With the help of a little heater, I was able to get all four sets of fillets done yesterday. They came out O.K., good enough since they will be sanded down anyway. Going from bare parts (fins, airframe, and coupler) to all glued up only added 5.06g! After sanding that will probably drop ~0.5g. Full up (minus parachute and charges) I'm at 195.93g (6.91oz) - right were I expected and want to be!
I'm definitely feeling better about the stability now - the fins don't look as crazy small attached to the short rocket, and with the nose-weight it is amazingly front heavy. It sure does look fast :smile:
Are you sure about the weight, even? How much is the span reduced? If it's cut down by a quarter, then the mass wouldn't have changed. Alternatively, reduce all dimensions by about 13% (1-sqrt(3/4) = 0.134) and the mass will be the same with less reduction in span. (I would have thought fin area would be more important than span, that's what the simple physics says, but I keep reading more experienced folks than I who state that span is the important figure, at least when it comes to minor adjustments. Obviously, a needle with a long span is not a good fin.)
Yeah, there is that, which is one reason that scaling down all the dimensions helps a bit. But what I'm asking about is this: scale down the fins (either in one dimension or both) so that four smaller ones weigh the same as three larger ones, will you get the same or lower CP as with the three? If not, then going to four does indeed add weight, and the extra filleting makes matters even worse. But if so, then subtleties like the fillets come into play. Has Rocket Chaser, who brought up the weight difference, tried it or is the increase in weight an unchecked assumption. I'm not trying to suggest which is the case, I'm just asking. I would try it in RockSim myself, but I'm at work; perhaps I'll try it tonight.
No problem! This is nothing, you should have seen the last MD thread I did - like 3 full pages of people arguing about stability calculations at mach+ speeds :lol:
For me its usually whats available - i.e. the common 5:1 length:diameter ratio that most of the FWFG cones are. I haven't made any of my own NC's yet simply because (a) I can't make them out of CF because of the trackers, so it has to be FG anyway (b) they're cheap, and the smooth insides and removable couplers are better quality than I could easily get (c) the thin-wall versions are already pretty light and less crazy-overkill. Doing one myself would be a lot of work for not very much gain. I was close to breaking down and making a 29mm one, but then RW put on sale the mongoose with its FWFG nose cone and it was problem solved
If I was designing my own, I would probably size them to be at least 3:1 for drag reasons (less than that I think is measurably less efficient), and build it however much longer than that as dictated by either the electronics/antennas I needed to fit in there, or like in this build, recovery stuff. On the high end I probably wouldn't go past 8:1, as it starts to get long and heavy without (I think) much gain in performance or useable internal volume.
Cool! I think a lot of people jumped on the mongoose 29! Based upon my last 29mm MD which was very similar in exterior form/mass to a stock mongoose 29 (except with a draggy conical NC) that got just under 10,000', I think a nicely finished, stock mongoose 29 should be capable of ~11,000 given a nice day and straight flight. Something about putting a rocket 2(+) miles up on (comparatively) inexpensive 29mm motors is just so awesome :smile:
I don't have many, but here's a few. I'm not always the best about finishing them though :lol::Very cool thread, definitely subscribed. It would seem you've figured out the configuration of my 38mm MD that you asked about. Parachute/shock cord in NC, tracker right below that, flight computer below that, and motor sitting directly below that. Result: zero empty space, gives a lot of room to work with when trying to hit optimal mass, just doesn't give a whole lot of room volume wise.
I like the idea of four fins. I feel it would be better for the performance of the rocket. You gain stability in the individual fin without sacrificing stability of the vehicle itself. I have a 54mm version of my 38mm MD that's not really optimized (made of fiberglass with a heavy FWFG NC), that was made more of a proof of concept than a record attempting rocket. I think when I build a heavily optimized version it will have four fins. I'll play around with it for a bit and fly the less optimized version in the mean time.
I think for my next project I may revert back to the ultra-light flying case, with a K1440 I think I could get to ~M3.2 Thank you! Yeah I think there's probably less than a cubic inch of airspace in this rocket - I got the 'chutes in today, and confirmed everything actually fits somehow :smile: There's always that moment where you know everything should work from plans and simulations, but you never really know for sure until you physically put everything together.
An optimized 54mm would be awesome, you could get some crazy altitudes out of that i expect! The nice thing about the larger size MD's is that the volume goes up, but the electronics stay the same size
Actually I should be able to set the I record with this - the TRA I motor record currently sits at only 10k and change. In regards to the finish, it seems to vary. With the flying cases despite them actually being polished they got no where near their expected altitude. On the other hand, my last 29mm MD (regular design) was not polished but went higher than even "polished" predicted! So we'll see, I'm optimistic
Yeah I was impressed with how light yours came out; my flying case was just a hair heavier, the AIM XTRA is nice but the unit itself and the battery to power it adds a lot of weight and uses up a lot of volume.
My dream rocket is a two stage MD with a Loki L2050 booster and a K1127 sustainer (or the soon-to-be-released red equivalents) - with a long (10+ sec) staging delay upwards of M3.5, and with a short or no delay up to mach 4.5 could be possible. Would be insanely difficult, but insanely awesome! Optimized for altitude I bet it could clear 75K @ blackrock
