Deployment Velocity

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gary7

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I am sure I have read about this on this forum before but can not find the thread now.

Here are my questions: Assuming I have adequate shock cord length to take the shock of deployment, what is the safest, maximum velocity for chute deployment? (Shock cord length does make a difference, doesn't it? But, I don't want this to be a discussion about shock cord length, I would like it to be about air speed at ejection/deployment.) None of us like zippers of course.
 

MarkII

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It depends upon the parachute type and construction, method of deployment, whether or not the rocket has a "zipper-less" design, and probably several other factors too. But in general, 20-25 feet per second should be the max. 20 fps is a rather harsh jolt, but it is usually tolerable; 25 fps is really pushing it right to the edge. At 30 fps you're looking at either a canopy shred, a zipper or both. This is for LPRs and MPRs; the limits for high power may be much broader.
 

MarkII

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A moderately long (not excessive) shock cord can be helpful in allowing the system to decelerate to an extent after a motor-initiated deployment. If you use an altimeter for deployment at apogee, the timing will usually be close enough to ideal that a really long shock cord isn't necessary. That's assuming you haven't gone overboard with the black powder amount in your charge, though.
 

cjl

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In general, I would say that +/- 50fps is completely fine. That's about a 2 second window on either side of apogee. Higher than that is usually OK for many HPRs.
 

Handeman

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I think this is another question that is going to be answered with the famous phrase: "It depends".

It depends on a hundred different factors, size of the rocket, materials the rocket is made from, how the rocket is made, how the recovery system is made, the type of chute, the type of harness, size of the deployment charge, how the recovery system is packed, how the chute deploys, etc. etc. etc.

You already have two answers, one says 20 - 25 ft/sec and another that says 50 ft/sec. Both are probably right under specific circumstances. The problem is they don't clarify what those circumstance are.

I would like to give you a specific answer, but I can't. It depends!



Notice I didn't even mention the length of the recovery harness. :wink:
 
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ClayD

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First lets assume your talking about the drouge...
Apogee velocity, can be quite high.. in effect all you are aiming to do is break appart the aerodynamics of your rocket and control the decent with a drouge, or flutter down without a drouge.
apogee velocity, quickly becomes a terminal tumbling velocity- or max the drouge will allow.
you cannot control this velocity... it is whatever the rocket gods give you at that moment. Unless you have some sort of, guidance system or a perfect mathmatical quandry of a flight.


BUT, I assume your talking about main....

Zippers are not caused by the "falling" velocity, they are caused, when the airframe is askew the shock cord at any form of impact(snaping tight of the shock cord). this can happen at apogee, durning decent, and during main deployment.
The idea, *shock cord length* is to discharge as much ejection energy as it can while the chute needs to unfurl and tighten and "orient" the rocket prior to fully opening the cannopy. so the lenght, (not in discussion) is a balance between disapating energy and allowing the unfuling chute to properly orient the rocket before snaping open. D-bags do a good job of doing this from what i am told.
in essence, your after a gradual deployment to prevent zippers, a side effect is reducing the velocity between the time the charge goes off, and the time the canopy is at full drag force.

I hope that helps.
 

ClayD

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heres the same rocket, with and without drouge...
the way i pack my chute allows it to pull everything into orientation before the lines can be pulled open. so no worries for the 57fps under drougeless...
 

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MarkII

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Low power rockets, made from thin-wall paper tubing and other lightweight materials and usually using plastic parachute canopies and shroud lines attached with small stickers or tape, really can't handle high deployment velocities and strong opening shocks. Mid power rockets with sewn nylon parachutes, longer and usually stronger recovery harnesses and somewhat stronger structural materials may be able to withstand slightly higher velocities and stronger shocks, but I would still stick to the 20-25 fps limit for them whenever possible. To fly well on commonly-available E, F and G motors, MPRs still need to be built fairly light, and that factor lowers their DV threshold.

