Stability calculations
Orville R. Weyrich
be stable was to calculate the Center of Lateral Pressure (CLP) and to
calculate or measure the Center of Lateral Mass (CLM). The model rocket
was then expected to give a stable flight if the CLM was ahead (closer to the
nose) of the CLP by a distance of at least (either the body tube diameter
or 10% of the rocket's length).
This rule of thumb was said to be a crude worst-case estimate.
Recently I have noticed that the Estes Partiot kit has a CLP approximately
32.4 cm from the tip of the nose (22.0 cm from the tail). I measure the CLM
to be about 1.5 to 2.0 cm BEHIND the CLP (with a C6-5 and wadding installed).
The rocket flies great.
PLEASE -- ANYONE ABLE TO VERIFY THESE CALCULATIONS, MEASUREMENTS, AND
EXPERIENCE?
Has anyone described a more accurate way of predicting the stability of a
model rocket? Off the top of my head, I would say that because the body tube
and nose cone are cylindrical rather than flat, they exert a lesser
opposing/restoring force than their lateral area would predict.
This would suggest that their effective lateral areas should be reduced
when doing the CLP calculation by an amount proportional to (the drag
exerted by a cylinder / the drag exerted by a flat surface of the same
profile).
DOES ANYONE KNOW OF ANY METHODS BASED ON THE ABOVE CONSIDERATIONS THANT WORK?
WHAT ARE THE FUDGE FACTORS?
HOW DOES THE AIR-SPEED OF THE ROCKET AFFECT THESE CALCULATIONS?
WHAT ABOUT THE TRANSITION FROM LAMINAR TO TURBULENT AIR-FLOW?
ARE THERE COMPUTER PROGRAMS AVAILABLE THAT TAKE THESE THINGS INTO CONSIDERATION?
-------------------------------------- ******************************
Orville R. Weyrich, Jr., Ph.D. Certified Systems Professional
Internet: orville%wey...@uunet.uu.net Weyrich Computer Consulting
Voice: (602) 391-0821 POB 5782, Scottsdale, AZ 85261
Fax: (602) 391-0023 (Yes! I'm available)
-------------------------------------- ******************************
Bob Kaplow
>About 25 years ago, the standard way to determine if a model rocket would
>be stable was to calculate the Center of Lateral Pressure (CLP) and to
>calculate or measure the Center of Lateral Mass (CLM).
This would be the cardboard cutout method. IMHO it isn't worth
the cost of the cardboard. BTW, a string test isn't worth the cost
of the string. Both give incorrect results in too many cases, and
often error in "verifying" stability of an unstable rocket.
>
>Has anyone described a more accurate way of predicting the stability of a
>model rocket?
>
Yes. Use the Barrowman equations. The only in print source I am
aware of is one of the appendicies of Stine's handbook. I don't have
access to CALVIN, but I thought there was a Barrowman CP program
floating around there. There are programs available commercially as
well, but I've never used anything other than the ones I've written.
Bob Kaplow | UUCP: ...!decwrl!pobox.enet.dec.com!kaplow
Digital Equipment Corp. | ARPA: kap...@pobox.enet.dec.com
Elk Grove Village, IL | CIS: >INTERNET:kap...@pobox.enet.dec.com
There is no such thing as a free launch!
Lawrence Curcio
and as corrected by me. These are Barrowman calculations; they do not take
turbulent/laminar flow into account. Also, the CG calculations are
approximate.
The program tells you when a rocket is unstable (using the estimated CG and
calculated CP). It doesn't tell you when the rocket is over-stable, but it
does give you calibers. Also, the CP calculations work for 3 and 4 finned
models, but not for 6 finned models.
For further info, see _HANDBOOK OF MODEL ROCKETRY_ by GHS.
PS> Jack, I'm digging my way out. Any day now...
