Rifling and 3d printed rockets

[Gip-Gip]

Hello all!

I've recently gotten back into model rocketing after being into it as a kid. As my first rocket, I've modeled a "rifled" 3d printed rocket according to these specs:

Spoiler: Specs

Minimum Rocket Class: C
Minimum Rocket Impulse: 10N*s
Minimum Rocket Height: 100m
Minimum Rocket Time: 1.60s

s=vᵢt+0.5at^2, 100=0*1.6+0.5*a*1.6^2, 100=0.5*a*2.56, 100=a*1.28, a=78.1m/s^2
add 10 m/s^2 to acceleration to compensate for gravity
Minimum Rocket Acceleration: 88.1m/s^2

J=F*t, 10=F*1.6, F=6.25N
Minimum Average Thruster Force: 6.25N

keep in mind the mass in this equation is in KG, not G
F=ma, 6.25=m*88.1, m=0.071kg

Maximum Rocket Weight Total: 71g
Maximum Motor Weight: 24g
Maximum Body Weight: 47g

Tell me if any of my math seems off, I think I have a decent grasp of basic physics but I'm probably wrong

Either way, so far I have 3d modeled a body, excluding parachute, that should match this criteria fairly well. Back to the question at hand:

How does rifling, and by extension spin, affect rocket performance?

The wings and launcher rifling are configured to complete one rotation every meter, which seems like a decent spin rate considering the size and speed at which this rocket is traveling. But, also, I've noticed spin isn't really a common topic when it comes to model rockets, and my guess is that it has more of a negative than positive impact.

Attached are pictures of the rocket, it's prototype(not finished) launcher, and various other details you may/may not find interesting. I'm still waiting on a replacement main board for my 3d printer, so I'll have plenty of time to revise the rocket before printing and launch

**EDIT** slimmed the upper portion of the rocket and adjusted the spin rate to 2 revolutions/meter

 

[mbeels]

But, also, I've noticed spin isn't really a common topic when it comes to model rockets, and my guess is that it has more of a negative than positive impact.

Spin stabilization is a thing, and I believe some sounding rockets use it. I've seen a few examples on this forum, so it can be utilized. I think I remember that maximum altitude is reduced somewhat due to the energy it takes to spin up the rocket, but it helps reduce dispersion (the rocket's deviation from vertical). It's fun to see something different tried.

 

[Gip-Gip]

Spin stabilization is a thing, and I believe some sounding rockets use it. I've seen a few examples on this forum, so it can be utilized. I think I remember that maximum altitude is reduced somewhat due to the energy it takes to spin up the rocket, but it helps reduce dispersion (the rocket's deviation from vertical). It's fun to see something different tried.

It's actually *fairly easy to calculate the parasitic starting force required to spin the rocket.

t=Ia

First things first according to solidworks the moment of intertia in g*mm^2 is 1812.16 (1.8122e6 kg/m^2)

The inital acceleration is also fairly easy to calculate, since, effectively the rocket turns 2pi radians per 1 meter. So, we simply multiply the minimum acceleration(minus gravity) and multiply it by 2pi to get the radial acceleration. Therefore, a=490r/s^2

t=1.81e6*490=8.87e8Nm

lastly, to get the force applied to the wings we simply do
t=F*r

where r is the radius of the rocket

8.87e8=F*0.042, F=

Honestly, I don't even need to figure out the math to know that's an unnecessarily high number

I'm probably doing the math wrong here, but there's a high chance if I'm to work with rifling I should probably work on making the pitch a little less severe

*EDIT* something wonky is going on with the program I'm using, I'mma figure it out and post a reply

 

[boatgeek]

This is a roundabout answer, but I made a spinning rocket several years ago, with fins canted about the same as yours (from a visual). OpenRocket said that the rocket only lost a couple of dozen feet (out of ~700) in apogee due to canting the fins. The delay prediction was dead on, so I think the final apogee with the canted fins was pretty accurate. So, long story short, it probably won't cut your altitude all that much. Once the rocket is spun up, the fins aren't really adding much drag.

