Overbuilding

[cjl]

If two rockets, differing only in mass but otherwise identical, are accelerated from rest by the same amount of impulse, which one has the greatest KE at the end of the motor burn?

Since impulse is proportional to change in momentum, both will have the same momentum at the end of the burn. This means that the heavier object will have less kinetic energy than the lighter one.

The point of my question is that you can't assume the one with the greater mass will be travelling as fast. That is, you can't assume the heavier one received the additional impulse to get it up to the same speed as the lighter rocket. A more logical assumption is that they both used the same motors. Hence, the heavier one will obviously not gain as much speed when propelled by an identical motor. So, given, two X-15's, one heavy and one light, both flown on C6 motors, it's not readily apparent which one will have the most KE. You need to find the burnout masses and velocities, then calculate the KE.

The heavier one will always have the lower kinetic energy, assuming negligible drag loss during burn.

 

[troj]

BTW, my point in regards to pointy balsa nosecones is to nothing but demonstrate that one cannot assume that Estes-type rockets are automatically "safe", due to the materials they're made of.

We need to get folks focused more on recovery system reliability, and safe rocketry recovery, across the entire range of rocketry, from Micromaxx on up.

-Kevin

 

[plano-doug]

Since impulse is proportional to change in momentum, both will have the same momentum at the end of the burn.

I agree with that. m1*v1=m2*v2=k neglecting drag.

This means that the heavier object will have less kinetic energy than the lighter one. The heavier one will always have the lower kinetic energy, assuming negligible drag loss during burn.

That may be intuitive for you, but not necessarily obvious for me However, crunching thru it a bit more, I get this:

KE1 = 0.5*m1*v1^2
KE2 = 0.5*m2*v2^2
= 0.5*(k/v2)*v2^2
= 0.5*k*v2
= 0.5*m1*v1*v2 < KE1 (v2 < v1)

But I'm still not convinced

Doug

.

 

[Ironnerd88]

Troj, I totally agree. I would hate to get hit by a ballistic Mean Machine. Recovery systems really get neglected. I have had very few issues with Estes-type plastic 'chutes. They just need to be checked once in a while, and folded and packed correctly every time.

I guess that's not on the Level III test...


CJL and Plano Doug

How does all that math work if the two rockets are in a ballistic recovery from about 700 feet? The issue of the article was recovery failures, after all (as in 3 out of five flights had recovery failures).

 

[plano-doug]

How does all that math work if the two rockets are in a ballistic recovery from about 700 feet? The issue of the article was recovery failures, after all (as in 3 out of five flights had recovery failures).

The energy discussion was regarding the ~horizontal flight of the X-15.

For the ballistic impact case, if we assume the rockets both reach terminal velocity on the way down, which is reasonable if they reach nominal altitude, then the heavier rocket will be moving faster since it has a higher ballistic coefficient (ie, more mass, same drag). The effect of impact comes down to the rate at which the rocket crumples. That is, assuming a given velocity, a rocket will have a momentum, m*v, which will impart a force, F, that is a function of the time, t, which it takes to decel from terminal v to 0. F=m*v/t. If it doesn't crumple, t=0, and F=&#8734;. Ouch. If it crumples, then F will be much, much less. IOW, it won't hurt so bad.

For the heavier rocket, F will be higher than for the lighter one, but, since the heavy one doesn't crumple, the added mass is insignificant compared to the fact that it doesn't crumple.

That said, I wouldn't want to get hit by either one. So having a durable shock cord and reliable deployment are the most important factors.

Doug...oversimplifying the math a bit...

.

 

[ScrapDaddy]

If two rockets, differing only in mass but otherwise identical, are accelerated from rest by the same amount of impulse, which one has the greatest KE at the end of the motor burn?

The point of my question is that you can't assume the one with the greater mass will be travelling as fast. That is, you can't assume the heavier one received the additional impulse to get it up to the same speed as the lighter rocket. A more logical assumption is that they both used the same motors. Hence, the heavier one will obviously not gain as much speed when propelled by an identical motor. So, given, two X-15's, one heavy and one light, both flown on C6 motors, it's not readily apparent which one will have the most KE. You need to find the burnout masses and velocities, then calculate the KE.

