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did Apollo go against the rotation of the Moon?


fommil

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When I land on the Mun in KSP, I always approach from the right side so that I will be inserted into a counterclockwise orbit (the Mun also rotating counterclockwise). If I was to visualise this, with the return journey as well, it would probably look a bit like a pushbike's chain on the two gears.

But when I see visuals of Apollo 11, it always looks like the lander approached in a clockwise manner (it looks like a 9 on top and a 6 on the bottom). Is that just for marketing or is there some science behind it? Does the Moon actually have a clockwise rotation (I thought it must be counterclockwise to hold one face toward us),  or is there some other good reason to do it this way? I'd have thought that approaching a rotating body from the wrong orbit would just mean having to pay a lot of extra deltav (and again on takeoff), but perhaps there is an advantage I've not considered.

I've also seen versions like this that have both on it: now I definitely would hope that the return journey is the one that brings back the couterclockwise insertion into the Earth's atmosphere!

ede8c646234163.584cc1f041041.png

Edited by fommil
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Yes, they went around the moon retrograde. This was so they would have a "free return trajectory" in case anything went wrong (and on Apollo 13, it did).

If you go around the moon in a prograde manner, if you don't brake into orbit you will probably get gravity boosted into a  higher orbit. But if you go around in a retrograde manner, you get the opposite effect. So the point was that if for some reason they were not able to brake into lunar orbit, they would automatically end up heading back to Earth for re-entry.

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You can also do a high-energy free-return where you circle the moon in a counterclockwise fashion but with a very high apogee, so you don't really get affected by the moon's gravity on the outward journey but you do a close pass on the return journey that will nudge you back into an Earth entry interface trajectory. This is useful if you want to take maximum advantage of the Oberth affect and you want to frontload some of that energy using your launch vehicle during the TLI burn and use less dV for a counterclockwise low lunar capture burn.

But this sends you back into a free return with a much higher Earth entry velocity than you would get from a retrograde free-return, so your heat shield's mileage may vary.

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4 minutes ago, sevenperforce said:

You can also do a high-energy free-return where you circle the moon in a counterclockwise fashion but with a very high apogee, so you don't really get affected by the moon's gravity on the outward journey but you do a close pass on the return journey that will nudge you back into an Earth entry interface trajectory. This is useful if you want to take maximum advantage of the Oberth affect and you want to frontload some of that energy using your launch vehicle during the TLI burn and use less dV for a counterclockwise low lunar capture burn.

But this sends you back into a free return with a much higher Earth entry velocity than you would get from a retrograde free-return, so your heat shield's mileage may vary.

Very high apogee? how does that affect trip / return time?

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1 minute ago, sevenperforce said:

You can also do a high-energy free-return where you circle the moon in a counterclockwise fashion but with a very high apogee, so you don't really get affected by the moon's gravity on the outward journey but you do a close pass on the return journey that will nudge you back into an Earth entry interface trajectory. This is useful if you want to take maximum advantage of the Oberth affect and you want to frontload some of that energy using your launch vehicle during the TLI burn and use less dV for a counterclockwise low lunar capture burn.

But this sends you back into a free return with a much higher Earth entry velocity than you would get from a retrograde free-return, so your heat shield's mileage may vary.

That sort of free-return also takes longer. Not a good thing when we are talking about life support and radiation exposure.

Anyway, the point is that they were not worried about which way the moon was rotating. The moon only rotates once per month (as opposed to the Earth's one per day), so it's not a big deal.

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4 minutes ago, StrandedonEarth said:

Very high apogee? how does that affect trip / return time?

Definitely takes much longer, as @mikegarrison notes. Which is not ideal for life support, obviously.

However, it is a way to get a free-return while also reducing the size of the insertion burn. I've done it a number of times in KSP when I have extra margin on the TLI stage but I still want a free-return. Obviously in KSP I could leave the TLI stage attached since you get unlimited restarts and infinite throttling, but it doesn't look as pretty. IRL, it could have been used for something like Artemis 1 to increase an uncrewed Orion's cislunar capabilities while also setting up for a re-entry test.

