Maximum Velocity for commercial accelerometer

Does anyone know what the maximum velocity standard accelerometers are designed for? Typically they spec the maximum acceleration being that is a function of the
sensor used. But the internal algorithm that calculates velocity from the acceleration measurments has alot of trade offs.
For instance if they are using 8bit variables then the velocity variable can go to +-127. They can control how large they need the velocity value to be in several ways I can see. First off they can sample slower or more simply accumulate velocity fewer times a second than they are sampling and storing. Then they can also reduce the bit count of the sample. From say 8 or 10 to 6bits.
This velocity value is used to determin apogee. So a 16bit variable would give better resolution, but the trade offs that need to be made to use an 8bit variable might have little or no impact on the practicle use of the accelerometer.
Just wondering RDH8
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Robert DeHate wrote:

I'm sure it's stored in a larger variable (probably 16 bits or more). RAM is cheap in microchips nowdays, and microchips can store variables in 16bits easily, even if it's an 8-bit microchip.
It's really dependant on the code that's controlling everything, but for the types of flight computers commercially available for rockets, it's probably well over what you can ever get a rocket to do. I imagine it'd be really negative advertisement if a company that makes them ever had to acknowledge a bug in their software that limited use to "slower rockets".
Hope that helps, Dave
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This seems to be a closely guarded secret for most altimeters but the subject has been discussed on the RDAS list. If my memory serves, the 1G offset is stored as a 10.6 bit fixed point value. Thus, the ADC reading is shifted before subtracting the offset.
The RDAS uses a 10 bit ADC so the maximum value for acceleration after removing the zero offset is 512 counts. (nominally 50 G's) The maximum sample rate is 200 SPS. A little math reveals the maximum amount of time you can sustain this acceleration before overflowing the 32 bit variable used.
2^31 / (200 SPS * (512 << 6)) = 327 seconds
Or was that an unsigned variable and 655 seconds? No matter.
Robert DeHate wrote: > Does anyone know what the maximum velocity standard accelerometers are > designed for? > Typically they spec the maximum acceleration being that is a function of the > sensor used. > But the internal algorithm that calculates velocity from the acceleration > measurments has alot of trade offs. > > For instance if they are using 8bit variables then the velocity variable can > go to +-127. > They can control how large they need the velocity value to be in several > ways I can see. > First off they can sample slower or more simply accumulate velocity fewer > times a second than they are sampling and storing. > Then they can also reduce the bit count of the sample. > From say 8 or 10 to 6bits. > > This velocity value is used to determin apogee. > So a 16bit variable would give better resolution, but the trade offs that > need to be made to use an 8bit variable might have little or no impact on > the practicle use of the accelerometer. > > Just wondering > RDH8 > >
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Interesting topic!
It also got me wondering, what's the most G's experienced by a high power sport rocket so far? That might even be a cool contest, to see who can pull the most G's and still get their rocket back in one piece.
h
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snipped-for-privacy@aol.com wrote:

Hmm... Airstart the sustainer after apogee, with altimeter based deployment??? <G>
David Erbas-White
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On 2 Jun 2005 17:27:54 -0700, " snipped-for-privacy@aol.com"

If you want a simple way to acheive neck-snapping levels of acceleration, with three-digit gees, there's no reason to go HPR, and every reason to avoid it. If you have RockSim or something similar, design the fastest HPR rocket you can, and then compare your simulation results to a sim of an Estes Mosquito on an A10-3T motor. Then, modify the fins on the Mosquito to be a nice, small, clipped delta shape, and re-run the sim again.
F=ma, so a=F/m. Increasing "F" is one way to get high "a", but it's a heck of a lot easier to reduce "m". Especially when you consider that high "F" motors tend to be high "m" motors, and require high "m" building techniques, as well.
- Rick "Micromaxx Black Brant downscale?" Dickinson
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"I've always been taught that if you code an arbitrary limit, try to
make it a power of two, or at least avoid powers of ten, so people
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Rick Dickinson wrote:

Good point. On the other hand, it's more of a challenge to do it with a high power rocket.
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Here's some I've flown but until now never simulated:
USR J880 with fins glued on: 100g's. This was a SU motor with 3 plywood fins 3" root, 1.5" span, 1.5" LE sweep, 1/8" thick. A single layer of 2 oz glass went tip to tip. A birch nose cone was turned and bored out to fit over the 29 mm 'stinger' containing the delay/ejection. Launched in 1992 from a tower - no lugs. Stock Estes Gnome on EX D40: 205g's. The Gnome will survive once, the second flight breaks the fins off. Vulcan G500 with fins: 380g's. I flew several of these from 1990-1992. 4 plywood fins, all were fire and forget. Tower launched. I usually removed the ejection and fitted a balsa nose cone into the top of the motor and allowed it to lawn dart. Liftoff weight 150g, propellant weight 62.5g, 139 N-s in .28 second burn with a nearly square thrust curve.
Tom
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:)
Is that motor one with slivers of propellant as the grain geometry?

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Jerry Irvine, Box 1242, Claremont, California 91711 USA
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Can I get one of those for a Mosquito?
:-)
    Bob Kaplow    NAR # 18L    TRA # "Impeach the TRA BoD"         >>> To reply, remove the TRABoD! <<< Kaplow Klips & Baffle:    http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org
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kaplow snipped-for-privacy@encompasserve.org.TRABoD (Bob Kaplow) wrote:

Here is a makable motor that fits in a 38-1440 case (~23") It has about 300G's peak 3/4 into the 0.7s burn. Pretty high G's for a larger motor. Monolithic grain bonded to the liner.
Thrust curve 0.0s 264.6# 0.5s 606.1# 0.7s 199.9# 1.42# BK pro 0.7s burn #-s 298.28 # 426.11 isp 210.05