High power rockets are designed for recovery deployments under a broader range of conditions and deployment techniques and often use stout harnesses/shock cords and parachutes that are constructed very robustly. As I mentioned before, you usually have more latitude with them.

As both Handeman and I mentioned, there are many variables to consider here. But if you want a simple rule of thumb, a ballpark figure that is generally true of most designs in most situations, stay below 25 fps and definitely do not exceed 30 fps for low power and most mid power rockets. As I said, high power rockets have a wider variety of component materials, construction techniques and deployment methods, and consequently that category of rockets has a broader range of acceptable DVs. I have no doubt that a well-built HPR probably can handle a 50 fps deployment velocity, but that speed will easily strip the chute from a low power rocket and will likely produce a zippered airframe in a mid power rocket built with paper tubing, or a cut shock cord in a fiberglass one.

Let me repeat that these are broad guidelines, though. With LPRs and MPRs, it is important to nail the motor delay to within a few percentage points of ideal in order to insure a low-speed deployment and a relatively gentle parachute opening. With rockets that are designed to fly with motors in the A-F and most of the G motor range, you may not be able to use certain motors because their available range of delays and delay adjustments simply will not meet the needs of that particular rocket. For example, I really cannot launch an FSI Orbit on an AeroTech E11 reload, because the only available delay for it is 3 seconds, which will always be too brief for a safe recovery system deployment in this particular rocket. You can use a delay adjustment tool to shorten a delay, but you cannot use it to lengthen one. A flyer can get around this problem by replacing motor-initiated deployment with electronic deployment, but this may not be an option for some designs, especially rockets that are launched on A motors and some B motors.

Let me repeat one more time that these are general guidelines, valid for most rockets in most launch situations. There can be any number of special cases that are able to exceed these ranges, though.
 
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MarkII

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Also I will say that a slightly early deployment can be less problematic than a late deployment. In the first situation, the rocket is slowing down as it reaches apogee, so even if the deployment is a little early (emphasis on "little") the rocket is quickly losing speed anyway, so the shock will be less. In the second situation, on the other hand, the rocket is accelerating again as it plunges back to terra firma. Normally the parachute's job is to prevent the rocket from gaining this high velocity during the return back to the ground, but in the case of a late deployment, the parachute is also being asked to arrest the speed that the rocket has already gained. This is a much more stressful process.
 

blackjack2564

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My 6in rocket weighing in at 53lbs flies with a 24in drogue and the descent rate is 65 to 80ft depending on some things. [According to on board data] the Skyangle main had never had any issues dealing with this over the years.

I fly 4in rockets on M's drogueless with descent rates as high as 106ft/sec. popping Topflights for mains. No issues with these, BUT the chutes are packed very carefully and deploy gradually, not snapping open suddenly. That's the trick.

As stated before, there are numerous parameters to deal with. If you could be more specific with what you are trying to accomplish I might be able to help.
 

gary7

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BUT the chutes are packed very carefully and deploy gradually, not snapping open suddenly. That's the trick.

If you could be more specific with what you are trying to accomplish I might be able to help.
Thanks blackjack2564.

Mine was a question in general for single deploy rockets using motor ejection. For instance, Rocksim may state an optimal delay time for any given rocket on "x" motor. You also get the velocity at the time of ejection/deployment. Optimally, one would like a very slow speed for deployment of course. But some simulations give speeds in excess of 100 ft/second. Thus the question: what is the safest, maximum velocity (feet/second) for chute deployment? This is regarding mostly MPR and HPR rockets.

I suppose the next question has to be asked then: how do you pack that chute carefully so that is will gradually deploy once you are happy with the deployment velocity? I have reviewed more than I can imagine the different ways to pack a chute, trying to use what methods I think are best and that I am comfortable with. One more thing to look up on this forum, again.
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bobkrech

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I am sure I have read about this on this forum before but can not find the thread now.