Without that, all that can be concluded with certainty is that the denser but otherwise identical rocket has a great ballistic coefficient.

Doug

.

Acctually I belive it is which one has the most PE when the rocket reaches apogee

 

[mjennings]

I agree with the majority the Bulletproof article was silly. While I don't have the experience level or flight count of many who have already posted. Out of a life time fleet of about 30+ rockets, I have only ever had 2 fin breaks. An Estes V2 that came in with a wadded chute, that my brother caught, and a gravity rider when it cleared the rod 6 feet off the ground and proceed to land on concrete. And all of it is wood or white glue. I have glass on one rocket, my Gemini DC where I had to cut in to the tube to add some nose weight and over the patches I put on due to tube burn through. I've used epoxy twice, on the above FG repair and on my USS Odyssey cause the directions said to and it was the best way to keep the long motor mount assembly from grabbing.

I've seen my share of failures at all levels from 1/2A up to K's all of them were scary. (I think all the L & M's I've seen were successful)

While I agree with those calling for a retraction, I don't think we'll see it. An article that directly counters the overbuild article may get more readership than an editors we shouldn't have printed that note.

I find the article weight break out poorly done, in that the altimeter should have been a separate weight, and his failure modes listed can have other root causes than frangible materials.

I don't think this is the end of rocketry as we know it. Since many LPR / MPR only builders, myself included can not justify the cost of armor plating their rockets. The guys who fly HPR and dabble in the low end can since to go to this kind of length is easy with the scraps from bigger projects.

I know NAR is try to include the HPR boys and girls but some times lately Sport Rocketry has felt like High Powered Rocketry.

 

[mkadams001]

Lets be clear: by the current "simplification" of our safety code Overbuilding and bulletproofing is not a direct violation of the code but is so far outside the spirit of the code it's beyond rediculous. But this IS a SAFETY Issue that can no longer be ignored.

First Fibreglass is NOT plastic it is spun GLASS. while laminating epoxy can be considered a plastic, the combination is NOT. Further fibreglass, carbon fiber and other reinforcing fabrics have been proven to be totally unnecessary over standard paper and cardboard construction materials.

The mass of the bullet has absolutely NO barring on this argument. a bullet is a bullet regardless of size. model rockets...regardless of mass SHOULD be self-destructing on impact to absorb as much of their kinetic energy as possible rather then expel it.

The Cover Headline reads:

"BulletProof your next Rocket"
What the heck do you think that translates to when read by the masses. It means this construction method is perfectly fine for all model building construction. IT IS NOT! This issue, it's cover headline and the article itself must be retracted.

G Harry Stine mentions in "40 years of Safety" report that the reason we've maintained such an outstanding safety record for all that time can be directly atributed to the fact we build our models to "Self-Destruct" on impact.

Bulletproofing is counter to the aims and methods of model Rocketry. As such the practice must be at least controlled if not eliminated.

We have hard evidence and flight tested proof that overbuilding, and the use of these products involved are totally unnecessary in the first place. encouraging such behavior will only contiune to erode the already dangerously shrinking safety margins maintained at our launches.

Since it's clear we can't really settle anything here I think this well be my last post on the matter. I think we've presented the facts and answered those who disagree. I'll close by asking everyone to be sure to send an e-mail to Tom Beach the Editor of Sport Rocketry requesting action to remove and retract this Hobby hurting article and Cover HEADLINE.
Thanks for helping protecting the hobby we've loved for more the 45 years.

So, now that it is agreed that there is no violation of the rocketry code in regard to overbuilding it now becomes the spirit of the code that is being violated; Good luck with that. I think that there is a reason for the simplicity of the code to allow the builders to design, build, and, fly rockets using a variety of materials and techniques.

Within this thread, for the first time, I have heard about testing the flimsiness of a rocket by launching it into a glass pane, that a rocket must self destruct upon impact to the ground or an object, that plastic must be thin, etc. I have never heard of any of these requirements to demonstrate the safety of a rocket. What I have read is for a rocket to be safe relies on the design and construction of a rocket to be stable, to match the motor with the rocket and flight conditions, to inspect your recovery systems, and to preflight your rocket for a safe flight. I do a preflight every time I fly, I check the fins, the recovery system, the nose cone fit, I build my motors without interruption, and make certain of the retaining system. The reason for doing this is that I take my rocket flights seriously, I'm flying heavier rockets than a 4 ounce rocket that can do damage if something fails so, I don't want anything to fail that I am able to control.

