How do I keep radial decoupled stages from colliding with my craft?

[JohnWittle]

It really seems like what I need is a radial decoupler with a couple hundred thousand ejection force, instead of a measly couple hundred. If I'm not pointed perfectly straight up, then it seems like no matter what, I can't "get out from under" my falling radial stages without having some kind of a collision. I got around this in my most recent craft by using multiple layers of radial decouplers, each firing at the same time, which gave me both much needed spacial separation at time of ejection and additional ejection force... but obviously this is a stupid solution. I experimented with using sideways-pointing ullage motors, but they're a poodle to set up and it really seems like nobody else has this problem.

Is there some special trick to it? I mean, from a purely physics-based perspective, it seems like as long as the center of mass of the radial tank+engine is aligned with the decoupler, then on ejection it should have the same velocity as my craft plus a slight outward momentum pushing it away... but it seems like that never happens. It feels like once i've ejected the side-tanks, I have a limited amount of time to accelerate away from them before they fall into my craft. Even though that doesn't make any sense.

[Streetwind]

There are tiny solid motors called Separatrons. They can be mounted on radial stages to push them away. Often one or two are enough, since you merely need to tilt the upper end outwards to prevent collisions in 99% of all cases.

[bewing]

Putting one fin on the outside edge of a radially decoupled booster (at the back end) pulls it away from your ship reliably. The bigger the booster, the bigger the fin you may need. If you want to get really tricky, there is also a question of timing if you are not using struts all over the place. When a booster flames out, and it has no struts on it, it will wobble fairly violently. If you hit the decouplers at the right moment on an outward wobble, the boosters will fly nose-outwards away from your ship. You can also use a couple of tiny fins at the top end of a booster, and angle them sligtly so that they produce a tiny outwards force. That will also pull the booster away from your stack when it decouples.

[Hodari]

For most of my ships, I don't have much problem with this.  Just make sure you do the following and should be fine:

1.  Be pointing prograde and not at too steep of an angle if you're still in the lower atmosphere.

2.  Just before separating the boosters, make sure you switch SAS to stability assist mode and not 'hold prograde' or anything else.  Don't turn the ship AT ALL during this stage.

3.  Once you have separated, wait a couple seconds until you've completely cleared those boosters before you start the engine for the next stage.

As noted above, you also want to make sure that you have enough struts to ensure that the boosters aren't wobbling at all.  If all of the above still aren't enough to keep them from colliding, sepatrons would be the next option to try, but I very rarely find that I need them.

[MrOsterman]

1 hour ago, Hodari said:

sepatrons would be the next option to try, but I very rarely find that I need them.

Yeah.. but they're Perty.... watching them fire to push those other stages away....

[Foxster]

Couple of tips...

Put the decoupler at the top and strut the tank at the bottom. This usually means moving the decoupler and/or tank around with the offset tool. This pushes the top of the ejected tank away on decoupling. 

Mount the radially mounted tank low so that the top of it is just above the bottom of the tank it attaches to. This makes the craft tall but that usually isn't a problem for a lifter. Doing this minimises the overlap between tanks and pretty much guarantees no collision on decoupling. 

If you have to use sepratrons then place them angled so they fire just away from the remaining tank, or they can cause that tank to overheat and explode. 

Edited June 28, 2016 by Foxster

[BeeGeenie]

4 hours ago, Foxster said:

Put the decoupler at the top and strut the tank at the bottom. This usually means moving the decoupler and/or tank around with the offset tool. This pushes the top of the ejected tank away on decoupling. 

This guy knows what's up. You want the force of the decouple to push the top of the booster away from the ship, so it spins out and away, like so:  |[]|   \ [] /   __  []  __

                          

You still have to be careful to stay near prograde, but it's much safer than leaving the decoupler/booster in their default configuration, with the decoupler attached to the middle of your booster stage.

[Stoney3K]

3 hours ago, BeeGeenie said:

This guy knows what's up. You want the force of the decouple to push the top of the booster away from the ship, so it spins out and away, like so:  |[]|   \ [] /   __  []  __

                          

You still have to be careful to stay near prograde, but it's much safer than leaving the decoupler/booster in their default configuration, with the decoupler attached to the middle of your booster stage.

To be more exact: Put the decoupler slightly above the empty CoM of your booster.

If you put the decoupler exactly in line with the empty center of mass, the decoupling force will push the booster straight outward, and aerodynamic drag will start pulling it retrograde (away from the direction your craft is going). The more you put your decoupler above the CoM of your empty booster, the faster it will spin due to the decoupling force (and less force will be applied to push the boosters sideways). Think of a lever with the decoupler acting on the end.

There are a few common causes to the 'boosters crashing into my rocket'-problem:

* Decouplers that are placed underneath the empty center of mass. This causes your boosters to tilt inward, and the nose cones will crash into your ship.

