Why is This "Rocket" Stable, and Why did I Build It?!

[BagelRabbit]

Okay, here's a question for you rocket junkies.

Recently, I constructed a model rocket based on the general shape of a soccer field.

(yes, a soccer field. Don't ask.)

This rocket (can you even call it a rocket?) was basically a gigantic flying board with some support dowels and LEGO figurines playing soccer on top. There was also a giant soccer-ball top, made from a Styrofoam ball.

...oh, and it flew on a very powerful H135 motor. This thing was big enough to get this board going pretty fast, and lofted it a fair distance into the air... at least, considering the fact that we're talking about flying a board. (The soccer field only reached about 100 feet in the air, which may be some sort of record for lowest-altitude intentional high-power rocket flight.)

Here's my favorite picture of the endeavor by far:

After it reached apogee, it came down fluttering like a leaf. Because of the fact that the entire rocket was built like a parachute, I didn't put any recovery device on... but all of the LEGO figures remained on through the relatively gentle landing.

So, that was a thing.

The second part of my question is rhetorical. I have no idea why I built this rocket, or why anyone let me fly it.

The first part, however, is actually not a bad question. We're talking about an asymmetric, finless rocket that didn't have too much nose weight, here. And yet it was capable of boosting surprisingly straight up in the air. It didn't spin much throughout the boost, so that's not much of a factor either, by the way.

So,

Why is this rocket stable?

-Upsilon

(P.S. I have a good general idea of why it's stable, but I'm wondering whether you're thinking what I'm thinking here.)

[NFUN]

It looks pretty symmetric. How is it asymmetric?

[BagelRabbit]

It looks pretty symmetric. How is it asymmetric?

Most rockets are very pleasant and have these lovely circular cross-sections. This one, however, is a pretty obvious rectangle. While this isn't truly "asymmetry," it's still enough to mess with some designs.

But not this one, apparently.

[dryer_lint]

I'd say you got insanely lucky with the positioning. Don't change anything about it!

Seriously though, that's amazing. Shouldn't it be suffering from a ton of drag with that shape?

[BagelRabbit]

Seriously though, that's amazing. Shouldn't it be suffering from a ton of drag with that shape?

I've flown heavier rockets to about 1,700 feet on that motor. According to the rocketry club's best guesstimates, this thing got about 110 feet in the air.

So yes, a ton of drag.

[orcman]

My first guess* is that it is stable because:

1. the center of drag is behind the center of mass (axially)

2. the mass is spread pretty evenly left-to-right and front-to-back

3. the drag is also distributed evenly as above.

4. CoM, CoT and CoD are all in a line axially.

The rocket has symmetry, just not the usual radial symmetry.

As for why you built it... I'd take a stab at the fact you like soccer? possibly the world cup?

* It really is a guess - I'm not a rocket scientist

[longbyte1]

It's cardboard, so it's pretty light and easygoing in terms of TWR.

The drag wasn't negligible, but your motor was powerful enough to overcome it.

[Tommygun]

I think you may have just recreated a Tail Retarding Device by accident.

On the plus side, your soccer ball is well suited for low level strike packages.

Edited February 2, 2015 by Tommygun

[Bill Phil]

Well, the center of drag and the center of thrust are lined up correctly, of course.

Looks like that's the rocket's neutral position, albeit with high drag.

[HafCoJoe]

Lego dudes!!! Awesome! Why did you build it? Why not...

[Neil1993]

The position of the center of pressure and center of gravity! The center of pressure is probably aft of the center of gravity. This means that the angle of attack will tend to correct itself, rather than increase, when the rocket receives an angular displacement. The field under the ball was probably what pushed the center of pressure far back enough

[GigaG]

The position of the center of pressure and center of gravity! The center of pressure is probably aft of the center of gravity. This means that the angle of attack will tend to correct itself, rather than increase, when the rocket receives an angular displacement. The field under the ball was probably what pushed the center of pressure far back enough

I'm not into model rocketry, but this certainly seems like it could make sense.

This reminds me of something-

https://twitter.com/maven2mars/status/558719381421572096

MAVEN's solar panels are bent for a similar reason, to stabilize the spacecraft as it rushes through the upper Martian atmosphere. It won't be aerobraking AFAIK, but apparently the atmosphere could still disrupt the spacecraft if its panels were flat according to this blog (go to "MAVEN completes Periapsis Lowering Maneuver.")

[BagelRabbit]

The position of the center of pressure and center of gravity! The center of pressure is probably aft of the center of gravity. This means that the angle of attack will tend to correct itself, rather than increase, when the rocket receives an angular displacement. The field under the ball was probably what pushed the center of pressure far back enough

You're getting close.

