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Choosing the best delay when the "right" one isn't available?

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neil_w

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Recently it has dawned on me that I am not entirely sure the best way to choose the right delay. Talking single-use motors here, where I can only choose from a set of standard delays. Is it better to err on the short or long side? How long is *too* long?

This problem is most acute when simming with Aerotech SU motors, e.g. the E20 or E30, where delays of 4 and 7 seconds are available (let's not mention the F44, which comes in -4 and -8 only). For whatever reason, a lot of my rockets seem to show ideal delays of about 5 seconds. In that case, using a -4 would give a decent low-speed deployment, but I've read that early deployment can result in the parachute not being ejected properly, or becoming tangled. Choosing a 7 second delay means a full 2 seconds late, or deployment north of 30 mph, which seems quite late to me.

If the flight trajectory is less than perfectly vertical, then a shorter delay would be better, although I don't know how much shorter.

So what's the right way to approach this?
 

Nytrunner

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I've heard that ejection on the way up is better because the rocket is slowing down instead of speeding up.

(I'll wait for the counterpoints to start rolling in)
 

K'Tesh

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Seems to me that going with a zipperless design would be a decent solution. In that case, I'd be more inclined to go with the longer delay. But that is simply based on my intuition, and nothing else.
 

mbeels

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I'm not sure what the right way to approach this is, but I tend to prefer the shorter delay. My sims are usually slightly optimistic, or my surface finish not ideal (actually, this has gotten better since I learned to set my surface finish to matte), and there usually seems to be more wind then I expected. It also avoid those few heart stopping moments when the rocket arcs over, points its nose down, and starts accelerating.

I haven't had any issues with the parachute not deploying properly or getting tangled if the ejection charge pops while the rocket is still going up. I'm using less plastic parachutes these days (and more nylon chutes), maybe that helps.
 

neil_w

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Erring a bit on the short side was my guess as well, but I feel like I’ve seen multiple mentions here recently of the perils of too-short delay.

I’m not too worried about zippers really; I don’t think 1/4” elastic is too likely to cause one under normal circumstances. I suppose I worry more about the survival of the parachutes, particularly plastic ones, of which I still use plenty.
 

dhbarr

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A tad early IMO is better than a tad late. Extremely early is worse than a tad late. Acceleration is constant for this ballistic trajectory, so I guess... whichever one sims closer, rounded in favor of the way up?
 

rharshberger

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IMO anything off more than 2 seconds either direction up or down is asking for damage, so I would go with a 4 if a 5 is the simmed recommended or with a 7 if the 4 wasn't available as both are withing the 2 second leeway I give myself.
 

boatgeek

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I'm in the "round up or down to the nearest delay but not more than 2 seconds off" camp. If it's halfway between the options (5.50 optimum delay in your example) then go with the shorter delay per @mbeels' reasoning.
 

mooffle

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Another reason to go with shorter: If the parachute ejects after apogee then the rocket is falling ballistically toward the ground and may fall into the parachute as it comes out. The parachute can then act as a nosecone and keep the rocket stable and falling ballistic (especially in the case of an over stable rocket).
If the delay is short and the rocket pushes into the chute there is still an apogee event where it can fall out of the parachute and/or the change in air direction can open it back up, it might still get tangled but a tangled chute seems much better than effectively no chute.
 

neil_w

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I'm really annoyed at myself that I bought some E30-7 motors. Shoulda got -4.
 

GalantVR41062

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In open rocket you can simulate your flight and weather conditions, then input the ejection event at X amount of time after motor burnout instead of apogee. This will give you a velocity at the point of ejection charge of to early or to late.

I have had both happen, sometimes with and sometimes with out some type of failure. Flying my SLS ARCAS on a Estes G40w-7 resulted in a early ejection charge, the rocket went up another 100' then started falling. The JLCR did keep the main held in a bundle so no damage on that flight.

We flew my daughter's largest park flier on a Loki G66r motor this winter. After some chuffing the motor lit and burned, then about 1 sec after burnout the ejection charge went off, I drilled the delay grain to much. The fishing swivel hook holding the main onto the harness bent straight and the chute landed a ways away, and the rocket and NC landed in the snow unhurt.

If you want to get down to the science of what's to much use electronics mounted solid in/on the rocket with a G meter, the flight scratch mini comes to mind. Then see the forces of the ejection events.

~John
 

mtnmanak

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I am going to assume the answer to this question is "no", but, is there any way to adjust the delay on single use Aerotech motors? I assume that would be an unauthorized modification, but I am not educated enough on that subject and can't seem to find anywhere that is discussed.
 

FredA

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Drill it and go fly if a reload.
SU -- you're pretty much stuck.

Gravity is gravity - short or late are about the same as far as deployment speed is concerned.
 

CoyoteNumber2

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You can adjust the delay on a single-use motor, through the touch hole with a drill bit.
 

SDramstad

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After burnout you have both gravity and drag slowing you down so you are decelerating at more then 1 G. On the way down gravity is pulling you down at 1 G BUT, drag is working against that so acceleration is less then 1 G. Seems to me then that you are going faster 2 seconds before appogee then 2 seconds after. My preference is slightly long over short.
 

Steve Shannon

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After burnout you have both gravity and drag slowing you down so you are decelerating at more then 1 G. On the way down gravity is pulling you down at 1 G BUT, drag is working against that so acceleration is less then 1 G. Seems to me then that you are going faster 2 seconds before appogee then 2 seconds after. My preference is slightly long over short.
1.,It takes time for the parachute to deploy. If you eject two seconds before apogee and if I takes one second to deploy, you’re within one second of apogee.
If you deploy two seconds after apogee and it takes a second for the chute to unfurl, you’re going 32 fps faster.
2. You’re correct that drag works against downward acceleration due to gravity after apogee and with downward acceleration before apogee, but because velocity is near its minimum near apogee it doesn’t have nearly as much effect as gravity.
 

ThirstyBarbarian

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Looks like lots of responses already, so I will just echo others. I aim for the closest to the calculated ideal, and when it’s about even, I opt for shorter rather than longer. And if I’m doing something odd, like a low altitude flight, or a long burn that could weathercock, I opt for shorter. I’ve had a few flights that deployed earlier than ideal, but I don’t remember ever having damage as a result. I’ve had more go a bit too long, and some of those suffered damage.
 

SDramstad

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1.,It takes time for the parachute to deploy. If you eject two seconds before apogee and if I takes one second to deploy, you’re within one second of apogee.
If you deploy two seconds after apogee and it takes a second for the chute to unfurl, you’re going 32 fps faster.
2. You’re correct that drag works against downward acceleration due to gravity after apogee and with downward acceleration before apogee, but because velocity is near its minimum near apogee it doesn’t have nearly as much effect as gravity.
I do agree with both of your points but you have to realize something. The chute will not open if the rocket isnt moving. From my skydiving days you learned real quick that a low speed "Hop and Pop" takes a lot more time to open. So while the forces are lower at apogee. It takes a lot longer to open. Some speed is your friend. :eek:
 
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