SS Boosters are temporary propultion to be built under Orbital Space Stations to deploy them above the other inner planets.
These are attached to the back of the space platform, so do not need defences as the station above will deal with asteroids while in flight, and the booster is designed to be removed after deployment.
The only difference between SS Booster 1 and SS Booster 2, is that 2 uses 'Advanced thruster fuel', 'Advanced thruster oxide', and 'Advanced oxide asteroid crushing'. If you set these to the basic recipe, it will still work, it just takes a lot longer for the tanks to be filled at Nauvis before it can be deployed.
Currently in use with space-age vanilla 2.0.28