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School science project help


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Thought some of you more sciencey types may be able to help here.

My 7 y.o. daughter has to build a "toy that moves" for a science project. It could be anything powered in any form. She elected to build a water rocket. The type where you pump a partially filled bottle full of air until the cork blows. Like this http://en.wikipedia.org/wiki/Water_rocket

She has to give a speech about the rocket, how she made it etc but she also has to describe what sort of energy is involved. That's fine for the kids who have wind/battery/mechanical toys, but I am not clear at all what the energy souce is for a water rocket from a technical point of view. Hydraulic? Help me out.

Also, through serendipity, we found that adding detergent to the water increases the height the rocket can obtain. Can anyone proffer an explanation?

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Affentitten,

The energy is from compressed air. Water's incompressible, and it is its expulsion via the compressed air above it in the rocket that provides the thrust. Adding detergent probably helps because it's a surfactant, making water more slippery, hence, able to flow faster through the exhaust nozzle. This increases the specific impulse, translating into more thrust, thus, greater height at apogee.

Regards,

John Kettler

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Adding detergent probably helps because it's a surfactant, making water more slippery, hence, able to flow faster through the exhaust nozzle. This increases the specific impulse, translating into more thrust, thus, greater height at apogee.

I hadn't thought of that. Could it also be that adding the detergent also slightly increases the specific gravity of the water, thus increasing the reaction mass slightly? I ask because I have no information on the subject.

Michael

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I hadn't thought of that. Could it also be that adding the detergent also slightly increases the specific gravity of the water, thus increasing the reaction mass slightly? I ask because I have no information on the subject.

Michael

I like John's explanation - 'tis truly elegant. How much does the sg change with the addition of the detergent? I suspect that the increase wouldn't translate to a noticeable change in maximum altitude.

Back to the original question of where the energy comes from - it's put into the mass of air in the bottle by compression in the pump; is the joule value of the stored energy the latent heat of the air mass? Boyle's Law PV=nRT, P has increased whilst T has remained almost constant. n has remained constant, R is a constant (8.14?), therefore my reasoning ability has run out and I can continue no further - help?

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Michael Emrys,

Considering that a surfactant/detergent goes into solution in water, your suggestion may indeed be part of the explanation.

costard (attn. Affentitten),

Your suggestion opens up another potential avenue of inquiry for Affentitten's daughter: the effects of water temperature on water rocket performance, as well as the related investigation into surfactant loading, since it's well established that hot solutions can carry higher concentrations than cold ones.

Regards,

John Kettler

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Good idea SeaMonkey, using a low viscosity oil (a vegetable oil would be environmentally preferable) should provide some interesting observations.

I cocked up, by the way, with the statement "n hasn't increased". It has, of course, increased with the increased mass of air pumped into the bottle. :o

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Good idea SeaMonkey, using a low viscosity oil (a vegetable oil would be environmentally preferable) should provide some interesting observations.

Including the interesting observations from Mrs Affentitten when little Affentitten comes back with her school uniform deluged in canola oil.

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It's hard to measure the height with the naked eye. From all the trial runs we'd probably be getting about 8 metres or so. The launch platform is actually the wooden holder from on old yard glass I got for my 21st birthday. It's a bit dodgy so launch angle can vary from the vertical about 15 degrees! Also the test rocket has just been a plain PET bottle and it tends to tumble. The improved vehicle has fins and a nose cone, so it should be a bit more stable. But we haven't test launched it because of the fear of wrecking it.

I noted on Wiki that the record altitude for a water rocket is over 600m! But having checked out the YouTube video it has about as much in common with with our rocket as a paper plane does with an SR-71.

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Affentitten,

Even when I was a kid, water rockets, to include dual stage versions, were readily available in toy stores. Seems to me a great deal of time and trouble could be saved by going this route and instead playing with some of the other factors mentioned. Also, this provides an opportunity to create a simple theodolite to measure rocket altitude.

Regards,

John Kettler

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Mate, she's 7 years old. The idea of the moving toy project was primarily the home made construction. She's already feeling like a winner because most of her chums have made cars out of Lego or simple sailing boats that have to be pushed. Instead she'll get to take the class out on the soccer pitch and blast everyone with with water, soap and orange food dye (her idea to simulate rocket blast). In doing this together we have watched a few YouTube NASA videos about rocket launches, shuttle take-offs and so on and she's developed a basic understanding of flight stability, realising that rockets have fins for the same reason arrows have feathers.

She has to give her speech and demonstration next week, but I think from a pedgagogical point of view it's already mission accomplished!

And as I expected, I've learnt stuff too by raising the issue here. Regarding the soap issue and surfactants: if the propellant is escaping the nozzle more quickly, will this not actually mean a reduced thrust time? Or is that compensated by the greater velocity?

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Even when I was a kid, water rockets, to include dual stage versions, were readily available in toy stores.

The two-stage variety was after my time, but I had a couple of the single-stage pump ups. I think I could get mine up to 10-12 meters.

I also had a water rocket that you didn't have to pump, so it was even better. The interior was divided into two chambers. One held citric acid/water and the other you filled with bicarbonate of soda. While it was still inverted, you attached the "launching pad", which was sealed to the bottom of the rocket. You then turned the whole rig right side up and the liquid and powder would mix creating gas in a couple of minutes. Pull a lanyard attached to the latch holding the rocket to the base and away it would go!

I kept increasing the propellent until I got it to go up to at least 15 meters. But then one day I overloaded it and we had an explosion on the pad. The side of rocket opened up in a vertical slit two or three inches long and that was all she wrote. They should have used stronger materials. Pity. We might have gotten some real altitude out of it.

Michael

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Affentitten,

The kinetic energy of the reaction mass is 1/2*m*v^2 - so any increase in the velocity of the escaping water/soap mix gives an exponential increase in the KE, whereas an increase in the mass gives a linear increase in the KE. This is why you noticed such a difference when you added the soap.

Given that the walls of the rocket are holding the air in (at increased pressure), would it be fair to say that the energy of the rocket is stored in the walls? I've forgotten my spring equations - something about the modulus of elasticity of PET, etc?

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Given that the walls of the rocket are holding the air in (at increased pressure), would it be fair to say that the energy of the rocket is stored in the walls? I've forgotten my spring equations - something about the modulus of elasticity of PET, etc?

Given that the air inside the bottle is more elastic (compressible) than the walls of the rocket, that strikes me as barking up the wrong tree as much as my thought about the detergent increasing the mass of the water by a significant amount.

Michael

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The wiki page on bottle rockets is pretty good on the 'why does this work' question. It even mentions soap, and suggests that salt (disolved in the water to raise the density of the water) would be an interesting experiment/comparison.

PV=nRT is the basic equation to do with where the energy is stored. The more you can increase the pressure (by squeezing more 'n's into the bottle) the more energy is stored, and the higher the bottle should fly. Once released from the launch platform the compressed gas expands, which expels the water, and the bottle moves away from the expelled water in a newtonian reaction.

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