5' MIRPS Pilot chute for 11lb rocket as main chute. Anything I'm missing? Linked.

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Hi
You may want to rethink the mesh chute. We had a guy from our club a few years ago who used a mesh chute for his main. The chute didn't open. The only thing we could think of was that the decent was to slow for air to fill the chute. It came out of rocket all clear just didn't inflate. Sometimes the cheap way ends up costing you more.
Have Fun
 
I thought the mesh lines chutes were designed for high speed deployment. Generally used as a drogue.

At what velocity are you intending on popping it?
 
I think the mesh chutes were designed as a higher speed drogue. What velocity are you going to deploy it at?
 
Good info, I figured there had to be a purpose for the mesh. It is a pilot chute- I'm a bit new to the game and didn't register that speed of deployment could be critical to chute deployment. Probably won't take the chance then.
 
The point of the mesh is to prevent any kind of fouling. From what I have read and the statements of members who are sky divers (and one pro rigger), I believe all chutes on the market for humans use mesh on the pilot chute. And what we are using are drogues that slow some until the main releases.

The post with a failure said that it was the main chute. And leaves out two very critical pieces of information. One is the speed under drogue. The other is altitude. As the speed under drogue decreases, the amount of distance to inflate the main increases.

For a drogue, I would see no issues with using mesh at all. The speed will probably be nominally higher for the size of chute given the mesh. But it will be much less likely to become fouled. All the same, even a complete failure of drogue can allow for a good final recovery.

Kirk
 
The fine print in the parachute listing states the inflated diameter is 3'. The descent rate of a 3' diameter chute for a 12 pound rocket is ~42.5 fps which is way too fast for a main chute which should slow the rocket down to ~ 15 fps. For that speed you will require an 8' diameter chute.

https://www.rocketreviews.com/descent-rate-calculator.html

Bob
 
I assumed that parachute diameter is uninflated. Am I incorrect? It's kind of impossible to get an accurate measurement of an 'inflated' parachute, and I know all of the 12-18" parachutes I've purchased from companies such as Top Flight are sold by their 'uninflated' diameter.
 
Most chutes sizes are given with a measurement across the arch of the chute because that is the easiest to measure. Check https://spherachutes.com/, they state that their chutes are measured across the top of the chute in the product description so there is no doubt, which is a very good thing. Since they are hemispherical, a 48" chute would be equal to a circle with 96" circumference. 96/pi = 30.56" diameter.

The formula I use to calculate the size chute I need is 2 * rocket weight in lbs/( rho * Cd * (Decent velocity in ft/sec)squared ). Where rho is .00273 at sea level or .00211 at 4000 ft, and Cd is coefficient of drag of the chute. Most chutes have a Cd of 0.75 or less. A disk-gap-band style is only 0.52. That calculation will give you surface area of the chute you need for the given decent velocity. When you have the surface area, divide by pi and take the square root to get the radius, multiply by 2 to get the diameter.

Using that calculation, a 3.2 lb rocket at sea level with a 0.75 Cd chute dropping at 15 ft/sec would need a 16.00 square foot chute = 2.55 ft diameter = 30.6 inches, or a 48" Spherachute would work just right.

An 11 lb rocket with a 0.75 Cd chute dropping at 15 ft/sec would need a chute with 55 square foot surface area = 4.72 ft diameter or 56.66 inch diameter which would be about a 90" Spherachute.
 
cbrarick said:
Handeman:

Please check your math, or the formulas. I'm not certain where you went wrong....or perhaps it's the spherachute reference??
Just seems enormously big for a small rocket, but I don't try for a really soft landing

https://topflightrecoveryllc.homestead.com/descent_rates.html
So topflight would say 58 inches
https://www.b2rocketry.com/
The b2 website gives a 52 inch classic 2
https://www.wildmanrocketry.com/ProductDetail.aspx?product=2967
or a 50 inch recon chute.
Click to expand...


