Is overbuilding turning into a thing we should reconsider?

[jlabrasca]

"Science" is flying with "someone looking over your shoulder" the whole time . . . "Fun" is "running like Hell", after you lit the fuse !

Okay, one of us is doing science wrong.

 

[cbrarick]

https://web.archive.org/web/20141228112258/https://www.rocketmaterials.org/

https://web.archive.org/web/20030801000000*/https://www.rocketmaterials.org/

Dave F.

well played, sir; well played!

 

[Rex R]

science fun is saying 'that's odd' AS you're running like the wind!
Rex

 

[boatgeek]

Experience is one of those things that's a double (maybe triple) edged sword. On the one hand, I can look at certain things and say "No way in heck is THAT going to break!" and go ahead and use it. I don't really feel the need to do a stress analysis on a subsonic fiberglass rocket with 1/8" fins because I know it'll hold up. Is it heavier than it needs to be and take more impulse to get to the desired altitude? 100%. Do I know that it will arrive in Huntsville for SLI intact no matter what the FedEx shipping apes do? 98% sure on that.

This may sound a little strange coming from an engineer, but I put a lot more value on experience than calculations. That's both my experience (1/4" epoxy radius fillets work great on 54mm body tubes!) and others' experience. I'm a lot more likely to trust What Would Crazy Jim Do than a computer program if the two disagree, mostly because WWCJD allows me to take advantage of experience of success and failure. I think the main reason I trust experience more than calcs is that I have a very jaundiced opinion of how well I can calculate all of the loads a rocket will experience. That probably comes from my personal work experience (Exactly what wave slap pressure will a 95' boat experience in the Bering Sea? I dunno, but 3/8" plate has worked well in the past!). Anyway, that's why I really like the "Can I fly XX on a # motor to Mach Y?" threads. I almost always learn a lot from those.

Where that all falls down is exactly where the OPs started, when you think that 1/8" G10 fins are required for flights over Mach just because that's what people always seem to do. Then you see someone throw a J into a cardboard and plywood 54mm rocket and it goes to M1.6 and comes back just fine. If you push to lighter/thinner than the consensus, you have a higher risk of failure. As long as everyone from the flier to the RSO to the pad manager understands that and is willing to accept it, I think everyone is OK. Maybe that PSII flying on a K goes from the far pads in case it decides to rain parts. Of course, when you get to making things really heavy, you have more issues with recovery harnesses and the like.

I've said it before and I'll say it again; there are tons of ways and methods to enjoy this hobby. Heck, Low and slow lends itself to better recovery east of the mississippi in rocket-eating-tree country!

S'truth.

 

[DaveW6DPS]

Why? (in regards to 150% to 200% margin)

Because there are a number of variables in the real world, including things like tolerance for the specifications on motors, fiberglass tubing, epoxy, and everything else related to rockets. Add in the potential for years of temperature variations, cumulative effects of force and stress (and falling over in the shed and hitting solid objects). We expect reliable performance from commercial motors, but there are the occasional chuffs and puffs that provide more shock loading than the clean curves seen on thrust charts. Or I could have said that these are standard margins that I have used in a career in power plants (which is the case).

Is this static loading? What are you using as the limiting criterion?

Dynamic loading during boost. Rarely close to exactly on axis, and also rarely (never?) constant.
My limiting criterion is any deformation or damage that would prevent relaunching without repair.

How? (To ensure adequate strength.)

I start with a general idea based on experience and then validate with calculation. If I am building a kit I tend to view it critically and try to validate that it will be strong enough. I have used some materials enough that I don't go through much evaluation. As I mentioned, a four inch fiberglass rocket with three centering rings and three fins, bonded with epoxy, has more than enough strength to stand up to any 54mm commercial motor I have used. I do look at the thrust curve, and especially peak thrust, to ensure the motor is not expect to exceed my expectations.

The seat-of-the-pants thing is, I think, the question.

