My Big and Little Berthas

Big Berthas are one of my favorite classic Estes models. I decided to build some downscale Berthas when I got the idea on rec.models.rockets. I started with a mini-bertha (the blue one), built a BT-50 sized one (which has since been lost) and a tiny BT-5 sized one (the bright red one). They have all flown on mini motors, The Micro size one almost files out of sight on a 1/2A.

I decided to build a biggest one after I started building HPR models. It is 4" in diameter, about 5 feet tall, weighs about 5 pounds and has a 38mm mount. It has not flown yet but should fly great on an I motor. The middle-sized ones are BT-70 and 3" upscales. Neither has flown and the BT-70 one will be a test vehicle for a Magnetic Apogee Detector and PerfectFlite timer and altimeter.

Following is a chart of scale information for every size of tubing I know of. It can give you the information you might need if you're trying to build your own Bertha (Big or Small).

My Fleet	Description	BT Size (OD)	BT Length	NC Length	Scale

	T-2+ (MicroMaxx)	0.281	3.09	0.43	0.17
	TT T-3	0.375	4.12	0.57	0.23
	TT T-4	0.448	4.93	0.68	0.27
*	BT-5	0.544	5.98	0.83	0.33
*	BT-20	0.736	8.09	1.12	0.45
	Rogue T-22	0.850	9.35	1.30	0.52
	T-.908 (ST-8)	0.908	9.98	1.39	0.55
LOST	BT-50	0.976	10.73	1.49	0.60
	29mm MT	1.210	13.31	1.85	0.74
	BT-55	1.325	14.57	2.02	0.81
	BT-56	1.300	14.29	1.98	0.79
	BT-60	1.637	18.00	2.50	1.00
	Rogue T-45	1.750	19.24	2.67	1.07
	Aerotech 1.9	1.880	20.67	2.87	1.15
	T-2.04 (ST-20)	2.040	22.43	3.11	1.25
^	BT-70	2.217	24.38	3.38	1.35
	54mm MT	2.260	24.85	3.45	1.38
	BT-80	2.600	28.59	3.97	1.59
*	3" LOC/PML	3.100	34.09	4.73	1.89
*	4" LOC/PML	4.000	43.98	6.10	2.44
	T-4.476	4.546	49.99	6.94	2.78
	5.54" LOC	5.540	60.92	8.45	3.38
	6.15" PML	6.155	67.68	9.39	3.76
	7.67" PML	7.672	84.36	11.71	4.69
	11.6" PML	11.660	128.21	17.79	7.12

My Fleet indicates the ones I have built (*) or that are under construction (^)

Description is what the tube is commonly called

Tube OD is the outside diameter of the body tube, all scale data is based on the original Big Bertha being an Estes BT-60 which has a diameter of 1.637"

BT Len. is the length of the cut body tube

NC Len. is the approximate exposed length of the nose cone

Scale is the scale factor, how many times the original size the model will be

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