Modified Go Devil 29 build thread

[GrouchoDuke]

I had already started on my Mongoose 29 when the Go Devil 29 came out and then went on sale for Black Friday. I couldn't resist the $29.99 on-sale price tag, so I picked one up. One of the great things about the Mongoose 29 & Go Devil 29 stock designs is you can fly any motor that'll fit in them and not worry about CG. Since they're min-diameter rockets, they go pretty high even with big fins and a long body tube.

The kit comes with a red fiberglass body tube, black 1/16" G12 fins, a red & black Von Karman nosecone, and a 1/16" bulkhead. I don't plan on using the bulkhead (it's sized to fit inside the coupler), but I'll use everything else. The kit does not come with any recovery gear, so you get to pick that yourself. For now, I have a 12" chute in the sim file. That fits well in the nosecone, but a 15" chute might help minimize ground rash on landings...if it fits.

For my Go Devil 29 I wanted to keep the ability to shove any motor in it, but I also wanted to modify it a little bit to increase performance. I wanted the avionics to be drop-in swappable with my Mongoose 29, where I put an EasyMini, TBeacon, GPS, battery & Featherweight magnetic switch inside the 3" long coupler tube. Since the body tube is fiberglass and doesn't have the RF restrictions that a carbon tube has, I also wanted to have room for my Eggfinder in the body. The Eggfinder and EasyMini don't stack well together inside a coupler tube, so I made sure the body tube was long enough to accommodate any combo of EasyMini/TBeacon/Eggfinder.

To keep in line with the "shove any motor in it" mindset, I didn't taper the back end of the body tube boat tail area. I'm going to use the back end of the body tube as the thrust ring (i.e. where the motor pushes). I'm using friction fit for motor retaining. Spiral wound masking take seems to work well to friction fit motors on rockets in this size/speed regime.

I messed around with the design for a while and came up with slightly reduced chord fins and I reduced the length of the body tube from 30" to 21". My Mongoose avionics will slide into the coupler and if I decide to print a new sled for an Easymini & Eggfinder, that'll fit too. It's likely that other small altimeters would fit fine too - with their own custom sled, of course.

The 3d printed sled itself won't be load bearing, but the recovery 300# Kevlar harness body loop will pass through it. The sled will be held in by a single #3 set screw that they sell at Lowes. I'm 3d printing a recovery harness mount for both the body & the nose cone. There will be a loop of 300# (just less than 1/8") Kevlar for the nose cone and another loop for the body. A single line of 300# Kevlar will connect the two using Palomar knots. The body recovery harness mount will have a hole in the middle of it so that I can fit a rod through it to push out motors in case they get stuck.

To keep the avionics bay mostly free of ejection charge and motor gasses, I'm putting a removable plate in between the motor & the body recovery harness mount (I'll likely just 3d print a 29mm diameter, 3-6mm tall disk to put in there). On the other side, I'll poster putty the holes in the top of the avionics sled and only leave a small hole to equalize the pressure between the nose cone and the avionics bay. I'm also doing some final research on using Corrosion-X on the avionics to hopefully cut down on ejection gas corrosion. I'll tape off any sensors (e.g. the altimeter chip) before I do that, but from the tests I've seen that seems like a pretty good way to get some extra protection since it won't be perfectly sealed.

For static ports, I'm planning on 3x 1/16" holes. This is way more than needed for the small payload & avionics area (only about 10 cubic inches), but I like the idea of having 3 static ports for overall robustness.

For nosecone retention, I'm just going to friction fit it. The max deceleration of the rocket (just after burnout with an I224) is about 13.5Gs. Those nosecone & stuff in that area only weighs a couple ounces. A 5-6lb nosecone separation force leaves a good margin built in even without including the drag force on the nosecone. A fairly small BP charge (0.1 or so) should be plenty to pop off the nosecone.

It'll be a single deploy setup. With the high altitudes it'll do, that works for me here in the wide open desert. It'd be severely limiting if you have trees & limited footprint to contend with.

For the fins, I plan on doing only minimal shaping. That's a tradeoff to keep them from getting dinged up too fast on landing. Knife edge G12 trailing edges probably wouldn't last too long against the rocks in the desert. I might do tapered leading edges with some high temp epoxy for reinforcement. Maybe.

For stability, I aimed for 1.5 calibers subsonic in OpenRocket for the worst case motors. I'll double check RASAero II before I cut any parts. Based on what I've seen between them, that'll keep it plenty stable up to the Mach 1.7 that an I224 should push it to.

My local Tripoli prefecture (why can't they just call it "club?") has a standing waiver up to 4800 ft AGL and we can get call in windows up to 15,200 ft AGL. On a G80 it sims to over a mile, so for any high power motor it'll require a call in window. With the non-super optimized fin design & length, it'll stay below our max altitude on everything that'll fit in it. It'll still do fun launches on F & G motors (probably E too) for non-club launch days.

Here's the OpenRocket file as it stands today:
https://www.dropbox.com/s/797hgtcm1q9233u/Go Devil 29 - GrouchoDuke v2.ork?dl=0

I don't plan on building this one super fast, but I'll post here as I make progress. I'm looking forward to another fun rocket in my quiver!

