Box Fin Modification for Estes AstroCam

[BABAR]

My first flight of the AstroCam suffered from too little wadding (my fault), scorched chute. I caught it on the way down, had it hit the pavement it probably would have broken a fin, not to mention what would have happened to the camera. Anyway, there was some rotation on the upward part of the flight. My Tank Killer rocket with box fins has always looked like flies on rails, so I figured I'd try it out with this one. Just raw balsa, but turned out pretty well.

 

[BABAR]

You can hear me get a little nervous when it hit apogee at 3 seconds but because I used a C6-5 didn't deploy for another two seconds. Got a bit of a zipper from the shock cord. I think this will do well on a C6-3. Not sure I am willing to risk it in my field on a Quest D.

 

[jqavins]

Are those pylons a little off normal? It looks sort of like the cheaters are where the fins should be and the fins just a little offset.

Whether they are or not, it gave me an idea. Just for kicks, what if the pylons for a box fin were deliberately off normal, the way we've some fins mounted? (Forgive the background. I'm fooling with gradient fills and transparency, making a crude attempt to look like a poorly maintained lawn.)

 

[BABAR]

Are those pylons a little off normal? It looks sort of like the cheaters are where the fins should be and the fins just a little offset.

Whether they are or not, it gave me an idea. Just for kicks, what if the pylons for a box fin were deliberately off normal, the way we've some fins mounted? (Forgive the background. I'm fooling with gradient fills and transparency, making a crude attempt to look like a poorly maintained lawn.)
View attachment 462051

You are spot on, as usual.

It wasn't intentional.

The critical intent was to make sure pylons and box fins were perfectly aligned with longitudinal rocket axis, i.e. flight path.

Cheaters were indeed put right where four 90 degree evenly spaced box fins would be.

I SWAGed a box fin size, the 4 fins were cut with pretty much exact rectangles, sanded as a stack to get each one identical, and glued them, keeping the box on a flat surface.

Pylons (inside fins) hemi-spans (i think that's the right lingo) were measured based on the box size and body tube diameter, stacked and sanded as perfectly even as I could, and were glued to the body tube aligned with the cheaters using an Estes fin Jig, so between the cheaters and the fin jig, they should be (and pretty much were) in perfect potion and alignment. Fillets were done with Titebond Trim and Moulding thick glue.

The best laid plans of mice and men, however....

As measured, the fin box SHOULD have slipped snugly and perfectly over the fins. Somehow I miscalculated, I might have not allowed for the tight corner space or maybe the box fin thickness. In any case, option included trimming the (already otherwise perfect) pylons or letting them "swing") slightly. I had my doubts that I could perfect evenly sand or trim them (and being off the slightest bit would CANT the box, ruining the whole point of the exercise), but I figured the "swing" wouldn't change the longitudinal alignment, so I went with that. Looks like I guessed right!

As for your offset fin idea, guarantee you it would work.

If you look at the Fliskits Tiddlywink, most of my Helis and Gyskelion, the find are ALWAYS perpendicular to the flight path but also always eccentric to the centerline of the rocket. They work great as fins, they are likely slightly less efficient as they have more fin area closer to the body of the rocket. For Helis and AirBreakers they have a mixed effect on recovery. They provide more surface area for the blades. @Rktman may chime in, more blade area usually good, but if it is at the OUTER TIP of the blade the inertia might slow it from initial spin up. For AirBrakes it is a definite win, and I personally like dual proposing the fin area, used for stability on boost and incorporated into the blades for recovery. Also, for helicopters without a swivel, standard fins likely add a small but probably not negligible drag to rotation which is eliminated when they are incorporated into the blades.

Many of my Asymmetrical fin designs have used fins mounted perpendicular to the FLIGHT path but completely catywampus to the central axis.

See Lucky 7 and Double Mach diamond here. Post 6_8

https://www.rocketryforum.com/threa...ights-overall-a-good-day.161391/#post-2036158
I think this would be a perfect design for NewWay square rockets, just make the fins long and glue them flat on the sides. Perfect alignment every time.

For a round rocket if you went this route, cheaters and a custom fin jig would be very helpful, it would be difficult IMO to "eyeball" these. If done in a balanced fashion, should not even add significant spin.

 

[Rktman]

You're correct that having the fins incorporated into the ends of the rotors don't improve duration time. (Found that factoid buried in an NAR report or somewhere here on the Forum, just can't recall exactly where, or the reason why the increase in area doesn't help).

 

[BABAR]

You're correct that having the fins incorporated into the ends of the rotors don't improve duration time. (Found that factoid buried in an NAR report or somewhere here on the Forum, just can't recall exactly where, or the reason why the increase in area doesn't help).

Interesting. Opens the door for a reversed Gyskelion/TiddlyWink, with the hinge on the Tail end, so the fin components are on the INSIDE of the rotor when deployed.

All my models since Gyskelion have been 4 sided due to rotor retention configuration. This opens some doors for more reliable deployment AND less probability of Fin Breakage AND can discard the canard-like forward rotor stops.