I don't think that through the use of a composite material to reenforce a Estes Guardian and increasing its weight only two ounces (yes it is double, but that is a flawed why of looking at it) justifies a retraction of an article or proclaiming that the rocket is now a lethal weapon. It is no more lethal weapon that any other 4 ounce rocket. What I am seeing is for those who think that using composites in the fabrication of a rocket is somehow offensive and violates a personal code of modeling ethics. And since there are no rules being broken then the only recourse is to wave the safety flag.

Somewhere along the way a self destructing rocket design has been created. How is this accomplished? Not one of my rockets has ever self destructed I guess that I missed this revolutionary safety design, was it Mission Impossible or Estes that brought this technology about?

Just because an article is in a magazine or has a headline on the cover does not mean that the editor or publisher endorsed the article.

Some of you may thing that my defense of the builder in the article would imply that I support the bullet proofing of rockets; I do not. However, there are some people who enjoy building heavy duty rockets. I don't want them to lose that right through, what I believe to be, some individuals need to police and enforce a personal modeling code.

It is unfortunate that your club is suffering from reduced safety standards at your launches.

 

[ScrapDaddy]

So, now that it is agreed that there is no violation of the rocketry code in regard to overbuilding it now becomes the spirit of the code that is being violated; Good luck with that. I think that there is a reason for the simplicity of the code to allow the builders to design, build, and, fly rockets using a variety of materials and techniques.

Within this thread, for the first time, I have heard about testing the flimsiness of a rocket by launching it into a glass pane, that a rocket must self destruct upon impact to the ground or an object, that plastic must be thin, etc. I have never heard of any of these requirements to demonstrate the safety of a rocket. What I have read is for a rocket to be safe relies on the design and construction of a rocket to be stable, to match the motor with the rocket and flight conditions, to inspect your recovery systems, and to preflight your rocket for a safe flight. I do a preflight every time I fly, I check the fins, the recovery system, the nose cone fit, I build my motors without interruption, and make certain of the retaining system. The reason for doing this is that I take my rocket flights seriously, I'm flying heavier rockets than a 4 ounce rocket that can do damage if something fails so, I don't want anything to fail that I am able to control.

I don't think that through the use of a composite material to reenforce a Estes Guardian and increasing its weight only two ounces (yes it is double, but that is a flawed why of looking at it) justifies a retraction of an article or proclaiming that the rocket is now a lethal weapon. It is no more lethal weapon that any other 4 ounce rocket. What I am seeing is for those who think that using composites in the fabrication of a rocket is somehow offensive and violates a personal code of modeling ethics. And since there are no rules being broken then the only recourse is to wave the safety flag.

Somewhere along the way a self destructing rocket design has been created. How is this accomplished? Not one of my rockets has ever self destructed I guess that I missed this revolutionary safety design, was it Mission Impossible or Estes that brought this technology about?

Just because an article is in a magazine or has a headline on the cover does not mean that the editor or publisher endorsed the article.

Some of you may thing that my defense of the builder in the article would imply that I support the bullet proofing of rockets; I do not. However, there are some people who enjoy building heavy duty rockets. I don't want them to lose that right through, what I believe to be, some individuals need to police and enforce a personal modeling code.

It is unfortunate that your club is suffering from reduced safety standards at your launches.

Well, estes did make the CATO....... Which can be found at https://www.rocketreviews.com/reviews/all/oop_est_cato.shtml

 

[cjl]

I agree with that. m1*v1=m2*v2=k neglecting drag.

That may be intuitive for you, but not necessarily obvious for me However, crunching thru it a bit more, I get this:

KE1 = 0.5*m1*v1^2
KE2 = 0.5*m2*v2^2
= 0.5*(k/v2)*v2^2
= 0.5*k*v2
= 0.5*m1*v1*v2 < KE1 (v2 < v1)

But I'm still not convinced

Doug

.

