Overbuilding

[smurf]

Originally posted by stymye
Smurf,I am totally with you on the durability issue

but I'm not on the idea that we all should learn to fiberglass and eventually "advance" to exotic carbon fiber artistry.like thats the ultimate goal

mabey some people consider that their goal ..but it's not for me

I feel it's harder and more important to learn how to build a light yet strong structure with accurate fit between joints and planning.
thats engineering !

building a cf rocket may look cool but it's not really that hard, you just need the right tools, glassing is easy,again just a matter of the right tools and a bit of know- how.

A person could build a rocket with no glue at all.. now >that< would be far more impressive to me personally. thinking outside the box... bottom line- it's not all about materials

Stymie

I didn't mean that the fiberglassing and carbon is the goal, what I was trying to say is that it's one more tool in the shed for when you need to solve a problem. If you have the skill then when it comes time to design you'll be able to apply another solution to the problem and maybe come up with something better.

I agree with John on the not going whole hog and building 40# monsters when it isn't necessary. It really is about coming up with an eloquent solution to achieve your design goal whether they are duability, speed, altitude or beauty.

 

[denverdoc]

Originally posted by smurf
Stymie

I didn't mean that the fiberglassing and carbon is the goal, what I was trying to say is that it's one more tool in the shed for when you need to solve a problem. If you have the skill then when it comes time to design you'll be able to apply another solution to the problem and maybe come up with something better.

I agree with John on the not going whole hog and building 40# monsters when it isn't necessary. It really is about coming up with an eloquent solution to achieve your design goal whether they are duability, speed, altitude or beauty.

I wish I could credit the suggestion to whoever gave it to me, but along these lines, I try to add one and no more than one new technique per build. This way you are acquiring a good tool set w/o making a total mess of things and/or risking failure--well at least the risk is manageable as you have a proven skill set underneath... Sorry, babbling here
John

 

[Bigander]

Originally posted by denverdoc
Then enroll in some community college courses, its not our problem you dont get it.
JS

Srry it was late at night and i could see he was making a good point and i wanted to quickly post and sign off.

 

[maricopasem]

Originally posted by denverdoc
I try to add one and no more than one new technique per build.

This is what has worked for me. I only add one new "experimental" aspect to a scratch rocket I build so I know what causes failure the rare times that happens. If I put in too many new things I just don't know which one was the disaster.

 

[denverdoc]

Originally posted by Bigander
Srry it was late at night and i could see he was making a good point and i wanted to quickly post and sign off.

Thats cool, I thought it was a thoughtful post by smurf and you were dissing it,
JS

 

[new2hpr]

Originally posted by maricopasem
This is what has worked for me. I only add one new "experimental" aspect to a scratch rocket I build so I know what causes failure the rare times that happens. If I put in too many new things I just don't know which one was the disaster.

Like another quote I like (don't know who to attribute it to):

If it stops working, remove the last improvement.

-Ken

 

[denverdoc]

Originally posted by new2hpr
Like another quote I like (don't know who to attribute it to):

If it stops working, remove the last improvement.

-Ken

Thats good.

 

[SRP Crew]

Here's a question. When do you know when you are actually overbuilding?

 

[denverdoc]

When carrying it causes either a hernia or heart attack, and its 75mm.
J

 

[Stymye]

....when it could be used for a bunker buster

 

[maxpower]

Originally posted by SRP Crew
Here's a question. When do you know when you are actually overbuilding?

Good question. Perhaps you will only know if you under built it

 

[denverdoc]

or when you stick it in the trunk, and your front wheels no longer make contact

or the biggest motor in that diameter can't trip the altimeter into sense apo mode

or when guys want to use it as a static test stand

 

[denverdoc]

Originally posted by MaxPower
Good question. Perhaps you will only know if under built it

In all seriousness, it is a good question, and the underbuilding flip side much more obvious. I think Smurf made some good points re durability and learning.

On a local club forum same issue came up, one of the senior members in the club thought a lot of what we do these days borders on the ridiculous, not from a weigh point of view, but that rockets didn't need CF, chopped fiber fillets, etc. So one way to overbuild is not so much thru weight as expense--that good ole FG and a decent epoxy is all thats needed.

