Bare Minimum: M2245 to 50,000 feet

[OzHybrid]

That's a massive range of materials you can get designs made in.

 

[Neutronium95]

How much heavier is the Al fincsn than CF fins? Seems like I could add a lot weight at the wrong end of the rocket.

The current version is 415g, according to my CAD right now. If I can figure out how to cleanly hollow out the fins a bit, that mass number will come down even more. It looks like a carbon fiber fin can will be about half the mass if I have the exact same geometry. However, that mass will go up a bit from things like an aluminum leading edge, and from thicker fins, if I end up needing to go with beveled plate instead of the tapered airfoiled fins of the current design.

I did have the chance to hold one of the fin cans that the Tu Wein group used on their Hound rocket last year, and it was astonishingly light, since the fins were mostly hollow. I'm not sure if I'll be able to be that aggressive with weight savings on this project.



Finally, I'm just not that concerned about weight. From playing around with RASaero, this rocket is relatively insensitive to changes in weight. The fin can weighing 100g more than is possible with carbon fiber won't make or break this project. And this design will probably result in a drag reduction in comparison to what's currently feasible for me with carbon.

And here's a quick screenshot of my firat attempt at putting a scalloped leading edge on the fin can, a la Bare Necessities:

 

[watheyak]

I was wondering how you could do hollow fins with this 3D printing method. But after seeing the Tu Wein photo I get it. Genius!

 

[Neutronium95]

I also have not made any firm choices about the fin can yet. I still want to make a carbon fiber fin can, but I really like having a backup option, just in case my super ambitious carbon fin can design doesn't work out.

The biggest stumbling block right now is currently the aerodynamic heating. I'm aware of a few projects that have had similar heat loads, but I want to have some sort of analysis that I'm confident in, instead of just blindly following what other people have done before me. I'm pretty sure that the right combination of high temp resin and surface coating will work, it's just a question of what resin and what coating.

 

[OzHybrid]

The current version is 415g, according to my CAD right now. If I can figure out how to cleanly hollow out the fins a bit, that mass number will come down even more. It looks like a carbon fiber fin can will be about half the mass if I have the exact same geometry. However, that mass will go up a bit from things like an aluminum leading edge, and from thicker fins, if I end up needing to go with beveled plate instead of the tapered airfoiled fins of the current design.

I did have the chance to hold one of the fin cans that the Tu Wein group used on their Hound rocket last year, and it was astonishingly light, since the fins were mostly hollow. I'm not sure if I'll be able to be that aggressive with weight savings on this project.
View attachment 617585


Finally, I'm just not that concerned about weight. From playing around with RASaero, this rocket is relatively insensitive to changes in weight. The fin can weighing 100g more than is possible with carbon fiber won't make or break this project. And this design will probably result in a drag reduction in comparison to what's currently feasible for me with carbon.

And here's a quick screenshot of my firat attempt at putting a scalloped leading edge on the fin can, a la Bare Necessities:View attachment 617586

Bit heavier, but it's not getting turned into a shaggy dog.

 

[OzHybrid]

Has anyone considered chrome plating a section of a tip to tip layup finished fincan. Continuous covering and plating could be considered as a surface finish rather than metal. So sort of plate the whole fin can area. Could be interesting.

 

[Neutronium95]

I think I might have a concussion from banging my head against the CAD for a few hours tonight, but I think I'm starting to get the hang of this.

 

[Neutronium95]

A little bit more CAD shenanigans. Fixed the fin thickness and also tweaked how I was doing the scalloped leading edge, which let me make it a bit more aggressive.


I explored a few more options as well, from more aggressive:



To maximum edgelord:




Next up is actually tweaking the design to make sure that it'll print correctly. I'm not quite sure how well things like the sharp leading and trailing edges will print. Then it's time to hit the books and figure out how feasible and expensive my carbon fin can ideas are.

After all of the tweaks and changes, I've also gotten the mass below 370g. I don't know if there's enough meat in the fins to really hollow them out much, but the printed fin can is looking more and more like it might be pretty competitive weight wise with the composite options. Especially the less optimized ones with flat plate fins and big fat fillets like I usually do for min diameter flights.