OK, how about this:

If k is larger than 1, then m2 is larger than m1, and Ke2 is smaller than Ke1. If k is less than 1, then m2 is less than m1, and Ke2 is larger than Ke1.

 

[cjl]

Troj, I totally agree. I would hate to get hit by a ballistic Mean Machine. Recovery systems really get neglected. I have had very few issues with Estes-type plastic 'chutes. They just need to be checked once in a while, and folded and packed correctly every time.

I guess that's not on the Level III test...


CJL and Plano Doug

How does all that math work if the two rockets are in a ballistic recovery from about 700 feet? The issue of the article was recovery failures, after all (as in 3 out of five flights had recovery failures).

In 700 feet, the majority of model rockets will reach terminal velocity. In that case, the heavier one will fall faster. If the rocket does not make it very high (only a couple hundred feet at most), they will probably not reach terminal velocity, but the heavier one will still almost definitely have more energy.

 

[powderburner]

I'll be honest, the scariest flight I've ever seen was a low power rocket on a D or E.

I'm not saying what you saw wasn't scary, and I certainly was not present to see it.

Nor was I present to see the high-power rocket that speared through the SUV windshield.

NONE of these should have happened. If these were at organized launches, the RSO should have caught most of the bad designs, or bad motor choices, or bad recovery system rigging, or many other problems.

It may also be time to re-evaluate the recommended launcher set-back distances; using launch control system leads that are twice as long (and which, BTW, gives you a better overall view of the flight) would be a start. Maybe someone needs to look at this stuff again?

We ALL need to be more aware of safety.

And the overbuilding "craze" needs to be discouraged. Reinforcing is absolutely fine where it is needed, but we have a world of rocketeers out there who don't have a clue WHEN it is needed. They add fiberglass automatically because they think their friends will laugh if they don't. This spells out a fairly obvious solution such as we need more structural analysis. We also need to know that a rocket that has been beefed up to fly on a hard K should not be allowed at any organized meet when powered by an H, I, or maybe even a soft J--with reduced power levels, that reinforced airframe is simply not needed, and is nothing but risk and danger. Save it for Blackrock, when you have another K.

I get the impression that there is some degree of "showing off" involved here. Be honest: can you hear a faint echo of "Hey, look at this!" behind some of this overbuilding? When was the last time that you saw anyone do some calculation of airloads, or tube structural buckling, or fin root shear and bending loads? When was the last time you saw a strain gauge on a rocket? When was the last time you saw someone build a set of ground-test specimens and pull-test them to destruction?

They call it rocket SCIENCE for a reason. We should stop treating it like rocket gambling.

 

[cjl]

I'm not saying what you saw wasn't scary, and I certainly was not present to see it.

Nor was I present to see the high-power rocket that speared through the SUV windshield.

NONE of these should have happened. If these were at organized launches, the RSO should have caught most of the bad designs, or bad motor choices, or bad recovery system rigging, or many other problems.

While it's true that the RSO should catch most things, there are some failures that neither the RSO nor the flier could have reasonably been expected to catch, and occasionally, stuff simply doesn't work. It is not reasonable to expect to catch everything (although I agree that I have seen flights that should not have been allowed).

It may also be time to re-evaluate the recommended launcher set-back distances; using launch control system leads that are twice as long (and which, BTW, gives you a better overall view of the flight) would be a start. Maybe someone needs to look at this stuff again?

To be honest, I'm not sure what would be gained by increasing setback distances. The impact zones for a ballistic flight are large enough that a setback launcher would not help (unless it was set back much more than 2x), and the standoff distances are already more than large enough to prevent any problems with shrapnel from a cato. I am always in favor of safety, but I don't think increased standoff distances are needed.

We ALL need to be more aware of safety.

And the overbuilding "craze" needs to be discouraged. Reinforcing is absolutely fine where it is needed, but we have a world of rocketeers out there who don't have a clue WHEN it is needed. They add fiberglass automatically because they think their friends will laugh if they don't. This spells out a fairly obvious solution such as we need more structural analysis. We also need to know that a rocket that has been beefed up to fly on a hard K should not be allowed at any organized meet when powered by an H, I, or maybe even a soft J--with reduced power levels, that reinforced airframe is simply not needed, and is nothing but risk and danger. Save it for Blackrock, when you have another K.

