Looking for acceleration/time data of parachute deployments on big rockets

[Colin Burfeind]

I'm looking for anyone's acceleration/time data of parachute deployments, especially main, on large rockets. Basically looking for good data of how many G's are pulled, and what that curve looks like during the deployment shock load. I'm working on a shock load mitigation system for a university rocket, but we lost acceleration data on past launches, so I need data to start characterizing the expected shock load without mitigation properly.

Ideally data would be logged from an accelerometer in the main airframe of the rocket, and the main parachute had been packed properly in a deployment bag. Preferably this rocket is an L3 or Class 3 sized rocket. Data in the form of a CSV file or just a high resolution plot would work. Thanks in advance.

 

[Steve Shannon]

Usually 30 gees or less.

 

[tfish]

I'm not sure of the number..but a rocket that accidently deploys it recovery system at Max q..and the major components stay tethered together..has been designed correctly.
Yes the chute(s) will shred..which helps dissipates energy..
Everything tethered together makes seeing and watching it much easier then several small things raining down.

Tony

 

[Maxwelljets]

I'm looking for anyone's acceleration/time data of parachute deployments, especially main, on large rockets. Basically looking for good data of how many G's are pulled, and what that curve looks like during the deployment shock load. I'm working on a shock load mitigation system for a university rocket, but we lost acceleration data on past launches, so I need data to start characterizing the expected shock load without mitigation properly.

Ideally data would be logged from an accelerometer in the main airframe of the rocket, and the main parachute had been packed properly in a deployment bag. Preferably this rocket is an L3 or Class 3 sized rocket. Data in the form of a CSV file or just a high resolution plot would work. Thanks in advance.

Read through the parachute bible by Knacke. It has info on just about everything related to chutes, including shock loads.

That said, especially on an amateur scale, this problem is quite difficult to characterize because it depends on so many different variables. Chute folding/packing method, deployment velocity, chute shape, and chute size all play a role in determining inflation time. It's never going to be the same twice.

Additionally, avionics bays typically end up swinging quite a bit in flight, which means the accelerometer data collected during chute deployment is usually inaccurate enough that it won't be particularly useful for your exact question. It's quite difficult to discern signal from noise. That said, I have included a CSV of 3 axis accelerometer data for a recent flight I did. The rocket weighed about 20 lb or so on descent, and the main chute was a 66" homemade toroidal chute with a Cd of about 1.9.

Here's a graph from the AIM XTRA software showing acceleration around main deployment:


As you can see, the accelerometer was not aligned vertically before the main deployed, so it didn't capture the full data. That said, you can still see how the deployment shock of the ejection charge and the recovery harness snapping taut is not trivial either. In most cases, the chute opening shock is smaller than the shocks related to ejection.

 

[G_T]

The drag force generated by a chute is going to be a function of the square of the velocity. Therefore the G loading is also going to go as the square of the velocity, until the velocity is too large and you shread the chute or the airframe or break the line.

I don't think there is enough time resolution in the graph, and the data appears to be smoothed a little.

Rocket was descending roughly 114fps under drogue at main deployment. Main was piloted tethered d-bag setup, with a high drag toroidal chute. Final velocity was about 24fps. And clearly that G scale on the graph is a little uncalibrated.

Gerald

 

[G_T]

Same rocket etc, different flight. Different altimeter. This one shows higher G loading on deployment, which I find more likely to be closer to reality.

Gerald

 

[lakeroadster]

Read through the parachute bible by Knacke. It has info on just about everything related to chutes, including shock loads.

That said, especially on an amateur scale, this problem is quite difficult to characterize because it depends on so many different variables. Chute folding/packing method, deployment velocity, chute shape, and chute size all play a role in determining inflation time. It's never going to be the same twice.

Additionally, avionics bays typically end up swinging quite a bit in flight, which means the accelerometer data collected during chute deployment is usually inaccurate enough that it won't be particularly useful for your exact question. It's quite difficult to discern signal from noise. That said, I have included a CSV of 3 axis accelerometer data for a recent flight I did. The rocket weighed about 20 lb or so on descent, and the main chute was a 66" homemade toroidal chute with a Cd of about 1.9.

Here's a graph from the AIM XTRA software showing acceleration around main deployment:
View attachment 560823

As you can see, the accelerometer was not aligned vertically before the main deployed, so it didn't capture the full data. That said, you can still see how the deployment shock of the ejection charge and the recovery harness snapping taut is not trivial either. In most cases, the chute opening shock is smaller than the shocks related to ejection.

Thanks for posting the link to the parachute bible by Knacke.

@Colin Burfeind Page 5-52 (pdf page 132) of the above document shows a maximum parachute force of 3,740 lbs... for a 200 lb torso dummy.... that's a factor of 18.7 : 1

 

[sriegel]

Thanks for posting the link to the parachute bible by Knacke.

@Colin Burfeind Page 5-52 (pdf page 132) of the above document shows a maximum parachute force of 3,740 lbs... for a 200 lb torso dummy.... that's a factor of 18.7 : 1

That example is also for a 29-foot chute at 250 knots, rather larger and faster than most model rocket airframes.

See page 5-50 for a relevant equation.

Fx = (CdS)p q Cx X1

where

Fx is opening force
(CdS)p is the drag area of the chute, area x drag coefficient
q is dynamic pressure at line stretch
Cx is opening force coefficient at infinite mass. See Table 5-1 for typical values. 1.7-1.8 is reasonable for a typical hobby chute (flat or domed)
X1 is opening force reduction factor (note: close to 1 for highly-loaded chutes like a small drogue and as low as 0.02 for lightly loaded chutes. I typically use 0.07 - 0.1 in my calculations).

I use Knacke's Method 2 (starting on page 5-56 in my copy) as my standard math. Feel free to message if you have questions on it. Watch the notation carefully!! Knacke has a couple very tricky usages of different variable terms that caused me some headaches at first.

 

[Colin Burfeind]

Read through the parachute bible by Knacke. It has info on just about everything related to chutes, including shock loads.

That said, especially on an amateur scale, this problem is quite difficult to characterize because it depends on so many different variables. Chute folding/packing method, deployment velocity, chute shape, and chute size all play a role in determining inflation time. It's never going to be the same twice.

Additionally, avionics bays typically end up swinging quite a bit in flight, which means the accelerometer data collected during chute deployment is usually inaccurate enough that it won't be particularly useful for your exact question. It's quite difficult to discern signal from noise. That said, I have included a CSV of 3 axis accelerometer data for a recent flight I did. The rocket weighed about 20 lb or so on descent, and the main chute was a 66" homemade toroidal chute with a Cd of about 1.9.

Here's a graph from the AIM XTRA software showing acceleration around main deployment:
View attachment 560823

As you can see, the accelerometer was not aligned vertically before the main deployed, so it didn't capture the full data. That said, you can still see how the deployment shock of the ejection charge and the recovery harness snapping taut is not trivial either. In most cases, the chute opening shock is smaller than the shocks related to ejection.

The knacke paper is awesome. I'm really excited to dig into it. Thank you very much!