Tutorial:Rocket Assembly
Most people use sub-assemblies for commonly used part-groups like boosters, but they can do so much more. The trick is to place an arbitrary and unneeded part as the first part when building a module intended to be used as a sub-assembly(it doesn't have to be a probe core, it can be any root part). Once the module is complete, pick up the piece you intend to attach to whatever ship you are building later, and drag the whole thing to the sub-assembly window.
If you create two sub-assemblies of similar weight (e.g. 5 ton) and place them opposite each other on the rocket, they will normally not balance each other exactly. However in general if the weight and size match each other, the torque caused by the difference can be cancelled out by the torque of your SAS.
- Note: If you use realistic aerodynamics such as FAR, you may want to use identical fairings around the parts since the shape will affect torque severely in atmosphere.
As you can see, this allows for greater diversity while building your main vessel without having to rebuild each item every time. Creating a variety of modules at different weights should allow you to build ships for just about any occasion at your whim. You can also use sub-assemblies to save entire stages to make the whole process even faster. Create a launch stage using a test payload of a given weight, and try to get into orbit. If it works, you can save that launch stage as a ##-ton launch stage. Eventually, you may be able to build all of your ships almost entirely through sub-assemblies.
Place the first arbitrary piece. Note it's weight if you plan on balancing modules. I used the stack separator to show me the weight of only the pieces after the arbitrary piece.
Build your module by building out from that first piece.
Detach the module by grabbing everything that you plan to mount on the vessel later. The green attachment point will be the point where it attaches to the rocket.
If you plan on being able to attach modules in a radial fashion, weight balance will be very important. You can attach other things to your module to help change it's weight. Using fuel as ballast is generally the best way to go, as you can usually use the fuel. If you do use some sort of ballast to get your module's weight exact, be sure to also include a decoupler so you can separate your module from the ballast.
Although the location of the center of mass will change the amount of torque applied, and the centre of mass. The effective torque this causes on the vessel may be offset by parts that can counteract torque.
Here I'm using a small RCS tank for ballast, as I didn't need much weight. This landing/return module weighs 8 metric tons.
Your attachment points on your rocket should have some means of disconnecting the payload modules unless they are self-disconnecting.
Here I have attached the 8-ton lander/return unit on one side, and an 8-ton rover on the other (I had to use quite a bit of ballast). Ideally I would have used a heavier module opposite the lander and required less ballast, then placed the rover on top.
checking center of mass for launch stage...
...and checking center of mass for orbital stage.
Secure those payloads with struts or they'll wobble... a lot.