==========================================================================
10 REM STABCALC-1
20 REM ROCKET STABILITY CALCULATIONS
30 REM BY G. HARRY STINE
35 REM CORRECTIONS BY LARRY CURCIO
40 REM 1979
50 DIM T(99)
60 DIM F(99)
70 DIM X(99)
75 A$="NO":B$="NO":C$="NO":D$="NO":E$="NO":REM CORRECTED FROM STINE
80 CLS:PRINT "ROCKET STABILITY CALCULATIONS"
90 PRINT "ENTER ROCKET NAME OR NUMBER"
100 PRINT "CIRCLE DIMENSIONS USED: MM IN"
110 PRINT "ROCKET DIMENSIONS LIST"
120 PRINT "NOSE LENGTH:";
130 INPUT X(1)
140 PRINT "NOSE BASE DIAMETER:";
150 INPUT D1
160 PRINT "ANY TRANSITIONS?"
170 INPUT A$
180 IF A$="NO" THEN 490
190 PRINT "TRANSITION 1 FRONT DIAMETER:";
200 INPUT D2
210 PRINT "TRANSITION 1 REAR DIAMETER:";
220 INPUT D3
230 PRINT "TRANSITION 1 LENGTH:";
240 INPUT L1
250 PRINT "DISTANCE, NOSE TIP TO TRANSITION 1 FRONT:";
260 INPUT X(2)
270 PRINT "ANY MORE TRANSITIONS?"
280 INPUT B$
290 IF B$="NO" THEN 490
300 PRINT "TRANSITION 2 FRONT DIAMETER:";
310 INPUT D4
320 PRINT "TRANSITION 2 REAR DIAMETER:";
330 INPUT D5
340 PRINT "TRANSITION 2 LENGTH:";
350 INPUT L2
360 PRINT "DISTANCE, NOSE TIP TO TRANSITION 2 FRONT:";
370 INPUT X(3)
380 PRINT "ANY MORE TRANSITIONS?"
390 INPUT C$
400 IF C$="NO" THEN 490
410 PRINT "TRANSITION 3 FRONT DIAMETER:";
420 INPUT D6
430 PRINT "TRANSITION 3 REAR DIAMETER:";
440 INPUT D7
450 PRINT "TRANSITION 3 LENGTH:";
460 INPUT L3
470 PRINT "DISTANCE, NOSE TIP TO TRANSITION 3 FRONT:";
480 INPUT X(4)
490 PRINT "FIN 1 ROOT CHORD:";
500 INPUT C1
510 PRINT "ROOT 1 TIP CHORD:";
520 INPUT C2
530 PRINT "FIN 1 SEMI-SPAN:";
540 INPUT S1
550 PRINT "FIN 1 MID-CHORD LINE LENGTH:";
560 INPUT S2
570 PRINT "NUMBER OF FINS, FIN 1:";
580 INPUT A1
590 PRINT "RADIUS OF BODY AT FIN 1:";
600 INPUT R1
610 PRINT "FIN 1 ROOT-TO-TIP LE SWEEP DISTANCE:";
620 INPUT X(5)
630 PRINT "DISTANCE NOSE TIP TO FIN 1 ROOT LE:";
640 INPUT X(6)
650 PRINT "ANY MORE FINS?"
660 INPUT D$
670 IF D$="NO" THEN 1030
680 PRINT "FIN 2 ROOT CHORD:";
690 INPUT C3
700 PRINT "ROOT 2 TIP CHORD:";
710 INPUT C4
720 PRINT "FIN 2 SEMI-SPAN:";
730 INPUT S3
740 PRINT "FIN 2 MID-CHORD LINE LENGTH:";
750 INPUT S4
760 PRINT "NUMBER OF FINS, FIN 2:";
770 INPUT A2
780 PRINT "RADIUS OF BODY AT FIN 2:";
790 INPUT R2
800 PRINT "FIN 2 ROOT-TO-TIP LE SWEEP DISTANCE:";
810 INPUT X(7)
820 PRINT "DISTANCE NOSE TIP TO FIN 2 ROOT LE:";
830 INPUT X(8)
840 PRINT "ANY MORE FINS?"
850 INPUT E$
860 IF E$="NO" THEN 1030
870 PRINT "FIN 3 ROOT CHORD:";
880 INPUT C5
890 PRINT "ROOT 3 TIP CHORD:";
900 INPUT C6
910 PRINT "FIN 3 SEMI-SPAN:";
920 INPUT S5
930 PRINT "FIN 3 MID-CHORD LINE LENGTH:";
940 INPUT S6
950 PRINT "NUMBER OF FINS, FIN 3:";
960 INPUT A3
970 PRINT "RADIUS OF BODY AT FIN 3:";
980 INPUT R3
990 PRINT "FIN 3 ROOT-TO-TIP LE SWEEP DISTANCE:";
1000 INPUT X(9)
1010 PRINT "DISTANCE NOSE TIP TO FIN 3 ROOT LE:";
1020 INPUT X(10)
