by Nitpicker » Tue Jan 06, 2015 6:54 am
Chris Peterson wrote:
This might be of interest.
M-933-72-17-T6.jpg
If I'm reading this correctly, the initial descent orbit insertion (DOI-1) was performed by the CSM while it was still docked with the LM. This placed the pair in a highly eccentric orbit (59 / 14.5 nmi). Then we have separation, followed by a short CSM burn that placed it in a slightly different orbit (61.5 / 11.5 nmi). In this orbit, it was lower than the LM at times. Then, the CSM performed another burn to circularize its orbit (required for redocking). Here, too, the CSM perilune (54 nmi) is below the LM apolune (59 nmi). Finally, the LM performs DOI-2, increasing its apolune and decreasing its perilune (59.6 / 6.2 nmi). Still, we have orbit segments where it is higher than the CSM.
These various orbits were maintained for more than two hours after undocking, so there were ample opportunities to photograph the CSM between the Moon and the LM. I think these images were taken shortly before the PDI burn that braked the LM out of lunar orbit and began the final descent.
I've been looking at this a little more closely ... these low lunar orbits have a period of about two hours, so the source images for the APOD, one orbit before landing, are most likely to have been taken about half an hour after the "CSM SEP" burn and about an hour before the "CSM CIRC" burn. This makes more sense, as I calculate the LM to be only about 18 vertical kilometres above the landing site, which in turn is just less than 5 km higher than the arbitrary zero vertical datum of 1,730 km from the centre of the Moon (
which I suspect was used for the Apollo missions). This would put the LM's altitude, relative to the zero datum, at about 23 km, or 12 to 13 nautical miles, which is not too far off the quoted "RESULTING PERILUNE" of the "DOI-1" burn, which occurred about nine orbits earlier. (I would expect the LM to be near its perilune, one orbit before landing.) Also, the quoted "VELOCITY CHANGE" of 1.0 ft/s generated by the little "CSM SEP" burn, would have separated the two craft by 700 metres (my earlier estimation) after 38 minutes and given the CSM a slightly lower perilune, so that is a close match as well.
Note that my estimations were not made with terribly high precision and are predicated on the lens focal length being equal to the nominal 60 mm, which may not be exact in this image. But I'm confident I'm in the right ball park.
[quote="Chris Peterson"]
This might be of interest.
[attachment=0]M-933-72-17-T6.jpg[/attachment]
If I'm reading this correctly, the initial descent orbit insertion (DOI-1) was performed by the CSM while it was still docked with the LM. This placed the pair in a highly eccentric orbit (59 / 14.5 nmi). Then we have separation, followed by a short CSM burn that placed it in a slightly different orbit (61.5 / 11.5 nmi). In this orbit, it was lower than the LM at times. Then, the CSM performed another burn to circularize its orbit (required for redocking). Here, too, the CSM perilune (54 nmi) is below the LM apolune (59 nmi). Finally, the LM performs DOI-2, increasing its apolune and decreasing its perilune (59.6 / 6.2 nmi). Still, we have orbit segments where it is higher than the CSM.
These various orbits were maintained for more than two hours after undocking, so there were ample opportunities to photograph the CSM between the Moon and the LM. I think these images were taken shortly before the PDI burn that braked the LM out of lunar orbit and began the final descent.[/quote]
I've been looking at this a little more closely ... these low lunar orbits have a period of about two hours, so the source images for the APOD, one orbit before landing, are most likely to have been taken about half an hour after the "CSM SEP" burn and about an hour before the "CSM CIRC" burn. This makes more sense, as I calculate the LM to be only about 18 vertical kilometres above the landing site, which in turn is just less than 5 km higher than the arbitrary zero vertical datum of 1,730 km from the centre of the Moon ([i]which I suspect was used for the Apollo missions[/i]). This would put the LM's altitude, relative to the zero datum, at about 23 km, or 12 to 13 nautical miles, which is not too far off the quoted "RESULTING PERILUNE" of the "DOI-1" burn, which occurred about nine orbits earlier. (I would expect the LM to be near its perilune, one orbit before landing.) Also, the quoted "VELOCITY CHANGE" of 1.0 ft/s generated by the little "CSM SEP" burn, would have separated the two craft by 700 metres (my earlier estimation) after 38 minutes and given the CSM a slightly lower perilune, so that is a close match as well.
Note that my estimations were not made with terribly high precision and are predicated on the lens focal length being equal to the nominal 60 mm, which may not be exact in this image. But I'm confident I'm in the right ball park.