# Calculating Parachute Snatch Forces & Canopy Opening Forces

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#### JimHeaney

##### New Member
Hey everyone, I am putting together a knowledge wiki for new members of our college rocketry team and I am wondering how everyone calculates the parachute snatch/opening force for HPRs.

Looking over old documentation of our past rockets, it looks like students arrived at a number with no info of how they got to it, or they used a factor of safety of 4-6 times the normal drag force of the parachute, which seems like a bit much (but then again, no such thing as too safe!). I'd imagine this is a good enough approach for an L1 or L2, but I foresee the team starting to build bigger rockets in the future and I want to make sure to give them a good base of knowledge to build upon.

I did some digging and found some old documentation from NASA and some research groups, but they all tend to either gloss over it without quantifying the forces, or do such a deep dive into math that I feel like I am back in my Differential Equations Class! One calculator that I did come across was the Parker College OSCALC, has anyone used this in the past?

#### Steve Shannon

##### Well-Known Member
TRF Supporter
It really all depends on how quickly your chute opens and how fast your rocket is traveling and how slow it ends up.
1. Empirical data that I have personally observed indicates that parachutes exert an acceleration of 30 gees in a rapid chute opening where a rocket has some velocity at deployment. These were not high speed deployments like you would experience from drag separation.
2. 44 years ago I was told that a man rated parachute takes about 250 feet to open. Of course that’s variable but it’s not out of the question. If you provide a beginning velocity and use a typical target velocity of 15-25 fps you can easily calculate the acceleration. Without diffeq’s.

#### mikec

##### Well-Known Member
The forces felt by the system also depend a lot on the elasticity of the shock cords involved. I'm not sure I'd believe anything but an actual measurement with an appropriate accelerometer.

People are constantly being surprised by high forces at apogee deployment causing unwanted main deployment, usually when using low-elasticity kevlar shock cords. In my experience, these forces are much higher than main chute deployment forces under normal circumstances.

#### JimHeaney

##### New Member
If you provide a beginning velocity and use a typical target velocity of 15-25 fps you can easily calculate the acceleration. Without diffeq’s.
I didn't even think of that approach, that definitely is a lot easier than quantifying a parachute opening! Thanks, Steve.

#### Handeman

##### Well-Known Member
TRF Supporter
People are constantly being surprised by high forces at apogee deployment causing unwanted main deployment, usually when using low-elasticity kevlar shock cords. In my experience, these forces are much higher than main chute deployment forces under normal circumstances.
It's the Kevlar cord and oversized ejection charges. All you need at apogee is to get the drogue into the air stream, its drag will do everything else, but some want enough charge to have the two halves hit the end of the cord hard.
Using shear pins makes this worse since you have to have enough force to shear the pins and then you end up with enough force the two halves hit the end of the cord hard. The apogee joint only need enough friction to not separate at burnout, which is very little to none on most rockets.

#### ksaves2

Ditto that. No reason to use a heavy blast for an apogee charge even if the rocket is single deploy. Main will have enough time to deploy if packed properly. Shear pins on a sustainer would possibly be considered on a rocket that is going to be traveling so blinking fast that the static port wouldn’t be able to equalize the internal/external pressure quick enough. Most of us never cross over into that realm where we’d have to think about that.

For an apogee charge I just ground test to get the parts to separate. Not to blow the pieces to the end of the harness. If using dual altimeters, sure put ”extra“ in the backup if you must but if the primary works nominally, it will just vent into the air so won’t do any harm.

I will revert to shear pins on a main charge because a nosecone, parachute and harness is relativistically less mass than the two pieces of rocket one is splitting at apogee. Even then, I’m ok with a ground test that blows out the main laundry with just a third of the harness coming out of the rocket after shearing the pins.

I have more of a tendency of using NC main charge shearpins than most because I’ve witnessed the loss of a sizable rocket where the aggressive apogee charge popped the main at a considerable altitude. NC was friction fit and I don’t know what the flier did for testing. This was before RDF tracking was common. The flier took off by car and was never seen of again!! No, no, no. Cheap attempt at humor, the rocket was never seen again. Had no tracker but lost the dual electronics and pricey motor casings. Was a dual cluster. Felt sorry for the guy but lesson learnt for me. On small, light DD rockets of course friction fit generally works fine. Kurt Savegnago

#### scschulte

##### Member
Excellent comments! I've launched my PML Endeavour (piston set-up) 22 times since Oct 2009 -- and now always use two 4/40 shear pins - I've had the main open at apogee several times without pins and I'm not climbing any more trees! (LDRS-33…) Great comments on not overdoing the ejection charge charge for drogue-- especially when firing the charge at apogee. I am new to The Rocketry Forum and very impressed!!

Steve on Saturday 11 July 2020

#### Kelly

##### Usually remembers to get the pointy end up
I've heard that people assume up to 100g for opening forces, but I've never bought that. It's usually pretty cheap - in terms of weight or  - to overdesign the recovery components, though, so a bit of overdesign may be warranted.
As far as calculating the force, as an upper limit, you could assume no shock absorbing by the harness/shock cord etc. Also assume that the parachute opens instantaneously. The force exerted by an open parachute is fairly straightforward to calculate knowing its Cd (there are many online calculators), so if you assume a maximum velocity for the rocket at deployment, this will lead you directly to the maximum deployment force expected.

#### JoePfeiffer

##### Well-Known Member
You can get a reasonable upper bound on the force when the main deploys by calculating the drogue decent rate, and then calculating the main parachute's drag at that speed. I'll start with that, put in a safety factor of 2 or 3, then use the next heavier-duty shock cord and hardware I can find.

#### tfish

##### Well-Known Member
There might be some useful info in this post....

Tony

#### tfish

##### Well-Known Member
Hey everyone, I am putting together a knowledge wiki for new members of our college rocketry team and I am wondering how everyone calculates the parachute snatch/opening force for HPRs.

Looking over old documentation of our past rockets, it looks like students arrived at a number with no info of how they got to it, or they used a factor of safety of 4-6 times the normal drag force of the parachute, which seems like a bit much (but then again, no such thing as too safe!). I'd imagine this is a good enough approach for an L1 or L2, but I foresee the team starting to build bigger rockets in the future and I want to make sure to give them a good base of knowledge to build upon.

I did some digging and found some old documentation from NASA and some research groups, but they all tend to either gloss over it without quantifying the forces, or do such a deep dive into math that I feel like I am back in my Differential Equations Class! One calculator that I did come across was the Parker College OSCALC, has anyone used this in the past?
As the Range Safety Officer (RSO) I like to see recovery systems designed and built to 'withstand' deployment at Max Q. Let me explain. I would like to see the major parts of the rocket stay tethered together. That way you/we only have to watch for 'one' major component and not multiples. Yes the chutes will more then likely shred. The shredding of the chutes will dissipate some of the energy and the shredded chute may act as a streamer somewhat. The recovery system can also contain "break points" These can be sewn in to the shock cords or made bu bundling shock cord and placing masking or electrical tape around them.

Tony

#### dhbarr

##### Amateur Professional
As the Range Safety Officer (RSO) I like to see recovery systems designed and built to 'withstand' deployment at Max Q. Let me explain. I would like to see the major parts of the rocket stay tethered together. That way you/we only have to watch for 'one' major component and not multiples. Yes the chutes will more then likely shred. The shredding of the chutes will dissipate some of the energy and the shredded chute may act as a streamer somewhat. The recovery system can also contain "break points" These can be sewn in to the shock cords or made bu bundling shock cord and placing masking or electrical tape around them.

View attachment 424047
Or by stitching hook-and loop segments, thought I'd seen that somewhere around here before.