Free Return

This exercise is to start from low Earth orbit and plan and fly a trajectory that does a single prograde burn that has the ship travel to the moon to sling around and return to Earth where either a second burn would reinsert a low Earth orbit or do a reentry. A mid-course correction is done on both the outbound and return legs. To save time the low Earth orbit is already aligned with the moon's orbit.

This tutorial is designed to be used with Orbiter running in Window mode with the scenario "Free Return" which is available here. Extract it into your Orbiter folder and it will then be found in your "Scenarios/Flytandem Tutorials" folder when you start Orbiter.

Getting the right trajectory will at one point in the adjustments be a bit "fiddly". It is important to have a good idea of what's going on so you can find the right combination of thrust and timing and not just be blindly stabbing at various combinations.

The trip out and back is like a large Fig 8 with us first orbiting counterclockwise around the Earth (as seen from above the northern hemisphere) then the trip out to the moon where it's a clockwise pass around the moon followed by again counterclockwise when we get back to Earth.

Now if we had just barely enough speed leaving the Earth to arrive at the moons distance from the Earth we would have the minimum amount of speed as we approach the moon and the moon would in effect drag us along to some extent in the direction it's going so when we do fall back to Earth we'll have a fairly large amount of prograde velocity and consequently a fairly large Pe. As we increase the initial speed we left the Earth, (TLI or trans-lunar injection) a couple things happen...
1. We must do the TLI burn to leave Earth a bit sooner because we are getting there quicker and the moon is therefore not as far around it's orbital position where it would meet us. Without this advance of the burn we would get well in front of the moon and essentially miss it.
2. The return has us not dragged along following the moon as much. We are leaving it relatively faster and also more in a direction opposite it's orbit around the Earth and the resulting fall to Earth gives a lower Pe which is what we want.

So in summary, when we get to the adjustment part, think of the combination of going in the following adjustment loop ......
1. increasing the TLI (more deltaV)
2. advancing the TLI (make it sooner) to stay near the moon
3. checking to see what our return Pe is.
(repeat 1 thru 3)

At some point we accept the values and do the manoeuvre burn because increasing the TLI velocity any more causes us to return with too low a Pe no matter what the timing of the TLI.

Bring up Transx in both left and right MFD's (LeftShift-J, RightShift-J)
Bring the moon into view with the following steps....
In the left MFD...
click "VW" as needed to be in the "Setup" view as shown in the upper right.
click "VAR" to toggle to "Select Target".
click "ADJ" to toggle to "Planets/Moons".
click "++" to toggle until it shows the target as "Moon".
What you have now is the Earth at center (see the upper left states "MAJ:Earth") and the large blue circle in the Moon (see the "TGT:Moon" in the upper right)

Turn on Manoeuvre Mode.... In the left MFD click the"VW" button until it toggles to the "Manoeuvre view" as seen in the upper right of the MFD. Then click the "++" button as needed to toggle the Manoeuvre mode "On".

Prograde to the Moon...
Click "VAR" to be in "Prograde vel."
Click "++" until you see the yellow dashed orbit enlarging to meet and barely exceed the blue circle by a little bit. Remember to change the increment to a lesser value such as "Medium" and smaller as needed using the "VAR" button. For the sake of this exercise make the Prograde vel = 3.053k for now.

Now advance the time (date) of the prograde burn. Click the "VAR" as needed to get "Man. date" (which stands for "Manoeuvre date").
Set the increment to something appropriate. Since we know it takes only 90 minutes to orbit the Earth let's switch to a small increment like "Ultra". Click "ADJ" as needed to get "Ultra". Remember the "Reset" increment if you need it.
Click "++" as needed to swing the orbit counterclockwise to get the two dashed yellow radius lines to be on top of one another. It might be with a "Man. date" value somewhere around 53752.7796 as in the following image.
Set the next Stage.
This will be your encounter with the moon. In the right MFD click "FWD". Now you have the moon in the white circle and you manoeuvre trajectory in green. To see details of the encounter click "VW" to toggle to the "Encounter" view (shown in top right of MFD)

