Can’t save subassembly “THE SELECTED PART IS NOT ATTACHABLE!”

To satisfy a contract you must activate the stage with the component to be tested while all the check marks are green. The check marks simply indicate that the conditions are right to start the test. Once you activate the stage you should be awarded the contract and that is instant. Recovery of the vehicle is not required for the contract, although you do recover funds.

• Do not decouple the boosters while also igniting the next stage. Put the decouplers and the next engine(s) in separate stages. Decouple the boosters and don't ignite the next stage until they drifted away to a safe distance. Even when the boosters have a notable spin, the relative velocity between core and boosters will be quite low, so a collision will likely be harmless. When you use a design with fuel crossfeeding where stages are separated while others haven't burnt out yet, put thrust to 0% while decoupling, and back to 100% afterwards. This makes the launch a bit less fuel-effective, but a lot safer.

• Place the decouplers as close as possible to the center of mass of the boosters to minimize rotation. The offset mode in the vehicle editor is very helpful for this, because it makes it very easy to adjust the mounting height of the boosters and the decouplers. Keep in mind that with a liquid fuel booster, the center of mass is usually further down because empty fuel tanks are quite light compared to the engine. But with solid fuel boosters, the center of mass stays in the middle.

• Replace your standard radial decouplers with the TT-70 Decouplers (those which are mounted on a girder assembly). The extra distance between booster and core might prevent a collision. To increase the distance further, you can place girder segments between decouplers and boosters. But then you will definitely need additional struts between booster and core to prevent the boosters from bending inwards during launch.