That is truly the main part of the question. How strong is too strong? That is subjective. Do I reinforce Estes kits with carbon fiber and fly on an A motor? No. Do I take a lot more care, and prefer more margin on a 25 pound rocket that I may launch on an L motor? Yes, definitely!

Which is worse too strong, or not strong enough?

I could not estimate how many flights I have seen where recovery systems failed for various reasons and to various degrees.
I have seem many rockets fly out of their fins.
I have seen quite a lot of rockets go unstable due to shifting CG because things were not anchored strongly.
I have seen unstable rockets due to the CG being to far back, or too far forward, of the CP.
I have seen a very few airframe failures under boost.
I have seen hazards to people and property because the flight path took a rocket, with and without recovery failure, over areas that rockets should not fly over.

I have yet to see a catastrophe due to "over-building".
Increasing the risk of structural failure to incrementally reduce the impact force makes no sense to me.

 

[DaveW6DPS]

...If you push to lighter/thinner than the consensus, you have a higher risk of failure. As long as everyone from the flier to the RSO to the pad manager understands that and is willing to accept it, I think everyone is OK. Maybe that PSII flying on a K goes from the far pads in case it decides to rain parts...

The central issue. Does everyone accept the increased risk of failure?

If I am the RSO and have concerns that a PSII rocket on a K may rain parts, it doesn't fly from any pad.

The idea that any poorly built rocket can fly if it is on the "far pads" is extremely non-conservative.

 

[jlabrasca]

I start with a general idea based on experience and then validate with calculation.

What do those calculations look like? How are you calculating yield strength under a dynamic load?

edit: again, because this is the internet, I don't mean that like "oh yeah? how do you do something that I think can't be done!" -- I am really asking. If there is a resource for this kind analysis I'd be grateful to know about it.

 

[Steve Shannon]

What do those calculations look like? How are you calculating yield strength under a dynamic load?

edit: again, because this is the internet, I don't mean that like "oh yeah? how do you do something that I think can't be done!" -- I am really asking. If there is a resource for this kind analysis I'd be grateful to know about it.

I can tell you what I have done.
I look at the maximum thrust that’s anticipate I’ll fly in a rocket. I double that and then I calculate the amount of shear stress the epoxy joints must withstand for that thrust. That’s static, as if the rocket is constrained from movement while the motor exerts its greatest thrust x2. I compare that to the shear stress properties of the epoxy. I’ve never seen an instance where my epoxy cannot withstand the shear. If I did I would either add centering rings or use thicker centering rings to increase the area and reduce the shear stress.
I’ve done similar calculations for plywood compression stress and bending stress using properties from a document available from a plywood building trade group. Designing using half inch plywood for the N2000 I’ve never even come close to exceeding the capabilities of the materials.
That’s what I do for the motor mount. My doubling of the thrust is simply to give me a little extra in case my preparations aren’t perfect.
For recovery I’ll use 50 gees of deceleration as my design target. That’s not unusual if it’s windy or if something goes slightly wrong. A person is foolish to design expecting perfection. I usually will use half inch wide tubular Kevlar or 3/4 inch wide tubular nylon because I want the width to help distribute forces along the mouth of the tube. I prefer TN because it stretches.
I don’t recall building any rockets with more than three centering rings. I’ve never used a thrust plate. I use a quarter inch radius fillet for my calculations.
I’m not sure if that answers your question.

 

[afadeev]

said:
Without proper lubrication, folks start treating this hobby as if it was a job, bringing science and material tests and strength analysis into a recreational activity.

Don't forget that a segment of the sport population do derive recreation and satisfaction from doing just what you (seemingly) deride.

You are absolutely right, and it appears my failed attempt at poking fun at ourselves was too cumbersome.

I do also derive an absurd amount of fun from over-planning and over-thinking with this hobby. That's part of the charm.
But I draw the line at making fun of, or ridiculing others' peculiarities.

