Sarah,
What I like about both of these types of rockets is how they can promote inquiry learning. Children can watch them launch as the prompt, wonder and explore how they work and then investigate a factor of their choosing that arises from their own questions.
Both types of rockets are reliable, and have enough variables that can be changed to keep a 2nd class cohort going for the week!
If you want to include some Irish in this, use the Curious Minds Irish language version of the balloon rocket activity which is also in the downloads for this module.