Center of Gravity?

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
Strattocoaster
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Center of Gravity?

Post by Strattocoaster »

Is it possible to get the center of gravity to far forward?
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bugwubber
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Re: Center of Gravity?

Post by bugwubber »

Here's my opinion:
Fins and Rocket Body = Lever
Center Of Mass = Fulcrum point

Assuming: gliding body, in atmosphere

The longer a lever end is from its fulcrum point, the less work is required to move the end of it.

So, the further forward the rocket's COM is, the less force your rocket's fins and rocket body have to apply against the air passing over them to keep your rocket in alignment with the airflow. Additionally, since COM is a fulcrum point, the counter lever action of the part of the rocket body in front of COM is reduced. Additionally, you can reduce the size of the fins since they have to do less work, thus decreasing drag.

So, in theory a COM at the tip of the rocket, would be best.

In practice though,

The rocket body has mass. The longer you make the body, the more you pull the COM rearward. Adding more mass to the nosecone will pull the COM forward, but it will take increasingly more mass to move the COM forward a given distance. So, you will hit diminishing, and then negative returns on altitude acheived for a given launch pressure.

Take my daughter's rocket Go Fetch. It is a bottle with a tennis ball on top. If I replaced that tennis ball with a steel hemisphere, I would effectively move the COM of that rocket to within an inch of the top of the rocket. I can guarantee you, that rocket with the COM less that 1 inch from the tip of the nosecone, would not travel as high as the tennis ball with its more rearward COM.

I also wonder that as the last bit of reaction mass is expelled and the final pressurized airburst is released, if you might run into temporary stability issues. But nothing to base that on.

So my final answer is yes, you can move the COM too far forward for optimal results.
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Strattocoaster
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Re: Center of Gravity?

Post by Strattocoaster »

water rocket.pdf
(13.36 KiB) Downloaded 466 times
maybe this will work???
Strattocoaster
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Re: Center of Gravity?

Post by Strattocoaster »

as a jpg?
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water rocket.JPG
water rocket.JPG (54.09 KiB) Viewed 99 times
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ptx
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Re: Center of Gravity?

Post by ptx »

Strattocoaster wrote:as a jpg?
Hi

Nice concept.

From my understanding moving the CG (center of gravity) up is good, however moving it too much up may result in a over-stable rocket and be higly influenced by the wind/breeze. To keep it stable just maintain the CG over the CP (center of pressure) by approximately 1 rocket diameter.

On your concept initially your CG will be high due to the water mass on top, however as soon as all water is expelled from the rocket you will loose this possible advantage.
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bugwubber
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Re: Center of Gravity?

Post by bugwubber »

Strattocoaster wrote:as a jpg?
Is the idea to have tiny holes in the pipe so the air and water mix evenly as pressure is released?

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arjan
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Re: Center of Gravity?

Post by arjan »

Hi strattocoaster,

There was a guy named Alex (forum name saskalex) here on the forum who did about the same thing, storing the water in the top of the rocket.
Alex managed to fly a unofficial multi stage record with that rocket. WO:

http://www.wra2.org/forum/viewtopic.php ... cial#p8318

Are you planning to fly this rocket as a sustainer in a multi stage configuration or just as a single stage?
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Re: Center of Gravity?

Post by Strattocoaster »

Bugwubber, no holes or water air mix.

Sustainer is my original thoughts but wil probably try single stage first.
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U.S. Water Rockets1
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Re: Center of Gravity?

Post by U.S. Water Rockets1 »

Strattocoaster wrote:Bugwubber, no holes or water air mix.

Sustainer is my original thoughts but wil probably try single stage first.
Good idea. You should make sure it is stable and the deploy and other components all work before you do a multiple stage configuration. Only NASA can get away with "All Up" testing. 8)
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Re: Center of Gravity?

Post by arjan »

Did you think about a way to prevent the water sloshing out of the 2 litre bottles during launch?
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Re: Center of Gravity?

Post by Strattocoaster »

I drew it as not having a top. I figure It would have a top which would limit the sloshing. Plus, it wouldn't be full to allow air bubbles when filling without spilling.
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Re: Center of Gravity?

Post by bugwubber »

Strattocoaster wrote:I drew it as not having a top. I figure It would have a top which would limit the sloshing. Plus, it wouldn't be full to allow air bubbles when filling without spilling.
guess if you just drilled a large 1" hole in the thickest part of the bottom of a the 2 liter, you could keep the rigidity of an intact bottle and almost all the weight savings of a topless bottle, while benefiting from slosh containment and free air flow.

Fascinating idea.

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Re: Center of Gravity?

Post by U.S. Water Rockets1 »

bugwubber wrote:
Strattocoaster wrote:I drew it as not having a top. I figure It would have a top which would limit the sloshing. Plus, it wouldn't be full to allow air bubbles when filling without spilling.
guess if you just drilled a large 1" hole in the thickest part of the bottom of a the 2 liter, you could keep the rigidity of an intact bottle and almost all the weight savings of a topless bottle, while benefiting from slosh containment and free air flow.

Fascinating idea.

Bugwubber
Baffles inside the rocket can make some really cool sounds as the air rushes through them!
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Re: Center of Gravity?

Post by Alien Space Agency »

Neat ideas. Now an idea for CCSF3.
Nothing can stop a curious mind from inventing- Anonymous
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Re: Center of Gravity?

Post by Alien Space Agency »

Neat ideas. Now an idea for CCSF3.
Nothing can stop a curious mind from inventing- Anonymous