Flight Question Orbital rendezvous trouble

[yankeeman1028]

Whenever I try to rendezvous and dock with any spacecraft in orbiter using the "go play in space" method, I can get to within 120-40 km anytime. The trouble is, after that, the ship starts moving away again. How can I get close enough to start a docking aPproach? Do I repeat the rendezvous method several times? Or can I orient my ship in a certain direction and thrust? How do I get within 5 km of a ship?

 

[Goth]

The approach is not nice. You've not done a correct rendezvous and so you approach the ship being still too far away.
Be sure that your altitude in the intersection point is the same of the target altitude.
Also, keep watching the DTMin value and be sure it's ~= 0.
If it's not, thrust prograde or retrograde untill it's equal to zero.

 

[Cras]

Can you refresh my memory on what the "go play in space method" is? My guess is that it is using the stock Sync MFD, but I want to be sure.

 

[Goth]

Yes, that's it.

 

[Cras]

while it is a stock MFD, I always found that method to be hard. Or maybe more tricky is the right word, because you always had to stay on top of it.

May I ask yankeeman1028, are you set on doing the rendezvous this particular way, or are you looking for the another solution, because there are some add-on MFDs that can make your rendezvous much easier to manage, and be more economical in the fuel department, which becomes very important when stepping up to a vehicle like the Space Shuttle, which has little room for error.

 

[Goth]

I started with that method and I must say that it's useful to understand how things work. So I would suggest to keep going with it, but then, when everything is clear, it's of course better to use other MFDs which could speed up the job and make it easier.

 

[Cras]

I started with that method and I must say that it's useful to understand how things work. So I would suggest to keep going with it, but then of course it's better to use other MFDs which could speed up the job and make it easier.

Excellent point. Understand the basics first, then you know what is going on under the hood when you step up to more complex and even automated MFDs.

 

[C3PO]

Excellent point. Understand the basics first, then you know what is going on under the hood when you step up to more complex and even automated MFDs.

And turn off non-spherical gravity. That way you can actually hit the ISS from ½ an orbit away. :thumbup:

 

[Cras]

And turn off non-spherical gravity. That way you can actually hit the ISS from ½ an orbit away. :thumbup:

Another great point. Non-spherical gravity definately does add a bit more difficulty to everything. That is for darn sure.

 

[C3PO]

Non-spherical gravity definately does add a bit more difficulty to everything.

None of the MFDs include non-spherical gravity, so you need experience to counter the effects. Or at least a good understanding of orbital mechanics.

 

[clive bradbury]

Take a look at my tutorial for the basic techniques:

[ame="http://www.orbithangar.com/searchid.php?ID=5472"]Soyuz to MIR Tutorial[/ame]

 

[Ripley]

Or Tex's video, here:
http://www.orbiter-forum.com/showthread.php?t=2055

 

[yankeeman1028]

Thank you everyone. I understand the concept behind the sync orbit mfd method, I just cannot get it to work. And thanks for the non spherical gravity hint. What are some of these mfds?

 

[Cras]

Rendezvous MFD, and IMFD. Take a look at DVTools MFD as well. DVTools is a fantastic add-on, and helps you plan things out, then you use the data it gives you to enter into other MFDs, like IMFD, to then perform the actual burns.

 

[Blacklight]

I still have yet to figure out IMFD, even with all the tutorials and manual reading I've done. I'm still at a complete loss for how to use it and I've never been able to get to where I've tried to go with it. No matter what, I can never, EVER get it's numbers to in any way match any tutorials. And it doesn't help that most tutorials are for older versions where options were different or nammed different things in the MFD.

I have no problem with rendevous, unless I'm using something where the fuel is scarce. Then I'm doomed. I've never, ever hit the ISS with the shuttle.
I've been flying around with Orbiter for YEARS. I've been to the Moon. I've built space stations. But I've never, ever, successfully left the Earth/Moon system and successfully done anything other than become another permanent satellite in orbit around the sun.

