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  #1  
Old 03-25-2016, 12:02 PM
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blackshire blackshire is offline
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Default "Cardboard Cluster-schtupp"

Hello All,

The following isn't my project, but since it was "Bernardo Tech's" project (see: https://www.youtube.com/watch?v=HP3...RyQGy3vHvv hcQ ), I figured it would fit here. His all-cardboard (except for its peanut butter jar nose "cone"), 3 D12 motor-powered, rear-ejection parachute recovery rocket was an exercise in building a model rocket--except for the motors, of course--using only common household materials. He didn't name his rocket, but a name like "Cardboard Cluster-schtupp" seems apt for it. Also:

Fortunately--despite his having ignored most of the safety rules--no one was hurt, the dry field from which he launched it wasn't set ablaze (he sat the rocket directly on the ground among dry foliage, with sheets of wadding under it, after it fell off its launch pad when the launch lug came off...), and its onboard keychain camera actually survived the rocket's un-braked (the rear-ejected parachute and cluster motor mount ripped away), *literally* core-sampling impact. Now:

I'm not an "every jot and tittle" stickler where the Safety Code is concerned myself. (I know where the rules are over-cautious, and flying alone far away from others [and away from flammable foliage], as I have done here in Alaska, there is much less that can go wrong.) But I have never tempted fate as Bernardo did, because I do not aspire to leave this body before its "due date," nor do I wish to leave it in a maimed condition (minus any limbs or sensory organs...). BUT:

This doesn't mean, however, that a project like his *couldn't* be conducted safely. Had he even "string tested" his rocket with its motors installed to check its stability, launched it from a proper launch pad (he could have staked-down the legs of his Porta Pad, tightened its elevation adjustment properly, and used a more sturdily-attached launch lug), he could have flown his rocket--even with its parachute failure--with no worries about possibly harming anyone or anything in the event of a failure. His video is, nonetheless, a good educational one on how *not* to do model rocket R&D (Research and Development) work.
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  #2  
Old 03-30-2016, 11:07 PM
olDave olDave is offline
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Default Really?

Quote:
Originally Posted by blackshire
I'm not an "every jot and tittle" stickler where the Safety Code is concerned myself.


Not trying to pick a fight, but I stopped reading right there.

The NAR safety code is there to protect not just all the people in the vicinity, but YOU as well. It is not a good idea to ignore it. Please do not brag about such an attitude on an open forum where new rocketry hobbyists will see it.

I don't know of any part of the safety code that I would suggest ignoring.
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Old 03-31-2016, 01:07 AM
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blackshire blackshire is offline
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Quote:
Originally Posted by olDave
Not trying to pick a fight, but I stopped reading right there.

The NAR safety code is there to protect not just all the people in the vicinity, but YOU as well. It is not a good idea to ignore it. Please do not brag about such an attitude on an open forum where new rocketry hobbyists will see it.

I don't know of any part of the safety code that I would suggest ignoring.
I'm sorry to read this (and I did read your entire posting). Your reply demonstrates your self-imposed ignorance about what I was saying (and your comment about me bragging is entirely based on that, as I never said or implied anything of the kind); had you continued reading, your reaction would have been different (at least in degree, if not in kind). Now:

When one flies model rockets in the middle of nowhere, with no flammable foliage around, there are safety code rules that relate to the safety of spectators and objects that could be hit (but not to the safety of the launching person) that simply don't apply in such isolated locales. (If a person walks around naked outside, in a place where there is no one else to see him or her, does it matter that s/he is breaking the public nudity laws?)
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  #4  
Old 03-31-2016, 07:06 AM
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Quote:
For the implication P → Q, the converse is Q → P. For the categorical proposition All S is P, the converse is All P is S. In neither case does the converse necessarily follow from the original statement.
- Wikipedia

If you are following "every jot and tittle" of the NAR safety code, you probably are flying safely.

This does NOT imply that in order to fly safely, you must follow "EJaT" of the NAR safety code.

Experienced and knowledgeable rocketeers know how rocketry works, what causes lead to what effects: the whys and not just the whats of the safety code. They know that in some particular circumstances, certain restrictions which might be important in most launch situations become irrelevant. They are every bit as qualified to make a determination as to what is safe and what is not as the safety code's authors were. (The code did not, after all, get handed down from heaven on stone tablets, did it?)

Now it is true that most people tend to overestimate their own competence and knowledge; one may not be qualified to judge safety to the degree one thinks one is. Off-code flying should not be undertaken lightly. But please, let's acknowledge that the same kinds of experience and understanding that produced the code can allow some rocketeers to safely deviate from it.
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  #5  
Old 03-31-2016, 09:09 AM
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blackshire blackshire is offline
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Quote:
Originally Posted by Rich Holmes
If you are following "every jot and tittle" of the NAR safety code, you probably are flying safely.

