Openrocket Motor Lengths

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Ilina

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Do Openrocket motor lengths include the length of the casing or is it just the length of the grains? I'm looking at the Aerotech K750 and the Openrocket length is 10.51", while the total length of the RMS 75-1280 casing including closures seems to be 10.259". I'm not sure if the missing length is due to the nozzle sticking out past the aft closure?
 
The files come from Thrustcurve, which has had data entered by various people using various methods of measuring motor length (I've observed all three of total length, length ahead of thrust ring, and length ahead of thrust ring but not including forward closure protrusion, and there may be other ways people measured them). For the sake of fitting the motor to the rocket, best to go by the actual case if you have it in hand and by the manufacturer's drawing if you don't. For the sake of cg in the sim, the variations in supposed length can probably be shrugged off as less than the uncertainties elsewhere.
 
The files come from Thrustcurve, which has had data entered by various people using various methods of measuring motor length (I've observed all three of total length, length ahead of thrust ring, and length ahead of thrust ring but not including forward closure protrusion, and there may be other ways people measured them). For the sake of fitting the motor to the rocket, best to go by the actual case if you have it in hand and by the manufacturer's drawing if you don't. For the sake of cg in the sim, the variations in supposed length can probably be shrugged off as less than the uncertainties elsewhere.
Thank you!
 
The files come from Thrustcurve, which has had data entered by various people using various methods of measuring motor length
To expand on this a little: When it's available, most people lately use whatever length is reported in either the certification letter or the vendor press release information. For example, the AT M2700 TMT letter gives "overall length" and "outside the MMT" and troj entered the overall length. I haven't checked to see how TMT defined the lengths relative to the actual hardware.

Of course, most of the content on thrustcurve is from old RASP files that predate thrustcurve, where it's unclear what length was being used. You should always rely on actual drawings if you can get them and if you really need to know the dimensions in great detail.
 
To expand on this a little: When it's available, most people lately use whatever length is reported in either the certification letter or the vendor press release information. For example, the AT M2700 TMT letter gives "overall length" and "outside the MMT" and troj entered the overall length. I haven't checked to see how TMT defined the lengths relative to the actual hardware.

Of course, most of the content on thrustcurve is from old RASP files that predate thrustcurve, where it's unclear what length was being used. You should always rely on actual drawings if you can get them and if you really need to know the dimensions in great detail.
The TMT certification document listed the length of the AT RMS 75/10240 M2700W as 48.52 inch / 1232.5 mm

See: Tripoli.org > M2700W-PS > Certification ( this is the official TMT certification document )

Also see: ThrustCurve.org > AeroTech M2700W-PS ( this is where Open Rocket gets the set of Thrust Curves ).

The motor length in ThrustCurve.org and Open Rocket matches the Cert Doc so a lawyer would say that the length is correct.

However, the certified length does not match any of the dimensions on the AT / RCS > RMS Motor External Dimensions > RMS-75/10240 page ( see below ).

There are three lengths listed, all measured from the fwd end of the aft closure where the motor would fit flush into a motor mount:

Length of the case only.................: 45.17 inch / 1147 mm
Length to end of std closure..........: 46.18 inch / 1173 mm
Length to end of threaded closure: 46.95 inch / 1193 mm

The certified length does not match any of the above so an engineer would say it is totally wrong !

I noticed a number of similar differences in the Open Rocket / ThrustCurve.org data.

I don't know what the solution is but I don't imagine many lawyers use thrust curves in their court cases so maybe the thrust curve metadata needs to make the engineers happy ?

I dunno ...

-- kjh

Screenshot_20250116_160825.png
 
Sorry if I confused things with the M2700, which was just a convenient example. TMT said the "outside the MMT length" is 1.60", and 48.52-1.6 = 46.92" which is close to your "length to end of threaded closure" (there's no such thing as an unthreaded "standard" closure for 75mm, BTW.) AT doesn't give you the dimensions of the nozzle but it looks to me like TMT measured the total length of the motor from the end of the nozzle to the front of the closure, which makes sense.

IMHO the Thrustcurve numbers are perfectly adequate for the typical purpose they are used for, though they could be more consistent certainly.
 

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