Variation of Orbits

[DarkEnergy]

I have recently been attempting to land on Phobos, which I was able to do in the DGIV scenario. I wanted to do one better, so I decided to go from Mars to Phobos myself.

My problem is that after I sync orbits and align the planes, Phobos' orbit keeps moving, making it impossible to keep a certain DtMin in the MFD. I can get within a couple kilometers of Phobos but I can't get it to the point that the DGIV scenario is at.

Why are the orbit's apsides drifting around?

 

[Tommy]

This is probably due to non-sherical gravity sources. Phobos is a tough target by any means. In the scenario included with the DGIV you start with your orbit matched to Phobos', and getting to that point is tricky. As you've noticed, Phobos has very little gravity, and won't capture your craft if you have more than a very slight R-Vel. Landing on Phobos is more like a docking manuever than a nomal capture/landing. First you'll have to match orbits. Sounds like you can get the planes aligned, and a sync within a few kilometers is fine. Use intersect mode when syncing. If approaching from a lower altitude, try to be the last to arrive at the intersect. If coming in from a higher orbit try to be first. Now for the next step - eliminating the relative velocity.

If you are using one of the scenarios with a Prelude base in Phobos (recommended) you can use VTOLMFD to help you eliminate R-Vel. Otherwise SurfaceMFD (reference Phobos) can be used. As you approach Phobos ensure you HUD is in Orbit mode, referenced to Mars. Point the nose at the center of Phobos. If you are coming in from lower orbit you will be travelling slower than Phobos and should be "behind" Phobos. Roll so that the Prograde Velocity marker is below the nose reticle (directly below center of screen, off screen OK). Watch your verticle speed (VS ) and vertical acceleration (VACC) on SurfaceMFD (ensure it is referenced to Phobos) You should have a negative VS. When VS is approaching zero (when we are closest to Phobos) use the main thrusters to maintain zero VACC, as close to zero VS. Up to 100 m/s VS is OK. At the same time, use the hover engines to move the Prograde Velocity marker unil its near, or on, Phobos. Maintain a small (100 m/s or less) negative VS until you are close to Phobs. Use translational RCS to keep the Prograde Velocity marker pointed in the direction you want to go and find the Prelude Base. Using small bursts of RCS, manuever yourself to the pad and land. Remember to keep the vertical speed and ground speed shown by SurfaceMFD small, less than 100 m/s total. patience is essential, use short bursts of 10x time accell rather than increase velocity.

If you are coming in from above Phobos you will be traveling faster than Phobos and should have timed your arrival to be in front of Phobos. Use the same procedure as above EXCEPT: Once pointed at Phobos, roll so that the RETROGRADE indicator is ABOVE the center of the screen, and use the retrograde indicator to target your "heading" around Phobos.

 

[DarkEnergy]

Wow, thanks for all of the advice. I realized going in that Phobos' gravity is not sufficient for an orbit and how you need to "dock" with it, but I could never get to that point. This advice will really help.

Before I checked back in to see if anyone had answered, I tried a different method using encounter MFD and transfer MFD. I was able to have a perfect landing that way. It is probably more sufficient fuel-wise as well to just catch it in an elliptical orbit, right?

 

[Tommy]

A Hohman Transfer will almost always be the most fuel eficient way to get from low mars orbit to Phobos. The technique I outlined above works just fine with a Hohman Transfer. Using Transfer and Encounter, rather than Sync, will probably result in more accuracy during the transfer burn, and result in less course correction later, which will save fuel. It would also make timeing of the transfer burn much easier, using just Sync leaves a bit of guesswork on where to establish the Ap (rendezvous point).

Personally, I like to set my AP a bit higher than Phobo's, and time it so I reach intersect one before Phobos, and reach intersect 2 just after Phobos. It's only a few more m/s than a strict Hohman transfer, and it creates a psuedo-free-return trajectory. As I approach my Ap I pass in front of Phobos, and at my AP Phobos is passing beneath me. If I do nothing I'll pass behind Phobos on my way back down to Pe, but instead I burn prograde (reference to Mars) just before my AP until my Pe matches Phobos'. From there, SurfaceMFD (referenced to Phobos) helps me monitor my R-Vel. No real reason for that flightplan except that I like the elegance of it, and it allows a "psuedo orbit" of Phobos to help locate the landing site.