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
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
Contents
Click Link to see Images
I can't upload any images for this page. So I put the images on my google drive but can't display them on the page. Click the Image:xyz links to see an image.
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.
3 Orbit Shamrock Cycler
Image: 3 orbit Shamrock Cycle. This tutorial demonstrates setting up and maintaining the Shamrock variant.
The Cycler starts with a typical 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 around the Moon and fall back to Earth. And then carry on around the Earth toward the Moon again.
Unfortunately the next time our highly elliptical orbit intersects the Moon's orbit, the Moon isn't there! Don't despair. The Moon has gone around 1/3 of it's orbit. On our next orbit the moon has finished 2/3 of its orbit. On our 4th orbit, the first of the next cycle, the Moon is back!
When looked at in a rotating Earth-Moon reference frame, this looks like a 3 lobed Shamrock. See diagrams.
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 RealSolarSystem Releases.
Any ship with a reasonable fuel supply can be used for this mission. I recommend a lower thrust engine for easier adjustments on low delta-V burns. I used the following
- Ravenspear Mk4 (stock space plane)
- remove everything except the cabin
- Add
- Rocomax 64 fuel tank
- LV-909 "Terrier" Engine
Mission phases
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 Lunar 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, appropriate for setting up a Hohmann figure 8. Set eccentricity to 0.001 to stabilize the orbit display in map mode.
First Figure 8 Hohmann
Image: Start the first figure 8 from Lunar orbit
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 700m/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 out-of-plane components to your burns 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. ... to intercept 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. So 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. Tip: Keep your mouse over the AP marker and display the AP time as you complete your burn.
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 a third 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
Pre Hohmann Earth PE setup Burn
When you get the Moon into a good position on the 3rd orbit, revert to your saved ellipse setup burn and setup for the final burn of this cycle.
Warp ahead to just before your final Earth PE prior to re-intercepting the Moon.
Setup a maneuver 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 and radial velocities 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 70m/s.