So you're saying that I should not be able to fly my level 3 rocket (currently under construction) on a J or K motor, despite the fact that they could safely lift it, just because I am building it with the strength required for an N motor? That's ridiculous. Just because I built a rocket to handle an N motor doesn't mean that I only ever intend on flying it on Ns.


If I build a rocket that is reinforced to take a large K motor (say, a K1275R), and it ends up weighing 5 pounds, there's absolutely no reason why I shouldn't be able to fly it on an large H or I motor if I want. It is capable of lifting off safely and stably on an H or I motor, and therefore, it should be allowed. I agree that overbuilding should be discouraged, but be reasonable.

I get the impression that there is some degree of "showing off" involved here. Be honest: can you hear a faint echo of "Hey, look at this!" behind some of this overbuilding? When was the last time that you saw anyone do some calculation of airloads, or tube structural buckling, or fin root shear and bending loads? When was the last time you saw a strain gauge on a rocket? When was the last time you saw someone build a set of ground-test specimens and pull-test them to destruction?

They call it rocket SCIENCE for a reason. We should stop treating it like rocket gambling.

Well, I haven't put a strain gauge on my rockets, but I can post some solidworks analysis (both finite element and computational fluid dynamics) and material test data that I have done for my level 3 if you want.

 

[powderburner]

I was retained as an expert witness...

As was I. (Don, I think you pointed them at me? Thanks a bunch?) They wanted an aerospace engineer with rocketry experience. As a bonus, I was also local.

Eyewitness testimony varied somewhat, but included statements about the qualifications of the crew conducting launch operations, that the same X-15 had previously flown safely and normally, that winds were not bad/moderate/gusty, that on its last launch the X-15 turned at low altitude to follow a more-or-less horizontal trajectory, distance from the launcher to the accident victim, and other aspects of the accident.

I put together an EXCEL spreadsheet simulation of the Estes X-15 launch. I used motor data from the NAR website, and calibrated a "normal" flight profile to the altitudes claimed in the manufacturer's data.

I then altered the spreadsheet so that the rocket made a 90 degree turn at the top of the launch rod and moved horizontally. I accounted for the portion of motor impulse which had been expended to that point, and assumed that the 90 degree turn was accomplished without additional impulse expenditure (i.e., the rocket did NOT momentarily "flip upside down" and thrust upward to kill the vertical motion before beginning its lateral path, but simply turned due to unknown reasons and delivered full remaining impulse to accelerate horizontally). My simulation showed that, at the cross-range distance stated for the location of the accident victim, the X-15 (and C motor) had attained sufficient velocity to indeed be quite dangerous.

The model rocket (at the moment of impact) had levels of kinetic energy directly comparable to a hunting arrow (which frequently strike deer-sized targets and penetrate completely) or a .380-caliber bullet. I am not a medical specialist but this rocket seemed quite capable of causing the type and extent of physical injury reported.

The lawyers defending the church scout organization and Estes have given permission for me to report what I learned, and I am in the process of getting something written up (one of many current projects). When completed, I will offer this info to the NAR safety people and to any website that wishes to post it. I think there are some important safety lessons to learn.

And back to this discussion, I would like to thank everyone involved for being able to remain (relatively) calm about the subject of overbuilding. This is another important subject that needs discussion. We all need to think more about how we are building and what we are doing.

 

[cjl]

The model rocket (at the moment of impact) had levels of kinetic energy directly comparable to a hunting arrow (which frequently strike deer-sized targets and penetrate completely) or a .380-caliber bullet. I am not a medical specialist but this rocket seemed quite capable of causing the type and extent of physical injury reported.

Kinetic energy isn't the only consideration however. Momentum, bluntness of the nose, and rocket strength all matter too.

I'm not trying to downplay the injuries or anything. However, you need to look at more than kinetic energy alone when considering the potential danger from a rocket.

