In my Soda-bottle rockets post, I showed a picture of the very simple soda-bottle rocket launcher I’ve used for the past 10 years (with pointers to the plans for making it).
One of the teacher blogs I occasionally read has just posted a video of launches with that design: Omega Unlimited: Cub Scouts and Rockets Do Mix.
I’ve still not gotten around to doing a soda-bottle rocket experiment in the physics class. Partly this is because we haven’t covered pressure yet—but force/area is a simple enough concept. I think that the problem should be optimizing the amount of water to put into a soda-bottle for launching at a fixed pressure to get maximum height. We’ll have to make a number of ridiculous simplifications (like no friction for the rocket or the water) and reasonable ones (like that water is incompressible), in order to make the problem tractable. Even so, the problem may have to be computational, as the pressure drops as the water leaves the bottle, so that both the thrust and mass of the rocket change with time.
Perhaps we should discuss this in class tomorrow and plan to test it experimentally next week. The launcher design is not ideal for experimentation, as the launch pressure is determined by the friction fit, which is not as repeatable as using a gauge and a trigger or using a burst disk. It might be worth buying a $26 triggered launcher for the experiment, especially as that would allow going up to 60 psi (414kPa), while the current design releases somewhere around 200kPa.
Perhaps, if we have time we could play around with old launcher tomorrow, and work on the theory and programs for 3 weeks while waiting for the new launcher. I would normally say 1 week, but I’m not sure the new launcher would get here on time. Two weeks is out, because my son and I will be on our way back from the Oregon Shakespeare Festival in Ashland, together with his dramatic literature class (I volunteered to chaperone—I’ve wanted to see the Oregon Shakespeare Festival for several years, but have never been willing to take the time and spend the money—volunteering makes the trip a bit cheaper and gives me a less self-indulgent reason to go).