The launching pad

FIRST VERSION

Looking at figure 1, it can be noticed that the plank (launching pad) presents a hole.

A piece of an inner tube with the valve is fixed on the plank so that the valve passes through the hole and is accessible from the lowest part of the launching pad itself.
We fixed two bars with two bolts on the launching pad creating a sort of tongs clenching the bottleneck and keeping the bottle pressed on the valve, which at this points acts as a cap.
The free ends of the bars are joined with a spring.
The bars, before unlocking the rocket, clench the bottleneck, which has its opening just in proximity of the inner tube valve. After releasing the rocket, the spring recalls both the bars making the bottle free.

SECOND VERSION

It's similar to the first one, except for the valve which is now inserted on the bottle bottom (head of the rocket) and covered by the nosecone.

We made use of easy to find materials in building of the rocket .

Some photography shows the two versions of the launching pad