A pole saw (a long pole with a saw blade on the end) may be the way to go. Now at most parks, etc., the powers that be are going to take a pretty dim view if you start hacking away too much at the tree itself, so the better alternative is probably to slice away the shock cords and shroud lines, and just hope the rocket drops out.
Shock cords are easy to replace, it's the rocket itself you want to get back.
Needless to say the powers (parks dept. cops, etc.) that be are also likely to get rather steamed if they see you out there pole-sawing away at a tree in a public park without their prior knowledge and consent, so you definitely better call and check with the parks department first. If you tell them what you want to do and that you will take every effort not to damage the tree, they will usually be OK with it - in fact, depending if they're in a good mood or not, they might even help you out a bit.
If you really, really, really want to get the rocket back, here's a 21-foot pole saw... figure you're holding it at a height of about 5-7 feet, that means you could reach a rocket up to 28 feet in the air (which is pretty far up.)
But the pole saw costs $149, so unless you're losing a lot of rockets in the trees, I dunno...
https://store.yahoo.com/cspoutdoors/haposaw.html