 

[GrouchoDuke]

More OpenRocket design tweaks today. I weighed all the components and added the weights (and/or densities) to the sim file. Since I don't plan on keeping this rocket polished, I dropped the finish down to "smooth paint." As expected, that cut the predicted max altitudes a bit. It's still over a mile on a G80 and over 11k on an I224. That's not overly impressive, but great for my "kick around" min diameter build.

The weight prediction is now just under 10oz without the motor & using the EasyMini/GPS/TBeacon avionics instead of the Eggfinder.

OpenRocket shows this config to have plenty stability (1.9 calibers for the worst case motor I checked), so I thought about making the fins a bit smaller. I'm keeping them this size though -- they'll be stronger if I keep the tips fairly big. Gotta have this thing survive bounces thing off the desert floor! That stability is without any nose weight, so that's nice too.

I'll try to check it over in RASAero II tonight, but I'm not expecting any issues to come up.

Here's the latest OpenRocket file:
https://www.dropbox.com/s/g262cq7zwyx1kmb/Go Devil 29 - GrouchoDuke v3.ork?dl=0

 

[GrouchoDuke]

I checked the Cp results in RASAero II. Both the standard and Rogers modified Barrowman results show a more aft Cp than OpenRocket does, although not by much. So, for this specific rocket, I can use OpenRocket's Cp results as a worst case all the way to the expected max speed of Mach 1.7. I tweaked a few internal things around in the design & started cutting parts.

Sadly, I don't have a real rocket workshop...I have more of a garage & office with a bunch of random tools. So, for accurate parts I lean on my 3d printer a lot. I made a fin marking & sanding guide that's exactly the shape of the new fin design. I can slide each fin into the guide to mark it for cutting. I cut the fins about 1/16" big using a Dremel diamond wheel. After that, I put all three fins together into the guide and sanded them to size. That makes quick work of getting them all within a hair of each other.



For the body tube, I 3d printed a cylinder that slips tightly on the outside of the body tube (I uploaded it here for anyone to modify & use: https://www.thingiverse.com/thing:2022914 ). I used that to mark the tube. From there, I cut it with a diamond wheel and sand it to size using the 3d printed cylinder to verify that things are square. I double checked the squareness of the back of the body tube with a CTI 29mm motor case & rear closure.



I used a shortened 3d printed fin guide to hold the fins in place for a photo of all the parts. Progress!

 

[ColumbiaNX01]

Thats cool. I like that. Nice toys!

 

[ksaves2]

So you are going to use the Easy Mini deployment altimeter and the Tbeacon? It looks like you will have to add a GPS receiver to the Tbeacon to get it to work. Do you have all the bits to test fit? I will warn you if you go with a small GPS receiver, the
performance is not that good. You'll need a battery for the GPS and a battery for the Tbeacon or a compatible GPS chipset you can power both the Tbeacon and the GPS off the same battery and use a larger capacity. Best of luck. Kurt

 

[GrouchoDuke]

Thats cool. I like that. Nice toys!

Thanks!!

So you are going to use the Easy Mini deployment altimeter and the Tbeacon? It looks like you will have to add a GPS receiver to the Tbeacon to get it to work. Do you have all the bits to test fit? I will warn you if you go with a small GPS receiver, the
performance is not that good. You'll need a battery for the GPS and a battery for the Tbeacon or a compatible GPS chipset you can power both the Tbeacon and the GPS off the same battery use a larger capacity. Best of luck.

Thanks Kurt. Yeah, I have everything wired together & tested on my Mongoose 29. I haven't flown the TBeacon yet though. The EasyMini is for deployment & altitude recording. The GPS I'm using is this one: https://www.readytoflyquads.com/micro-ublox-m8n-gps-w-18x18x For that price I'm not expecting that it's a real UBlox M8N, but it seems to work well. I've tested the GPS in all orientations and with things covering it up - so far so good. I don't need it to work throughout the flight, so the typical high speed dropouts won't be a problem. I don't plan on pinging it for coordinates until after the EasyMini has deployed the parachute. The TBeacon will work as a normal radio beacon if the GPS doesn't work, so I should be able to fox hunt to it if I have to.

I have tested the same GPS model on my Quicksilver where I paired it with a quadcopter flight control board and an FrSky XSR transceiver (that talks to my FrSky Taranis r/c controller on the ground). At least on the (much slower) speeds of the Quicksilver, I've gotten great telemetry with coordinates throughout the flight.

I tried quite a few different batteries. Because of the tiny space in my (cut way down) Mongoose 29 build, I went with a single battery for everything. The TBeacon has inputs for its own battery, but there just wasn't space for it. Even the 1S 60mAh battery that comes with the TBeacon didn't fit in the avionics bay of my Mongoose. I'm using this 1S 150mAh battery (with the protection circuit removed) - https://www.adafruit.com/products/1317 It lasts about 1.5 hours with everything running and a moderate amount of TBeacon transmissions.

For other TBeacon info, I'm also running a beta build of his version 0.57 software. The big change for me there is that all of the "startup" transmissions it does are at about 1mW instead of the full 100mW power. With this firmware, it doesn't transmit at 100mW until I ask it to via a tone from my ham handheld.

