I195J Thrust Curves

[atestani]

Aerotech I195J motor data is inconsistent and I would like to understand why (if possible).

This is the thrust curve from ThrustCurve.org:

Note the average thrust line is at 175N (not 195N as listed above the curve in the table of motor specifications).

This is what the thrust curve(s) look like in OpenRocket:

Compensating for the different Thrust axis scale, the yellow OR curve matches what is in ThrustCurve.org but OR uses the blue curve in simulations. Also, the "Show Details" tab in OR lists the Average thrust at 263N (different from 195 and 176 as above)

..and then over at RockSim it is this:


Can anyone explain what is going on here?

 

[jmasterj]

The curve on the aerotech instructions doesn't match any of those either:



Though it's pretty close to the blue one above. It would be useful to look at the certification document but it's not linked on thrustcurve.

 

[atestani]

Tripoli's web site only has motor certification documents back to 2018 and the I195J isn't listed.

Aerotech (RCS) resources don't include the I195J in their list of motor certification documents

 

[kjhambrick]

@atestani --

The attached 1999 data is from Sue McMurray when she was on the TMT team and when I was maintaining copies of the RASP.ENG files.

This is a picture of the thrust curve:

The raw TMT data and the converted RASP.ENG files are attached ...

Might be helpful ?

-- kjh

EDIT: Appended I195J.ENG inline
EDIT: Corrected Diameter -- was 29 should have been 38 mm ; also corrected attached aei197j,eng

;
; Thrust Curve gen'd by dosuess (AEH00432.DAP) for prog mkcurve
; TMT Comments:  MD10NOV98T75F4X13.0
I195J   38         297    6-10-14          0.3010   0.5702   AT    
         0.027     199.360
         0.031     190.500
         0.060     285.750
         0.063     299.040
         0.121     375.462
         0.152     368.816
         0.175     400.935
         0.229     447.452
         0.296     378.784
         0.321     385.430
         0.385     358.848
         0.425     378.784
         0.525     349.988
         0.542     363.279
         0.617     347.773
         0.706     362.171
         0.767     344.450
         0.779     352.203
         0.892     296.825
         0.902     301.255
         1.002     243.662
         1.015     243.662
         1.121     202.683
         1.140     204.898
         1.246     159.488
         1.256     166.133
         1.363     110.756
         1.373     116.293
         1.488      56.485
         1.500      58.700
         1.606      15.506
         1.748       0.000
;Total Impulse     426.50 (N-Sec)
;Avg Thrust        244.00 (N)
;Specific Imp      144.49 (Sec)

 

[neil_w]

1) I have no idea why those curves are so different
2) OR uses the data straight from Thrustcurve.org. Note that there are three motor files for that motor. You might inquire with John Coker if he knows anything about what's going on with that motor.
3) You can use any of the curves for simulation in OR if you like (although it seems like the blue one it's using is probably "most correct")

More info here: https://openrocket.info/tutorials/motor-selection.html

 

[atestani]

@atestani --

The attached 1999 data is from Sue McMurray when she was on the TMT team and when I was maintaining copies of the RASP.ENG files.

This is a picture of the thrust curve:
View attachment 634233
The raw TMT data and the converted RASP.ENG files are attached ...

Might be helpful ?

-- kjh

EDIT: Appended I195J.ENG inline

;
; Thrust Curve gen'd by dosuess (AEH00432.DAP) for prog mkcurve
; TMT Comments:  MD10NOV98T75F4X13.0
I195J   29         297    6-10-14          0.3010   0.5702   AT     
         0.027     199.360
         0.031     190.500
         0.060     285.750
         0.063     299.040
         0.121     375.462
         0.152     368.816
         0.175     400.935
         0.229     447.452
         0.296     378.784
         0.321     385.430
         0.385     358.848
         0.425     378.784
         0.525     349.988
         0.542     363.279
         0.617     347.773
         0.706     362.171
         0.767     344.450
         0.779     352.203
         0.892     296.825
         0.902     301.255
         1.002     243.662
         1.015     243.662
         1.121     202.683
         1.140     204.898
         1.246     159.488
         1.256     166.133
         1.363     110.756
         1.373     116.293
         1.488      56.485
         1.500      58.700
         1.606      15.506
         1.748       0.000
;Total Impulse     426.50 (N-Sec)
;Avg Thrust        244.00 (N)
;Specific Imp      144.49 (Sec)

