Aerodynamics curiosity - Rocketarium Terraformer - Why does it spin?

[curtisheisey]

I recently obtained a Rocketarium Terraformer, which I think is a wonderful little oddroc, https://www.rocketarium.com/Rockets/Terraformer . It is a symmetrical design with three trianglar shaped blades/fins. They are non-canted, and the motor mount is not canted. The top edges of the triangle blades are flat. It spins slowly on its ascent, gaining rotational speed. At apogee, it flips over and spins quickly down. It flies to about 300-400 feet on an Estes D12. I got to wondering about this rocket, and why does it spin on the way up? The edges are flat, and there are no apparent edges for any kind of aerodynamic gradient. It strikes me as something that would not spin.

Any aerodynamics experts out there that can comment on this?

The only reasons that come to mind, are that in actual construction, no one can get it perfectly symmetric and balanced. This might be the reason. Or there are so many edges and surfaces on the bottom that it would catch some sort of gradient.

 

[Blast it Tom!]

I'm not an aerodynamics expert, but you may just have it with (B), unavoidable asymmetries. Couple that to a meta-stable system and the asymmetry provides the perturbation to get the motion going. That is, if you were to have a perfect rocket, motor, and quiescent atmosphere, no rotation would occur. Much like balancing a marble on a bowling ball, once it starts it ain't stopping, and in fact accelerates up to where the drag sets the top rotational velocity. Interesting model!

 

[OverTheTop]

^^^ What he said. It is likely there is a slight asymmetry somewhere that starts the rotation, then subtle difference in airflow across the parts contribute to helping it along a little bit more.

[edit] Thinking about it, it could also be some roll-pitch coupling, setting up something like a dutch roll motion.

It is similar to windmilling, the spining of rockets falling flat as they descend, I think. I have had a look at the maths for that and there is nothing that says it should happen, except perhaps in higher-order calculations.

 

[GlenP]

The unsteady vortex shedding from the edges of the short delta wing fins would be unsymmetrical. Look at the swirling whirlpool waves around the edges of a paddle in the water for example. That introduces a little side force on the fins that are not in sync with each other and will initiate rotation. One question, does it always rotate in the same direction? That might be due to a slight bias in the construction. Maybe it rotates the other direction in Australia?

 

[OverTheTop]

I was wondering about Coriolis effects too .

 

[curtisheisey]

Thanks for all the thoughts. I was thinking about Newtonian physics for the motion, but GlenP made a good point that the eddies/airflow off the triangles are non-deterministic. If it is metastable with respect to some rational moment, it would not take much to get it rotating, even for a perfectly constructed Terraformer in perfect conditions.

Interesting Q about the direction of rotation. I need to paint the bottom of one triangle and get some more videos. I took some photos and videos and posted on FB in the NAR group, but I need to get some better videos to really analyze the motion.

 

[kuririn]

I have several launches on mine, and can confirm that it spins in a clockwise rotation.
It starts spinning before it leaves the rod. IIRC Rocketarium recommends that you use a short rod.
If it spins in a counter clockwise rotation in the Southern Hemisphere then I suspect that it would be the Coriolis Force in effect. Any Aussie or Kiwi owners out there?
Have you tried asking Rocketarium since its' their design?
Here's one of several videos of the Terraformer by my buddy Ron. Go to the 16:06 mark.

 

[curtisheisey]

After thinking about this, the model only weights 45ish grams, and Coriolis force is very small, so I would conjecture that aerodynamic forces are stronger that Coriolis effects.

 

[curtisheisey]

Yeah, I emailed Pavel at Rocketarium and asked the question, but I didn't hear back.

 

[dpower]

I modified mine to angle the triangles, to force a spin. IIRC, it spinned, until I put an F39 in it.

 

[Steve Shannon]

The only reasons that come to mind, are that in actual construction, no one can get it perfectly symmetric and balanced.

I think that’s probably correct.

 

[mh9162013]

The only reasons that come to mind, are that in actual construction, no one can get it perfectly symmetric and balanced.

That's my thought, too.

How many onboard cam rocket videos do you see with 3FNC or 4FNC designs where the rockets spin on ascent?

EDIT: Typo fix

 

[curtisheisey]

How many onboard cam rocket videos do you see with 4FNC or 4FNC designs where the rockets spin on ascent?

Most. But there are a very few that don't

 

[mh9162013]

Most. But there are a very few that don't

So I think you have your answer.

 

[kuririn]

If anybody would like to pay for my airfaire and hotel to Sydney, I'll be happy to launch mine and see whether it spins in the opposite direction from here.

 

[BABAR]

Maybe we are thinking this backwards. Instead of thinking “why does this rocket spin”, think, “why don’t all rockets spin?” (Understanding that most actually do, some certainly more than others.).

standard rockets have fins that stick out straight laterally aligned with the long axis. While frequently they DO spin, there is a drag force that tends to keep them from going too crazy.

the Terraformer fins don’t really have a flat lateral fin component to RESIST rotation, so seems like once a spin starts, there is nothing to stop it. I still don’t know WHY there may be a vicious cycle, meaning that once it starts spinning maybe it sets up a positive feedback vortex that speeds it up.

i build a lot of asymmetric fin rockets, and most corkscrew, although not built with any intent for such and I really haven’t kept track of which way they go. But seeing lots of onboard rocket videos, aside from @Ronz Rocketz rockets which usually go pretty straight, MOST standard low power rockets have a surprising amount of spin.

i do know that my box fin rockets are one of the few low power designs that I have seen that have a strong tendency NOT to spin, but they have a lot of fin surface area perpendicular to the flight path. Terraformer has essentially NO fin orientation perpendicular to the flight path.

