Calculating Drogue Parachute Snatch Forces

[AllDigital]

I know this question has been asked in at least a few threads, but the situation is slightly different and I want to ask it again to check my math.

We are experimenting with a new spring latch deployment system on a large 8" 200 lb rocket. This won't use pyros, so you don't have the typical ejection forces caused by pyros. It is a dual deploy "two out the top" system with both drogue and main coming out the nose end of the upper airframe. In this scenario, at apogee, an electric latch releases a spring loaded nose cone and as the nose falls away it pulls out the drogue. The drogue is a seven foot chute on a forty foot tether to a second latch in the upper airframe (the "main latch"). The question is... what is the maximum snatch force we should design for on that main latch at deployment?

I've got dozens of files of high resolution accelerometer data with pyro dual deploy recovery, but it is all worthless when trying to accurately determine (negative) gee forces created by a drogue deployment, because so much chaos is going on immediately following drogue pyro discharge. It is also highly variable depending on lots of factors.

For this test situation, a reasonable scenario would assume that the drogue ejected late and the vehicle is going 200 fps. The drogue ejects and takes three (3) seconds to deploy from full speed to drogue descent speed (75 fps). In that scenario, Acceleration = -1.3 g's (200fps to 75fps in 3 sec), so Force = mass (200 lb) * acceleration (-1.3g) = 1,156N = 260 lbs. of force on the "main latch".

If the drogue was faster to decelerate the vehicle, say two seconds instead of three, then the force would be 380 lbs. (-2 g's) Or if it opened really late and was now traveling at 300 fps the snatch force would be 466 lbs. Good timing, good packing, and shock cords will all help minimize Force, but we do need a design point.

We have a lot of options on the latch strength from 300 lbs to > 1000 lbs, but we don't want to overbuild. Also, failure of the second "main latch" is not catastrophic, it just means the main will come out early and recovery will be a very long walk.

Does the math above look right? Does anyone have actual deceleration timing data from drogues at different speeds (versus main chutes)?

Thanks,

Mike

 

[jderimig]

You are assuming constant acceleration in your rough calculation, or average force. The acceleration at the end of the event is zero so the acceleration at the start of event must be higher. If you assume a constant Cd*A (simplest first model) then then the drag force or acceleration is a parabola with 0 at the end and an area under the curve of your average. From that you should be able to solve for the peak.

 

[user 30299]

I started looking into the snatch forces for drogues a couple of weeks ago. I'm still trying to digest all the information and math. You may want to also search under Pilot Chutes. And that tends to send you to pilot chutes for Base Jumpers. Snatch forces are critical for them.

A couple of professors created an "Opening Shock Calculator" for parachutes.

Here is a link to a Google drive where you can get the manual and program : https://drive.google.com/drive/folders/1GXzWbYhkBrSO_XuhR9oY0B0t0cthny3q

I have attached the manual so that you can see if this will get you what you want. I'm still working thru this too.

Sorry I could not be more helpful.

 

[jderimig]

You can also determine the initial shock experimentally. Take your droque and a force gage for a ride in your car at 200 ft/s. Sounds like alot of fun at 136mph.

 

[AllDigital]

@QFactor the research you found is excellent. The manual alone is a wealth of information. I really like the mass ratio scatter plots. Those really help provide a range for planning/design purposes.

With my specific situation, I've decided to "over build" first (with a 3-4X margin) and then try smaller and lighter latches on subsequent test flights.

I am still contemplating @jderimig's suggestion of rigging a load cell to my car

 

[user 30299]

@QFactor the research you found is excellent. The manual alone is a wealth of information. I really like the mass ratio scatter plots. Those really help provide a range for planning/design purposes.

With my specific situation, I've decided to "over build" first (with a 3-4X margin) and then try smaller and lighter latches on subsequent test flights.

I am still contemplating @jderimig's suggestion of rigging a load cell to my car

Good to hear this was helpful!

 

[user 30299]

AllDigital,

I think you will need one of these for your load cell testing. Best to get to that 136 mph as quickly as possible, and have "velocity" to spare . . . .

SC TUATARA: 316 MPH
SSC Tuatara: 316 MPH

SSC Tuatara Specifications

Engine	5.9-Liter Flat-Plane Crank V-8
Horsepower	1,750 Horsepower
Torque	1,280 Pound-Feet
0-60 MPH	2.5 Seconds (est)
Top Speed	286 MPH (316 unverified)
Price	$1.6 Million
Production	100

 

[David Schwantz]

Nice car. But I don't think I could even get in the thing

 

[manixFan]

QFactor - Wow, at a recent club meeting we talked about this subject regarding a very heavy nosecone and the forces on the parachute at opening. That PDF is very interesting reading, and the program looks pretty reasonable to use.

Thanks for the link,


Tony

 

[user 30299]

QFactor - Wow, at a recent club meeting we talked about this subject regarding a very heavy nosecone and the forces on the parachute at opening. That PDF is very interesting reading, and the program looks pretty reasonable to use.

