OpenRocket - Override Stage Mass/CG + [Never Launch] Motor for Variable Nosecone Weight

[BigMacDaddy]

Bit of a crazy idea here that came up as a comment on Facebook (maybe this has already been shared / discussed here but figured I would throw it out there for TRF).

One limitation of OpenRocket is that when you override the stage mass and CG (and check to override subcomponents) you cannot play around with different nosecone weights to find optimal stability for a given motor configuration.

I usually do this differently -- I use OR to calculate the CP of the design and then add nose weight till I get CG sufficiently in front of the forecasted CP location. However, there are a number of instances when it would be nice if you could have some components (particularly nosecone weight) that was not included in the subcomponents that were overridden by the stage mass / CG.

Turns out that the only subcomponents that are not overridden by the stage mass/CG are motors. So the idea I shared was to put an inner tube in the nosecone, make this a motor mount, check "Never" for the ignition option, and change out motors to play with different nosecone weights. This is only so useful since it only lets you use motors in the database (as far as I can tell there is no way to manually set or override the mass). I was thinking it might be more useful if someone setup various weights in the motor database - I would be happy with 1/4oz increments through 4oz or so.

Anyway, below are some screen captures to show this works.

 

[bad_idea]

I do this. I have 999 dummy motor files from 1g to 999g that I swap around. I suppose when I start building larger rockets, I'll need to make some heavier dummy motors.

Edit to add: I don't do this to avoid cg overrides, I use it to account for variable weights in various configurations when running sims. But I like the idea of using them your way too.

 

[BigMacDaddy]

I do this. I have 999 dummy motor files from 1g to 999g that I swap around. I suppose when I start building larger rockets, I'll need to make some heavier dummy motors.

Edit to add: I don't do this to avoid cg overrides, I use it to account for variable weights in various configurations when running sims.

Cool. Can you share an example of a dummy motor file? I am having issues creating them and importing into OR... Thanks!

 

[bad_idea]

This is the text of the 762g file, saved as 762.eng:

Z762 1 1 P 0.762 0.762 Configurable_Mass
0.001 0.0001
0.002 0.0001
0.003 0.0000

Since we don't have any Z motors in the hobby, naming the impulse as "Z" makes it easy to search up my dummy motors in the motor menu. The brand is "Configurable_Mass."

I learned through trial and error some of what I had to put into the file.

 

[neil_w]

I feel your pain in what you're trying to do. I have an old feature request filed to add this capability: https://github.com/openrocket/openrocket/issues/689

A semi-reasonable workaround, which I think is cleaner, is to add a mass object to make the OR model match the desired total mass. Adjust it's position until the CG matches the measured CG. Then you can add noseweight or whatever without any issue, because you haven't used any overrides.

 

[BigMacDaddy]

I feel your pain in what you're trying to do. I have an old feature request filed to add this capability: https://github.com/openrocket/openrocket/issues/689

A semi-reasonable workaround, which I think is cleaner, is to add a mass object to make the OR model match the desired total mass. Adjust it's position until the CG matches the measured CG. Then you can add noseweight or whatever without any issue, because you haven't used any overrides.

Thanks - Yes, I do something like this also. In my case, I play with the thickness of the NC and boattail which are usually ABS (similar weight to styrene) in my models till I get the Mass/CG in the place of my as-built model (usually starts out somewhat close when I set all the parts to be proper materials and thicknesses to start).

Anyway, the engine workaround was just another way to do something similar that some people may find useful.

 

[BigMacDaddy]

This is the text of the 762g file, saved as 762.eng:

Z762 1 1 P 0.762 0.762 Configurable_Mass
0.001 0.0001
0.002 0.0001
0.003 0.0000

Since we don't have any Z motors in the hobby, naming the impulse as "Z" makes it easy to search up my dummy motors in the motor menu. The brand is "Configurable_Mass."

I learned through trial and error some of what I had to put into the file.

Thank you - got it working for 1/2oz increments in the range I use. Just in case

 

[bad_idea]

My most common use of the fake motors is to insert a JLCR into rockets that I might fly without a chute release on smaller motors. So it might fly on a C motor with an empty, massless fake motor tube placed right where the parachute is, then on a D or an E it would fly with an unlighted motor named Z023 which happens to mass 23g loaded into that tube (or whatever the JLCR's mass is, I don't have it in front of me now).

The second thing I use it for is to simulate removable nose weights when larger motors require it. So I have several rockets I sim with two fake motors, one for removable weight, one for the JLCR.