Re: Launch at balloon burst
Yes it would weigh a lot and that would reduce the maximum altitude that the balloon could take it to. On the plus side though there would be no need for the air pressure trigger so a bit or weight is saved and there would be no need to make provision to launch the rocket before the balloon bursts. If the balloon bursts before that rocket is fired the whole mission will fail so a significant margin for error must be allowed. It could be that the current safe launch altitude is less that the maximum attainable by the same balloon with heavier payload. Also the vertical launch has to be the best way to get maximum vertical distance form the rocket engine doesn't it?
Maybe just using a pipe from the neck of the balloon connected to a pressure switch of some kind as the rocket motor trigger is a sensible compromise?