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-   -   Rocketry (http://www.physicsforums.com/showthread.php?t=60754)

 Quantum_Prodegy Jan22-05 10:15 PM

Rocketry

Hey everyone,

I'm building a rocket with my friend from scratch. I have calculated the specific impulse of the rocket engine that we have designed and it turned out to be around 235 newton-seconds. I was wondering what the maximum weight of rocket this engine would get off the ground, and how to figure it out. If you need more information I probably have it I just don't know what information you will need. I'm a grade 11 student and haven't learned this kind of thing in school.

Thanks!

 Janitor Jan22-05 11:26 PM

Given your units of newton-seconds, did you mean that you calculated total impulse, not specific impulse?

 mrjeffy321 Jan23-05 01:13 AM

well, to lif the whole rocket off the ground (meaning to accelerate it upward, even for the shortest possible time), at least some time during the rockets burn, preferably near the start, the force of the engine needs to be greater than the total weight of the rocket, otherwise, it wont go anywhere, so matter how big of a total impulse you have.
so in other words, what is the highest, peak, force that rocket engine will give you, then divide by the acceration due to gravity, and that will give the the absolute maximum mass the rocket can lift (and it wont go very high if you plan on maxing out the mass, techincally, if you do "max out" the mass, it wont go anywhere, because there the net force on the rokcet is zero and it is already at rest, so it will stay at rest).

 Quantum_Prodegy Jan23-05 05:33 PM

ok, that makes a lot of sense. No, i don't plan on maxing out the mass of the rocket, i'm thinking now that overall it won't be more than 8 to 10 lbs. So...

acceleration due to gravity is 9.8m/s^2
is the force the mass of the rocket in kg?

 mrjeffy321 Jan23-05 06:00 PM

depends on how you are measuring your force, in Newtons (my favorite), or Pounds of thrust?

if in newtons, use g = 9.8 m/s^2, this will give you the weight of the rocket in kilograms

if in pounds, use g = 32 ft/s^2, this will give you the wieght of the rocket in pounds

F = m*a, where m is the mass of the rocket (either kg, or lb) and a is the acceleration of the rocket (either due to gravity or due to the rockets thrust).

1 kilogram = 2.2 pounds

 Quantum_Prodegy Jan23-05 11:19 PM

my physics teacher would kill me iff she found out i had forgotten the F=ma equation already.... :eek: xmas break wasn't THAT long.

hehehe, thanks for your help guys...don't know why i didn't remember that. :grumpy:

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