That is sort of the issue when people start talking parachute sizes. The hobby guys really don't have standard. If I tell you that you need a 48" chute for a 3.2 lb rocket you tell me I'm wrong, but if I tell you that you need a 30 inch chute for a 3.2 lb rocket you might tell me that's too small. The 48" Spherachute has a diameter of 30.6 inches, so both of those are the same. It depends on how you measure the chute.

If you take a flat octagon and make a chute from it, is it measured from point to point, or flat to flat? If you do use flat to flat, would that chute have the same Cd as the same diameter elliptical chute? If you can't answer these types of questions about the chute you are talking about, you can't make an accurate judgement as to what "size" the chute needs to be.
 
I sense some experiments in my future to accurately determine the cd of certain chutes.
 
Handeman said:
Most chutes sizes are given with a measurement across the arch of the chute because that is the easiest to measure. Check https://spherachutes.com/, they state that their chutes are measured across the top of the chute in the product description so there is no doubt, which is a very good thing. Since they are hemispherical, a 48" chute would be equal to a circle with 96" circumference. 96/pi = 30.56" diameter.

The formula I use to calculate the size chute I need is 2 * rocket weight in lbs/( rho * Cd * (Decent velocity in ft/sec)squared ). Where rho is .00273 at sea level or .00211 at 4000 ft, and Cd is coefficient of drag of the chute. Most chutes have a Cd of 0.75 or less. A disk-gap-band style is only 0.52. That calculation will give you surface area of the chute you need for the given decent velocity. When you have the surface area, divide by pi and take the square root to get the radius, multiply by 2 to get the diameter.

Using that calculation, a 3.2 lb rocket at sea level with a 0.75 Cd chute dropping at 15 ft/sec would need a 16.00 square foot chute = 2.55 ft diameter = 30.6 inches, or a 48" Spherachute would work just right.

An 11 lb rocket with a 0.75 Cd chute dropping at 15 ft/sec would need a chute with 55 square foot surface area = 4.72 ft diameter or 56.66 inch diameter which would be about a 90" Spherachute.
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You wrote diameter but you mean radius........

TBSSJoe said:
I assumed that parachute diameter is uninflated. Am I incorrect? It's kind of impossible to get an accurate measurement of an 'inflated' parachute, and I know all of the 12-18" parachutes I've purchased from companies such as Top Flight are sold by their 'uninflated' diameter.
Click to expand...
You assumed correctly however the listing for that chute mention that it has a 3' inflated diameter. Regardless, it is too small to be the main chute for a 12 pound rocket.....

Bat-mite said:
If you still want a cheap, military surplus, 60" chute, try these.

White with spill hole: https://aeroconsystems.com/cart/all-parachutes/60-inch-white-parachute/

White/green with no spill hole: https://aeroconsystems.com/cart/all-parachutes/60-inch-pilot-parachute-green/white/

I haven't used them, but lots of folks here have. $12.00 plus shipping for each.
Click to expand...
Excellent chutes. Have several. I think you will need 3 for a 12 pound rocket to get 15 fps descent rate.
A 5' diameter chute has an area of ~20 square feet so 3 chutes have an area of ~60 square feet. The diameter of an equivalent single chute is 2 x sqrt (60/pi) ~ 8.7'. As you loose ~15% effective area when you deploy 2 or more chute due to the angle of attack difference, so it's more likely equal to the 8' spherachute previously mentioned.

Bob
 
Hey Handy:
ut
I never claimed to be the sharpest knife in the drawer, which is why I just use the manufacture's decent rate charts (the links I provided). It's a whole lot easier as they have done the math using the way they measure chutes. I'm not a chute rigger and I don't scratch build chutes so it's not an area I have particular expertise in (talk to me about red sparkey EX motors). You bring up an excellent point, that there appears to be no standard as to how chutes are measured. My experience is that all three of the chutes I use would have been a 60........but I don't use spherachutes.
See you on the field! Stop by and I'll buy you a diet coke!

