Difference between revisions of "Tutorial:Basic Docking"

Latest revision as of 20:09, 2 May 2019

Specifications

Difficulty: Medium

Time: 10 - 30 minutes

For Version: 0.18 (Approach possible since 0.14.0)

Overview

Assumptions and terms of reference

This tutorial makes several assumptions to make it easier to read. Whilst these assumptions have been made, the principles described here will hold true for any docking maneuvers for two vessels in orbit around the same celestial body.

It is also assumed that the player who follows this tutorial has gotten to grips with the basic skills including advanced orbiting and has a ship capable of carrying out these maneuvers with enough delta-v etc.

The celestial body shall be known as Kerbin. The vessel to be docked with shall be known as the 'target'. The vessel to perform the docking maneuvers shall be known as the 'ship'.

For the sake of realism, this tutorial assumes that the target is on an elliptical and slightly eccentric orbit. It is also assumed that the ship is beginning from an approximately circular equatorial orbit below the target.

Finally, it is assumed that the target is "Set as Target" in-game.

Step 1 - Matching orbits

To match your orbit you will first need to match the inclination of the ship's orbit with that of the target. To do this, launch at the time of day when the launch pad is underneath the orbital path of the target. Once you've reached a comfortable altitude, burn in the direction the target will be going when it reaches your position until you have a comfortable orbit. You may need to make further corrections; the most efficient place to do this is at one of the ascending or descending nodes as shown on the orbital map.

There are two ways to measure how far you are from the target's inclination. First, you can hover over either the ascending or descending nodes with your mouse, which will give you however many degrees difference there is. Watch that when you're burning to match inclination, and hit X when it is close or exactly 0. NaN is considered even better, but usually not worth the effort and might not appear at all. However, you will not be able to do this while your mouse is using the maneuver planning interface. The best way to do that is to arrange the camera until the circle of the target's orbit occludes itself and appears to become a straight line. Match the projected path with that line, and that will give you the exact maneuver you need to execute in order to match eccentricity.

Don't expect the node to change the sign because the relative inclination on the ascending node is by definition always positive. So usually the relative inclination will approach 0° and at some point start going back. As soon as it goes back stop to burn and wait for the craft to be closer to a node or reorientate to match the required inclination change. The relative inclination won't change to 0° if the craft isn't burning exactly on the node.

Once the inclination has been matched, you will need to execute either a Hohmann or a bi-elliptical transfer burn.

Note: Depending on whether the target will be ahead or behind the ship after the orbits have been matched will depend on how to place the transfer burn. We will assume the target will be ahead.

Transfer burn

The transfer is best carried out at the semi-major axis of the target's orbit (periapsis is or apoapsis). This will reduce complications found in creating two matched elliptical orbits. Based on our assumption that the target will be in front of the ship after the burn is completed, it is best to place the periapsis at the altitude of the periapsis of the target and the apoapsis between 90 and 95% of the altitude of the target's apoapsis. This will allow orbital mechanics to bring your ship around past the apoapsis area faster than the target, and this make up the time. Once the periapsis of both vessels (Target and Ship) has been matched, at least one set of Intercept markers should appear in the orbital map. Using these is a good way of working out if the ship is ahead or behind. Perform and adjustment burn at the periapsis to move the target position and ship position as close together as possible here.

Intercept

It is likely that a correction burn will be necessary, because the ship and the target will still be several kilometres from one another at intercept. This is particularly the case when the Target has an orbit high above Kerbin. The room for error in burns decreases as the elliptical shape of the orbit elongates. (i.e. with a low periapsis and a high apoapsis). I have found through practice that a correction burn with about 5% of the orbit left to go until intercept if a good time for a reasonably efficient correction burn. It would be most efficient to burn at the opposite part of the orbit from the Intersect point. If you could do this accurately, the chances are that a correction burn at this stage would not be necessary anyway. So for the sake of ease, it is often worthwhile to sacrifice a small amount of efficiency here.

Step 2 - Matching velocity

Once the separation is showing as less than a kilometer (preferable 0.0 km) at the intercept, ensure that the target is selected on the navball as opposed to orbit or surface.

Note:This should occur automatically once the ship is close to the target anyway.

As late as is comfortable, (i.e. when the ship and the target are as close as possible) burn against the velocity shown on the navball to ensure the ship and the target are no longer moving relative to one another.

Step 3 - Local approach

The chances are the ship and the target have stopped some (hopefully small) distance apart. Point the ship towards the pink target marker on the navball. Move very slowly in this direction, either with a tiny burn, or with RCS. Move the ship to a position where it is lined up with the target as you require before another tiny burn to kill the relative velocity again. You should now be within a few metres of the target, and roughly lined up with the target.

Note: This is a good position to transfer things such as Kerbals via EVA if there are no docking clamps on the ship / target.

Step 4 - Final precision approach

The game incorporates a special UI for docking. This can be used, but it is not mandatory.The Docking mode will use RCS for maneuvering. It is highly suggested that RCS is used in the Staging view too if this is preferred. Move towards the target very slowly (suggested less than 0.5 m/s). As you approach, visually maneuver the ship's docking port to match that of the Target. One good way to do this is to right click on the docking ports you are using (on both ships) and maneuver so that the ship is pointed north with a pitch of 0° and the target is pointed south (heading 180°) with a pitch of 0°. If you need to make adjustments with the other ship, use the [/] key's to switch ships without going to map view. Once they are touching, kill the relative velocity and you are docked!