This does NOT imply that in order to fly safely, you must follow "EJaT" of the NAR safety code.

Experienced and knowledgeable rocketeers know how rocketry works, what causes lead to what effects: the whys and not just the whats of the safety code. They know that in some particular circumstances, certain restrictions which might be important in most launch situations become irrelevant. They are every bit as qualified to make a determination as to what is safe and what is not as the safety code's authors were. (The code did not, after all, get handed down from heaven on stone tablets, did it?)

Now it is true that most people tend to overestimate their own competence and knowledge; one may not be qualified to judge safety to the degree one thinks one is. Off-code flying should not be undertaken lightly. But please, let's acknowledge that the same kinds of experience and understanding that produced the code can allow some rocketeers to safely deviate from it.
Thank you (including for bringing back memories of my first geometry course, in high school; the seven of us and Mr. Singleton [who actually *looked* like an ancient Greek mathematician, and I mean that as a compliment] did a lot of work with the P and Q logical "truth tables")! I've been deeply involved in model rocketry since 1969, and my occasional deviations from the letter of the safety code were slight and carefully considered. I also employed (without being aware of it, until later) a logical principle that was attributed (in a James Blish novel) to--appropriately--Mr. Spock: "A difference which makes no difference *is* no difference." If a small variance (to use a building code term) from a safety code rule was more convenient, but made no difference to actual safety (in an isolated area), I would use it. Also:

Even G. Harry Stine broke the safety code rules (which hadn't yet been formulated, so technically he didn't break them :-) ), as he wrote in his "Handbook of Model Rocketry." He fired model rockets in the desert to see if they flew according to Tartaglia's Laws of Gunnery, which include firing at a 45 degree elevation for maximum range (the safety code forbids launch elevations lower than 60 degrees above the horizontal [30 degrees down from the vertical])--they did. As well, even professional rocketeers break their own safety procedures on occasion. One example of this occurred at Michigan's Keweenaw Rocket Range (see: https://en.wikipedia.org/wiki/Keweenaw_Rocket_Range ), in 1971:

The launch crew were preparing a Nike-Apache (carrying a payload to investigate stratospheric warming that helped trigger the onset of spring, in a wave moving southward), the last vehicle of the campaign, for launch. After numerous delays, they reached a point where they had to either launch it or return it to Wallops. At the end of this time window, the winds were howling, and exceeded the published safety margins. The project managers at the Wallops Flight Center and the Goddard Space Flight Center told the rocket crew at Keweenaw not to launch in those conditions. But when the winds held steady at all altitudes, they persuaded the Wallops Range Director to permit the launch, even though the winds--and the launcher aiming settings--were well outside the accepted safe limits. The rocket flew successfully and landed on-target, but the Goddard managers were furious when they learned what the Keweenaw crew had done. Glen Swanson and Peter Alway covered this incident--and the history of the Keweenaw Range--here (see: http://www.gt.org/keweenaw-rocket-base/spcprtmi.html and http://www.gt.org/keweenaw-rocket-base/index.html ).
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http://www.lulu.com/product/cd/what...of-2%29/6122050
http://www.lulu.com/product/cd/what...of-2%29/6126511
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  #6  
Old 03-31-2016, 11:06 PM
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luke strawwalker luke strawwalker is offline
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Quote:
Originally Posted by Rich Holmes
If you are following "every jot and tittle" of the NAR safety code, you probably are flying safely.

This does NOT imply that in order to fly safely, you must follow "EJaT" of the NAR safety code.

Experienced and knowledgeable rocketeers know how rocketry works, what causes lead to what effects: the whys and not just the whats of the safety code. They know that in some particular circumstances, certain restrictions which might be important in most launch situations become irrelevant. They are every bit as qualified to make a determination as to what is safe and what is not as the safety code's authors were. (The code did not, after all, get handed down from heaven on stone tablets, did it?)

Now it is true that most people tend to overestimate their own competence and knowledge; one may not be qualified to judge safety to the degree one thinks one is. Off-code flying should not be undertaken lightly. But please, let's acknowledge that the same kinds of experience and understanding that produced the code can allow some rocketeers to safely deviate from it.


Absolutely! Couldn't agree more...

There are times when the demands of safety go beyond the safety code, and times when things that aren't completely compliant with every point are perfectly safe to fly at the time and conditions they're flown.

