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
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