See list attachedMay 12, 196969-PA-T-76APA/Chief, Apollo Data Priority CoordinationG mission lunar descent is uphill – all the way
Just in case you didn't know, I thought I would send you this note about some nominal G mission landing site characteristics which I thought were kind of interesting. First of all, apparently this landing site (2-P-6) is about 9,000 feet lower than the mean lunar radius. The significance of this, of course, is that all ascent and descent targeting – in fact, all lunar altitudes – are referenced with respect to the landing site radius. That is, the 60 mile circular, LOI orbit is targeted with respect to the landing site and thus is lower by 9,000 feet than you might have assumed. But more important, the insertion altitude after ascent which is nominally 60,000 feet above the landing site is really only 51,000 feet above the mean lunar surface and, of course, less than that over the bumps.
Another interesting characteristic is that the approach to this landing site is even lower. Specifically, the estimated slope of the lunar surface as the spacecraft approaches the landing site is about 1° up- hill. This in itself appears to be tolerable, although it does perturb the descent trajectory a little causing the approach angle to be low – that is, toward the visibility washout direction. Something we do want to look into about this was brought out by Bernie Kriegsman (MIT) the other day. One of his computer runs showed that during the final portion of the descent trajectory under automatic control, the spacecraft would actually stop descending and would achieve a positive altitude rate prior to landing. The dispersion that caused this was a 1° slope uncertainty in the lunar datum, which when added to the aforementioned estimated slope resulted in a 2° uphill grade. We are going to have to cross-check this to see if this is really what happens. If it is, we are going to have to look in to the effect of this on how the crew would respond and how the landing radar works under this condition.