Handeman said:
That is sort of the issue when people start talking parachute sizes. The hobby guys really don't have standard. If I tell you that you need a 48" chute for a 3.2 lb rocket you tell me I'm wrong, but if I tell you that you need a 30 inch chute for a 3.2 lb rocket you might tell me that's too small. The 48" Spherachute has a diameter of 30.6 inches, so both of those are the same. It depends on how you measure the chute.

If you take a flat octagon and make a chute from it, is it measured from point to point, or flat to flat? If you do use flat to flat, would that chute have the same Cd as the same diameter elliptical chute? If you can't answer these types of questions about the chute you are talking about, you can't make an accurate judgement as to what "size" the chute needs to be.
Click to expand...
 
I came up with a 60" chute by using the calculations in Open Rocket. I think another critical thing is the cd of the chute. A parasheet like what top flight sells won't be as efficient as a hemispherical chute. Somewhere online I got 1.5cd for a hemispherical chute, which would put me around 15fps descent rate. That CD is most likely inaccurate.

You assumed correctly however the listing for that chute mention that it has a 3' inflated diameter.
Click to expand...

It's my understanding that any parachute will have a different diameter when inflated. It's also my understanding that pretty much everyone uses the 'uninflated' diameter of the chute to determine sizing, including openrocket and the above mention descent calculators- so the inflated diameter is not relevant, right?
 
Handeman said:
Since they are hemispherical, a 48" chute would be equal to a circle with 96" circumference. 96/pi = 30.56" diameter.
Click to expand...

bobkrech said:
You wrote diameter but you mean radius........

Bob
Click to expand...

I assume you meant the line above. Diameter is right. Circumference is 2 * pi * Radius. Just dividing a circumference by pi gives you diameter.
 
TBSSJoe said:
I came up with a 60" chute by using the calculations in Open Rocket. I think another critical thing is the cd of the chute. A parasheet like what top flight sells won't be as efficient as a hemispherical chute. Somewhere online I got 1.5cd for a hemispherical chute, which would put me around 15fps descent rate. That CD is most likely inaccurate.



It's my understanding that any parachute will have a different diameter when inflated. It's also my understanding that pretty much everyone uses the 'uninflated' diameter of the chute to determine sizing, including openrocket and the above mention descent calculators- so the inflated diameter is not relevant, right?
Click to expand...

AAAAAAARRRRRGGGG!!!!!!!!:facepalm:
A hemi is less efficient than a flat chute.
This has been proven by testing doe by the US military and the FAA and parachute manufacturers.
If you take two pieces of material the same size (they have the same square footage of material) and make them into parachutes one flat and one hemispherical and hang the same amount of weight from them the hemi will drop faster.
And in order to make a hemi as strong as a flat chute it requires more reinforcement because when chutes open the tend to want to flatten out and it puts more stress on the structure when it can't.
 
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TBSSJoe said:
What're you ARRRRGGGHH'ing about?

I'm talking diameter vs diameter.
Click to expand...

Well, that's your problem right there. One should be talking 'canopy surface area' (i.e. 'S(o)') vs 'canopy surface area' (i.e. 'S(o)'). So, *which* "diameter" are you talking about? 'Constructed Diameter' (Dc)?, 'Projected Diameter' (Dp)?, 'Inflated Diameter'? (to which there isn't a term for that in the commercial/military arena - because they realized decades ago that trying to discuss something as irrational as 'inflated diameter' is nuts (something this hobby has yet to realize)). (with some slight consideration given to the fact that Dp is somewhat close to that).

There (really) is only *one* diameter that has any relevance to performance discussions and that is 'NOMINAL' Diameter (i.e. 'D(o)' or just 'Do') and I haven't seen any talk here that's even mentioned it -- the "diameter referenced to total canopy surface area S(o)" (or, the diameter of a circle with the exact same area as the canopy in question). This very well may not be related to *any* other number you can put your finger on (i.e. Dc, Dp, etc etc.) -- or that you could measure (either with a tape or via photo analysis).

A 'flat circular' will have a Dc equal to Do (and that is the only one that will).

Once you get the discussion centered on Nominal Diameter, THEN (and only then) can you discuss relative performance characteristics - because then Cdo (Cd referenced to Nominal diameter) makes sense. Any other 'Cd' value is meaningless (and, fundamentally, a waste of time).

