the merge blocks are the Hard Point and the Command & Control Interface

There is a bug in SE that prevents them from working exactly like the vanilla merge blocks do however you can do the following.

1) Build your missile seperatly, and then fly it to the hardpoint and attach it.

2) use a scaffold block behind the Hard Point to attach the C&C to. then grind the scaffold out after the fact. This method also works with projectors. Use the scaffold block to "attach" the missile to the mother ship then grind it out after you weld up the projected missile. I have included a Sprue to use as a scaffold. it is made from 3 small pipes, so it will be very cheap and quick to make and then grind out again later.

3) with a projector. Build the missile with the hardpoint attached and save your blue print. then the hard point can be used as your Attach point for projection. you will need to grind out the Hard Point each time before you can re project a new missile.