Sizing Three Parachutes verses One Parachute ?

Title says it all. Would three parachutes of .33 the square footage of one appropriate sized parachute be equal/more/ or less descent rate wise?

Need to know if three equals more or less than one in reality and RockSim.

Scott Evil said:

Title says it all. Would three parachutes of .33 the square footage of one appropriate sized parachute be equal/more/ or less descent rate wise?

Need to know if three equals more or less than one in reality and RockSim.

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Presuming the same Cd for the parachutes, then yes, each chute being 1/3 the area of a single chute should be about the same. (That is, 57% the diameter of the single chute.)

The only caveat that I have is that multiple parachutes stretch away from each other at an angle. For example, Apollo 13:


The loss in total drag should be approximately the cosine of the angle that the chutes form relative to vertical. Even a 30* tilt only loses ~14% of the overall drag. So 60% the diameter of a single chute should be a pretty good bet, even with some angle to each chute.

Since all three chutes are angled WRT to the direction of descent, there will be some gain in descent velocity. Not a lot, but some.

Greg

Absolutely,,, insanely cool.......
This forum is just unbelievable sometimes...........
My greatest compliments to you all...........

Teddy

As seen with the Apollo if there is more than one chute the airflow around them pushes them apart. The length of the bridle has a big effect-- long bridle they can get apart with a small angle.
I am sure there is a program to sim it. A drop test would give real data. Or just make the chutes a size larger, better too big than small

M

TheAviator said:

<snip> The loss in total drag should be approximately the cosine of the angle that the chutes form relative to vertical. Even a 30* tilt only loses ~14% of the overall drag.

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That (somewhat) idealized drag reduction scenario can only be achieved when 'Le' (effective suspension line length) is increased beyond that of a single canopy (typically taken as 'Le/Do' = 1.0 (with 'Do' being 'Nominal Diameter' - or the diameter of a circle of fabric with the same area as the canopy in question)). Knacke (see the tech resources thread) has reported using a 'Le/Do' figure of the square root of the number of canopies in the cluster (in this case: 3 canopies = 1.73x single canopy 'Le'). This can be any (reasonable) combination of either suspension lines or risers - but the total *effective* length 'Le' would need to be (at least) 1.73 to the confluence point. Those of the Apollo ringsails look to be even longer (and, in this picture here - something like 2.5 times the *inflated* (or *projected*) diameter ('Dp' - which would be less than 'Do'). NASA has some technical notes on the NTRS server that document all the construction dimensions of these canopies (something that Knacke doesn't get into (to any great detail) - see immediately following...

A good (recent find) is here:
https://lhldigital.lindahall.org/cdm/ref/collection/parachute/id/834
(shows the canopy dimensions about page 30)

... and a more detailed treatise here:
https://tinyurl.com/apollo-ELS
(dimensions around page 25)


That said, unless there is some really compelling reason to cluster the canopies, the potential downsides outweigh any benefits (packing volume (and, thereby, weight - since the very fact of a reduced 'Cdo' confirms clearly you will need more fabric for the same drag), fouling in deployment, etc). Clusters are pretty, to be sure - but the scale of the recoveries that the hobby engages in (i.e. small) can be far more reliably handled with a single canopy. (Now, saying that, I will still cluster the canopies on my Saturn V <g>).

-- john.

Thanx for all the info guys. Sooo... if I understand correctly, I will actually gain descent velocity due to the chutes being at an angle from their deflection from each other.

The reason I'm asking here is that I picked up three custom colored 36" CrossFire chutes from Top Flight thinking that the 6lbs max for each chute would be ok for the field at MWP in my Eagle Claw (18ish lbs.). RockSim is telling me otherwise though...30+fps with the main(s) out and I guess that it's factoring the 48" X-Form drogue in as well. I have no idea what the CD is for these chutes (I can barely SPELL cd). If it's greater than the RS standard and a respectable descent rate, well...cool. If not, I'll pop an online order in for bigger 'chutes. I'm VERY impressed with the quality and service of Top Flight LLC,they had those 36 inchers at my door not long after my screen saver kicked in...(Kudos Folks! :headbang: )

The necessity for three parachutes? Honestly, it's all ego. I fly a rocket once or twice a year and at least few hours of travel is usually involved, go big or stay home is my current opinion.