 

[powderburner]

RSOs do not (can not) catch everything. But they should probably be catching and questioning much more than many currently do. And yes, I understand that when an individual launches, there is no RSO at all...

I should have expanded more on "setback." I agree that more is not automatically better, but the NAR published some good material on proper launch field layout with respect to wind direction, and proper location of observer and parking areas. We need to pay more attention to these recommendations. Setback alone will only gain you so much, in terms of improved safety.

And yes I am standing by the statement that, for given weather conditions, field size, crowd size, etc, it is improper to launch an excessively built rocket. The extra airframe is nothing but risk. Whether you can proceed to launch is a decision that your launch director and RSO should make. And I believe that excessive size/weight/hardness is an entirely reasonable criteria, especially in a large crowd where we all know darned well that half the people are not watching.

I would like to see any and all structural analysis posted here on TRF. Be aware, however, that due to variables like dirty workshops (and contaminated bonding surfaces), material prep and handling variations (and consequent variations in bond strength, tensile strength, etc), variations in substrate materials over which reinforcing wraps are applied, variations in laminate orientation and in laminate edge overlap/gapping, variations in vacuum bagging and final resin content, and many other un-quantified "shop" factors, there is a wide range of resulting material performance and airframe strength. Even if you can consistently build good structures, there is no assurance that some other guy looking at your build pix will understand the subtleties and the consequences of not getting everything right. I maintain that without real-world test samples, and without FAR more descriptive and instructional documentation than are currently shown, there is a dangerously wide band of potential construction variability.

At an absolute minimum, there should be some criteria for building a standard-sized component with standard materials and using standard test force/load levels to see if Joe-Bob can even achieve a minimum level of proper and safe construction. (See my previous comments about rocket SCIENCE.)

 

[Pat_B]

When was the last time you saw someone build a set of ground-test specimens and pull-test them to destruction?

Ironically, I do this sort of thing all the time in order to see how LIGHT I can make a rocket. It isn't until you've had a chance to go superlight that you can really appreciate how strong some of these materials are.

Here's a nose cone strength testing jig that I use for my styrene nose cones. I have something similar with a digital force gauge to test the flexibility of my fiberglass body tubes so that I know how light I can go.

 

[powderburner]

Kinetic energy isn't the only consideration however. Momentum, bluntness of the nose, and rocket strength all matter too.

I'm not trying to downplay the injuries or anything. However, you need to look at more than kinetic energy alone when considering the potential danger from a rocket.

If you have studied wound ballistics (and reported/claimed bullet terminal ballistic performance) or attempted to make your own corellations of any of this data, you will know that what you say is quite true. Weird stuff happens when high-speed objects strike flesh and bone.

And of the ballistic objects which I compared (a hunting arrow, a pointed plastic rocket with a metal nose ballast insert, and a .380 bullet), the X-15 is among the most pointed and penetration-capable shapes. And while pure kinetic energy isn't everything, it is nonetheless a major factor in the equation.

 

[powderburner]

Ironically, I do this sort of thing all the time in order to see how LIGHT I can make a rocket. It isn't until you've had a chance to go superlight that you can really appreciate how strong some of these materials are.

Here's a nose cone strength testing jig that I use for my styrene nose cones. I have something similar with a digital force gauge to test the flexibility of my fiberglass body tubes so that I know how light I can go.

YOU are the kind of guy we need writing articles for the magazine!

 

[SecretSquirrel]

(Don, I think you pointed them at me? Thanks a bunch?)

I'm guessing that when they asked you to participate it was much like my experience. That is you were free to decline if you wished. I'm pretty sure Luca Brasi wasn't holding a gun to your head making an offer you couldn't refuse. I know I never feared waking up with a horse (or squirrel) head in my bed.

 

[cjl]

RSOs do not (can not) catch everything. But they should probably be catching and questioning much more than many currently do. And yes, I understand that when an individual launches, there is no RSO at all...

I should have expanded more on "setback." I agree that more is not automatically better, but the NAR published some good material on proper launch field layout with respect to wind direction, and proper location of observer and parking areas. We need to pay more attention to these recommendations. Setback alone will only gain you so much, in terms of improved safety.

