through a number of times to "converge" on a good solution (think of this process as starting with an estimate, then refining the estimate until the exact value is determined). During each guidance cycle, it recomputes all of the values along with a "miss" error. If the miss error is less than 2 percent of the total value and remains that way for several guidance cycles, the guidance solution is considered good. This is referred to as "converged guidance" and is what the PAO is passing along.
In the unlikely event that the miss error does not meet that criteria within the requisite five guidance cycles, the guidance solution is bad ("unconverged guidance"). Unconverged guidance means that the shuttle is flying to the wrong place. For that reason, the crew is trained to manually fly the ascent and all intact ascent aborts. They will take over control from the computers if the guidance does not converge within 10 seconds after the transition to second stage.