Lakeroadster's Astron Strutter

[lakeroadster]

This design evolved from the thread "Vern's Astron Scout @ 2.13 Scale?"

The Astron Strutter is a dual ring fin design, utilizing Poplar struts for ring supports.

I used Poplar struts for the outriggers on the Cygnus Prove Ship and I was amazed at how strong the final sub-assy was. The struts lead me back to putting The Astron Strutter back on the build pile.

It seems I've settled on BT-55 for most designs lately. The size allows for through wall fin attachment with 29 mm power. Additionally the 1.325 dia. allows me to turn the nose cone on my lathe easily from 2" x 2" pine stock (1.5" x 1.5") or even better from a 1-1/2" pine dowel.

 

[boomtube-mk2]

So, what percentage of the stability do the struts provide vs the rings?

 

[neil_w]

The Astron Strutter is a dual ring fin design, utilizing Poplar struts for ring supports.

Love it. You had me at "dual ring fin".

It seems I've settled on BT-55 for most designs lately.

BT55 is a fantastic size for LPR. Easy to work with, and comfortably straddles 18mm and 24mm motor mounts.

 

[lakeroadster]

So, what percentage of the stability do the struts provide vs the rings?

1/4" dia. struts are simulated in Open Rocket as (2) trapezoidal fin sets. A leading edge set, and a trailing edge set.​

​

To determine ring fin stability in Open Rocket you have to do the Ring Fin Hack... basically add 6 normal fins, based on the geometry of the ring fin... see below screen grabs.​

To answer your question I re-set the nose ballast to zero, selected the D12-3 motor and re-ran the simulation.

• -0.483 Stability Caliber: Front Struts only
• 1.07 Stability Caliber: Front and Rear Struts
• 1.22 Stability Caliber: Front & Rear Struts + The Inner Ring
• 1.32 Stability Caliber: Front & Rear Struts + Both Rings

I take this all with a grain of salt. On this simulations Open Rocket throws an error message for "Too many parallel fins". Uhm, Yep.. I hear ya lil buddy.

What has helped me to build some confidence is past swing tests of my scratch design / built oddrocs. Thus far the swing test results have indicated the simulations are pretty accurate.

Typical Oddroc stuff. My SOS is: the swing test proves if the rocket is flight worthy. And the actual flight proves if it's really stable.

 

[Daddyisabar]

Trust in SWING! If you were in Germany in the early 30's they would have labeled you a Swingkinder! You would have had to hang out with a young Werner Von Braun at the rocket club.

 

[lakeroadster]

I spent a few hours today creating CAD models and started the drawing package. More to follow.

 

[jqavins]

This puts me (back) in mind of something that's on my virtual build pile. A ringtail design, with the ring well out behind the body, supported on a truss made from CF rods or tubes, diameter TBD.

 

[OzHybrid]

I spent a few hours today creating CAD models and started the drawing package. More to follow.

View attachment 509053 View attachment 509054 View attachment 509055

What are you using to create your drawings. They look classic old school. (In a good way)

 

[lakeroadster]

What are you using to create your drawings. They look classic old school. (In a good way)

Autodesk Inventor.

 

[Dotini]

This puts me (back) in mind of something that's on my virtual build pile. A ringtail design, with the ring well out behind the body, supported on a truss made from CF rods or tubes, diameter TBD.

These sorts of rocket do have practical annoyance with fussy attachment of the ignition clips.

 

[jqavins]

Now that you mention it, I recall a discussion of that subject before. But there's an easy way to overcome it: long igniter leads. Use leads long enough to go all the way down to below the ring and out to the side. Long lead igniters can be a bit expensive, but for LP engines one can simply solder an extension onto ordinary igniters. Something like this stuff. (That's the first hit I found, so there are certainly plenty of other choices.)

 

[BABAR]

So, what percentage of the stability do the struts provide vs the rings?

Interesting question, but this mind sims extremely stable with no contribution from struts. call me a cowboy but I wouldn’t even bother to swing test this one.

what I LOVE about ring fins is the whole ring is at max distance from the tube. Standard perpendicular fins get essentially zero stability from the root of the fin and max stability at the tip. Ring fins are essentially all “tip”

 

[lakeroadster]

... what I LOVE about ring fins is the whole ring is at max distance from the tube. Standard perpendicular fins get essentially zero stability from the root of the fin and max stability at the tip. Ring fins are essentially all “tip”

Well, that, and they just look awesome. I blame @Dotini, I never was "into" ring fin rockets until I started reading his posts her on TRF.

call me a cowboy but I wouldn’t even bother to swing test this one.

"Yippee ki-yay"

I see folks writing that frequently on the ole interwebs. I don't know when "running a simulation" became a substitute for real world testing? One fat fingered mistake in a simulation can result in a bird that sim's stable, but isn't. But a swing test (in accordance with Vern's tutelage), now that's an actual real world test of the flight ready bird.

Bottom line for me though is I'm now compelled, by my shiny new NAR membership, to swing test these types of rockets, pre-launch:

"5. Launch Safety:...... If I am uncertain about the safety or stability of an untested rocket, I will check the stability before flight."​

 

[jqavins]

I blame @Dotini, I never was "into" ring fin rockets until I started reading his posts her on TRF.

You clearly had't browsed through this thread.

 

[lakeroadster]

You clearly had't browsed through this thread.

Sure I have, I mean @neil_w threads are all required viewing for scratch design / builders.

But the difference is, well in my opinions anyway, @Dotini isn't merely adding ring fins just for the "cool" factor... he's using them for R&D as to their efficiency, and further more using them as a recovery method. That adds to the awesome factor.

I need to give credit where credit is due here and include @BABAR.... Ring fins that induce the Magnus Effect... way cool.

 

[jqavins]

But the difference is, well in my opinions anyway, @Dotini isn't merely adding ring fins just for the "cool" factor... he's using them for R&D as to their efficiency, and further more using them as a recovery method. That adds to the awesome factor.

OK, that's fair.

 

[BABAR]

"5. Launch Safety:...... If I am uncertain about the safety or stability of an untested rocket, I will check the stability before flight."​

no argument there. “Certainty” is often IMO never 100%, and I suspect there are a number of experienced rocketeers who HAVE conducted swing tests that “certified” the rocket was stable, only to experience skywriting when actually launched.

there is certainly no harm in doing a swing test (well, I guess it is possible to hit someone with the rocket while swinging it around your head, but not if you are careful!). In some cases, even though many scratch designs have unique features (in this case your struts), the rocket “looks” enough like other stable designs that I personally feel sufficiently “certain” that the rocket well be stable Based on experience and mindsim.

https://www.rocketreviews.com/scratch-two-stage-fiddle-faddle-by-kathy-miller.html
i look at your design and I see decent sized fins which are all or nearly all behind the tail end of the motor, which to me (unless your rings are oak wood) pretty much assures CG is gonna be ahead of CP. your ring fin chord lengths aren’t huge, but especially the rear ring is so wide that even a short ring is gonna provide a lot of “dig” into free air with minimal off axis tilt, so lots of stability.

so this bird? It’s cool, it’s unique, but stability isn‘t to me a question. Some of your other triumphs, like the hammerhead or X-wing, THOSE are rockets that definitely need something more than mind-sim before they go on the pad.

you really want KISS principle on this one, keep it light, spit the motor, and go featherweight recovery. Make a bouncy rubber nose cone (shuttlecock?), and your only challenge is tracking it on the way down!

looking forward to lots of good flight reports from you!