 

[Rktman]

Interesting. Opens the door for a reversed Gyskelion/TiddlyWink, with the hinge on the Tail end, so the fin components are on the INSIDE of the rotor when deployed.

All my models since Gyskelion have been 4 sided due to rotor retention configuration. This opens some doors for more reliable deployment AND less probability of Fin Breakage AND can discard the canard-like forward rotor stops.

Would the fins then function as the rotor stops (or actually BE the rotor stops? My first heli was a ring fin and the ring functioned as the rotor stop). Uses a sliding piston up top to release the rotors. (First rocket I built as a BAR, didn't realize helis existed until AFTER I had started the build, or I might have used an easier to craft retention/deployment system. Turned out well though).

 

[BABAR]

Would the fins then function as the rotor stops (or actually BE the rotor stops? My first heli was a ring fin and the ring functioned as the rotor stop). Uses a sliding piston up top to release the rotors. (First rocket I built as a BAR, didn't realize helis existed until AFTER I had started the build, or I might have used an easier to craft retention/deployment system. Turned out well though).

View attachment 462096

Great minds think alike, if not synchronously.

Integrating the fin into the rotor is much more challenging with the fin segment at the rear. Much easier and more reliable to use it as a rotor stop alone.

I was disappointed with estes flip flyer. Seems the could have made it both easier and better simply by extending the plastic rotors the length of a single long body tube, integrate the custom plastic retention ring Into the base of the cone, blow the cone with no chute but a swivel on the shock cord, and as @mbeels likes, Bob's your Uncle it comes down as one piece.

 

[BABAR]

great flight, unfortunately the camera timed out, I think I forgot to turn it off, and I theeeeenk that if you do that and the battery runs out, it doesn't "close" the video. So the video was there, and listed as a video, but it wouldn't play. It was too bad, because it looked from the ground like zero rotation.

Learning curve, definitely need to pack the shock cord from rocket to chute first, then the chute, then the cord from the chute to the nose cone. Got all tangled at deployment, while the up video would have been absolutely awesome (i could see the box fins, this did not roll one iota), the DOWN video would clearly have required strong doses of Dramamine!

 

[jqavins]

I don't always look at who posted before I read, but I'm getting to recognize your launch site.

Nice flight.

 

[BABAR]

Vindication!

Looks like the box fin configuration provides significant stability on boost

I am crummy at editing, so just jump to 58 seconds.

 

[Rktman]

Vindication!

Looks like the box fin configuration provides significant stability on boost

I am crummy at editing, so just jump to 58 seconds.

Nice, arrow straight ascent and zero rotation!

 

[Greg Furtman]

My first flight of the AstroCam suffered from too little wadding (my fault), scorched chute. I caught it on the way down, had it hit the pavement it probably would have broken a fin, not to mention what would have happened to the camera. Anyway, there was some rotation on the upward part of the flight. My Tank Killer rocket with box fins has always looked like flies on rails, so I figured I'd try it out with this one. Just raw balsa, but turned out pretty well.



View attachment 460584

@BABAR I picked up an Astrocam too. Not to impresses with the rocket though. The box fins seem really stabile. I do have a question though. Is there a reason that the fins going out to the box aren't perpendicular to the body tube I not fitting into the exact center of the box corners? Just wondering.

 

[Greg Furtman]

@BABAR How big is the square. I'm guessing about 2.5" x 2.5". Am I close?

 

[BABAR]

Don’t have access at the moment, but my standard fin diameter is 2 inches wide, so maybe you can eyeball it from there.

please let me know your results, I hope I have ignorantly stumbled on a way for reliably making stable camera boost lift vehicles ( non rotating). . I think box fins are sort of like ring and tube fins, not as efficient as linear fins but less prone to weathercocking.

if you build the box first it is easy to keep it straight, and if you use the cheaters and perfectly rectangular “spokes” the alignment pretty much is self-perfecting.

 

[Greg Furtman]

I laid out the square fin using TurboCAD. Here's what it looks like. The proportions seem to be about right.

 

[BABAR]

I laid out the square fin using TurboCAD. Here's what it looks like. The proportions seem to be about right.

Looks exactly like mine except without the offset ( I.e., better!)

 

[BABAR]

I suspect as built this is likely over stable, which has two down sides

1. Tendency to weathercock. Theoretically overstable rockets tend to do this. However, it has been published that tube fins are less prone to this,and I suspect ring rockets including square rings are also resistant. I think the reason is that the cross sectional area (what a cross wind would “hit”) is proportionately less than the surface area exposed to the forward to back airflow during flight (not sure if this is all or in part due to the internal “spokes” which are shielded from cross winds by the outer structure. Despite this, while I am pretty sure my box fin birds are over stable (since many of my designs don’t lend themselves to sims, i tend intentionally to mind-sim them to be a bit over stable), I haven’t experienced any significant weather cocking even in light to medium winds.

2. Drag. Clearly (although @Dotini does well with his rings!) my big boxers with all that surface area and corners and vortexes is pretty certainly more draggy than conventional 3 or 4 fin designs.

my tank killer flies perfectly on D12s, and my BoxCam does well on C6-3, it zippered on a C6-5.