Here are my questions: Assuming I have adequate shock cord length to take the shock of deployment, what is the safest, maximum velocity for chute deployment? (Shock cord length does make a difference, doesn't it? But, I don't want this to be a discussion about shock cord length, I would like it to be about air speed at ejection/deployment.) None of us like zippers of course.
The acceleration of gravity is 32 ft/s/s. If you are off apogee by 1 second, you rocket velocity is 32 fps; 2 seconds, 64 fps; 3 seconds, 96 fps.

Your rocket should be able to withstand at least a 100 fps deployment because that is within the allowable ejection charge timing variation of a motor of +/- 3 seconds.

Bob
 

MarkII

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Thanks blackjack2564.

Mine was a question in general for single deploy rockets using motor ejection. For instance, Rocksim may state an optimal delay time for any given rocket on "x" motor. You also get the velocity at the time of ejection/deployment. Optimally, one would like a very slow speed for deployment of course. But some simulations give speeds in excess of 100 ft/second. Thus the question: what is the safest, maximum velocity (feet/second) for chute deployment?...
In a perfectly vertical ascent in still air, at the instant it reaches apogee the rocket will be motionless, with a 0 fps velocity in every plane. RockSim appears to calculate the optimum delay by first calculating the time to apogee and then subtracting the time to motor burn out. To get a "perfect deployment" in RockSim (symbolized by a parachute with no arrows in it), the rocket appears to need to be moving no faster than 3-4 fps. This is a RockSim thing, though; no one, not even me, has maintained that the rocket must be moving no faster than 5 fps at recovery system deployment. I wouldn't be all that hung up on achieving a "perfect deployment" in RockSim.
 

gnunes

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This is an old thread, but it's a question that I need to answer. I am the (relatively inexperienced) teacher supervising a TARC team at my school. The team's rocket is about 275 grams (with egg, without motor). The motors that get the rocket to 800 feet all have either a 4 second or 7 second ejection delay. That means we have to choose to deploy the parachute either 1 second before apogee (upward velocity around 32 fps) or 2 seconds after (downward velocity around 64 fps). What is the recommendation?
 

UhClem

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What is the recommendation?
Since you have a target altitude, you want a delay time that causes ejection to happen after apogee. Otherwise the peak altitude will depend on the precise delay time of the particular motor used. That is a random variable over which you have zero control.
 

teepot

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I think 2 seconds after plus the time it takes the chute to open insures a zipper.
 

les

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One thing you can do is use a longer shock cord, and "Z" fold the cord. Zig-zag sections of the cord and secure with masking tape. Do this multiple times. Some of the energy then gets absorbed in breaking the tape.
 

G_T

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Just to put a number above what most seem to be providing, I routinely deployed main after dropping a moderately heavy L3 rocket (83# on the pad) down fast - to reduce drift - at around 105fps . I used a deployment bag, drogue intended for very high speed deployment, z-folded and tape for 1" nylon webbing, z-folded shrouds, etc. I timed it as 1 sec between release of main and fully open main. The deceleration was considerable - as are the forces. This is not the method to use without fairly robust recovery hardware! I replaced the 1" nylon every other flight, as it would show some breakdown after the second flight, and kept an eye on the shroud lines. It was very reliable. I'd deploy at about 700-750'AGL, and it would be at about 24fps by the time it got to 500'.

Gerald
 

shockie

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Could ALL the variables be defined and then put into a spreadsheet where the user enters the values for the variables? Or based upon some user input other variables could be read from say an internal table of values?
 

rcktnut

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This is an old thread, but it's a question that I need to answer. I am the (relatively inexperienced) teacher supervising a TARC team at my school. The team's rocket is about 275 grams (with egg, without motor). The motors that get the rocket to 800 feet all have either a 4 second or 7 second ejection delay. That means we have to choose to deploy the parachute either 1 second before apogee (upward velocity around 32 fps) or 2 seconds after (downward velocity around 64 fps). What is the recommendation?

With a rocket of that weight use a good quality elastic shock cord, properly protected from ejection charge, no worries, even at a much higher velocity.
 
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