It's not perfect, but it fits. One and a half hours means I can't mess around on the ground before launch & I'll need to high tail it out to find the rocket. For high launches, I'm setting a 30 minute no-go timer once I turn things on. That should be easy with my local club since our launches aren't that big. That could be an issue at a huge launch where your rocket had to sit on the pad for a while though.

This build will have much more room for electronics than my Mongoose 29. I should have room for a much bigger battery if I decide to put one in. I do like the idea of being able to swap my avionics back & forth between my Go Devil & Mongoose though, so we'll see. I might pick up another EasyMini & have a dedicated avionics sled for this one with the Eggfinder & a bigger battery. For now, I'm planning on first flight with the TBeacon though.

Since running lipo batteries to zero is really bad for them, my plan is to ditch the battery if it gets too low. I'm hoping I can get to the rocket before it goes dead though.

EDIT: I'm toying with designing & making my own GPS tracker to fit in a 29mm coupler tube using a LoRa telemetry radio & small UBlox GPS. I'm kinda hoping someone else will do it first & make it available for cheap.

 

[ksaves2]

Ok, I see the device can be set to transmit the position based on time. You might want to do that but would need to keep in mind that will drain your battery faster. Can it be set to transmit every so many seconds or is it only going to do it periodically
every 30 minutes? If you have a crazy sight unseen flight, unless you can copy down several of the lat/long positions quickly to get a trend line before the rocket lands, you could conceivably lose the whole shebang if you can't get into the ground footprint of the tracker and have no idea where to start. Getting a tiny GPS tracker to perform is going to be a compromise.

As I understand, the device sends out a 100mW signal? If so, that's pretty powerful and is that the power level the voice announcement is sent? If it's at that level, I'd get your ebay setup completely put contained ematches on your Easy Mini and turn everything on in that confined ebay and ping that Tbeacon and get it transmitting as much as possible. If the Rf doesn't cause the Easy Mini to cycle or pop the matches after 30 minutes or more, you should be able to fly. A 1mW signal is pretty
low and might not cut it for tracking. It would mean you would have to ping the rocket in flight to get the high powered position, it would have to hear you and you'd have to receive the position.

The Achilles heel is the fact you have to ping the device and perhaps it won't send a "timed" position out more frequently than every so many minutes. If you could get it to transmit lat/long every 5 or 10 seconds that would be helpful.

One other bit of advice, you might want to try to fly it within sight or with a short "out of sight time" so you increase your chances of a visual on descent. You'll be able to fully test the reliability of the system and your reception rates thus
increasing your confidence before really punching it up high. You sling it up there and there's a tracking problem, it's just as bad as if you didn't put a tracker in there in the first place. Be careful

Also, use a high quality receiver with a true signal strength meter. Th UV-5 brands are full deflection if a signal is received and full off if no signal. There is no intermediate readings. Kurt

 

[GrouchoDuke]

Thanks for the tips! I have a couple different radios I'm using for reception. One of them is an SDR that's connected to a laptop that'll record all the audio. That way I can review the audio later if I need to. My ham HT is a Baofeng UV-5X3. It's a fairly new model and seems to work pretty well. Granted, it's not the best out there by a long shot, but reports are that it's a lot better than the older UV-5 models. I've done some range tests with it (with squelch on & off) and it seems like when the TBeacon is in the air, its 100mW transmissions will have effectively a "forever" range. Normal ham CQs with it always get really positive results from people on both simplex freqs & repeaters. On the ground with the TBeacon will be different. I'm definitely planning on getting some coordinates from it while the rocket is on its descent.

I don't have a truck-mounted ham radio...that might be a nice addition for rocket hunting.

Yeah, the TBeacon can be set to transmit pretty much whenever you want. It can transmit every X seconds with a variable delay on how long it waits to start transmitting after power up. You can also vary the number of times it reads you the coordinates for each transmission cycle it does. After some practice listening to it, it usually takes me once to copy down the coordinates & I use a second time to verify them. The 1mW (1.3, I think) output is only for the startup transmissions it does (like when it first turns on and again when it gets a GPS lock). I also modified an iOS app of mine ("GPS Location") so I can enter the coordinates & get a trend line. It'd be pretty easy to do that with any handheld GPS too. The TBeacon doesn't transmit altitude, so that's a bummer. For any timer-based or ping-based transmission it does, those are at 100mW.

I haven't tested it yet doing 100mW transmissions right next to the EasyMini & e-matches. That's part of the plan though. Having this Go Devil build be able to accept any motor is also part of my test plan for the avionics since I pretty much can't fly anything besides a P29-6GXL on my Mongoose. It'll fly 3k or so on a small F. On the dry lakebed here, it'll be easy to find that even if the tracker doesn't work.

Note: for people reading this in the future, the low power startup transmissions on my TBeacon are currently only in the beta version of the 0.57 software. Please check this yourself with whatever software you're using.

 

[ksaves2]

Probably first time you punch it hard, have it set to beacon as frequently as you can tolerate say every 5 to 10 seconds. Get a copy on some positions for a trend line if you need one. Other thing would be to get an idea of a flight profile with
multiple simulator runs. You can run a stopwatch and if the rocket appears to be flying the profile (and not get cocked off at an ungodly angle) you can try to ping after the expected apogee to get a fix and ping sometime after the expected main opening.