Interesting... yet another average thrust number = 244 N

 

[kjhambrick]

Like the old saying goes ... a man with one watch always knows what time it is, but the man with two watches is never quite sure

I suppose the same goes for thrust cirves

-- kjh

 

[Buckeye]

Wow, this is horrible data for the I195.

Thrustcuve is essentially crowd-sourced, so the data can come from anywhere. In the Thrustcurve flight simulator, multiple curves are averaged.

Remember these motor discrepancies when somebody complains that the simulation doesn't match with "reality."

 

[neil_w]

Best course of action is to report to John Coker.

 

[cls]

I remember being surprised by several I195 flights, a lot more kick off the pad than expected, faster burn too. Anecdotes, no real data, sorry.

 

[rcktnut]

That one is all over the place, best course of action is to report it to Aerotech!! Looks like according to cls the thrust curve on the Aerotech instructions could be the correct one. Latest file for RS. Looked at the motor cert list (Sport Rocketry) and that lists it at 426.1 N-sec total impulse and a propellent mass of 272 grams. Cert by TRA.

 

[atestani]

I put a pointer to this thread on the AeroTech Open Thread to see if anyone there has any answers.

 

[MClark]

I was TMT after Sue McM.
We typically did 3 motors for each test. Average of three gave total impulse, burn time etc. Prettiest curve was used.
Slight differences in the igniter make big differences in the start of the curve. A smooth as cast surface makes a difference.
Everything makes a difference.

 

[atestani]

I was TMT after Sue McM.
We typically did 3 motors for each test. Average of three gave total impulse, burn time etc. Prettiest curve was used.
Slight differences in the igniter make big differences in the start of the curve. A smooth as cast surface makes a difference.
Everything makes a difference.

Understood... but we have multiple curves and average thrust numbers. Can you advise which is correct and what should be on thrustcurve.org and in OpenRocket?
Thanks

 

[MClark]

I don’t have any of the information in a form I can read. On a dead computer.
With the way motors are stored and ignited the data will always be all over the place. Oxidized surfaces, dust, moisture, size of igniter, materials in igniter, how much tape on nozzle (and an hundred other factors) all effect burn.

People do sims and take the output as absolute truth when it’s only as good as the inaccurate information put in. Why did my flight not match the sim? Welcome to the real world!

Every motor is like this, if thrust curves found, motor packaging, thrust curve.org, cert paperwork all match it’s because it is the same test fire. Motors fired in real world but multiple people are all over the place.

 

[Neutronium95]

In the end, it doesn't really matter that much. As long as the impulse and average thrust are pretty close, then the sim should be reasonably accurate.

If a small change in impulse or average thrust would be the difference between breaking a waiver or having an unsafe rail exit speed then that's already a too thin margin in my book. Find a different motor.

 

[kjhambrick]

I was TMT after Sue McM.
We typically did 3 motors for each test. Average of three gave total impulse, burn time etc. Prettiest curve was used.
Slight differences in the igniter make big differences in the start of the curve. A smooth as cast surface makes a difference.
Everything makes a difference.

I don’t have any of the information in a form I can read. On a dead computer.
With the way motors are stored and ignited the data will always be all over the place. Oxidized surfaces, dust, moisture, size of igniter, materials in igniter, how much tape on nozzle (and an hundred other factors) all effect burn.

People do sims and take the output as absolute truth when it’s only as good as the inaccurate information put in. Why did my flight not match the sim? Welcome to the real world!

Every motor is like this, if thrust curves found, motor packaging, thrust curve.org, cert paperwork all match it’s because it is the same test fire. Motors fired in real world but multiple people are all over the place.

Thanks Mark !

Sue sent data to Fred Brennion, Tim Van Milligan and I for this batch of tests ( September 1999 ) and there was a followup set in November 1999.

Sue sent a summary spreadsheet and a bunch of indivirual test stand files and I wrote a script called `dosuess` to automate the conversions for RASP from the spreadsheet summary and the individual files.

These results were from the Sep 1999 set.

I'll dig around in my TMT directory to see if I can find anything for I195J in the Nov 1999 set ... maybe there was a better run in the later set.

And Yes !!!

It is VERY important to remember the old GIGO rule, especially for sim's ... they're only as good as your input and the thrust curve and the environment and all things unimaginable

-- kjh

 

[jmasterj]

In the end, it doesn't really matter that much. As long as the impulse and average thrust are pretty close, then the sim should be reasonably accurate.

If a small change in impulse or average thrust would be the difference between breaking a waiver or having an unsafe rail exit speed then that's already a too thin margin in my book. Find a different motor.

This one varies from 250N to 425N in initial thrust. That's huge! Perfectly safe rail speed on the latter could be totally anemic on the former. This difference matters a lot because of the rail exit speed.

 

[kjhambrick]

This one varies from 250N to 425N in initial thrust. That's huge! Perfectly safe rail speed on the latter could be totally anemic on the former. This difference matters a lot because of the rail exit speed.

That is a GREAT point !

Rail exit speed is MUCH more important than apogee to me.

-- kjh

 

[AeroTech]

I was TMT after Sue McM.
We typically did 3 motors for each test. Average of three gave total impulse, burn time etc. Prettiest curve was used.
Slight differences in the igniter make big differences in the start of the curve. A smooth as cast surface makes a difference.
Everything makes a difference.

Mark is right on. Black Jack is slow burning and the method and speed of ignition can greatly affect the curve. Soot and slag can accumulate in the nozzle. Also it’s probably more affected by batch-to-batch particle size variations of the metal and AP.

 

[atestani]

This thread has gotten off track. Of course, there will be performance and use condition differences from motor to motor in actual use. The original question asked was why there are different thrust curves and values for an I195J in existence for the same motor and which one was the "official" one.

TRA, NAR, NFPA require the use of "certified motors" and do not allow any modification to those motors (unless the manufacturer allows it). It would seem logical that the specifics of a particular certified motor be documented and consistent. I tried to get a copy of NFPA 1125 (code for motor manufacturers) to see what the documentation requirements are but the account creation portion of the website doesn't work. If someone has NFPA 1125 please correct me, but I can't imagine that it allows 4 wildly different specifications for a specific HPR motor.

I195 is a certified motor, what is the one thrust curve we should be using? How many other motors have this problem?

 

[jmasterj]

I195 is a certified motor, what is the one thrust curve we should be using? How many other motors have this problem?

Still a reasonable question, but you're ignoring what you've been told above: that one thrust curve is either the middle ground of several tests or even an average of several tests. I too would like to know which I195 thrust curve to use for my sims, but to play rail exit speed safe you could just use the one with the least initial thrust.

 

[atestani]

Still a reasonable question, but you're ignoring what you've been told above: that one thrust curve is either the middle ground of several tests or even an average of several tests. I too would like to know which I195 thrust curve to use for my sims, but to play rail exit speed safe you could just use the one with the least initial thrust.

I'm not ignoring that, I'm just saying when all the averaging, etc. is done, there should be one "official" (nominal) thrust curve for each particular motor. When I design I do the sim with what (I assume) are the nominal specifications for a motor. I then apply my own safety factors on key parameters, e.g. rail exit speed, thrust/weight ratio, ground hit speed, etc.

