Why are my Mun orbits different from Apollo's?

[Rus-Evo]

Hi all,

Whenever I see an orbit diagram of the Apollo spacecrafts earth and moon orbits, it always looks like a figure 8 (ie looking from the north pole, you would leave the right side of the earth, pass from right to left and enter Mun orbit from the Muns left). I'm sure many of you will have seen this diagram.

But what usually happens to me in the game is that I leave Kerbins orbit on the right hand side but also enter the Muns orbit on the same side (unless something goes wrong). In fact I almost always enter an SOI in an anti-clockwise orbit. Is there a reason for this? I actually do want that orbit so its not a bad thing.

Any thoughts on this? It could be just the way I am setting up my burns.

[FenrirWolf]

It's just a matter of setting up the proper burn. IIRC, a free-return trajectory (the figure-8 path you mentioned) takes a bit more delta-v than a typical hohmann transfer on the initial burn, but the reward is that little to no further course corrections will be needed to complete the flyby and swing back around to reenter the atmosphere.

Anyway, what you want to do is make a maneuver node and at least get an encounter of some kind set up with the Mun. After that, click on the Mun and click the button that says focus view. The camera will change to be centered on the Mun, and you'll also see your path relative to the Mun now instead of Kerbin. Try adjusting your maneuver node some more until you see that you have a retrograde flyby around the Mun. Keep checking both Kerbin and the Mun until you've got a path that both gives you a good flyby but also sends you happily back home afterwards.

[FREEFALL1984]

While its possible to have a figure of 8 shaped shaped free return trajectory it is really tough, this is because KSP works on a sphere of influence based system rather that true n-body physics which would take into account the moons influence even if you're simply in a parking orbit above the earth (wherever that place is) so as soon as you even begin to approach the moon the moons gravity starts to pull the orbit towards it allowing a nice gradual curve and a perfect figure of 8 shaped trajectory, the only way to do it now is to have the moons SOI effectively throw you around the back side of the mun, which means a very slow approach to the muns soi and a low pass across its surface (less than 10km) basically meaning you're actually treating it as two separate highly elliptical orbits.

Hope that helped

[el_coyoto]

As it has been said, you need a free return trajectory if you want the 8 shaped trajectory.

These are a bit less efficient than pure hohmann interceptions, but are safer in case of an engine malfunction.

They are a bit tedious to setup in KSP, but I usually start by planning a burn that brings me to a ~14500 km Kerbin apoapsis, then mess around with the node until I get an encounter with the Mun and the trajectory I want.

This helped me a bit with my first free return trajectory.

[Kasuha]

The thing with free return trajectory is, it is not "fuel saving" trajectory. You need to enter Mun SOI at speed, your Kerbin apoapsis would be about halfway between Mun and Minmus. People usually pull their apoapsis just to Mun and from that trajectory Mun is too big, figure-8 trajectory has Mun periapsis under surface.

[SRV Ron]

The thing with free return trajectory is, it is not "fuel saving" trajectory. You need to enter Mun SOI at speed, your Kerbin apoapsis would be about halfway between Mun and Minmus. People usually pull their apoapsis just to Mun and from that trajectory Mun is too big, figure-8 trajectory has Mun periapsis under surface.

It is just very difficult to set up that maneuver in KSP, but not impossible, compared to doing so using math models on glorified calculators.

[FenrirWolf]

I don't think anyone's claiming it is a fuel-saving maneuver. The point of it in real life was to eliminate possible points of failure. Easier and less risky to perform just one burn in LEO to take care of the whole trip. Even if the plan is to actually circularize the orbit upon arrival and go in for a landing, being on a free return until that point was definitely handy for Apollo 13.

[KSK]

Minor nitpick - Apollo 13 didn't start out on a free-return trajectory. One of the first things they did after the accident was to make a course correction burn to get back onto a free return. But the point about eliminating points of failure still stands.

[Kasuha]

My point was that people often pull their apoapsis right to the Mun orbit, not higher. That's why they usually don't get a free return trajectory.

In KSP, free return trajectory is hard to set up and largely unnecessary because we don't need to be concerned about safety that much.

Fun thing is that there are in fact two more kinds of free return trajectories.

In the simpler case, you just peek into the Mun SOI from low apoapsis and then get back, without circling Mun. It's not very exciting one particularly because it has rather high apoapsis.

