Project Juno: 3"/54mm MMT HPR sport flyer

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Well-Known Member
Nov 27, 2011
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NB Canada
Greetings TRF.

This will be a bit of a long introduction, but I do have a question for the community so if you don't want to read the background just scroll to the bottom of this post ;)

It's been a little while since I have built a new HPR rocket. My last HPR build project was the simultaneous construction of a Wildman Punisher and DarkStar Jr. The Punisher (named New Horizons in a nod towards the NASA mission) was my Canadian Level 3 rocket back in 2016. Besides those two, my only other HPR rocket is my Shape Shifter Jr., which was used for my level 1 and 2 certification flights in 2014. In the last few years, all three of those rockets have flown many, many times with only a couple minor mishaps (not counting a motor CATO in the Punisher in 2017 - fortunately the damage was minimal, and it has since been repaired and re-flown several times). While all three of those rockets are still going strong, it is now time to bring some fresh blood to the fleet, and so I present to you project Juno:
This rocket project has evolved quite a bit since it's inception (though yes, I also realize it is just another 4FNC). Before beginning the complicated process of going for my level 4, I knew I wanted to get more experience with level 3 motors. I've had this project code named the "K or L motor project" in the back of my mind for quite a while, and this year seems like a good time to finally build it. The design for Juno originates from a different MPR project that never made it past the planning stages, so I decided to take the design and upscale it (with some tweaks) for this project.

To date the largest motor I have flown is a mid-size J in New Horizons, so Juno is designed with the intent to fly it on a K (but will still mostly see a diet of I and J motors for budgetary reasons). Running sims in OR currently suggests similar performance to the Punisher, though I'll feel more comfortable about pushing this one a bit harder since it will include a few features that I have not yet incorporated into my existing fleet including redundant dual deploy and GPS tracking (though I am planning to add GPS tracking to the rest of my HPR fleet by next year).

Like New Horizons, the name Juno was chosen as a nod towards the mission currently in orbit around Jupiter. I like to name rockets after existing spacecraft and the name "Juno" has a nice short punch to it. Naming it Cassini was also considered, but I might save that for my level 4 project someday.

Up until recently, the design for this project was actually based on the Wildman Interceptor AAD-98, but pricing things out made it clear that going that route would result in the project being somewhat over budget unless I ditched the GPS tracker which I am not willing to do.

So now on to the current design: The plan is to have Mike at MAC Performance Rocketry kit this up for me. I've already been in contact with him and he has agreed to do it for a good price (including a FWFG NC). Ideally, I will have this ordered by the end of the week. GPS tracking will be accomplished with a Featherweight tracker in the NC, and dual deploy will be done using an RRC3 with an RRC2+ serving as backup, both of which will be located in the mid-section av-bay. I'll be using a 4' Rocketman chute for the main and a small (TBD) Top Flight chute for the drogue. Motor retention will be accomplished with an AeroPack (probably the flanged version 'cause it looks cool and the rear CR/thrustplate on MAC kits provides a good place to bolt it too). General construction will be accomplished with some left over Rocketpoxy from my previous projects. I am also planning to foam the fincan with PML 2-part expanding foam. The plan is to get some custom Stickershock once I've finalized the paint scheme.

Currently I am targeting the maiden flight for labour day weekend next year when our club hosts its big annual HPR launch.

QUESTION about the fins (aka please critique my design):
So before I give the go ahead to Mike to kit this up for me, I'd appreciate any feedback from others on my design.

For me, my main concern at the moment is the fins: I've never worked with canvas before, so perhaps I am just underestimating it, but currently my fin span is 9 cm (or ~3.54 inches) which seems like a bit much for 1/8" fins going over mach (which the OR sims say this rocket will do on a K). I would also like build this rocket strong enough to survive a flight on an L (which could potentially push it above mach 1.5) since my launch area would be capable of supporting a flight like that. The current fin design was chosen partially for aesthetics and partially out of a desire to keep the stability of this rocket at or around 2 cal when loaded on the pad with pretty much any motor I could put in it. As such, I do not really want to make the fins smaller, but would it be worth increasing the thickness to 3/16" (or requesting that they be made out of G10)? I apologize if I am over thinking this, but this will be my first attempt at breaking mach and also my first self designed HPR so I am being a bit cautious.

On that note, is there any rule of thumb to consider for designing fins that will break mach? I've seen folks on here using Aero Fin Sim software to help determine the speed at which fins will fail, but it's Windows only (Mac user here) and only available in the US so that's not really an option for me I guess.

I've attached the latest .ork for those interested. Thanks for looking and stay tuned!


  • Juno.ork
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Following up on my question about the fins. A bit of digging through the forum turned up a spreadsheet that supposedly calculates the max safe speed for a given set of fins (see this post) based on a model published in an issue of Apogee's peak of flight newsletter. One key factor that I am missing though is the shear modulus for canvas phenolic. A bit of searching online turned up nothing, but based on a value I found online for Young's Modulus, I guesstimated that it would be somewhere around 400,000 psi. Punching that value into the spreadsheet for 1/8" fins suggests that problems may start to arise just below mach 1... Opting for 3/16" thickness suggests things will be good until around mach 1.8 or so. (Using a similar analysis for G10 based on these values suggests that 1/8" G10 fins would be good up to around mach 1.5, and 3/16" would be good to over mach 2).
Are you asking Mike for canvas, fiberglass, or carbon fiber fins?
At the moment the design I sent him has 1/8" canvas fins. My back of the envelope calculations have made me think I definitely want to upgrade that, but am a bit torn by either asking for 1/8" G10 or 3/16" canvas.
Update: I ordered this about a week ago :). It's in the mail and should be delivered by the 22nd. I made a few changes to the fin shape, but decided to ultimately stick with 1/8" canvas for the fins after getting Mike's opinion (he's flown 1/8" canvas to Mach 1.4 without issue). I also shortened the booster by 1" and the payload by 2" so that it could be manufactured out of a single length of tube.

I spent some time today playing around with ideas for the paint scheme in OR... my favourite so far is pictured below, and is somewhat inspired by the BSD/SBR Thor (which I just think is such a good looking design) and the Space Shuttle launch stack orange and white look. Currently considering flipping the white and orange (and switching the black decals to something metallic), or switching the orange to a different colour (maybe a red or silver).

Screen Shot 2019-10-12 at 7.34.42 PM.png


  • Juno_FINAL.ork
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