1030 PRINT "DISTANCE, NOSE TIP TO CG:";
1040 INPUT X(11)
1050 REM CALCULATION OF NOSE ******************************************
1060 PRINT "ENTER NOSE SHAPE: 1=OGIVE, 2=CONE, 3=PARABOLA OR ELLIPSE ";
1070 INPUT U
1080 IF U=1 THEN 1110
1090 IF U=2 THEN 1130
1100 IF U=3 THEN 1150
1110 N2=X(1)*.466
1120 GOTO 1160
1130 N2=X(1)*.665
1140 GOTO 1160
1150 N2=X(1)*.5
1160 N1=2
1170 PRINT "NOSE NORMAL FORCE: ";N1
1180 PRINT "NOSE CP: ";N2
1190 IF A$="NO" THEN 1600
1200 REM CALCULATION OF TRANSITION 1 **********************************
1210 PRINT "TRANSITION NO. 1: "
1220 T(1)=(D2/D1)*(D2/D1)
1230 T(2)=(D3/D1)*(D3/D1)
1240 T(3)=(T(2)-T(1))*2
1250 T(4)=(D2/D3)*(D2/D3)
1260 T(5)=1-(D2/D3)
1270 T(6)=1-T(4)
1280 T(7)=1+(T(5)/T(6))
1290 T(8)=(L1/3)*T(7)
1300 T(9)=X(2)+T(8)
1310 PRINT "TRANSITION 1 NORMAL FORCE: ";T(3)
1315 PRINT "TRANSITION 1 CP: ";T(9)
1320 IF B$="NO" THEN 1600
1330 REM CALCULATION OF TRANSITION 2 **********************************
1340 PRINT "TRANSITION NO. 2: "
1350 T(11)=(D4/D1)*(D4/D1)
1360 T(12)=(D5/D1)*(D5/D1)
1370 T(13)=(T(12)-T(11))*2
1380 T(14)=(D4/D5)*(D4/D5)
1390 T(15)=1-(D4/D5)
1400 T(16)=1-T(14)
1410 T(17)=1+(T(15)/T(16))
1420 T(18)=(L2/3)*T(17)
1430 T(19)=X(3)+T(18)
1440 PRINT "TRANSITION 2 NORMAL FORCE: ";T(13)
1450 PRINT "TRANSITION 2 CP: ";T(19)
1460 IF C$="NO" THEN 1600
1470 REM CALCULATION OF TRANSITION 3 **********************************
1480 PRINT "TRANSITION NO. 3: "
1490 T(21)=(D6/D1)*(D6/D1)
1500 T(22)=(D7/D1)*(D7/D1)
1510 T(23)=(T(22)-T(21))*2
1520 T(24)=(D6/D7)*(D6/D7)
1530 T(25)=1-(D6/D7)
1540 T(26)=1-T(24):REM CORRECTED FROM STINE WHO HAS T(14)
1550 T(27)=1+(T(25)/T(26))
1560 T(28)=(L3/3)*T(27)
1570 T(29)=X(4)+T(28)
1580 PRINT "TRANSITION 3 NORMAL FORCE: ";T(23)
1590 PRINT "TRANSITION 3 CP: ";T(29)
1600 REM CALCULATION OF FIN 1 *****************************************
1610 PRINT "FIN NO. 1:"
1620 F(1)=(S1/D1)*(S1/D1)
1630 IF A1=3 THEN 1650
1640 IF A1=4 THEN 1670
1650 F(2)=F(1)*12:REM CORRECTED. STINE USES 13.85
1660 GOTO 1680
1670 F(2)=F(1)*16
1680 F(3)=2*S2
1690 F(4)=C1+C2
1700 F(5)=(F(3)/F(4))*F(3)/F(4)
1710 F(6)=1+F(5)
1720 F(7)=SQR(F(6))
1730 F(8)=1+F(7)
1740 F(9)=F(2)/F(8)
1750 F(10)=S1+R1
1760 F(11)=R1/F(10)
1770 F(12)=1+F(11)
1780 F(13)=F(12)*F(9)
1790 F(14)=X(5)/3
1800 F(15)=2*C2
1810 F(16)=F(15)+C1
1820 F(17)=F(16)/F(4)
1830 F(18)=F(14)*F(17)
1840 F(19)=C1*C2
1850 F(20)=F(19)/F(4)
1860 F(21)=F(4)-F(20)
1870 F(22)=F(21)/6
1880 F(23)=X(6)+F(18)+F(22)
1890 PRINT"FIN 1 NORMAL FORCE: ";F(13)
1900 PRINT "FIN 1 CP: ";F(23)
1910 IF D$="NO" THEN 2550
1920 REM CALCULATION OF FIN 2 *****************************************
1930 PRINT "FIN NO. 2:"
1940 F(31)=(S4/D1)*(S4/D1)
1950 IF A2=3 THEN 1970
1960 IF A2=4 THEN 1990
1970 F(32)=F(31)*12:REM CORRECTED. STINE USES 13.85
1980 GOTO 2000
1990 F(32)=F(31)*16
2000 F(33)=2*S5:REM CORRECTED FROM STINE ***
2010 F(34)=C3+C4