TBSSJoe said:
A hemispherical of the same diameter as a PARASHEET (I said it) is not going to have the same square footage of fabric though, right?
Click to expand...

Depending on your meaning of 'diameter', no, it won't -- odds are it (the hemi) will have more - quite a bit more (which is why the 'hemi' is considered obsolete today --- too inefficient).


TBSSJoe said:
Here is something stating a hemi chute will have ~1.42cd. Not sure how credible it is.
Click to expand...

Not very. In fact, if you encounter any 'cd' above 1.00, you should consider it with the deepest suspicion, because there simply isn't very many canopies that can even get close to that number (by that, I mean '1.00', not '1.42' - e.g., the 'annular' is about the only one, topping out at 0.95) - and I include *every* *single* canopy available in the market focused on this hobby. When you start to see performance claims specifying 'Cdo' (and backing that up with some certified wind tunnel data), then (and only then) can any discussion relating 'performance' of canopies used in this hobby pass into the realm of "rational". Not until.

-- john.
 
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TBSSJoe said:
I'm talking diameter vs diameter.
Click to expand...

jcato said:
Well, that's your problem right there.
....
So, *which* "diameter" are you talking about?
Click to expand...

Joe (is it??), I'm aware my previous response was a little 'gruff' - and this wasn't in any way directed towards you (hard to tell,,, I know) - more towards the (seemingly) continued confusion on how to express canopy sizes and some exasperation in me because of it. 25 years ago some of these same kinds of discussions (and questions, and confusions) were being discussed and I would hope the hobby could have moved beyond that by now. However, you were asking simple and honest questions and my response wasn't recognizing and responding to that. Sorry if it came down too hard.

As much as anything, I fault the merchants in the hobby for not trying to 'steer' folks to a better understanding and working to limit confusion - and it just doesn't seem that is happening. It's unlikely I'll respond much different in the future, because the commercial/military arena have settled on some rational standards (in expressing canopy sizes and their measurement and performance) and that should spill over into the hobby, IMO.

-- john.

p.s. As a good preliminary design rule (and a 'reality check' of the more rigorous design methods), you will generally find that sizing your canopies to between 3.0 and 3.5 s.f. of canopy per pound of recovered weight will get you close most every time --- 3.0 giving something on the order of 20 FPS and 3.5 closer to 15 FPS. FYI. --jhc.
 
Hi John,

Thanks for the input- it is appreciated. I can see how 'diameter' isn't the best, or even a good method for determining appropriate parachute sizing. The reason I am sticking to it is because it's apparently the standard. This conversation did show me I need to put some significant thought into parachute sizing before purchasing one simply based diameter.

Very new to the high power side of this hobby, learning lots. Thanks.

--Joe
 
jcato said:
-- john.

p.s. As a good preliminary design rule (and a 'reality check' of the more rigorous design methods), you will generally find that sizing your canopies to between 3.0 and 3.5 s.f. of canopy per pound of recovered weight will get you close most every time --- 3.0 giving something on the order of 20 FPS and 3.5 closer to 15 FPS. FYI. --jhc.
Click to expand...

John, sorry for bring up the whole "diameter" thing again, but what do you mean by "sizing your canopies"? Are you talking about the s.f. of the surface of the canopy, or the s.f. of the circle the canopy forms when it opens?

I ask because if you use a Spherachute 48" chute, it has a open diameter of 30.5" and has about 10.2 s.f. of canopy area and 5.1 s.f. of area in the circle the chute forms when it opens. If you use a elliptical chute with the same diameter and the same 5.1 s.f area in the circle it forms when it opens, it will have 30% less canopy area, about 7.1 s.f

Both those chutes form circles of about 5.1 s.f. when open and will both drop a 3 lb rocket at the same rate. If you look at the s.f. of the canopy surface, the hemispherical is 30% larger then the elliptical.

