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jbrracer

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You would have to also post your open rocket file so that we can pick through it to be able to answer that question, but quite often it's because form factor and actual weight in the sim does not match the real world item.

If those are correct, then play with overall surface finish. Most of us get to within 5% easily, and many get within 1%
The weight length and diameter are all correct. Ive even adjusted the barometric pressure....



PS The max altitude via the TRS is 6507 via the filter data.
 

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Charles_McG

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Ok, to get a reasonable match between sim and flight, I had to set all surfaces to polished. And most importantly, set the aft finset to 'rounded'.

Altitude comparison, up to apogee.
1605660709119.png
 

Banzai88

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Ok, to get a reasonable match between sim and flight, I had to set all surfaces to polished. And most importantly, set the aft finset to 'rounded'.

Altitude comparison, up to apogee.
View attachment 438718
I had to set both fin sets to rounded to match and set for all a Polished finish.
 
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Charles_McG

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One more - a blow up of the first 5 seconds. The anomaly coincides with the regressive part of the motor burn. I'm thinking something shifted in the body and pushed air into the avbay.

1605661317331.png
 

Banzai88

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One more - a blow up of the first 5 seconds. The anomaly coincides with the regressive part of the motor burn. I'm thinking something shifted in the body and pushed air into the avbay.

View attachment 438720
Probably most likely the forward recovery bundle. But that would also seem to indicate that the av bay needs the hardware penetrations sealed to avoid any further issues.
 

jbrracer

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Probably most likely the forward recovery bundle. But that would also seem to indicate that the av bay needs the hardware penetrations sealed to avoid any further issues.
I didn’t know sealing the eBay was a Necessity like that. I had over sized holes for passing ejection igniters though and a unused hole from an after thought / change of plans. When loading the payload bay should I be shoving the bundle down ?
 

TheBru

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Are you kidding me? The rounded fin leading edge makes 800. - 1000 foot difference?!?
The finish of the materials generally makes a bigger difference for me but the fins help to get it all a bit closer to reality when needed.
 

heada

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I didn’t know sealing the eBay was a Necessity like that. I had over sized holes for passing ejection igniters though and a unused hole from an after thought / change of plans. When loading the payload bay should I be shoving the bundle down ?
The fore and aft end of the av-bay should be air tight. You don't want any gases going from those compartments into the av-bay. When the charges fire, you're creating a large pressure area that is full of smoke and soot from the charges. The pressure sensor really doesn't like large spikes or nasty residue.

The nosecone should be tight enough and there should be no air leaking that when you cover the vent hole in the forward airframe, pulling on the nose creates a vacuum and you can almost lift the rocket.
 

Banzai88

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Are you kidding me? The rounded fin leading edge makes 800. - 1000 foot difference?!?
Yes. If you get the form factor, actual weight, and actual CG 99% sim vs. real world, it's quite easy from that point to mess with surface finish (Set for all or mix finish for various components), the parachute CD (adjust to match recorded descent rate, drogue and main), and launch conditions in the launch tab to get within 1% sim vs. real world.

Play around with the actual launch conditions for the day (average wind speed and direction) in the launch tab, then plot/export a sim flight and choose GROUND TRACK, and you can sim to your heart's desire current or different conditions and have a reasonable idea of where your rocket will land, too.
That methodology has been used more than a few times to help folks find rockets, especially L3 birds that achieve near-waiver altitudes, that have an uncommanded main at apogee and drift for miles!

I didn’t know sealing the eBay was a Necessity like that. I had over sized holes for passing ejection igniters though and a unused hole from an after thought / change of plans. When loading the payload bay should I be shoving the bundle down ?
Yes, for several reasons. You don't want things like ejection spikes messing with your avionics, specifically the baro sensor. Good chance to blow it out or overstress it and contribute to a failure. Further, BP residue is corrosive, especially to electronics traces on a circuit board. Goes without saying that that's not something you generally want in contact with your avionics.
Lots of folks seal the hardware pass thoughs with neoprene washers or maybe a dab of RTV. Others use a mix of things, and for wire pass throughs or temporary plugs, a hunk of poster putty comes in handy!