If of course you get a visual at any time, that's icing on the cake.

The problem of automatic voice transmission at a 100mW output might drain that 150mah battery fast. if doing high rate reporting, let the GPS get a local fix if possible from another power source especially if it takes minutes. Once it gets a fix, shut it
off, connect up the battery in the ebay to the Tbeacon/GPS and turn it on at the last possible minute. Once it gets a fix again (it shouldn't take very long now), launch it as soon as possible so you don't run out of battery capacity. Once that battery starts draining, the power output is going to drop. Even with 50mW that ground footprint on level ground should be pretty good. Again, I'd suggest you work up to more aggressive flights once you're convinced the configuration is reliable for a totally sight unseen flight profile. With the small size of that rocket, it will be easy to achieve a total disappearing act.

You have one advantage over a digital data reporting system like the NMEA trackers use like an EggTimer or Missileworks. If your received "analog" voice signal is a little scratchy, your brain is pretty remarkable in copying and decoding that audible report. You could even use headphones to improve your decoding ability! Digital data? Unless it's "good enough" to decode you don't see the report or in my case I don't see the icon on my live map move.

Another thing. If your flights really make it to the extreme, borrow a 70cm band Yagi antenna for your receive station. If you have some idea of the direction of the rocket, you point it in the general direction and
the higher effective output of your H/T will increase your chances the Tbeacon will "hear" your request for a ping. That can offset the effects of polarization and differences of antenna position if the rocket
is flopping around under drogue. The Yagi also increases your ability to hear the Tbeacon.

I like to get my data on a live map whether APRS or using NMEA GPS trackers but not "every" single position gets plotted. More than enough make it through though so I've walked right up to every flight.

So far, no one has made a small enough streaming digital GPS tracker for small rockets and this Tbeacon , though designed for R/C, looks promising for your locating purposes. Kurt

Kurt

 

[dhbarr]

Wow, so Tbeacon + Mini8 = sub24mm GPS? Definitely watching this one!

 

[GrouchoDuke]

...You can run a stopwatch and if the rocket appears to be flying the profile (and not get cocked off at an ungodly angle) you can try to ping after the expected apogee to get a fix and ping sometime after the expected main opening.

Yeah, I'm using sim data, aviation winds aloft data and other things to prep for flights. My local club's FAA waiver only has a mile radius at higher altitudes, so it gets pretty tricky. Lots of planning. I have other semi-close options for launching, so that can help too.

Even with 50mW that ground footprint on level ground should be pretty good. Again, I'd suggest you work up to more aggressive flights once you're convinced the configuration is reliable for a totally sight unseen flight profile. With the small size of that rocket, it will be easy to achieve a total disappearing act.

The developer of the TBeacon says there have been tests in urban environments without line of sight out to 3km. Others have reported more than that inside a forest. Out here in the desert, it should do pretty well even when it's on the ground. We'll see when I do it!

If your flights really make it to the extreme, borrow a 70cm band Yagi antenna for your receive station.

I've thought about picking up one of the Arrow Yagis, but haven't done that yet. I did ask the TBeacon developer to add APRS ability, but that's not in his current plan for it. Like you said though, the digital range would likely be less than the voice range. Either would do fine for airborne range though. User reports of airborne tests of LoRa sound very promising for really long telemetry range. (Hint, hint, hint....a tiny EggLoRaFinder would be niiiiice.)

Wow, so Tbeacon + Mini8 = sub24mm GPS? Definitely watching this one!

Well, I can't promise it'd fit in a 24mm body tube. I haven't checked that specifically. It does fit inside the MadCow 29mm coupler though (with about a 25.4mm ID). That fit is with a battery & EasyMini in there along with the TBeacon, GPS, magnetic switch and 3d printed sled. I have it all fitting in less than the 3 inch coupler length on my Mongoose. Cramming all those things inside that coupler was just about the hardest part of my Mongoose build. It's tight!

 

[dhbarr]

Well, I can't promise it'd fit in a 24mm body tube. I haven't checked that specifically. It does fit inside the MadCow 29mm coupler though (with about a 25.4mm ID). That fit is with a battery & EasyMini in there along with the TBeacon, GPS, magnetic switch and 3d printed sled. I have it all fitting in less than the 3 inch coupler length on my Mongoose. Cramming all those things inside that coupler was just about the hardest part of my Mongoose build. It's tight!

I was just going based off the published size of the constituent components: 18x18, 20x30. An antenna-up 3D NC sled seems super achievable.

 

[GrouchoDuke]

I was just going based off the published size of the constituent components: 18x18, 20x30. An antenna-up 3D NC sled seems super achievable.

Yeah, it seems like it's doable to put them in a nosecone pretty easily. Just remember that a 70cm band quarter wave antenna is about 6.8" long. For 900MHz, that'd be just over 3", but the TBeacon only talks in the 430-ish MHz range. That's why I'm poking mine out the top of the av-bay & letting it get some fresh air on the way down.

Another thing with the TBeacon for tiny space applications is the beeper it has on it. It sticks up about 4mm on one corner of the board (grrr). The other side of the board has the radio daughter board (RFM22), so the overall thickness of the TBeacon is about 7mm. You could maybe grind down that beeper to save a mm or so, but you can't desolder it. On the plus side, the beeper is very loud & you can set it to go off whenever you ping it with another radio.