 

[ADSWNJ]

For both Yankeeman and Backlight ... the trick is to reel in your target like landing a big fish on a sea fishing expedition! This is particularly important with "realistic" OMS engines and fuel quantities such as on the Shuttle.

A brief method on this fishing technique (as questions if you need more):

1. You must launch very close to aligned planes right off the deck. E.g. you need to be +-1 degree. (I think the Shuttle using 100% of its maneuvering capability would run the tanks dry doing just 4% of plane change). You need to just be very accurate on your launch azimuth and keep it aligned as you climb up to orbit.

2. Align your plane with ISS using the Align Plane MFD. (Anti Normal on Ascending Node, use the Normal+ or Normal- autopilot and bump the orbit on the node points so you are precisely aligned.

3. Start fishing with Sync MFD. Push out the number of orbits to 18, to see which is closest). You want to start with an orbit underneath the ISS (e.g. at 200 x 200 km). This means you will be lapping the globe faster than the ISS, you you are chasing it down.

4. When you get the closest orbit starting to come down (from 18 down to around 10 orbits), gradually (say 20-30km at a time) raise one side of your orbit, then circularize again. As you do this, your relative catch-up speed reduces, so your closest orbit will go back to say 15 or 16. Reel it in, then let it go slack, fishing style!!

5. You goal is to continue ratcheting up to match the ISS orbit in a zig-zag like this, ever so gently, so your final approach is maybe a 10 secs delta time for the whole orbit (your "T" on Orbit MFD is within 10 secs of ISS's "T" time). This way, you need a tiny retro burn to come up on station, rather than the Buck Rodgers scream-in and burn to a stop that you can do on other ships - e.g. DG.

6. Optional extra for ++ brownie points ... if your target has some eccentricity, then you can really get smart and align not only your PeA and ApA, but also align your Argument of Periapsis (AgP). The easiest way to do this is to get circular first, then do a Prograde bump exactly at the ApT of the target (i.e. squeeze out your new Periapsis into the egg-shaped target orbit with everything lined up. (You can also "move" the AgP by doing Prograde burns a few tens of degrees away from the Peri or Apo points - but you'll need some experimentation to make this work out!). Why do this? Well - it helps keep you on station better for the final docking maneuver, as you are floating up and down on the same eccentric orbit as the target.

Good luck guys + let us know how you get on.

 

[Dude]

It sounds like you're getting into orbit, aligning planes, etc. correctly, but let's be sure of that. I'm assuming that since you apparently haven't rendezvoused with anything yet, you're probably trying to get to ISS or Mir with their orbits unaltered from the standard Orbiter distribution. Both these craft are in low Earth orbit and have nearly circular orbits. If you're trying to do something other than this, I'd suggest putting that plan aside until you've at least done a few rendezvouses with one of Orbiter's "built-in" satellites.

When you try your rendezvous maneuver, you should be starting from an orbit where you're in a good position to do it. That means you should be at the same radius (or within a few km) of your target, and your orbital plane should be the same (or as close as possible) to your target. Specifically:

1) In Orbit MFD, your craft should show your periapse and apoapse radii (PeR and ApR) as being within a few km of each other, and close to your target's PeR and ApR values.

2) In Sync Orbit MFD, the relative inclination between your craft and your target (Rinc) should be very low, as close to zero as possible. If it's anything under over one degree, align your orbital plane to your target before going further.

You won't have to do this later on, but for the moment I want to be sure you're reading the right data off the MFDs in #1 and #2 above, so ... to double check those things, also look at values in Orbit MFD for your craft versus your target for these parameters -- SMa, Ecc, Inc, LAN, T, and Vel. These should *all* be close to being the same between you and your target, and all of them should fall into line automatically when you've put yourself into a good orbit from which to do your rendezvous maneuver. There will be small differences, but that's fine. If *any* of those show a dramatic difference between you and your target, something's wrong.

