What's mass override in Rocsim?

I downloaded some Rocksim files and one of them had the mass override feature on for some of the parts and I'm not sure why that feature was used. I read the manual but it was rather generic.

I'm trying to add some mass to simulate a payload section. I would assume I would want the mass override off. Otherwise, it appears to actually change the mass of my sample payload??

In any event, what's this feature used for?

When you put together a bunch of parts to make a rocket in Rocksim, the program plugs in the weights of each of those parts as it knows them from its database. When it's done, the rocket is the sum of all of those weights.

When you build a rocket in real life, the rocket's weight is not really the sum of the weights of all the parts. There's the extra weight of the glue and/or epoxy, the fillers and primers, and the paint. There's also changes because you've sanded off some material, and perhaps cut a few holes in some of the parts.

In short, what your rocket really weighs is not what Rocksim expects it to weigh. You can make your simulation more accurate by actually weighing your rocket, and telling Rocksim to use that weight rather than the one that it expects. You can also do this override part-by-part. And you can also override the measured center of gravity - measure the rocket's real balance point and plug that it for the CG rather than just using the point that the program expects.

Mass override is also one way of accounting for the masses of all sorts of little things you don't want to bother putting into the simulation: screweyes and Nomex pads and snap swivels and the like.

And let's not forget the weight of fiberglass when you do your large rocket projects.

I don't Rocksim LPRs, only my HPRs.

My rockets almost never come out to the weight Rocksim projects. I feel it far easier to build the rocket, then override Rocksim's final weight (changing the weight of each part is waaaay too painstaking). Besides, overriding the completed weight accounts for all the epoxy and paint. I also balance the rocket fully prepped (without motor) and plug that into the CG override. With the correct weight CG and Rocksim's calculated CP, which is a reflection of rocket design and not weight, I can sim which motors will work with a particular rocket and still keep the CG at least 1 caliber in front of the CP. If nose weight needs to be added to fly a particular motor, I'll know that before flight time and prep accordingly.

Originally posted by MarkM
changing the weight of each part is waaaay too painstaking

Click to expand...

You are correct, fussing with a long list of itty bitty part weights can be a PITA. You should try it with a full-size production aircraft with tens of thousands of components. Fun.

The one advantage I can think of (for entering the detailed data) is that you can better model the pitch/yaw inertia of your rocket, and thereby get a better simulation of vehicle dynamics. This comes through as how much wobble (and lost altitude) your rocket may have. FWIW. Not having ever seen the "guts" of rocsim, I don't even know for a fact whether that software uses distributed mass data and inertia in its flight performance equations, so if you put in a whole bunch of detail weight data rocsim might not even be using it.

Pat, the over-ride thing will only become important if your actual rocket is **significantly** different from the rocsim model. Rocsim itself already has some (unknown?) amount of error due to estimated drag coefficients, estimated atmospheric conditions, and other similar math shortcuts. Being off a little on the weight isn't going to be a huge problem.

Thanks from the very informative responses everyone. It makes sense. I downloaded a Rocksim file from rocketreviews.com and the creator of that file had overidden the mass. Make sense for his use, but perhaps it might be better to turn that feature off when distributing a Rocksim file to others.