In other news, I printed the nosecone recovery harness mount. It weighs just over 2g and gives a ton of surface area to tug against. I'll slop it in place with epoxy one of these days...

 

[ksaves2]

Yeah, I'm using sim data, aviation winds aloft data and other things to prep for flights. My local club's FAA waiver only has a mile radius at higher altitudes, so it gets pretty tricky. Lots of planning. I have other semi-close options for launching, so that can help too.

AKS: Great idea. The better you develop a picture of the expected flight profile the better off you'll be. Of course you likely don't have to go through all that trouble if you use a motor that keeps the rocket in sight or nearly in sight the first testing flights.



The developer of the TBeacon says there have been tests in urban environments without line of sight out to 3km. Others have reported more than that inside a forest. Out here in the desert, it should do pretty well even when it's on the ground. We'll see when I do it!

AKS: You likely know that the 70cm band propagates better than the EggFinder 900Mhz/33cm band. Plus the Tbeacon 100mW output should carry well.


I've thought about picking up one of the Arrow Yagis, but haven't done that yet. I did ask the TBeacon developer to add APRS ability, but that's not in his current plan for it. Like you said though, the digital range would likely be less than the voice range. Either would do fine for airborne range though. User reports of airborne tests of LoRa sound very promising for really long telemetry range. (Hint, hint, hint....a tiny EggLoRaFinder would be niiiiice.)

AKS: Do your preliminary flights with a duck antenna and see how it goes. APRS takes another chipset likely and the developer proceeded on a different path. Might not be able to keep the device as small as it is. If it's comparable to a Tele-GPS not much gained unless it were as reliable and cheaper.

Well, I can't promise it'd fit in a 24mm body tube. I haven't checked that specifically. It does fit inside the MadCow 29mm coupler though (with about a 25.4mm ID). That fit is with a battery & EasyMini in there along with the TBeacon, GPS, magnetic switch and 3d printed sled. I have it all fitting in less than the 3 inch coupler length on my Mongoose. Cramming all those things inside that coupler was just about the hardest part of my Mongoose build. It's tight!

AKS Looks good. Kurt

 

[GrouchoDuke]

Do your preliminary flights with a duck antenna and see how it goes. APRS takes another chipset likely and the developer proceeded on a different path. Might not be able to keep the device as small as it is. If it's comparable to a Tele-GPS not much gained unless it were as reliable and cheaper.

The TBeacon has a CPU on it already. I think it's just a coding problem to add APRS. Doable, but it'd require other hardware on the ground (an HT-to-PC USB dongle works fine, so does a USB SDR connected to a smartphone or computer). As long as the TBeacon survives the 70+ G launches on my Mongoose, it'll work fine for what I need. I'll put real time telemetry in my bigger rockets. I'm just happy to have a way to get any GPS coordinates out of a 29mm rocket.

 

[GrouchoDuke]

Fin work today. I cleaned up the body tube & fins with denatured alcohol. I then sanded a simple roundish leading and trailing edge on the fins.



I masked the area around where the fins attach, then sanded the body tube & bottom of the fins a little (good info on epoxy sticking here: https://www.rocketryforum.com/showt...what-your-government-doesn-t-want-you-to-know ). My 3d printed fin guide got a triple check on its fit. The fins are now doing the mating dance with Proline 4500 and the body tube.

 

[GrouchoDuke]

I got some private message questions about things I should have talked about here. Since they were similar questions to what I had when I started down my min-diameter rocket building road, I figured I should post in the thread what I'm thinking and what I learned from digging around this forum & other places...

Why rounded fins?

OpenRocket isn't the best for estimating the drag on different airfoil shapes. OpenRocket also warns you that its supersonic calculations may not be accurate. RASAero II has many more options for fin airfoils. I don't know for certain that RASAero's fin drag predictions are better, but I expect that they are given all of Chuck's work on RASAero II. For me, OpenRocket is easier to use for design tweaking & big picture optimization. OR also has some auto-optimization tools (as long as you're smart with the constraints you put on it).

Once I have a design fairly close in OpenRocket, I move it to RASAero II to look at Cp and drag predictions. For Cp, I look at both the standard Barrowman and the Rogers Modified Barrowman results. From those, I use the worst case for Cp across the speed range I expect (plus about 5-10% in speed) and make sure the CG will work.

I haven't used RockSim, so it's tough for me to talk about that. I haven't even tried the demo of it. With Chuck's great recent work on RASAero and the fairly-friendliness of OpenRocket (both available for free), I couldn't justify spending the $123 on RockSim. Gotta draw the line somewhere, right? (I may be missing something completely awesome - I don't know. YMMV.)

