A couple of years ago, before any of the lunar flights, GCD started looking into improvements in the LM descent guidance and navigation (G&N) computer programs to compensate for possible problems in rough terrain, landing radar performance, descent targeting by the ground, etc. Actually, they were quite successful; they conceived the so-called delta guidance, prefilter, and terrain model package which substantially increases the LPD capability at a very reasonable descent propellant cost. Since then we have performed two lunar landings, including the pin-point Apollo 12, which have pretty well eliminated the original need which the modifications were to satisfy.

But, delta guidance does provide a chance to make a big ∆V saving in the earlier braking phase of descent by compensating for the inability of the descent engine to throttle near the max-thrust setting. So the decision had to be made - is the ∆V saving (i.e., 90 fps which is equivalent to 300 lbs payload to the moon’s surface, or to 20 seconds of hover time) valuable enough to extensively revise the LM G&N program and to modulate the descent engine through the non-throtteable zone up to 10 times?

An additional data point to be considered before making that decision is the fact that about one-half of that ∆V savings can be obtained in other ways. One way is to change the targeting, which has no effect on the on-board guidance or procedures at all, but is not so conservative about protecting against simultaneous DPS valve failures and a low performing DPS engine. A second approach is to develop a procedure for throttling the DPS engine down only once during the braking phase for a period to be determined at the start of descent based on either on-board or ground-computed estimates of actual DPS performance.

The decision is - do not implement delta guidance (tearing up the LGC program is not worth the 40 or 50 fps extra that it would provide); do implement one or a combination of both of the alternates noted above.

Some small program and display changes may be implemented to provide an on-board capability - either auto or manual - to throttle the DPS.

Incidentally, there is one survivor from this delta guidance program change “package”. There appears to be unanimous agreement that we should add the terrain model of the specific landing site we’re going to in place of the present “billiard ball” moon. This will eliminate some objectionable pitch excursions and will make the LPD work better.

Terms & Abbreviations


Descent Propulsion System.


Guidance and Navigation.


Guidance Control Division.


Lunar Module Guidance Computer.


Lunar Module. Earlier it was known as the Lunar Excursion Module and abbreviated “LEM.” Even after the name change, it continued to be pronounced “lem.”


Landing Point Designator. A set of marks on the LM window that helped the crew locate the designated landing area.