When you're at this point, just before you're going to sync orbits, do a quicksave then post the scenario file here (you'll find it in <Orbiter root directory>\Scenarios\Quicksave), and let us know which vessels are your spacecraft and your target. That would be helpful.

As others have said, turn off non-spherical gravity in the Launchpad. But be aware that there are two big things that cause an Earth satellite's orbit to fluctuate. One is that the Earth isn't a perfect sphere, but is somewhat fatter at the equator than the poles. The second is gravity from the Sun and Moon pulling the orbit in various directions ("perturbing" it). Turning off non-spherical gravity only turns off the first of these effects. It helps a lot, but don't expect your target to be *exactly* where you predict it will be for your rendezvous.

One thing that can be frustrating when using Sync Orbit MFD is that the moment you start doing your burn, the periapse and apoapse of your orbit change. There are two ways to rendezvous -- do a retro burn to make your orbital period shorter, so that you get to the same point faster than before; useful if your target is ahead of you and you need to catch up to it. Because you're in low earth orbit, you don't have a lot of room to reduce your periapse distance, so that method is really only useful if your target is *slightly* ahead of you. So I'm assuming that you're doing the other method, doing a prograde burn to make your orbital period longer, and by doing that allowing your target to catch up to you. So, from a circular orbit, just before you do your burn you might be 40 degrees past your periapse (true anomaly, "TrA" on Orbit MFD) ... but the moment you start burning, your apoapse increases and the point you're at right now becomes the new periapse ... which means that "Ship periapse," "Ship apoapse," and "Manual axis" all change to different references on Sync Orbit MFD. The way I usually handle this is to use "Manual axis" to choose my burn point, but switch to "Ship periapse" immediately after starting the burn. Once I've completed the burn, I switch back to "Manual axis" -- the periapse of the new orbit and the rendezvous point may shift a degree or two away from each other, but rotating the MFD's intersection point ("R+" or "R-") takes care of that.

 

[Cras]

For Rendezvous MFD, it is an older MFD, and will not like when you switch focus. It will reset itself which can be quite a bad thing when you are in the middle of rendezvous,

but what you do is set the target, and then you adjust the time you want to intercept the target. It will then show you your relative path toward the target, but most important of all, it will tell you how fast you will be going relative to the target at the moment of intercept, so you can plan an approach that you will be able to then complete when you get there. A nice small deltaV is essential here for realistic vehicles such as the Space Shuttle.

Set the time to something like 2400 seconds, then adjust until you get a path that is good with a good intercept relative velocity. Then you switch to the burn vector view, which is just like the ones found in IMFD and LTMFD, and you that is how you burn. When I use it, I use the autopilot for the first burn (which will not work if you are using it in an external MFD), and then use Attitude MFD to align my attitude at 0-0-0 on the LVLH (or velocity vector, however Attitude calls it). Then I make little corrections to keep the dV required at zero using translational RCS.

Rendezvous will require a constant monitoring of that BV screen, as it will show you drifting off course quickly and often, so you need to continue making little burns to keep things all nulled out and stay on its planned course.


The alternative, is use IMFD. As you approach your target from a lower orbit, slowly raise your orbit higher and higher and higher, until you are only a bit under. Then you can pull up IMFD, and use the Target Intercept program. Set the target to the Station you are shooting for, then you have two values to adjust, the Tej and the Tin. Tej is when to start the burn, Tin is when you are to reach your target. The screen will also show information such as how much deltaV will be required to match up the orbits when you finally intercept the target, use it just like you use the relative velocity value in Rendezvous MFD. Also may attention to the resulting orbit from the burn. In IMFD, it displays your current orbit in green, and your planned orbit in blue. If you are not careful, you can input values in Tej and Tin that will result in a burn that will dip your perigee well into the atmosphere and you will de-orbit. Getting close to the target, and being in an only slightly lower orbit will help you avoid this, but watch the blue lines in the IMFD screen to make sure you are not dipping your orbit too much and dooming your mission. You will want to just put yourself in a nice ascent where you will drift up right under the station.