For this rocket, I looked at the difference that different fin airfoils would make in the performance. This modified Go Devil 29 build was never meant to be a 100% optimized design. I have a Mongoose 29 for that, so I wanted this one to be more of my "beater" min-diameter rocket. I still wanted a little better performance than the stock Go Devil 29, but I wanted to make sure the rocket was fairly strong. The desert floor out here in NV is either hard packed dry lakebed or hard sand with rocks. The difference between rounded fins and an airfoil shape or hexagon shape was only about 400 feet for the biggest motors I plan to put in it (the CTI I224 or I243). Since the rocket will land tail first & could take a hit to the fins, I chose to give up that 400 feet and keep the trailing edge of the fins big. Rounding the edges does help some over leaving them square and shouldn't hurt the strength too much, so I went with rounded.

Aside from that, it only took a few minutes to round the fins. Strong & lazy won for this rocket.

Are fillets enough to hold the fins on?

I didn't do any real engineering on this. All I have is anecdotal evidence from other builders' posts. I dug around for a long time before building my Mongoose 29. Most of what I found here was for 38mm min-diameter rockets. What I gathered is that for low aspect ratio fins for 38mm and smaller rockets, fillets are plenty for up to at least Mach 2. Somewhere above that, you'll have to start considering tip-to-tip reinforcement or other strengthening methods.

That said, how you build the fillets is important. For ballpark drag considerations, there are a couple references and old NACA papers that recommend keeping fillet radius in the range of 4% to 8% of the root chord. That's not a hard rule. There are several things that play into it, so think of it more like a WAG. Keep in mind that when you add fillets, you're basically doing an anti-area rule shaping on the rocket. In general, you can think of bigger fillets as causing more drag. For structural strength, bigger fillets are generally better. So, it's a trade off. I planned my fillets to be in the top half of the 4-8% range.

Solid engineering: TLAR.

Since min-diameter rockets have all their fin joints pretty close to the motor, I'm using a high temp epoxy for everything. I went with Proline 4500, but I really wanted Cotronics 4525. The Cotronics epoxy was just too expensive though (about $100 for their smallest kit). I haven't been able to find a real data sheet on Proline 4500, so it's a bit of an unknown. People on the forum here seem to have good luck with it though. I'd love to see some real specs on it.

For mixing epoxy, getting the right ratios is important. I went with a Gemini-20 scale and I love it. It only measures up to a couple ounces, but that's more than enough for this. Here's where I got it: https://www.amazon.com/dp/B0012TDNAM/?tag=skimlinks_replacement-20

What about flutter?

FinSim is the only hobby-use (i.e. affordable) tool I've seen for fin flutter predictions unless you feel like coding something up yourself. I tried to buy AeroCFD and also get some of the free tools from the same developer, but we got lost sending emails back & forth for his approval process. I gave up (for now?), so I don't have FinSim.

This Go Devil 29 build should top out around Mach 1.7. I kept the 70deg leading edge sweep of the fins. As a design tradeoff, I also left the span the same as stock (36mm). The G12 fins shaped like that should be fine out to Mach 1.7, but I didn't do any engineering on it. For any fast rocket, I recommend doing some real flutter analysis. For this one, I skipped it. (Cross your fingers.)

I don't see a recommended motor list on MadCow's site...which motors do you plan to use?

Madcow just lists "D through H" for this rocket. This rocket should fly well on both mid and high power motors. For me, I don't see much fun in flying it on a D motor (which should top out in the 3-400' range). A small F, like the CTI F36, should push this rocket into the 2000' range. I'm calling that about the smallest motor I plan on using. On the big side, I wanted to be able to put in anything that'll fit. That included all of Aerotech's 29mm DMS motors and everything up to CTI's I224 and I243. It should be a lot of fun to launch motors in the G-H range with lots of smoke. I'm guessing those will be most of the launches I do because of our local FAA waiver restrictions. Motors like the long burn Apogee F10 should also be really fun to fly. On a G80, this should go higher than probably 95% of the launches at my local club...so that seems cool.

So, Madcow says up to H and I'm planning an I224. What's up with that? This is a newly released kit that doesn't have any forum threads (except for this one, I think). I haven't talked with Madcow about how big of a motor they've flown on it. So, I don't have any previous info to go on. The body tube & nosecone are very strong. I don't expect any problems from them. The fins are 1/16" G12. They're the only "I don't know for sure" in the build. Their strength is one of the reasons I decided to keep the fins thick instead of tapering them to an airfoil shape.

What are you going to use for launching?

I don't have a tower yet, so I plan on using a 3d printed fly-away rail guide. I have my own design for one and also picked up one of the 29mm ones here: https://cart.amwprox.com/index.php?option=com_virtuemart&view=category&virtuemart_category_id=69

I was really hoping someone would post some data on a tower vs fly-away guides. I haven't been able to find any though. I'd like to build a tower, but that hasn't made it to the top of my priority pile yet.

 

[AeroAggie]

On behalf of the newbs, thanks for the great write up! Looking forward to seeing this one completed.

PS...got one of those fin guides in 38mm?

 

[GrouchoDuke]

On behalf of the newbs, thanks for the great write up! Looking forward to seeing this one completed.


PS...got one of those fin guides in 38mm?

You're welcome! I haven't seen a Go Devil 29 build thread so....FIRST!


(Talk ta ya more on PM about fin guides.)


Today I started the fillets. I waited to sand the fin & much of the body tube fillet area until just before applying the epoxy. To get started, I picked up a piece of Sharkbite PEX tubing at Lowes that had about a 0.87" outer diameter. That works out perfectly for the big fillets I want. A 5 foot section of the tubing was a couple bucks and it works great to form the radius of the fillets.


