Bottle Rockets~ Failure

Our Rocket did best on it's first launch. How sad. We spent the entire class trying to build the perfect parachute buy folding and cutting a ton of trash bags. Everything else about our rocket was fine, as Mr Blake said, we just had to figure out how to get the nose cone off so the parachute could deploy.
We built our rocket out of two 2L bottles, filling them up at a constant amount of one L. We marked the fill line and the PSI that seemed to work, 60 PSI. Everything worked as planned, except for one of the fins and one of the tabs for the nose cone. To start off, we tried launching the rocket as is from yesterday. We started out with a slightly higher one than the day before, 6.51 seconds! They fell off after out second launch today because the rocket landed on them. We came back into the classroom to hot glue gun them back on. Yesterday, we made 6.41 seconds! I thought for sure our new design would extend the amount of time by at least a couple seconds! We put mini fins at the top of our rocket to prop up our nose cone so it can fall off. We also chose to use (out of the many parachutes we attempted to build) a small round circular parachute with many strings attached to the rocket for stability. This was a bad idea. The strings kept getting tangled up and our parachute didn't deploy.
Getting our rocket to stay in the air for more then 10 seconds is actually really really REALLY difficult. We had to account for air resistance, but also include fins to stabilize the rocket. Gravity was tricky because our rocket was so heavy, it would hit the ground so hard after accelerating so fast. This was frustrating because we would have to repair our rocket every time after launch. Plus, getting a parachute to launch is basically the hardest thing ever in life. Never. Again.
Final thoughts & other things learned: I will never ever do this again. There is so much work involved and it's pretty difficult to launch a rocket when we didn't really have a lot of time to build and or test it.