 

[diyaerospace]

A few weeks ago someone at FAR needed to use the lathe at the launch site to sand the ID of his printed metal fin can to fit onto the motor.

I wonder who that could be?

The ID of my fincan was 2.136", which is less than the WM airframe ID of 2.152".
For my future fincans I will use the Wildman airframe ID.

Walter

 

[OzHybrid]

I wonder who that could be?

The ID of my fincan was 2.136", which is less than the WM airframe ID of 2.152".
For my future fincans I will use the Wildman airframe ID.

Walter

Test fit everything before you turn up at the launch site and fix it before the launch day.

 

[OzHybrid]

A little bit more CAD shenanigans. Fixed the fin thickness and also tweaked how I was doing the scalloped leading edge, which let me make it a bit more aggressive.
View attachment 617719

I explored a few more options as well, from more aggressive:

View attachment 617720

To maximum edgelord:

View attachment 617721


Next up is actually tweaking the design to make sure that it'll print correctly. I'm not quite sure how well things like the sharp leading and trailing edges will print. Then it's time to hit the books and figure out how feasible and expensive my carbon fin can ideas are.

After all of the tweaks and changes, I've also gotten the mass below 370g. I don't know if there's enough meat in the fins to really hollow them out much, but the printed fin can is looking more and more like it might be pretty competitive weight wise with the composite options. Especially the less optimized ones with flat plate fins and big fat fillets like I usually do for min diameter flights.

If that fits perfectly to your body you're good. If not it's going to be a PITA to machine. If you print a thin web all the way to the top where it's been scalloped and remove it once you get the can back from the printer and have checked the inside diameter, you'd be able to machine it as a complete bore and manually remove it afterward. Do you know what the minimum thickness that can be printed is? You'd make the LE and TE and tip thickness that dimension as a minimum, otherwise their slicer won't print to the full dimension of the fin.

 

[Adrian A]

A little bit more CAD shenanigans. Fixed the fin thickness and also tweaked how I was doing the scalloped leading edge, which let me make it a bit more aggressive.
View attachment 617719

I explored a few more options as well, from more aggressive:

View attachment 617720

To maximum edgelord:

View attachment 617721


Next up is actually tweaking the design to make sure that it'll print correctly. I'm not quite sure how well things like the sharp leading and trailing edges will print. Then it's time to hit the books and figure out how feasible and expensive my carbon fin can ideas are.

After all of the tweaks and changes, I've also gotten the mass below 370g. I don't know if there's enough meat in the fins to really hollow them out much, but the printed fin can is looking more and more like it might be pretty competitive weight wise with the composite options. Especially the less optimized ones with flat plate fins and big fat fillets like I usually do for min diameter flights.

You should consider scalloping the back edge too, unless you have a tailcone that matches the OD of the back of the fin can.

BTW, I looked up the mass of the CF fins in my 54 M record design, and they are about 100 grams including fillets, so that's still a pretty substantial difference. I would need to grow the fin height by 10% to make up for the impact to stability if they were 370 grams.

 

[Neutronium95]

You should consider scalloping the back edge too, unless you have a tailcone that matches the OD of the back of the fin can.

BTW, I looked up the mass of the CF fins in my 54 M record design, and they are about 100 grams including fillets, so that's still a pretty substantial difference. I would need to grow the fin height by 10% to make up for the impact to stability if they were 370 grams.

The rear end matches up with the motor thrust ring. I'm borrowing a gen 1 case from a friend for this project. I've thought about trying to integrate a tailcone into the design, but haven't found any good solutions. The CTI tailcone tapers so abruptly that it provides essentially no benefit, and the nozzle doesn't stick out of the case enough to make something that attaches to that either.

A quick, back of the envelope square cube scaling from 54mm to 75mm would put your tailcone mass at around 270 grams. All of the masses are very preliminary as well. My current plan is to build the whole nose section before making a final decision about the specific details of the fin can, like the exact fin geometry. The nose section is probably going to be a bit heavier than is ideal, since I'm planning on having a relatively thick fiberglass nosecone, and if I want to stick to a single sleeve diameter, the Soller composites sleeves will force me to have a relatively large aluminum nosecone tip.

