Model Rocket Design Notes

Dimensions

Engines

Type	Diameter	Length	Casing
MicroMaxx	6.3mm	26mm	1mm
Mini	13mm	45mm	1.6mm
18mm	18mm	70mm	2.6mm

Tube	Inside Dim.	Outside Dim.	Length
	mm/in	mm/in	cm/in
MM Stand	4.00 / 0.157	4.50 / 0.177	1.35 / 0.53

Body Tubes

Tube	Inside Dim.	Outside Dim.	Wall	Circumference	Estes OD	ASP OD
	mm/in	mm/in	mm/in	mm/in
T-MM	6.45 / 0.254	7.16 / 0.282	0.355	22.49 / 0.886		7.1
T-10.5	10.72 / 0.422	11.38 / 0.448		35.75 / 1.41
BT-5	13.2 / 0.518	13.8 / 0.544		43.35 / 1.707	13.7	13.7
BT-20	18.0 / 0.710	18.7 / 0.736		58.75 / 2.313
BT-50	24.1 / 0.950	24.8 / 0.976		77.91 / 3.064
BT-55	32.6 / 1.283	33.7 / 1.325		105.87 / 4.168
BT-56	33.1 / 1.304	34.2 / 1.346		107.44 / 4.230
BT-60	40.5 / 1.595	41.6 / 1.637		130.69 / 5.145
BT-80	65.7 / 2.588	66.0 / 2.600		207.35 / 8.163

Scaling

Src	Dst	Ratio
BT-5	T-MM	51.88%
	BT-60	300%
BT-20	T-MM	38.29%
	BT-5	73.8%

Limits

Ground Hit Speed

"From how high would I be comfortable just dropping a rocket?"

Assuming a standing start and no air resistance yields freefall numbers like:

30mph	44fps	30.1'
23.86mph	35fps	19.0'	Tripoli safety code limit
17.05mph	25fps	9.7'
15.00mph	22fps	7.5'
13.63mph	20fps	6.2'
10.23mph	15fps	3.5'
6.82mph	10fps	1.5'

Electronics

Jolly Logic Altimeter Two

49mm x 18mm x 14.5mm (diameter 23.114mm)
9.9g (0.35oz)

Stability Notes

Velocity off rod should generally be at least 15 m/s (very rough rule of thumb).

Stability (CP-CG) should be 1--2 caliber for "normal" rockets.

"Normal" here is fineness at/above 10:1.

Base drag of stubby rockets requires special attention

"Stubby" is more or less rockets with fineness below 10:1
OpenRocket accounts for base drag in its simulation, but not in calculation of CP for stability
To account for this, add a 0g cone at the end of the rocket with fore diameter 0mm, aft diameter same as the body tube, and length π*diameter
Discussion in Apogee Newsletter 154

Component Notes

Nosecones

Nosecone plug length should be approximately the diameter of the tube.

Couplers

Coupler construction video from Apogee.

Tube couplers should be ~2x the diameter of the tube.

At what power levels?

Fins

Orient grain of balsa wood along leading edge of fins.

Engine Retention

Minimum diameter engine hook

There's a motor retention technique for minimum diameter rockets shown in Tim van Milligan's book that uses a piece of piano wire bent into three sides of a rectangle, two long sides and one short, where the short side goes over part of the bottom of the engine, staying clear of the nozzle of course, and the long sides are buried in fin fillets. I've started using that method on larger than minimum diameter rockets, with the ling sides going through pinholes in the aft centering ring. It seems like that should work nicely for MicroMaxx.