That said, I think you'll need more stabilization off of the pad. I don't think that the rocket will have enough spin or speed to be stable from a launcher the length of the body. Another thing to consider is taking away the large nose cone. The overhanging nose cone will shadow the fins a bit, making them less effective.

 

[dhbarr]

I'm no expert on rifling, but spin rate seems to me to be a function of how fast you're planning to go? As in I would want to design for the thrustcurve of a particular motor.

 

[Gip-Gip]

I'm no expert on rifling, but spin rate seems to me to be a function of how fast you're planning to go? As in I would want to design for the thrustcurve of a particular motor.

Yes, but it still spins once every meter due to the way the fins and rifling are curved. The speed may be different but the rotations/distance are the same, if that makes any sense.

If it helps, in the CAD program I modeled the fins and rifling to follow a thread with a 1 meter pitch

 

[Gip-Gip]

Alright, so apparently the developers of solidworks thought you just move the decimal point by 3 places when converting between KG/M^2 and g/mm^2. I'm fairly disappointed to say the least

No worry, I'll find the moment of inertia when the wings break off during launch

Eitherway, I'll make the upper bit narrower so I don't starve the wings of air and I'll see how that goes

 

[Steve Shannon]

Depending on angular velocity, ensuring that the mass of the rocket is balanced about its centerline is important. I had a polycarbonate rocket with an aluminum fin can. Each time I flew it one of the fins picked up a little kick to the side. I usually pounded it back into shape and flew it again. One time I thought “I’ll just fly it with that fin like that and see it it spin stabilizes.”
It did, really well, right up until the body folded on the way up.
Also, keep in mind that a rifling rate of 1 turn per meter is a spin rate of over 20,000 rpms at Mach. You probably want to keep the linear velocity much lower than that.

 

[dhbarr]

Also, keep in mind that a rifling rate of 1 turn per meter is a spin rate of over 20,000 rpms at Mach. You probably want to keep the linear velocity much lower than that.

This is much cleaner version of what I was inartfully trying to say.

 

[Gip-Gip]

Also, keep in mind that a rifling rate of 1 turn per meter is a spin rate of over 20,000 rpms at Mach. You probably want to keep the linear velocity much lower than that.

Now that you mention that, it probably wouldn't hurt to turn the spin rate down a bit. Dividing the spin rate by 10(1 rotation every 10 meters) brings it down to a maximum ~700 rpm, maybe 1 rotation every 20/50 meters?

 

[boatgeek]

You won't get the full nominal rotation rate--there will be slip as the air going past spins up the rocket. How much? That's a great question and really hard to answer. I have a smaller LPR rocket with about 2-degree canted fins, or about 2m/rev. You can definitely still see the fins rotating while it's around 50-100 m/s, so the spin rate isn't unreasonable. Your rocket is probably heavier and higher moment of inertia than my smaller one, so it will spin up slower. I wouldn't go less than 2m/rev.

 

[Kelly]

Alright, so apparently the developers of solidworks thought you just move the decimal point by 3 places when converting between KG/M^2 and g/mm^2. I'm fairly disappointed to say the least

Well, assuming you move the decimal in the right direction, that IS the correct conversion factor...

 

[Gip-Gip]

Well, assuming you move the decimal in the right direction, that IS the correct conversion factor...

That it doesn't. Instead of converting 1kg/m^2 to 0.001g/mm^2, it instead converts 1kg/m^2 to 1000g/mm^2. Kinda a big oopsie

I spent a solid two hours trying to figure how it was mathmatically possible for a small rocket to require 600 meganewton/meters of torque to spin it up to a few thousand rpm...

 

[Gip-Gip]

Aight, revised the rocket to have a higher center of mass and modeled the 24g motor, 13g chute and 10g payload to go along with it

The rocket modeled as is weighs exactly 71 g, a final velocity of 125m/s, and has a theoretical spin rate of 2 rev/meter, or a maximum 3.75krpm. Also, without factoring in the parasitic losses from spinning, and if the parachute didn't deploy, it would take 12.7 seconds to decelerate due to gravity (dividing the final velocity by 9.8 and adding it to the rocket burn time) and the max height would be around 900m

Whether or not the rocket breaks itself before it gets near half that height is a question I cannot answer yet, but my 3d printer board should be arriving soon enough!

