SyncMFD Unsuccessful Synching of orbits

[Jason210]

I've tried to get Delta Gilder to rendezvous with the ISS a few times now, without success. This is what I'm doing:

• Aligned orbital planes. Can't get relative inclination better than 0.01
• Matched the periapses of the two orbiting objects to the nearest mile.
• Controlled my speed by raising / lowering my vessels apoapsis using pro/retro burns at the periapsis.

With estimates / trial and error regarding speeds I can get to within 100km of the ISS. Although on my vessel's Orbit MFD the orbit radius indicator of my vessel appears to be aligned with that of the ISS as it crosses the periapsis, such visual alignment is evidently not good enough.

I need a way to fine adjust the both the alignment and synching. Any advice appreciated. I have read Smitty's tutorial, btw. Not really sure what the DTmin parameter is though. The bit I get frustrated with is that all the burns have to be done at the Peri and App.

Any help advice appreciated.

 

[Jarvitä]

The ISS is in an almost perfectly circular orbit, so there's really no point in syncing the periapses. For your first try, just focus on aligning orbital planes and arriving within a ~100km or so.

Then you just have to kill relative velocity (using the docking HUD and MFD), then thrust towards the station. Give yourself a high velocity direct approach vector (around 50 m/s), and keep the velocity vector focused on the station, using the main engines for corrective burns if necesarry. At around 20km out, kill the relative velocity again and proceed with the normal approach and docking pattern.

 

[fireballs619]

I suggest you follow Tex's great DGIV to ISS tutorial. The man does a fine job of explaining the process, and should help to get you there. It was a little ambiguous in your post, but are you using the Sync MFD also?

 

[N_Molson]

I use the Intersect 1 / 2 mode. It usually give me some good results.

During the last 5 orbits, adjust DTmin to near-0 each time you pass the marker bar. Non-spherical gravity sources and tidal forces perturbate your orbit, so you need to make corrections.

During the last orbit, make a final plane alignement.

When roughly at 90° from target interception, make a final DTmin adjustement.

A RInc of 0.01 is fine, that should'nt be a problem.

 

[jthill]

You're getting within 100km using only the Orbit MFD? That's good work. If you're sure you're at the right altitude you can fine tune that by calculating how much distance you're gaining or losing each orbit, look at how many seconds different your orbit periods are.

The Sync MFD does a lot of the necessary arithmetic for you and puts up other useful numbers, the Space-Shuttle-to-ISS tutorial scenario shows how to do a basic approach. You can vary almost everything about that to taste (I like doing my rendezvous at tg apoapsis from a much lower overtake orbit, lalala), like any skill the book's a starting point.

 

[Jason210]

Thanks for all the help. I try again after work tonight and see if I can apply some of it.

@fireball 619 & jthill
I'm using the default Sync Orbit MFD also. I think I am not using it so well. I'll have a look at Tex's tutorial also.

---------- Post added at 08:43 PM ---------- Previous post was at 05:57 AM ----------

Yes, it worked fine. It was very satisfying to meet up with the ISS and see the orbits merge on the MFD as I made translation burns.

It was sheer luck that I had sunlight to do it, though.

Now for the next challenge, getting back to a specific location on earth...

Cheers guys!

 

[Cras]

I never use Sync MFD myself. I use Rendezvous MFD instead, and get much better results.

Launch on the right heading, align the planes to at least 0.01, make sure the orbit is about 70-100 km lower than the ISS, then wait until I catch up. Once you get to something like 800km-500km in distance Rendezvous MFD will come alive and then set the time so that the arrival speed is managable, something like 20-50 m/s, and then use its bv display to get me there, doing little corrections at first but then the course tends to stay true.

Approach the station from behind and below on the R-bar every time. Open up RPOP and use it to get the y-axis lined up, then focus on controlling the vertical and horizontal rates closure rates.

Also I lock the vechicle into LVLH 0-0-0 using Attitude MFD which is an aboslute must have MFD in my book. I always have it running during orbit ops.

 

[TMac3000]

This topic is another of my favorites.

I'll tell you the dirty little secret: you can get from the runway to ISS in less than 2 hours without any of that align and synchronize business.

The trick is to know how long it takes your craft to get into orbit, calculate how far the ISS will travel in that time, and launch while it is behind you, at the right time so that ISS has just a little bit of overtake. For a DG-IV or XR2, I find the best launch distance to be about 3500 km.

1) On the ground, bring up Synch Orbit MFD and Map MFD. Wait or accelerate time until the station's orbital path is directly overhead (i.e., you have a launch window), and the station itself is about the correct distance behind you.

2) Launch and turn to the right heading. For the ISS, this will be either 042 or 132 depending on whether you are following the northern or southern leg.

3) Proceed to orbit as normal.

If you do it right, you should complete orbital insertion less than 100 km from the station. Just match velocities and then burn toward the station. Start with an approach speed of about 100 m/s, and pare it down as you get close to the entry boxes.

A few caveats: it can take some trial and error to find the right distance to launch. The idea is let the station overtake you by about 100 km. 3500 km behind your launch point is usually a good baseline, but you will have to vary it depending on how long it will take you to turn to heading and ascend, as seconds can make a big difference.

Happy docking

 

[C3PO]

I never use Sync MFD myself. I use Rendezvous MFD instead, and get much better results.

RendezvousMFD is fantastic to really nail those approaches, but it has it's limitations. If you're using the Deltaglider (or similar) you can do the transfer at very high Eccentricity. SyncMFD or even TransX are better in those cases.

 

[Loru]

You can even perform randez-vous with stock "TransferMFD".

 

[Jason210]

This topic is another of my favorites.

I'll tell you the dirty little secret: you can get from the runway to ISS in less than 2 hours without any of that align and synchronize business.

The trick is to know how long it takes your craft to get into orbit, calculate how far the ISS will travel in that time, and launch while it is behind you, at the right time so that ISS has just a little bit of overtake. For a DG-IV or XR2, I find the best launch distance to be about 3500 km.

1) On the ground, bring up Synch Orbit MFD and Map MFD. Wait or accelerate time until the station's orbital path is directly overhead (i.e., you have a launch window), and the station itself is about the correct distance behind you.

2) Launch and turn to the right heading. For the ISS, this will be either 042 or 132 depending on whether you are following the northern or southern leg.

3) Proceed to orbit as normal.

If you do it right, you should complete orbital insertion less than 100 km from the station. Just match velocities and then burn toward the station. Start with an approach speed of about 100 m/s, and pare it down as you get close to the entry boxes.

A few caveats: it can take some trial and error to find the right distance to launch. The idea is let the station overtake you by about 100 km. 3500 km behind your launch point is usually a good baseline, but you will have to vary it depending on how long it will take you to turn to heading and ascend, as seconds can make a big difference.

Happy docking

Interesting you should mention that because I tried this as an experiment before I read your post! Once I was in orbit I made a burn to align the planes, and then was about to switch to sync orbit MFD when I noticed on the Orbit MFS that the ISS was just 70km in front of me! I was able to catch up with it and dock in less than half-an-hour. It was just luck though so thanks for the tips!

 

[TMac3000]

Any time Always glad to help:tiphat:

---------- Post added at 12:28 PM ---------- Previous post was at 12:26 PM ----------

PS: For a proper SCRAM ascent in XR2, the launch distance is about 2500 km:tiphat: