Need parachute advice (HPR)

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BigBlueDart

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I'm just about finished with a 4" Iris build. Just putting together a few pieces for the avionics bay. The rocket weighs in at right about 8 lbs with everything but parachutes, motor, and a yet-to-be-determined payload (probably only ever a few ounces, but I doubt I'd ever go above 2 lbs). The MMT is 54mm and the longest motor I could fit in there would be a 5-grain. I think the burnout weight of the largest K I'd be flying is about 1.5 lbs.

So, the main question is what parachute(s) would folks recommend for main and drogue? The other question I have is in calculating descent weight, do I include the weight of the parachutes themselves? And then the third question, aside from weight and how much space it takes up, are there any drawbacks to using a slightly oversized parachute (one that I could potentially use for heavier rockets down the road)?
 

Steve Shannon

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I have had excellent results with Rocketman chutes, but all of the manufacturers make good chutes.
Yes, you include the chute weight in the descent calculation.
The main drawback of using an oversized chute is the drift range, which is especially true if your main comes out at apogee.


Steve Shannon
 

mpitfield

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For drogue I would use a 12" classic Elliptical from Fruity, it will bring it down quick. Having said that the approach I take is to take a best guess then dial in the recovery gear through post flight analysis. This can include visual, preferably video, as well as reviewing the altimeter data. I have several 12", 15", 18" and 24" Classic Elliptical chutes and this allows me to adjust as required. Keep in mind different chute designs will have different decent rates or CD, so you need to know the chute CD.

To calculate the decent rate weight, just load your rocket with all the recovery gear, all electronics, battery, etc., as well as the dry motor hardware, estimate for any consumables, BP e-matches, weigh it on a scale, then I usually add a pound then round to the nearest pound. So if it adds up to 12.2 pounds, based on my formula I will estimate at 13lbs. So long as you are close then you should be fine for decent rate calculations.
 

Lowpuller

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Almost identical to my recent L2.

I used an 18" drogue and a 60" Top Flite regular canopy.
 

Wayco

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Most of my rockets use Top Flight or Recon chutes. I do a ballpark estimate on weight and use Top Flight descent rate chart for a size: https://topflightrecoveryllc.homestead.com/descent_rates.html
My guess is your rocket will weigh around 10 lbs.
For my bigger rockets, I like the Wildman Recon chutes, easy to pack and pretty much bulletproof. Check out his 50": https://www.wildmanrocketry.com/ProductDetail.aspx?product=2967
Or you could go with a Top Flight 58": https://topflightrecoveryllc.homestead.com/page1.html It's about half the price and you can pick a color.

I also use Missleworks RRC3's for altimeters, and after the first flight, it will tell you how fast your descent is.
 

Handeman

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I would recommend mpitfield's advice on dialing in your drogue size. You really have to adjust as needed to get the right flight profile and rocket part positions on the drop.

As for main size, we fly on farm fields so the soil is relatively soft and forgiving. I don't buy chutes, I make my own. In my 4" DD rocket that weighs about 10 lbs without motor, I use a golf umbrella skin as the main chute. This is about a 53" main and works very well. I pop the main at 300'.

Rocketry 201 016 c01.jpg
 

crossfire

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Check out our Crossfire line of chutes. They pack up very well. I am sure you would be happy with a CF chute.
 

BigBlueDart

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Regarding TFR parachutes, it seems like their sizing is a bit off, even for their recommended descent rates (17-22 fps), which are higher than what some others recommend. For instance, the 70" standard parachute says it is rated for 12-18 lbs, but if I run that through a descent rate calculator for a standard flat/circular parachute and a rocket weighing 12lbs, I get a descent rate of 21.88 fps. That's the upper end of their recommended descent rate at the lower end of their recommended rocket weight for that parachute. Am I missing something, here? Any argument why TFR standard chutes would have a Cd above 0.75?

Also, since you mentioned the Crossfire chutes, what Cd do they have? And do you have the weights of your parachutes listed anywhere?
 

dhkaiser

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Any argument why TFR standard chutes would have a Cd above 0.75?

Had my Altimeter3 tell me my rocket decent rate was 13 ft/s when OR said it should be more like 18 ft/s. I adjusted the TFR Cd in OR from .75 to 1.3 then OR said decent rate = 13 ft/s. Later Gary told me the CD Par9-Par24 = 1.09, Par-30-58" = 1.34, Par70-120" = 1.40 and the XT chutes are .98
 

Coop

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So, the main question is what parachute(s) would folks recommend for main and drogue? The other question I have is in calculating descent weight, do I include the weight of the parachutes themselves? And then the third question, aside from weight and how much space it takes up, are there any drawbacks to using a slightly oversized parachute (one that I could potentially use for heavier rockets down the road)?

