# How to calculate your own launch windows

There are a number of tools you can use to calculate a launch window. This page covers two - the default Transfer MFD that comes with Orbiter, and my own TransferX MFD.

### Calculating a launch window using the Transfer MFD

The example I'll use is a hypothetical journey from Earth to Mars. Set the Transfer MFD up as follows.

• REF = Sun. This is the body around which Earth and Mars orbit
• SRC = Earth. This is where you'll start
• TGT = Mars. This is where you'll end
• HTO - Bring up the HTO bar, and swing it around (EJ+/-) to the place where the Earth is (the green bar). Then use the DV+ button to create an ellipse that just touches both orbits as below. The solid blue line shows where your craft will intersect Mars orbit, and the dotted yellow line shows where Mars will be at the time - quite a few million miles away.

Now use the EJ+ and EJ- keys to swing the whole thing around until the Mars intersect line is on top of the dotted yellow Mars line, as follows. This now shows the following. If you eject from Earth orbit when Earth is where the dotted green eject line is, then you should arrive at Mars orbit at the same time as Mars does. In short, you have found a launch window.

Now all you need to do is calculate how long it will take Earth to travel from its present position at TrL=146.98 to the required ejection longitude at TLe = 170'. For Earth this is easy(ish) as Earth moves roughly 1 degree longitude per day - as we all know, there are 360 degrees in a circle, and 365 days in a year. Add this number to your present MJD to give the MJD of the eject time.

You can edit a scenario file to this MJD if you don't want to wait.

With other planets it's a bit more complicated. A good approximate method is as follows.

• Bring up the Orbit MFD and set REF to the Sun (or the body your current planet orbits) and set the target to the planet you're on.
• Take note of T - this is the length of time in seconds to do one orbit.
• Divide this number by 360 to get the time to orbit by 1'
• Multiply by the number of degrees forward the eject time is (my earlier example with Earth is 20')
• Divide by 86400 to convert from seconds to days.
• Add this to the MJD number, and you'll have the MJD for your orbital window.
• Manually edit a scenario file to have this date if you don't want to wait for it.

If you don't want to do all this calculation, you can use the TransferX MFD instead....

#### If you don't find a launch window

Sometimes you don't find a launch window from one planet to another using the Transfer MFD. If you don't find one, edit the scenario file to move the MJD a whole orbit into the future or past, and try again. In most cases you should find a window on the second attempt.

### Calculating a launch window using the TransferX MFD

TransferX is an add-on of mine. The method of using it to find a transfer orbit is similar, and avoids the calculations and complications needed with the Transfer MFD.

• Set MAJ=Sun
• Set MIN=Earth
• Set TGT = Mars
• Press VAR once. This should set the current variable to Prograde velocity
• Press ++, and a whole lot of extra lines and figures will appear. Keep pressing ++ until the yellow hypothetical orbit just touches both planet orbits At this point you have three yellow targeting lines. The one touching Earth's orbit is the eject time, whilst the other two show the distance between your craft and Mars at closest approach. As you can see, the distance is considerable!

• Use VAR to select the Eject MJD variable. This is the date of the launch window. Move it around until the two target lines representing your course and Mars's are aligned. This will make the MFD look as follows. You can now read off the Eject MJD straight from the display. Edit a scenario so that it has an MJD that's a day or two before that (to give yourself time to take off and get into position), and you're done. In this case 51973 looks good.

Unlike the Transfer MFD, you can easily look for launch windows in the past or more than one orbit into the future - just change the MJD until you find a window.

Once you're done, edit a scenario file with the MJD you want. I tend to edit it to a date a couple of days earlier than the window to give me time to take off and get organised.

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