 

[dr wogz]

The more spin, the more drag you'l induce in trying to spin it..

or

In trying to spin said rocket, you'll cant one or all fins a degree or two (or 6 or 16 or..). this would add some drag to the initial thrust phase; getting the air to act & spin the rocket. Once achieved, the spin may then be 'churning' the air inducing more drag as it coasts (rotational inertial vs. upward inertia)

or so my initial gut / thought tells me.

 

[Gip-Gip]

The more spin, the more drag you'l induce in trying to spin it..

or

In trying to spin said rocket, you'll cant one or all fins a degree or two (or 6 or 16 or..). this would add some drag to the initial thrust phase; getting the air to act & spin the rocket. Once achieved, the spin may then be 'churning' the air inducing more drag as it coasts (rotational inertial vs. upward inertia)

or so my initial gut / thought tells me.

That is true, spinning does have parasitic force losses, I wish I could calculate them but I don't really have an accurate way to calculate the moment of inertia for the rocket

Eitherway, I'm making accommodations for a telemetry recorder circuit aboard so I can see how far it actually goes and what kinds of forces act upon the rocket

 

[DaveW6DPS]

Spin stabilization requires a good balance. The axial CG must essentially match the axial CP.

The amount of stabilizing force depends on the rate of spin. Since the rate of spin will be low to start with, ensure your rocket is aerodynamically stable to keep it on track until the rotations come up to speed.

 

[Gip-Gip]

Alright! Here's the revision 2 design!

Spoiler: Center Of Gravity

The center of gravity is directly in the center of the model more-or-less. I haven't fine tuned it at all, not really a huge benefit to doing so until I find the center of pressure. Hopefully I'll figure out a way to calculate the CP, I have the ability to really finely balance the rocket with cad

Spoiler: Payload Capsule

The payload capsule is designed to carry the 10g cargo(the small circle at the top) and it houses the 13g parachute(the larger bottom circle) during flight. The nose is a simple 45 degree angle but I can make that narrower if I need to

To attach the capsule to the upper body is a small taper, both to ensure a snug fit and automatic 3d-printer clearance compensation

Also, there are dual shock cord attachment holes to keep the center of gravity equally balanced

**NOTE** The chute is attached to the upper body but is stored inside the capsule during flight. Once the top, and by extension the chute, is ejected the none elastic chute cords yank it out of the capsule. The capsule, by contrast, is attached to the upper body via a shock cord.

Spoiler: Upper Body

The upper body is where the capsule and wadding sits. Wadding is packed below the capsule down to the ribbing below

Spoiler: Lower Body

The lower body is where the rocket motor sits, starting from the triangular motor stops pictured up in the upper body picture, to the butt end of the rocket. along the lower portion you will see ribbing to both reduce weight and keep the rocket in place

This is also where the spiraled wings sit, which rotate the rocket around the Y axis to an ideal maximum of over 3krpm

Spoiler: Launcher

The launcher is of a really simple design. It is made to be locked in a vice, for ease of design, and features a rifled "barrel" to both provide the rocket with initial spin and remove the need for launch lugs

I'd really appreciate thoughts on the design, this is my first time working on a non-kit rocket so I'm basically going by what I read at this point.

Thanks for the discussion so far!

 

[Gip-Gip]

Spin stabilization requires a good balance. The axial CG must essentially match the axial CP.

The amount of stabilizing force depends on the rate of spin. Since the rate of spin will be low to start with, ensure your rocket is aerodynamically stable to keep it on track until the rotations come up to speed.

Out of curiosity, by axial CP do you mean the CP and the CG should be the exact same in every way? Or should they both be along the same axis as the rocket?

 

[Alan15578]

Out of curiosity, by axial CP do you mean the CP and the CG should be the exact same in every way? Or should they both be along the same axis as the rocket?

He probably means that the inertial axis should be aligned (e.g. spin balanced) to the aerodynamic axis (e.g the line of symmetry). But I would not take bets on which statement makes the most sense to the reader.