OpenRocket question - is weight of RMS hardware taken into account?

[Dave S.]

I had a question about OpenRocket. If you are simulating an RMS motor in the rocket is the weight and Cg only taking into account the weight of the reload - or the reload and the case hardware also?

I did an example calculation comparing the weight reported of a simulation empty, compared to that where a DMS motor a (38mm H219) is used and the weight seems to be different by an amount very close to what eh Apogee web site reports for the weight of the DMS. That is correct if you are using a DMS motor.

However, when I compared the weight of the empty rocket to that reported when using an H242 DMS kit, the increase with the DMS kit is close to the weight reported on the Apogee site for the DMS kit itself.

What about the reload hardware itself? You would think that should be taken into account in OpenRocket- is it?

Obviously OpenRocket will not know if you are using the motor case intended for the motor or perhaps a longer one w/ spacers, but at least the weight of the intended motor case should be used in the simulation. Is it?

Journey 75 - 38MM Weight Comparison


DMS Comparison:
56.3 oz = 3.519 lbs w/ no motors per OpenRocket

65.5 oz = 4.094 lbs per OpenRocket w/ H219 DMS motor
H219 DMS motor weighs 256 grams per Apogee = 0.565 lbs
Difference between OpenRocket weights = 0.575 lbs [close to actual DMS weight per Apogee - good!]


RMS Comparison:
H242T DMS kit = 279 grams per Apogee = 9.841 ounces = 0.615 pounds [needs 39/240 case]

65.8 oz = w/ H245 RMS motor per OpenRocket
Difference between OpenRocket weights = 65.8 - 56.3 = 9.5 oz [close to actual DMS weight per Apogee - bad!]

 

[manixFan]

You need to look at the details for the motor you pick in OpenRocket, not what is reported elsewhere. When you are selecting a motor, click the "Show Details" button and see what is listed as the Launch Mass and Empty Mass. Those are the values that OR uses. You can then verify it is using the Launch Mass by checking the 'Mass with no motors' vs 'Mass with motors' values in the upper right of the View screen. If the weight of the motor is wrong in the ThrustCurve.Org database, then it will also be wrong in OR. I've had several instances where data in the TC database was either obsolete or just entered incorrectly.


Tony


For the H242T, here are the weights that Open Rocket will use:


You can see here if you compare the 'Mass with no motors' to the 'Mass with Motors', it varies by exactly 269 grams.

 

[manixFan]

You can test with motors of different loaded weights to make sure the change in CG makes sense.

Here's the GC with no motor:



And with the H242:



I should also have added that I have had to make new motor files and them add them to OR in order to be able to run sims with the correct values.


Tony

 

[neil_w]

The short answer is "yes", weight of the case is included, as reported in the data from thrustcurve.org.

 

[JoePfeiffer]

As others have said, yes OR does consider the mass of the casing. But I'm having a hard time understanding the comparison you're making, at least in part because you're going back and forth between ounces, pounds, and grams instead of using consistent units. So...

Journey 75 - 38MM Weight Comparison

DMS Comparison:
56.3 oz = 3.519 lbs w/ no motors per OpenRocket

1596 grams

65.5 oz = 4.094 lbs per OpenRocket w/ H219 DMS motor

1856 grams

H219 DMS motor weighs 256 grams per Apogee = 0.565 lbs
Difference between OpenRocket weights = 0.575 lbs [close to actual DMS weight per Apogee - good!]

1596 + 256 = 1852, so pretty close. Thrustcurve.org says the H219 weighs 261 and 1596 + 261 = 1857, so that's good.

RMS Comparison:
H242T DMS kit = 279 grams per Apogee = 9.841 ounces = 0.615 pounds [needs 39/240 case]

Do you mean H242T RMS? thrustcurve.org shows a weight of 264 grams. I don't know where apogee gets their 279.

65.8 oz = w/ H245 RMS motor per OpenRocket

Do you mean H242? 1865 grams

Difference between OpenRocket weights = 65.8 - 56.3 = 9.5 oz [close to actual DMS weight per Apogee - bad!]

1865 - 1596 = 269. Should be 264, of course, but it's close. I'll guess only showing the Imperial weights to three significant digits accounts for it.