Choosing the Right Radial Decoupler

[swjr-swis]

8 hours ago, Snark said:

The fact is, mounting the radial decoupler as high above the radial booster's CoM as possible will do the best job of getting the booster to peel out and away from the central stack, by making it rotate outwards.  I do this all the time, and it works great, and have never had a single problem with it.  I've seen this advice echoed by other folks in the forums as well, so I'm not the only one advising this.

It does work, but sometimes too good. Intertia can carry a heavy booster parallel to the center stage for longer than you may wish, especially if you're getting into the thinner upper atmosfere and drag doesn't counter it enough anymore. Any kind of rotation while the CoM of the booster keeps up with the center stage can make the other end of the booster hit the center, with catastrophic results.

With lighter boosters, it's the drag that sometimes kills you, as the smallest rotation putting the booster nose out of prograde can initiate a sudden flip in midair, again crashing it into other parts of your craft.

And sometimes, and I admit this may be a personal thing I have with some of my designs, the shape of a complex craft really needs my boosters to let go with an absolute minimum of lateral or rotational movement.

I've had very good results just taking the trouble to figure out the exact CoM of my booster assemblies, and placing that on the decouplers. Most of the time I can even completely dial down the ejection force of the decouplers, cause they just 'slide' down without endangering the rest of the craft, and whatever tight space I left in the configuration of the craft for the exhaust is enough for the booster to fall through. They tend to start rotating only a good distance down/away from my craft. It's very worth the extra effort, and very satisfying to watch.

[Snark]

1 hour ago, swjr-swis said:

It does work, but sometimes too good. Intertia can carry a heavy booster parallel to the center stage for longer than you may wish, especially if you're getting into the thinner upper atmosfere and drag doesn't counter it enough anymore. Any kind of rotation while the CoM of the booster keeps up with the center stage can make the other end of the booster hit the center, with catastrophic results.

With lighter boosters, it's the drag that sometimes kills you, as the smallest rotation putting the booster nose out of prograde can initiate a sudden flip in midair, again crashing it into other parts of your craft.

Point taken, though in practice I find that it's not an issue for me; I never get collisions like that.  I always set up my stages so that the center core's engine ignites on the same stage that jettisons the radial boosters, which means I'm not dependent on drag to decelerate the boosters-- even with no atmosphere at all, the ship's own acceleration is plenty to slide it up out of them.  I attach above CoM, but not super far above CoM, so most of the ejection force goes to pushing them outwards rather than rotating them.  A bit of rotation is plenty.

Jettisoned boosters hitting the central core used to be a lot more of a problem in the old pre-1.0 aerodynamics.  First, the spent boosters had the same mass/drag ratio as the heavy core, so they didn't have that differential aero acceleration.  Second, there was no such thing as body lift, so being angled to the airstream didn't help separate them.  The new aero makes this much friendlier, causing them to do exactly what I want them to do:  quickly move out away from the ship, and back behind it.

[JoeSchmuckatelli]

After reading this - I decided to go try the TT-38K (the small one) and it worked - catastrophically.  The rocket design I am comfortable with (at my level of tech) has two 'Skippers' flanking a 'Mainsail' (the 3 engines are level with one another).  The Skippers are supposed to burn approximately 3/4 of the Mainsail burn, then I drop them.  I have - till now - used the Hydraulics almost exclusively - because they've never blown up my ship.  Trying out the TT-38s resulted in detonation of the lower fuel canister and Mainsail engine upon separation.

 

So I tried out the TT-70 with strut connectors - and that worked fine.  Was a bit hairy - but no explosion - which is a good thing.

 

Based on this - I may stick with the TT-70 for future builds.  But I will say - there's something majestic and comforting with the way the Hydraulics eject the radial stages.

[Kryxal]

I love using the standoff one for the 2.5m parts and the BIG boosters ... in fact, I made a Kickback booster subassembly.  Decoupler, booster, nosecose, sepratron angled just inward of straight up, and I just need a strut for stability and it clears off nicely.