Spin Stabilization

[DynaSoar]

This rocket isn't swing stable. Several swing tests with C11 through E engines installed show it to be neutral stable at best. If there's software able to model it, I haven't seen it.

It's 16" of 24mm tube with an engine hook, 2" * 1/8" lug, Apogee plastic nose and 4 itty bitty fins. Recovery is a simple kevlar cord glued inside, and a 3 foot mylar streamer, with an elastic loop on the nose for shock absorbtion.

The fins are 1/8" basswood, 1" high by 1.25" wide. But the fins are beveled at 30 degrees on one side on the leading edge, and the other side on the trailing and outer edges.

Flight tests show it to do exactly what the fins dictate -- it spins like mad. White on one side and black on the other, it's a grey blur in flight. It gives good straight flights. They're so straight, and the fins are so small, that there's not even a flutter sound. It just drills itself straight up at at least 10 revs per second. No weathercocking noticed coming off a 4 foot rod.

Not the sort of thing to be test flying at a group launch. Technically it's probably against rules until it's test flown because it can't be shown to be stable should an RSO ask. But this one's tested now, and it's passed fine.

The next version will be identical, except the fins will be only half an inch high. At that point they'll be more a set of vanes than fins. The new model should be at least as stable as this one, if not more so, because there'll be only half as much fin weight at the tail (CG forward), but every bit as much spin.

 

[rstaff3]

Hmm, interesting. I was under the impression that it is virtually impossible to spin a modroc fast enough with even enough thrust for spin stabilzation to work. Maybe that was in regards to pure spin stabilization (i.e. no fins), but it is possible to spin stabilize a model with marginal stability. I reckon you may have experimentally shown the latter.

It might be interesting to see what Rsim says about the design. I have had stable models that flunked a swing test. Maybe the bevelled fins work against a spin test in this case?

Anyway, this sounds like a fun type of rocket to experiment with. U be a rokit scientist, dude!

teflon, you listening?

 

[GL-P]

I know that longer rockets tend to fail the swing test. I don't know why.
Spinning rockets are really cool. Anyway, at that size, I don't think you'd be breaking any rules. Just say "It's spin stabilization. I don't have a program to sim it but it's flight proven." Anything to keep flying!

 

[fehskens]

>I know that longer rockets tend to fail the swing test. I don't know why.

I suspect it has something to do with the fact that unless you're swinging it on a really long tether, the nose and the tail are flying at extreme (and opposite?) angles of attack.

len.

 

[GL-P]

Your guess is as good as mine.

 

[Stymye]

a spinning rocket will not self stabilise per say... but it will follow a straighter path because a disturbance(and the change in angular momentum it imparts) has less effect on a spinning rocket "compared" to a non spinning, or slow spinning rocket.

certain roll rates should be avoided,(especially the rockets roll-coupled resonant rate)this often happens with "fin -canted" models.

 

[sandman]

Went way back in the search to find this design from two years ago.

https://www.rocketryforum.com/showthread.php?s=&threadid=1093&highlight=spiral

Maybe I'll upscale it! Maybe a BT-56 size.

The original design I did 40 years ago had a rubber band shock cord so...maybe that helped! Those are made to be wound up!

 

[teflonrocketry1]

RockSim using the RockSim equations predicts that both designs will be stable even without the spin. See attached RockSim version 5 file that can be opened with the Demo version of the software. Halequin1.rkt is the 1 inch high fin version.

Bruce S. Levison, NAR #69055, Section #535 Tri City Sky Busters

 

[teflonrocketry1]

Halequin2.rkt is the 1/2 inch high fin version. Note, all the simulations were done with a 15mph wind speed.

Bruce S. Levison, NAR #69055, Section #535 Tri City Sky Busters

 

[Stymye]

something to note-
one downside to fast spinners is they do a great job of tangeling a chute.

 

[DynaSoar]

Originally posted by stymye
a spinning rocket will not self stabilise per say... but it will follow a straighter path because a disturbance(and the change in angular momentum it imparts) has less effect on a spinning rocket "compared" to a non spinning, or slow spinning rocket.

Gyroscopic.

Enough rotational momentum is the key. Tough to do inside the body. Wonder if I could get a flywheel up to speed then launch it? It'd might have to stick out, like a bicycle wheel, but still... the possibilities boggle.