2020 F(35)=(F(33)/F(34))*F(33)/F(34)
2030 F(36)=1+F(35)
2040 F(37)=SQR(F(36))
2050 F(38)=1+F(37)
2060 F(39)=F(32)/F(38)
2070 F(40)=S3+R2
2080 F(41)=R2/F(40)
2090 F(42)=1+F(41)
2100 F(43)=F(42)*F(39)
2110 F(44)=X(7)/3
2120 F(45)=2*C4
2130 F(46)=F(45)+C3
2140 F(47)=F(46)/F(34)
2150 F(48)=F(44)*F(47)
2160 F(49)=C3*C4
2170 F(50)=F(49)/F(34)
2180 F(51)=F(34)-F(50)
2190 F(52)=F(51)/6
2200 F(53)=X(8)+F(48)+F(52)
2210 PRINT"FIN 2 NORMAL FORCE: ";F(43)
2220 PRINT "FIN 2 CP: ";F(53)
2230 IF E$="NO" THEN 2550
2240 REM CALCULATION OF FIN 3 *****************************************
2250 PRINT "FIN NO. 3:"
2260 F(61)=(S5/D1)*(S5/D1)
2270 IF A2=3 THEN 2290
2280 IF A2=4 THEN 2310
2290 F(62)=F(61)*12:REM CORRECTED. STINE USES 13.85
2300 GOTO 2320
2310 F(62)=F(61)*16
2320 F(63)=2*S6
2330 F(64)=C5+C6
2340 F(65)=(F(63)/F(64))*F(63)/F(64)
2350 F(66)=1+F(65)
2360 F(67)=SQR(F(66))
2370 F(68)=1+F(67)
2380 F(69)=F(62)/F(68)
2390 F(70)=S3+R3
2400 F(71)=R3/F(70)
2410 F(72)=1+F(71)
2420 F(73)=F(72)*F(69)
2430 F(74)=X(9)/3
2440 F(75)=2*C6:REM CORRECTED FROM STINE WHO HAS S6
2450 F(76)=F(75)+C5
2460 F(77)=F(76)/F(64)
2470 F(78)=F(74)*F(77)
2480 F(79)=C5*C6
2490 F(80)=F(79)/F(64)
2500 F(81)=F(64)-F(80)
2510 F(82)=F(81)/6
2520 F(83)=X(8)+F(78)+F(82)
2530 PRINT"FIN 3 NORMAL FORCE: ";F(73)
2540 PRINT "FIN 3 CP: ";F(83)
2550 REM CALCULATION OF TOTAL ROCKET **********************************
2560 PRINT "TOTAL ROCKET CALCULATIONS"
2570 REM STINE LEAVES THIS LINE OUT
2580 M1=N1+T(3)+T(13)+T(23)+F(13)+F(43)+F(73)
2590 PRINT "TOTAL ROCKET NORMAL FORCES"
2600 M2=N1*N2
2610 PRINT "NOSE MOMENT: ";M2
2620 M3=T(3)*T(9)
2630 PRINT "TRANSITION 1 MOMENT: ";M3
2640 M4=T(13)*T(19)
2650 PRINT "TRANSITION 2 MOMENT: ";M4
2660 M5=T(23)*T(29)
2670 PRINT "TRANSITION 3 MOMENT: ";M5
2680 M6=F(13)*F(23)
2690 PRINT "FIN 1 MOMENT: ";M6
2700 M7=F(43)*F(53)
2710 PRINT "FIN 2 MOMENT: ";M7
2720 M8=F(73)*F(83)
2730 PRINT "FIN 3 MOMENT: ";M8
2740 M9=M2+M3+M4+M5+M6+M7+M8
2750 PRINT "TOTAL ROCKET MOMENT: ";M9
2760 X(12)=M9/M1
2770 PRINT "ROCKET CP IS LOCATED ";X(12);" BEHIND NOSE TIP."
2780 PRINT "ROCKET CG IS LOCATED ";X(11);" BEHIND NOSE TIP."
2790 B1=X(12)-X(11)
2800 PRINT "ROCKET STABILITY MARGIN IS: ";B1;" DIMENSIONAL UNITS."
2810 IF B1<0 THEN 2830
2820 IF B1>0 THEN 2850
2830 PRINT"UNSTABLE ROCKET!!!"
2835 PRINT "NEUTRAL STABILITY: INSUFFICIENT!!!":GOTO 2920
2840 GOTO 2920
2850 B2=B1/D1
2860 PRINT "ROCKET STABILITY MARGIN IS: ";B2;" IN CALIBERS."
2870 IF B2<1! THEN 2890
2880 IF B2>1! THEN 2910
2890 PRINT "QUESTIONABLE STABILITY. HEADS UP!!!"
2900 GOTO 2920
2910 PRINT "STABLE ROCKET DESIGN."
2920 END
Chris Tavares
> BTW, a string test isn't worth the cost of the string.
A rocket that flies stably on the string test is known to be stable. A rocket
that doesn't is not necessarily unstable. It's a useful test for at least half
of the number line.
