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Rocketry, using gravity instead of fins to balance rockets?

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http://www.nakka-rocketry.net/fins.html

 

 

So as i read and understood this, i have a question. If i empty half or a third of a rocket, the CG will go down and i will use the gravity to balance my rocket, is that possible ? im about to show u this picture where i thought that because 1 side of the rocket is empty, the other side will be pulled downwards acting same as the fins. or i think it might be even better, but the main disadvantage is that we can only use half of the rocket so the fuel we have is less.

 

http://img823.imageshack.us/img823/4710/cpcg.png

 

I was reading exghaust pressure and stuff and saw nozzle with 30 degree inward and 12 degree outward is best ? or im reading wrong thing ? so is it worth for me to do that with my nozzle ? if its true, i will melt a piece of metal to shape my nozzle.

 

 

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Presumably you are considering small rockets that travel through the atmosphere because once you leave the atmosphere fins will do nothing. IMO you need something that will either automatically maintain stability which fins do well or, if trying to maintain stability in other ways, a system which can respond very rapidly. Such a system (again IMO) would be complicated requiring feedback to ensure the right amount of correction was applied. I can't see controlled burning of rocket fuel doing this. Perhaps the method that has worked for arrows for centuries is the best system?

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An example to think about. I've a small rocket I've made that stands about 4.5 feet tall with an airframe of about 1.5" diameter. It's made of a carbon fiber tube to withstand launch forces since the entirety of it is filled with a fast burning composite of ammonium perchlorate, so fast in fact that it burns all of its fuel in under 2 seconds. This rocket leaves the launch rod at mach 2.2 and coasts to an average altitude of 16,000 feet in seconds. Which force do you think has a fast enough reaction time to keep the rocket stable instantaneously, the force of the air resistance on the fins or the force of gravity on the body? For very slow rockets gravity might assist you in stabilization but not enough for fast moving rockets.

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I remember watching rockets launch on TV and noticing that the direction the rockets' nozzles were under servo control. Perhaps there are electronics you could adapt for this purpose?

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I remember watching rockets launch on TV and noticing that the direction the rockets' nozzles were under servo control. Perhaps there are electronics you could adapt for this purpose?

 

That's called 'gimbaling'

 

There is also the option of having fins in your rocket exhaust, sure you lose some of the the power but it allows controllable flight in vaccum and atmosphere (atmosphere only while rocket is firing) without RCS rockets.

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I think your idea is to lower the center of gravity to stabilize the rocket?

 

Edit: After reading Enthalpy's and doG's replies, I think this is all wrong:

I assume this would work, but only so much. It would be like crouching on a tight-rope, or like making a car low for stability.

 

The rocket is still unstable though, because the propulsion occurs below the center of gravity.

The point is, the nozzle is pushing toward the center of the fuselage, and if the rocket begins to tip off balance it will continue to tip further.

If you put the nozzle above the center of gravity (so that the nozzle is pulling the rocket up from the top instead of pushing it up from the bottom) then if it begins to tip off balance, it will tend to correct itself... it is stable.

I would say it is useless to use only the bottom half of a rocket for fuel like this, because if you have the top half empty, why not just make the rocket shorter??? This will lower the center of gravity even further.

 

 

Edit: So instead... the force of gravity couldn't be used to stabilize a rocket, because the rocket would essentially be in freefall with respect to the force of gravity? If the center of gravity is off-center of the rocket thrust's axis, whether in front of or behind the nozzle, it would provide torque to tilt the rocket. Then uh... would moving the CG away from the nozzle help reduce this??? Regardless, as doG says the CG to CP relationship is important, and I don't think you could make a rocket stable enough just by moving its CG without using fins.

Edited by md65536

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Mamma mia!

 

The centre of gravity has no relationship with the engine's thrust in stability, because the thrust is axial, not vertical.

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Mamma mia!

 

The centre of gravity has no relationship with the engine's thrust in stability, because the thrust is axial, not vertical.

Sorry but that's incorrect.

 

Stability

CP to CG Relationship

 

The CG (Center of Gravity) and CP (Center of Pressure) are very important fundamental design and flight parameters of any rocket, and have an important relationship to each other. The general relationship between the CG and CP is as follows: the center of pressure must be a minimum of 1 body diameter BEHIND the center of gravity on a rocket fully prepped for launch to ensure stability. Now we’ll explain why it must be this way, and what you can do to make sure your rocket meets this requirement, whether it’s a kit or a scratchbuilt design....

 

 

I will also mention that the relationship of CG to CP is on the Level 2 High Power Rocketry Certification test...

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An example to think about. I've a small rocket I've made that stands about 4.5 feet tall with an airframe of about 1.5" diameter. It's made of a carbon fiber tube to withstand launch forces since the entirety of it is filled with a fast burning composite of ammonium perchlorate, so fast in fact that it burns all of its fuel in under 2 seconds. This rocket leaves the launch rod at mach 2.2 and coasts to an average altitude of 16,000 feet in seconds. Which force do you think has a fast enough reaction time to keep the rocket stable instantaneously, the force of the air resistance on the fins or the force of gravity on the body? For very slow rockets gravity might assist you in stabilization but not enough for fast moving rockets.

 

 

 

 

Very very true, so fins are only way ?

 

 

 

 

my rocket design

 

http://img839.imageshack.us/img839/733/rocket1design.png

 

 

 

 

 

can you tell me how big my fins shall be please ? and if its okay i use aluminium foil or kitty litter to make a nozzle. the dirham is a currency i got that fits in the tube exactly so will do the blocking, my rocket will not have a parachute.

 

 

 

 

 

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Very very true, so fins are only way ?

See http://exploration.grc.nasa.gov/education/rocket/rktcp.html for some rough CP calculation. Try different fin sizes. You can always add weight to the nose to move the CG forward to adjust the CG to CP relationship.

 

Side note: For safety I'd not use the needle on the nose cone. Parachute ordinance can and does fail and the rocket will come down like a ballistic missile when that happens. There's no need for it to be an arrow as well.

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Replying to my:

"The centre of gravity has no relationship with the engine's thrust in stability, because the thrust is axial, not vertical."

 

Sorry but that's incorrect.

You believe what you want... Give a thought at how big the moment of two coaxial forces is, and whether it depends on how far apart the coaxial forces are.

 

By the way, the British admirals had refused rear screws for boats because this, they said, would have made boats unstable - despite the invention had been demonstrated in front of them. Next, they lost the US independence war.

Edited by Enthalpy

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See http://exploration.g...cket/rktcp.html for some rough CP calculation. Try different fin sizes. You can always add weight to the nose to move the CG forward to adjust the CG to CP relationship.

 

Side note: For safety I'd not use the needle on the nose cone. Parachute ordinance can and does fail and the rocket will come down like a ballistic missile when that happens. There's no need for it to be an arrow as well.

 

Thank you, i can't take it off but i hope it break the air better ?

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