How does KSP decide which decoupled part is the ship?

[Eogen]

I am having a problem where when my ship decouples a spent fairing, the KSP engine decides that the decoupled fairing is the ship, and the rest of the probe is a new entity that I must switch to from the tracking station.

I need to decouple from Stage2 before deploying Fairing1a, and I want to continue to be able to later control Stage2 (which has multiple Stage-3 probes.)  Stage2 and each Stage3 have Control Pods (Octo2's)

• Stage2 || DeCoupler1a || Fairing1a || DeCoupler2a || Stage 3a
• Stage2 ||
• Stage2 || DeCoupler1b || Fairing1b || DeCoupler2b || Stage 3b

Here's what is happening:

1. I pilot the ship to the point where I want to deploy Stage3a.
2. I fire DeCoupler1a.  This leaves the "Active Ship" as Stage2, with Fairing1a and above moving away as a new, separate entity.
3. I go to the tracking station, and switch to the new entity.
4. I activate the Fairing1a, and Stage3a is exposed.
5. I fire DeCoupler2.  This leaves the "Active Ship" as the Fairing1a and spent Decoupler2a.  Stage3a is now moving away as a new separate entity. 
6. The only controllable entities (Stage2 and Stage3a) are not active, and instead the "Active Ship" is a couple of dead parts with no control pods.
7. I go to the tracking station, and switch to the Stage3a entity, and activate its engine.

I would rather not end up in control of the fairing stage after DeCoupler2 fires, and avoid the extra trip to the tracking station.  So how does KSP decide which half of a ship to "stay with" when a decoupler fires?  During ascent, I don't have to revert to the tracking station to not follow the spent engine and tank from (the unshown) Stage1.  How can I make KSP follow the same algorithm when discarding the fairing?

Thanks in advance.

p.s. If you're wondering, the Decoupler1's are there because if you fire Fairing1a when it is in close proximity to Fairing1b, then both Stage3's end up being hit by the fairing debris, and their instruments end up damaged.

Edited May 9, 2020 by Eogen

[Superfluous J]

Not an entire answer for you, but you don't have to go to the tracking station to switch vessels. If they're within 2km you can use the [ and ] keys to switch vessels.

 

[bewing]

Generally, the focus stays with the part of the ship that contains the root part.  You can change the way the focus moves with the reroot tool.

Edited May 9, 2020 by bewing

[Eogen]

7 minutes ago, bewing said:

Generally, the focus stays with the part of the ship that contains the root part.  You can change the way the focus moves with the reroot tool.

Sure, but AFAIK, I cannot have multiple root parts.  So the root part of the entire original ship is the Octo2 control pod in Stage2.  

You can think of this question then, as "How can I pre-designate what the root part of Stage3a will be when it disconnects from the main root part of the entire ship in Stage2?"

Edited May 9, 2020 by Eogen

[Eogen]

18 minutes ago, bewing said:

Generally, the focus stays with the part of the ship that contains the root part.  You can change the way the focus moves with the reroot tool.

I accept that.  However, in the above case, the root part is in Stage2.  I do not know of any way where I can pre-define what the new root part of Stage3a will be once it is disconnected from the main root part of Stage2.  And if I put the root part in Stage3a, then I am in the same boat when it comes to launching Stage3b, because I will have launched the root part as a piece of Stage3a.

 

(sorry for the duplicate post.)

Edited May 9, 2020 by Eogen
apology

[AHHans]

55 minutes ago, Eogen said:

You can think of this question then, as "How can I pre-designate what the root part of Stage3a will be when it disconnects from the main root part of the entire ship in Stage2?"

You cannot. The root part of a decoupled section will be the part in this section that was "closest" to the root part of the ship before the decoupling. Or in other words it will be the part that is/was directly connected to the fired decoupler.

So in your example, when you fire decoupler-1a then fairing-1a will be the root part of the decoupled section.

[Edit:] So the recommended way to deal with this situation is to do what @Superfluous J suggested:  fire the decoupler and then use "[" or "]" to switch to the vessel that you want to control. (And since a recent patch (1.9.0 I believe) "[]" will not switch to "debris" - i.e. parts without a control point - anymore.)

Edited May 9, 2020 by AHHans

[jimmymcgoochie]

Unless you're decoupling landers with their own fairings to survive an atmospheric entry, I don't really see why you need multiple fairings and decouplers either side of them. What exactly are you using this rocket for?

Why not use a single fairing on stage 2 under the stack adapter to enclose the whole thing? Unless it's a really narrow stage or you have really wide payloads, a single fairing makes a lot more sense than using several smaller ones that can collide with each other and break stuff.