> Yes. Use the Barrowman equations. The only in print source I am
> aware of is one of the appendicies of Stine's handbook.
The equations were printed in a recent American Spacemodeling after having been
otherwise out of print for years. They are also available in a Tech Report from
NASA Goddard.
--
c...@pdp.sw.stratus.com --If you believe that I speak for my company,
OR c...@vos.stratus.com write today for my special Investors' Packet...
David Utidjian
I am in the process of rewriting stabcalc.bas in C, mainly because
I have problems downloading to a machine that has a basic interpreter. This
is after making several attempts. Also upon inspection of stabcalc.bas I
found several errors. I am waiting for a reply to a letter from Larry Curcio
to confirm this since, I think, he was the last person to modify stabcalc.bas.
At the moment there seems to be considerable interest in stability prediction
methods, especially for homebrew designs and HPRs.
At present the programs on calvin are a hodge-podge of BASIC, Pascal,
C, and what I assume are IBM binaries. It would be nice to have the source
code for the binaries posted so that they can be downloaded and compiled on
the various types of hardware/operating systems that the people who want to
use them have available. It is rather unwieldy at the moment to use the
various programs because of this. I am not qualified to comment on the
suitability or the correctness of the programs for their intended use. But
I am a user and in that sense I feel qualified to comment on the condition
of the programs on calvin. I must say that it is great that the programs
are even available for access by anonymous ftp. For this I owe, we all owe
a great deal of thanks to the people who have posted the programs and made
all of this available to all of us.
I think that the programs would be much handier if they were available
in the original source format and in both C and in BASIC. If that is really
possible. This would allow most everyone to use them with the minimum of
difficulty.
I would appreciate any and all input that people may have regarding
stabcalc.bas. Should any bells and whistles be added? What about supersonic
speeds? What are good sources of theory and practice regarding stability?
Are there any other programs already written that can cover this topic?
Any suggestions for the other programs?
Thanks for your time and attention in advance...
BTW could anyone out there send me a copy of the Barrowman equations
please? My copy of HBofMR is stashed in New Jersey at the moment.
-Dave-
Utidjian at your service...
email: utid...@remarque.berkeley.edu
snailmail: David Utidjian
2560 Bancroft Way #55
Berkeley, CA 94704
Lawrence Curcio
The only reason that binaries were included is that the source is in
TURBO III, and most people don't have that compiler.
