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OpenRocket 3" vs 2.6" ???

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dhkaiser

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Recently I have built and simulated two rockets, A Mac Performance 3" Scorpion and a Madcow 2.6" DX3, both single deploy. I have a question regarding the OpenRocket simulations. The larger Scorpion with a AT I280 motor sims out to 4747ft weighting at 78.4oz. The smaller and lighter DX3 sims out to 4876ft at 66.3oz on the same motor.

I would think the smaller rocket, weighting 12 oz less, would fly much higher than the larger heavier rocket. Being relatively new to rocketry I figure I am missing something? Any ideas?
 

RocketFeller

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I imagine it boils down to larger, higher-drag fins on the DX3, but I'm not familiar with the two kits.
 

dhkaiser

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The fins on the DX3 are 2 1/2" tall whereas the Scopion's are 3" tall. If anything the Scorpion's fins are thicker as well. Presently at a loss.
 

dhkaiser

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Here are a couple of screen shots of the two sims...

3vs2.6#1.jpg3vs2.6#2.jpg
 

markkoelsch

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The lighter, smaller rocket goes faster. Drag increases by the square of the velocity. Might go faster, generate more drag, and slow down.
 

dhkaiser

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The lighter, smaller rocket goes faster. Drag increases by the square of the velocity. Might go faster, generate more drag, and slow down.
So if I used a slower burning motor of the same total impluse it wiould go higher?
 

rharshberger

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Also there is something called Optimum weight, add weight to the nose of the lighter rocket until the sim stops showing gains in altitude, at that point you have reached optimum weight for altitude. Drag and other variables come into play as others have stated.
 

Peter Olivola

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Maybe. There's another issue. Trying optimizing for weight in your simulations. The motors and diameters you're using fall into the category of it being possible to build too light for optimum altitude.

So if I used a slower burning motor of the same total impluse it wiould go higher?
 

scsager

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Try adding a little nose weight to the DX3 to increase the stability to at lease 1 caliber.
 

markkoelsch

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So if I used a slower burning motor of the same total impluse it wiould go higher?
Maybe. Optimum weight will vary based on motor. Too heavy and it will not go as high, and the same is true if it is too light.
 

dhkaiser

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Try adding a little nose weight to the DX3 to increase the stability to at lease 1 caliber.
I have tried that and the altitude decreases. I also tried adding an upper BT with the same result, less altitude.
 

dhbarr

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I see the screenshots, have you posted the sims?
 

TopRamen

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Something I learned from working with the 13mm stuff is that sometimes inertia becomes a factor unto itself.
An Estes 13mm such and such would fly fine on a 13mm with a short delay, and deploy at a reasonable altitude, yet my FG or CF versions would seem to fly much higher, even to the point of damage at deployment.
Weight was trumped by mass and velocity, though surface area and dimensions for calculating CP remained constant. Openrocket actually seemed to calculate for this to some degree, but even in the "POST REAL WORLD DATA" mass and CG override, it is still just a SIM, and only marginal in it's representations of flight data, but the correlation was obvious, as apparent by the plot chart of the flights and the velocity and altitude at deployment of recovery gear.
I'm no rocket scientist, so I digress, but it all makes sense in my Mindsim, which typically is 98-99% flight Effective, so there's nothing to worry about other than the times when I have based my expectations on computer simulations. That's when things go wrong.
If my Rocket screws up, that's on me, even if I relied on a computer to make me screw it up.
 
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rocketguy101

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Go download a couple of old Estes and Centuri technical reports about altitude prediction -- they are the grand-pappies of the modern computer sims. Both were written by Douglas Malewicki, but present the results in different formats.

Estes TR-10 http://www.oldrocketplans.com/pubs/Estes/estTR-10/TR-10.pdf

Centuri TIR-100 http://www.oldrocketplans.com/pubs/Centuri/TIR_100.pdf

The Estes report disucusses the "ballistic coefficient" which impacts the coast time and maximum velocity. The Centuri report's graphs depict optimum mass more clearly to me than the Estes charts.
 
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Bat-mite

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Two things:

In answer to your question -- yes, a slow-burn motor will get you more altitude than a fast-burn motor of the same total impulse.

