r/KerbalSpaceProgram • u/CoolPeter9 Barely exits atmosphere • 13h ago
KSP 1 Question/Problem does decoupler height matter at all?
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u/AmiSimonMC 13h ago
It will. Too high and the bottom of the srb will risk hitting your engine, too low and they might hit your fuel tanks. I usually aim middle and a bit high, because if the top is further away after decoupling, aerodynamic forces will push the srbs away
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u/22TheFenix22 13h ago
Yes, *if* you use the decoupler with any force. To avoid using separatrons, i usually place the decoupler at around 1/3rd of the distance of the SRB, so it has a leverage point
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u/Equivalent-Session68 12h ago
I generally put it in the middle but a bit toward the top and also have the booster lower than the main engine so that there isn't a risk of it hitting the engine.
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u/human84629 13h ago
We’re all assuming some struts are involved in this design 😅
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u/TheAshenHat 11h ago
If no struts then i would say about 3/5 of the way up the booster. Keeps it stable and i have only had a few go inward during decoupling(usually when i’m doing something funky with the gravity turn lol).
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u/extremely-cynical 13h ago
I think you generally want it near the middle, keeps it from wobbling too much during launch.
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u/Millerboycls09 13h ago
Using decouplers as struts
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u/TheNerdyCroc Believes That Dres Exists 12h ago
A U T O S T R U T
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u/ThellraAK 4h ago
I been playing again pretty heavy for the last few weeks and until this post I forgot about the wobbles.
Autostrut and I just put my asparagus stages all the way at the bottom of the stage above it and it just works...
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u/InterKosmos61 Dres is both real and fake until viewed by an outside observer 13h ago
Yes, in the first example the SRB will turn outward and fall away from the core, while in the second one it will turn inward and collide with the core.
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u/Apprehensive_Room_71 Believes That Dres Exists 11h ago
I place the radial decouplers at center of mass of the SRBs and then place the SRBs as far down as possible, preferably with their nozzles at the same line as the main booster nozzle(s). This generally works just fine. I rarely have to engage in any other shenanigans.
If needed for more throw, Sepatrons are your friend.
Though there are some truly nifty radial decouplers with built-in solid rockets for very positive separation in one of the Space Y mods. They are glorious and come in a couple flavors, extra spicy and ghost pepper. Fantastic parts to have.
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u/Goobmeister21 6h ago
Screw separatrons, just decouple your side boosters mid barrel roll and they will fly off no issue
voila problem solved
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u/DrStalker 2h ago
If needed for more throw, Sepatrons are your friend.
Jeb says it's better to just spin around as fast as you can before hitting the stage button and letting the rotational momentum help fling the boosters away.
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u/Apprehensive_Room_71 Believes That Dres Exists 11h ago
To be clear, I use Ferram Aerospace, I don't use stock aerodynamics. It does make a difference in various ways. Especially with fairings and nose cones.
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u/WazWaz 9h ago
I'm a bit confused by what you're describing - we don't get to place the decoupler on the SRB before placing it on the main body.
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u/Sensitive-Offer-5921 9h ago
You gotta do some root part shenanigans
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u/Apprehensive_Room_71 Believes That Dres Exists 9h ago
No you really don't. I am not doing that. See my reply to WazWaz.
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u/DooficusIdjit 7h ago
Place the decoupler. Put on the booster. Take the assembly off. Hit your symmetry, put it back, then scooch it. That way, it’s exactly where you want it on the booster, and it’s offset where you need it on the main part.
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u/primalbluewolf 7h ago
we don't get to place the decoupler on the SRB before placing it on the main body.
Pretty easy with subassemblies.
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u/Apprehensive_Room_71 Believes That Dres Exists 9h ago
I place the decouplers on the rocket about where they should be, then place the boosters about where they should be, then use the movement tools to get things exactly where I want them.
I am not literally building it as booster, decoupler then rocket. I am telling you where I place things, not order of operations of assembly.
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u/agate_ 9h ago
This is the right way to do it, with or without mods. I like to put decouplers near the geometric center of the boosters, so they're just above the center of mass of the boosters when empty (the engines are heavier). This minimizes booster wobble, and gives a good "Korolev cross" separation with no risk of hitting the main booster.
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u/RealHoldenBloodfeast Jebediah 13h ago
Yes, it affects where the connection point is and where the separation force applies. Always keep it towards the middle or the top
A decoupler in the middle of the booster means it will hold more stable and less wiggly without struts/autostrut. If you put it high, the separation will make the booster will fall away from the core stage like a peeling banana. If you put it low, the top of your booster will be pushed into your core stage
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u/Weary-Support-7383 Enjoying tea with the Kraken 13h ago
It can give your booster some angular velocity when it decouples, meaning that your booster will sort of spin away. I recommend putting it slightly above the middle so that your booster flies away from your main craft when you detach it, and also having a decoupler really far from the center of the booster can make it wobble while it's attached.
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u/Freak_Engineer 11h ago
Yes, of course! It blows the part away with an impulse, put it too far down and the boosters will deflect into your rocket instead of away from it.
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u/danj729 6h ago
The decoupler causes a small explosion when it gets triggered. If you put the decoupler at the top, it means the explosion will push the nose end of your booster away from the core, at which point air resistance will hit the angled booster and carry it further away from the core of your rocket.
If you put the decoupler on the bottom, then the small explosion will cause the engine end of the booster to push away while the nose remains close to the rocket. With the booster angled this way, air resistance is more likely to push the boosters closer to the core.
So I try to put the decouplers high and then use support struts to hold the bottom end of the boosters to my core so I don't get a wobble.
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u/ExpensiveFig6079 6h ago
I find the decoupler being JUST above CoM has tendency to get the discarded booster to rotate its nose away from the main.
THEN air drag and "lift" cause it to fly away from the main. It's subtle but trying to build biggish before seperatrons is tricky like this.
I get a similar effect with 2 45deg tails fins on that booster they also through drag steer the discarded booster away.
Note I use FAR always (ymmv)
Also once I start adding physical visual struts to kill wobble they I think reduce that effect, and I believe but are not certain the angle o the strut changes stuff... (experiments would involve getting to space with test right and checking depature speeds and rotations, AFTER separation while not under thrust.
(hmmm I might do just that)
Auto struts are TBMK different again
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u/FirstRyder 12h ago
Decouplers have a force - the two parts are pushed away from the point of the coupler.
In the first example the top of the booster will move out, but because it isn't at the center of mass the bottom will also move in. If you are accelerating this is fine because it won't be fast and your ship will be out of the way before it hits.
In the second example the bottom of the booster will move out and the top will move in, which has a high chance of hitting your rocket.
In the middle (at the dry center of mass) the whole thing will move away while remaining parallel. But definitely better to err on the side of the first example.
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u/FarmerArtistic4499 12h ago
I always attach at the top of the booster when I can. It rotates the top away from the ship, and either air hits the angle of the booster and pushes it away from the ship or it spins away from me in space and I can just accelerate for half a second and be fine
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u/l0st-c0nnecti0n 13h ago
more so in atmosphere. less in vacuum. regardless, putting them towards the top of the booster should be normal practice
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u/davvblack 13h ago
it does sort of matter, but in any situation that it does you can add struts on the other end and and it fixes it. i generally keep separators in the middle of the boosters just cause that’s where they snap to without the move tool.
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u/gozulio 13h ago
yes. A decoupler imparts a force on the part it's decoupling when activated. You can take advantage of this effect to have boosters "peel away" from your craft in such a way that the discarded part doesn't strike and destroy other parts of your craft. An part allowed to move and flex (not fixed in place by struts) will do so about the point at which it's attached to the rest of the craft.
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u/HAL9001-96 13h ago
yes even if you strut the decoupler is the poitn where hte ejection force happens when decoupling so if you don't use sepatrons and you decoupel at the bottom wiht sturts the booster wil lstart rotating towards you while if you decouple at the top it wil lrotate away from you as the nose gets pushed off
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u/recoil-1000 13h ago
I put them in the middle and add a tiny set of fins on each boosters nose at a slight outwards angle, that way they naturally curl outwards and peel away
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u/VolleyballNerd Reaching for the stars 13h ago
Yes. It has a decoupling strenght, so it pushes the place its attached away from the main rocket. Drag and aero forces make it go even further. When you attach it on the upper edge it's that edge that will be pushed away, causing the booster to be rotated away from the main rocket.
