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A video and a question about Mun approach


Sarxis

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I like watching these old space videos: 

 

My question is about approaching the Mun - is injecting into orbit more efficient in terms of west to east or east to west?  It never occurred to me that there could be a difference.  I typically set up standard 0 degree inclination 'West to East' orbits if I don't need anything special.  But I just remembered that the Apollo missions seemed to do 'East to West' orbits around our Moon.  Does it really make a difference in dV?  Does it make a difference in return/approach velocities?

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In KSP it doesn't make a difference, the revolution velocity of the Mun is too low.

In real life, it is more efficient to go in a West to East orbit (prograde) for a Moon landing just like it is more efficient to launch to the East: you add the body's rotational velocity to yours. Though being tidally locked to the Earth, the Moon rotational velocity is small.

Now, Apollo used free-return trajectories to ensure they would go back home if something happened on the way to the Moon (cf: Apollo 13). These involve using a gravity assist from the Moon to slow down, and therefore to pass in front of the Moon as opposed to behind normally. This implies a East-West orbit.

Edited by Gaarst
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8 minutes ago, Gaarst said:

Now, Apollo used free-return trajectories to ensure they would go back home if something happened on the way to the Moon (cf: Apollo 13). These involve using a gravity assist from the Moon to slow down, and therefore to pass in front of the Moon as opposed to behind normally. This implies a East-West orbit.

That was something else I was thinking about: I don't think the Mun allows for free return trajectories, does it? Never really tried it! Seems like I always have to slow down my ships no matter how close I skim the surface.  Did the Apollo missions adjust their incoming velocities to allow for free return? (Thinking about it, yes, I guess you are still in an elliptical orbit around Kerbin when going to Mun via Holman)

Just watched Apollo 13 again and I thought about if they needed to do a burn on the dark side of the Moon to get back to Earth. 

Edited by Sarxis
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11 minutes ago, Sarxis said:

That was something else I was thinking about: I don't think the Mun allows for free return trajectories, does it? (Never really tried it!) Seems like I always have to slow down my ships no matter how close I skim the surface.  Did the Apollo missions adjust their incoming velocities to allow for free return? 

Just watched Apollo 13 again and I thought about if they needed to do a burn on the dark side of the Moon to get back to Earth. 

A free return trajectory around the Mün is entirely possible; I use them for most trips I make around there. You have to plot your trans-Münar injection burn a little later than normal, and pump a little more ∆v into it, but it will take you around the Mün and then take you back to Kerbin with a periapsis of whatever you think is reasonable (I've found 25-30km to work the best).

9cdrWui.png

As you can see, for only 853.0ms-1 you can fly to the Mün, go around it, and then drop back into the sea. When you reach the Mün you will need to adjust your trajectory to fit your needs better, but in the event of an emergency, this will take you home without any expenditure of fuel.

Edited by CommanderSmith
Image + description of trajectory shown
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17 minutes ago, Sarxis said:

That was something else I was thinking about: I don't think the Mun allows for free return trajectories, does it? Never really tried it! Seems like I always have to slow down my ships no matter how close I skim the surface.  Did the Apollo missions adjust their incoming velocities to allow for free return? (Thinking about it, yes, I guess you are still in an elliptical orbit around Kerbin when going to Mun via Holman)

Just watched Apollo 13 again and I thought about if they needed to do a burn on the dark side of the Moon to get back to Earth. 

Adjustments are always done to have the safest reentry possible or shorter transfer time (especially important for Apollo 13), but the TLI is done so that even if the CSM doesn't do a single maneuver, it will reenter Earth.

You can do a free return trajectory with the Mun, you just need to come in front of the Mun and a lot of tweaking until you find the right spot.

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1 minute ago, Gaarst said:

Adjustments are always done to have the safest reentry possible or shorter transfer time (especially important for Apollo 13), but the TLI is done so that even if the CSM doesn't do a single maneuver, it will reenter Earth.

You can do a free return trajectory with the Mun, you just need to come in front of the Mun and a lot of tweaking until you find the right spot.

So in the movie Apollo 13, when the one NASA guy says 'We get them on a free return trajectory', he means 'we get them back/keep them on their free return trajectory'.

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Basically, but apollo 13 also had issues that the life support would only last so long, so they needed to get them home faster than just letting the trajectory resolve itself. That little disaster had a lot of effect on how NASA did it's emergency planning. 

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Just now, Sarxis said:

So in the movie Apollo 13, when the one NASA guy says 'We get them on a free return trajectory', he means 'we get them back/keep them on their free return trajectory'.

Actually, I've read up a bit and it seems like Apollo 13 was not on a proper free return trajectory at the time of the incident. Because of the land site, free return trajectories like the previous missions were impossible; so the TLI got the CSM in a free return orbit that didn't pass around the Moon. They then adjusted the trajectory for landing; and when the accident happened, the changed trajectory to a circumlunar path that would get them back to Earth.

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23 hours ago, CommanderSmith said:

As you can see, for only 853.0ms-1 you can fly to the Mün, go around it, and then drop back into the sea.

I'm glad someone else uses this number. Since about 0.22 or so I've always made my Mun transfer nodes exactly 853m/s, and then adjust the timing of the node from there.

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22 hours ago, Gaarst said:

when the accident happened, the changed trajectory to a circumlunar path that would get them back to Earth.

Intuitively, this makes sense to me - if the CSM passed between Earth and Luna, it would have been accelerating towards Earth at Earth gravity minus Luna gravity, whereas by passing behind Luna, Luna and Earth's gravity combined to result in a higher acceleration, therefore higher return velocity, and therefore a shorter trip. Basically like a slingshot home.

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17 hours ago, regex said:

I'm glad someone else uses this number. Since about 0.22 or so I've always made my Mun transfer nodes exactly 853m/s, and then adjust the timing of the node from there.

They also work up to at least 872m/s which gets you home a little faster if you have limited life support (a la Apollo 13)

I made a Kos script once which did a complete mun mission and it found the Munar burn point by getting into an 80-82km orbit, waiting until Mun was directly overhead then warping forward a preset time and making an 872m/s node which meant the Pe at Mun was reliably between 10-20km and Pe on return was 40-50km.

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