Someone wants to optimize materials to bear just the right margin of simulated shear/compression/bending stress for a record attempt - good for them.
Someone wants to over-build so that their rocket can withstand been tossed around in the back of a pick-up truck, or just because they feel like it - why not!
Someone wants to fool around with all-threads in their L1 cert rocket (like that college kid) - have at it.

It's the creeping judgmentalism that I find unbecoming.

I've said it before and I'll say it again; there are tons of ways and methods to enjoy this hobby.

Indeed.
Grab a drink, chilax, build a rocket whichever way you feel like, as long as it's safe to fly.
Maybe even fly it. Or not.

Live and let live!

Over and out.
a

 

[cbrarick]

The central issue. Does everyone accept the increased risk of failure?

If I am the RSO and have concerns that a PSII rocket on a K may rain parts, it doesn't fly from any pad.

The idea that any poorly built rocket can fly if it is on the "far pads" is extremely non-conservative.

With all due respect, sir, I disagree.
While there are some rockets that probably should never fly- especially level 3 or high flyers - I've seen a few in my days - I'm all for something that isn't an overt danger flying from far away pads. My criteria would be that it couldn't reach the flight line UNDER ANY CONDITION. If that means I sentence you to lug a pad half mile away, so be it, if you want to fly it that's what you'll do.
There's always going to be something new and novel that people want to fly; who am I to say no as long as I provide Rocket Safety? My job isn't to determine if they built it to my overbuild standards.
Otherwise we're left with 3 fins & a nose cone.

I guess I'll kick the bear and say that college teams, who like to do difficult things just because, should do smaller scale proof of concept....

PS at the New York LDRS with disco I think I RSO'd about everything that went on the disco pads except for the snowmobile. The one question I asked of all these odd rocks was to prove they couldn't make the flight line. a K in a 20 pound project, with the motor firmly retained was more then safe even if it was in a grandfather clock or a space saucer...

 

[Steve Shannon]

With all due respect, sir, I disagree.
While there are some rockets that probably should never fly- especially level 3 or high flyers - I've seen a few in my days - I'm all for something that isn't an overt danger flying from far away pads. My criteria would be that it couldn't reach the flight line UNDER ANY CONDITION. If that means I sentence you to lug a pad half mile away, so be it, if you want to fly it that's what you'll do.
There's always going to be something new and novel that people want to fly; who am I to say no as long as I provide Rocket Safety? My job isn't to determine if they built it to my overbuild standards.
Otherwise we're left with 3 fins & a nose cone.

I guess I'll kick the bear and say that college teams, who like to do difficult things just because, should do smaller scale proof of concept....

PS at the New York LDRS with disco I think I RSO'd about everything that went on the disco pads except for the snowmobile. The one question I asked of all these odd rocks was to prove they couldn't make the flight line. a K in a 20 pound project, with the motor firmly retained was more then safe even if it was in a grandfather clock or a space saucer...

I agree. As long as a rocket looks like it will stay together, is stable, has a workable recovery system, and follows the safety codes, let people learn from the flight.

 

[Bat-mite]

As I wrote in a private conversation with jlabrasca, I think there is a line that we all fall somewhere on. Hopefully we all fall to the right of center, but how far tp the right we need to be is a matter of preference. Obviously the far left shouldn't be on the field. I have never felt the need to go to the extreme right.

|_________________________________________________________________________________________________________|
I don't know if this rocket is stable,.................................................................................I've engineered this thing to optimal perfection
durable, or able to fly on the motor.................................................................................and now I am ready for a job with SpaceX.
I've selected.

 

[boatgeek]

The central issue. Does everyone accept the increased risk of failure?

If I am the RSO and have concerns that a PSII rocket on a K may rain parts, it doesn't fly from any pad.

The idea that any poorly built rocket can fly if it is on the "far pads" is extremely non-conservative.