I used the tubing as a guide to see exactly how big the fillets would be. Because of the radius of the body tube, the fillet width along the body is slightly different than the width (height?) along the fin. I taped off everything that wasn't going to get filleted and lightly sanded the surface trying to go in all directions (that part is tough when the fins are already on). Like before, I didn't remove much material. I just scuffed up the surface & got all those molecules dancing around. I cleaned off the area with a rag.




Once it was taped & sanded, I mixed a total of 13g of epoxy (11g & 2g, for the 5.5:1 ratio). That was 30-40% too much for two fillets, but close enough. Here's that scale in action. Yes, grams to 3 decimal places.



I slopped on the epoxy and used the PEX tubing to smooth out the fillets.



After about 10 minutes, I removed the tape & did a little bit of cleanup. Proline 4500 dries with a gloss finish. It looks really nice, but so far I haven't been good enough to not need some sanding on my fillets. When it's sanded, it has an almost metallic-looking dark gray appearance. Anyway, I'll sand these down later.



Others have done all the fillets at the same time. I prefer to do 2 at a time though. This takes longer, but it works for me. I'll get the others done next!

 

[ksaves2]

Why sand the finished fillets? They look good enough from the photo. You going to paint it or something? Why bother? You eventually ain't going to be seeing it once you really kick it up with the I motor and will have to rely on your tracking solution to find it. I foresee a completely sight unseen flight with the large motors so painting it fluorescent orange t'aint gonna help much.:wink: Kurt

 

[cerving]

I have a Mongoose 29 and and I can attest that it gets to ridiculous altitudes on relatively small motors. Broke 10,000' with an AT DMS I205. It would have been a lot better if the fly-away rail guide (a 38mm with a shim) hadn't gotten stuck up on the rail a little bit, causing it to go about 20 degrees off-vertical. Would have made it to nearly 12,000' most likely.

 

[ksaves2]

I have a Mongoose 29 and and I can attest that it gets to ridiculous altitudes on relatively small motors. Broke 10,000' with an AT DMS I205. It would have been a lot better if the fly-away rail guide (a 38mm with a shim) hadn't gotten stuck up on the rail a little bit, causing it to go about 20 degrees off-vertical. Would have made it to nearly 12,000' most likely.

I had a cardboard, plywood finned 38mm MD rocket blast through a rail guide that bound on the rail. 2oz glass cloth in a 1/3rd, 2/3rds and full span lamination.
Yeah, twelve pieces of cloth that was laborious to do. But man oh man, just a few paint chips off a couple of fins. 8600 feet and flew arrow straight.
The rail guide ended up in the garbage after I removed the buttons for recycling. Oh, it was my fault for using the dirty rail. I should have sat out the launch
and waited for a clean rail to clear and fly from. Kurt

 

[GrouchoDuke]

I have a Mongoose 29 and and I can attest that it gets to ridiculous altitudes on relatively small motors. Broke 10,000' with an AT DMS I205. It would have been a lot better if the fly-away rail guide (a 38mm with a shim) hadn't gotten stuck up on the rail a little bit, causing it to go about 20 degrees off-vertical. Would have made it to nearly 12,000' most likely.

Yeah, both the Mongoose 29 & Go Devil 29 should be real screamers. Sorry to hear about the rail guide sticking - hopefully the dedicated 29mm one won't do that. Is your Mongoose cut down some? That seems pretty awesome for a stock one.

Why sand the finished fillets? They look good enough from the photo. You going to paint it or something? Why bother? You eventually ain't going to be seeing it once you really kick it up with the I motor and will have to rely on your tracking solution to find it. I foresee a completely sight unseen flight with the large motors so painting it fluorescent orange t'aint gonna help much.:wink: Kurt

Honestly, those first set of fillets look better in the photo than they are. The fillets themselves are pretty good, but the edges aren't smooth. At the very least, I'll have to sand the edges so the transitions to the fins & body tube are nice.

The main problem with those transitions was I put the tape lines a little too far away on the body tube. I just did the second set of fillets and the came out much better. The still need a little bit of touch up around the front & back, but not much.



This rocket is definitely not getting any paint. Like you said - no need! (The red & black look good anyway.)

 

[AeroAggie]

...A 5 foot section of the tubing was a couple bucks and it works great to form the radius of the fillets.


I slopped on the epoxy and used the PEX tubing to smooth out the fillets.

View attachment 310435View attachment 310436View attachment 310437

!

Did you drag the PEX through the epoxy and the epoxy is thick enough to hold the shape, or did you tape a segment of tubing in place as a caul plate? Did you put any release agent on it? Those fillets look like you cauled it, but you left that picture out.

 

[GrouchoDuke]

Did you drag the PEX through the epoxy and the epoxy is thick enough to hold the shape, or did you tape a segment of tubing in place as a caul plate? Did you put any release agent on it? Those fillets look like you cauled it, but you left that picture out.

Yeah, I couldn't get a photo while doing that. Setting up something to take video might be the best way to show the technique. It's pretty simple though.