 

[OverTheTop]

Remember the swept fins leading edges see a much reduced normal velocity and stagnation temperature as they are not normal to the airflow. With the rocket at M3 and decent sweepback the leading edges will probably only see a bit above Mach 1. Your material needs there might be much less than you expect.

If you can get the sweep back enough and stay inside the Mach cone it would only see subsonic velocities at the LE. I suspect in this rocket that would make the fins impractically long, but you still get some improvement in conditions.

Anything directly facing the airflow will see the full stagnation temperature.

 

[Neutronium95]

I finally got around to ordering the fiberglass sleeves for making the nosecone and avbay. I think that I might be able to make the fiberglass parts for the flight by the end of the year.

I'm also considering doing a "test flight" before flying the M2245. I have a M3500R that I wanted to fly at BALLS this year, but I ran into some issues at the launch site. The main reason I'm considering this is because I'm having some doubts about my current 75mm min diameter rocket. I'm just not sure it'll hold up to Mach 3, or if it'll run into dynamic stability issues. I'd like to launch in February, so that a friend of mine can get footage of the flight. I really want to get some good pictures and video of the M3500 flight, so that in the future people can see what it was like. I doubt I'll be able to get a nice carbon fin can made the way I want to in time for a launch in February, so that flight will probably use a printed Aluminum fin can.

One other thing that I need to get a handle on before I commit to any specific fin can design is dynamic stability. Pretty much all of my knowledge on the topic comes from this series of lectures on YouTube, unfortunately they're a lot better at explaining the basic concepts than at showing what are good ranges of values to target, and how to adjust designs to hit those targets. To start out I managed to get Openrocket to give me the information I need to calculate the damping ratio, and when I did so it came out a good ways below the rule of thumb that I was told. I'm not certain how useful that information is, since Openrocket and RASaero start to diverge quite a lot beyond Mach 2, in terms of stability. Are there any examples of dynamic stabilty analysis for projects that went above Mach 3 on here? Beyond that, are there any good books or papers I should read? I have a copy of Advanced Topics in Model Rocketry, but I can't seem to find it right now.

 

[Porkchap]

Are there any examples of dynamic stabilty analysis for projects that went above Mach 3 on here? Beyond that, are there any good books or papers I should read? I have a copy of Advanced Topics in Model Rocketry, but I can't seem to find it right now.

Dynamic stability analysis whowhatnow? Man, just put her on the rail and SEND that beeyatch already! You got people waiting over here. If you're scared, go to church first...

 

[Neutronium95]

Dynamic stability analysis whowhatnow? Man, just put her on the rail and SEND that beeyatch already! You got people waiting over here. If you're scared, go to church first...

I wish I could convince myself to just send it. Unfortunately with the price and availability of M2245s I have one shot at this flight, and I want to be confident that I've done everything within my capability to ensure a successful flight when the button gets pushed.

 

[Neutronium95]

I'm also starting to waffle on the nose cone construction.

I was initially planning on using Soller Composites sleeve over a printed male mandrel, but I'm having second thoughts. First the relatively large minimum diameter of 1.6" for the 3" diameter sleeve will require a very large machined nose tip. Secondly, if I go with a relatively thick wall at the base, it will be absurdly thick at the tip.

I'm now thinking about making a proper female mold, and using either a bladder like in this thread, or a male plug cast out of silicone that will apply pressure when heated due to thermal expansion, as shown in this video.

Also, after after a recovery failure on my 54mm testbed rocket, I'm rethinking recovery. I'm leaning towards an Adrian A style parachute cannon on top of the avbay, but am considering a bunch of options right now. I'm going to fly one or two of them in the test rocket before making a final decision on this build.

 

[Neutronium95]

After a few weeks of bashing my head against the wall, and another recovery failure on the testbed rocket, I'm finally approaching a design that I like.

It uses a parachute cannon, like what @Adrian A has used on a bunch of different high performance rockets. The big difference is that my design actually separates the nosecone into two parts. This lets me use the soller sleeves in it's preferred size ranges. The CAD is still very preliminary, but I'm at a stage where I'm pretty confident that everything will fit, I just need to flesh out the details more and more.



