Decoupler and separator

Decouplers and separators are parts that allow detachment of one vehicle section from another. A typical example is the Rockomax Brand Decoupler. Sometimes "decoupler" is used as a generic term for both types of staging devices, decouplers and separators, because both behave in a similar manner.

Mode of operation

Decouplers remain attached to one section of the vehicle when staged. This asymmetric detachment is the distinguishing feature of decouplers when compared to stack separators, which detach from both sections symmetrically. On many models, the attachment affinity of the decoupler can be determined by an arrow decal which points towards the explosive end of the decoupler. The momentum it imparts is defined by the name ejection force which is given in multiples of 10 Ns.

By default, neither decouplers nor separators provide automatic crossfeed, and thus isolate fuel consumption within the respective stages. But both crossfeed and and staging properties can be tweaked - and these can be usefull at emergency decoupling options or when additional fuel is necessary, for example upon unplanned disassembly.

Force Percent - bar for tweaking the ejection force
Decoupler: Disable/Enable Staging
Enable/Disable Crossfeed

Sequencing

All actions in the same stage as the decoupler or separator are executed simultaneously, including those on the detached section, even if that section is isolated from all command modules by the decoupling event itself.

Types

Decouplers exist in both inline and radial designs.

The first unlocked type in career mode is the stackable small TR-18A Stack Decoupler and is mandatory for further developing of any part. It is in the first researchable node Basic Rocketry, which means that at the beginning with the career no decouplers or separators are available. The sandbox mode has already all decouplers unlocked.

The symmetry modifier in the VAB

Radial decouplers, such as the TT-70 Radial Decoupler, attach to the sides of a rocket and eject stages horizontally. This can be useful for setting up asparagus staging, among other uses. As the ejection force may not be enough to avoid collision afterwards Sepratron I can be used to push detached stages away. Those decouplers also allow the usage of the symmetry modifier of the VAB combined with adding booster stages.

Separators, such as the TR-XL Stack Separator are a special type of vertical decouplers, which separate from both sides, leaving three craft: Both sides of the separator and the separator itself. Separators mass slightly more than the similarly sized decouplers. Each one separator and one decoupler cost more than their same sized counterparts. The tiny sized decoupler and separator cost the same.

The TT18-A Launch Stability Enhancer can also be classified as a decoupler, although one side is fixed to the ground. But the sound effects are similar and they share the same functionality, separating two parts. It was also be officially defined as a decoupler, but was changed into strut.

Application notes

Post firing concerns

Care should be taken to ensure that jettisoned engines on radial stages have finished burning before detachment, as there is the risk that they will collide with the rest of the rocket. The main reason for that is, that the decouplers are usually placed below the center of mass of decoupled stages which means that on separation they induce torque pointing the stage “inward”. If the engines are shutdown this is usually not a large problem because the rocket can clear it fast enough, but if they are running they will push the stage inward.

Also the decoupler normally stays on the spend stage the center of mass is closer to the rocket than the center of thrust. This will also induce torque pointing inwards but this torque is only induced by running engines. If they are stopped they won't induce torque this way. Almost empty booster stages are also relatively light causing a higher acceleration and overtaking the rocket. Both effects cause jettisoned stages which are still running to crash into the rocket in most cases.

Leaving stacked decouplers attached to an engine renders that engine ineffectual. Although the engine will ignite, it will provide no thrust because it's blocked by a part. Jettisoning a stacked decoupler shortly to soon is not a large problem as long as the rocket doesn't rotate.

Synthesized separators

Synthesized Stack Separator using 2 TR-18A Stack Decouplers. (Note the opposing orientation)

It is possible to create a separator out of 2 properly oriented decouplers (at the small expense of additional weight and cost over a bonafide separator). It may be helpful to include both decouplers in the same launch stage.

Explosive decoupling

As decouplers and separators have to be developed first, it is not possible to have multiple stages at the first launches of Career Mode. However there is an art of staging by placing RT-10 Solid Fuel Boosters next to each other, either connected at the side or arranged vertically so the exhaust of the upper booster fires onto the lower (they don't have to be directly joined). The idea is to build up heat through contact and rocket exhaust to cause overheating resulting in an explosion, decoupling the segments below it approximately when desired, like normal staging.

KSP YouTuber Scott Manley exploited explosive decoupling for a technique he calls "rocket surgery" describing it as more an art form than a science. Using this he was able to go all the way to Minmus and return with only Start technology in his video "Kerbal Space Program - 0.22! Lets Do Science! Lots Of Science".

Although explosive decoupling is possible with other engines, the first decoupler is already in the first research node. The decouplers are the more flexible and reliable staging method and become available before other engines capable of explosive decoupling. So it is particularly useful in the start of the career mode in version 0.22 when there is no decoupler but the RT-10.

As a means of propulsion

Because of the physics it is possible to use decouplers and separators as a means of propulsion although usually not for productive usage. Those are called “separator cannons” or “staging cannon”. Since decouplers and separators all have a “decoupler force”, this force can be applied to launch something. The more the explosive force and the less payload mass in this “decoupler rocket”, the more altitude or speed will be gained from the separation. The TR-XL Stack Separator is the best candidate because it has the highest ejection force.^[1]

Decouplers

Part	Radial size	Cost ()	Mass (t)	Max. Temp. (K)	Tolerance (m/s)	Tolerance (g)	Ejection impulse (kgF∙s)^{[Note 1]}
TT-38K Radial Decoupler	Radial mounted	600	0.025	2 000	8	50	250
TT-70 Radial Decoupler	Radial mounted	700	0.05	2 000	8	50	260
Hydraulic Detachment Manifold	Radial mounted	770	0.4	2 000	8	50	450
TD-06 Decoupler	Tiny	150	0.01	2 000	6	150	50
TD-12 Decoupler	Small	200	0.04	2 000	6	150	100
TD-25 Decoupler	Large	300	0.16	2 000	6	150	150
TD-37 Decoupler	Extra large	375	0.36	2 000	6	150	200
TS-06 Stack Separator	Tiny	215	0.01	2 000	6	150	50
TS-12 Stack Separator	Small	275	0.05	2 000	6	150	100
TS-25 Stack Separator	Large	400	0.21	2 000	6	150	150
TS-37 Stack Separator	Extra large	500	0.48	2 000	6	150	200