The other one is much more interesting and I would even say useful for KSP purposes. Instead of figure-8 where you enter the Mun SOI from Kerbin, you enter it "towards Kerbin", i.e. after you turned around at your apoapsis. This allows you to set up the Mun periapsis on Mun side facing Kerbin. The advantage of this approach is that you can perform ejection burn on Kerbin day side, land directly from low periapsis on Mun on day side, and return to Kerbin on day side as well.

Edited April 14, 2014 by Kasuha

[MarvinKitFox]

... it always looks like a figure 8 ...

The reason Appolo used that type of orbit, was for the apollo 13 scenario. Where they need to get back to earth using the minimum effort, both in delta-v and computing resources. On the figure-8 orbit you could virtually just sit back and arrive back at earth 5 days after launch.

In KSP, a similar orbit is also nice, but for totally different reasons!

1)less delta-v!

Yes, really. You need to burn about 15m/s more to line up the entry, and when you are in orbit you are orbiting against the surface rotation, so you need to throw away another 30m/s to counter that.

BUT, the figure-8 orbit causes your circularisation burn around Mun to be much less, as you have just gone through a negative gravity slingshot relative to Mun.

Coming in from behind, you experience a positive slingshot, right when you dont want it!

2) faster transit time.

Not so important, unless you are running one of the life-support mods, when every hour counts.

Mind you, it is *way* more difficult to line up a figure-8 orbit correctly. Your speed needs to be accurate to <5m/s, and your time window for the burn is measured in single-digit seconds.

Doing your trans-munar injectionburrn the normal way, your errors get eaten by orbital mechanics. Plus, you usually get a second encounter chance if you are a bit early, when you are drifting down from apoapse. A speed error of 20m/s on the normal launch simply means you have a bit of an eccentric orbit at your destination.

With figure-8, a 20m/s error means you pass Mun on the wrong side, and start heading out to jool the slow way.

[Kasuha]

1)less delta-v!

Yes, really. You need to burn about 15m/s more to line up the entry, and when you are in orbit you are orbiting against the surface rotation, so you need to throw away another 30m/s to counter that. BUT, the figure-8 orbit causes your circularisation burn around Mun to be much less, as you have just gone through a negative gravity slingshot relative to Mun. Coming in from behind, you experience a positive slingshot, right when you dont want it!

You don't save any delta-v by using free return trajectory. You're coming to Mun pretty fast and you need to kill all that velocity. Doing so at low periapsis reduces spent dv thanks to Oberth effect, but you can brake at low periapsis from a much cheaper transfer burn as well and you brake less as you come in at less speed.

Your speed needs to be accurate to <5m/s, and your time window for the burn is measured in single-digit seconds.

Okay, you need to set up the speed correctly and setting up the maneuver is not exactly simple but once you know the trajectory, the space where and how you achieve it os pretty wide. Even if you miss the maneuver by a minute you can still get back on track by applying right amount of thrust in the right direction.

[FenrirWolf]

Minor nitpick - Apollo 13 didn't start out on a free-return trajectory. One of the first things they did after the accident was to make a course correction burn to get back onto a free return. But the point about eliminating points of failure still stands.

good catch, I always forget about that part

[AlexinTokyo]

I don't agree that a free-return trajectory is particularly difficult to set up in KSP. It's a little bit fiddly, and you need to time your burns fairly close, but it isn't overly hard.

It does make a great early mission in career mode; as you can get high- and low-altitude Munar science without having to worry about having enough tech to build a lander, and without having to pack enough ÃŽâ€v to burn out of a Munar orbit.

I think it makes a perfect bridge mission between a Kerbin orbital hop and a Minus landing.

[Rus-Evo]

Thanks for those answers. They were quite useful.

I was under the impression that if I ever did not do an insertion burn in Mun SOI then I would pop back out into Kerbins SOI, which made me think I was already on a free return trajectory (not ever having not burned into Mun orbit I haven't tried though).

[blizzy78]

I was under the impression that if I ever did not do an insertion burn in Mun SOI then I would pop back out into Kerbins SOI

That is always the case. You need to shed excess velocity to stay in the SoI.

[FenrirWolf]

You'll always end up back in Kerbin's SoI, but not necessarily on a free return unless you aimed properly. In some cases you'll find yourself on an escape to interplanetary space, or just in a Kerbin orbit that doesn't intersect the planet or its atmosphere.

[Claw]

When I first started setting them up, I thought they were difficult. So I started making them my default Mun shoot for a bit and got used to them. I mostly fiddle with the green prograde and blue angular (or whatever they are called) markers now.

I suppose it's like most things in this game. Difficult at first, but with practice it gets easier.