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 recommend a lower thrust engine for making lower delta-V orbit adjustments. I used the following

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

Initialization

Skip the Launch

Your mileage may vary. These steps worked for me when starting 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 4,738,000 which becomes a PE of 3,000,000 meters. Set the inclination to -158 degrees. This matches the inclination of the Moon's orbit but leaves your vessel orbiting in the opposite direction. Set eccentricity to 0.001 to stabilize the orbit display in map mode.

First Lunar Hohmann

The first step toward an Earth-Moon Cycler is building a Hohmann figure 8. In this case we're starting from the Moon and looping around the Earth.

In Map mode, at 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.

This completes the setup phase. The next maneuvers are repeating parts tuning the Cycler's orbit.

Repeating cycle of Cycler orbit adjustments

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

Using Lunar PE as the start of the cycle, you vessel will perform two highly elliptical orbits around the earth before re-intercepting the Moon on your third orbit. ... if all goes according to plan ...

Set the Ellipse duration

The first recurring Cycler maneuver is a retrograde burn that reduces the AP of your highly elliptical Earth orbit. Coming from the Moon back to the Earth the orbit has too much energy and the next ellipses will take too long. The Cycler's non-Moon-intercepting ellipses need to have a duration just below 1/3 of the Moon's orbit duration. The result is intercepting the Moon in your 3rd orbit around the Earth. 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 elliptical orbit duration now without navigation assistance.

At Earth PE setup a retrograde burn that brings the PE to AP time down to 4d 21h. This is just a guess! Save the game as you may need to adjust this timing. Zoom out to see the whole orbit. Hover over the AP marker to see the time to AP.

Check your intercept

Perform the burn and check that your AP will occur in about 4d 21h. Warp around to just after AP. Warp around to just after the next AP. And warp around one more time to see the position of the Moon relative to your orbit's AP. Your goal is that the Moon should be 0 to 15 degrees counter clockwise when you get to AP. If you're very lucky final orbit will already intercept the Moon.

What to do if the Moon is in the wrong position when you get to the third AP? Revert to your saved state prior to setting up the AP time. Readjust your orbit duration. - If the Moon passed you, reduce your PE-AP time - If the Moon didn't reach you, increase your PE-AP time

Saving each variant lets you track how your burns change your arrival position relative to the Moon.

Pre-Hohmann setup burn

When the Moon is in a good position on the 3rd orbit, revert to your saved position and setup for the final burn of this cycle. adjust your ellipse timing to arrive at your final ellipse with the Moon in about the right location.

Warp ahead to just before your final PE prior to re-intercepting the Moon.

Setup a manuever at Earth PE. Futz with the controls until you get a figure 8 Hohmann around the Moon AND the next Earth PE is about 3,000,000 meters. You may need to adjust both your prograde velocity and your radial velocity to obtain a good solution.

Rinse and repeat

Because we're just eyeballing these maneuvers your first attempts will likely result in fairly high delta-V. After a few attempts I got it down below 100m/s.