I should probably mention for non-model-rocketeers that despite the fact that the CG and CP (Center of Pressure, same as Center of Drag) are aligned, rockets will still be unstable if the CP is in front of the CG (if the fins are in front).

According to conventional calculations, this rocket should be unstable. The main drag-producing bit is in the back, but there's still some serious drag in the front (caused by the soccer ball). In addition, the center of gravity is pretty far back, as a result of the motor that I used (solid rocket fuel is heavy). The rocket balances right at the base of the soccer ball, which is very slightly behind the Center of Pressure.

And yet, the thing flies relatively stably.

So, what's causing the added stability? This is the real question.

-Upsilon

[Neil1993]

So, what's causing the added stability? This is the real question.

hmmmm.....

I may have to throw this in a CFD program when I get home... But I think I might have a theory...

[YNM]

Good placement ? High TWR (not accounting drag) ? CoG, CoP and CoT gets a good arrangement ? Combined with large, symmetrical drag "pressure"... ?

Were there any winds upon launching ?

[-Velocity-]

Well, how did you calculate the center of pressure? That would be my guess as to why it's actually stable, you're using the wrong assumptions for how drag works. Think about the incredible vortices that the big rectangle creates as it flies through the air. The conventional assumptions for drag may not apply.

Now, I am not an aeronautical engineer and it's been a long time since I had a course on fluid dynamics. I'm sure there are more knowledgeable people on these forums for this topic than me. But think about this- if that big rectangle generates a huge amount of vortices behind it, shouldn't that mean that the air there is moving faster? Shouldn't that, through Bernoulli's principle, mean that you form a large low pressure area behind your rocket? That would mean that there is a large force applied, through the differential in air pressure, on the surface of the rectangle. Because this force is behind the center of gravity, and points away from the center of gravity, it acts to stabilize the rocket.

Even without the action of vortices, there should be a difference in pressure anyway, just from the air being compressed in front of the rectangle by its forward motion, and expanding behind it to fill the void left by the rectangle's forward motion. That pressure acts against the huge surface of the rectangle, and stabilizes the flight of the rocket.

Anyway, I think that something like that is responsible for the stability of the rocket.

Anyway, I'm impressed your rocket actually flew and remained in one piece! A long time ago, I went to this high powered rocketry event near Huntsville, AL. One of the people launching rockets that day had built a small rocket with HUGE forward-slanted fins as his rocket for H and I motor qualification. On launch, the fins sheared off- leaving just stubs glued to the rocket- almost before the rocket even managed to leave the launch rail. As the fins came slowly fluttering back to the ground, the rocket, freed of the ridiculously oversized fins, zipped away almost horizontally into the far distance. (Luckily, all the fins sheared off, evening out the drag, before it could turn more than 90 degrees.) I couldn't stop laughing at it. In my defense, I was only 13 at the time, but that probably made it even worse for the guy, to get laughed at like that by a kid. Though... it was a pretty dumb rocket design

Edited February 4, 2015 by |Velocity|

[Neil1993]

Well, how did you calculate the center of pressure? That would be my guess as to why it's actually stable, you're using the wrong assumptions for how drag works. Think about the incredible vortices that the big rectangle creates as it flies through the air. The conventional assumptions for drag may not apply.

Now, I am not an aeronautical engineer and it's been a long time since I had a course on fluid dynamics. I'm sure there are more knowledgeable people on these forums for this topic than me. But think about this- if that big rectangle generates a huge amount of vortices behind it, shouldn't that mean that the air there is moving faster? Shouldn't that, through Bernoulli's principle, mean that you form a large low pressure area behind your rocket? That would mean that there is a large force applied, through the differential in air pressure, on the surface of the rectangle. Because this force is behind the center of gravity, and points away from the center of gravity, it acts to stabilize the rocket.

If he was using standard rocketry methods, he would have calculated stability using the Barrowman Method or something similar.

As for your theory about the low pressure behind the rocket, I agree that it might be the cause, but I still want to verify with some simple computer models. Unfortunately, I've been really busy at work and haven't had a chance to experiment yet.

[goldenpeach]

why did you build it?

-because KSP can give some really bad idea.

[pincushionman]

I bet we could simulate this…badly…with The Ball, a structural panel or two, snd some Separatrons?

[Neil1993]

I bet we could simulate this…badly…with The Ball, a structural panel or two, snd some Separatrons?

The simulation would be better if we did it with the Stayputnik and FAR