Safe ft/s deployment speed

[Area66]

as the title said, what is safe deployment speed ?

 

[CarVac]

That's not enough information.

How tough is the rocket?

Is the parachute prone to open suddenly, or does it open gently?

Is it 'zipperless' configured?

How much does it weigh?

Are you using skinny shock cords or wide straps?

 

[GregGleason]

It does depend on the rocket, and how you define "safe". Does safe mean wrt to those on the ground, or wrt to the rocket, itself?

Also, if the chute "snaps" open, that puts a considerable load on the recovery train vs a slow opening chute. A slow-opening chute, at a higher speed, may have less damage on the same rocket than a quick-opening chute at lower speed.

In most cases, a deployment velocity of 6 fps and below will have no to minimal damage. Beyond that, it's a continuum of stress and damage, affected by many variables.

Greg

 

[Area66]

here the number, this is a spreadsheet from the rocket I will fly tomorrow with different delay posibilities , weight are with and without motor and it's onces

View attachment 129586

 

[Salvage-1]

going off your table, go for the delay that works out to give you the lowest speed. That means if the delay is 'off' slightly, there should only be a marginal change in deployment velocity.

 

[fyrwrxz]

as the title said, what is safe deployment speed ?

The *safest* speed is 0 fps.

(yes- there is one in every bunch..)

 

[timro]

Any simulation will provide you with an estimate of the vertical velocity. Although it's guaranteed to pass through 0 fps, it's highly unlikely you will ever have a case where your rocket will be truly motionless at deployment.

Getting the vertical velocity as close to zero as possible is the goal so that the true velocity is minimized. I think you're asking "how high" can I get away with and as others posted before, details of the scenario will impact the safety margin (both to the rocket and anybody on the ground).

This is something you can learn the hard way like I did... dual Egg lofting rocket flying on E15-4, which was way too short. From the flight trajectory I later estimated it was going at about 50-60 ft/s at deployment.

When it deployed, the shock cord / bulkhead mount ripped and the nosecone and payload section came down in a flat spin while the rest of the rocket recovered just fine on the parachute. The eggs were in a bag... and hardboiled. It certainly made peeling them easier

 

[bill_s]

I think it's said 25 fps for low power and 50 for high power (tough) rockets. 32 is one second off of exact apogee so it's hard to assure much less, and any horizontal travel increases it enough that 50 actual is probably common even when shooting for much less. I have found even with rear ejection more things can go wrong than just zippers.

 

[Area66]

I think it's said 25 fps for low power and 50 for high power (tough) rockets. 32 is one second off of exact apogee so it's hard to assure much less, and any horizontal travel increases it enough that 50 actual is probably common even when shooting for much less. I have found even with rear ejection more things can go wrong than just zippers.

this is the type of reply I'm looking for, thanks.

 

[bobkrech]

The lowest deployment velocity is V deployment = g x delta t where g = 32 ft per second per second multiplied by the time before or after apogee in seconds.

The maximum safe deploy velocity depends on the strength of the rocket and/or minimm altitude required to deploy the recovery system and slow the rocket down to less than 20 fps.

The recovery system of a rocket using motor ejection should be designed so that a safe deployment can occur if the ejection charge goes off 5 seconds before or after apogee because 1.) the maximum allowable error in a certified delay is +/- 3 seconds; and 2.) you could be off in your delay esitimate by 1/2 increment which could be 2 seconds, for a total of +/- 5 seconds.

If your deployment is off by 5 seconds, the minimum velocity is 5 x 32 = 160 fps assumming the rocket has no horizontal velocity. Deploying a standard chute at 160 fps can result in an instanteous deployment shock of 100 G. In a 1/4 pound model rocket that's 25 pounds, in a 3.3 pound maximum model rocket that's 330 pounds; in a 10 pound L1/L2 rocket that's 1000 pounds. None of these G-loads are hard to design for, however many rocket will not survive this level of loading unscathed. A deployment retarding system employing a slider to slow the opening of a parachute will greatly reduce the g-loading.

Heavier rockets should employ electronic apogee or dual deployment to reduce the shock loading and limit the deployment velocity and therefore the g-loading.

A properly designed electronic apogee deployment will occur within 2 seconds of apogee resulting in a low g-loading. In dual deployment systems, the typicial descent velocity is around 80 fps before the main is delployed which is equivalent to 2.5 seconds after apogee or a 25 G shock loading if no retarding system is employed. Employing a slider or reefing line can reduced the shock load by factors of 2 to 3.

Bob

 

[Area66]

Thanks Bob, very informative.

 

[NAR29996]

Heavier rockets should employ electronic apogee or dual deployment to reduce the shock loading and limit the deployment velocity and therefore the g-loading.



Bob

What is the fastest, then would you consider safe for a dual deploy rocket falling with a streamer (deployed at apogee)? I just ran a OR sim on a PML mini BBX, and the sim suggested that the rocket would fall at about 120 FPS before main deployment. That seems fast to me, but this is what this kit comes with, and I am presuming that many who have it fly it stock. I guess I'm looking for recommendations as to if I should use a small parachute for the drogue. BTW, this is the CPR 3K version.

 

[bobkrech]

What is the fastest, then would you consider safe for a dual deploy rocket falling with a streamer (deployed at apogee)? I just ran a OR sim on a PML mini BBX, and the sim suggested that the rocket would fall at about 120 FPS before main deployment. That seems fast to me, but this is what this kit comes with, and I am presuming that many who have it fly it stock. I guess I'm looking for recommendations as to if I should use a small parachute for the drogue. BTW, this is the CPR 3K version.

Look at the attached spreadsheet to see what the shock loads will be for a given deployment scenerio. If you input your rocket weigh, and the 2 chute sizes, you can determine the shock load of a normal deployment. Once you know the apogee event descent velocity as a function of time and distance, you can select a drogue/main chute combination that generates an acceptable deployment G-loading. If the main deployment G-loading is over 25 G, I would strongly suggest using a sliding ring opening retarder to stretch out the opening time to reduce the G-loading.

You have to be careful when simming apogee drogue or streamer deployments. AFAIK no sims include the cross-section of the rocket as a drag surface and may tend to overpredict the drogue descent velocity. You want to do 2 additional descent velocity calculations: the ballistic descent velocity, meaning the streamlined no deployment core sample velocity. This is the fastest descent rate you can have, and second, input to any chute calculator, the sideways cross-section converted to an equivalent circular chute diameter: A = pi x r^2 so D = 2 x r = 2 * sqrt(A/pi), which represents a drogueless apogee deployment. If the calculated drogue descent rate is greater then the rocket on the side descent rate, you will have to make sure the rocket descends in a vertical orientation so the drogue controls the descent rate.

Bob

View attachment parachute deployment loads.xls