I work with a lot of Boy Scouts who want to complete the Space Exploration merit badge. It's a great program, and one of the coolest requirements is building, launching, and recovering a model rocket. Unfortunately, my Scouts are in the city and finding a location to launch rockets is ... well ... tricky.
In the past, we'd drive out to the countryside and use either a park or a friendly farm to launch the rockets off. It never used to be a big deal ... but someone got it into their head recently that these rockets pose a significant fire hazard (despite over a decade of fun without a single problem).
The biggest argument I've heard against the rockets is the nature of the engine. Basically, model rocket engines are black powder-based. A small electric igniter lights the engine and it produces a lot of heat and smoke to propel the rocket.
The parks and farms that have since banned us from launching rockets finally conceded that, if I could find a way to launch rockets without burning anything, they'd let me bring the Scouts back.
Every reference I can find to "cold rocket engines" seems to refer to older 1970s freon-based engines. I don't want to go that route, but I'm looking for some ideas for cold or low-temperature rocket engines. Even if it's something I'd have to build myself ...
Any ideas or suggestions?