Thanks for the link,


Tony

You're very welcome!

It was heavy, nose cone rockets that started me on the search for more info on chute opening forces and drogue chute snatch forces. I have three rockets that come down very fast, and nose cone first, before the main chute opens. When their chutes open, the whole rig whipsaws the other direction.

Attached are pictures of my Lotus 5 (Composite Warehouse kit). This is the one that really sent me on the search. It's HED, tips the scale at 13 lbs., after the motor burn, no drogue (at this time). Even after the first separation, that nose cone takes over and it's coming in straight as an arrow.

 

[Johnly]

What a great program find!
It's now added to my library.

 

[sriegel]

A couple of professors created an "Opening Shock Calculator" for parachutes.

This looks like some awesome stuff. Many thanks for sharing it to the group!

 

[Tractionengines]

What about a slider (ring) to slow opening time. This allows the deceleration time to be extended, and forces change over time. With the ring near the chute at start, it is greatly "reefed", so minimal force. As the chute opens forcing the ring to slide down the shroud lines the canopy drag increases it opens more, but the speed is decreasing so peak force on line is lower.
If used on the main on a standard dual deploy, it also gives some time for the "recovery string" to organize below the main chute, hopefully preventing a zipper as the main takes the load, and slows the decent.
Thoughts?

 

[user 30299]

What about a slider (ring) to slow opening time. This allows the deceleration time to be extended, and forces change over time. With the ring near the chute at start, it is greatly "reefed", so minimal force. As the chute opens forcing the ring to slide down the shroud lines the canopy drag increases it opens more, but the speed is decreasing so peak force on line is lower.
If used on the main on a standard dual deploy, it also gives some time for the "recovery string" to organize below the main chute, hopefully preventing a zipper as the main takes the load, and slows the decent.
Thoughts?

When you read up on the slider rings, it seems to be a mixed bag of results with rockets. You can find past comments/threads on TRF for them. That may be one of those devices that works well for some people - and not at all for other people. But it's a good question for the parachute makers . . .

 

[Tractionengines]

Yeah. It's a balancing act that "like everything" has a learning curve. Some (most) of which is trial-and-error.

Too small a ring placed too close to the chute and it can prevent opening at all. Too big a ring and it slides/falls down very fast not giving much cushioning, or way too big goes up over the edge of the skirt holding it closed...but when they work, it can look really nice with a slow opening main chute as it fills with air.
Mike

 

[rharshberger]

AllDigital,

I think you will need one of these for your load cell testing. Best to get to that 136 mph as quickly as possible, and have "velocity" to spare . . . .

SC TUATARA: 316 MPH
SSC Tuatara: 316 MPH

SSC Tuatara Specifications

Engine	5.9-Liter Flat-Plane Crank V-8
Horsepower	1,750 Horsepower
Torque	1,280 Pound-Feet
0-60 MPH	2.5 Seconds (est)
Top Speed	286 MPH (316 unverified)
Price	$1.6 Million
Production	100

View attachment 497855

View attachment 497856

Built in Richland, WA, a 4 wheeled rocket....

 

[Handeman]

I know this question has been asked in at least a few threads, but the situation is slightly different and I want to ask it again to check my math.

We are experimenting with a new spring latch deployment system on a large 8" 200 lb rocket. This won't use pyros, so you don't have the typical ejection forces caused by pyros. It is a dual deploy "two out the top" system with both drogue and main coming out the nose end of the upper airframe. In this scenario, at apogee, an electric latch releases a spring loaded nose cone and as the nose falls away it pulls out the drogue. The drogue is a seven foot chute on a forty foot tether to a second latch in the upper airframe (the "main latch"). The question is... what is the maximum snatch force we should design for on that main latch at deployment?

Mike

I'm wondering what makes you think the nose cone will fall away from the rocket? The rocket and nose cone will fall together! There will probably not be enough differences in drag or gravity to make the nose cone fall any faster or slower than the rocket, and unless you impart enough inertia into the nose cone with your spring release, similar to a pyro charge, it is unlikely the nose cone will pull anything out of the BT.

I've seen multiple AT G-Force and similar rockets flying on G motors with small 0.75g ejection charges eject the nose cone a few feet and then everything falls down together without the chute ever coming out of the BT. When the rocket is picked up and tilted down, the chute falls out of the BT.

You might want to put a small drogue (18") on the nose cone to add drag to it so it separates from the rocket as they both fall. That should pull your larger drogue out of the BT.

good luck

 

[David Schwantz]

I have several rockets where the NC comes down below the booster. The 1/3 scale V2 for one. It does have 25lbs of nose weight though

 

[AllDigital]

You might want to put a small drogue (18") on the nose cone to add drag to it so it separates from the rocket as they both fall. That should pull your larger drogue out of the BT.

