1/4 Patriot Rocksim data accuracy?

[BravoIndiaLimaLima]

Hello, I'm starting my first HPR build, the 1/4 scale Madcow Patriot, but am struggling with simulations. I understand that people commonly add nosecone weight to this model, the Rocksim file comes with 12 oz. already in there. So, my thought was that I would make new fins that stick out .5 inches further, which seemed to make it stable even without the additional nosecone weight. But, my confusion comes in with other components of the Rocksim file. It doesn't have shock cord or eyebolt in the file, but does have a 7 oz. mass object toward the aft end of the rocket. I'm not sure what that is for. Maybe an average weight and balance location of the missing objects plus epoxy/paint weight? I'm concerned about this, because when I remove that mass object to make it more like I think it should be, that throws my stability out of whack again. So, I'm wondering if anyone has experience with this one, and what you have done to get accurate simulations?

Thanks,
Bill

 

[Rex R]

the only thing that really matters for simulations is the outside of the rocket, everything else is for the benefit of the builder. build the rocket then find the cg & overall mass then use the mass/cg overrides to see if it is stable. make sure to measure the nose cone length and adjust the design file as needed(MC has apparently changed the kit from time to time and not changed the file, as I recall the nose on mine differed by 3").
Rex

 

[BravoIndiaLimaLima]

Hmmmmm, I'm trying to wrap my head around this. That makes sense to build, find the center of gravity, then adjust the sim file to match. I guess I was just hoping to validate in advance that I am thinking along the right lines by making slightly bigger fins for this rocket. Of course, I don't think I really have anything to lose, because if I still need to move the CG forward, I can always add the nosecone weight later. Obviously, once I epoxy some fins in, then that's set and unchangeable, at least for this airframe.

 

[Rex R]

if it helps I needed/used 15 oz. of nose weight...and have used blocks of cheese as additional weight for bigger motors. the patriot has such small (span) fins that the stable/safe cg usually ends up near the center of the model, so increasing the span will help. the sim programs will give you a nice ballpark figures for mass etc. but you'll still want to measure mass and cg after building/ painting for the motor selection, so I wouldn't sweat it much if your actual weight comes out differently than what the program says(I doubt that you'll be flying the computer ).
Rex

 

[markkoelsch]

Post the file?

 

[BravoIndiaLimaLima]

Sure, here is the Rocksim file from Apogee. If someone could explain what that 7 oz. mass is in the end of the rocket, I'd appreciate it.

Bill

View attachment patriot.rkt

Post the file?

 

[Buckeye]

If someone could explain what that 7 oz. mass is in the end of the rocket, I'd appreciate it.

Probably a motor retainer and/or motor reducer adapter.

Design files from others reflect their build, not yours. Also, material databases are iffy. Do you have the rocket kit in front of you? Weigh each piece and override the mass and CG for each component in the Rocksim file.

 

[BravoIndiaLimaLima]

I have the kit in front of me, but will have to use a scale at work to weigh everything. I played with the fin design more, and got the sims to run more stable with larger fins. Since I have access to a laser engraver, I think I am going to cut some new fins to see if I can get by without all that nosecone weight. But, this does bring up an interesting point. When I pick an motor and simulate the launch, it has no idea which casing the reload is in, not that they likely weigh all that differently.

I am going to try to make the sim more accurate for my specific configuration. Interestingly, a friend of mine is building the DX3 for his first HPR. Besides having a longer upper tube, longer nosecone, and bigger fins, it's really similar to my Patriot kit. But, in the simulations, he has been getting far higher altitudes than I have on the same motor. I have a feeling that we'll both get some surprises when we actually finish the builds and launch.

Thank you, everyone, for your help. This has clearly been a learning experience already.

 

[Buckeye]

I have the kit in front of me, but will have to use a scale at work to weigh everything. I played with the fin design more, and got the sims to run more stable with larger fins. Since I have access to a laser engraver, I think I am going to cut some new fins to see if I can get by without all that nosecone weight. But, this does bring up an interesting point. When I pick an motor and simulate the launch, it has no idea which casing the reload is in, not that they likely weigh all that differently.

I am going to try to make the sim more accurate for my specific configuration. Interestingly, a friend of mine is building the DX3 for his first HPR. Besides having a longer upper tube, longer nosecone, and bigger fins, it's really similar to my Patriot kit. But, in the simulations, he has been getting far higher altitudes than I have on the same motor. I have a feeling that we'll both get some surprises when we actually finish the builds and launch.

Thank you, everyone, for your help. This has clearly been a learning experience already.

When you select a motor in the simulation, it includes the mass of the casing.

 

[dhbarr]

More mass can often lead to higher altitudes.

With suboptimal mass the air slows down your coast faster, with overoptimal your coast starts out lower and slower.

Try adding weight to the nose of your sim and see if you trade off some max speed for more altitude.

 

[Forever_Metal]

I have the kit in front of me, but will have to use a scale at work to weigh everything. I played with the fin design more, and got the sims to run more stable with larger fins. Since I have access to a laser engraver, I think I am going to cut some new fins to see if I can get by without all that nosecone weight. But, this does bring up an interesting point. When I pick an motor and simulate the launch, it has no idea which casing the reload is in, not that they likely weigh all that differently.

I am going to try to make the sim more accurate for my specific configuration. Interestingly, a friend of mine is building the DX3 for his first HPR. Besides having a longer upper tube, longer nosecone, and bigger fins, it's really similar to my Patriot kit. But, in the simulations, he has been getting far higher altitudes than I have on the same motor. I have a feeling that we'll both get some surprises when we actually finish the builds and launch.

Thank you, everyone, for your help. This has clearly been a learning experience already.

I had the same issue with my MC 4" Patriot. After assembly and mock-up I was worried by how much weight I'd have to add to get it stable (12 oz to start). Adding the 12ozs added another issue in that the motor I'd chosen was too slow off the pad and might cause a problem getting up to speed. So off to a bigger (and heavier) motor. And now more weight again... Ended up being 14-15 oz when all was said and done.

What I did was I built my OR file with all of the individual components measured as close as I could get with calipers and a 10nths ruler (even the retainer). Then I pre-weighed every individual piece (down to a 10th of a gram) and overrode the weight of each component if needed. Just to check I weighed all pieces together and compared to what OR said I had. I kept that number until I finished it to ready for color paint (2 coats of smooth primer). I then guesstimated where the additional weight would be centered (more for the fin-can due to adhesive weight+paint) and added mass object accordingly. Once I got the overall weight right and matched to the fie i checked CG empty and loaded to see where OR said it should be and overrode again. It was a long drawn out process of back and forth (i'm a little weird like that), but it did help in the long run. Ended up keeping the components as delivered just tweaked the motor selection a bit to make the flight more stable and altitude aware (not going to the moon... yet).

fm

 

[Rex R]

from the location of the 88g(3.1oz.) mass object, I would say that it is to represent the fin filets. I like the AT H242 for my patriot, gets it up off the rail nicely w/o going over high.
Rex