Also, the sims do not assume the rocket is going straight up, unless you have completely zeroed out the wind, etc. The rocket showing less altitude (A) might be weathercocking enough to lower the altitude.

AB WC.png
 

rocketguy101

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^^^^^^^^^^ Good point!

Plot the simulations, and use the flight side profile to see how much weather cocking is taking place.

2016-06-18 11_15_53-Edit simulation.jpg
 

dhkaiser

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^^^^^^^^^^ Good point!

Plot the simulations, and use the flight side profile to see how much weather cocking is taking place.
OK, did that and I am more confused then ever. Has the rocket drifting over a mile away before the chute is deployed by the chute release. Shows little weathercocking...

flight side.jpg
 

Banzai88

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How are you simulating the Chute Release?
Not very well evidently. :) I set the deploy at 300 ft. Obviously I need do something else. Not sure what.
This is how I do it. I've done it now with about a 1/2 dozen different rockets on multiple flights in all sorts of winds. Assuming that your form factor, mass, and CG are correct in the sim, I've always achieved within about 1% of what the sim spits out.

http://www.rocketryforum.com/showth...-items-for-my-wish-list&p=1557687#post1557687
 
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Buckeye

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Your OR simulations are wrong, somewhere. A good first check of simulations is to use thrustcurve.org before you spend a lot of energy making the design in OR. Since these are both simple 3FNC, a Cd of 0.6 is good enough. With I280:

3.0 rocket = 3353'
2.6 rocket = 4172'

Corrected OR sims should not be much different than the numbers above.
 

mikec

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Your body tube is unfinished on the DX3 and the fins are square. The Scorpion is polished and airfoiled. That makes a significant difference in the Cd estimations.
 

dhkaiser

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Your body tube is unfinished on the DX3 and the fins are square. The Scorpion is polished and airfoiled. That makes a significant difference in the Cd estimations.
Wow. Now that is interesting as I downloaded the DX3 file from Madcow, says all rights reserved Madcow Rocketry. That will teach me to just accept a sim file as good just because it comes from the manufacturer!
 

Buckeye

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Wow. Now that is interesting as I downloaded the DX3 file from Madcow, says all rights reserved Madcow Rocketry. That will teach me to just accept a sim file as good just because it comes from the manufacturer!
Madcow sells you an unfinished tube and square fins. He doesn't know how you plan to finish the rocket.

Every sim file needs to be rebuilt to match your build. No free lunch.
 

mikec

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Madcow's files are for Rocksim, not Openrocket. The results for Rocksim for the same surface finishes and fin profiles are probably different. I think OR is quite pessimistic about square profile fins.

As noted, you should always check the file and update it for the specifics of your build if you want the most accurate possible results.
 

Banzai88

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MadCow files are a raging mess. Often weights are WAY off (especially for the nose cones), centering ring thickness is often off, and parts fit and sometimes actual location is often off, too. I LOVE their rockets, but their sim files are no better than a good starting point. I've emailed them corrected ones for several that I've bought and built, but they've never been put up on the web pages.

There's correcting it somewhat in build for planning, but once you have the form factor correct, overriding the actual weight and CG gets things right.
 

rharshberger

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Madcow's files are for Rocksim, not Openrocket. The results for Rocksim for the same surface finishes and fin profiles are probably different. I think OR is quite pessimistic about square profile fins.

As noted, you should always check the file and update it for the specifics of your build if you want the most accurate possible results.

Best thing to do with any downloaded .rkt or .ork is to go through the components and remove ALL overrides then adjust to fit your rocket.
 

Buckeye

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Wow. Now that is interesting as I downloaded the DX3 file from Madcow, says all rights reserved Madcow Rocketry.
Madcow's files are for Rocksim, not Openrocket.
MadCow files are a raging mess.
I suspect that you downloaded Madcow's rkt file and read it into free-ware OpenRocket. Do so at your own risk. OR conversion errors can occur.

Madcow has one 2.6" DX3 Rocksim file on their site that I can see. It is for the cardboard version with 2 body tubes. The body tubes are "gloss."

Your ork shows one body tube made of fiberglass and it is "unfinished."

The problem is not with Madcow. This is OR and/or user error.

As for the "raging mess", are you confirming the Madcow design files in native Rocksim or converting to OpenRocket?
 
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