This might seem obvious but you will need a pump that can handle the pressure. It's one thing to pump up a bicycle tire once a month and another to pressurize a two liter bottle to 70+ pounds over and over. Once you start getting serious about rockets you'll want a good quality pump.
Ideally the pump will have a enough of a safety margin of pressure that it can handle far above the maximum you'll use on your rockets. Metal bike pumps are preferable over plastic pumps. Check all of the high pressure parts, make sure they didn't skimp on them.
Our heavy duty plastic pump gave out after a few months of rocketry. Right in the middle of rocket pressurization a plastic plug at the base of the pump snapped and shot off like a bullet, ricocheting off a nearby shed and across the lawn.
Your pump should also have an easy to read pressure gauge. Even if it's off by a few pounds it's better to know the approximate pressure and ensure that you don't go too high and blow things up. You'll also want to extend the length of the air hose like you did with the launch string. There are three good reasons for this:
There are other options. A friend tried a cigarette lighter powered car pump and said it worked but was really slow and noisy. I've heard of folks using pressurized air tanks and a valve to "charge" the rocket. I tried the big air compressor in our garage but the valve setup wasn't very good. Also, launching close to the house or cars isn't a good idea.
I've been toying around with another idea but haven't yet taken it from drawing board to workshop. The idea is to build upon the design used by the air powered pumpkin and potato launchers. Instead of pressurizing the bottle slowly you release a large compartment of pressurized air attached to it. You wouldn't need gaskets or tie wraps to hold the bottle in place.
I think the challenge is to make the air path short enough for the air to quickly go from reservoir into bottle. Maybe the base of the launcher is a PVC reservoir (3" or larger pipe) with a an electronic sprinkler valve to facilitate the launch. As I said, something to noodle over.
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