I see your point here, and I don't think I was very articulate. Let's say that the flier brings a PSII with a K loaded to the RSO. The flier can describe how they reinforced the model to take the loads, but the RSO has a nagging doubt that it may rain parts just because he/she hasn't seen this combo before or they don't know the flier well. Then it might go to the far pads. I agree with the "how do we make sure nothing reaches the flight line" approach as well.[edit] I guess I'm saying that I don't want an RSO to deny a flight solely because of a nagging doubt if they can find another way to mitigate the risk. [/edit]

What do those calculations look like? How are you calculating yield strength under a dynamic load?

edit: again, because this is the internet, I don't mean that like "oh yeah? how do you do something that I think can't be done!" -- I am really asking. If there is a resource for this kind analysis I'd be grateful to know about it.

One of the great moments in the loads calculation class in college was when we built a bridge pretty much exactly to calculated loads. It promptly fell to flinders because we didn't account for off-axis loads and overall stability of the structure. As I remember, every group had the same issue except the students who had come back to college after working at the state DOT for a couple of decades. Especially in an unpredictable loading regime like flight, it's really easy for un-accounted-for loads to creep in and ruin your day.

 

[jlabrasca]

I’m not sure if that answers your question.

Thanks, Steve. It kind of answers my question.

I revisited this thread

Centering Ring Thickness

Thinking about how I'd start a solution to a problem like: "The outer tube is pinned and constrained against translation. The inner tube is secured to the outer tube by filleted adhesive butt joints on the inner and outer circumferences of two annular plates, but is otherwise unpinned. The inner tube is filled with a rigid cylinder and will not bend. Given the dimensions and relevant physical properties of all of the materials and components, compute the maximum force that can be exerted on the inner tube, parallel to its long axis ..."



... I couldn't even figure out how to write the problem statement.

It gets worse when you start looking at the high strain rate behavior of polymers and composite materials under dynamic loading, where sometimes the yield strength increases with strain rate, and sometimes it decreases with strain rate.

Anticipating the complaints about overthinking or analysis-paralysis, I agree that this kind of analysis isn't necessary for the kinds of rockets most of us build, or ever expect to build. However, I think it is the kind of analysis you'd have to do if you were going to figure out how to build-in an N:1 margin (for example).

Which is to say, I am skeptical about numbers like "200% of the expected load..."

Again, I need to demur and disclaim: I am trained as a solid-state physicist, not a mechanical engineer or a materials scientist.

 

[Ez2cDave]

well played, sir; well played!

I suggest that anyone, who is interested, should go to the links I posted and "archive" the information they want from that excellent but, unfortunately, gone website !

https://web.archive.org/web/20141228112258/https://www.rocketmaterials.org/

https://web.archive.org/web/20030801000000*/https://www.rocketmaterials.org/

 

[DaveW6DPS]

With all due respect, sir, I disagree.
...I've seen a few in my days - I'm all for something that isn't an overt danger flying from far away pads. ...
There's always going to be something new and novel that people want to fly; who am I to say no as long as I provide Rocket Safety? ...

I agree. As long as a rocket looks like it will stay together, is stable, has a workable recovery system, and follows the safety codes, let people learn from the flight.

Maybe you should both read my comment again!

What I said, and what I maintain, is that if I think a rocket will "rain parts" or is poorly built, I will not sign off on the flight.

If the flyer can reasonably explain that the rocket is sturdy and stable, then I have no problem. "New and novel" is not the same as poorly built!

Using pad distance to compensate for a poorly built rocket is non-conservative. It is wrong, dangerous, and should not be allowed.

 

[Steve Shannon]

Maybe you should both read my comment again!

What I said, and what I maintain, is that if I think a rocket will "rain parts" or is poorly built, I will not sign off on the flight.

If the flyer can reasonably explain that the rocket is sturdy and stable, then I have no problem. "New and novel" is not the same as poorly built!

Using pad distance to compensate for a poorly built rocket is non-conservative. It is wrong, dangerous, and should not be allowed.

