Two-Stage Piston Rocket

I've made a small two-stage rocket that uses two motors touching to ignite the upper-stage, and I've made a more complicated one that uses electronics to ignite the upper-stage. I decided I would try to make something even more complicated. I wanted a rocket that electronically ignites the upper stage, but I wanted the upper stage to have some more speed and power, so I came up with a solution: Putting a piston inside of the rocket. Here's how it works:

• Even though this rocket has electronics ignite the upper-stage, it is completely different in design from my other high-power two-stage rocket.

• The electronics bay, which holds the electronics and battery, is in the first stage. After a predetermined time after launch, the electronics ignite the upper stage motor.

• The piston is also in the first stage. The way it works is that it captures gas from the just ignited upper-stage motor, and uses that gas to propel the piston upwards. The upper-stage is temporarily attached to the piston, so the piston accelerates the upper-stage, until the piston reaches its maximum extension, at which point the upper-stage flies off. This very successfully gives the upper stage more speed.

Here is a video where I explain exactly how it works: