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recomend a good decent rate calc.

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stickershock23

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SO I tried 3 different ones, all with the same data.

One said 31 feet a second.

one said .25 meters a second (ia a little over 1 foot a sec if I guestimate correctly)

and the last said 19 feet a sec.


Kind of hard to make an educated guess with that kind of data.

anyone recomend an accurate Calc?? (sorry I dont like rocsim)
 

stickershock23

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so a dozen hits and nobody has an answer. must be everyone has had the same bad luck I have.

I guess its time to go seat of the pants... Its duel deploy, I'll just go way to big on my main.. its 12 lbs without the motor, so 13 max. 5.5" nike smoke by polecat.

I was guessing a 48" maybe 60"

anyone?
 

brianc

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Decent rate is going to depend on (at least) the chute type, air density
and rocket weight.

For example, I like the hemisperical chutes from Spherachutes.
This chart - http://spherachutes.com/chart.asp
shows your 60" might be too small for 12 lbs at sea level (and
certainly too small in Utah)...

I also like Top Flight chutes - http://topflightrecoveryllc.homestead.com/descent_rates.html
suggests you may need at least 70" for their standard chute...

What kind of chute are you proposing to use?
 
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stickershock23

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I also like Top Flight chutes - http://topflightrecoveryllc.homestead.com/descent_rates.html
suggests you may need at least 70" for their standard chute...

What kind of chute are you proposing to use?
Had not decided, thats why I was poking around,

I"ve been pretty partial to my local gal "Deby" at Recovery Technologies www.rocketparachutes.com. Her online chart for 18fps only goes to 10lbs but I'd guess somewhere around 80-84" at 1000' above sea level which is what our site is. Here's the chart http://www.rocketparachutes.com/sizer.html
Great charts thanks


I tried this one the first time. compared to Elcheapo and Brian c's data that seemsto be accurate. I will stay away from the other two sites I visited.

my 78" is in my Der red max, seems Im goingto have to swap those onsite (flying both next weekend) or I have a pair of top flight 48" chutes, I can parallel.

Honestly even though I'm LIII I fly mostly small stuff ( I like to watch the entire flight and make lots of flights) so when I decided to fly my nike smoke. I was kind of rusty.

Thanks guys
 
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DaveHein

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SO I tried 3 different ones, all with the same data.

One said 31 feet a second.

one said .25 meters a second (ia a little over 1 foot a sec if I guestimate correctly)

and the last said 19 feet a sec.
Could you post the links for the descent rate calculators you used? The second one is so far off I wonder if there is was a error in entering the data? If this rate calculator is really that bad people should be warned not to use it. Otherwise, they might select a parachute that is way too small.

Dave
 

Zeus-cat

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MarkM

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I guess its time to go seat of the pants... Its duel deploy, I'll just go way to big on my main.. its 12 lbs without the motor, so 13 max. 5.5" nike smoke by polecat.

I was guessing a 48" maybe 60"

anyone?
52" or 60" SkyAngle

I use the 52 on my all fiberglass 4" rocket which weighs about the same as your Nike. I have soft recovery areas so usually bring my rockets down between 20-25 fps on the main. I'd probably go with the 60 for the Nike since you'll probably be flying that rocket on 3" motors which will add more weight. The great thing about the SkyAngles is they have the built-in reefing slider. So when you fly smaller motors, just reef the chute in some and basically make it like a 52.
 

stickershock23

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Could you post the links for the descent rate calculators you used? The second one is so far off I wonder if there is was a error in entering the data? If this rate calculator is really that bad people should be warned not to use it. Otherwise, they might select a parachute that is way too small.

Dave

yep right here

http://www.rocket-simulator.com/descent.php

and I couldnt find the third one. the first one I used is the same one recomended by fox racing guy. that one seemed to be on!


Mark M. Yeah I think I either going to use parallel 48" top flite or my 78"
semi hemi chute.

thanks everyone for the input i feel much better!
 