 

[Ronz Rocketz]

Maybe we are thinking this backwards. Instead of thinking “why does this rocket spin”, think, “why don’t all rockets spin?” (Understanding that most actually do, some certainly more than others.).

standard rockets have fins that stick out straight laterally aligned with the long axis. While frequently they DO spin, there is a drag force that tends to keep them from going too crazy.

the Terraformer fins don’t really have a flat lateral fin component to RESIST rotation, so seems like once a spin starts, there is nothing to stop it. I still don’t know WHY there may be a vicious cycle, meaning that once it starts spinning maybe it sets up a positive feedback vortex that speeds it up.

i build a lot of asymmetric fin rockets, and most corkscrew, although not built with any intent for such and I really haven’t kept track of which way they go. But seeing lots of onboard rocket videos, aside from @Ronz Rocketz rockets which usually go pretty straight, MOST standard low power rockets have a surprising amount of spin.

i do know that my box fin rockets are one of the few low power designs that I have seen that have a strong tendency NOT to spin, but they have a lot of fin surface area perpendicular to the flight path. Terraformer has essentially NO fin orientation perpendicular to the flight path.

The Aerospace One I flew last Saturday was a bit of a glider (like the SST). Finished the decals and can’t wait to get a cam on it. The drag on one side caused it to veer over so the cam should counter weight it. I’ll probably stick with the C5-3 and see if it stays in the park.

 

[Donnager]

I modified mine to angle the triangles, to force a spin. IIRC, it spinned, until I put an F39 in it.

Get the Rocketarium Turbo Vortico. It's 29mm. I launched mine once on a G25. The RPM's when the motor went out were.....'high'.

 

[kuririn]

I flew my original Terraformer on an E30 and the struts splintered.
IIRC an E28 is one of the recommended engines.

 

[neil_w]

i build a lot of asymmetric fin rockets, and most corkscrew, although not built with any intent for such and I really haven’t kept track of which way they go. But seeing lots of onboard rocket videos, aside from @Ronz Rocketz rockets which usually go pretty straight, MOST standard low power rockets have a surprising amount of spin.

I believe the asymmetric drag from the camera housing contributes greatly to spin. And therefore I would be reluctant to conclude purlely from on-board camera footage that "most rockets spin".

In fact I believe the opposite is true. For sure, most of my rockets do not spin. That's not a brag, just a statement of fact. If they do rotate at all it's very slight.

 

[BABAR]

Neural disease

I believe the asymmetric drag from the camera housing contributes greatly to spin. And therefore I would be reluctant to conclude purlely from on-board camera footage that "most rockets spin".

In fact I believe the opposite is true. For sure, most of my rockets do not spin. That's not a brag, just a statement of fact. If they do rotate at all it's very slight.

for classic 3FNC and 4FNC nose cone rockets I think it is challenging to detect a certain small amount of spin. The asymmetric camera mounting would be a logical culprit, but while most LPR camera flights have some ”rotation” (”spin“ term may exaggerate the degree), some don’t. For @Ronz Rocketz seems like the ones with more finnage don’t. Many of your sci-fi fantasy rockets and scale models also have considerable surface area, and you take great pains for good alignment. I’ve seen mixed flights with Estes AstroCam rocket, it comes with a plastic fin can so the alignment SHOULD be built in perfect. My Box fin rockets seem to have an inherent “anti-spin” tendency, maybe it’s just easier to make them straight, although I would think the opposite would be true.

anyway, it is a different fin orientation from standard.

 

[dpower]

Get the Rocketarium Turbo Vortico. It's 29mm. I launched mine once on a G25. The RPM's when the motor went out were.....'high'.

I've already got one! Had to reinforce the exposed plywood seam on the bottom, but it's been solid since. Great fun, I usually fly it on F motors.

 

[markg]

I flew my original Terraformer on an E30 and the struts splintered.
IIRC an E28 is one of the recommended engines.

I shredded mine on an E18. Rebuilt with added epoxy and tried again on a D9 but it stuck on the rod. Will try again with a D9 at next launch, then move up to E18 again if/when successful.

 

[PSLimo]

That looks like it would make a fun low and loud upscale...

 

[cls]

... wondering, do all of these spin the same way? Clockwise or counterclockwise?

If so, I would guess design. If not, I would guess variation in materials and construction.

We need a mass launch of hundreds of these, to collect the data, ahem...

 

[Grog6]

Sillier things get funding.

 

[Hobie1dog]

this is a pretty cool design thanks for posting it up

 

[kuririn]

... wondering, do all of these spin the same way? Clockwise or counterclockwise?

If so, I would guess design. If not, I would guess variation in materials and construction.

We need a mass launch of hundreds of these, to collect the data, ahem...

Some small random construction deviations would result in a clockwise spin, others would result in a counter clockwise spin.
If they all spin clockwise when launched in the Northern Hemisphere and counter clockwise when launched in the Southern, then we know the cause.
The blades and struts appear uncanted.
And a slow, not rapid spin is imparted.
(Compared to the family of Vorticos).
Doesn't take much force to get a rocket spinning.
So I suspect the Coriolis Force.
Look at how it "spins" the water in your toilet sink.

 

[OverTheTop]

I'm not convinced the Coriolis force is strong enough. I guess we could measure direction and speed of rotation of a number of these and look at the statistics to work it out. Wind-tunnel might work too.

 

[kuririn]

I'm not convinced the Coriolis force is strong enough. I guess we could measure direction and speed of rotation of a number of these and look at the statistics to work it out. Wind-tunnel might work too.

Guess you'll just have to buy a kit and launch it down under.