Imagine an epoxy with the perfect viscosity for making super easy fillets...that's just about how Proline 4500 is as soon as you mix it. It's really easy to work with. The process is basically: tape, sand, mix, slop, swipe, de-tape.

I cut little 2" seconds of the PEX tubing using a brass pipe cutter like this one: https://www.lowes.com/pd/BrassCraft-1-8-in-to-7-8-in-Copper-Tube-Cutter/999903207 After I drip the epoxy into place, I just do a single swipe with the PEX tubing from the front to back of each fillet keeping the tube as parallel to the body tube as I can. The rear end of the fillet takes a little extra twist & scoop motion to deal with the sharp ending of the fin. I throw away those PEX tubes after each set of fillets I make. The epoxy does stick to the PEX some, but I've been able to just wipe the excess off with a rag or paper towel (watch the fuzz) in between each swipe. When cut with a tubing cutter, the PEX edge is smooth enough that the fillets have a great shape right away.

If you mess up your fillet shape, you can add more epoxy if you need to and swipe through it with the PEX again -- however, if you're going to do that, it has to be pretty quickly after mixing the epoxy. "Pretty quickly" is probably within about 5 minutes of mixing it.

I'm probably done shaping both fillets within about 5 minutes of mixing the epoxy. If there are any bubbles or missed parts, I try to fix those right away. The epoxy flows a little bit after making the fillet shape, but not much. After I'm done shaping the fillets, I pull off the masking take. I've been doing this no later than about 10 minutes after the epoxy was mixed. In the 10 minute post mix timeframe, the Proline epoxy still has just enough flow left in it to smooth out where the tape lines were.

To make sure I know how the fillets will flow after I lay them down, I keep the rocket's body tube as level as possible and I keep the two fillets I'm working on pointing up.

There are definitely others with better skills at this than me. This technique works for me though.

It's also worth noting that you shouldn't add any fillers to Proline 4500. It's just right as-is.

 

[GrouchoDuke]

I just did the last fillets. I added a little more tape on the body tube around the front & back of the fillets to keep the overflow down where the fillets taper off. They came out pretty good I think.



One thing I didn't mention before is that you really have to be careful when you pull off the tape. It's very easy to have a curled up end of a piece of tape drag across or stick to your fillets while they're still wet. If that happens, it makes them not so pretty.

 

[ksaves2]

Yeah, both the Mongoose 29 & Go Devil 29 should be real screamers. Sorry to hear about the rail guide sticking - hopefully the dedicated 29mm one won't do that. Is your Mongoose cut down some? That seems pretty awesome for a stock one.

Honestly, those first set of fillets look better in the photo than they are. The fillets themselves are pretty good, but the edges aren't smooth. At the very least, I'll have to sand the edges so the transitions to the fins & body tube are nice.

The main problem with those transitions was I put the tape lines a little too far away on the body tube. I just did the second set of fillets and the came out much better. The still need a little bit of touch up around the front & back, but not much.

This rocket is definitely not getting any paint. Like you said - no need! (The red & black look good anyway.)

My gosh, your photo taking abilities are better than mine. The little dongles at the front and back aren't going to matter much and the longitudinal edges of the
fillets look good. Trust me. Don't risk gouging up the fins with sandpaper. Fly the thing several times and if you don't lose it and it still "bothers" you then
sand the thing. Kurt

 

[GrouchoDuke]

My gosh, your photo taking abilities are better than mine. The little dongles at the front and back aren't going to matter much and the longitudinal edges of the
fillets look good. Trust me. Don't risk gouging up the fins with sandpaper. Fly the thing several times and if you don't lose it and it still "bothers" you then
sand the thing. Kurt

Thanks. I definitely get your point. I did some quick sanding on the first set of fillets earlier today. It looks like I can get the big chunks off pretty easily. The fin side of all the fillets look pretty good, so I probably won't need to do anything with them. The body tube side is where the mess is. I should be able to protect the non-mess areas with some tape & knock down the bumps pretty easily.

 

[GrouchoDuke]

I designed a recovery harness retainer/mount/thing to fit in the body tube. It'll go below the bottom of where the Eggfinder would go, if I decide to add that later. For the body harness attachment, I'm going to make a loop of 300# Kevlar that'll go around this mount and through the avionics bay. I printed it in PETG for a grand total of 7 grams. The photos show the bottom of the mount facing up.



FWIW, the hole in the middle is so that I can shove a rod through it in case I need to push out a motor that way.

 

[GrouchoDuke]

I ordered a Top Flight PAR-12TM and PAR-15TM for the Go Devil. My plan for now is to bring it down fast with the 12" chute, but I'll see how the 15 fits & maybe use that in certain cases. The Blackhawk 29 comes with the PAR-12TM and it's a similar size & weight...it's carbon though. Seems like it'll be fine.

I also got mostly ready to attach the harness retainer in the body. It'll go almost 6" down into the body tube. I masked off the area above that to hopefully cut down on me accidentally getting epoxy where I don't want it. I need to figure out a good way to get some sandpaper down in there. After that, I'll epoxy in the retainer. I might use a die grinder extension rod I made a long time ago for working on cylinder head ports. We'll see.

Here's a photo shot down the top of the body tube. If you use a little imagination, you can see the tape going down about 5.5" and the retainer below that.