24" vs 27" parachute question

[Bill S]

I've been using Rocksim to design a rocket, and I am running into issues concerning parachute size. I was looking at an Estes 24" plastic parachute. I add it in Rocksim, but it doesn't appear to change anything in the general tab. I had to use mass override to get the correct mass. Rocket weighs about 6.89oz coming down.

I've selected medium descent rate, and ran 10 sims. 3 out of 10 times, the descent rate is way too high - 18 to 19 fps. The others are about 13fps. Avg drift is 223 feet.

I change the option to low drift rate and Rocksim suggests a 27" parachute, which doesn't appear to actually exist. This improves things so that only 1 time out of 10 is the descent rate too high (17 fps), with avg drift being 266 feet.

Trouble is, nobody sells a 27" parachute. I haven't seen any 30" plastic parachutes, and nylon is probably going to be too heavy, lest I redesign the rocket again.

So my questions are these:

How accurate is Rocksim concerning parachute size and descent rate?

Given that I am generally launching my rockets on a grassy field, and the typical area rockets drift into are farm fields/grassy fields, is it likely that the higher descent rate with some of the 24" parachute launches will cause damage to my rocket?

 

[Eric]

Something is wrong with the SIM. A 7oz rocket coming down on a 24" chute will be close to 12ft/sec. That's a nice slow descent. Fly it and if it drifts a bit to far, you can always add a spill hole.

 

[mikec]

I've selected medium descent rate, and ran 10 sims. 3 out of 10 times, the descent rate is way too high - 18 to 19 fps. The others are about 13fps.

Certainly descent rate can vary, maybe by that much, depending on thermals and parachute behavior. But I don't think Rocksim could possibly model that complex behavior. It almost seems like it's just making these numbers up, or at least randomizing them on the basis of some assumed probability, that frankly seems off to me.

18 to 19 fps could end up being just fine. I typically use a 24" chute in a 1 lb MPR rocket, and yours is less than half as heavy. At a guess I would use more like 18".

 

[Bill S]

Okay. I'll go with the 24" parachute and see how it goes. I can always attach an 18" if I need to (after checking first in Rocksim to confirm that the change in weight doesn't affect stability much).

 

[BABAR]

Not very hard to make a 27 or 30 inch chute out of a garbage bag

 

[Buddy Michaelson]

Okay. I'll go with the 24" parachute and see how it goes. I can always attach an 18" if I need to (after checking first in Rocksim to confirm that the change in weight doesn't affect stability much).

Hello Bill, if you’d like, I can make you a custom 27” Parachute for $22.00 shipped to you, your choice of color as well. The material is 1.1 ripstop with braided nylon lines, ultra light and incredibly strong.

 

[Bill S]

Hello Bill, if you’d like, I can make you a custom 27” Parachute for $22.00 shipped to you, your choice of color as well. The material is 1.1 ripstop with braided nylon lines, ultra light and incredibly strong.

What would the weight be like?

 

[bill_s]

I used to see the rocket catching thermals and the drop rate slowing to near zero with large chutes and light rockets, particularly at a field where I flew an Estes Excutioner, 8 oz. with 24" chute. That had thin plywood swept back fins and landing could be hard seeming, knowing how delicate they look. But with such slow descents, wind is also a big factor how hard it hits. Drift can be huge. Their Big Daddy is only 5 oz. with 24" so it comes down even slower on its swept back pointy balsa fins, that would be comparable to your 27". OTOH the D-Region Tomahawk came with an 18" chute and weighed 9 oz., with stronger fins. I prefer my rockets to come down much faster if I know they can take it. 1 pound on 18" 6-line for example, comes down like a rock. I did do about 2 pounds on a 24" but had issues with that rocket.

Judge what the rocket can take, and go for a harder landing if you think it can take it.

 

[Buddy Michaelson]

What would the weight be like?

The weight of the parachute? About 1.03oz, the decent rate would be 14.5oz at 15fps

 

[BABAR]

Most of my scratch built rockets with streamer or chute recovery are built with FORWARD swept fins or ring fins that don’t extend aft of the base of the body tube, thus the motor casing takes the brunt of the initial impact.

Personal preference, I think they look cooler and I get less damage. However, Forward swept fins are not as efficient at adjusting CP.

YMMV.

 

[Bat-mite]

Keep in mind that plastic chutes tend to stick together, can melt, and often don't open successfully unless coated with baby powder. Nylon chutes "want" to be open, and have none of those weaknesses. Of course, they cost more.

 

[Bill S]

The weight of the parachute? About 1.03oz, the decent rate would be 14.5oz at 15fps

Okay, thank you. An Estes 24" Nylon parachute is .9oz (claimed by Estes), so that isn't bad.

 

[AfterBurners]

Okay, thank you. An Estes 24" Nylon parachute is .9oz (claimed by Estes), so that isn't bad.

Another option would be a Top Flight thin mill. Lighter and can pack into a smaller body tube.

 

[AfterBurners]

How accurate is Rocksim concerning parachute size and descent rate? Very accurate

Given that I am generally launching my rockets on a grassy field, and the typical area rockets drift into are farm fields/grassy fields, is it likely that the higher descent rate with some of the 24" parachute launches will cause damage to my rocket? No it won't. If you were on a dry lake bed now that would be didn't. Just an FYI - You can actually program RS to have thermals at certain altitudes along with speed and a whole bunch of other varies. You can easily change the size of your chute in RS and also account for a spill hole which would make your rocket come down more stable without much swinging from side to side, which will cause damage, but maybe not so much on a grassy field.

 

[Zeke Johnson]

If the weight of Buddy's chute is not too much, and if it will fit in your airframe, I would recommend his.

#1 - Buddy's chutes are top notch quality.
#2 - The nylon chute will not melt and shred like the plastic Estes chute.
#3 - Buddy's chutes just look cool. It really makes a typical "Estes Style" rocket stand out when it's coming down with a "real" parachute.

Zeke

 

[Buddy Michaelson]

Okay, thank you. An Estes 24" Nylon parachute is .9oz (claimed by Estes), so that isn't bad.

If I had to accurately guess, it would take about 5-5.25”^3 so 5” in a 1” body tube. About 1.75” in a 2” body tube. My hexagon Chutes are cut with a hot knife, than folded all around the edges, and than bias tape sewn on where the lines attach too, incredibly strong but light weight. And for $17.00 plus $5.00 in shipping you can’t really go wrong. My Chutes also have a adjustable slider, so you can change the drag on the chute in about a half second, to play with the decent rates. On windier days slide the slider up the lines about 6”.

 

[Bill S]

Thanks. I am going to have to build the rocket first and get a few test flights in with a 24" parachute before I decide how I want to proceed.

 

[jnmiller]

LOC precision has a 28" parachute