TransX surrogate


Tutorial Publisher
Tutorial Publisher
Oct 19, 2007
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San Bernardino
Almost everything I've learned with TransX has been thru trial and error. (emphasize error) Recently I've been playing around with ways to reduce the guesswork and more specifically plan the type of trips that TransX isn't really set up for. In any event, I haven't been able to see how to make it work, without a surrogate ship.

For example if you are sitting on the moon and want to plan a trip to Mars, you don't really have the same tools available as if you were sitting on the Earth to plan it. Moreover if you want to utilize dropping through the Earth's gravity well when going from moon to mars then the phase of the moons orbit when doing the TEI is critical to having your Pe at the Earth properly oriented. But how to know when the correct moon's phase (launch date) is? Just eyeball it?

Using a surrogate :cheers:(for lack of a better word) ship that is just sitting anywhere on the Earth is a convenient way to do it. Simply make a plan for this ship to leave Earth and have a low Pe for the TMI burn. Graphically look at the orientation of the Pe with the orientation at zero (ecliptic) and then you know the direction of the Ap which is where you will be dropping from when you drop in from the moon to this Pe. If you want you can swing the orientation a full 180 so that the orbit is clockwise around the Earth which gives two windows a month to leave the moon. It's a bit more fuel when leaving the moon to go clockwise around Earth but could be a savings over what options are otherwise available with only 1 window a month.

There is a bit of an art to it but it is very workable. You advance the time which brings the moon toward the correct phase, approximate the launch date and be sure that the surrogate ship plan uses this date. Actually you make it about 3.5 days later to account for the time to drop from moon to low earth Pe. But when you change the date of the TMI for the surrogate, it will require changing the transfer velocities a bit. And this will slightly affect the orientation of the Pe. Then advance the date putting the moon exactly into phase and set up a TEI plan for the ship on the moon.

If there is a large plane change in the surrogate's plan then it gets more complicated. And the moon doesn't orbit Earth on the ecliptic so you may be rounding the Earth at not an ideal inclination. But artfully using the various views especially toggling between ecliptic and plan and doing the moon-earth transfer with a tiny bit of plane change then seeing the result by toggling between ecliptic and plan view will show if there is a change giving an idea how close you will be to the ecliptic with planned TEI. So if there turns out to be a lot of plane change for the surrogates plan to Mars, you still could end up with some trial and error.

All in all the point is to reduce the trial and error for this type of gravity well kickoff when going moon to Mars. Or moon to Venus. I've done about a dozen randomly dated transfers over the next couple of decades and they have mostly been very predictableand efficient using an average of 1500 m/s from low moon orbit to mars.

BTW this surrogate system can also be used when doing the TMI from low lunar orbit too. Meaning going directly moon to Mars with no gravity well burn at the earth. The problem had always been to know what direction to launch from lunar surface for this to be a prograde burn (no plane change) to head to Mars. Use the surrogate and raise the Pe up to the distance of the moon to get the orientation and approximate velocity values of the TMI. Moon being in proper phase is very helpful. Then roughly set up this as an eject orientation for the moon with roughly ths same plane change in the plan and launch onto a heading to follows this eject orbit. Then once in orbit cancel the plan and replace it with a specific manoevre. It's amazing how well this can provide a nice "prograde from orbit" manoevre burn with not very much plane change.