So the question to you is, when you say sizing your canopies to between 3.0 and 3.5 s.f. of canopy per pound of recovered weight what do you mean by s.f. of canopy? If you mean the s.f. of the circle formed by the chute when it is open, you are probably right.
 
Handeman said:
John, sorry for bring up the whole "diameter" thing again, but what do you mean by "sizing your canopies"? Are you talking about the s.f. of the surface of the canopy,
Click to expand...

Yes. 'S(o)' - canopy surface area.

Handeman said:
... or the s.f. of the circle the canopy forms when it opens?
Click to expand...

No. (and this is describing 'Dp' (Projected Diameter) - then again, if we're talking a 'rigid skirt' canopy like the Spherachute design (assuming here a true hemisphere), then that would also be 'Dc' (Constructed Diameter)).

See what I mean --- this thing just rapidly descends into chaos and confusion - which 'S(o)' and 'Do' fundamentally removes - the point being: 'sizing your canopy' based on the *actual* amount of canopy fabric in the design. Since a flat circular is the most common canopy design in the hobby, there's no real change needed -- just figure the area of that circle and you've got 'S(o)'. If we're talking a hexagonal or octagonal, reduce that to a circle with the same area - again, relatively simple.

Handeman said:
I ask because if you use a Spherachute 48" chute, it has a open diameter of 30.5"
Click to expand...

... 'Dc' - and I would imagine the actual inflated (or what you refer to as "open") diameter (i.e. 'Dp') would be slightly less.

Handeman said:
... and has about 10.2 s.f. of canopy area and 5.1 s.f. of area in the circle the chute forms when it opens. If you use a elliptical chute with the same diameter and the same 5.1 s.f area in the circle it forms when it opens, it will have 30% less canopy area, about 7.1 s.f

Both those chutes form circles of about 5.1 s.f. when open and will both drop a 3 lb rocket at the same rate. If you look at the s.f. of the canopy surface, the hemispherical is 30% larger then the elliptical.

So the question to you is, when you say sizing your canopies to between 3.0 and 3.5 s.f. of canopy per pound of recovered weight what do you mean by s.f. of canopy? If you mean the s.f. of the circle formed by the chute when it is open, you are probably right.
Click to expand...

But then, 5.1 s.f. (I'll call this 'Projected Area' or 'Ap') results in 1.7 s.f. / lb, whereas the 7.1 sf and 10.2 sf figures results in 2.4 and 3.4 sf/lb, (and Nominal Diameters 'Do' of 3.0 ft & 3.6 ft) respectively (note that we don't have effective descent rates in these examples). The less efficiency of the hemispherical design would be accounted for in the Cd (should be Cdo) values being less than that for the elliptical (not sure where your figures for the elliptical are coming from - is this Nakka?).

One of the main benefits of Nominal Diameter (and the use of Cd values tied to that -- i.e. Cdo) is there is a direct correlation of the actual *efficiency* of any particular canopy design (i.e. just how much drag it creates for each square foot of fabric) and furthermore allows *direct* comparisons of one design over another (something virtually (and, for all intents, "literally") impossible in the hobby arena at the present hour). A canopy with a Cdo of 0.80 is 7% more efficient (and will utilize 7% less s.f. of fabric that weighs 7% less and will take 7% less volume in the recovery bay) than a canopy with a Cdo of 0.75 -- no matter if that canopy is 1 s.f. or 10 s.f. or 100 s.f. This would make canopy selection incredibly simple - simply pick the one with the highest Cdo value (assuming one wanted maximum drag per s.f.) - and don't worry about how they 'measure' the canopy size (over the top, 'open' (????) or whatever other term the 'marketing dept' comes up with). Plug that Cdo value into the terminal velocity equations and output the s.f. of the canopy needed. (This would, of course, require merchants to (determine and) publish the (*accurate*) Cdo values and publish the actual s.f. of the canopies they offer - which doesn't appear to be close to happening).

