Lunar orbit injection question

[Millitron]

Is there a way to make a single burn for TLI, and be captured by the moon into a stable orbit? Usually when I do the Horizon method, I end up either in a hyperbolic lunar orbit, or on a collision course. Is there a way to aim it perfectly so I am captured without any extra burns? Do I just need to have exceptional aim and luck, or is there some trick to it?

[Hypocee]

Nope! Gravity conserves energy, so any orbit that comes in from outside the Sphere of Influence will also by definition escape at the same velocity. You always need to make a burn to be captured.

[SteevyT]

Well....technically you wouldn\'t leave the Mun\'s SOI if your trajectory was one that intersects the surface once you are captured. But you would hit the surface at escape velocity.

[Ydoow]

Well....technically you wouldn\'t leave the Mun\'s SOI if your trajectory was one that intersects the surface once you are captured. But you would hit the surface at escape velocity.

I think we all learned something about Lithobraking here.

[Cepheus]

Funny thing, this is near exactly the purpose of a Low-energy-transfer. It uses lagrangian points, and is meant to put you into a nice orbit. HOWEVER, there are no lagrangian points in KSP.

[closette]

I\'m guessing that it might be possible to enter the Mun\'s sphere of influence (SOI) and orbit in the current framework. But probably not if you\'re coming from a simple trans-munar elliptical transfer from Earth, because if anything you are going too slowly at Kerbin\'s apoapsis when the Mun comes by flying by you at ~540 m/s (all relative to Kerbin). For that speed to be less than the Munar escape speed you\'d have to be less than 44000m above its surface.

To pull this off, you\'ll need to enter the Mun\'s SOI at a radius of 2430km (see the Wiki) with as little kinetic energy as possible in the Mun-spacecraft reference frame, and yet still having just enough angular momentum to make it around the Mun and not crash into it. At that distance you would need to be moving at less than 230 m/s relative to the Mun, and ideally at 90 degrees to the spacecraft-Mun line.

One way might be to orbit Kerbin in a slightly smaller circular orbit than the Mun\'s, and use this to catch up to it, gradually. That\'s a big orbit, but sounds worth a try. It might even make a good challenge.

[Kosmo-not]

The best you can do it achieve an elliptical orbit that extends past the Mun\'s SOI. Whatever speed you enter the Mun\'s SOI with is the speed you\'ll have exiting it.

[closette]

That\'s right. I just looked back at the original question, and although you could arrange to have negative energy on entering the Mun\'s SoI, you would still have to modify the orbit just a wee bit (at periapsis, ideally) to lower your apoapsis and thus prevent yourself from leaving it.

I thought that 3-body gravitation was complicated, but one still has to think carefully about KSP\'s simplified 2-body model as well.

[samstarman5]

As a wise man once taught me, math shows anything is possible. But what is true on paper, nature will always throw you a curve ball.

With that out of the way, Closette\'s idea is possible. I\'ve seen a shot where someone actually expanded a Kerbin orbit to be closely parallel to the Mun\'s, where the Mun would be bound to capture you at a lesser speed than just a typical TMI. However, such a maneuver would require more time and delta-vee than an ordinary TMI, so it would defeat the purpose of trying to avoid a burn to establish Munar orbit.

Another thought would be keeping Periapsis inside the Kerbin atmosphere, and have Apoapsis positioned over the orbital path of the Mun further along to where you make a return trip to Kerbin, lose velocity skipping through the atmosphere, and then on the outward leg to the Mun, you achieve orbit.

As I type this, it sounds wonderful, but I have to think you would lose too much velocity to re-attain the Mun\'s orbit. And yet there is a part of my brain that likes this. I\'m reminded of the science in the story 2010 where the Leonov used Jupiter\'s upper atmosphere to slow down to attain orbit of Io. What was it Dr. Heywood Floyd said? 'It\'s fire on paper, but the people who wrote the paper are safe back at home.'

[Kosmo-not]

I was thinking about it, and it\'s possible to achieve an orbit around the mun that will decay with time. The idea involves entering mun\'s SOI at orbital velocity and nearly perpendicular with the apoapsis leading the mun\'s direction of orbit. Then, whenever you leave mun\'s SOI, you are slowed down a little by Kerbin before re-entering mun\'s SOI. You will either end up crashing into the mun, or be ejected out when the mun has orbited Kerbin enough.

[samstarman5]

I return with this thought that obviously there is something to this being possible. Otherwise, how would Mars, Jupiter and so on outward have captured their rogue satellites? It\'s not like they had boosters to decelerate.

This is a real brain-teaser.

[NovaSilisko]

I return with this thought that obviously there is something to this being possible. Otherwise, how would Mars, Jupiter and so on outward have captured their rogue satellites? It\'s not like they had boosters to decelerate.

This is a real brain-teaser.

Because of gravitational influence with the other moons. The same applies to the Earth-moon system - there\'s occasionally bits of fluff getting caught in orbit for a few months.

[Hypocee]

And tidal drag plus billions of years.

[VincentMcConnell]

Is there a way to make a single burn for TLI, and be captured by the moon into a stable orbit? Usually when I do the Horizon method, I end up either in a hyperbolic lunar orbit, or on a collision course. Is there a way to aim it perfectly so I am captured without any extra burns? Do I just need to have exceptional aim and luck, or is there some trick to it?

No. You have to make a L.O.I. Lunar Orbit Insertion burn. If your Periapsis is on the near side of the moon (which it should be if you\'re using the Earth Orbital Approach) burn retrograde until you make an orbit around the moon. Correct it by trimming -- The same way you do on Earth.