How are you able to predict the height without knowing the aircraft weight? I bought regular D rockets from Hobby Lobby and removed the ejection charge. I'm igniting it with a servo connected to a 3S battery. It's super basic but it should work fine.
By the way, I used only 1 cell of lipo (used the balance connector) and the gear servo to arm rocket.
Have a video of my Bell-X1 being launched statically.