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u/Microdoser_Ltd Lithobraking since 2013 13h ago
Not if you strut it properly and use the devices built for when you separate, separatrons.
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u/Lambaline Super Kerbalnaut 12h ago
decoupler up top for a smooth ride
decoupler at the bottom for big booms :)
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u/Previous-Big-6281 Kerbal Aircraft Engineer 12h ago
If you have struts at the bottom, then no, it doesn't matter unless you are using the Ferram Aerospace Research Mod, structurally speaking. If it's at the top, it'll be more likely to fall away; if it's at the bottom, it'll probably fall into the rocket.
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u/TF2fanatic102 12h ago
Yes. Higher up, top of the booster will fall away first in a sort of "korolev cross" style, but too high up and the booster might slam into the main engine. If you put it at the bottom, the top of the booster will likely slam into the fuselage and break stuff. Decoupler in the middle is typically fine, but I usually put the decoupler closer to the top just to be safe.
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u/JovialKatherine Always on Kerbin 12h ago
Decouplers apply an impulse/force at the point they are attached. having it at either extreme will induce a rotation/pitch change instead of pushing the booster away.
Your best option would be having the decoupler at or just above the center of mass of the empty booster. Most of the force will be put into pushing the whole booster away, and the slight pitch change of the nose will pull the booster away in an atmosphere.
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u/hstarnaud 11h ago
Yes, the decoupled object has its own center of gravity and gets pushed where the de-coupler is. It doesn matter for your ship but it affects if the decoupled object will spin or how it will be pushed. Generally speaking you want the de-coupler aligned with the center of mass of the object that gets pushed away by the de-coupler so it gets pushed straight, without spin.
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u/david4069 11h ago edited 8h ago
If you are able to use mods, SpaceY Heavy Lifters has rocket-assisted radial decouplers that work well when placed just above the (empty) radial booster's center of mass, so that the top of the booster is pushed away a bit more than the bottom. The ones in the mod are large, but if you have a part scaling mod like Tweakscale, you can scale them down as needed to fit smaller rockets. You can remove some of the solid fuel and reduce the thrust as needed if it is too powerful for your radial booster.
It's available on CKAN also. The mod and it's companion, SpaceY Expanded add rocket parts from 5m to 10m.
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u/Electro_Llama 11h ago
Yes, the offset tool is your friend, and hold LeftShift to bypass the limits for offsetting. I like to make the bottoms slightly lower than the engine so it rests on the boosters on the launch pad.
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u/acestins 10h ago
Yes, to a degree.
Radial decouplers have an ejection force that they impart onto the booster when activated. This force generates a torque on the booster the further away it is from the boosters center of mass (it makes it spin). Too much torque and instead of pushing the booster away, it rotates and the opposite end crashes into your rocket.
The best method is to place the decoupler slightly above the boosters center of mass, just a bit. Most of the ejection force will be imparted onto the booster, but a slight torque will still be generated, pushing the top of the booster away a little, allowing the boosters to fall away while the main body flies away.
What I've started doing is disable the force, and add separator rockets (the tiny solid rocket motors), and adjust their power and placement so the center of thrust pushes the booster straight sideways.
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u/bigorangemachine KVV Dev 9h ago
Yes it absolutely matters.
Maybe more with my builds because they are complicated layer cakes at this point.
You want to attach a radial decoupler at the CoM of an empty booster.
The detached booster can / or \ striking the main booster.
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u/ppoojohn 9h ago
Yes bottom placed decouplers will cause the side booster to spin towards your rocket and the air will push it into it
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u/dahbakons_ghost 9h ago
it does matter but only for style points and stability.
decoupling at the top is more likely to give you a pretty decouple with the top falling away nicely. it doesn't REALLY matter though. unless your worried about collision on your main stage.
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u/Supplice401 8h ago
I put it at the bottom, since most of my rockets are pretty booster heavy, I put sepetrons at the top with 0.8 units of solid fuel.
