• 0
Stewcumber

Please help with my first orbital rendezvous!

Question

I'm trying to put up my very first space station in orbit around Kerbin. I've got the core of the station "ODS1 (Space Station)" up at a circular 150,000m orbit. I've got the living quarters "ODS1 (Living Quarters)" about 5km lower, but accelerating away from the SS by 13m/s. I also have a 2.6km intersect coming up.

ExuRklc.jpg

o0lyqKI.jpg

mrqMgbm.jpg

As you can see, my faster, lower ODS1LQ is now infront of ODS1SS. I'm stuck as to what to do next- the guides online don't seem to explain the burning to the target very well :(! I'm trying to use a guide by Entropius:

DWMawWp.png

If I slowly burn towards the "Target Prograde" icon I end up flying in large circles around the space station and messing up my orbit. I'm not sure how to "drift" over to the ODS1SS. If I burn prograde to orbit to get my orbit a little bigger, I will get even further in front of the ODS1SS? :/

Interestingly I've just noticed my target / prograde icons are the opposite of the Entropius guide (I guess because my LQ is infront of the target not behind...) so I would be... pulling retrograde?

irEpryG.jpg

So what should I do once I'm at that 2.6km intersect?

Edited by Stewcumber

Share this post


Link to post
Share on other sites

5 answers to this question

Recommended Posts

  • 1

I could give you an explanation of how I do it, but I think this video will help you more than enough

 

Share this post


Link to post
Share on other sites
  • 0

The videos are always more helpful than the words. But you already have a video (above) -- so I'll give you a few more words. They may trigger an "aha!" moment, or maybe not. :wink:

1. Yes, while you are doing a rendezvous maneuver like this, your orbit will be temporarily messed up while you are maneuvering. It gets fixed again when you are much closer to the target. (For orbits that are down very close to the atmosphere, you have to be careful that you don't accidentally put your orbit down too low while you are maneuvering.)

2. This is something you already know, but I'm trying to trigger the Aha! moment: "prograde" points exactly in the direction you are moving. The target marker points exactly toward the target. So if the prograde marker is exactly on top of the target marker, that means that you are moving directly toward the target. Which is what you want.

3. The reason your target marker is opposite to what the picture shows is: at your current point in your orbit, you are moving away from the target. This will usually be the case for half your orbit as you approach a rendezvous. So it's OK, but you can wait until the other half of your orbit -- then the two orbits will be forcing the ships closer together, and your display will look right. (If you really wanted to, you could flip your ship around and burn right now, cancel out your movement away from the target [burn retrograde until your target velocity is zero], point at the target and burn to get your prograde vector on top of the target vector, and begin your approach. But it would take more fuel, so it's better to wait.)

4. All of us end up going in big circles around our target the first time we do this rendezvous stuff. In physics, it's called Coriolis forces that keep you from going straight. You may figure that it shouldn't matter much at only 5km separation -- but in fact it makes a difference all the way down to 30 meters separation. That's what makes it hard: you point your ship, and burn, and then you don't quite go straight. You go in a big curve instead.

5. Generally, you want to start your rendezvous maneuvering a couple minutes before your closest approach point.

 

Edited by bewing

Share this post


Link to post
Share on other sites
  • 0

I hate to throw another video at you (except that it's mine :D) but this answers your question exactly: How do you go from "getting within a few km" to "parked meters away at 0m/s relative velocity"

It's not the best way (The pictures you have above detail that) but I think it's the best way to learn. My suggestion: Do it this way until you realize WHY it's not efficient and you'll understand the efficient way.

Also, it's not really all that inefficient.

 

Edited by 5thHorseman
Oops forgot to add the actual video :D

Share this post


Link to post
Share on other sites
  • 0

What my personal strategy is this:

1. Once I have an intercept within 5km (which you have), I change the navball mode to "target" (click on "orbit"). I then time warp until I'm within five kilometers the target.

2. I point retrograde on the target-mode navball. Wherever you burn, it 'pushes' the retrograde marker away. So, position you ship so that it will 'push' the retrograde marker on top of the the:targetretro:.

3. As you glide closer, the retrograde marker will drift away from the:targetretro:. Just keep the retrograde marker centered by occasionally burning to push the retrograde marker. As you do this, you should be pointed roughly retrograde, so you should be slowing down as you burn to stay on course.

4. When you get within 100m, point directly retrograde and kill your velocity. Then move in using RCS for docking!

PS: If you run out of velocity while approaching, enable RCS and tap the 'N' key a few times to gain speed. This saves you from having to turn around prograde then back.

Share this post


Link to post
Share on other sites
  • 0

Thanks for the help guys! I watched Scott Manley's video this morning and I finally understood the concept of burning retro/prograde relative to your target, instead of just burning straight at the /away from the target. So now I've docked my living quarters and fuel tank :)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now