Pass on the correct side of the Moon.
The left side of the right MFD ( over half way down) shows the Inclination. If it's a low number like 0-90 then you are passing on the right side of the moon (behind the moon in it's orbit) and will sling with extra energy to a larger orbit. What we want is to pass on the left side where you will be slung retrograde (wrt Earth) to return to Earth which will be an inclination of a number probably between 160 and 180 degrees. You can change which side you pass the moon either with the date of the TLI manoeuvre (trans lunar injection) or with the strength. The date is VERY sensitive even on Ultra, so let's choose to change "Prograde vel" to get things started.
In the left MFD...
click "BCK" (if needed) to ensure you are in stage 1
click "VW" (if needed) to get into Manoeuvre view.
click "VAR" (if needed) to get the variable "Prograde vel."
click "ADJ" to bring the increment to a small amount like "Super".
Play with the "++" and "--" adjustments and watch the right MFD to get the green orbit passing somewhat outside (perhaps a diameter or two) of the edge of the moon as shown in the following image. Check your inclination is somewhere between 170 and 180 degrees.

Create the return to Earth Stage. (Stage 3)
In the RIGHT MFD...
Click "VW" as needed to see the "Setup" view.
Click "VAR" as needed to get "Select Target".
Click "ADJ" as needed to get "Planets/Moons".
Click "++" to see the target as "Escape".
Click "FWD".
If in the right MFD you happen to click the "BCK" then "FWD" buttons or toggle the "VW" button between "Setup" and "Sling Direct" view a dashed yellow sling trajectory curve will appear. You really won't need this but no need to try to eliminate it as well. Just ignore it.

This is the adjustment loop outlined under IMPORTANT CONCEPTS: near the beginning of the tutorial. Position the MFD's with the arrows to be able to see the left MFD completely and have access to view most of the right side so you can access the right side FWD and BCK buttons.

Quick Commentary: It's not too tough to have a moon sling that is a full moon diameter from the moon and return to a low Earth Pe. The challenge is to get a fairly close pass at the moon yet still have the return Earth Pe allowing a reentry.

Here's the adjustment loop.

1. Increase the "Prograde vel." by just a couple of m/s. (left MFD)
2. Back up the "Man. date" using "Ultra" increment and the "--" button and a very tiny short click or two. (left MFD)
3. Look at the right MFD and toggle "FWD" and "BCK" to check the pass at the moon to be sure it's still on the same side, not too far away and not touching the moon's surface and to check the return Pe which is the value after "Focus PeD:"
then repeat the above 3 steps until the "Ultra" "Man. date" adjustment is simply too much of a change causing the Pe to drop into the Earth. It would be NICE if Transx had a smaller "Man. date" increment than "Ultra".

I found that for this scenario, a value of "Prograde vel" at about 3.068k and a "Man. date" of about 53752.7781 gave a pretty good pass just 1000 Km above the moon and a return with a Pe of less than 2,000 Km above the surface. The TLI will be very timing sensitive and there will likely be a need for midcourse corrections no matter what you have.

Like in other previous manoeuvres you've done, Click the left MFD "VW" to get the bullseye target, center the green "x" and center the burn. BurntimeCalc MFD is VERY helpful in knowing the burn duration. If you had Limited Fuel checked the time to start the burn is about 78 seconds before "T to Manoeuvre" drops to zero. Clean up the last of the burn with some LIN RCS. Then turn the Manoeuvre mode "Off".

Let time warp at 1,000 and maybe 10,000 for a bit and watch your progress in Stage 1 in the left MFD. When about 2/3 of the way to the moon do turn the Manoeuvre mode "On". Have the right MFD on Stage 3 and as you set the manoeuvre "Prograde vel" you can watch to see what the effect has on the return to Earth Pe. Smart to keep it around 7 to 8M for now.

Enjoy the ride past the moon and don't warp too fast when in the moon encounter Stage. Warp 100 is plenty. After you get past the moon speed up the warp to 1000 and after the moon encounter stage switches to the return stage with the Earth at center (there is only 1 stage now), slow to real time and do your second MCC. Turn on the Manoeuvre mode, adjustment is probably best as a combination of "Outward vel." and "Prograde vel." for this manoeuvre. Set the Pe where you like it whether is to plan on doing an orbital insertion burn later at the Pe or to drop directly into the atmosphere for a reentry. Setting the Pe (seen as "Hyp Ped" to 40 or 50 Km above the surface is best ie: 6.415M or 6.420M.