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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Finless Rocket

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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LamdinRockets
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Finless Rocket

Post by LamdinRockets » Sat Apr 14, 2018 10:21 pm

I am developing a rocket ( near flight ready), which may not need fins.

The CG of the rocket (not currently measured precisely) sits at least the diameter of the main body above the CP position.

This was achieved through shoving everything of significant mass right to the top of the nose.

Am I missing something? Because from my view the rocket could produce a stable flight without fins.

Thoughts?



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Alien Space Agency
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Re: Finless Rocket

Post by Alien Space Agency » Sun Apr 15, 2018 9:36 am

In the coast phase it would be stable. Your problem would be the boost phase.


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anachronist
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Re: Finless Rocket

Post by anachronist » Sun Apr 15, 2018 6:48 pm

Yes, the CG needs to be ahead of the CP when the rocket has water mass in it.



LamdinRockets
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Re: Finless Rocket

Post by LamdinRockets » Sun Apr 15, 2018 10:58 pm

I don't think that's the case, at least when the rocket is full, because the boost phase is very short and to bring the cp behind the CG at this point would require unrealistically large fins.



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anachronist
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Re: Finless Rocket

Post by anachronist » Sun Apr 15, 2018 11:12 pm

All I can say is, experiment. Yes, the boost phase is short, but when your rocket is traveling at 180mph for about 0.1 sec, it'll be about 25 feet in the air, plenty of time for wind forces to turn the rocket before it becomes stable. But there's a chance, with a rocket that is well balanced along its central axis, that it will go straight long enough for the water mass to disappear and your rocket be stable.

I am reminded of an experiment my brother in law did at a civil war artillery range (he has his own cannons). Instead of firing a lead ball, he attached the ball to a metal tube the same diameter as the ball, with the idea that the projectile would be very nose-heavy with a lightweight cylinder trailing behind. Good in theory, but in practice the projectile waggled tail as it flew, evidenced by the projectile hitting the target on its side rather than straight on. The barrel wasn't rifled so there was no spinning stability, just aerodynamics. Fins would have helped. The cylinder of a rocket body doesn't really provide a good weathervane effect.

I'd say, try it out. That's what this hobby is all about after all. Build a finless test rocket with some ballast in the nose and see what happens.



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