DTMIN is really large yet all looks good

[rthorbur]

Hi. I'm attempting to dock with MIR and I've run into an interesting situation. I'm at the point where the ships are in very close to the same orbit but I need to adjust my orbit so I catch up to MIR in a few orbits.

My orbital parameters look fine (see below) but my DTmin is absolutely huge ( > 2k ).

INC: me = 3.49 MIR=3.50 (close enough I believe)
ApA: me = 386.3k MIR = 386,3k (This is actually MIR's Alt at my ApA not MIR's APA)

PeA: me=227.4 Mir = 281.9K
Vel: me=7.789k Mir=7.74 k

I believe these are all the relevant parameters.

My hypothesis is that DTmin is trying to tell me that my orbit is very close to the same as the of MIR but I'm WAY behind it so a burn at ApA to reduce DTMIN would simply not work as it would radically affect my PeA dipping it into the atmosphere which not not a good thing.

Is my hypothesis essentially correct? Assuming I am correct would it be fair to say that my mistake was that I should have waited for MIR to be closer to KSS (with the orbit of MIR crossing KSS of course) before I launched (I checked and when I launched the orbit of MIR crossed KSS but MIR was over Madagacar)?

Sorry for the long winded question but I'm trying to use it as a learning experience to get a better handle or orbital dynamics.

 

[flytandem]

I would like to help but I'm much better with an actual scenario to work with. Can you post the scenario you are asking about? But likely others will be able to jump in with advice before I get to see the scenario, and get some time to play with it.
cheers
rob

 

[tblaxland]

INC: me = 3.49 MIR=3.50 (close enough I believe)

That looks OK but the orbits also need to have the same (or close) longitude of ascending node. When you go into the Align Planes MFD, what is RInc showing?

My hypothesis is that DTmin is trying to tell me that my orbit is very close to the same as the of MIR but I'm WAY behind it so a burn at ApA to reduce DTMIN would simply not work as it would radically affect my PeA dipping it into the atmosphere which not not a good thing.

Is my hypothesis essentially correct? Assuming I am correct would it be fair to say that my mistake was that I should have waited for MIR to be closer to KSS (with the orbit of MIR crossing KSS of course) before I launched (I checked and when I launched the orbit of MIR crossed KSS but MIR was over Madagacar)?

Assuming your RInc is low, then you may well be correct. The position of the station in orbit is not too critical, it just means you need to wait more time to catch up to you. In that regard, I recommend increasing the list length in Sync Orbit MFD by pressing the LEN button and entering 18, that way you will be able to see rendezvous opportunities further out in time.

 

[Pquardzvaark]

Assuming RINC is low, another method to catch-up or slow-down to Mir is to circularize your orbit (get your eccentricity near zero, or ApA and PeA close to each other). This means Sync MFD won't work so well because your orbits won't intersect, but don't worry, that's for later.

Then go to Orbit MFD and get Mir as the target and you'll see two lines protrude from the middle and each intersects a line.

The yellow is Mir, you're the green one. If the yellow line is ahead of the green one, you need to catch up to Mir, in which case you want your orbit lower than Mir's. If the yellow line is behind the green one this means Mir is behind your craft, in which case you want to slow down, so make your circular orbit be at a higher altitude than Mir's.

Over time, you'll see the two lines getting closer and closer, and once they form a narrow wedge is when I'd recommend you open up Sync MFD. By that point you should be able to get DTmin close to 0 and intersect in less than 5 orbits.

I hope this and the above are sufficient.

 

[rthorbur]

Thanks. I tried it again but this time waited for another orbit of MIR so that MIR was far closer to KSS than my previous attempt (before takeoff). Sure enough I was able to get DTMIN very low right off the bat. In fact I only had to wait 1 orbit to get to MIR (after adjusting DTMIN to zero of course).

The downside was the my Rinc was far larger (9 deg or so) when I got to orbit than previously so I had to do a long burn to align the orbits (90 seconds or so at full thrust).

It seems like the orbit of MIR is a bit of a pain in the buttocks for launch from KSS. You have to use allot of DV to either align the orbit in space or to play catch up. There doesn't seem to be a happy medium.

I own a DGIV (Prius model ;-) ) so I hate to waste the fuel.

As and aside: It took me a full 1/2 hour to dock once I got within a few hundred meters. I haven't got the hang of using the docking interface but I will. I can only do it VFR atm and as luck would have it MIR and I were in the Earth's shadow. Go figure. I had to wait until we got out of the shadow.

Thanks again. I will load up my scenario tomorrow with the large DTMIN and use your suggestions. I figure I've got to know how to deal with that type of situation also.

 

[ar81]

Try [ame="http://www.orbithangar.com/searchid.php?ID=3074"]Tutorial: DG to ISS[/ame]
You are in the process of synchronizing orbits.
This tutorial tells you how to understand and work with DTMIN.

 

[n122vu]

A good tip for launch - bring up Align Planes MFD and target MIR. Wait until RInc is as low as it can be. Ideally, you want it at least below 1 degree, but depending on the target's orbit and your location, that may not be possible. Launch in the necessary heading (Azimuth Calculator comes in handy, if you need it), adjusting heading as you go.

Take a good look at ar81's tutorial also to help understand how to work with DTmin, etc.