Surely this wouldn't fly...

So, I was playing around with OpenRocket, and I ended up with this crazy design that has the fins mounted closer to the top of the rocket than the bottom, and yet it is stable. Granted, it has a lot of noseweight, but surely this wouldn't fly safely. Or would it? If this isn't just some weird OR glitch, I'm going to have to build this just to mess with the LCO at my club. Of course, I'd bring the OR printout, just in case.
Anyway, here it is:

The motor in this screenshot is an AT E18-7W it's stable with up to an F39.
It started out as a Low 'N' Slow design, hence the name.

I tend to agree that the model looks unstable. Is there some error that is not transparent? It looks like the total weight is on the order of 11 oz, so I would guess that the nose weight is not too much. How much is the nose weight? One time I made a short stubby model that had clear plastic fins that extended far aft into the base region, but did not extend radially into the base region (smaller than the body diameter). Rocsim stated that the model was stable, but when I flew the model, it was clearly unstable. I concluded that the design was outside the normal boundaries of Rocsim and that there was something inherently unstable about the model that the code was not capturing.

On the other hand it looks like the c.g. (blue dot) is way ahead of the red dot (one diameter or more). I'm assuming that the red is the center of pressure. On that basis the model should be stable. It might be that your nose cone weight is sufficiently heavy to offset the aft weight of the motor. Rocsim reports how high a model needs to go before it reaches the minimum velocity to be stable. Do you have such a prediction?

The nose weight is 7.5 oz. Originally I was just messing around with the design, so I had not added the recovery device. With the parachute and shock cord, it weighs in at 11.7 oz. Showing 1.26 calibers of stability with an F39.
OR does not calculate how high the model has to go to reach a stable velocity, but the simulation does calculate the velocity off the rod. With a 36" rod (oops, I forgot the launch lug... oh well, OR doesn't seem to care) it says 39.4 MPH.

Probably, the nose weight explains it. That is almost a half-pound. I'm looking at a 2 oz fishing line sinker and it is fairly big. Your nose weight would be almost 4 of those sinkers. I'm looking at at an AT SU G-motor and it is about 4.5 oz and your motor is probably smaller. So, it may be stable, but convincing the RSO may be another thing. If I were to get it past an RSO, I think as the designer and builder I would want it on a far-away pad and call it a heads up flight. I think 4 large sinkers may take up more room there your drawing shows.

You're right; I forgot to adjust the size of the mass component. Even properly adjusted, it is still stable.
Getting past an RSA would certainly be... interesting. But so long as I had the OR printout, I think they would be fine with it. If I do build this, I might bring my own pad and set it up farther away using extension wires from the club's leads, like they do with EX loads.
Definitely a heads-up flight! :duck:

The CG looks almost unbelievably far forward, like the engine isn't included or, at least, is the after-firing weight. Any weight overruns such as due to paint and glue will have an exaggerated effect. If you can show an RSO it balances just behind the nose cone, should be OK

I may have to move the fins a little farther back, so I can paint it florescent orange-- it sims to over 1700 ft! :eyepop:
I do have some extra A10-3T's laying around. Maybe a smaller version is in order before I use up an F39. It'd be safer, and I could test it in isolation with my own launch equipment.

I had a play around with the design in Open Rocket. If I enlarge the nose weight to fill most of the nose cone, which is likely to be the case with that much of it, the CG moves back so that it is less than 1 calibre ahead of the CP, and that's assuming Open Rocket estimated the rest of the masses correctly. Personally I trust programs like this to get the CP right, then I find the CG of the actual model and check that it is far enough ahead of the calculated CP.

In theory there's no reason why this can not be stable. Consider a more conventional design with about half the body length and the motor up near the fins - this would be stable enough. All you've done is extended the body tube, moving the CP back a little; moved the motor back, taking the CG back; then stuffed enough nose weight in to bring the CG back where it needs to be.

It's another example of the theory that anything will be stable with enough weight in the nose and enough thrust in the tail. Which means build the rocket, find out where the CG really is, add more nose weight if needed, and if necessary use an E28 instead of an E18.

Yup, the size of the noseweight is somewhat of a problem. I'm building a little one right now, and since I've already glued the noseweight in the nose cone (I couldn't fit any more...) I can adjust the location of the fins based on that. I'm thinking that I might not need to build a larger one; the little one's so much cheaper to fly.

Very intersting design. I guess you can make a lot of rockets fly with more nose weight, but the you take a hit on performace.

I think the original design looks a little weird, due to the fins further forward than midpoint. Airplanes have wings around midpoint, swept back, or even further back. Question is are you shooting for particular look, or just trying to prove it can be done.

bill_s said:

I think the original design looks a little weird, due to the fins further forward than midpoint. Airplanes have wings around midpoint, swept back, or even further back. Question is are you shooting for particular look, or just trying to prove it can be done.

Click to expand...

Well, both sort of. I'm shooting for the look of "no way that thing is stable," while trying to prove that it can be done. However, I've decided that proving that it can be done with 13mm motors is proof enough for me.

I built something similar a few years ago... my fins are aft of the mid point of the rocket, but are much smaller:



I used a ton of nose weight and flew it on an E9-8... straight as an arrow and out of sight! Unfortunately a tree grabbed it... it was recovered but not without some damage, and I never put it back together. I may some day... but even though the CG/CP relationship is OK I wouldn't normally fly a rocket with fins that small.

As long as you double check your CG/CP relationship and make sure your liftoff velocity is OK it shouldn't be a problem!