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Rocksim indeed lets you use a launch tower that is 5000 feet high, and enter 150 ft/sec of cross-wind. When I ran it, it predicted that such a situation would result in just under 300 N*m of torque when it…   Read more»

Well, I just ran it through rocksim, and rocksim seems to think the total force would peak at ~220lbf (that’s not too far from my value of ~72lbs on a single fin, considering all of the assumptions that I had…   Read more»

New calculations:(these ones are right, I promise)New dynamic pressure: 86.4 PSINew lifting force on a single fin: 72.6lbsNew thickness required (ignoring flutter): 0.019 inches of top, aerospace grade CF.New peak stress within 1/8″ thick fin in this scenario: 3500PSI (which…   Read more»

Ok, 76 lbs is a lot more reasonable than stacking 10 SUVs on the end of the tube. I think i’ll use rocksim to double check. It provides restoring torque as an optionin the graphs. I’ll see if it let’s…   Read more»

Well, after some calculations…Ah, the cool refreshing sound of real engineering.Very interesting results. 150 ft/sec of instant shear is a pretty conservative worst case, but it shows the margin we have here. What did you assume for the distribution of…   Read more»

On a related note – it’s amazing how fast you forget things once you’re out of school for the summer. Things like how to properly read the table in the back of the aerodynamics textbook. (apparently, 1.9270 and 1.9270*10^-3 are…   Read more»

8 or 10 layers sounds like a bit much – you only need to build it up to roughly 1/8″ plate. Anyone know how many layers there are in 1/8″ plate CF?

I’m with Chris on this – dadoed slots in the airframe with relatively thin, low mass cores properly glued in, then 8 maybe 10 or even more layers of tip to tip, vacuum bagged and with a proper cure process.W

That’s why I mentioned starting with relatively thin plate, and then adding quite a few layers of T2T. That in essence makes the entire fin can one piece of CF, and that should allow for these kinds of loads.

Just let me know when you guys get around to flying it…

Great calculations, but if a layman like me understands the thoughts here, I think you are talking about stress on the fins – not the joints. The joints – not the fins – are what is worriesome. That is why…   Read more»

Well, after some calculations, it appears that 0.083″ fins are all that is needed at a minimum, at least for my design. That is using the assumption of a launch at 4k ASL (black rock, roughly), and that max Q…   Read more»

Well, my rough single stage sim to 40k or so with a 3kg rocket would pull around 50G on its way to M3.2. The acceleration shouldn’t be a problem – Adrian’s I powered shot did fine with greater acceleration than…   Read more»

Gosh, I go camping for a few days and you guys have it all figured out! :DSpeaking for myself, I’m not interested in a multi-staged project. If this was recovered, then maybe stick a stage below or above it, but…   Read more»

Good comments on the fins; the Mach cone angle at M3.5 is about 16 degrees. So these would need to have a really long root chord and short tip length. That alone will do wonders to reduce the flutter risk….   Read more»

I believe that is accurate, though we only touched on supersonic flight in my Vehicle Design and Performance class last semester. Once school starts up again, I’ll be taking 3000 level aerodynamics, and I could ask my professor about that…   Read more»

I think the leading edge of the wing creates its own shock wave, and not enough sweep creates a lot of wave pressure drag. By sweeping the fin back, the effective mach number- what is perpendicular to the leading edge-…   Read more»

Well, I’ve put together some rough sims, and even with some fairly bad assumptions, I’m able to get some sims as high as 80k (with something like 40 seconds supersonic) on a 2-stage M3000 to J570, and 42k for a…   Read more»

They should definitely be within the mach cone, IMHO. Even if you assume a ridiculously short rocket (say, 1 foot longer than the casing), that allows for something like a 17 inch semi span before you hit the shock wave…   Read more»

Seems like before you get to the fin material, elasticity, strength, etc, you need to figure out if you are going to keep the fins behind the mach cone (71 degree sweep at Mach 3) or stick them into the…   Read more»