After the last recovery failure on my 54mm submin, my confidence was pretty rattled. I decided that the best course of action would be to build a test nosecone, and fly it once or twice to confirm that the recovery system will work as intended. I'm also not certain exactly how I'm going to lay up the final nosecone, but the test parts will be made with my usual method of fiberglass sleeves over a 3D printed male mold. I'm aiming to launch at the first FAR weekend in March, so I have two weekends to make a first revision part, and then make a second set of parts if I run into issues and need to change the design.

 

[JPL]

I've seen several people I know choose 3D printed aluminum for fin cans on high performance projects. I've been aware that it is probably the best all around method of making a fin can available to me, but I've been avoiding it because I want to build the fin can myself. Tonight, I decided to throw together a quick CAD model of my planned fin can, and get a quote for it. It comes out to around $300 printed in Aluminum. That's a lot less than I'd need to spend if I went and acquired the prepreg and other supplies to make a fin can the way I want to. It's the most cost effective option, and it will save me a ton of stress about aerodynamic heating. I can probably optimize the design further with it as well, maybe by hollowing out the fins and fillets a bit, like on the printed fin cans that Tu Wein use on their rockets.

View attachment 617317

This is far from the finished design, but it's close enough to get a preliminary quote.

Right now from where I'm standing, the only reason to pursue a composite fin can is to be able to say that I made every part of the rocket myself. In all other aspects the aluminum fin can will probably be better. I need to do some more work on figuring out exactly what it'll take to make a prepreg fin can, and if there are any other options to consider.

While I chase my tail in endless circles about the fin can, I'm going to try to make actual progress on the nosecone soon. I'm planning on testing out the 3" Soller sleeves at several different diameters, so that I can characterize exactly how thick they'll be at various points on the nosecone, which will allow me to design a male plug that will lead to a perfect Von Karman profile. The practical minimum diameter of the sleeves will also determine just how big the aluminum nosecone tip will need to be.

Do you mind sharing the fin can file? Im working on a similar project.

 

[Neutronium95]

Do you mind sharing the fin can file? Im working on a similar project.

This is the current state of the fin can. It's very much a work in progress, and as such has not been tweaked for manufacturing concerns, had any lightening features added, or been properly analyzed for fin flutter.

I'd suggest taking inspiration, but I absolutely would not use this as is on a high performance project.

Here is a link to the Onshape file. There's probably a better way to make it, but feel free to take inspiration from my process.

 

[Neutronium95]

The recovery test rocket is coming together nicely. This week I laid up the fiberglass parts of the nosecone, got most of the printed parts designed, and threw together the booster section. It's been a long time since I've built a traditional TTW fin rocket, and it was my first time assembling the motor mount and fins outside of the rocket, filleting, and then installing the whole assembly into the tube. It's kinda ugly, but it'll work fine.

Cross section of the updated nosecone CAD


I laid up the two nosecone parts with my usual Soller sleeves over a 3D printed plug with lubricated single use latex release layers for mold release. I hung more weight off of these than on previous builds, and I think that lead to better consolidation of the parts, since the wall thickness was a bit thinner than I expected. Not an issue for this test rocket, but something to consider for the full send flight.




I borrowed a technique from a friend, and built alignment features into my printed centering rings. I also chose to use a printed motor retainer, I'm a bit nervous about trusting it, so I'll probably friction fit the motor as insurance.

I threw together the fin can this evening with a whole bunch of 5 minute epoxy. I just slopped it in there to form the fillets. They're ugly, but this rocket is not entering a beauty pageant.


And here is the motor mount assembly glued into the body tube. I had to trim down the bits of tube that fit in between the fin and centering ring. I should probably rethink this little detail for future builds, but this will work fine.

 

[kjhambrick]

I borrowed a technique from a friend, and built alignment features into my printed centering rings ...

That's genius !

Thanks to you and 'your friend' for the GREAT idea !

-- kjh

 

[Neutronium95]

I'm making progress, but it's slow going. I'm on track to be ready to fly this thing at Mudroc, but I don't have the time to rebuild it after a failed test flight, so I'm just going to test the recovery system on an I motor and then send it on the M2245. I might bring the M3500 to refly it at Mudroc as a victory lap if it survives the M2245 flight.