Over on composites there has been a thread about yellow glue. Seems to hold up fine for some while others wouldn't touch it. Personally I like it, and use a fair amt of CA as well.

Bob Krech has offered an interesting analysis of why there should be stretch in our recovery bridles, ie we dont need 30 to 50ft of tubular kevlar in every rocket, and that using such a no give material increased the deployment jerk.

I dont know that every rocket needs massive amts of allthread to tie the CR's together or that theres a need for more than 3 CR's in most cases. But I have never flown an N or O.

Personally I like to glass all my rockets as it fills the spirals and offers some good purchase for the fins, but I'm not big on TTW fin mounting so I want a better substrate. The list goes on and on as to where and when things might be overkill. I do a lot of simming as to max gee's, max q, fin flutter issues and then take my best guess, and always say a brief prayer before launch.
John S

 

[SRP Crew]

These remarks are so funny! I couldn't stop laughing.

But John you couldn't be closer to the truth here.

I've seen so much variance in how people have built their rockets
(just on this forum) that it really brings out this important question.

Yes most of us can sim and do small testing, but really, who has a wind tunnel etc. at our disposal, to really get the real facts.

So, we rely on our experience and hope it works again and again.

"Tried, tested and true"....Are we at the bottom of overbuilding or at the top of it.....etc.

Without the facts, It's a tough call for the majority I believe.

This is one of the better threads I've seen in a long time, by the way

 

[denverdoc]

Originally posted by SRP Crew
I've seen so much variance in how people have built their rockets
(just on this forum) that it really brings out this important question.

Yes most of us can sim and do small testing, but really, who has a wind tunnel etc. at our disposal, to really get the real facts.

So, we rely on our experience and hope it works again and again.

"Tried, tested and true"....Are we at the bottom of overbuilding or at the top of it.....etc.

Without the facts, It's a tough call for the majority I believe.

This is one of the better threads I've seen in a long time, by the way

A windtunnel could answer so much of what we do. Especially if it went supersonic, just crank her up til the fins buzz, or strip off. At what point does the velocity of a rocket actually contribute to heating vs cooling? I know its in the neighborhood of M2.1 but local velocities might be a lot higher, now a good CFD program might answer that as well. but who has access to these tools?

I think the habit to watch for is having a fluke or multimodal failure cause you to change your habits so that it becomes the norm for all rockets, instead of thinking about things before the build of each, just as the thread starter suggested. In other words a one size fits all mentality.
John

 

[heada]

Originally posted by denverdoc
A windtunnel could answer so much of what we do. Especially if it went supersonic, just crank her up til the fins buzz, or strip off. At what point does the velocity of a rocket actually contribute to heating vs cooling? I know its in the neighborhood of M2.1 but local velocities might be a lot higher, now a good CFD program might answer that as well. but who has access to these tools?

...

John

For another project I'm working on (ramjet) I have been looking into building a supersonic wind tunnel. It turns out that theres a very good reason that only people like NASA and Boeing have them. They're really, really hard to build. You can't use a prop or fan very well and the best way is to use highly pressurized gases. Building a wind tunnel that can reach speeds in excess of 400 mph is going to be a daunting task.

https://facilities.grc.nasa.gov/8x6/index.html

-Aaron

 

[denverdoc]

Originally posted by heada
For another project I'm working on (ramjet) I have been looking into building a supersonic wind tunnel. It turns out that theres a very good reason that only people like NASA and Boeing have them. They're really, really hard to build. You can't use a prop or fan very well and the best way is to use highly pressurized gases. Building a wind tunnel that can reach speeds in excess of 400 mph is going to be a daunting task.

https://facilities.grc.nasa.gov/8x6/index.html

-Aaron

Yea i did some back of the napkin calcs one dreamy day, and decided it was way out of my realm, then searched around and saw the size of the facilities involved--nope not gonna fit in my backyard.

