Complete and clueless newbie needs help with docking

[RAZZIRAZZ]

Hi all,
I need some help (to say the least) since I know next to nothing about space flight and am taking baby steps as I go along.

Okay, that said, I figured out how to go into orbit, how to align orbits and basically how to "begin" to dock with the ISS. I emphasize "begin" because I get up to being 50k or so away from the ISS and I have the Docking MFD in my left panel but that's when my problems start.

For one, my relative velocity doesn't stay put and if I don't keep on giving thruster bursts it goes into the green (minus bar) and instead of getting closer to the ISS, I'm naturally slipping further away from it. So, for one, how do I keep my relative velocity steady?

The other thing is that I don't understand the rationale or the usage of the "velocity vector" itself.

For instance, I'm using Jared Smith's docking tutorial and at one point I'm told to "quickly rotate your ship to align your nose indicator in your hud to point directly at the velocity vector." Okay I do that but then my relative velocity starts to decrease and if I use my main thrusters to keep it up, the velocity vector starts to slip away along with my RV. <shrug> I just don't get what I'm supposed to do.

Anyway, any little bit of help is appreciated and thanks in advance.


JimR

 

[Urwumpe]

You should remember that you are, at 50 km, still too far away for direct travel. You need to still use orbital mechanics until you are about 15 km away, better start doing so only 500m away.

If you travel fast enough, you can also use a proportional guidance approach, from about 25 km distance on (important is: Time to docking has to be significant shorter as orbit period, ie 1/10th). This buzzword means, you just keep the angle between velocity vector and target constant. This brings you very close to the target.

The relative velocity indicators in Orbiters docking HUD give you the magnitude and direction of your velocity vector relative to the target. The "[+]" marker means, you increase the relative velocity, when burning main engines that direction, the "[-]" marker means, you reduce relative velocity there. (Of course, it is opposite when you burn retro engines)

I recommend using RendezvousMFD, it is annoyingly accurate sometimes. You will arrive just like the MFD shows you, before you activate it's autopilot. Exactly like that.

[ame="http://www.orbithangar.com/searchid.php?ID=1199"]Rendezvous MFD 050621[/ame]

Otherwise, I have written a small article on OrbiterWiki on the orbital mechanics approach to rendezvous.

http://www.orbiterwiki.org/wiki/Rendezvous

 

[Mission_CDR]

[ame="http://www.orbithangar.com/searchid.php?ID=2663"]Atlantis to ISS Full Tutorial[/ame] this is what I used to learn how to dock, watch it enough times so that you caould do it in your sleep.

 

[ar81]

[ame="http://www.orbithangar.com/searchid.php?ID=3074"]Tutorial: DG to ISS[/ame]
I also learned with Jared Smith tutorial.
I made this tutorial so you have graphical awareness of what Jared explainedin his tutorial.

 

[Wedge]

Docking

Here's a long description of the "noobie docking"
There is a smaller checklist at the end to remember the tricks.

Let's say you are 50K from ISS, your orbits are aligned, but it seems you won't get any closer to ISS than 50K.
Turn on the docking mode, switch HUD to docking mode.
There are two indicators on the Hud, thats to only thing you'll need.
One of them is ISS indicated by a green box, forget it from now.The other is the relative speed vector indicator.
ISS and you are going at very high speed, so you need to change referential to prevent your brain from boiling from the mental stress.
Imagine there is nothing in space but you and ISS. And ISS is dead still in space, no movement whatsoever.

The relative vector indicator indicates the direction you're NOT going (using ISS as a referential.) The crossed circle is showing the direction you must accelerate in to stop moving relatively to ISS. Do that...Rotate your ship to face exactly the relative velocity indicator.
Fire thrust at maximum throttle. Your (relative)speed will decrease, make sure you are at zero.

Ok, relative speed means ISS and You are moving at exactly the same speed and direction. So all you have to do now is to point your nose toward the station and fire a burst to accelerate towards ISS. You should see cross coming in view, that's the relatvie vector indicator too, it shows where you are going (still using ISS as referential) point it to ISs using liner rcs , wait till you are 10K from ISS, decelerate to 20knots or so.Approache gently, and dock using linear and rotation rcs.

It might be hard to understand at first, but it's actually quite simple. First you stop moing away from ISS, then you move towards it.
Here's the "checklist"

-Wait until you are "close" to ISS.
-Point ship towards the crossed circle
-Main throttle to 100% until relative speed is 0
-Point towards ISS
-Fire main throttle to 100% until you reach 200 or so speed
-Wait till you are closer to ISS, and adjust speed and direction with Linear RCS, and retro bursts.
-once you are 1klick from ISS, it' all about precision burst with RCS.
Good luck.

 

[astrosammy]

Also make sure that the relative inclination is as close to 0 as possible, so you are approaching ISS from behind, below, front or above, not from left or right. Approaching from left or right makes the docking much harder. It's also much easier if you rotate ISS with the scenario editor so the docking port is facing pro- or retrograde (Don't worry, that is no "cheating", the real ISS has also such an attitude).

 

[ar81]

The reason why your relative velocity changes in time is the following:

Just like when you fly an airplane you exchange speed for altitude.
So if you are below the station, you will be travelling faster and if you are above you would be travelling slower.

So as soon as you leave your relative velocity as zero, such principle starts to act. Your velocity will change because of the difference of altitude.

Even if you try to be leveled with the station, as you go around the planet, you will be at a different altitude a few minutes later. It seems very unintuitive at the beginning, but in time you get used to it.