When loading the recovery bundle, upper or lower bay, it's good practice to make sure its as far down as it will easily go. ANY shift of weight aft will shift the CG. Uncomanded, uncontrolled during launch is NOT the time for that to happen and it lead to anything good.
It's also good practice in your sim to place the recovery bundles as far aft in the rocket sim as any aft bulkhead will allow when doing build CG estimates, since that's most likely where they'll end up under thrust anyway. Gives you a 'worst case' scenario of CG when in the planning/build phase of a rocket, and thus the greatest margin for safety because you have more accurate information.
 
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Buckeye

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I didn’t know sealing the eBay was a Necessity like that. I had over sized holes for passing ejection igniters though and a unused hole ... ?
Well, then. I take back my comment about no weird pressurizations. However, the unfiltered baro spike at apogee is not as bad as you would expect for open holes into the avbay. Even a well-sealed bay can show this ~500 ft anomaly at apogee. This spike can also come from an acceleration jolt to the baro, in addition to pressure from the ejection charge.
 

Charles_McG

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I plan on Continuing this on other flights, I’d be happy to tag you in them and have your opinion for sure!
Well, I love working up my own data. I don't know that I would want to routinely work up other folks' flights. Friends and customers, maybe :).

I had to fiddle with your sim a fair bit to get a match to the flight. And in directions I'm not used to tuning. I normally have to add mass and drag. Is there a reason you saved the sim as Rocksim rather than OpenRocket? I'm also suspicious that rounding the aft finset had the bigger impact on altitude of the two. I don't know if it's just due to size and angle, or if something screwy is going on because the object order doesn't match the fore-aft order and is breaking an assumption in the calcs.

And I'm not taking the time to figure it out.
 

Buckeye

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I would not spend too much time fiddling to match the altitude of the flight beyond 5% or so, considering the altimeter standard atmosphere model is always wrong by some amount. They measure too high in cold weather, too low in hot weather. It was cold at the OP's launch, with serious weather fronts moving in, and barometric pressure changes. Plus, I believe OR's implementation of Standard Atmosphere is messed up, while RockSim looks correct.

 

jbrracer

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Well, I love working up my own data. I don't know that I would want to routinely work up other folks' flights. Friends and customers, maybe :).

I had to fiddle with your sim a fair bit to get a match to the flight. And in directions I'm not used to tuning. I normally have to add mass and drag. Is there a reason you saved the sim as Rocksim rather than OpenRocket? I'm also suspicious that rounding the aft finset had the bigger impact on altitude of the two. I don't know if it's just due to size and angle, or if something screwy is going on because the object order doesn't match the fore-aft order and is breaking an assumption in the calcs.

And I'm not taking the time to figure it out.

Ill revert the fins to as flown change the order in open rocket and post it here.
As for saving as rocksim I just saved it and didnt pay much attention to it. Ill save fas OP next

Thanks again!
 

Charles_McG

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I don’t go too bonkers with sims - but the first pass on this sim came out at 3300’. With fiddling it jumped up to 4600’ - but I don’t know what I did. Surface and fin tuning got it what I would call ‘close enough’.

If you’re overlaying sim and flight data, they need to be moderately close. Sometimes the tweaks themselves feel instructive. Flight data with both baro and accelerometer is more interesting. I’ve got a few flights logged with the same rocket and motors - and the motor variability pops right out on the accelerometer data.
 

Banzai88

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Slightly more conventional layout for the sim file. Presented as a suggestion on technique. Information added on the various click tabs for each component. Sims to ~3% of actual performance....well within the noise of motor and weather condition variation.
 

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cerving

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I've found that to get OR to be reasonably accurate, you have to weigh every component, make sure the finishes are correct (don't used "polished" when it's "regular paint", for example), and the temperature, launch altitude, rail angle, and other launch parameters need to be correct. YMMV of course...
 

Banzai88

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I've found that to get OR to be reasonably accurate, you have to weigh every component, make sure the finishes are correct (don't used "polished" when it's "regular paint", for example), and the temperature, launch altitude, rail angle, and other launch parameters need to be correct. YMMV of course...
Weighing each component helps with build planning, but doing the master override on CG and actual weight is still required to ensure an accurate, as built, sim because it captures glue and paint and all those other fiddly little bits that would make building the sim tedious and time consuming.
 
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