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calculating thrust/lift help please

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calculating thrust/lift help please

Unread postAuthor: JDP12 » Wed Oct 01, 2008 8:23 am

say i had some source of propulsion- whether it be compressed air, CO2, or a rocket engine, how can i calculate out how much weight it will be able to lift how high?

Thanks
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Unread postAuthor: mark.f » Wed Oct 01, 2008 8:39 am

Well, in flight, you could calculate theoretical thrust (with the absence of an atmosphere) using inertia of the exhaust gases, which could again be guesstimated by stoichiometry and gas flow through a fixed orifice, but other than that basic attempt I'd say it's beyond both mine and your skill-level.
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Unread postAuthor: homedepotpro » Wed Oct 01, 2008 8:40 am

well, you could measure how fast it accelerated an object with a known mass. then you know how much the force its created, you calculate its lift against the force of gravity which is 9.8 m/s^2. Have you taken and higher level science classes?
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Unread postAuthor: psycix » Wed Oct 01, 2008 1:19 pm

You can experimentally record it by putting it upside down on a scale.
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Unread postAuthor: jonnyboy » Wed Oct 01, 2008 2:11 pm

@homedepotpro

Isn't he trying to figure out what its lift is? I like the scale idea. If you have seen mythbusters they use a light stand thing with an arm to hold the rocket out and some sort of force gauge underneath.

BTW how much does gravity change as you go up in altitude. If its anything significant that could really affect your answer.
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Unread postAuthor: Ragnarok » Wed Oct 01, 2008 2:25 pm

Yes, such a thing is possible, but it's really more a job for a computer simulation than some back of the envelope scratchings - although that could suffice.

I suggest getting on the line to D_Hall, as he's the aeronautical engineer.
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Unread postAuthor: jimmy101 » Wed Oct 01, 2008 2:31 pm

If compresseed air or CO2 is your energy source then you should add water as the "ejected mass". A Google search should turn up thrust calculators for water rockets, for example;

http://www.et.byu.edu/~wheeler/benchtop/sim.php
http://physics.gallaudet.edu/tools/scwatroc.html
http://findarticles.com/p/articles/mi_q ... _n17172742
http://www.sciencebits.com/taxonomy/term/6 (scroll down a bit)

For Estes rocket engines a little physics is all you should need. The rockets are rated by the total impulse and pulse length. I'm sure a Google search will turn up a ballistics calculator for Estes engines.

If you are homebrewing your engines with a solid propellant you need to measure the impulse and burn time of the engine. Again, Google will, I'm sure, find everything you need to know.
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