Coming off a successful first HPR build (PML Quicksilver) and first-flight L1 cert a couple of weeks ago, I've decided to do something a little different for a Level 2 rocket. One of my favorite rockets as a kid was the Estes Hi Flier, but I haven't seen any supersized versions on here (maybe my google-fu is weak?). I was planning to hold off on the build thread until I get all the parts in, but I'd like to get some feedback while I'm waiting on those.
The parts
Scale Factor Challenge
If you look at the original Hi Flier, the fins are quite large in comparison to the body. When you scale it up, you realize just how big those suckers are. I did all the dimensional scaling in excel and plugged those numbers into OpenRocket. Bear with me here, I can't get the picture of my *ork file to upload at the moment. I'll add pics when I can get them to upload correctly.
I rounded off some numbers to make the math easy:
The first challenge is stability. Open Rocket says bare tube, nose cone, and fins are ajust a touch over 1 caliber. Add in all the guts and a motor and that number goes negative in some cases. With the fin length being about 1/3 of the body length, the Cp is pretty far forward. This is easily mitigated with nose weight (which I don't want to do) or a longer body tube. I'll probably take some artistic/engineering license and stretch the overall length a bit to get the stability under control.
The second challenge is fin material and size. I want to use carbon plate, but it's just not in the budget for fins this size. Lots of rockets this size use plywood and G10 with great success, but their fins generally aren't as big. I ran the numbers in FinSim and it ain't pretty. Theoretical flutter speed for 1/4" plywood at 21.5" x 7" is just under 900 ft/s. Divergence speed is a little better at 1200 ft/s.
I can improve flutter margin by making the fins stiffer and/or smaller. Since solid carbon plate is out of the budget, I need to make them a little smaller. If I make the fins about 16.5" x 5", flutter speed improves to 1475 ft/s. I think that's ample margin for a K550 which should push to around 1000 ft/s (Mach 0.94).
The third challenge is sourcing material. FinSim material properties are based on aircraft grade plywood. It seems most kits and scratch builders tend to use Baltic Birch plywood, which as far as I can tell, comes in 5 ply for 0.25". This is not aircraft grade, which would normally be something like 10-12 ply birch. Most of what passes for aircraft plywood at the hobby stores here is also 5 ply, and it just feels awfully flexible. I remember way back when Midwest Products aircraft plywood really was the 12 ply good stuff, but I can't find that here any more.
I have some carbon fabric and a vac system, so I'm leaning toward getting 0.25" 5-ply hobby plywood and bagging it in plain weave carbon.
So the start of my build thread is not so much building as fishing for some feedback on successes/failures with large plywood fins, or substitute material suggestions. I know, tl;dr, Pics or it didn't happen, etc.
Cheers!
The parts
- 4" FW glass tube (red)
- 4" 5:1 FW glass nose cone (black)
- 54mm phenolic motor mount
- fins - TBD
- chute(s) - TBD
- Eggfinder/timer/TRS, model is TBD
Scale Factor Challenge
If you look at the original Hi Flier, the fins are quite large in comparison to the body. When you scale it up, you realize just how big those suckers are. I did all the dimensional scaling in excel and plugged those numbers into OpenRocket. Bear with me here, I can't get the picture of my *ork file to upload at the moment. I'll add pics when I can get them to upload correctly.
I rounded off some numbers to make the math easy:
- Length - 62"
- Diameter - 4"
- Fins - 21.5" x 7"
The first challenge is stability. Open Rocket says bare tube, nose cone, and fins are ajust a touch over 1 caliber. Add in all the guts and a motor and that number goes negative in some cases. With the fin length being about 1/3 of the body length, the Cp is pretty far forward. This is easily mitigated with nose weight (which I don't want to do) or a longer body tube. I'll probably take some artistic/engineering license and stretch the overall length a bit to get the stability under control.
The second challenge is fin material and size. I want to use carbon plate, but it's just not in the budget for fins this size. Lots of rockets this size use plywood and G10 with great success, but their fins generally aren't as big. I ran the numbers in FinSim and it ain't pretty. Theoretical flutter speed for 1/4" plywood at 21.5" x 7" is just under 900 ft/s. Divergence speed is a little better at 1200 ft/s.
I can improve flutter margin by making the fins stiffer and/or smaller. Since solid carbon plate is out of the budget, I need to make them a little smaller. If I make the fins about 16.5" x 5", flutter speed improves to 1475 ft/s. I think that's ample margin for a K550 which should push to around 1000 ft/s (Mach 0.94).
The third challenge is sourcing material. FinSim material properties are based on aircraft grade plywood. It seems most kits and scratch builders tend to use Baltic Birch plywood, which as far as I can tell, comes in 5 ply for 0.25". This is not aircraft grade, which would normally be something like 10-12 ply birch. Most of what passes for aircraft plywood at the hobby stores here is also 5 ply, and it just feels awfully flexible. I remember way back when Midwest Products aircraft plywood really was the 12 ply good stuff, but I can't find that here any more.
I have some carbon fabric and a vac system, so I'm leaning toward getting 0.25" 5-ply hobby plywood and bagging it in plain weave carbon.
So the start of my build thread is not so much building as fishing for some feedback on successes/failures with large plywood fins, or substitute material suggestions. I know, tl;dr, Pics or it didn't happen, etc.
Cheers!