Daunting? Your're looking at twice the speed of a really fierce hurricane like Katrina. Keep us posted, and best of luck.
John

 

[WillCarney]

Originally posted by denverdoc
Will,
No doubt. I think the reason some of us (myself, Robert DeHate, others) get so riled over the issue is that it can contribute to failure--in other words when a potentially 20#'er becomes 40, the stresses on everything double during deployment, leading to higher failure rates in many cases. Not suggesting that this happened with your L3. But if can.
John

My failure was of my own doing and nothing to do with the over
built issue. I used a kids parachute and I only had one knot in
the light shroud lines. That knot came loose at deployment of the
main. I have since repaired the rocket and the chute has loops
sewn in 9/16 tubular nylon this time. If the rocket had not been
overbuilt it would have been a write-off. The fins for instance
were two sheets 1/8 plywood 5-ply birch with fiberglass
inbetween them and on the outside. So basically the fins were 3
layers of fiberglass and two layers of 1/8 inch plywood. The body
tubes were 1/8 inch thick shipping tubes and had one layer of
Kevlar-Easyglass with epoxy and a second layer of easyglass-
epoxy over that. It was overbuilt for a M motor. On landing only
one fin had cracked loose a little from the motor mount and
cracked through about half way. The upper body tube was
slightly dented. Still repairable without replacing the fins or BT.

I will sometimes overbuild to fly larger motors. I intend to fly my
L3 rocket on some pretty strong EX M motors. I have a 24, yes
24 EX case that will handle G-H motors. I also have a 29 mm case
to fly H motors and am looking for a 38 mm case for K's and a 54
mm case for M's. It's all a matter of having fun.

William

 

[denverdoc]

Originally posted by WillCarney
My failure was of my own doing and nothing to do with the over
built issue. I used a kids parachute and I only had one knot in
the light shroud lines. That knot came loose at deployment of the
main. I have since repaired the rocket and the chute has loops
sewn in 9/16 tubular nylon this time. If the rocket had not been
overbuilt it would have been a write-off. The fins for instance
were two sheets 1/8 plywood 5-ply birch with fiberglass
inbetween them and on the outside. So basically the fins were 3
layers of fiberglass and two layers of 1/8 inch plywood. The body
tubes were 1/8 inch thick shipping tubes and had one layer of
Kevlar-Easyglass with epoxy and a second layer of easyglass-
epoxy over that. It was overbuilt for a M motor. On landing only
one fin had cracked loose a little from the motor mount and
cracked through about half way. The upper body tube was
slightly dented. Still repairable without replacing the fins or BT.
....

William

Will, Not to be argumentative, especially as it is all about having fun, but is its possible the knot would've held had the rocket been a lot lighter? Thats the Tao of rocketry re overbuilding IMHO. Not saying it shouldnt have been double knotted, but also some would say no knots are better than two.
JS

 

[WillCarney]

Originally posted by denverdoc
Will, Not to be argumentative, especially as it is all about having fun, but is its possible the knot would've held had the rocket been a lot lighter? Thats the Tao of rocketry re overbuilding IMHO. Not saying it shouldnt have been double knotted, but also some would say no knots are better than two.
JS

No, not to be argumentative, I think it would have failed even
with 20 lb rocket. (my lift off weight was 46lbs)

After I thought about it I don't even remember that it was even
as tight as it should have been, it was probably just lightly tied
together. It was a single knot with the ends loose. It would
have failed no matter what the weight of the rocket. I wanted to
have it sewn but was in a hurry for the cert flight and did not get
to it. I flew it without double checking the knot for secureness
and tightness.

It was a bad call on my part pure and simple, we all learn from our
mistakes and move on.

The rocket being overbuilt did save it that I am sure, but I am
pretty sure the knot would have failed regardless of rocket
weight. It won't next time I try.

William

 

[denverdoc]

Originally posted by WillCarney
No, not to be argumentative, I think it would have failed even
with 20 lb rocket. (my lift off weight was 46lbs)


It was a bad call on my part pure and simple, we all learn from our
mistakes and move on.

The rocket being overbuilt did save it that I am sure, but I am
pretty sure the knot would have failed regardless of rocket
weight. It won't next time I try.

William

Amen to that, and one of the reasons and speaking of overbuilding, I confess to using a pair of massive snap swivels (well at least they aren't forged) to take the knots out of the loop so to speak.
J