Congrats on the build completion and here's hoping for a successful flight for you. I'll be skipping around the order in which you asked the questions, because linear thought is not my forte.

You mentioned a payload, as well, at times. So, my recommendation is to size the main, in particular, for the descent weight. This will be the airframe and recovery components, burnt-out motor and casing, and payload. If it's at 8 now, without the chutes, motor, and payload, you're going to be looking at a recovery weight of somewhere around 12lb, if the figures you mentioned hold true.

As far as a particular recommendation: "Chute XXX from manufacturer YYY" that's a little harder for me to do for you. There's a good many styles out there to choose from. Topflight, Crossfire, and Rocketman have already been mentioned. Others one might consider are--in no particular order-- Spherachutes, Skyangle, and Fruity Chutes. They each have their own advantages and disadvantages, fans and detractors, and levels of cost and performance. But they all work.

Comparing them is not quite as straightforward as one might initially think. Not everyone uses the same reference measurements for their Cd calculations, and not everyone uses the same recommended landing speed. Some use 15-20. Some use 17-22. If you like the look of Chute A, with a recommended landing speed of 17-22, but you want to hit the ground at 15 --or let's say you have a delicate project, and want to land even slower... 12 or 13... it often is just guessing, based upon the published data.

That all said....

Try this:

Cd * REF * 0.5 * rho * Velocity^2 = Mass.
Where all values are in FEET and LB.

REF is reference area. For circles, r^2 *pi. But, being as chutes are typically sized on diameter (D), it may be best to think of it as (D/2)^2 *pi. If chute size is in INCHES (as most are in rocketry, with Rocketman being a noted exception), then you must divide this all by 144 (sq inches to sq ft.)
rho is 0.002377 slugs/ft (I know; it's an odd name for a measurement value, but it is, actually, a real thing it means "the mass of accelerated by 1 ft/sec when 1 pound-force is applied. In air, at sea level, this value is 0.002377)

Let's look at a 48" chute from Nooneinparticular. They say it's good for 13.2 lb at 22FPS, but do not publish their Cd. You want to know what this chute will recover at 13 FPS.
So:
Cd * REF * 0.5 * rho * Velocity^2 = Mass.
Cd * ((48/2)^2 * pi)/144 * 0.5 * 0.002377 * 22^2= 13.2
Cd * 1809.216/144 * 0.5 * 0.002377 * 484 = 13.2
Cd * 12.564 * 0.5 * 0.002377 *484 = 13.2
Cd * 7.23 = 13.2
13.2/7.23 =Cd
13.2/7.23 = 1.825
Cd = 1.825

Now that we know the Cd, we can use it to find the recoverable mass at any given speed. In this case, 13 FPS.

Cd * REF * 0.5 * rho * Velocity^2 = Mass.

1.825 * 12.564 * 0.5 * 0.002377 * 13^2 = Mass

1.825 * 12.564 * 0.5 * 0.002377 * 169= Mass
4.60 = mass.

So that chute will recover 4.6 lbs at 13fps. Seems rather low, given the relatively high Cd, but the recoverable mass is dependent upon the square of the velocity, remember.

I know--it's a lot of math, but maybe it will help you in your chute quest.


Later!

--Coop
 

BigBlueDart

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Thanks for the responses, everyone. I think I'm feeling a lot more comfortable going with the TFR PAR-70 as I originally planned. I also plan to use a 24" x-form as a drogue. Down the road I expect I'll try out some products from the other vendors, but right now cost has been the biggest inhibitor to me getting this project off the ground. TFR's standard chutes look like the best value.
 

Handeman

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Try this:

Cd * REF * 0.5 * rho * Velocity^2 = Mass.
Where all values are in FEET and LB.

REF is reference area. For circles, r^2 *pi. But, being as chutes are typically sized on diameter (D), it may be best to think of it as (D/2)^2 *pi. If chute size is in INCHES (as most are in rocketry, with Rocketman being a noted exception), then you must divide this all by 144 (sq inches to sq ft.)
Another area where you have to be careful is diameter (D). Some manufactures use the open and flat diameter of there chute when they say it is 60" diameter and use that to calculate the Cd. The actual diameter of the chute under canopy might be more like 52". Some manufactures will use the canopy diameter, or the 52" when calculating their Cd even though they sell it as a 60" chute. Without knowing exactly what values the manufacture used it is hard to compare advertised Cd.

Another reason to do the calculations yourself as Coop suggests. You will get more consistent results.
 

kweaver

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Where do I get information and buy crossfire parachutes?
 

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