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Glom
2009-Jul-28, 08:14 PM
Up to Apollo 12, the Descent Orbit Insertion burn was done by the LM after separation from the CSM. From Apollo 13 onwards, it was done by CSM before separation. After separation the CSM returned to the higher circular orbit.

Why the switch? Yes it means the LM can burn less fuel, but this is greatly increasing the overall fuel budget since now the heavy CSM must also be point into the descent orbit and returned to circular orbit afterwards.

What gives?

matthewota
2009-Jul-28, 08:18 PM
The CSM fired its engine to lower the perilune to about ten miles. This increased the LM's fuel budget considerably.

Glom
2009-Jul-28, 08:23 PM
The CSM fired its engine to lower the perilune to about ten miles. This increased the LM's fuel budget considerably.

Yeah, but at the expense of the overall mission budget.

Nick Theodorakis
2009-Jul-28, 08:26 PM
Yeah, but at the expense of the overall mission budget.

Would it be more prudent to give the LM more flying time in the event it had to move around for a suitable landing site, as 11 did? It seems the CSM mission profile was more predictable and could get by with less reserve.

Nick

ToSeek
2009-Jul-28, 08:35 PM
I can't find immediate support for this online, but I believe it also allowed for more payload on the lunar modules - I don't think they could have carried the rovers otherwise.

Glom
2009-Jul-28, 08:38 PM
I can't find immediate support for this online, but I believe it also allowed for more payload on the lunar modules - I don't think they could have carried the rovers otherwise.

So it is the case that allowing more payload for the LM in particular was more valuable than keeping the propellent requirements on the CSM down?

It can't have been the rovers specifically though because I believe Apollo 13 was to be the first mission to use this technique.

Larry Jacks
2009-Jul-28, 08:49 PM
So it is the case that allowing more payload for the LM in particular was more valuable than keeping the propellent requirements on the CSM down?

That sounds reasonable. From the beginning, the LMs were far more weight sensitive than the CSM. I've read that the LM-5 vehicle used on Apollo 11 was the first one that was actually light enough to successfully land. As Grumman built each LM, they managed to improve the design and reduce the basic empty weight. However, Apollo 11 had the easiest descent profile in the sense that they weren't going after a restricted landing space. Later Apollo missions were more demanding and needed all of the margin they could get. The "J" missions (15-17) incorporated design improvements to improve the descent engine's Isp (IIRC, by about 3 seconds), increase duration on the surface, and carry additional equipment like the rovers. Still, they needed all the help they could get. It seems better to use some of the surplus capacity of the CSM to increase the LM's chances of success.

Tuckerfan
2009-Jul-28, 11:29 PM
Part of the reason that A11 had an unusual descent was that no one had taken into account the amount of thrust provided to the LM as it pulled away from the CSM as air leaked out from the dock. In follow up missions, they used this thrust to their advantage, in order to have added fuel available to the LMs on the way down.

ugordan
2009-Jul-28, 11:37 PM
Tuckerfan, can you give some references to that?

Tuckerfan
2009-Jul-28, 11:42 PM
Tuckerfan, can you give some references to that?

Its discussed in the official biography of Neil Armstrong First Man. It may be mentioned in Chaikin's A Man on the Moon as well.

ugordan
2009-Jul-28, 11:48 PM
Thanks, I'll have to check that out. Looks like even the tiniest of perturbations messed with their downtrack landing error.

Tuckerfan
2009-Jul-29, 12:05 AM
Thanks, I'll have to check that out. Looks like even the tiniest of perturbations messed with their downtrack landing error.

It all has to do with the distances involved. A one inch error in leaving Earth orbit would have had them missing the Moon by millions of miles (and they didn't do a course correction burn enroute).

kleindoofy
2009-Jul-29, 02:03 AM
Here's something on that venting:


102:36:18 Armstrong: (To Houston) Our position checks down range show us to be a little long.

102:36:21 Duke: Roger. Copy. (Heavy Static)

[In a post-mission analysis, Apollo Descent and Ascent Trajectories, Floyd Bennett notes that, at PDI, Eagle was about 3 miles farther downrange than planned, due to "small delta-V inputs to the spacecraft state in coasting flight. These inputs were from uncoupled RCS attitude maneuvers and cooling system venting not accounted for by the propagation of the predicted navigates state at PDI."]

[Journal Contributor Ron Wells calls attention to discussions in Gene Kranz's Failure is Not an Option which attribute the positional error at PDI to residual pressure in the tunnel between the LM and CSM at undocking at 100:12:00. In a 2002 e-mail, Kranz elaborates, "Floyd's note was correct on the velocity-induced position error at the start of the descent. There were several interrelated navigation problems, i.e. known deficiencies in the R2 lunar potential model, cross- and down-track propagation errors, and errors induced by maneuvering of the spacecraft. The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock. Page 82 of the Apollo 11 post mission report says ....'because of uncoupled attitude maneuvers such as hot fire tests, undocking impulse, station keeping, sublimator operation and possible tunnel and cabin venting. The net effect of these perturbations was a sizeable down-range miss.' To my recollection, the trajectory reconstruction determined that with the exception of the tunnel venting, most of the other perturbations were essentially self canceling. Further the post mission review indicated that the delta P gauge was too gross, the markings misleading and the tunnel had to be vented earlier in the timeline and the valve left in the tunnel vent position rather than returned to off."]

http://history.nasa.gov/alsj/a11/a11.html "The First Lunar Landing"/time code