Rocketology: The rocket always goes the same height

[rahulanand]

While doing the Rocketology project, my son and I found that the rocket always went roughly the same height. The little variations that we say could be attributed to the reaction not finishing, I think.
To step back, we used a Fuji canister (hard to find film canisters these days), and barely at most a 1/2 teaspoon of baking soda. So we started with that, and 6tsp of vinegar. From there we went down to 1 tsp of vinegar. No change.
Next we tried with 1/4 tsp baking soda and 5tsp - 1 tsp of vinegar - same height as previous set of experiments.
Next we mixed equal parts talcum powder and baking soda and used a 1/4 tsp of the mixture (in effect, 1/8tsp of baking soda) and 5tsp - 1tsp vinegar - same results.
Is this expected? I think it should be, because the canister's lid will always pop off at a (roughly) constant pressure. So it should always go the same height. The only observed difference might be how long it takes to build that pressure, i.e. how long it takes to launch. Am I missing something?

[theborg]

rahulanand,

Thanks for the question and welcome to the forum. Thrust is a function of the amount of mass being ejected out the back of the rocket (i.e. the total mass of the fuel, wet and dry components) and how fast it is being ejected (largely driven by the pressure). I would say your assessment is correct that the pressure is roughly the same because the film canister top pops about the same pressure. If there isn't an appreciable difference in the mass of propellant the thrust will be the same...or at least within the error margin of your ability to measure height. I should say the mass should be the measure of "reacted" material. I'll take a look at the project and see if I can come up with any other explanation.