Originally posted by astronboy
OK, For those without rocsim:
18" BT-60
PNC-60AH (Mean Machine nc)
BT-50 heavyweight engine tube (od is .99)
Centering rings are .125" lite ply.
I doubled the engine block just in case.....
engine tube runs more than 1/2 the length of the rocket as a stuffer tube, in order to keep the recovery system up front, and to stiffen the airframe...
3g of noseweight brings the rocket up to oprtimum weight for this motor.
There is a 1.6" long paper boat tail from the rear of the BT-60 to the BT-50.
G Harry's fin rules are as follows:
root = 2 x Diameter of airframe
clipped tip is 1 x D
span of fin is 2D
with of root of fin is .1 root length (max)
with of fin material at fin tip is .1 tip length (max)
My fins have a 3.274 root, a 1.637 tip
Span is 3.5 (Harry says that you can add to span if needed)
I am now leaning towards .25 balsa fins sanded to .125 at the tip, and then doped with silkspan for strength.
Here is a jpeg of the design
Phred
I've been building several BT60 based higher power but Estes part birds similar to this design. It scales up in power well. I'm trying to break Mach with as little huigher powered material as possible.
Some suggestions:
1/8" basswood fins. You can get a better edge than balsa and it's easier to get a good flat finish. Glue a very thin strip of the same material along the root edge before connecting to the bird, to give it 3 times the body contact surface and way more for the fillets to grab.
Using a stuffer tube is a necessity since your engine is 40% of the body length. Very good for strength, but leaves little volume to be compressed by the ejection charge. It will be a powerful event in this bird.
If you think your upper body might need integrity, you could line it with a layer of BT60 cut with a slit down the side and slid in, or a piece of tube from Reynolds aluminum foil. The latter fits inside BT60 perfectly but isn't as heavy as BT60 coupler.
Put a shock absorber in your shock cord. Tie two loops in it a foot apart. Between them tie a piece of strong elastic that's 6" when slack. This will give a buffer zone of less "give" before the cord reaches maximum length, but won't cause "Estes Dents" due to its not being the full length of the cord. Alternatively, place a loop of elastic on the nose connection and attach the rest of the recovery stuff to that.
Keeping the recovery gear forward in the tube is good. Sometimes I use a 50/60 or 55/60 centering ring glue between the top of the MMT and the forward end to keep stuff from sliding down during boost. In 24mm birds I use a 20/50 centering ring with a piece of bamboo skewer glued across it, and also use that for shock cord mounting.
Consider cutting a PNC60 up to use as a tail cone. Only slightly better flight dynamics than a conical tail, but looks very sharp, and you can notch it easily if desired. It can also be notched for the exansion of an engine hook. And you CAN use a hook, or maybe 2 or 3, for motor retention. See my "cheap 29mm motor retainer" elsewhere for the idea. It can fit any MMT size. You just use an inch of the same motor tube slotted once to fit over the motor tube.
I don't know about the thrust curve of thre G37. Does this thing sim to Mach+? If it's close, take a look at a conical cone like PML's "1.5" urethane cone. That's always an option for later, if you put removeable connectors on all your recovery gear, or even just use a slip-loop to connect your shock cord. This also allows swapping in of a payload section later, a good idea since this is something you might want to get an altitude/speed measure on later.
And as mentioned elsewhere, a shiny mylar streamer. You can see the flash in the sun when everything else is still too small to see. Build it to be removeable, just like a chute, since mylar doesn't take heat well and you might want to replace it later.
I really like the combination of BT60 airframe and 24mm MMT for strength, internal volume and weight, as well as availability of various parts, including heavier style parts of very similar size if desired. Highly versitile, but excellent with plain LPR stock. I've built several birds on it (up to 7' long) and have more planned.
Build for the future! There's always more G37s. And others.
"The future is tomorrow!" -- Yoyodyne Propulsion Systems
