Altitude Measurement Methods -
SEAM and DEAN compared

This comparison refers to my SEAM altitude measurement method and Dean Wheeler's Rocket Apogee Correlations (select flight equations). Both methods should be studied before reading this comparison report.

On 03. May 2001 my friend Norbert Pfanner and myself made some comparative measurements for the 2 mentioned altitude measurement methods. We used my newest rocket, "GOLLUMP 4f" for this purpose. Each of the 4 launches below were made with the following data:

The person taking the SEAM measurements was 4 m above launch level, in a distance of 56 m (that was too short) and later 83 m away from the launcher. The launching person took the time measurements that we needed for Dean's method. 

The results are given in this table:

	S E A M						D E A N
Units:	m	m base	°	m Drift	° Drift	m	sec	sec	sec	sec	m	m	m/s
Rocket	m	r	alpha	s	beta	h SEAM	Tup	Tdwn	Ttot	dT	h Corr	h simple	Vt	Cd	Comment
Gollump 4f	4	56	62	12	40	118,0	4,48	5,37	9,85	0,89	117	116,3	44	0,25	Tup probably too long - distance time-taker to Launcher only 20m
Gollump 4f	4	83	50	27	15	118,5	3,88	5,80	9,68	1,92	111	112,3	29	0,57	probably most correct measurement of Tup.
Gollump 4f	4	83	53	36	90	114,1	4,00	5,78	9,78	1,78	113	114,6	31	0,5	Tup=3,44 measured was too short - observation apogee insecure. Tup=4s guessed
Gollump 4f	4	83	58	37	155	110,0	5,75	3,81	9,56	-1,94		109,4			Lost sight of rocket (watched watch!) - almost hit: in r=40m, impact <2m left of me! Tup (??) too large.
Hubschrap 1	4	56	52	14	75	78,0	3,99	4,35	8,34	0,36	83	82,6	52	0,24     ?	shattered into 3 pieces, bottle thread ripped out at impact. PET fins undamaged! Helicopter blades did not unfold. 3-4 bar better!

Meaning of variables
hSEAM	=	altitude determined with my SEAM method
hCorr	=	determined with Deans' Rocket Apogee Correlation pdf file
hsimple	=	Deans's "simplest correlation":  hsimple [m] = 1.23 * Ttot2 - 3
Tup	=	up time = apogee time
Ttot	=	total flight time = Tend
Tdown	=	down time = Ttot - Tup
dT	=	Ttot - 2Tup
vt	=	terminal speed of the rocket, derived from Tup and Ttot
Cd	=	211.7 * mass [gram] / (Diam [cm]^2 * vt [m/s]^2)

All data were entered right after each launch into my trusted Psion 5 palmtop computer. The cells marked grey are the ones which contained formulas.

Conclusions:

1. To be honest: we did not expect such a good agreement of the results (yellow)!
2. It was more difficult to take the apogee time than the total time. The calculated Cd value greatly depends on the apogee time.
3. There is room for more exact data  to be gathered like launch tube dimensions and function (my LT sometimes gets pushed out (a few cm) or even thrown out entirely.
4. Next time we will have more people with watches to get more reliable and comparable T_ap time measurements.
5. As described elsewhere, the DEAN measurement can NOT be used for rockets with parachute recovery as long as you do not know its Cd number. SEAM measurement can be used in all conditions.
6. Both SEAM and DEAN method proved to be working well. But the comparison needs to be continued in the other value ranges.
7. Thank you, Dean, for making your flight equations in the SI system. That shows far-sightedness.
8. I cannot more than recommend Dean's method to every water rocketeer who is interested in the altitude of his or her rockets.

To the launch report with pictures.

Stand / Last Revision:  17.05.01