That’s fine and I support your authority as an RSO to make that call. And I appreciate your efforts to keep the range safe. I don’t think either of us were saying that everything should be allowed to fly, no matter how sketchy the construction.

 

[DaveW6DPS]

That’s fine and I support your authority as an RSO to make that call. And I appreciate your efforts to keep the range safe. I don’t think either of us were saying that everything should be allowed to fly, no matter how sketchy the construction.

And yet I got push-back over this comment:

The central issue. Does everyone accept the increased risk of failure?

If I am the RSO and have concerns that a PSII rocket on a K may rain parts, it doesn't fly from any pad.

The idea that any poorly built rocket can fly if it is on the "far pads" is extremely non-conservative.

 

[Steve Shannon]

And yet I got push-back over this comment:

Not so much push-back as discussing the nuanced details.

 

[jlabrasca]

The central issue. Does everyone accept the increased risk of failure?

If I am the RSO and have concerns that a PSII rocket on a K may rain parts, it doesn't fly from any pad.

The idea that any poorly built rocket can fly if it is on the "far pads" is extremely non-conservative.

On the grounds that we do not fly fireworks; that our rockets are intended to be recoverable and reusable I agree that the RSO ought to disallow anything that is likely to shred. I don't even think in needs to be framed as a safety issue, although safety is the overriding concern.

I don't know what to think about the folks who want to launch things that are not rockets on rocket motors. How do you RSO a snowmobile?

I think you are presenting false choice when you juxtapose a rocket built to be indestructible with a rocket built to shred. Your assertion that you'd rather rockets be too sturdy than too fragile doesn't really compare to equivalent categories of risk.

A rocket that "rains parts" is not exactly as dangerous as a rocket comes towards the flight line as an intact projectile. It is impossible to say that the shred won't ever be as great a danger as a falling javelin, but the latter is a danger that can be mitigated by sending the heavy and robust rockets to the pads farthest from people.

As for how to address the OP's question about the culture of overbuilding (for want of a better term); I got nothing.

edit: It occurs to me that I am basing my estimate of relative risk on a really small sample of HPR launches.

A question for the more experienced fliers here: Have you seen more HPR shreds, or more HPR lawn-darts?

 

[Steve Shannon]

Probably more lawn darts but it’s pretty close in my experience.

 

[boatgeek]

Lawn darts or other unsafe recovery by maybe a 2:1 margin over shreds. Motor mayhem (spat motors or CATO that results in fire out both ends of the rocket) somewhere in between.

 

[Ez2cDave]

Hopefully we all fall to the right of center, but how far to the right we need to be is a matter of preference. Obviously the far left shouldn't be on the field.

Truer words were never spoken . . .

 

[DaveW6DPS]

...
I don't know what to think about the folks who want to launch things that are not rockets on rocket motors. How do you RSO a snowmobile?

The same way you RSO three fins and a nose cone. Is it sturdy enough for the installed motor? Is it aerodynamically stable (and can the flyer demonstrate this)? Does have a thrust to weight ratio that will make it stable off the guide? Etc....

I think you are presenting false choice when you juxtapose a rocket built to be indestructible with a rocket built to shred. Your assertion that you'd rather rockets be too sturdy than too fragile doesn't really compare to equivalent categories of risk.
A rocket that "rains parts" is not exactly as dangerous as a rocket comes towards the flight line as an intact projectile. It is impossible to say that the shred won't ever be as great a danger as a falling javelin, but the latter is a danger that can be mitigated by sending the heavy and robust rockets to the pads farthest from people.

This is the misconception that worries me!
Did you not see the photos of a lightweight RTF rocket that punched through a trailer roof? Did you not understand the fatality that I mentioned was with a similar model rocket? The problem with thinking that light weight equals safer is that it simply is not true. An incremental difference in impact energy is not the issue. Is it safer to be run over by pick-up than a semi? Is it worse to be killed falling 30 feet than ten?

https://www.pe.com/2015/11/16/boy-scouts-inland-scouting-official-killed-in-rocketry-accident/

edit: It occurs to me that I am basing my estimate of relative risk on a really small sample of HPR launches.