1 hour ago, mikegarrison said:

If you go around the moon in a prograde manner, if you don't brake into orbit you will probably get gravity boosted into a  higher orbit. But if you go around in a retrograde manner, you get the opposite effect.

With the high-apogee prograde free-return, you actually are still being gravity boosted into a more energetic orbit, but your trajectory is being altered so that you end up going back through Earth's atmosphere with that higher energy.

You can also use this kind of a maneuver to do a more energetic Oberth maneuver at perigee for a deep space mission. The faster you're going when you start the final ejection burn, the more help Oberth will give you, but there's an obvious limit to how high your apogee can be. Accordingly, you could use this kind of trajectory to get a lunar gravity assist added to your ejection energy before your final ejection burn. Don't know whether this has ever been done in real life.

26 minutes ago, mikegarrison said:

Anyway, the point is that they were not worried about which way the moon was rotating. The moon only rotates once per month (as opposed to the Earth's one per day), so it's not a big deal.

Yep. The rotational direction doesn't matter but the orbital direction does, obviously. That's where the gravity assist (positive or negative) comes from.

5 minutes ago, sevenperforce said:

You can also use this kind of a maneuver to do a more energetic Oberth maneuver at perigee for a deep space mission.

Apropos of nothing (somewhat), this is essentially the maneuver depicted in the xkcd comic that first introduced me to Kerbal Space Program:

six_words.png

 

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I think that same XKCD is what first brought KSP to my attention as well.

I think I had heard of it before but only in the context of "little green men trying to get to space" or something, which made it sound more like a tycoon management game. But that comic intrigued me.

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2 hours ago, sevenperforce said:

Apropos of nothing (somewhat), this is essentially the maneuver depicted in the xkcd comic that first introduced me to Kerbal Space Program:

six_words.png

 

This was an real planned probe for close flyby of the suns pole using Jupiter flyby to lower Pe and change inclination. 
It would work both in real life and in KSP. Problem was the Jupiter flyby who takes time and would require an RTG

They did it with multiple earth and venus flybys instead as I remember. 

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50 minutes ago, magnemoe said:

This was an real planned probe for close flyby of the suns pole using Jupiter flyby to lower Pe and change inclination. 
It would work both in real life and in KSP. Problem was the Jupiter flyby who takes time and would require an RTG

They did it with multiple earth and venus flybys instead as I remember. 

Are you talking about the Parker Solar Probe?

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42 minutes ago, mikegarrison said:

Are you talking about the Parker Solar Probe?

Yes, but did not remember the name of it :) 

Has done the free return trip tricks in KSP , here the reasoning was power, did not have solar cells and was worried I would run out of power. Plays with life support so capsules always draw some power. 

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22 hours ago, mikegarrison said:

This was so they would have a "free return trajectory"

I missed this on my first reading, thank you that explains the reasoning really well, so it wasn't just an arbitrary choice. I feel like this is the most important thing I learnt all week :lol:... well, that, and remember to put the solar panels onto a Moho probe...

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On 9/15/2022 at 7:04 PM, mikegarrison said:

Yes, they went around the moon retrograde. This was so they would have a "free return trajectory" in case anything went wrong (and on Apollo 13, it did).

If you go around the moon in a prograde manner, if you don't brake into orbit you will probably get gravity boosted into a  higher orbit. But if you go around in a retrograde manner, you get the opposite effect. So the point was that if for some reason they were not able to brake into lunar orbit, they would automatically end up heading back to Earth for re-entry.

Also, as I just learned the other day, the surface rotation of the moon is so slow, being tidally locked, that it really doesn't matter much which direction you land from; at least not when weighed against the safety benefits of having the free return abort option

Edited by darthgently
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