How to get maximum flexibility from decouplers

[bcbarnes]

So I am still learning to play KSP 1.8. I watched a video by Matt Lowne (linked below - thanks for the work Matt)

In this video he designs a rocket that has as a first stage a single stack of 800 fuel tanks with a swivel at the end. Attached radially in a pair is another stack of 800 fuel tanks with reliant engines. Standard decouplers are used at the top, and fuel ducts are used to feed the center engine from the side tanks. Matt comments that when the side tanks are empty, the idea is to eject the two side stacks, and the center stack is still full of fuel.

I copied his design exactly, and was able to follow the video and get to Mun and back. Now, however, I want to do a rescue mission for a Kerbal around Minmus. I modified the design for a heavier payload (two seat capsule, external monopropellant, RCS thrusters, etc). I'm expecting to need more dV out of the first stage, and thought that I could just double up on the side tanks/reliant engines. But then, while out on a walk (where my head tends to go to new and diffferent places ...) I thought about adding a pair of solid boosters instead. This lead me to the problem of who is going to run out of fuel first - the side liquid engines, or the side solid engines.

I can also devise scenarios where it changes - a more vertical launch profile to maximize the initial AP, or a flatter profile to get circularization for "free". In one case, I'd run all liquid engines at max, and the two side engines are more likely to run out of fuel before the solids. In the other case, I would probably throttle down the liquid engines as much as possible, in which case the solids would run out of fuel first.

All of this was a long lead in to my question, and what I would like to be able to do. In decreasing order of happiness:

 

1. Have both the liquids and solids set to decouple as I anticipate based on my expected launch profile. I can manually switch the decoupler staging during flight to get what I want.

2. Be able to manually trigger the decouplers during flight by click in the stage stack during flight.

3. Be able to tell the decouplers to trigger automatically when a given (pre selected) engine is out of fuel.

 

I have KSP 1.8 and both DLC. Is there a way to accomplish #2 or #3 (I haven't looked into the robotics, etc. that the DLC's provide)? If not, does anyone know if KSP 2 will allow this. If not that, does anyone else think this might be a useful feature request??

 

Thanks for listening

Brian

 

 

[Pecan]

Action groups for manually activating decouplers regardless of position in the staging stack.  (Set in the VAB during construction or can now be set in-flight.  Allows number keys plus a few others to activate just about anything that can be activated).

For automatics you'd need a mod, probably an autopilot one such as MechJeb (MJ) or KoS.

PS:  Making a heavier vehicle doesn't mean you need more dV.  dV already includes calculation based on mass so as the vehicle gets heavier your dV (and TWR) goes down.  What you need is more fuel/better engines to bring your dV back up to whatever the mission requirements are.  Eg; it always takes 3,400 m/s to launch to orbit (roughly) but it's easier to achieve that much with a lighter vehicle.  As a separate issue note that you can 'tweak' (right-click and adjust in the VAB) SRBs to produce less thrust over a longer period, which may help with stage timing when mixed with LF.

Edited October 24, 2019 by Pecan

[bcbarnes]

Pecan, Thanks for the info and explanations. I haven't really understood the action group stuff yet. I guess I'll dig into it.

[Pecan]

2 hours ago, bcbarnes said:

Pecan, Thanks for the info and explanations. I haven't really understood the action group stuff yet. I guess I'll dig into it.

Action groups are very useful, especially for stuff which doesn't really belong in 'staging' - toggling ladders & solar panels, undocking, etc.  Look at the buttons at the top-left in the VAB and ask if you have any problems but note that in career mode (aka 'how to make learning hard for no reason mode') you need to upgrade a building or two before you have access to them.

[drpunk]

21 hours ago, bcbarnes said:

This lead me to the problem of who is going to run out of fuel first - the side liquid engines, or the side solid engines.

You can adjust the thrust and amount of fuel in solid rocket boosters which will both have an effect on how long the solid rocket boosters will fire for, so you could tune solid rocket boosters to always run out of fuel first.

I've no idea what button to use on PC, but you can bring up the info dialog for the decouplers from the staging (I put cursor over decoupler stage and press square on PS4), and the info dialog has a decouple button which you can use to decouple it outside of the order of the staging. You don't necessarily have to move them around in staging. You can do that with anything in the staging (fairings, parachutes, etc.).

While above is good for single item, if you require two or more things done at exactly the same time then you need to look at action groups as mentioned.

Edited October 25, 2019 by drpunk