CD TO MACH 10, ALT TO 2,320,000 FT Program name: ORBIT.BAS -COPYRIGHT 1983, 1984, 1986, 1991, 1996 JERRY IRVINE ENTER NAME OF ROCKET (50 OR LESS LETTERS)1 DO YOU WISH TO RUN ALT2 (SINGLE/BOOSTER STAGE) VERSION OR JERRY (UPPER STAGE) VERSION? (ALT2=1 JERRY=2) 1 THE TIME INCREMENT FOR THE RUNGE-KUTTA CALCULATIONS IS 0.10 SEC DURING COAST. ENTER TIME INCREMENT DURING BOOST (0.01 SEC OR 0.10 SEC) .1
ENTER ROCKET MOTOR BURN TIME (SEC): .7 PROPELLANT WEIGHT (LBS): ENTER ZERO TO INPUT GRAMS 1.42
ENTER CRITICAL ENDPOINTS OF ROCKET MOTOR THRUST CURVE IN TIME INCREMENTS OF NO LESS THAN0.1 SEC (ROCKET MOTOR BURN TIME MUST BE LESS THAN 40 SEC)
AT TIME=0.0 SEC THRUST=0.0 LBS AT TIME=BURN TIME THRUST=0.0 LBS
ENTER NEXT ENDPOINT ENTER TIME (SEC)= 0 ENTER THRUST (LBS)&4.6 ENTER NEXT ENDPOINT ENTER TIME (SEC)= .5 ENTER THRUST (LBS)`6.1 ENTER NEXT ENDPOINT ENTER TIME (SEC)= .7 ENTER THRUST (LBS)9.9 TOTAL IMPULSE POUND-SECONDS = 298.275 TOTAL IMPULSE NEWTON-SECONDS = 1326.7272 AVERAGE THRUST NEWTONS = 1895.324571 AVERAGE THRUST POUNDS = 426.107143 SPECIFIC IMPULSE (LB-SEC/LB) = 210.052817
HOW MANY MOTORS (OF THE SAME TYPE) DOES THE ROCKET USE? (IF MORE THAN 1, ENTER TOTAL PROPELLANT WEIGHT OF ENTIRE CLUSTER FOR PROPELLANT WEIGHT OF ROCKET) ENTER NUMBER OF MOTORS= 1 VERIFY THE ENTIRE THRUST CURVE? (YES=1 N0=2) 2 MALEWICKI CHART DATA (ALT v WEIGHT)? (YES=1,NO=2) 1 ENTER LOWEST REALISTIC WEIGHT OF ROCKET VEHICLE (LBS): 3 ENTER MAXIMUM LIFT-OFF WEIGHT OF ROCKET VEHICLE (LBS): 15 ENTER WEIGHT INCREMENT, DELTA W (LBS) .5 ENTER ROCKET OVERALL REPRESENTATIVE SUBSONIC CD, CDr (THE OVERALL REPRESENTATIVE SUBSONIC CD IS THE AVERAGE OF THE CD VALUES AT REYNOLDS NO. OF 10^6 AND 10^7) .5 ENTER MAXIMUM DIAMETER OF ROCKET (INCHES): 1.5 ENTER LENGTH OF ROCKET (INCHES): 30 ENTER TEMPERATURE OF AIR AT LAUNCH SITE (DEG F) (SEA LEVELY F, LUCERNE DRY LAKE F) 80 ENTER AIR PRESSURE AT LAUNCH SITE (IN-HG) (SEA LEVEL).92, LUCERNE DRY LAKE'.6575) 29 SPEED OF SOUND AT LAUNCH SITE (FT/SEC)= 1138.822251
1 CDA (IN^2)=0.883547 ORBIT.BAS COPYRIGHT JERRY IRVINE WEIGHT ALTITUDE BURNOUT BURNOUT MACH ALTITUDE COAST (LBS) (FT) ALT (FT) VEL(FPS) MAX (MILES) TIME (S) --------- --------- --------- --------- ----- ---------- --------- 3.0 16617.2 994.3 3717.1 3.3 3.1 23.1 3.5 17581.0 827.9 3071.3 2.7 3.3 25.2 4.0 17960.3 706.8 2608.2 2.3 3.4 26.6 4.5 18095.3 621.0 2283.7 2.0 3.4 27.6 5.0 17887.1 551.2 2021.2 1.8 3.4 28.3 5.5 17444.3 494.5 1806.8 1.6 3.3 28.6 6.0 17003.5 450.3 1646.0 1.4 3.2 28.8 6.5 16392.3 412.1 1503.8 1.3 3.1 28.7 7.0 15709.1 379.7 1381.6 1.2 3.0 28.5
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That's not really a fair question as stated, because the lighter the rocket, the easier to achieve high G's without any damage. Most any estes kit can handle more G's that your average high power rocket.
My personal "best" is 20.4 G's on a 53 lb rocket, using a cluster of home-made motors with 0.6 second burn time.
-- David

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Emanual Avionics was unable to withstand about 300G's.
I suggest keeping it under 50.
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Jerry Irvine, Box 1242, Claremont, California 91711 USA
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On 2 Jun 2005 17:27:54 -0700, " snipped-for-privacy@aol.com"

Or a variation - who can pull the most G's ... without breaking their accelerometer. :-)
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I'll bet that a Mosquito on an A10 exceeds the G forces on any HPR rocket.
    Bob Kaplow    NAR # 18L    TRA # "Impeach the TRA BoD"         >>> To reply, remove the TRABoD! <<< Kaplow Klips & Baffle:    http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org
We must have faith in our democratic system and our Constitution, and in our ability to protect at the same time both the freedom and the security of all Americans.
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kaplow snipped-for-privacy@encompasserve.org.TRABoD (Bob Kaplow) writes:

Maybe so but ROOOOOOAAAAAAAARRRRRRRRRR is much more exciting than pfft.
FWIW my 6ft long 54mm ID rocket has been known to pull 42G on fast blue thunder motors. Very nice!
Chris
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True, but the question was pulling the most Gs and surviving.
    Bob Kaplow    NAR # 18L    TRA # "Impeach the TRA BoD"         >>> To reply, remove the TRABoD! <<< Kaplow Klips & Baffle:    http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org
    Voting for "the lesser of two evils" is still voting for evil.
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About 150 G's, according to RockSim.
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