Lesson 4 - Ships that pass in the Night

Hopefully you have managed to shape your orbit to match that of SH-03 - Our next task is to syncronise our orbit with SH-03, and dock with it. In this part I'll give you some general synchronisation techniques at the end of the document. Reading them is not neccessary to accomplish this part but I it's always good to know how to synchronise in general situation, not just this particular one. Addon used for the optional tips is Arrive MFD.

You must download This save state or its EXE alternative. It had to be a only bit modified. Particularly I had to add a radio trasmitter to SH-03 which will help us during docking.

We are now orbting around the Moon, chasing SH-03 almost exactly - but how do we catch up? The first intuitve impulse is to do a prograde burn - after all, we want to go faster so we catch up, right? But let's examine what happens if you do this. Recall from your flight so far that a prograde burn raises your orbit - giving you a longer path to fly, and slowing you down. If we do a prograde burn we'll actually fall further behind!

What we need to do is take a short cut around the Moon - we need to fly a shorter path, and fly it quicker...

...a retrograde burn.

SyncMFD shows our orbit and that of SH-03, the current postions of both craft (our position as a dull green radius, and SH-03's as a yellow radius). The bright green radius is the reference line. The reference line is the best place to change our arrive times by doing pro- retro- grade burns, because only burning there won't change the reference line's position. All the calculations made by the MFD refer to this line and it can be switched between several locations.

While it's possible to synchronise with a target using all options from above, "Intersection 1 and 2" is generally preferred because only meeting with target (ie. having the same longitudes) at orbits' intersection guarantees that you will also be at the same altitudes at the moment of interception. That said, our situation is specific in this way because we have already equalised PeA and ApA with the our target so we are roughly at the same altitudes on whole current orbit

The pro of 2nd set of options (Sh ap and Sh pe) is that, as you may remember, the points in orbit at which it's most efficiently to change altitude of the oposite one (thus trying to synchronize) are our Peri- and Apo- apsis. These modes require some thought, ie. if we want to do a retrograde burn to catch up with the target then the point closest to target's orbit will be Sh apoapsis so we'll choose this one now. On the other hand if you will be doing a prograde burn to let the target catch you, you'll have to choose Sh periapsis.

On the right of the MFD are two lists of numbers - Sh ToR and Tg ToR (ToR stands for "Time on Reference"). These numbers show the time in seconds until each craft crosses the refernce line, for the current (0) and next four (1-4) orbits. To catch up with SH-03, we need to adjust our orbit so that we cross the reference line at (almost) exactly the same time. The two crossings which are currently the closest are highlighted in yellow. The DTmin is a difference in arrival times to the reference line for the highlighted pair. Use this parameter for precise burning.

Now, if you were precise enough with DTmin and Rinc, you should be within 5km of SH-03. at this distance, if we are careful and patient, just burning straigt at SH-03 will get us closer - for the most part, common sense reigns again. IF you happen to be much further like 40 - 200 km, you can't just burn towards SH-03 because at such distances the change of your orbital parameters will influence your relative motion non-linearly and you'd eventually start receding instead of closing. You can try Rendezvous MFD which will get you to the target but I won't describe it here. You can also try to resynchronise... and be more precise :)

During docking there are three types of misalignments combined together which you need to correct seperately. On the left I provide an easy to understand graphical overhead view and on the right - in-cockpit view and docking MFD's visualisation of the same situation.

Two important values in Docking MFD are:
TVEL - Tangential velocity - any velocity perpendicular to the approach path. TVEL of 0.00 guarantees that you won't change distance from the approach path so if you're aligned and on the app. path keep the TVEL at 0 m/s.
CVEL - Closing velocity. Can be positive (closing) or negative (receding).
The DST value is the distance to target.

Once you start moving your ship you'll see a yellow arrow appear in Docking MFD (tangential velocity indicator). This neat indicator shows us in which direction w.r.t target we are rotating / moving using translation mode.

To align longitudinally I had to rotate left thus making the arrow point in the direction of longitudal rotation indicator (red cross). You will usually have to combine 2 axis rotational/translational manoeuvres to set the proper arrow direction. Remember that the arrow shows movement w.r.t. target and that translational [Num 1] = left, [Num 3] = right, rotational [Num 4] = left, [Num 6] = right BUT despite their location in NumPad - translational and rotational [Num 8] = down not up, so it can be a bit confusing to move the arrow in the direction you want. It will point left if you press left but will point down if you press [Num 8] (intuitive up) and up when you press [Num 2] - just in the direction you'll move.
It's not necessary to increase the magnitude of the arrow to make it reach the crosses. Only direction matters and besides, it would be a waste of fuel and could lead to further inverse misalignment, saving only a little time if it succeded.
After you have aligned longitudinally, press [Num 5] to kill rotation. After aligning on approach path, you have to cancel out relative velocity manually. Here is where the arrow helps. Using translation mode, try to counter its direction so that it gets smaller and eventually dissapears and TVEL reduces to 0.

Remember that Docking MFD operates on logaritmic scales. It means that the smaller a value is the more sensitive the MFD becomes in displaying this particular value.

By moving towards last rectangle you already roughly aligned yourself on the approach path and by facing the target - longitudinally (still - roughly). The rotational alignment can be also roughly achieved thanks to the approach path rectangles.

Keep in mind that this type of synchronisation (a retrograde one) is sane only if the target is ¼ ahead of you max. In any other case there is a great danger that if you start a retrograde burn to try to synchronise, the PeA may fall below surface or at least into the atmosphere. For such cases use a prograde synchronisation.
Next time, we return to base.

Optional part

In this subsection I'll try to teach you some general synchronisation methods to regularise your knowlegde. Download Arrive MFD, extract the archive to Orbiter main directory and activate in the Launchpad > plugins tab. Warning! It will crash your Orbiter if you try to move its window. The correct playback for this subsection is named: RSfD-4-ships-that-pass-in-the-night2-optional.scn
We'll start from much higher orbit than SH-03.

We are 160 degrees before SH-03. First thing to do would be orbital alignment and after that - circulisation. Notice that our orbital period, distance to travel are already greater, velocity is smaller and we can use it all as adventages. It means that we don't have to equalise our orbits from the start but we can simply ... wait for SH-03 to roughly catch up with us and actually it's the best thing we can do if it's about saving fuel and time, when our orbital radii are much different.

As you may have already figured out there is also a possibility of an inverse situation, ie. when our radius is smaller than targer's. Then, we are the ones who chase the target, as it was in the very first case. As I have proven later you don't need to equalise orbital altitudes or even create the intersection from the start, but can wait until you are closer to the target and then create the intersection with a prograde burn.

Apart from the prograde instead of retrograde burn, the only notable difference is that the resulting RAnm will be (180 - X) where X is the usual value, because it always dsplays the distance to Periapsis and our intersection is at apoapsis - 180° 'round the clock ...

... so if you want the intersection to be 11° from your apoapsis, set RAnm to (180° - 11°) = 169°

Be aware that this method of synchronisation is widely used for eg. STS missions

The last thing I want to say is that it's possible to actually plan the synchronisation burn to know when you'll meet the target using TransX, before even igniting engines. Maybe I'll cover it in another tutorial in future.