Stomp Rocket

The goal of this lab is to connect the motion of a simple object with physics and chemistry you have learned previously. The object is a “stomp rocket,” a lightweight plastic tube which can be launched with surprising effectiveness by compressed air, which is generated by stepping down hard on a flexible plastic box connected to a tube the rocket fits over.

The basic task: measure the height to which the rocket rises, and use this number (and your background knowledge) to estimate energies, pressures, and other characteristics of the launch process.

The basic model: The stomp rocket is basically a cylinder full of an ideal gas (air) with a piston at each end. Rapid compression at one piston can create enough gas pressure to launch the other piston high in the air.

(a) Assemble your gear:

stomp rocket / launcher / survey tape / meter stick / protractor / anything else you think may be useful

Find a place where the top of the rocket’s trajectory can easily be observed with an altitude angle of 45 degrees or less, and where the rocket can easily be recovered after its flights. Use some practice flights to set up a trigonometric method for measuring the height of the trajectory.

(b) Make several measurement runs, i.e. launches where you take data. (If you look ahead, you’ll see that vertical launches will be easiest to interpret.)

(c) Measure the mass of the rocket (top-loading balance provided) and estimate the potential energy change of the rocket between launch level and the top of its trajectory.  Calculate how much work needed to be done on the rocket to launch it on the trajectory you observed.

(d) Now try varying the angle at which you launch the rocket.  Predict quantitatively how this will affect the potential energy of the rocket and hence the height the rocket will travel, then perform several runs to see if you measurements compare with your predictions.  Be careful to try to get a reproducible stomp so you can compare these measurements with your vertical launch measurements.

(e) Inspect the launch apparatus (if you haven’t already) to develop a clear picture of how it works. Use the dimensions of the apparatus (and any other information you need) to estimate the number of moles of gas expelled during the launch process.

(f) Use the dimensions of the rocket to estimate the average pressure exerted by the gas while the work of launching was being done.

(g) Do you think the gas process during launch is more likely to be isothermal or adiabatic? Explain. Notice there are two phases of the process, initial compression and subsequent expansion. Estimate the temperature change in the gas during each phase.

(h) How many whole or fractional cashews do you estimate is the minimum it takes to provide the energy a person needs to do the work that launches the rocket?

(i) Think about the sensitivity of your trigonometric method to determine the height the rocket traveled.  To what extent does deviation in measuring the angle to the highest point traveled translate to deviation in calculating the height traveled?  Can you think of a way to express this mathematically?

(j) How would you go about designing a better stomp rocket?  To address this, start by asking what the present model tries to be good at, then think about what direction of improvement you’d like to go:  Would you want it to go higher, carry a greater load, require less pressure, etc.  How, at least theoretically, could this be accomplished?