Lunar Orbit Return to Earth

TRANSX MANOEUVRE: This exercise introduces the basic transx function called the "Manoeuvre". It shows by turning on the manoeuvre mode you can see what a specific thrust amount and direction will do for you by showing a future trajectory. The time of the manoeuvre is adjustable and the direction/amount is done by allowing to adjust a burn amount in 3 axis specifically prograde(the direction you are moving in orbit), sideways and up/down. You then simply thrust when it tells you and turn off the manoeuvre mode to see how you did. Here, the objective is to get back to the Earth from a low lunar orbit. This tutorial associates with an Orbiter scenario called "lunar_return" which is available here. Extract it into your Orbiter folder and it will be in a scenarios subfolder called "Flytandem Tutorials" and then start this scenario in Orbiter in a window. To get Orbiter in a window, in the Orbiter launchpad as the program first opens, click the tab called "Video" and then click the "Window" button.
Keep the browser open and also in a window so you can scroll the webpage and run Orbiter at the same time.
At the start of the scenario you are in low lunar orbit going in a direction similar to the lunar orbit used in the Apollo missions. To make the return easier you are already in a fairly equatorial alignment meaning what you need is prograde thrust at the correct time and amount. In the future if you want to use what you've learned on your own lunar orbit, it's very helpful to first get a rough (within +/- 10 degrees) equatorial alignment perhaps using "Map MFD". You could be going either east or west along the equator for the easy return to Earth.

With the scenario "Lunar_Return" running bring up Transx in both left and right MFD's. "LeftShift-J", "RightShift-J"

In the right MFD...
click the "++" button until "Escape" appears.
click "FWD" to get to the next stage.
You now have a view of the Earth at center with the moon's orbit around it in blue and the blue radius line connecting the Earth position to Moon position.

In the left MFD...
click "VW" as needed to get "Manoeuvre mode".
click "++" until the Manoeuvre mode is "On" as shown in the following image.
Left MFD click "VAR" and you'll see it toggles between,
Prograde vel,
Man. Date,
Outward vel,
Ch. plane vel,
Manoeuvre mode.

The real fun and challenge of Orbiter is visualizing in 3 dimensions what is happening. Transx is best used by trying to visualize the motions and flight goals. Logic will then tell you what to adjust.

To get back from the moon, we first need to get away from the moon such that as the moon continues on its orbit around the Earth we are pushed away from the moon mostly in a direction opposite the moon's motion leaving us hanging in space and subsequently falling to the Earth. If we truly were hanging motionless there we would fall straight into the Earth, not what we really want. If we had pushed away from the moon harder than leaving us motionless we would fall to Earth and be going around the Earth in the opposite direction as its rotation. Let's push away to escape the moon but just with enough that are still going around the Earth like the moon but much slower and hence fall toward the Earth to orbit it in the same direction the Earth rotates.

HOW MUCH THRUST:To expand one's orbit to the point of escaping is best done with a Prograde thrust. In the left MFD...
click "VAR" as needed to get "Prograde vel"
click "++" a few times to expand the hypothetical orbit.
click "ADJ" to change the rate of adjustment per click.
click "--" to reduce the thrust.
By about 700 m/s it will show this amount of prograde as being enough to escape the moon. Notice an orbit in green then appears in the right MFD which is what your orbit would become if you did a burn with what you've set up so far.

WHEN TO THRUST: In the left MFD...
click "VAR" until "Man date" appears.
click "ADJ" until "Ultra" appears.
Then start advancing the timing of the hypothetical prograde manoeuvre by clicking the "++" button. Click and hold if you like. In the left MFD you'll see the yellow dashed radius line moving. This line shows where the hypothetical maneuver burn will be done. Watch the right MFD green orbit as you advance the left MFD a full 360 or more to see what the various timings cause. At some point you'll see where the result is an orbit near the Earth with one side fairly small. Try to find when the close side is at its closest (smallest Pe) Note: If you advanced it massively with the increment set at "Coarse" for example you can reset it to the current sim time by adjusting the increment to "Reset" and clicking the "++" button.

Back to the Prograde adjustment and adding prograde "++" shrinks the closest distance to the Earth shown by the number in the right MFD beside "Focus PeD:"

PLAY with both the Prograde and Man. date to get a Manoeuvre like what's shown in the next images which are the left and right MFD's. The Prograde used to get the following image is about 830 m/s and the Man. date is about 52006.8623.

Now all we have to do is burn a direction, amount and timed according to what we've set up. Using left MFD...
click "VW" as needed to see a bullseye target.
use ROT thrusters to center the "x".
See "T to Mnvre" for how long to wait until you apply thrust.
Advance the time carefully to bring the time to burn down to about 100 seconds. Pause Orbiter [Control-P], and read the next part about centering the burn.
You will want to center the burn with the middle of the burn timed when "T to Mnvre" = 0. BurntimeCalc MFD is helpful. Also an approximation is total burn being 70 seconds for every 1000 m/s but depends on the ships mass which changes if you are flying with limited fuel. Since a burn of over 800 m/s will probably be around 60 seconds try starting the main engine at full thrust burn when the "T to Mnvre" is dropping and passing 30 seconds. Watch the "Rel V:" drop as you burn and stop the burn as close as possible to "0". Use LIN RCS if you like near the end of the burn to tidy up the burn amount and note that you can shift the little "x" with lateral and up/down LIN. See the following image and note that the velocity is only .005961 m/s from zero which is quite low.

In the left MFD...
click"VW" as needed to get "Manoeuvre".
click "VAR" as needed to get "Manoeuvre mode"
click the "++" button to switch the "On" to "Off".

You are now on your way back to Earth. Go to external camera "F1" and rotate the camera angle "Ctrl-arrows" for a view of the moon if you like.
As you get closer to the Earth the Moon stage will have disappeared and both MFD's will be the Earth centered stage. Watch the Focus PeD (Periapsis Distance at the Earth). When you are within several Earth diameters of Earth you should change the PeD to be what you want, either a contact with the atmosphere or a non atmospheric pass.

Transx doesn't model the binary Earth Moon system well so there tends to be some minor differences in what is planned and what you get. Other planets are much more accurate and the worst I've found seems to be transfering between Earth and Moon. To push the periapsis to a higher value is easiest done by an outward burn during the trip back to Earth. You can do this in many ways.

Either set up a Maneuver like this tutorial showed in escaping the moon, or you can "burn on the fly" as in the following steps...

1. once half way back to Earth turn to a "Prograde" attitude "[" button on the keyboard. Then Killrot "5" after it's in position.

2. make sure the RCS is in LIN mode "/" button.

3. burn outward "3" or inward "1" to change the Focus PeD as shown in Transx.

4. Get ready to either do a reentry or a retrograde burn centered on Pe depending on what you want to do on your return to Earth.