My little simple 'rule of thumb' ignores several things (the actual Cdo and launch site elevation being two of the most relevant), but will get one 'in the ballpark' for a reasonably safe and practical recovery. Unless and until we could get the merchants to list the s.f. values for their canopies (something that a grass roots effort by the rocketry community could accomplish), it may not answer as many questions as it rightfully *could*. Spherachutes *does* list enough information that 'S(o)' (canopy surface area) can be determined - which is good to see. Haven't verified how many others may do the same.

-- john.
 
TBSSJoe said:
... I can see how 'diameter' isn't the best, or even a good method for determining appropriate parachute sizing.
Click to expand...

Understand, Joe, that 'diameter' is a *fine* method of expressing parachute sizes -- I see nothing wrong with it -- provided...

TBSSJoe said:
... The reason I am sticking to it is because it's apparently the standard.
Click to expand...

This is kind of my point -- well, this *is* the point: There doesn't appear to actually BE a 'standard' (in the hobby, at least) -- some measure it 'over the top' (for shaped canopies like the Spherachutes brand, e.g.) - some measure 'across the opening (aperture)' (as I think Fruity and some others). Then we get to polygonal canopies and some want to measure 'across the points' and some want to measure 'across the flats' (and, from what I've seen, NO one will tell you what the canopy fabric area is).

See what I mean?

<sigh> (Please, someone, make up your mind.)

Joe, if you'll look here:

https://www.rocketryforum.com/showt...hute-and-Recovery-Systems&p=295089#post295089

(this is post #1 from the Technical Reference 'sticky' thread at the top of this forum - this I call Knacke-1991 (Knacke-1978 is post #3)):

...the 'classic' terminal velocity equation for a parachute in stable descent is:

v(eo) = SQRT ( (2*Wt) / (S(o) * Cdo * rho))

where: v(eo) = equilibrium descent velocity, FPS

Wt = Total weight of system - load+parachute, lbs

S(o) = canopy surface area, ft^2

Cdo = parachute drag coefficient related to S(o)

rho = air density at target altitude, slugs/ft^3

(you can see this discussion on page 4-16 (or page 71 in the pdf) of the above document)

You will note that any term for 'diameter' does not exist in that equation. It's all referenced to 'canopy surface area' - S(o) and the Cd value is referenced back to that as well (Cdo). When you think about it, it makes sense -- as all the various and sundry canopy designs that have been developed would make something like 'diameter' a nightmare to quantify in the above equation - *specifically because* of the very thing this hobby is struggling with - "nobody can decide what is really MEANT by 'diameter'". Far simpler to simply add up the square footage of fabric used in the canopy (i.e. S(o)) and work the equation from that perspective. By using a drag coefficient related to that area (i.e. Cdo), then the equation is solved rather simply. This will (automatically) account for any special canopy 'features' (ribs, vanes, etc -- like is used in the guide surface canopies) - it's all simplified back to simply the total area of fabric in the canopy. Knowing that, it is a trivial exercise to express that area as a simple circle - the diameter of which is called 'nominal diameter' - D(o).

The benefit of that is that all the various designs of canopies (flat circular, conical, hemispherical, quarter-spherical, ringslot, ringsail, toroidal (or annular), etc) -- can ALL be compared to each other (to determine which design REALLY IS the best for a particular application) because now they are all quantified on a common footing (i.e. S(o) and Cdo).

That's the point (as far as parachute 'diameters') are concerned - as well as the reason that the commercial/military arena has standardized on this mechanism.

Note, also, that any perforations in the canopy (slots, etc - as well as the apex vent) is ignored in figuring S(o) - just add up the areas as though those 'holes' weren't there. The 'Cdo' will account for whatever effect this will have on performance (with the understanding that extremely large openings - like the missing central portion of the annular canopies (or the large vent on the Fruity IRIS ultra) would be subracted (or not counted) in figuring canopy surface area).

You might also like to look over pages 5-3 thru 5-5 (pages 82-84 in the pdf) on the Knacke-1991 text referenced above for a comparative look at the various canopies and their performance.

Maybe this clarifies things a little bit - as noted, I've got no problems with 'diameter', as long as there's some common footing for the definition of the term.

-- john.
 

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