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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Yr12 Projectile motion study - Mass distribution

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funnyfacefour
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Yr12 Projectile motion study - Mass distribution

Post by funnyfacefour » Thu Nov 22, 2018 4:14 am

Hey guys, i'm a year 12 student studying projectile motion at the moment and am wondering about the effects of mass distribution on the range of a rocket.

My rocket design was using a plain PET bottle, attaching plastic fins to the sides, evenly spaced, and a plastic nose-cone with no extra weight at the nose.
The way i tested weight distribution was using a cut up sock to make a wide ring that tightly fit around the bottle, duct taping multiple 10g weights to the sock and keeping the weights in place on the rocket with extra duct tape.
Then i did 5 repetitions of launching the rocket with the weight at the middle, 5 repetitions with the weight at the front-middle, and 5 repetitions with the weight at the nose.
For my method i set up a station with a 40*degree angle, filled the bottle with 250ml of water and used a low pressure air compressor to launch the model rocket.

My results found there was a decrease in range as the weight moved further away from the front (the nose).

My question is: What is the theory behind this idea, why does the range decrease as the mass is moved more towards the middle.
Really appreciate the response! I'll make sure to reference you and this forum in my reference list ^-^

Thanks guys, :D



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anachronist
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Re: Yr12 Projectile motion study - Mass distribution

Post by anachronist » Thu Nov 22, 2018 5:07 pm

I think there's more going on than just weight distribution. By changing the position of the weight, you are changing the center of mass relative to the center of pressure, which will affect the stability of the rocket. Moving the weight aft will decrease stability and if the rocket wallows a bit in the air as a result, the range will decrease.

In addition, your arrangement of the weight is external, therefore you are changing the shape of the rocket, and also its aerodynamics, when you move the weight. It may be that the body shape you get with the weight moved aft also increases the aerodynamic drag on the rocket, which will have a significant effect on range.



funnyfacefour
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Re: Yr12 Projectile motion study - Mass distribution

Post by funnyfacefour » Sat Nov 24, 2018 1:18 am

anachronist wrote:
Thu Nov 22, 2018 5:07 pm
In addition, your arrangement of the weight is external, therefore you are changing the shape of the rocket, and also its aerodynamics, when you move the weight. It may be that the body shape you get with the weight moved aft also increases the aerodynamic drag on the rocket, which will have a significant effect on range.
Oh that's true, I had used another bottle as a sleeve to cover the weight, but the sleeve changed shape as the weight moved up and down. I didn't know how much aerodynamics would affect the flight



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