-S

I love multiple chutes. It may be harder to "do them right" but they sure do look cool!

One word: Apollo.



Please post pics!

Greg

GregGleason said:

One word: Apollo.
Please post pics!

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OK. (You asked for it. <g>)




These last photos (below) are not Apollo (obviously) but show my 'affinity' to ringsails <g>.
First is a scan from the HPR article (Mar/Apr 1993) of our local group (Coastal Rocketry Assoc - southern GA) - and is included here to give a sense of scale.




"Hundreds" (per the caption above)??

30 gores * 10 sails ea. = 300 individual sails. 6 months (+-) construction time.




-- john.

jcato said:

OK. (You asked for it. <g>)
These last (below) are not Apollo (obviously) but show my 'affinity' to ringsails <g>.
First is a scan from the HPR article (Mar/Apr 1993) of our local group (Coastal Rocketry Assoc - southern GA) - and is included here to give a sense of scale.

"Hundreds"?? 30 gores / 10 sails ea. = 300 individual 'sails'. 6 months (+-) construction time.

View attachment 182155 View attachment 182156

-- john.

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But Oh So Cool!

Handeman said:

But Oh So Cool!

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Your not kiddin,,,,
Those are gorgeous....

Teddy

onebadhawk said:

Your not kiddin,,,,
Those are gorgeous....

Teddy

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Teddy, you say 'those' (if referring to that last canopy) - understand those last few pics are of one canopy. (I'm not sure I would have had the mental fortitude to make more than one <g>. It was one, tedious undertaking, I can tell you that). But, if you are referring to that last one, thanks for the compliment.

Worse, I've always viewed that as a 'study model' to gear up and do about a 9 footer that is an accurate scale recreation of the Apollo ringsail - but that is 68 gores * 12 sails per = 816 total sails - almost 3 times the work of this one. I should have started on that one immediately afterwards - as I was younger and more 'foolish' then. The thought of tackling something like that today is a little scary. <g>

Glad you enjoyed the pictures (all of them - Apollo included). Got some more (of my work), but it's kind of hi-jacking the thread to post here.

-- john.

GregGleason said:

I love multiple chutes. It may be harder to "do them right" but they sure do look cool!

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John's (conservative) rules for clusters:

A) ALL canopies should be _identical_ in every respect (diameter, line length, everything);
(you may get away with violating that rule to some degree, but once you do, all bets are off
(and *why* would one want to risk a failure in a recovery of this level of complexity???))
B) Follow Knacke's rules for sizing 'Le';
C) Don't pack the canopies individually -- spike and flake them all, lay down together an then pack AS ONE CANOPY
(if bag deployed, pack them all in ONE bag).

This last 'rule' assures that all the canopies reach line stretch (and start inflating) at the same instant - which helps considerably in deployment success.

I know there are Certified Parachute Riggers here in this forum - and they may chime in with some other advice (that could conflict with mine above). Listen to *everyone* in terms of recovery -- because the professional guys (civilian or military) are not *near* as bold as rocket hobbyists --- for good reason.

[Clarification and Correction]: -- Listen to *every bit of advice* in terms of recovery and then *thoughtfully* consider the wisdom so imparted - because there is a lot of some pretty bad advice out there - not just in recovery. Adopt an attitude of humility and don't think you can rewrite the laws of physics - because you can't. The pros know this ---- the amateurs don't.

-- john.

jcato said:

Teddy, you say 'those' (if referring to that last canopy) - understand those last few pics are of one canopy. (I'm not sure I would have had the mental fortitude to make more than one <g>. It was one, tedious undertaking, I can tell you that). But, if you are referring to that last one, thanks for the compliment.

Worse, I've always viewed that as a 'study model' to gear up and do about a 9 footer that is an accurate scale recreation of the Apollo ringsail - but that is 68 gores * 12 sails per = 816 total sails - almost 3 times the work of this one. I should have started on that one immediately afterwards - as I was younger and more 'foolish' then. The thought of tackling something like that today is a little scary. <g>

Glad you enjoyed the pictures (all of them - Apollo included). Got some more (of my work), but it's kind of hi-jacking the thread to post here.