Perhaps the most significant factor that is not usually accounted for is wind direction. If more attention were paid to wind direction, it could prevent many of the incidents.

And yes I am standing by the statement that, for given weather conditions, field size, crowd size, etc, it is improper to launch an excessively built rocket. The extra airframe is nothing but risk. Whether you can proceed to launch is a decision that your launch director and RSO should make. And I believe that excessive size/weight/hardness is an entirely reasonable criteria, especially in a large crowd where we all know darned well that half the people are not watching.

That's completely ridiculous. I would be infuriated if an RSO denied the launch of a completely capable rocket on the basis that it was "too strong", especially if that strength was not overkill, but rather because it was built to take larger motors. Calling a heads up flight would be fine, but there is no reason to prevent the flight of a perfectly safe rocket just because it's not being flown on a big enough motor (and I'm not referring to the motor's ability to physically lift the rocket).

Basically, if I'm understanding you correctly, you would not approve the flight of my L3 rocket on a K motor, but if I came up to you with the exact same rocket on an N, you would be fine with it. That's simply not reasonable (unless the K were insufficient thrust for the rocket's weight). If a rocket is considered safe on an M or N motor, then there's no reason to consider it less safe on a sufficiently high thrust K.

I would like to see any and all structural analysis posted here on TRF. Be aware, however, that due to variables like dirty workshops (and contaminated bonding surfaces), material prep and handling variations (and consequent variations in bond strength, tensile strength, etc), variations in substrate materials over which reinforcing wraps are applied, variations in laminate orientation and in laminate edge overlap/gapping, variations in vacuum bagging and final resin content, and many other un-quantified "shop" factors, there is a wide range of resulting material performance and airframe strength. Even if you can consistently build good structures, there is no assurance that some other guy looking at your build pix will understand the subtleties and the consequences of not getting everything right. I maintain that without real-world test samples, and without FAR more descriptive and instructional documentation than are currently shown, there is a dangerously wide band of potential construction variability.

I absolutely agree, which is why I'm having to put in some fairly large margins in some areas.

 

[Ironnerd88]

mkadams001---

The big issue I have is not that this rocket is "bulletproofed". I actually found it rather amusing that the guy would spend so much time and money "battle-hardening" a $10 rocket. Rather I have an issue with a lot of the other things that happen in the article.

I can see adding the plastic screws to the payload section, and beefing up the shock-cord mount. Although totally overkill in this example, it's not a bad idea at all. But then he DOUBLES the weight. Sure it's only a 104.5 gram rocket, there are lots of those flying safely every day, but not on an Estes C6 - with a max liftoff weight of 113 grams (please note that the 104.5g weight given in the article does not include the 25g motor which must be figured into the total lifted weight). A level 3 member should know to use a motor that is up to the task at hand.

He has three recovery failures in five flights with a bone-simple little sport kit. What rate does he have with his big complex rockets? Of particular interest to me was the fact that both his Bullet-proof and normal models had recovery failures, both fell to the gound at around 20 mph, and both survived just fine. Why bother with the whole bulleproofing concept -- apparently all it does is turn "okay" rockets into "garbage birds". Any twit can build a really heavy rocket with poor performance. There is nothing to be proud of here. Especially since the modified rocket had zero successful flights, while the maligned non-modded rocket managed 2 out of three.

Near the end of the article, we learn that the non-modified rocket had a fin-glue-joint failure after an otherwise perfect flight. If the fin didn't break and the paper tube didn't rip, he didn't glue it correctly. A level 3 rocketeer should know how to glue a fin to a tube.

We also find that he gets about three flights out of a Guardian before it needs repair? Really? With the author's recovery success rate I'm impressed he gets two flights. Keep in mind that the fist launch the bulletproofed rocket nosed into the ground before the ejection charge went off. This is due to high winds and weather-cocking. It sounds like he may have tilted the rod INTO the wind rather than with it - something else a level 3 guy should know about.

As for the self destructing rockets - Look about half way down page 7 of the "Handbook of Model Rocketry" (seventh edition). I read this book back in seventh grade (second edition) and every year since, and have found it to be an invaluable resource.