Yep, that is the plan. The spring release will only get the nose 1-2 feet clear of the BT. There will be a small pilot chute on the nose to give it enough drag to pull out the drogue. This is what we are testing. We'll have four cameras on it to get good data on actual behavior and a radio controlled backup chute triggered by BP charges, so we can push the limits with minimal risk.

I've also had other rockets that separated, but not far enough to pull out the laundry. Small pilot chutes can be your best friend.

 

[user 30299]

Here's a picture of three rockets where the nose cone is always leading (pulling) the body tube during the fall. Their harnesses are always fully extended before the Main opens. The Phoenix and Batray are 4" dia. rockets with under 1 lb. of weight in the nose - for stability. The Lotus 5's nose cone weighs nearly 2 lbs - and doesn't need any additional weight for stability; 5" dia. body tube, and HED. Once you have the separation and the nose cone starts heading "south", the body tube gets tipped down and acts like a drogue chute. You would think the weight of the body tube and motor case would prevent the drogue effect of the "empty" body tube - but they don't. These are short rockets, so their flight characteristics are a bit different.

These rockets are fun to launch, but boy when they come in nose first, and fast, you're praying hard for that chute to open. I usually have a Jolly Logic chute release on the Phoenix and Batray.

 

[David Schwantz]

Like the bat wings

 

[user 30299]

Like the bat wings

It's a Madcow kit. Great kit. Fins came that way. I added the tailcone. The holes in the fins give the rocket an awesome whistle when you launch.

 

[AllDigital]

Here's a picture of three rockets where the nose cone is always leading (pulling) the body tube during the fall.

I've launched hundreds of rockets and can say with confidence that recovery and deployment is the most unpredictable and difficult to repeat consistently. I am always in awe (and holding my breath) of manned capsule missions when all three chutes open perfectly.

We have lots of good software to model the "up" part of flight and general descent rates, but does anyone have good formulas or documentation or software on how airframes will behave in free-fall? I had not considered the example of a nose falling faster than the rocket and using the rocket as a drogue, but it makes complete sense -- I just haven't seen it because there is always some type of chute involved. What is the relationship between the CG and the CP that causes rocket "roll recovery" vs. lawn dart? When does the nose fall faster than the airframe? How to get an airframe to do a Starship belly flop?

My approach to deployment has always been excessive amounts of BP to ensure the laundry has no choice but to succumb to opening up, but that won't work with this HED approach. I resolve to get better at recovery in 2022.

 

[rocketman20]

I started looking into the snatch forces for drogues a couple of weeks ago. I'm still trying to digest all the information and math. You may want to also search under Pilot Chutes. And that tends to send you to pilot chutes for Base Jumpers. Snatch forces are critical for them.

A couple of professors created an "Opening Shock Calculator" for parachutes.

Here is a link to a Google drive where you can get the manual and program : https://drive.google.com/drive/folders/1GXzWbYhkBrSO_XuhR9oY0B0t0cthny3q

I have attached the manual so that you can see if this will get you what you want. I'm still working thru this too.

Sorry I could not be more helpful.

hey the link to google drie is not working and I couldn't find programme on net can you help me

 

[Handeman]

hey the link to google drie is not working and I couldn't find programme on net can you help me

That post was over 15 months old. I'm not surprised the link doesn't work. QFactor is still around, you might want to PM him and ask directly.

Here's a link to the Recovery Systems Design Guide. Chapter 6 talks about snatch and inflation forces. There is no software so you'll have to do the math yourself, but it is explained well. It also cover a lot of info that probably isn't applicable to rocketry, but it's pretty interesting.

 

[user 30299]

hey the link to google drie is not working and I couldn't find programme on net can you help me

Attached is the program in a Zip file.

 

[Sooner Boomer]

This might be interesting, too.

https://apps.dtic.mil/sti/pdfs/ADA063497.pdf
"Snatch force" is not really the technical term. Position->velocity->acceleration->jerk

-> is used to represent "change in", so change in velocity is acceleration, change in acceleration is jerk. If you can start with a more basic term, for example distance or velocity, along with the time the term is applied, you can derive higher terms. Sometimes this requires SWAG, deriving higher terms, then working backwards to see if the guesses are valid. It's fairly straightforward calculus/physics. I hate to use the expression "it's easy", because some people's brains lock up at math more complicated than balancing a checkbook. Personally, my bete noir was integrals.

 

[Handeman]

The attached also has a long discussion of snatch loads and how to mitigate them. They can get huge if the chute opens before the shrouds and lines tighten.

"This dynamic ‘twang’ is known as the snatch load, and if no attempt has been made to
restrain the canopy from partially inflating before this snatch load has concluded, this can be
the highest load the recovery system has to suffer. You might think that the small mass of the
tiny drogues used in HPR rocketry couldn’t produce a significant snatch load, but you’d be
surprised!"