Descent rate: OR vs Alt3

mccordmw said:

[FONT=&amp]That link I included says this:

"Typical values of [/FONT]drag coefficient[FONT=&amp] for a parachute is about 1.75, compared to 0.75 for a typical model rocket."

If that's correct, then the default in ORK of 0.8 is way low for a parachute. I've been using 1.4 since I saw that value some place.[/FONT]

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Coop said:

I would believe the recorded data. I've been doing a lot of this kind of work lately.

To accurately profile an unknown Cd, you need to know what the chute is recovering--that means weighing the complete assembly as it is recovered--that is, nosecone, shock cords, chute, rocket, left over dog barf, case, and spent motor--even that grasshopper that was a stowaway, if you want to get 5-decimal place nutty with it.

Is the 12.6 oz the weight of the SPENT motor, or the motor itself? I suspect it is the latter, as more people are using that figure for thrust:weight ratios for liftoff than they are for profiling their recovery systems...

I don't think the 0.4 Cd you determined is accurate. Cds in the 0.4-0.6 range are the realm of disk-gap-band parachutes, and I'm pretty extra-special sure TFR doesn't offer one of those. I suspect your determined Cd is so low because your assumed weight was incorrectly high.


Later!

--Coop

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AlfaBrewer said:

I got similar Cd numbers for the 7ft Rocketman (.55). As Coop correctly points out, I have introduced the error of using the full pad weight for descent, not the landing weight. I'm not sure yet how to do that in OR, but will dig into it.

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Buckeye said:

OR should be smart enough to reduce mass during the motor burn. You should not need to do anything. Plot mass vs. time to check.

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mccordmw said:

Drag being a function also of velocity would also affect your results.

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dhkaiser said:

Buckeye is correct, just plotted mass vs time and OR decreased the mass the proper amount during the burn.

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John Beans said:

This thread got me to wondering about how uniform the rules are about what 36" means (flight chord, or material width?), and how a shortening of shroud lines (via twisting) might affect that.

Seems like the chute manufacturers would want it to be flight width (not material width, to compensate for different shaped material and canopies), and that they'd also list drag coefficient at that flight width.

If OR/RockSim used different assumptions than the manufacturers for geometry, or if the shroud lines were twisted/shortened, you'd get very different results than expected...

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Banzai88 said:

When I started to get into high power, I started to wonder the same thing. What I discovered is that there IS no standard, and therein lies the rub. Some are publishing 'over the top', some are publishing 'opening size', others are using 'flat fabric size'.....making it almost impossible to mathematically compare x form, parasheet, vented parasheet, ellipsoid, hemispherical, cupped parabolic, or any other form directly with any other BEFORE actually buying one.

The consumer has to trust the manufacturer's data on weight/descent rate charts, which are often well off of real world performance. I treat those charts like political propaganda. :flyingpig:

I'm not 100% certain, but I think that I've been told that OR uses assumed surface area of the input size, and baseline assumes flat parasheet, and you can override that .80 to anything you want.

Sounds like we need a thread where people post their rocket weight, parachute make and model, advertised CD, altimeter or data logger used, and actual derived CD based on altimeter data.

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