11
$\begingroup$

Did the Apollo command module always pass directly over the lunar landing site? Or were they sometimes in different planes?

My inclination (pun intended) is that staying in the same plane would consume the least amount of fuel. Of course, manual adjustments occurred during the last few hundred feet, so they would land in a desirable spot.

Slightly related: Why didn't Apollo land on the Lunar poles?

$\endgroup$
  • $\begingroup$ There are several planes to think about. If the approach were in the Earth-Moon orbital plane, then they'd automatically be inclined by about 1.5 degrees wrt the Moon's equator, which is roughly the inclination shown in this image. Based on the excellent answers there, I can't answer your question, but I have a hunch the answer is "yes", at least for some of the missions. $\endgroup$ – uhoh Sep 5 at 6:26
  • $\begingroup$ having the CSM in a different plane to the landing site would make LM ascent and rendezvous even tricker. In addition the CSM had excess deltav, so it was easier to do it once there rather than once in the LM descent stage and once in the LM ascent stage (especially with the rocket equation cost to the descent stage for the ascent stage manoeuvrer) $\endgroup$ – JCRM Sep 5 at 10:07
  • 2
    $\begingroup$ Not only does staying in the same or similar plane consume the least amount of fuel, it means that the lander can make the CM rendezvous on each of its orbits. The larger the difference in plane, the more fuel will be consumed, but also the longer it will take to perform the rendezvous. You can trade fuel for time in various ways, but all out-of-plane scenarios take longer than the in-plane ones. This might be very important if the lander was leaking oxygen, or had some kind of battery failure, etc Having the CM pass overhead was also a backup in case Earth lost direct comms with the lander. $\endgroup$ – Innovine Sep 5 at 12:04
19
$\begingroup$

Because the lunar landings happened at some latitude, the landing sites were subject to longitudinal drift due to the Moon's rotation around its own axis.

Due to the small latitude of the first landing[1], less than 1 degree for Apollo 11, and the short stay on the Moon, The LM lunar landing site did not drift far from the CSM orbital plane. Hence, Apollo 11 did not have a CSM plane change maneuver

However all subsequent landings did this.

From the Apollo 17 mission timeline, five and a half hour before LM ascent in this case:

CSM plane change ignition (RCS). 179:53:53.83

CSM plane change cutoff. 179:54:13.88

(Other Apollo missions used the SPS rather than the RCS)

In conclusion, the planes were drifting apart, and a correction maneuver was needed before LM ascent to correct this on all Apollo missions except Apollo 11.

[1] Lunar landing coordinates: https://nssdc.gsfc.nasa.gov/planetary/lunar/apolloland.html

$\endgroup$
  • 1
    $\begingroup$ Since the latitudes were all generally low, and the moon's rotation slow, the drift rate was pretty slow. $\endgroup$ – Russell Borogove Sep 5 at 16:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.