Sinead,
I hope the balloon rockets work for you – I finally got them working for the first time last year, as part of the “Heavy Lifting Rockets” activity with children.
I found that the line needed to be taut. I used fishing line which was smooth, with a large diameter bit of straw on it. We found that the positioning of the balloon attachment to the line mattered, since the balloons would go in circles around the line if the straw wasn’t lined up with the opening / neck of the balloon. I also destroyed a lot of balloon pumps!