-- john.

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Yes John,,
I understand,,
The last 3 shots are of the same chute you built...
That my friend,, was a LOT of sewing and very careful laying out...
And that chute came out gorgeous...
I love big chutes,, I think there is a beautiful presence to them in the air,,, and slow final descents...
I have gotten myself in a bit of trouble in the past because of this for tying to put to big of a chute in too small of a space... lol
I would absolutely love to see a thread with chute pic's and your work constructing them...
If you start one please send me an email with the link so I don't miss it inadvertently....
Thanks for the pics John,,,,
I can't wait to see that 9 fter your about to start building...........lol
Take your time with it,,,
It doesn't have to happen overnight......

Teddy

jcato said:

OK. (You asked for it. <g>)

View attachment 182149View attachment 182150View attachment 182151View attachment 182152View attachment 182153View attachment 182154


These last photos (below) are not Apollo (obviously) but show my 'affinity' to ringsails <g>.
First is a scan from the HPR article (Mar/Apr 1993) of our local group (Coastal Rocketry Assoc - southern GA) - and is included here to give a sense of scale.

View attachment 182157


"Hundreds" (per the caption above)??

30 gores * 10 sails ea. = 300 individual sails. 6 months (+-) construction time.


View attachment 182155View attachment 182156

-- john.

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Wow. How much drag does the ringsail sacrifice compared to a normal parachute of the same open size, and of the same packed volume?

CarVac said:

Wow. How much drag does the ringsail sacrifice compared to a normal parachute of the same open size, and of the same packed volume?

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Using Knacke's numbers*, a hemi chute's Cd is 0.62 to 0.77, whereas a ringsail is 0.75 to 0.85 and is used in descent applications where v is < M 0.5.

Greg

* Ref.: NWC TP 6575, Table 5-2 "Slotted Parachutes", March 1991

CarVac said:

Wow. How much drag does the ringsail sacrifice compared to a normal parachute of the same open size, and of the same packed volume?

Click to expand...

GregGleason said:

Using Knacke's numbers*, a hemi chute's Cd is 0.62 to 0.77, whereas a ringsail is 0.75 to 0.85 and is used in descent applications where v is < M 0.5.
Greg
* Ref.: NWC TP 6575, Table 5-2 "Slotted Parachutes", March 1991

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{I dearly *love* soapboxes... :grin:}

(with consideration of the fact of the 'hemi' now being obsolete (as is the Flat Circular (i.e. your standard hobby canopy) - Cdo of 0.75-0.80*) - or the Conical (not obsolete) - Cdo = 0.75-0.90* (but this assumes 'bias' construction - thus allowing the fabric to stretch into the more normal hemispherical shape - improving that Cdo) -- and also noting the fact that, *of the slotted canopies*, the ringsail is the most efficient (in terms of drag - Cdo) - but I didn't build mine for that reason - I just simply wanted one). *Knacke - Table 5-1

All those trivial details are neither here nor there (but thanks for bringing this up, Greg ) - because what prompted my reply is this...

Folks will notice CarVac's question...

CarVac said:

Wow. How much drag does the ringsail sacrifice compared to a normal parachute of the same open size, and of the same packed volume?

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"How much.... compared to..."

I was using the technically more correct term "Cdo" (but Greg's use is fine for what he was trying to point out) - which means: "Cd referenced to NOMINAL diameter (Do)" (which is the diameter of a circle with the same (fabric - or 'surface') area as the canopy in question).

When you frame your technical specs that way -- then CarVac's question is easily (and rationally) answerable -- because *all* these numbers relate to each other and direct comparisons are trivially simple ---- a 'Cdo' of 0.80 is a more efficient canopy than a 'Cdo' of 0.75 - and one can use 7% less fabric (0.80/0.75) (and have 7% less weight in the recovery system) to create the same drag and, thus, have the same descent rate (and it doesn't matter a tinker's damn where that fabric is (in the canopy),... nor what shape it takes when inflated,... nor what that 'inflated diameter' is (as nearly impossible as that would be to measure),... nor...). None of it. Simply compare 'Cdo' to 'Cdo' and you have your answer.

