Tutorial:Free Return Trajectory
Going to the Mun is one thing. Going there and landing is another. But getting there, and back with just 2 burns? That's really quite challenging! This tutorial will provide you with all the tools but practice, to get you to the Mun, and then back for free.
Contents
Specifications
- Length: 30 minutes
- Difficulty: Hard
- For Versions: 0.13.1 (and above - although may work for older versions).
Steps
Building your rocket
You will need to build a rocket capable of a Munshot. This will most likely be a multi-stage rocket, but I'm not going to patronise a seasoned Kerbonaut. You can design your own rocket!
Launching
Launch on a Munshot (either 090 degrees or 270). You know the drill. But seeing as we're on an economy mission, I suggest you head East (090).
Establish an Orbit
Establish a circular orbit, between 120-160,000 m.
Make for the Mun
As per usual the plan is for a transfer orbit out toward the Mun. However, instead of going for the usual 12,000,000 ish Apoapsis, you're trying for more like 14,500,000m.
As soon as your Apoapsis hits 14.5M, cut your engines. You're not going to be needing them again!
Flight path
If you've done it right, you should come to within around about 500,000m of the Munar surface, probably doing just over 1000m/s. Don't worry, map view will show you in an escape trajectory, here you may panic. Don't. At this point steely nerves like Jeb are required. Hopefully, your slingshot will shoot you out behind the Mun, to move in the opposite direction from the Mun's orbit.
The aim is for your angular velocity to be as small as possible compared to the planet. This way you won't have enough angular velocity to make a full orbit around Kerbin when you are back in its Zone of Influence.
Again, don't worry if your mind is running ahead and you're thinking: 'Oh poo, the Mun moves at just over 500 m/s and I'm doing over 1500. This could work out badly for Jeb.'
The Mun's gravity will actually slow you down a lot as you move away, slowing you down a nice amount.
You will soon observe your projected orbit switch back to Kerbin, and you should either have no Periapsis, or one that is within the atmosphere. And - we all know what that means.. it means you get home, with nothing to do, but open your parachute. No home-wards bound burn required.
Table of Free Return Insertions
Note: This table is incomplete, and would benefit from addition! Figures in orange are not necessarily fully representative of successful range.
| Altitude at burn (m) | Successful altitude range at Apoapsis (m) |
|---|---|
| Above 75,000 | 14,250,000 - 15,000,000 |
Final word
In all, you used 2 burns.
1. Get into orbit 2. Transfer into Free Return Trajectory
This is a super efficient way of performing a Munshot. Those "who know" about these things from an expert perspective have said that on the instrumentation we have, this is impossible. PakledHostage and Rascal Nag on the forums proved that wasn't the case, and now we can all get out there and do it! Good luck all, please feel free to edit this tutorial if it needs it. It's been put together quickly so that people can get using it!
Thanks for reading