 

[Tractionengines]

The original question asked was why there are different thrust curves and values for an I195J in existence for the same motor and which one was the "official" one.

I195 is a certified motor, what is the one thrust curve we should be using?

I'm not ignoring that, I'm just saying when all the averaging, etc. is done, there should be one "official" (nominal) thrust curve for each particular motor. When I design I do the sim with what (I assume) are the nominal specifications for a motor. I then apply my own safety factors on key parameters, e.g. rail exit speed, thrust/weight ratio, ground hit speed, etc.

In that case...USE THE ONE PRINTED ON LABEL ON THE SIDE OF THE TUBE THE MOTOR COMES IN. (Shown in post #2)

Also remember NFPA does not specify a specific number be 100% accurate.

[Edit to correct references]
NFPA 1125, Model Rocket Motors.... min of 10 tested samples:

• Total impulse: 6.7% max standard deviation.
• Average thrust: 20% or 1N whichever is greater
• Time delay: 20% or 1.5 sec whichever is greater. Not to exceed 3 secs from labelled value.

NFPA 1125, HPR Motors.... across min of 2 tested samples:

• Total impulse: 6.7% max standard deviation.
• Average thrust: 20% or 10N whichever is greater
• Time delay: 20% or 1.5 sec whichever is greater. Not to exceed 3 secs from labelled value.

 

[kjhambrick]

This thread has gotten off track. Of course, there will be performance and use condition differences from motor to motor in actual use. The original question asked was why there are different thrust curves and values for an I195J in existence for the same motor and which one was the "official" one.

TRA, NAR, NFPA require the use of "certified motors" and do not allow any modification to those motors (unless the manufacturer allows it). It would seem logical that the specifics of a particular certified motor be documented and consistent. I tried to get a copy of NFPA 1125 (code for motor manufacturers) to see what the documentation requirements are but the account creation portion of the website doesn't work. If someone has NFPA 1125 please correct me, but I can't imagine that it allows 4 wildly different specifications for a specific HPR motor.

I195 is a certified motor, what is the one thrust curve we should be using? How many other motors have this problem?

Point taken.

I've looked at the official Tripoli TMT Combined Motor List and the I195 listed there does not match propellant mass ...

I am confused.

Maybe @Alan Whitmore knows what's what ?

HTH

-- kjh

 

[JohnCoker]

Ideally we'd have the cert. letter for each motor so that the stats were correct. As far as thrust curves go, the first RASP file appears to be produced from the actual test stand data, but that was a long time ago.
https://www.thrustcurve.org/simfiles/5f4294d20002e90000000031/

I can delete any data which is incorrect/out of date, of course. If someone wants to submit an updated thrust curve, that would be the best solution. (ThrustCurve.org shows the most recent data file by default.)

 

[atestani]

Ideally we'd have the cert. letter for each motor so that the stats were correct. As far as thrust curves go, the first RASP file appears to be produced from the actual test stand data, but that was a long time ago.
https://www.thrustcurve.org/simfiles/5f4294d20002e90000000031/

I can delete any data which is incorrect/out of date, of course. If someone wants to submit an updated thrust curve, that would be the best solution. (ThrustCurve.org shows the most recent data file by default.)

Thanks, John. The RAS data in the link you posted shows and average thrust of 263 N (not 195 N as you would expect). In fact, you even show this discrepancy and others on ThrustCurve, i.e.:


As noted in this thread there are 4 different curves including the one from Aerotech. I'd be happy to submit a new curve if I knew which one is correct... but it seems we should use the cert letter wherever that may be!

 

[AeroTech]

 

[jmasterj]

Looks like the blue one from the OP.

 

[kjhambrick]

Looks like the blue one from the OP.

Thanks Gary !

If I am not mistaken, that is the one I attached to post #4 that came from Sue / TMT in Sep 1999

-- kjh