After a bunch of playing around with the test parts, I came to the conclusion that the parachute cannon just wouldn't quite work. There just isn't quite enough room for everything. I could probably make it work by either shrinking the main, stretching the nosecone or going to a custom sized fiberglass tube for the cannon. Ultimately, I decided to go back to what I know and am comfortable with, which is cable cutters. I am going to attach the drogue to the apex of the main with a kevlar leader, to ensure prompt deployment of the main when the cable cutters fire. Since this is a new technique for me, that means the recovery flight is mandatory. I'm running low on cable cutters after a few failed flights and being horrifically disorganized, so I decided to make some @tfish style cable cutters.

I laid up another set of nosecone parts today. I'm keeping the two part nosecone for a few reasons. First it lets me use two different sizes of sleeve, which allows me to make the tip a smaller diameter. Secondly I have access to an oven I can use for curing the parts, but unfortunately I can only fit a 13" long part in it. I had initially planned on building an oven, but I need to limit scope creep or else this thing will never fly. I also laid up a coupler to connect the parts together, along with a more traditional coupler that will house the electronics. 3D printing is amazing for making molds relatively quickly.

Finally, I've been making more progress on the fin can CAD. I prefer the looks of these fillets, but they ended up adding some mass, so I might not stay with them. I also figured out how to hollow out the fins a bit. I'd like to go more extreme with the hollowing, but I'm currently being pretty conservative with the wall thickness, and the relatively thin profile of my fins means there isn't a whole lot of material to hollow out. I'd see much bigger weight savings with thicker fins.

 

[Neutronium95]

The new fiberglass parts came out pretty nice. There are a few imperfections, but nothing a bit of sanding can't fix. My dad took a ton of pictures of the process, so I'll try to post a full writeup of the process in the techniques subforum sometime soon. It'll be a bit tight, but I'm on track to launch the recovery test at FAR this Saturday. That should also be the point where I have a solid as built mass for pretty much the entire nose section, which will let me finally nail down the exact fin shape.

 

[Neutronium95]

I flew the test rocket twice yesterday at Rocstock. I've settled on cable cutter dual deploy, with the drogue attached to the apex of the main to ensure that it deploys. On the first flight, the rocket spun during drogue descent, and the shock cord ended up wrapped around the main burrito, preventing it from deploying when the cable cutter fired.

I changed how I rigged the parachutes for the second flight, but I have no idea how well that worked, since the I500 violently cato'd at ignition. The fin can got blown to pieces, and the rest of the motor got hurled through a bulkhead and into my avbay, where it killed my Blue Raven and possibly my Featherweight GPS.

In brighter news, the parachutes for the actual flight arrived from Valkyrie Recovery. I'm just not sure what to do right now.

Edit: forgot to attach the picture of the exploded rocket. I went back to the pads later and picked up even more pieces.

 

[pugachu]

I flew the test rocket twice yesterday at Rocstock. I've settled on cable cutter dual deploy, with the drogue attached to the apex of the main to ensure that it deploys. On the first flight, the rocket spun during drogue descent, and the shock cord ended up wrapped around the main burrito, preventing it from deploying when the cable cutter fired.

I changed how I rigged the parachutes for the second flight, but I have no idea how well that worked, since the I500 violently cato'd at ignition. The fin can got blown to pieces, and the rest of the motor got hurled through a bulkhead and into my avbay, where it killed my Blue Raven and possibly my Featherweight GPS.

In brighter news, the parachutes for the actual flight arrived from Valkyrie Recovery. I'm just not sure what to do right now.

Edit: forgot to attach the picture of the exploded rocket. I went back to the pads later and picked up even more piecse

Ouch,

Sorry about the CATO.... those always suck.

 

[Neutronium95]

@PosTart got some great pictures of the CATO.



Fortunately, the fiberglass parts are all pretty much unscathed. So I just printed a few parts to adapt my 3" tubefin into the role of recovery test rocket. My Valkyrie recovery order also arrived, so this next (and hopefully final) test should be able to use the exact same recovery system as the full up flight. This whole fiasco with the recovery test has taken a lot of margin out of my timeline, but I'm still on track to fly at Mudrock.

 

[311stylie]

Bro that sucks. A cato on a DMS? I fly a lot of I500s, just as you did I fly them for test/inaugural flights and I've used them for boosters in smaller 2-stage flights. Never had an issue. I guess that time comes for all of us.