I can update the source to a more recent version - it involves closing
all READ's with READLN's. (If you did that in TURBO III, you would
read an extra line - go figger). The PASCAL source is long for several reaons:
1) I used very similar routines for LAUNCH, BOOST, COAST, and DROP phases.
This was to reduce repeated decisions and increase speed. The tradeoff was,
in retrospect, a mistake.
2) There is a reasonably detailed user interface.
3) I tended to comment out code that I replaced over the years, rather than
to discard it.
4) PASCAL is prolix to begin with.
I have compared output with other sources, and things jibe when similar methods
are used (CAUTION - my default is 20 degrees F, most use 0 degrees. You can
reset temperature.)
As for STABCALC and RASP, I posted them only because there was demand for
them. I do not trust GHS programming and it is difficult to debug because
it is so awful. Sorry there is a problem. GHS violated a pattern that he
had established as he rewrote a routine over and over for want of subroutines.
The programs, RASP-79E and STABCALC do not work as published in the _HANDBOOK_
Personally, I think a restriction to C is a big mistake, but it might be
helpful to have directories by language. If it's any consolation, the thrust
curve stuff was originally written in C. I had to put it into BASIC to share
it. The updates went into the BASIC.
The reference for the PASCAL stuff is MANDELL, CAPORASO, and BENGEN. The
thrust curve stuff I wrote myself.
PLEASE don't say that source is not included. It is.
No one is error free, and it was never my intention to hide MY errors - only
to share my programs. I hope the poster will share his. It's getting pretty
damn lonely on calvin. (It was really good to see Kevin contributing!)
Regards,
-Larry
Joe Pfeiffer
Personally, I think a restriction to C is a big mistake, but it might be
helpful to have directories by language. If it's any consolation, the thrust
curve stuff was originally written in C. I had to put it into BASIC to share
it. The updates went into the BASIC.
I'd prefer to see the stuff kept by submittor, then organized however the
author wants under that. Once we get enough authors to make that worthwhile!
No one is error free, and it was never my intention to hide MY errors - only
to share my programs. I hope the poster will share his. It's getting pretty
damn lonely on calvin. (It was really good to see Kevin contributing!)
Right! Also, I liked seeing the postscript stuff that started
appearing -- more, more, more!
-Joe.
David Utidjian
Subject: Re: Stability calculations
ummary: apologies
Expires:
References: <QcJrALa00...@andrew.cmu.edu>
Sender:
Followup-To:
Distribution:
Organization: University of California, Berkeley
Keywords:
In article <QcJrALa00...@andrew.cmu.edu> lc...@andrew.cmu.edu (Lawrence Curcio) writes:
>Source for all binaries is included on calvin. They are in .pas files.
>The only reason that binaries were included is that the source is in
>TURBO III, and most people don't have that compiler.
>
Sorry Larry, You are quite right it has been a while since I looked
at those file in detail.
.... stuff deleted ....
>As for STABCALC and RASP, I posted them only because there was demand for
>had established as he rewrote a routine over and over for want of subroutines.
>
>The programs, RASP-79E and STABCALC do not work as published in the _HANDBOOK_
>
That is an understatement....
>Personally, I think a restriction to C is a big mistake, but it might be
>helpful to have directories by language. If it's any consolation, the thrust
>curve stuff was originally written in C. I had to put it into BASIC to share
>
I did not mean C and only C. C, however has some
distinct advantages over all other languages. It is portable, it is
relatively readable, and it is easier to translate into other languages
than the other way around.
Do you still have the thrust stuff in C? Could you post,
or email it to me?
>The reference for the PASCAL stuff is MANDELL, CAPORASO, and BENGEN. The
>thrust curve stuff I wrote myself.
>
>PLEASE don't say that source is not included. It is.
>
Again I apologize...
>No one is error free, and it was never my intention to hide MY errors - only
>to share my programs. I hope the poster will share his. It's getting pretty
>damn lonely on calvin. (It was really good to see Kevin contributing!)
>
>Regards,
>-Larry
There will be a rewrite of stabcalc.bas on calvin by monday.
Please anyone who is interested, check it for me and send comments.
I do not have any way to check it myself...