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Chute dimensions question
#1
My latest assembled rocket came with two different mylar plastic chutes, pre-assembled, but of different sizes.  Now, I am wondering why one would need or want a large chute or different chute than what was supplied?

I had the good fortune to buy ten such "adjustable" plastic chutes from JonRockets.com in a lot, with the idea that if one of my chutes melted or malfunctioned or shredded, that I could get back in the game if I had a few spares prepared and labeled in my "jump kit" tackle box.

Now I am wondering if there's a relationship between the size of motor or the diameter of the body tube and the size of the chute...12", 15" or 18" options.    Is there something that I'm missing here?
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#2
Perhaps the smaller chute is for higher altitude launches or launches in higher winds? Just so she doesn't drift too far away?
AMRS #54
WARS #24
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#3
Remember the Astron Drifter? You used the large chute when you wanted to go for duration with a slow descent and a gentle landing, the smaller one when you flew out of a smaller field, or when the winds were higher..
Greg Young - L3
TRA 00234
NAR 42065
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#4
It would help to know what size rocket you are talking about.  Most Estes-sized rockets can free fall to the ground with no chute.  Some people use a streamer to make them more visible and slow them down a tad.  But a small cardboard/balsa rocket will flutter down with no damage.

Most people like to see their bird land at about 15 fps.  There are calculators online that allow you to enter the chute dimensions and rocket weight, and these will calculate the descent rate.  When you start stepping up to heavy, hard rockets (like fiberglass, phenolic, PVC, what have you) there is a lot less wind resistance, and terminal velocity is much, much higher.  That's when rockets can break on landing, or hurt someone.

Try putting your two different chute sizes in this calculator, and see what descent rates you get.  You can also put in your expect max altitude, and it will tell you how long it will take the rocket to land.

The usefulness of this is that you can then multiply the descent time by the wind speed (in fps) to see how far it will drift before it lands.

As an example,say you have a four-ounce rocket with a 12-inch hexagonal chute.  You expect it to go 800 feet high.  You expect the wind speed to be 5 mph, which is roughly 7.5 fps.

You get a 17.54 fps descent rate, which is fine.  You also have a descent time of 45 seconds.  If you multiply 45 seconds X 7.5 fps (wind speed), you can expect your rocket to land 337.5 feet from the pad.

This, of course, is assuming the rocket when straight up, did not weathercock, etc.

Now, if you bump that same rocket up to a 16" chute, doing the same calculations, you expect to land 450' from the pad.  Considering that the minimum field dimension for an 800' launch is 400', then you could easily lose your rocket.

Hope this helps.
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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#5
It's like buying cereal:
"By weight, not by volume."

Go with a simulator, and see what kind of descent rate you get. The slower you descend, the more you'll drift. I find 17 fps is a good target number... not too fast so things tend not to break, not too slow so it winds up in Jersey. It helps once you get a feel for your field and the prevailing winds...


Later!

--Coop
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#6
OK Bat-mite-- I weighed the modified Der Red Max and plugged it into the formula link you provided. The chute is hexagonal and measured side to side. Now, the package for the skill level one rocket claims 600 ft height, but at least one advertiser on eBay claims 700 ft maximum, so I plugged that all in.

The website recommends at least at 16.2 inch chute to achieve a rate of 15 ft/sec decent rate.
Plugging 15 inch chute in comes in at a faster descent rate of something like 17.6 and a 16 inch chute still is in excess of the 15 ft/sec target.

It occurs to me that when I weighed the rocket with nose cone and a D12-5 motor installed, what I don't have is the weight of the parachute added in. Admittedly, it's only mylar plastic, plus six shroud lines, a few plastic hole reinforcements and recovery wadding as well. How much more impact do you think that might have on the mass/weight? If it's weighing 5.40 oz right now, should I guess a 6 oz total?

What do you guys think?
18 inch hexagonal plastic chute seems to be the way to go, right?
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#7
A couple of things...

Once the rocket is coming down, the propellant has burned out.  So a true descent weight would be measured with a burned out motor.

I should have mentioned that the simulators don't really handle terminal velocity*.  So when the rockets are below half a pound, the results aren't perfect.

But honestly, at that weight, there is so much drag that terminal velocity is ridiculously low.  I think you could safely let it tumble, or if you want to slow it a bit, use a streamer.  If you really want to use a chute, I'd go with a 12".

On a personal note, I watched a young lady launch a 4 ounce rocket to about 700 feet, and out came an 18" chute.  It drifted and drifted and drifted, then it hit a thermal and rose up another 20 feet, finally disappearing over some trees.  The thing might as well have been filled with helium.  [Image: rolleyes.png]

\\* Terminal velocity is the maximum speed at which a body will fall through the air. In a vacuum, all bodies continue to accelerate 32 fps2. But in air, there is a point at which everything stops accelerating.\\
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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#8
(08-13-2015, 07:58 AM)Greg Young Wrote: Remember the Astron Drifter? You used the large chute when you wanted to go for duration with a slow descent and a gentle landing, the smaller one when you flew out of a smaller field, or when the winds were higher..

I sure remember the Drifter -- and its very appropriate name.  Had one as a kid in the 60s, and when you are given the choiice of using the 12" or 24" chute, or course you choose the 24" chute!

Only one launch on my Drifter -- it just kept climbing with the chute deployed.  And I suspect somebody in Kansas found it when it finally touched down.  

I'd like to build an upscale of the Drifter, maybe BT-70 or 80.  I would probably stick with the 24" chute, too.   Rolleyes
Lee
NAR 55948, L2
In Build: Mars Snooper
Next: upscale Sky Hook (Panavia kitbash)
In Finishing:  Estes Honest John, Red Nova, Super Cobra
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#9
(08-14-2015, 11:46 AM)Bat-mite Wrote: But honestly, at that weight, there is so much drag that terminal velocity is ridiculously low.  I think you could safely let it tumble, or if you want to slow it a bit, use a streamer.  If you really want to use a chute, I'd go with a 12".

I use a 12" chute with my Red Maxes. Sure I crack an occasional fillet (easy enough to fix) but I haven't lost one yet on my small field. In Florida we get pretty strong thermals in the summer so they have to have spill holes as well.
Living life dangerously...launching C's on a B field.
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#10
Tom flew his Red Max (the one that I scorched with my own in a drag race). He had a Quest C6-5 in it which didn't blow the chute clear. The nose-cone flopping about created enough drag that it ended up with a perfect 'standing landing' and no damage.
Phill Ash
Secretary - SouthEastern Virginia Rocketry Association (SEVRA)
2015 Tot Impulse: 1398.8 Ns (L2, 45 Flights, 57 Motors)
2016 Tot Impulse: 190.3 Ns (L2, 14 Flights, 16 Motors)




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