SL_ST PNEUMO ROCKET 2

To create maximum momentum, I used a piston as a valve. 

As the piston moves, the air is compressed, the air has nowhere to go until the piston reaches the bottom. At this moment, a thin extended window opens, through which the air is quickly discharged into the cavity under the rocket projectile. 

I hope you can dig the projectile out of the ceiling.
It seems to me he may get stuck there =)