DaveHein

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yep right here

http://www.rocket-simulator.com/descent.php

and I couldnt find the third one. the first one I used is the same one recomended by fox racing guy. that one seemed to be on!
There is definately a problem with that descent speed calculator. I entered the parameters for a 3-pound rocket with a 3-inch chute, and it calculated a speed of 4.5 MPH. I would guess the actual speed would be at least 10 times that.

Dave
 

stickershock23

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There is definately a problem with that descent speed calculator. I entered the parameters for a 3-pound rocket with a 3-inch chute, and it calculated a speed of 4.5 MPH. I would guess the actual speed would be at least 10 times that.

Dave
Good so Im not stupid.... LOL
 

cjl

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There is definately a problem with that descent speed calculator. I entered the parameters for a 3-pound rocket with a 3-inch chute, and it calculated a speed of 4.5 MPH. I would guess the actual speed would be at least 10 times that.

Dave
Specifically, there's a problem with the unit conversions it is doing. It converts weights properly, but no matter what unit you select, it assumes the parachute diameter is entered in meters. Therefore, what it was calculating in this case was for a 3 pound rocket with a 3 meter (9 foot) chute. If you convert the numbers into meters before entering them, this one should agree with the others.
 

Handeman

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It did the conversion correct for me. I put a 53" chute in and it converted it to 1.3462 meters. The weight was also, right 8lbs to 3.6288 kg. The final result was way off. 0.30 mph or 0.44 ft/sec That would have had my main deploy at 400 ft taking a half hour to get down.

The copywrite on the site is 2003, must be coming back down on one of his chutes.
 

cjl

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Try entering the chute diameter in meters initially, rather than inches. It should work. From my experimenting with the calculator, that does seem to be the problem.
 

cobra1336

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So on my last build, still building. At 30 lbs. I need a 12 foot main chute ?
I know the drouge does not affect main chute size but what would you folks suggest for a drouge size ?
 

Handeman

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Try entering the chute diameter in meters initially, rather than inches. It should work. From my experimenting with the calculator, that does seem to be the problem.
That right there is a reason not to trust it. If it can't accurately convert oz or lbs to kg and inches to meters, then I wouldn't trust any of it's other calculations. 8 ft/sec for an eight pound rocket on a 53" chute doesn't sound right either even when the diameter and weight is entered in metric.
 

bobkrech

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The parachute is an example of an object falling at terminal velocity.

The terminal velocity of a rocket under chute is described by the equation:

V = square root [(2 x W) / (Cd x rho x A)] where V is the terminal velocity in ft. per second, W is the recovery weight of the rocket in pounds, Cd is the drag coefficient of the parachute, rho is the atmospheric density in slugs per cubic foot, and A is the area of the chute in sq. ft.

Drag coefficients range from 0.65 to 1.25 depending on parachute design. Many calculators simply use 1which is not a bad approximation for descent rates in the 15-20 fps range. The drag coefficient drops as the descent rate increases and you migh want to use 0.7 for a drogue in the 60-80 fps range.

At 70 °F and 14.696 psia, dry air has a density, rho = 0.002328 slugs/ft3.

The atmospheric density drops by 5% for every 1000' increase in altitude.

A simple calculator is here. http://www.digitaldutch.com/atmoscalc/

A pictorial description of terminal velocity is found here and there is a simple applet at the bottom of the page that does the calculation for you. http://www.grc.nasa.gov/WWW/K-12/airplane/termv.html This Java calculator is accurate and allows for altitude compensation.

Using the calculator with a Cd = 0.9 and an 8' chute you get ~15 fps at sea level and 16 fps at 5000' as the descent rate of for a 12 pound rocket.

Bob
 
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kandsrockets

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So on my last build, still building. At 30 lbs. I need a 12 foot main chute ?
I know the drouge does not affect main chute size but what would you folks suggest for a drouge size ?
You will need a 12' round chute or 18' parabolic chute or 16' X-form chute. Also I would recommend using a 18" or a 24" Drogue chute if you need a drogue. I fly mostly drogueless but I do have a few rockets that need a drogue.
 
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