When to separate SRBs? A not-so-obvious question.

[Snark]

46 minutes ago, wumpus said:

1.  Stability.  My SRB-based first stages are typically controlled by a pair of AV-T1 winglets on the North and South end of the rocket (which then provide control for an E-W pitchover).  The liquid rocket typically has vectored thrust and can do the rest needed to get the rocket back on course (note: you pay not only in that extra decoupler, but also in a much worse (I assume) aerodynamic profile.  Just slap on more controlled winglets unless firing it up for the reason below as well).

Perhaps, though for me that's generally not a reason.  The disparity in Isp is just too much.  You're right that stability is an issue when you don't have any thrust vectoring to play with, but my solution to that is generally to be judicious about building rockets that are aerodynamically stable (which tends to work out well for SRBs, since they lose mass fast after launch, so the CoM moves up rapidly as they burn).  I use reaction wheels to give the gravity turn its initial eastward blip when I get up to 20-30 m/s or thereabouts, and let aero stability and the gravity turn mostly handle things after that.  If I need a little extra help, I put on a couple of steerable fins on the north/south sides of the rocket, same as you.    (Though my default choice is usually the AV-R8.)

49 minutes ago, wumpus said:

2.  Re-usability.

I suppose, though for my part, that's something I've never cared about at all.  KSP economics make recovery pretty pointless unless one is into it for the "role-playing" aspect, which I am definitely not.  I just stage 'em and forget 'em.

[Kowgan]

9 hours ago, Plusck said:

Precisely: The boosters' thrust (494.4) is greater than the stack's TWR multiplied by the weight or mass of the boosters (giving a total of 382), so you keep them.

Because you want TWR(boosters) to be greater than TWR(stack). The TWR is the thrust divided by weight. Weight is mass multiplied by gravity.

Thus:

thrust(boosters) / (mass(boosters) * local gravity)  > thrust(stack) / (mass(stack) * local gravity)

multiply both sides by gravity:

thrust(boosters) / mass(boosters)  > thrust(stack) / mass(stack)

multiply both sides by mass(boosters):

thrust(boosters)  > (thrust(stack) * mass(boosters) ) / mass(stack)

And the actual units don't matter since they'll cancel each other out. It was also unnecessary to include gravity to start with (i.e. to differentiate between weight and mass) since gravity cancels out too for your purposes, but I thought it best not to start saying that TWR = thrust/mass because that's wrong by definition

Got it, the SRBs' result was still bigger than the rest of the vessel. But even though, when the SRB's fuel is about to end, and their thrust is much smaller than the ship's, I still got a SRB number higher than the ship:
http://puu.sh/nnIdD/71dbcf48df.png
http://puu.sh/nnIhZ/7a698767cb.png
Thrust (Boosters) = 42.7*2 = 85,4
Thrust (Stack) * Mass (boosters) / Mass (stack) = 696 * (2,073975 * 2) / 47.782 = 60.419, which is still smaller than the boosters' thrust. Even though, as you can see on the second image, I decouple them right away and they're slower than the vessel.
I'm guessing this method will tell me to drop the boosters once their thrust reaches (or gets very close to) zero.

Thanks for the advice, though. Good catch on the gravity cancellation.

Edited February 27, 2016 by Kowgan

[Rayder]

Just remember that all this is based on the OPs use of the variable thrust mod. This has no relevance to stock KSP - just stage when there's no fuel left. As for working out the TWR, it shouldn't be too complicated. The only thing that changes is the fuel consumption, which slows down as you burn to maintain the TWR. With those numbers, calculate the TWR of your vessel without SRBs.

Your other option is to just find the TWR of your vessel and then just set the SRBs to that.

On 2/14/2016 at 3:24 PM, Buster Charlie said:

I Kinda wish there was a stock checkbox in the VAB to auto stage when there was like 1% fuel left in the SRB so you didn't have to have the extra mass/parts count of a sepratron, I know you can kinda eyeball and do it manually, but if you're designing a unified launch system then it's good to have repeatability.

Although not stock, there is a perfect mod for this! KM Smart Parts. You can select a specific fuel in the tank it's attached to, and a specific percentage. You can also set it to either stage or fire off a specific action group. There's some other cool gadgets in there too like an altimeter, a timer and a proximity sensor.

Example: Have your main liquid stage only fire once the SRBs have cleared enough distance from your vessel!

Edited February 28, 2016 by Rayder