Tutorial: Earth-Moon Aldrin Cycler

Under Construction!!

I'm new to media wiki and the KSP wiki but fairly thick skinned about suggestions for improving my pages. I'm guessing that proper etiquette is to use the Discussion page to send me comments while I'm actively editing this page. That said, if you need to get my attention feel free to add a comment block right below this preamble. I ask that you not edit the rest of this page until I remove the Under Construction flag. Thanks, Larry Fast

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Buzz Aldrin is credited with inventing a class of orbits that allow a ship to continuously transit between two bodies with little or no orbit adjustments. This is an extension on the classic Hohmann Transfer orbit.

• Length: hours to days
• Difficulty: extreme
• For version: 1.8.1 with RealSolarSystem mods

Prerequisites

This tutorial assumes you already know how to

• use Kerbal's Orbit Maneuver node controls
• alter a circular orbit into an elliptical orbit
• change the plane of your orbit
• install the RealSolarSystem mods
• execute a Kerbin-Mun or Earth-Moon Hohmann transfer orbit

Why create an Earth-Moon Aldrin Cycler

IMO an Aldrin Cycler cruise ship is the next logical step for space tourism after LEO Hotels. Tourism has much deeper pockets than our current science and industrial uses of space. Just look at the size of cruise ships and Earthly hotels! My dream of the future is that space can lay claim to some of those delicious tourist dollars.

When visiting space, tourists want more than just a trip in a tin can. They want Windows! They also want private suites, open spaces to enjoy zero-G activities and adventure services like space walks. These services require high mass infrastructure that is best left in a stable orbit requiring little or no delta-V. Launching this equipment with the tourists and deorbiting it does not make sense. Thus the obvious first destination is hotels in LEO.

After LEO tourism gets rolling a trip to the Moon is the next obvious destination. But we still want windows and zero-G fun on our week long trip. Aldrin Cycles are orbits that transit between celestial bodies and require minimal delta-V to maintain stability. This lets us create an Earth-Moon cruise ship.

This tutorial demonstrates a low delta-V orbit where cruise ships could cycle repeatedly between the Earth and the Moon.

Lunar Cycler Background

In 1985 Buzz Aldrin laid out the first example of a stable orbit that would allow heavy assets to cycle freely between the Earth and the Moon. Over the years his original concept has been expanded to include Earth-Mars Cyclers and a variety of Earth-Moon Cycler orbits. The foundation for this tutorial is this AAS paper. [Circumlunar Cycler Orbits for a Manned Earth-Moon Space Station PDF] It describes a number of variants.

This tutorial demonstrates setting up and maintaining the Shamrock variant. The cycler start with a fairly standard figure 8 Hohmann transfer orbit to the Moon. Our objective is a free return to Earth and then future free returns to the Moon. So our cruise ship will simply drift past the moon and fall back to Earth. And then carry on around the Earth toward the moon again.

Unfortunately the next time our orbit intersects the Moon's orbit, the Moon isn't there! Don't despair. We can set the duration of our elliptical orbit such that our orbit intercepts the Moon on every third orbit.

The first diagram below shows our ellipses in the normal Earth centered inertial frame. Every third time we loop around the earth we get to visit the Moon. We can also look at this in a rotating Earth Moon reference frame. This is where we see the 3 lobed Shamrock.

The Shamrock cycler orbit is not clean & stable. It requires a series of maintenance burns. In Kerbal Space we have to eyeball our maneuvers so don't expect to hit the minimum delta-Vs described in the paper. Only real orbital mechanics can get those results.

Kerbal System Setup

While Aldrin Cycle orbits can be setup in the Kerbol solar system, I haven't done this yet. This tutorial currently requires the RealSolarSystem mods. When written, 1.8.1 was the newest Kerbal Space version able to run RealSolarSystem. See [Kerbal Forums Real Solar System].

Ship

Any ship with a reasonable fuel supply can be used for this mission. I used the following

• Ravenspear Mk4 (stock space plane)
• remove everything except the cabin
• Add
  • Rocomax 64 fuel tank
  • LV-909 "Terrier" Engine

Skip the Launch

Your mileage may vary but if you start from a clean Sandbox with the RealSolarSystem mods.

From Key Bindings use Mod+F12 Debug/Cheats Menu => Set Orbit to place your ship in orbit.

Due to the inclination of the Moon's orbit, it's easiest to start this operation from lunar orbit. Set the SemiMajor Axis to 3,000,000 meters and the inclination to -158 degrees.

First Lunar Hohmann

On the point in your Lunar orbit opposite the Earth setup a prograde burn of about 775m/s. Your target is an Earth PE of 3,000,000. If you're not familiar with Map mode and Maneuver nodes, you should read the other Kerbal tutorials about changing orbits.

It's important to ensure that your Earth-Moon orbits remain coplanar with the Moon's orbit. Check the plane of your orbit during each maneuver. Add an out-of-plane component to your burn to correct any errors.

Tip: I always turn on SAS and use the automatic controls to point my ship in the right direction.

Setting the Ellipse duration

This is the trickiest part of the whole operation. Kerbal won't show you the next Lunar encounter until it's less than one orbit in the future. Unfortunately we need to set the size elliptical At the first Earth PE we need to setup our ellipse timing to re-encounter the Moon in 2.5 orbits.

Set the Moon as your target. L-click the Mouse when pointing at the Moon and "Set as Target".

Setup a burn to bring the time of Earth AP for your elliptical orbit down to 4d 21h after Earth PE. I find warping to 10m before PE simplifies setting this up.

Save your sim state just before your first Earth PE. Zoom out to see the whole lunar orbit track.

Perform the burn and check that your AP will occur in about 4d 21h. Warp out to just after AP. Warp around to just after the next AP. And warp around to a few degrees before the third AP after your burn.

If the timing is right the Moon should be very close to your current position. Ideally your final orbit will conclude with a Hohmann figure 8 around the Moon. On the first attempt this is unlikely. Your goal is that the Moon should be 0 to 15 degrees counter clockwise when you get to AP.

Now you can revert to your saved position and adjust your ellipse timing to arrive at your final ellipse with the Moon in about the right location. Saving each variant lets you track how your burns change your arrival position relative to the Moon.

Pre-Hohmann setup burn

When you have a good solution, the next step is to do a pre-Hohmann burn at Earth PE on your final orbit before encountering the Moon.

Advance to the second Earth PE after your ellipse setting burn.

Adjust your ellipse to produce a figure 8 Hohmann encounter with the Moon. You may need to adjust both your prograde velocity and your radial velocity to obtain a good solution. Your objective is PE= 3,000,000 meters.

Rinse and repeat

When you reach PE at the Moon, adjust your orbit to get an Earth PE of 3,000,000 meters. That completes the whole Shamrock multi-orbit cycle.