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Old 03-16-2019, 06:09 AM   #1
Default Intercepting orbiting vessel in lunar orbit from earth

I've come across a couple threads on this topic; but can't seem to use IMFD reliably to actually intercept a space station orbiting the moon from Earth.

At first I used jarmonik's suggestion in one thread; to use planet approach to set AgP to zero, matching the PeA of course. Then doing an orbit insert of 0.8 eccentricity so as to use less dV when changing planes. After changing planes, I'd finish my retrograde burn at PeA to make my orbit circular, then go about syncing with the space station.

The first few times I tried this, it all worked great (other than my PeA altitude always being raised significantly when doing a plane change in normal or anti-normal attitudes for some reason), so I'd always do a little retrograde burn while up there in ApA; only to find that when nearing the PeA it would drop below the surface; so I'd use my vertical translation thrusters (while facing prograde) approximately 90 degrees in orbit before PeA to raise it back up to the space station's altitude.

However, since then I've tried intercepting the same space station but at a different time leaving Earth; but I can't for the life of me (using Planet Approach) get AgP anywhere near zero [50 degrees is the lowest]--unless I go into a retro-grade orbit (opposite the direction of the space station); or set the TEj to a time very close to PeT (which defeats the purpose of doing a mid-course correction to save dV) or setting the PeT to a later date which also has the same effect of increasing dV to be about as high as my initial dV to leave Earth in the first place! (well above 3K)!

I've seen TexFilms' videos of intercepting orbiting spaceships using TransX; but haven't come across any similar tutorials using IMFD--well other than dgatsoulis, who posted this video:

However, I can't seem to get anywhere using the Delta Velocity program to get my PeA to match the station, while getting the RIn near zero, much less try to get the periapsis and intersection node to coincide. (This is while in the Sun's SOI [which is where it seems dgatsoulis was doing his Delta Velocity program]). I've then tried skipping ahead and getting into the moon's SOI to try the Target Intercept program with the space station as the Target but that too just doesn't get me anywhere close to where I need to be.

Could it be that I just left Earth at the worst possible time for a nearly five day long cruise? More likely I don't know what I'm doing.

I've probably spent nearly 20 hours now trying to figure this out on my own... This must be a simple exercise as I see very little discussion on it; and I'm just too dumb to figure it out.

Last edited by skyeboy; 03-16-2019 at 06:13 AM.
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Old 03-16-2019, 06:59 AM   #2
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Intercepting a vessel in lunar orbit while coming from Earth is an advanced challenge. Looking at your description it looks like you are going to intercept the Lunar wheel station. This is in a Lunar polar orbit, which makes it especially difficult!

The method I use: Departing from low Earth orbit I aim for a Lunar orbit having the same inclination as the target vessel in Lunar orbit. Changing inclination later on would be much more costly (dV-wise) than changing apo- and periapsis.

I use TransX MFD, which quite bareback. Maybe other MFD's provide more convenient options for this.

While on route to the moon I adjust the trajectory to a low lunar periapsis for a good capture burn. Still not caring where the target will be in its orbit when I arrive.

Lunar orbit insertion (capture burn) is into a highly elliptical orbit first. Out at the apoapsis any orbital plane adjustment is easy, as the vessel is slow. As you already encountered, do not lower the periapsis to close to the lunar surface. Orbit perturbations from Earth's gravity may bend your trajectory below the lunar surface.

After that it becomes a straightforward orbit synchronizing task. Orbit speeds are low, so also dV for maneuvers are low. With a little patience you should be able to catch up with the target within a few orbits.
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Old 03-16-2019, 06:08 PM   #3

Yeah, I'm intercepting the Big Wheel on a 74 degree inclination with the equator, 17.5km above the surface. (Giving it 5km clearance with the highest mountains)

Sounds like you do pretty much the same thing I've been doing in the past... I use the delta velocity program in LEO to set my PeA to 17.5km and my equitorial inclination to 74 degrees; but in the Sun's SOI I can't seem to adjust any of the three dV vectors to get those aligned anywhere close--much less get my nodes to sync.

I've noticed if your LAN isn't matched, getting the same inclination is little help. I once tried inserting into Moon's orbit at the same inclination only to discover after capture that my relative inclination with the station was nearly 180 degrees--almost perfectly out of phase.

Maybe I just need to start all over again--perhaps the scenario files I'm using are based on one that got corrupted and it's throwing my navigation off? I've noticed that sometimes IMFD gives me odd readings until I restart the sim--or if I try running two or more IMFD programs at once. I'm just confused why I could get AgP down to zero while at a moon G-factor of 0.40--when I first tried these missions; but when I went back again using a different date; I can't make it work without making it very expensive; energy wise even though I'm using the same process--taking a five day trip to intercept the moon as it's intercepting the ecliptic.
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