The bottom radial decouplers ejects with just enough force to push the boosters away from the main engine, and the sepetrons firing will provide enough kick to achieve the Korolev Cross.
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u/GallonsOfPoo 8h ago
If you put them above the center of mass they tip out away from the rocket and are less likely to hit it
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u/RandomEnmusimp Cursed “spacecraft” designer 8h ago
Short answer, it depends on the mass to be decoupled, the mass of your craft, and decoupler strength.
Due to Newton’s third law, decoupling something at an uneven point will cause it to rotate instead of shoot straight off into the distance.
Depending on the mass to be decoupled, you could hit your own craft with the decoupled mass. Depending on the strength of the decoupler, that can increase the speed of the decoupled object, which could be the difference between it destroying part of your craft when decoupled and just knocking you a little off course.
The inverse is also true, it could also slam your craft into the decoupled mass if your craft is lighter.
All in all, try to line up your decoupler to the rough center of mass of whatever you’re decoupling. If that’s not possible due to something like a fairing or other booster in the way, use separatrons to force the decoupled item to go away before it has a chance to rotate into your craft or vice versa
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u/DooficusIdjit 7h ago
Yes.
Where it connects to the booster matters a great deal on takeoff, and on decoupling, and they’re often at odds with each other. Personally, I try to get it near the center of mass of the empty booster, then strut the tops to knock out the wobble factor.
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u/Greenfire32 6h ago
If you keep the attachment point on the left SRB the same, move the decoupler to the bottom of the rocket. That way when you eject the booster, it'll want to pivot away instead of into.
You can achieve the same effect on the right SRB by keeping everything where it is and putting sepatrons on the top of the booster pointing away from the rocket.
But the most stable configuration is where the SRB is attached in the center of mass on the rocket's center of mass and then you use sepatrons to eject it away from the rocket.
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u/ColaCat2200 6h ago
Yes. They have an ejection force, so it is usually optimal to put them at the top so the SRB separate well. Doesn't matter as much when working at the larger sizes with Seperatrons/Mites. However placing at the top does mean you need struts, so on very small rockets it can be worth placing it closer to the centre of mass, saving mass/drag from struts.
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u/ARS_Sisters 2h ago
When decoupler being triggered, they cause a pushing force that push away anything they're attached on
-If you put it on top of the SRB, the pushing force would force the bottom portion of the SRB to get pushed inwards into the main stage
-If you put it on the bottom of SRB, the pushing force would force the top portion of the SRB to get pushed inwards into the main stage
The best position should be on the SRB's center of mass, so the pushing force would allow the SRB to be jettisoned at perpendicular vector relative to main stage's body. Add a separatrons if you need more pushing force (generally, LOX fueled stage is heavier to push away from the main stage compared to solid fueled stage), add centrifugal spin as well if you really really want to eject them away
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u/zekromNLR 2h ago
If you are going to decouple with some force, you want the decoupler placed slightly above the empty center of mass of the booster. That way, it mostly just gets shoved sideways, but is also given a slight rotation outwards. This serves two purposes, it prevents any aerodynamic effects from rotating the booster's nose inwards to strike the core stage, and in fact it makes body lift aid in pulling the booster away.
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u/hymen_destroyer 13h ago
I tend to put them closer to the top. They push the booster out from where they are attached which can sometimes cause the top of the booster to impact the stack as it flips inwards
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u/BuzLightbeerOfBarCmd 8h ago
How do you control where on the booster the decoupler attaches? IME when you try to place a booster on a decoupler, it attaches automatically to roughly the middle of the booster and you can only drag it slightly up or down before it stops registering that the booster is supposed to be attached to the decoupler at all.
I just use sepatrons for my boosters. They barely add any weight or drag and solve the problem entirely.
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u/hymen_destroyer 8h ago
You can use the positioning tool to drag it up or down. I generally put it about 1/3 of the way down the booster and it has never caused any issues
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u/PangolinMandolin 13h ago
Decoupler at the top of the booster and the booster falls away.
Decoupler at the bottom of the booster and the booster falls inwards (meaning it hits your main stack and things go boom)