A question for the more experienced fliers here: Have you seen more HPR shreds, or more HPR lawn-darts?

I did my level one cert in June of 1999. My level two in 2001. In about sixteen years I have performed RSO duties at many monthly club launches, two LDRS, and an NSL.

I would estimate that there are more ballistic crashes and other recovery failures than shreds. There are also a significant number of crashes due to instability, which can be just as hazardous as a ballistic crash.

The weight of the rocket compared to rockets flying on similar motors (the criterion used to determine the safe launch distance) is negligible in the risk. A five pound rocket coming in ballistic may hit with less energy than a seven pound rocket, but in reality the difference is moot.

As I have said, the key to safety is keeping the rocket away from people, especially at high energy.

That same five pound rocket descending under a good parachute is still a serious hazard if it comes down on someone.

 

[cbrarick]

Maybe you should both read my comment again!

What I said, and what I maintain, is that if I think a rocket will "rain parts" or is poorly built, I will not sign off on the flight.

If the flyer can reasonably explain that the rocket is sturdy and stable, then I have no problem. "New and novel" is not the same as poorly built!

Using pad distance to compensate for a poorly built rocket is non-conservative. It is wrong, dangerous, and should not be allowed.

Geeze Dave, I did qualify it as "something that isn't an overt danger" If you're bringing me something that would NEVER work or had a BS recovery plan - no! no!
a shread or wonky flight far, far away is OK. We all know you can't predict everything...but I don't think we need to epoxy everything...

 

[jlabrasca]

This is the misconception that worries me!
Did you not see the photos of a lightweight RTF rocket that punched through a trailer roof? Did you not understand the fatality that I mentioned was with a similar model rocket? The problem with thinking that light weight equals safer is that it simply is not true. An incremental difference in impact energy is not the issue. Is it safer to be run over by pick-up than a semi? Is it worse to be killed falling 30 feet than ten?

Yes. I saw it. I did some suspect calculations trying to estimate its speed at impact a couple of pages back in this thread. That is what I am describing as "an intact projectile".

Since this was a lightweight LPR, is it your contention then that everything has to go from the far pads?

As others have observed, the same rocket constructed of FG might not have been stopped at half body length. A rocket with a metal-tipped cone would have been a through-and-through for the trailer.

I think this might be the second lap for me. I am getting off here.

 

[DaveW6DPS]

...a shread or wonky flight far, far away is OK...

That is where we disagree. A foreseeable shred, or other mishap, is not okay. From any pad.

 

[DaveW6DPS]

...Since this was a lightweight LPR, is it your contention then that everything has to go from the far pads?...

I have been trying to make two points.

Moving an unsafe rocket to a farther pad does not make it safe.

ALL FLIGHTS should be away from people! It doesn't matter what a rocket is made of or how much it weighs--it should not fly toward people.

 

[Steve Shannon]

I have been trying to make two points.

Moving an unsafe rocket to a farther pad does not make it safe.

ALL FLIGHTS should be away from people! It doesn't matter what a rocket is made of or how much it weighs--it should not fly toward people.

I completely agree on both points. The point I was trying to make is that not every rocket can be correctly assessed. In that case moving a questionable rocket to an away pad is an acceptable means of risk mitigation.

 

[Ez2cDave]

That same five pound rocket descending under a good parachute is still a serious hazard if it comes down on someone.

That is precisely why "overbuilt" rockets can be dangerous . . . The additional mass, in excess of the necessary minimum, even if descending under a good parachute, is a serious hazard if it comes down on someone.

In fact, a smaller diameter, "overbuilt", rocket could theoretically be more dangerous than a larger diameter, "overbuilt", rocket of the same weight. In ballistics, that is referred to as "sectional density".