Apollo Discussions > The Reality of Apollo
Lunar Orbit
ka9q:
The reason for the two LOI burns on 8, 10, 11 & 12 was simple engineering conservatism. Shortening the first LOI burn increases its margin against a dangerous overburn. The resulting elliptical orbit can be carefully tracked and compared with the expected orbit to analyze SPS performance and to reveal any unexpected platform drift, accelerometer bias, or errors in the gravity model. Then the more critical circularization burn can be done more safely.
With the experience of these four flights, NASA decided that they didn't need to be quite so conservative and it was safe enough to perform LOI in a single burn.
I knew about the CSM performing the descent orbit maneuver starting with A14. I've always seen that as an indication that the original spacecraft mass budget probably wasn't optimum. It allocated too much to the CSM and not enough to the LM. It's more efficient overall to put only the LM in the descent orbit than both the CSM and LM, but doing that with sufficient margin would entail a major redesign to enlarge the LM's descent propellant tanks. The CSM could be kept as-is, with some of its SPS propellants offloaded if necessary to conserve total mass.
The SPS did have a slightly greater Isp (314.7 sec measured during Apollo 15) than the LM's DPS (305.8 sec measured on Apollo 15 during the full throttle braking phase). This might have tilted the balance in favor of having the CSM perform the descent orbit maneuver, but I'd have to work out the detailed numbers.
ginnie:
--- Quote from: Bob B. on May 13, 2012, 10:36:02 AM ---
--- Quote from: Kiwi on May 13, 2012, 08:22:43 AM ---
--- Quote from: Coelacanth on May 12, 2012, 05:10:01 PM ---How low can one go and still be "safe", where you don't have to worry about the perturbations causing you to form a new impact crater?
--- End quote ---
I was a bit confused over this post until I read Ka9q's reply, because I thought you were asking our honourable Master of Ceremonies, LunarOrbit, in a rather clever way but in the wrong part of the forum, how low you could stoop as a poster before he went ballistic and dug a hole to bury you in because of "perturbations" from other members.
--- End quote ---
That's what I thought he meant too.
--- End quote ---
I thought the thread was about the moon getting closer and closer to the earth.
In fact, the moon is increasing its distance from the earth.
gwiz:
Just to add this mission to the debate. They've been orbiting at a mean altitude of 55 km up to now, but plan to go down to 23 km.
http://www.nasa.gov/mission_pages/grail/news/grail20120529.html
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