To spin or not to spin, that is the question.

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Senior Space Cadet

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I know general consensus is that rockets fly higher if they don't spin.
I'm not from Missouri, but I have to see for myself.
Here's the rub: so fins designed for spin create more drag, but spin not only makes the rocket fly straighter, drag at the rear also makes it fly straighter. If you put a cone shaped flair, at the back end of a rocket, would it not fly straight? So my thinking is, you wouldn't need as much fin surface area to make the rocket fly straight if you make it spin. Maybe way less.
There is also the question of whether altitude is the only priority? A rocket that flies beautifully, but not so high, might be a good goal.
Another question is, if your construction skills are less than perfect, and mine certainly aren't, is it possible that spinning will result in a much straighter, and therefore higher, flight?
So here's the test I'd like to do. Build two identical rockets, except that one has more or larger fins that don't make the rocket spin and one has less or smaller fins that do.
One obvious test I might try is build one with three fins at a slight angle and one with four fins inline with the body.
I know I'm going to get hammered on this, but I need to see for myself.
 

OverTheTop

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If you put a cone shaped flair, at the back end of a rocket, would it not fly straight?
Some rockets do have a cone-shaped flare at the rear to help stabilise them at higher speeds. IIRC it is for near the hypersonic end of things. Yes, frontal area takes a hit so the rocket is more draggy.

There is also the question of whether altitude is the only priority? A rocket that flies beautifully, but not so high, might be a good goal.
It depends what goals you set for your build and flight. A nice flight can indeed be a great outcome. You can fly for speed, altitude or cool flight, whatever you want to burn your APCP in :).

FYI quite a few rockets induce spin during flight. Some use rocket motors, some use canted fins, and others use wedges bolted onto the trailing edges of the fins. The spin allows the rocket to fly straight for longer once the fins exit the atmosphere and lose effect. It is also improved if you have a radially-balanced and stiff rocket. A rocket spinning in no atmosphere will degrade to a rotation about the axis of maximum rotational moment of inertia. In a rocket that is typically and end over end tumble.
 

Funkworks

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Ok, here's the hammering! 🤓

The motor provides a certain amount of energy. Whatever energy is used for spinning the rocket is energy not used to gain altitude. The higher the rpm, the more energy is wasted exerting forcing in the wrong "direction". This is straight out of high-school science, and valid.

A proper test, if allowed by the rules, would be to have only the angle of the fins varied. But then by how much? What RPM would think is best?

By increasing the mass or number of fins of the straight-fin rocket, you're only inhibiting it and tipping the scale in support of your preferred results. 😜
 

Winston

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Build two identical rockets, except that one has more or larger fins that don't make the rocket spin and one has less or smaller fins that do.
I built two otherwise identical 18mm rockets that are both 1 caliber stable, but one with slightly canted fins [only a few degrees is plenty] and plan to launch and video them simultaneously in a slight breeze to show how spin greatly reduces weathercocking, something I've dramatically demonstrated previously in a solo flight of a different, larger rocket using a 3D printed canted-fin can. They both also use 3D printed fin cans which makes identical characteristics other than fin cant easy to maintain. OpenRocket can predict spin rate for various cant angles, but changing the angle from zero, while inducing spin [and a lot of it with only small cant angles], doesn't seem to have any effect on lowering apogee or bringing the rocket's virtual CP rearward as spinning should, so I guess it doesn't take spin into consideration in those calculations. They remain identical with or without spin.
 
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Stefan2k4

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My concern with deliberately inducing spin, is what happens in the early portion of the flight as the rocket is being guided by the rod or rail and just after it exits the rail. If the launch lug is located on the outer surface of the body tube as it is on most model rockets it's going to restrict the rockets ability to rotate on it's longitudinal axis and perhaps cause it to want to rotate around the rod. I would think this might be able to cause the rocket to corkscrew or gyrate as it's leaving the rod. I would think that if you're going to design a spin stabilized rocket, this might need to be a design consideration and you might either need a design that allows the launch rod to either go through the center axis of the rocket or use something like a launch tower or piston launcher instead of a rod or rail.

Also, if you're designing a rocket that's going to spin at a high RPM, balance around the longitudinal axis might become more of a concern than it would normally be in your average model rocket.
 

OverTheTop

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My concern with deliberately inducing spin, is what happens in the early portion of the flight as the rocket is being guided by the rod or rail and just after it exits the rail.
Airspeed is not enough when it is on the rail to induce any significant rotation. It is not there for long enough.
 

Steve Shannon

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My concern with deliberately inducing spin, is what happens in the early portion of the flight as the rocket is being guided by the rod or rail and just after it exits the rail. If the launch lug is located on the outer surface of the body tube as it is on most model rockets it's going to restrict the rockets ability to rotate on it's longitudinal axis and perhaps cause it to want to rotate around the rod. I would think this might be able to cause the rocket to corkscrew or gyrate as it's leaving the rod. I would think that if you're going to design a spin stabilized rocket, this might need to be a design consideration and you might either need a design that allows the launch rod to either go through the center axis of the rocket or use something like a launch tower or piston launcher instead of a rod or rail.

Also, if you're designing a rocket that's going to spin at a high RPM, balance around the longitudinal axis might become more of a concern than it would normally be in your average model rocket.
You do not want a rocket to spin at a high rpm. They end up twisting themselves into a pretzel on the way up because of the balance problem you mentioned.
 
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