The professional recovery industry did it that way (thank goodness) for precisely that reason - so that *direct comparisons* are possible. Good for them! Now, if we could get the hobby to do likewise..., well, I wouldn't need as many soapboxes <g>.

(Did anybody notice these numbers and how much _lower_ they are than what you hear in hobby parachute advertisements? Now you know why - and now you know why I ignore claims touting drag performance as much as (or more than) 200% above those numbers. Ain't gonna happen. That dog just won't hunt.)

OK, enough already -- now where was that *other* soapbox I was looking for....

.....

But, thanks Greg (and CarVac) for setting this one up right in my path.

(Sorry, Kevin, but there's just no way I was going to ignore this one.)

-- john.

onebadhawk said:

That my friend,, was a LOT of sewing and very careful laying out...
And that chute came out gorgeous...

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Yes, it was (a lot of sewing) - but thanks for noticing.

onebadhawk said:

I would absolutely love to see a thread with chute pic's and your work constructing them...
If you start one please send me an email with the link so I don't miss it inadvertently....
Thanks for the pics John,,,,

Click to expand...

Well, if you think folks would be interested, we could do a 'Parachute Build' (or 'deconstruct') thread where I review the techniques I've developed (in the general sense) and then close out with the special points concerning (and focus on) the ringsail.

These photos recently published are scanned from prints (long ago - negatives... somewhere??) and I've just recently scanned some of the others -- but, I'd have to photo some of the close-ups and details and such (not a big deal) to go with the 'seminar' as well as try to find my patterns (for the ringsail - I run across them every now and then, but can't place them in my mind immediately). Sadly, I didn't take as many notes as I should when I was building all these canopies (20+ years ago) - but I do have the paper gore patterns - I was just cutting and sewing like a banshee (there for a few years). I've got one or two other things, so (again, if folks are interested), let me kind of organize things a bit.

onebadhawk said:

I can't wait to see that 9 fter your about to start building...........lol
Take your time with it,,,
It doesn't have to happen overnight......
Teddy

Click to expand...

"... ABOUT to start building..."???

Cute!



-- john.

CarVac said:

Wow. How much drag does the ringsail sacrifice compared to a normal parachute of the same open size, and of the same packed volume?

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I know you figured, "How could such a one line question result in such a two page reply??" <g> I had to get 'that other stuff' out of the way. Forgive me. I saw Greg's reply and figured, "He's got it covered." (and, understand, my reply was not directed in any way towards you - more to the (broad scale) propensity to engage in confusion (as too many of these performance claims tend to do)).

The short answer is: Not much.

-- john.

jcato said:

I know you figured, "How could such a one line question result in such a two page reply??" <g> I had to get 'that other stuff' out of the way. Forgive me. I saw Greg's reply and figured, "He's got it covered." (and, understand, my reply was not directed in any way towards you - more to the (broad scale) propensity to engage in confusion (as too many of these performance claims tend to do)).

The short answer is: Not much.

-- john.

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I understand the area benefit, but I imagine that below a certain size the amount of hemming and stitching and fabric tape needed to construct a ringsail would outstrip the packing volume benefits of the reduced fabric area. Much like the additional shroud lines used in toroidal designs.

CarVac said:

I understand the area benefit, but I imagine that below a certain size the amount of hemming and stitching and fabric tape needed to construct a ringsail would outstrip the packing volume benefits of the reduced fabric area. Much like the additional shroud lines used in toroidal designs.

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To be sure.

Not so sure that one can justify a ringsail over, say, a conical on sheer drag benefits alone - in fact, outside opening forces, there may not be much else. In hobby size recoveries, there's really no benefit at all - and opening forces can (and should) be better handled with reefing. Considering the incredible increment in the amount of labor involved, it's probably the worst choice. (If I were to try to market something like this, I doubt $1000 or $1500 for the size of my ringsail would even qualify as minimum wage - and I wouldn't work for minimum wage on this).

-- john.