My complaint to the editor was that NAR membership and readers of Sport Rocketry would have been better served by an article about Rocket Repair, or Avoiding Common Recovery Failures. I don't think a retraction is needed because it has shown us a lot of what not to do, and it has certanly sparked a lot of conversations (always good). I just think our magazine could (and should) do better.

 

[ScrapDaddy]

Where did you get 113 grams?

 

[mkadams001]

Ironnerd88

I agree with you; it is overkill and this L3 flyer has a problem with recovery. I guess he has never used making tape to friction fit a nose cone to a payload bay. He was using the wrong motors for the conditions.

What I don't want to see is an excessive use of the safety card to stop building methods that some disagree with. In the same handbook you referenced, look towards the bottom of page 6 and note the reference towards composites. Apparently the guru of rocketry recognizes alternate building materials.

As far as Sport Rocketry, they should print a wide range of articles including repair. We all will benefit from diverse range of topics.

 

[Mike Di Venti]

So after all this discussion, what does an RSO say/do when this person brings up 104gr estes guardian? He then states it's bullet-proof and goes into the details of how he built it.
Do you turn him away and say that's too dangerous to fly here?
At a larger field do you send him out to the 50ft pads with an estes D motor in it?
What would be the "right" thing to do?

Some back ground:
Around 2003-2005 I was L2 certified and had been having recovery issues.
I did start to make stronger rockets (all hawk mtn) and still had recovery issues. I realized that wasn't the problem. I was having a recovery problem. Actually I was having a 3yr old son problem that wanted to fly all the rockets RIGHT NOW and ALL of them RIGHT NOW.
In my haste to please my young son I had been neglecting the proper way to pack the chute, hook the chute up, set the altimiter, forget to double check, ect.
I mean it was so bad I couldn't get saucer to recover correctly! I took a couple months off from flying and while I was fixing all my rockets I had my epiphany.

Since then I take my time prepping my rockets. I prep them at home at night after my loving helpful son goes to bed. I've had a few other failures and I can contribute that to being distracted by some of my helpful friends, but it's a lot better now. When I get to the field I'll double check my more complex flights and I don't try to fly 30 rockets in a weekend anymore.

Sorry for the long post.

 

[MattieShoes]

In 700 feet, the majority of model rockets will reach terminal velocity. In that case, the heavier one will fall faster.

Wait, what? Gravity would be the same, drag would be the same -- they should have the SAME terminal velocity. So a heavier rocket does NOT fall faster, though it has more force behind it thanks to the added mass...

Right?

 

[cjl]

Wait, what? Gravity would be the same, drag would be the same -- they should have the SAME terminal velocity. So a heavier rocket does NOT fall faster, though it has more force behind it thanks to the added mass...

Right?

Nope. Drag coefficient is the same, but at terminal velocity, the drag force is equal to the weight. The heavier one weighs more, so it requires more drag to balance the weight. With the same drag coefficient and frontal area, this means it will fall faster.

 

[ScrapDaddy]

Nope. Drag coefficient is the same, but at terminal velocity, the drag force is equal to the weight. The heavier one weighs more, so it requires more drag to balance the weight. With the same drag coefficient and frontal area, this means it will fall faster.

There is one thing however, you can get past terminal velocity, just nit in one piece, there are circumstances in which this could happen, a downdraft perhaps but we all know one you somehow exceed teminal velocity, you can kiss your rocket bye-bye......

 

[cjl]

There is one thing however, you can get past terminal velocity, just nit in one piece, there are circumstances in which this could happen, a downdraft perhaps but we all know one you somehow exceed teminal velocity, you can kiss your rocket bye-bye......

Terminal velocity is defined as relative to the surrounding air, so a downdraft won't get you past it. If your motor was still firing as your rocket was aimed downwards, you could impact well above terminal velocity though (I've seen this happen on 2-stagers where a motor with a long delay was used in the bottom stage).

 

[ScrapDaddy]

Terminal velocity is defined as relative to the surrounding air, so a downdraft won't get you past it. If your motor was still firing as your rocket was aimed downwards, you could impact well above terminal velocity though (I've seen this happen on 2-stagers where a motor with a long delay was used in the bottom stage).

Doesn't an object desintagrate when it passes termanal velocity?