Apollo Discussions > The Reality of Apollo
Lunar Orbit
Bob B.:
--- 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.
Not Myself:
--- Quote from: ka9q on May 13, 2012, 06:24:43 AM ---To illustrate just how significant the perturbation from the earth can be, consider a non-nominal lunar orbit insertion burn.
If the burn doesn't happen at all, you come back home on your free-return trajectory (assuming you were on one).
If you burn too long, you impact on the near side a half orbit later, just as you'd expect. The astronauts watched the clock, ready to push the manual stop button if the computer didn't stop the burn as it should.
If you burn just a little too short, you'll go into an elliptical lunar orbit with apolune on the near side. Again, just as you'd expect.
But as you decrease the burn time further, something interesting happens. Although the apolune continues to increase, the perilune also decreases. There is a range of burn times, all less than the nominal time, that can result in lunar impact within a single orbit. The cause of this decrease in perilune is the perturbation from earth's gravity at apolune on the near side. You're at a higher altitude, increasing the earth/lunar gravity ratio, and the orbital period also increases, so the earth tugs on you for a longer time.
--- End quote ---
As I would guess they were gunning for a relatively close orbit, if the earth perturbations are still something to worry about, then should I take it that just about any lunar orbit requires relatively frequent course correction? Can you place yourself in an orbit where you can go a few weeks or months without worrying about it, or is that just not possible?
--- Quote from: ka9q on May 13, 2012, 06:24:43 AM ---Apollo had emergency "bailout" procedures in case a dangerously non-nominal burn had ever occurred.
--- End quote ---
I take it that's "bailout" in a figurative sense, since bailing out of the spacecraft would not seem to improve their situation a whole lot.
In any event, thanks for the information. What I was really wondering is whether you can park something in lunar orbit, and not worry about it for weeks or months at a time. My assumption was that the higher orbit would be the more stable, but perhaps that is not the case.
scooter:
I recollect the initial Apollo lunar orbits had an apolune (is that the right term?) of maybe 2-300km, and would be circularized an orbit or 2 later. Was this maybe a safety measure to preclude an overburn/"crater"?
Glom:
--- Quote from: ka9q on May 13, 2012, 06:24:43 AM ---To illustrate just how significant the perturbation from the earth can be, consider a non-nominal lunar orbit insertion burn.
If the burn doesn't happen at all, you come back home on your free-return trajectory (assuming you were on one).
If you burn too long, you impact on the near side a half orbit later, just as you'd expect. The astronauts watched the clock, ready to push the manual stop button if the computer didn't stop the burn as it should.
If you burn just a little too short, you'll go into an elliptical lunar orbit with apolune on the near side. Again, just as you'd expect.
But as you decrease the burn time further, something interesting happens. Although the apolune continues to increase, the perilune also decreases. There is a range of burn times, all less than the nominal time, that can result in lunar impact within a single orbit. The cause of this decrease in perilune is the perturbation from earth's gravity at apolune on the near side. You're at a higher altitude, increasing the earth/lunar gravity ratio, and the orbital period also increases, so the earth tugs on you for a longer time.
Apollo had emergency "bailout" procedures in case a dangerously non-nominal burn had ever occurred.
--- End quote ---
Damn n-body mechanics. Gives my PC a headache.
Bob B.:
--- Quote from: scooter on May 13, 2012, 02:34:45 PM ---I recollect the initial Apollo lunar orbits had an apolune (is that the right term?) of maybe 2-300km, and would be circularized an orbit or 2 later. Was this maybe a safety measure to preclude an overburn/"crater"?
--- End quote ---
The first four lunar flights – Apollo 8, 10, 11 and 12 – performed lunar orbit insertion (LOI) in two separate maneuvers using the SPS of the CSM. The first maneuver, LOI-1, was initiated after the spacecraft passed behind the Moon and crossed the imaginary line through the centers of the Earth and Moon at approximately 160 km above the lunar surface. The SPS burn was a retrograde maneuver that placed the spacecraft into an elliptical orbit that was approximately 111 × 315 km. After two revolutions and a navigation update, a second SPS retrograde burn, LOI-2, was made as the spacecraft crossed the antipode behind the Moon. On Apollo 8 and 10, LOI-2 was an orbit circularization burn that placed the spacecraft in a 111 km orbit. On Apollo 11 and 12, LOI-2 placed the spacecraft in an elliptical orbit approximately 101 × 122 km. This orbit became circularized at 111 km by the time of the LM rendezvous due to the effect of variations in the lunar gravitational potential on the spacecraft as it orbited the Moon.
Starting with Apollo 14, LOI placed the spacecraft in an approximate 107 × 315 km elliptical lunar orbit, similar to previous missions. However, rather than performing the LOI-2 maneuver, the SPS performed the descent orbit insertion (DOI) maneuver, which placed the CSM/LM into an elliptical orbit approximately 20 × 109 km from which the LM would begin the later powered descent to landing. In Apollo 11 and 12, DOI was a separate maneuver using the LM descent engine. The Apollo 14-17 DOI maneuver in effect was a combination LOI-2 and DOI that produced two benefits: conserved LM descent propellant that would have been used for DOI and made this propellant available for additional hover time near the surface, and allowed additional lunar revolutions of spacecraft tracking in the descent orbit to enhance position and velocity data for updating the LM guidance computer during the descent and landing phase. (Apollo 13 was planned as the first flight to employ this technique, but the mission was aborted prior to LOI.)
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