Midcourse correction maneuver

Flying to the Moon, when you don't have fuel to throw around, is a ballistic lob, a large scale stone throw across nearly 400,000 kilometres of cislunar space. Having achieved the throw, using the last firing of the S-IVB, it is important to know that you are going to end up where you intended at the end of the coast. Taking the analogy of throwing a stone further, if you could tell that the stone was travelling a little too fast or too slow to reach its target, you might want to impede it or give it an extra little push during its flight to achieve an accurate hit.

So it is with lunar or interplanetary travel. By knowing very accurately the three dimensional position of the spacecraft at an instant of time, and knowing the velocity vectors in three axes with which it is moving - a matrix of seven values collectively known as the state vector - you can use computers to calculate where the spacecraft is heading. (The seven values consist of three for position, three for velocity and one for time.) A little too fast and Apollo 15 would reach the Moon's distance before the Moon had got to where it was supposed to be. It would be deflected around the far-side at a higher altitude than intended. It would be more difficult to get into the desired orbit - and be costly in fuel. A little too slow, and their altitude around the Moon's far-side would be lower than intended - in a worst case, they may even impact the lunar surface - and again the desired orbit would only be achieved at the cost of maneuvering fuel. The midcourse correction maneuvers are intended to modify the spacecraft's velocity during the coast so that the final outcome will be a perfect arrival in the vicinity of the Moon, at an altitude of 110 km (60 nautical miles), from where they can achieve their lunar orbit as planned. The earlier a correction is made, the greater its effect on the final outcome for a given use of propellant.

On Apollo 15, there are four planned midcourse corrections during the translunar coast which can be used if required. The firing of the S-IVB was sufficiently accurate, that Mission Control do not feel it is worthwhile carrying out a correction on the first opportunity and are prepared to wait until the velocity error has built up to make the firing of the SPS engine worthwhile, allowing them to test its faulty control bank. Normally, the RCS thrusters could have been used to correct the velocity error at this first opportunity. (ap15fj)