* Boosters that are placed too high up. Drag will pull the tumbling boosters toward each other when they're jettisoned, so if they are too high up your craft, they get jettisoned away from your craft and will boomerang back. Place your strap-on boosters as low on the core as you can.

* Decouplers that are placed too high up. This will cause insufficient clearance sideways, and the boosters will pretty much 'roll off' your core instead of having enough sideways momentum to fall on their own. In the extreme case, the decouplers will only spin the boosters into a tumble and not push them outward, which will make them flop around a 180 and smash into your main engine on the bottom.

Short answer: Decoupler needs to go a small amount above the dry CoM. Liquid boosters have a center of mass that is quite low when they are empty, solids have a more uniform mass and their center of mass is more in the booster's center when they're burnt out. Also, use struts to keep the boosters from wobbling, one strut per booster attached to the bottom is more than enough.

Edited June 28, 2016 by Stoney3K

[Firemetal]

Seperatrons are the best option but if your strap-on boosters are too big, I'd suggest flea solid rocket boosters.

[CSE]

Like bewing, I find a single fin at the bottom of the radial boosters is normally enough to tilt them outward just slightly, without sending them into an immediate wild tumble.

A real-life solution is to use longer-burning boosters so that you will be decoupling them higher in the atmosphere, where the air is thinner.

The SpaceY mods offer some combined decoupler-sepatron and nosecone-sepatron parts, which are a particular help for large rockets.

[ineon]

You might find this helpful, I had a similar problem:

Your problem sounds similar, but not exactly the same. The solution that I found worked was using the larger decouplers that keep some distance between the main rocket and the boosters.

Edited June 29, 2016 by ineon

[klesh]

Sepratrons for me.  My favorite style is to put 2 of them on the flat top of a booster, at such an angle that their flames wont hit the main craft, but will still apply the majority of their force away from the main ship. I'll put my nosecones on the top of the boosters, so only the engine bell of the sepratrons poke out.  Looks great and functions swimingly.   Don't forget to tweak the burn time and thrust levels if you want a cinematic feel to the separation.  

[Loren Pechtel]

There are also the separator nosecones in the SpaceY parts.  They're easier to configure than separatrons.  They do burn towards the main rocket but I've never had any problems from doing this.

[Superfluous J]

I like putting 2 of the tiny starter fins on a booster angled slightly (5 degrees, using the rotate tool) outward so aerodynamics causes the boosters to magically just drift away. If you place them right on the COM of the dropped stage, and are dropping 4-6 boosters at the same time, it's quite cool to watch them all just drift away while maintaining their facing.

I'll try the "single fin on the outside" trick though, as that seems the most elegant.

[KamenRiderzero1]

There is a very nice super-sepatron, I think it's in an ISS set (I'm a touch too tried to go digging), designed for the Ariane 5. stacks nicely on other things, it's a 2.5 meter part and fires from both ends.

 

 

[Tiberius K]

Foxster and Stoney3K every time.

I've never used a sepatron for anything other than scaring kerbals on the runway.  If you just eye it so the decoupler is a little above what looks like the centre of drag of the shed boosters ... and use a single strut at the top to stop wobbling prior to decoupling you should be right if you are steady in your prograde marker.

If your decoupled boosters are enormous you might want to use a hardpoint or the extended decoupler

[kiwi1960]

I just put up with all the purdy explosions... Kiwi loves the big bangs and the fireworks...

Not sure if it would help, but once you have the decoupler in place, right click on it and increase the ejection force, on some, you can have 600 units... normal is about 100 from memory, but 600 should be enough to push it well away from your rocket... or.... it could make some purdy fireworks... if it does, please, post pics so I can enjoy the fireworks...

 

Edited June 30, 2016 by kiwi1960

[Warzouz]

On 28/6/2016 at 4:52 PM, Stoney3K said:

To be more exact: Put the decoupler slightly above the empty CoM of your booster.

True, this is the way to do it. You rarely need separatrons if you do that. I already launched a 80T space stations and still not used any seperatons even I usually use radial decouplers.

I would add to put the side engines below the central core engine. So when the booster will open like a flower, their bottom, even if the come closer the central core, won't hit it because the core would already have moved forward.

[Norcalplanner]

I'll add one point which hasn't been brought up yet - horizontal location of the boosters on the rocket. If you're only using two radial booster stacks, place them on the N and S sides of the rocket to minimize any aerodynamic effects from your rocket being slightly above or below prograde on ascent. If you have three radial stacks, place the initial stack (the one you're controlling with the mouse) on the E side of the rocket. If you have 4 booster stacks, place them on the NW, NE, SW and SE sides of the rocket.

[Gojira1000]

There are a lot of solutions, all of which have been listed. I like Sepatrons because I enjoy pyrotechnics and (in RSS at least) I'm sometimes shoving really huge boosters around large, barely-controlled rockets that are shedding stages in the atmosphere.

In stock I don't tend to need them much as I'll usually build single-stack rockets thanks to KW Rocketry and SpaceY