As a landowner, the club advisor and I have an understanding, that goes something like this... There are times when I will scrub a launch even though "officially" conditions are acceptable, if *I* as landowner think it's not safe... Sometimes the farm can get really dry when weather conditions are fickle and rain falls everywhere around us but here, leaving us in a dry pocket that makes conditions very hazardous for a fire... even though the surrounding area might have had just enough rain so there has been no burn ban put up. If I feel the LOCAL on-site conditions are too risky, I'll scrub the launch, regardless of the burn ban status...

At the same time, I don't have a problem with someone wanting to fly an experimental design or something that might not be absolutely PROVEN safe... they just have to present their case and "convince me" and set up a time when there won't be a risk to spectators...

One of the "jot n tittle" things of the safety code that's IMHO pretty stupid is the "current motor certification" rules... Even NAR lets you "officially" break this one IF you fill out the paperwork for the "old motor test program" BS... IMHO, "once certified always certified" should rule the day, unless a batch of motors is PROVEN statistically dangerous, in which case they should be officially recalled and an order NOT to use them should be issued along with the recall notification... Otherwise, it's just paper-pushing nonsense to "decertify" a perfectly safe motor simply because the manufacturer is out of business or no longer making that particular motor.

Later! OL J R
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  #7  
Old 03-31-2016, 11:28 PM
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Regarding the shock cord failure, when clustering three engines, is it a good practice to make two of them zero delay booster engines with no ejection charge? The combination of the three rocket ejection charges at the same time might have been a little too much for a typical shock cord.
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Old 04-01-2016, 01:16 AM
olDave olDave is offline
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Default In short: NO!!!

Quote:
Originally Posted by GlenP
Regarding the shock cord failure, when clustering three engines, is it a good practice to make two of them zero delay booster engines with no ejection charge? The combination of the three rocket ejection charges at the same time might have been a little too much for a typical shock cord.


A booster motor has no ejection charge, but at the end of the propellant burn (the thrust phase of motor operation) the propellant grain is designed to burst forward, to ignite the next stage. This would effectively produce the same thing as ejection charge gasses. Unfortunately this would occur waaaaaay too early in the flight, like 10 feet off the end of the launch rod.

A plugged motor might work. It would burn and provide thrust and then remain "dead" for the rest of the flight.

But for redundancy, you want all three ejection charges. If you only have one motor with an ejection charge and it happens to be the one that fails to ignite on the pad, your rocket will fly on the remaining motors and then will have no recovery system deployment.

You do not need to worry about overpressuring anything due to simultaneous ignition of ejection charges. Odds are that your multiple motors will not start at the same instant, or burn for exactly the same duration, or coast (while burning the delay charge) for exactly the same period of time. Odds are very very very small that your ejection charges would go off at the exact same time.

If it makes you feel better about it, select a second and third motor with a longer ejection delay and they can act as safety backups to the primary.

And the way to fix the shock cord problem is ..... use a longer shock cord.
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  #9  
Old 04-01-2016, 02:53 AM
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blackshire blackshire is offline
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Quote:
Originally Posted by GlenP
Regarding the shock cord failure, when clustering three engines, is it a good practice to make two of them zero delay booster engines with no ejection charge? The combination of the three rocket ejection charges at the same time might have been a little too much for a typical shock cord.
olDave is right. When flying a three-motor cluster model such as an Estes Cobra or Scrambler (or a "clone" of either one), even if you use a 12 volt car battery-powered ignition system (which ensures simultaneous ignition of all three motors [barring broken, shorted, or improperly installed igniters, of course]--at least as far as human senses can perceive), you will usually hear the motors' three ejection charges fire in quite rapid succession--"POP-POP-POP!--and not all together as one big POP! Model rocket motors have enough small variations between them (within tight limits, though) to have their propellant and delay charges burn for slightly different durations, which results in closely-spaced but discrete ejection charge firings, as olDave wrote. Also:

Even if, by chance (which is possible, but it would be a rare occurrence), all three motors' ejection charges fired simultaneously, cluster rockets' larger-diameter body tubes have a larger internal volume for the multiple ejection charges' gases to expand into, which makes it a "gentler" event. In fact, cluster models often have to employ a "stuffer tube" (a narrower tube centered inside the larger body tube by wide, flat centering rings, see pages 21 - 24 *here*: http://www2.estesrockets.com/pdf/28...ction_TR-TN.pdf ) to reduce the volume, so that the multiple ejection charges *can* pop the nose cone or payload section out of the front of the rocket (some cluster models have large volumes of "dead air" inside them, which can't be pressurized enough without stuffer tubes).
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  #10  
Old 04-01-2016, 09:59 AM
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That makes sense. The shock cord was probably too short and not strong enough, maybe the rear eject set up did not have enough space for a longer cord.
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