Three's company - 3F3NC scratch MP capable slightly-odd roc

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Nov 8, 2021
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Been mulling an idea over for a while and thought it might as well see the light of day. Maybe its an interesting idea, maybe its a terrible idea, maybe its been done many time before!!
This could naturally fit as an odd-roc, but part of the aim is to make something capable of big acceleration with dual/multiple deployment so decided to put it here in MP and fingers crossed get some advice!
The concept is basically a bit like the LOC viper 3, but without a forward section. Or looking at it the other way, like an Estes sidekick but with another airframe. So it is going to be three joined minimum diameter airframes, each with its own nosecone, but with common fins. The first version will be three times 18 mm, but who knows maybe a three x 24mm could be a future project if anything good came of the prototype version.
The things I like with this are....
i. the potential to use anything from (possibly) a single A motor, up to low E impulse with three Cs... or even Klima D motors for F impulse (lets shelve this one for now tho...) or indeed any combination of the above. So it should be flyable in almost any venue.
ii. using different motors with different delays to deploy different recoveries... most usefully... a drogue on a short delay, then a chute (and either a second chute or drogue in whatever order) so as to ensure good recovery while limiting drift, particularly for the higher impulses.
I fully realise this is not going to be a high efficiency design!
Here is a very rough mock up with rubber bands and cardboard fins to show the approx idea...
Here is a very rough, first iteration OR SIM, the NC is nothing like what I have in mind and i realise that OR will probably not simulate much of the highly unconventional airframe... possible nothing at all in which case its just giving me a vague idea of stability for a given lenght and fin dimension, which would still be a good place to start.
first on three A8s which looks OK
Here is the same rocket on three D9s, which I already definitely firmly shelved and not sure why I'm even doing the SIM...
The stability would need to be much higher esp if it was actually going to attempt to go anywhere near that velocity range (firmly shelved like I said), but something to work on. This doesn't include any chute mass as yet
There are clearly a number of challenges to work on, particularly to get it MP capable.

NC design - definitely the big one, as it needs to be aerodynamically sensible as well as functional for ejection. I am favouring individual NCs rather than a compound one because I think they will eject more cleanly (can even be ejected without retention on their own streamer). Possibly off-centred tips so they create a better aero. Or could do something wacky like rotated to make it spin/corkscrew/rip to shreds. Maybe the body tubes are cut at an angle to help throw the NCs out when the eject. Probably a 3D printing job to get them symmetrical.
Joining the fins very robustly - seems like there is plenty of surface area to work with so is achievable but needs some thought, maybe some small inserts. Epoxy will help but need to be careful with more aft mass.
The body tube transition - the SIM confirms my guess that the aft mass loading will need the airframe to be quite long, so assuming I use standard BT, then its two lengths. The join is an obvious weak spot... one solution could be to stagger this (can be seen in the rubber band mock up), which would bring the total length down a little but I think would be much stronger). Plus maybe another 3D printed insert to help with alignment, like an internal rod to get the separation spot on and help with an epoxy bead.
Thrust steer - because the axial separation is small, and the fin area is large my guess is this will not be a big problem. Plus I can easily test at low power by just flying it on e.g. a single C. It would make sense to impart a very slight motor cant by having a larger separation at the aft (ie from the fin insertion between the tubes) than at the fore.
The motor mounts don't seem too difficult although there could be some nice detail for the retainer.
Lots of design decisions to be made about the size and placement of the fins, to be iterated in the SIM, plus mateial etc. Currently thinking a pretty standard but large MP type shape made of ply with a small aft overhang, not far off the SIM above.

Thats were I'm up to for now. Will try and make some sketches of some of the ideas for details as above. Very open to feedback and ideas!

Also the name is a working title... so far its either that or "three's a crowd". was also thinking of trio like the old school choc bar. Prob some better ideas out there tho
Why not 3 nose cones?
You should be able to simulate this pretty easily in OR just by using pod's.
The Estes Side Kick is similar to what you're building here.

Looking forward to watching this build. :computer:

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The sidekick is definitely one of my starting reference points (also LOC viper) altho my guess is it will need much stronger bonding between the airframes than side kick if it is going to MP.
With the OR SIM I couldn't find a way to put on three NCs... Sounds like I need to spend longer fiddling. Good to know it's possible!
My thinking with needing a more complicated NC set up than three standard-ish NCs is that to make the airframe bond strong enough (as above altho accepting I may be over doing this) it will need to basically be beaded directly between body tubes at the fore. This would then not leave clearance for each NC to pop off separately. Particularly if there was a small cant in the body tubes.
That's not too say that the NCs would actually have to be massively different. Even just removing the shoulder on the points closest the other NCs so they don't bind might be enough (altho id be worried until id tried it). One for a static ejection test maybe.
I did another very basic rubber band mock up on this to show the idea about a simple outward angled NC (see picture) and this gave me another idea. This is to go without any NCs at all, since the angled cut off is probably moderately aero. With a bit of luck they might even hum!! And the ejection charges would just chuck the recovery out the front.
This would then be 3F0NC...
I'm thinking of just giving it a go like this because it simplifies the whole thing so much I could just get on and glue it together to see what happens...
Right this project is BACK on the build table after an extended hiatus....
I am going short with much bigger fins, heres the revised sim.
Thats for a C/C/B config to give a low E impulse. It is very stable with only one or two motors loaded and should go fine with a B, and maybe just about with an A (soft ground!). It still shows as stable with 3 Ds but there is no way I am ever going to try that. Definitely not.
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Been cutting fins to the sim pattern (more or less) from 1.5 mm ply
Put in basic leashes on each 18mm airframe
And motor mounts. Was going to try a neater retention using a central bolt but couldn't find quite the right size and didn't want to hang around
The fins were first tagged onto a dowel then a thin epoxy fillet. Feels pretty strong
Assembled just with rubber bands for now but ready for the airframes and fin assembly to be epoxied together, with thin fillets at the aft and thick ones at the front to help with mass distribution. Going without any nosecones at all to start with but can always add them on esp if it needs extra fore mass
Decided that if it's supposed to be MP capable then it might as well look like it's MP capable so fitted rail buttons to one airframe. Will fit a standard rod guide on a different side for LP launches.
First set of epoxy fillets at the fore done and it's starting to feel ok

Currently weighing in a little over 70g with the aft epoxy still to do and no recovery included yet

Been thinking about the lack of fore mass and noticed a thread about using chalk dust for ejection tracking, so might consider that to shift the CG forward and also give an ejection height as the minimum diameter means it's too small for my altimeter.
Finished application of several epoxy fillets in different areas, with thick ones at the fore + mid and medium in the fin joins with a thin one right the way down each tube join. Mostly ok with one ugly one but will keep that turned to the back in photos. Ran a little epoxy around the outer part of each front port (ie where the nosecones would be if there were any) to strengthen this against zippering. Filled all the remaining spirals.
Feels good and solid.
My only query at the mo is whether I should paper or coat the fins to strengthen against flutter. There is a little flex in the 1.5 mm ply but it's quite strong. I guess a layer of epoxy would not hurt.
Got it on the pad yesterday after a bit of faffing with packing the recoveries which felt tighter than I liked in the min diameter tubes.
Went for two out of three for the first flight with B motors, one slow with a short delay on a streamer drogue and one fast with a long delay on a para. Double checked CG and all looked good.
All went off nicely, debatably a touch of thrust steer after the fast motor burnout, but maybe more an extended stall combined with wind cocking as there was a breeze.
Both recoveries deployed fine and altho drogue streamer didn't do much the para did the trick with at least a second to spare.
View attachment VID20231217115921.mp4
Will b trying a set of Cs for a low E impulse and probably a fast single B with a short delay, pretty sure it won't go on an A. I don't promise not to try a set of Ds one day 😂.
Need to replace the streamer with a mini para or even just a kevlar square to at least slow it a little
Curious - with nose cones off, and flying thus configured, does this whistle? Three different tones (depending how deep the p-chutes) are packed in?

Does anyone build rockets for whistling capabilities? I often hear them whistle, but I think it is an inadvertent accident that they do that. I have been looking at making fippels but have yet to settle on method and material

Several years ago, the cover of "Sport Rocketry" (March-April 2017) had a picture of a three tubed rocket cluster and the tubes twisted around the center. They spiraled up to three nose cones.
Now that I looked at the photo, three 18mm tubes spiral up around what looks to be a 38mm tube, and sort of the same idea you have here.
Yours is less bulkier.
Are you starting with 18" or shorter tubes?


Thanks for the interest.
It has not yet whistled for me! But then so far I've not reached very high velocities and I think it would need to be going at quite a lick to make any noise. I'd be happy if it did so fingers crossed when I fly with a bigger thrust.
My guess is that the geometry, especially the short length of the free tube (because the recovery is quite close the open end) will not tend to produce a note easily. If I flew it will try motors and the third tube empty i think it might stand a chance (lower note shd require lower energy excitation off the top of my head). The relation can be calculated I think but I've not got round to trying it.
I've read a few threads on this and the most reliable method seem to be having a carefully sized gap between a pair of fins.
The triple spiral sounds great altho challenging to scratch with the curved tubes. Sounds like it might spin (for sure if the motor mounts are angled). Will have a look around for that issue.
I have used the standard 18" lengths minus a tiny bit for the angled cut off that passes for a NC.