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Old 02-28-2010, 09:21 PM
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BEC BEC is offline
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Join Date: Feb 2009
Location: Auburn, Washington
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Default LPR motor weight errors....

I was using OpenRocket 0.9.6 to show the effects of some changes to a given design (BMS School Rocket - a BT-50-based 4FNC) to some students on Friday and wondered why OpenRocket gave me better stability margins with an Estes B6 than with an A8.

After a little scrutiny I figured out that the weights of the two motors - which come from Thrustcurve.org and to them from NAR cert data - are wrong. Both the full and empty weights of the B6 are understated and show up as lower than for an A8.

A closer look shows errors (relative to weights I've taken myself) in many of them. Some are slight and some are not so slight.

Through a circuitous path that involved editing the thrust curve files within the archive (using jZip on the .jar file) I have been able to get the weights that are consistent with my own measurements into my copy of 0.9.6. I had to change the existing files in the package, not add others, to get it to work.

But that really begs the question of how one gets the correct data in these online sources. Any ideas??

Also, I think there is an issue in that in the RASP file format "propellant weight" really is the weight of everything that gets burnt, but in the NAR cert data it literally the propellant weight, not that of the delay element or the ejection charge, and in little black powder motors that distinction can make quite a difference percentage-wise....

After looking at the cert data on the NAR site for Estes A8, B4, B6 and C6 motors and my own weights it looks to me as if this difference in interpretation of "propellant weight" may be a key bit. Now I need to go back to Thrustcurve.org and see what happened there.


On WECO motors - a tooltip pops up when selecting them in OpenRocket (maybe it's been added recently) which explains where the data came from - a combination of a different site and the author's own weights.

And on these as Quest A8-3s - the NAR cert info shows the propellant weight as 3.6g but in the RASP file it is shown as 0.0063 kg. Doing the calcs based on samples of spent cases and new motors in hand.... that latter value is close enough. Hmmmmmm........
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