What Size Chute?

[SacEsq]

Just finished the Estes/NCR Big Brute (see it here: http://www.rocketryforum.com/showthr...stes-Big-Brute) It is a big one and the fins seem thin' not as rigid as I would like. Therefore I am going to order a new chute from CATO (black and red ) and was wondering what size I should get.

Thoughts?

[Wayco]

How much does it weigh?

[rcktnut]

My BB is 28 oz. without motor, I use a Top Flight 30" thin mil, has worked great for years. Launched 2 weeks ago, very hard ground, landed without any damage.

[SacEsq]

How much does it weigh?

Have not weighed it yet. I will weigh it and post here later tonight.

[SacEsq]

My BB is 28 oz. without motor, I use a Top Flight 30" thin mil, has worked great for years. Launched 2 weeks ago, very hard ground, landed without any damage.

Yeah I was thinking a 30" chute as well. Thanks

[Rocketbuilder]

If you don't already use it, this is a great resource: http://www.rocketreviews.com/parachu...alculator.html
Just enter the weight, and it gives you three different parachute diameters and the descent rate of the rocket on those chute sizes.

[SacEsq]

If you don't already use it, this is a great resource: http://www.rocketreviews.com/parachu...alculator.html
Just enter the weight, and it gives you three different parachute diameters and the descent rate of the rocket on those chute sizes.

THX ... now the chutes from CATO have a spill hole ... will it take that into consideration?

[Coop]

Has to do more with the Cd of the 'chute. The above calculator assumes a Cd of 0.75. Higher Cd in the same size 'chute will have a slower descent rate. Take the Fruity Chute Iris, for example --high Cd (2.2), yet,has a spill hole. The spill hole decreases pulsations in the 'chute under descent. I don't know the Cd of CATO 'chutes (don't see it listed), but I'd bet the manufacturer could answer the question as to what descent rate to expect with a given 'chute and the rocket's mass.

All the above said, that size rocket, 24-36" should be about right...


Later!

--Coop

[o1d_dude]

Jason, I don't remember. Were the CATO chutes flat or semi-hemispherical?

The drag of flat chutes is typically much less than a shaped chutes so basically a shaped chute can be smaller in diameter yet still provide a slow descent. For example, one of my big birds uses a 36" flat chute but recovers more reliably on a 30" shaped chute.

Regardless of the chute size, it seems the rockets always find the only rock, creek, or tree on the field. Sometimes a combination of the three.

[SacEsq]

Jason, I don't remember. Were the CATO chutes flat or semi-hemispherical?

The drag of flat chutes is typically much less than a shaped chutes so basically a shaped chute can be smaller in diameter yet still provide a slow descent. For example, oone of my big birds uses a 36" flat chute but recovers more reliably on a 30" shaped chute.

Regardless of the chute size, it seems the rockets always find the only rock, creek, or tree on the field. Sometimes a combination of the three.

The CATO chutes are shaped. Still think I will go 28 or 30. And you are right about rockets finding trees etc. Lol

[rcktnut]

I forgot to mention: Great job on your Big Brute!!! How do you like that Ultra Cover paint? I've never used it, think it is time.

[SacEsq]

I forgot to mention: Great job on your Big Brute!!! How do you like that Ultra Cover paint? I've never used it, think it is time.

THANKS!

Just weighed the Big Brute and it is everything its name implies ... 1lb 12.3oz (803g) or just about the same as rcktnut said his weighed. This weight is without the motor as I have a number of 29 mm cases (29/40-120, 29/60, 29/180 and 29/240) but only have reloads for the 29/40-120 right now. Need to figure out weight with the big motor to also calculate if nose weight is necessary. Anyone know the CP for this rocket?

[SacEsq]

If you don't already use it, this is a great resource: http://www.rocketreviews.com/parachu...alculator.html
Just enter the weight, and it gives you three different parachute diameters and the descent rate of the rocket on those chute sizes.

Well according to that link my rocket weighing in at 803g will have a decent rate of 6 mps with a chute of 77.84cm (just a bit larger than a 30" chute. If I got to a 93.41cm chute (just a bit larger than a 36" chute) I get a decent rate of 5 mps.

So the question becomes ... what is an ideal decent rate for most rockets without taking into consideration wind and drift?

[rcktnut]

CP for the big brute is 27 inches from the tip of the nosecone. I have no weight in the nc. It will fly fine on any of the 240 motors, I beleive the heaviest reload for that would be the H97 at 4.9 oz. With that load mine is at +1.07 stability margine, flies fine. Of course you have to check your CG, just saying that mine flies well with the + 1 margine.

5-7or 8 mps would be in the safe range for decent.

[o1d_dude]

I've found the descent rate charts to be a little optimistic in their predictions.

A lot of what's a good rate will have to do with the field. Our field tends to feature a lot of hard ground while Snow Ranch has a more forgiving soft ground...if you avoid the rocks. The TCC field is variable depending on whether the field has been disked, has greenery, or is sand-like (all different times of the year).

I recall you saying the fins seemed "thin" so I'd err on the side of risking a longer chase rather than a faster descent.

[bill_s]

I've seen the Estes Executioner catch thermals where it comes down about 2/3 of the way and then starts drifting like mad and can almost appear to stop descending. It would help to do morning not late afternoon launches ... it's 8-9 oz. without engine with a 24" parachute and that online calculator says 15 fps. Their D-region Tomohawk has better plastic fins with little sweep and a fin can and descends faster with 18" and at about 9 oz.; good thing because it also goes higher.

As I mentioned elsewhere, I think the .75 Cd is for measuring the parasheet flat. In operation a 24" actually runs maybe 19" at about Cd=1.4. Samller spill holes make very little difference in Cd. Another way to check parachutes is to run your rocket sim with a "rocket" the dia. and Cd of the parachute, and weight of your actual rocket. You may have to bump up the motor a bit to get enough altitude to reach terminal velocity on the descent.

[Coop]

Regardless of the chute size, it seems the rockets always find the only rock, creek, or tree on the field. Sometimes a combination of the three.

This needs etched on a stone tablet.


Later!

--Coop

[GregGleason]

Descent rates can be tricky. If you fly where the recovery area is soft, then you can get by with faster descent rates. Also, if your rocket has a lot of fragile parts, you might want a slower descent rate. So you start with a rule of thumb, then adjust. If you start with a slightly larger chute than needed, you can reef it to suit the desired descent rate.

Where I fly, the pads are on asphalt (it's at an airport) and the rockets sometime land there. So we have to account for that in chute sizing. So I shoot for a descent rate between 11 and 14 fps (3.4 and 4.3 mps).

Obviously, one doesn't want to oversize your chute so that it flies on into the next county.

Greg

[evil ed]

This needs etched on a stone tablet.


Later!

--Coop

It is.
The rocket landed on it and broke a &*#$%** fin.......

[cwbullet]

This needs etched on a stone tablet.

Funny. Caveman style.