bandman444
Well-Known Member
Which is the correctthrust curve for the J570W?
Its a big difference, 450 pounds initial thrust vs 25 pounds initial thrust?
Its a big difference, 450 pounds initial thrust vs 25 pounds initial thrust?
Aerotech Data vs. Thrustcurve
- Thread starter bandman444
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Interesting. If you look up the certified motors on Tripoli website, they list the maximum thrust of J570 at 973.1 Ns or 218.7 lbsec. in their certified list.
BUT, if you look at the cert file here
it lists maximum thrust as 1142.5 Ns or 256.7 lbsec. The thurst curve looks very much like the one from Thrustcurve.org.
Not to say the Manufacture is wrong, but....
I would use the curve from Tripoli or Thrustcurve. YMMV
BUT, if you look at the cert file here
it lists maximum thrust as 1142.5 Ns or 256.7 lbsec. The thurst curve looks very much like the one from Thrustcurve.org.
Not to say the Manufacture is wrong, but....
I would use the curve from Tripoli or Thrustcurve. YMMV
The J570 has a total impulse of 973.1 Ns and a maximum thrust of 1142.4 N (not Ns). On a side note, the plural form "Newtons" should IMHO be avoided because of its ambiguous nature in our context.
The J570 has quite a pronounced erosive spike at the beginning of the burn, as can be seen on the thrust curve provided by Aerotech. This is typical for long motors with WL or many other propellants.
Besides the fixed motor parameters (propellant, geometry), there are many variable factors that will likely influence this spike (type of igniter, igniter location, temperature,..).
Due to the transient nature of this spike, the measured thrust curve will be influenced by the nature of the thrust stand (mass, stiffness, dampening). On a motor with a more gradual rise/decline in thrust, this is less of an issue.
An analogy: If you hit a steel door with a hammer, it will sound differently than a wood door hit with the very same hammer. On the other hand, if you just lean against it, you won't notice a difference, unless it is a really flimsy one.
The electronic data acquisition system has also its particular characteristics, but often the electronics are so "fast" (high bandwidth) compared to the motor and thrust stand that its influence is negligible.
Last but not least, the data is post processed on the computer to put it in a more useful format for end use. This typically involves things like noise reduction (low pass filtering) and a final reduction of data points (the original RASP can only handle max. 32 of them). Both of these steps have the potential to smooth away the spike at the beginning, depending how exactly they are performed.
Because you can't know exactly what your average J570 will do really in your rocket, you should be very conservative. In other words, make sure the rocket can structurally handle the peak and that the rocket will still launch safely if it is not present (launch weight, rail length).
Reinhard
The J570 has quite a pronounced erosive spike at the beginning of the burn, as can be seen on the thrust curve provided by Aerotech. This is typical for long motors with WL or many other propellants.
Besides the fixed motor parameters (propellant, geometry), there are many variable factors that will likely influence this spike (type of igniter, igniter location, temperature,..).
Due to the transient nature of this spike, the measured thrust curve will be influenced by the nature of the thrust stand (mass, stiffness, dampening). On a motor with a more gradual rise/decline in thrust, this is less of an issue.
An analogy: If you hit a steel door with a hammer, it will sound differently than a wood door hit with the very same hammer. On the other hand, if you just lean against it, you won't notice a difference, unless it is a really flimsy one.
The electronic data acquisition system has also its particular characteristics, but often the electronics are so "fast" (high bandwidth) compared to the motor and thrust stand that its influence is negligible.
Last but not least, the data is post processed on the computer to put it in a more useful format for end use. This typically involves things like noise reduction (low pass filtering) and a final reduction of data points (the original RASP can only handle max. 32 of them). Both of these steps have the potential to smooth away the spike at the beginning, depending how exactly they are performed.
Because you can't know exactly what your average J570 will do really in your rocket, you should be very conservative. In other words, make sure the rocket can structurally handle the peak and that the rocket will still launch safely if it is not present (launch weight, rail length).
Reinhard
bobkrech
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Thrustcurve.org data varies greatly in it's accuracy as you don't know the source of the data.
IMO the best engine file data is obtained during NAR motor certifications and is published on each motor certification paperwork. The thrustcurves obtianed from several firings are critically analyzed and test stand artifacts are removed from the sim data.
In general the Total Impulse is the most impiortant oarameter followed by the burn time. A reasonably accurate sim can be obtained by dividing the total impulse by the burn time to obtain the average thrust. Using measured average thrust and the burn time gives a fairly accurate sim. The most accurate sims are obtained by digitizing and fitting the thrust curve into 16 or 32 points which is the data presented in the NAR certification paperwork using John DeMar's excellent Thrustcurvetool program.
If you don't have digited data you can expand the manufacturers or certification data and hand digitize it yourself. Just make sure the integral of the curve is the specified or measured total impulse of the motor.
Bob
IMO the best engine file data is obtained during NAR motor certifications and is published on each motor certification paperwork. The thrustcurves obtianed from several firings are critically analyzed and test stand artifacts are removed from the sim data.
In general the Total Impulse is the most impiortant oarameter followed by the burn time. A reasonably accurate sim can be obtained by dividing the total impulse by the burn time to obtain the average thrust. Using measured average thrust and the burn time gives a fairly accurate sim. The most accurate sims are obtained by digitizing and fitting the thrust curve into 16 or 32 points which is the data presented in the NAR certification paperwork using John DeMar's excellent Thrustcurvetool program.
If you don't have digited data you can expand the manufacturers or certification data and hand digitize it yourself. Just make sure the integral of the curve is the specified or measured total impulse of the motor.
Bob
bandman444
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The thrustcurve data matches the cert data much better than Aerotechs published data. What does that mean?
Is there anything gained or lost by the HUGE difference?
Is there anything gained or lost by the HUGE difference?
billspad
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It means that the source of the thrustcurve data was most likely the TMT certification sheet.
The best way to find out is run a sim with the thrustcurve data and then modify the engine file to match the Aerotech data and run it again.
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Yes. See this thread:
https://www.rocketryforum.com/showthread.php?t=24730
I narrowed down (for the most part) the simulation vs. altimeter comparison issue to the motor impulse discrepancy.
Correlating sims to real flights is nearly impossible due to the motor variation and misinformation out there. NAR has some standard deviations listed for thrust curves. Use them to bound your solution. It would be nice if all motors included the variance and statistics.
