First Time Questions

[Pippen]

After doing his first rocket from a kit last summer, my (then) 9 year old announced that what he *really* wanted to was build one from scratch. Oh ye of little faith wasn't too keen on the idea given my lack of experience and his age but this summer he repeated the request so that's what we're doing. For the past few days we've been talking his ideas through and gathering materials and today he started in. I have to say I was pleased because so far it was actually easier getting him through this than through a kit with instructions. And I think he's getting a lot more from it because we've had to talk through things (like stability) and make adjustments instead of just following instructions.

Questions:
1) What he drew up for a fin idea were swept back bird wing shaped fins similar to what's on the little Estes Mini Meanie. Is there a basic primer around on different fin shapes and why one might be better than the other?

2) He wants to do a payload--I found a clear plastic tube that fits. Is there anything we need to know about building a payload rocket from scratch outside of basic design/how it works?

Thanks for any help on this--I really hope he gets a success out of this.
Lisa

 

[cjl]

Fin shape doesn't matter too much as long as you have enough area far enough rearward that it is stable (I'm assuming you don't care too much about optimizing this for altitude). Just make sure they stick out far enough to have some decent area - fins that are long but skinny (close to the rocket) do very little.

As for a payload rocket?

Make sure the payload is securely fastened, but other than that, you can basically treat the payload section just like any other section of tube.

Can you give any more specifics on the rocket? Dimentions?

 

[Stefan_Jones]

Welcome to the hobby!

Here is a very good on-line guide to construction:

https://www.ninfinger.org/~sven/rockets/catalogs/centuridesign/cendesign.html

Technical reports:

https://www.ninfinger.org/~sven/rockets/EstesPubs.html

 

[Pippen]

Originally posted by cjl
Can you give any more specifics on the rocket? Dimentions?

Specifics...well, first and foremost in the eyes of a 10 year old it's going to have flames, be called Inferno, and will likely carry a Lego Anakin Skywalker.

As for the rest of the boring but necessary details ;-) , most of the parts came out of an Estes Trans Wing Super Glider kit. He's using only one of the body tubes and it looks to be 9 inches long and just barely 1" in diameter. The payload material is clear plastic, and feels similar to what I've seen in kits before--it's also 1" in diameter and 4 3/4" long. For the transition between the payload and the BT, we planned to use a second one of those plastic doohickies that fits into the nosecone and wrap it with tape so it fit tightly.

Here's the plan:

 

[Pippen]

Here's what he's done so far. Only the motor mount is glued in.

 

[Pippen]

Here he is cutting the payload. He sanded the end smooth afterwards.

 

[Pippen]

Oops...let's see if I can't get that right this time.

 

[sylvie369]

Originally posted by Pippen
Oops...let's see if I can't get that right this time.

Take THAT, plastic RTF designers!

That's a heartwarming photo - a boy building a rocket using a coping saw and a vise - no matter how ragged those edges come out.

 

[adrian]

My main concern is the fins. How do you plan to make them, and from what material? If they're balsa, the long, thin tips are likely to snap - in fact, they may even break while you're working on them.

Take a look at the old Space Cruiser Excalibur on JimZ Rocket Plans. This has fins similar to what your son is planning. Rather than being a single piece of balsa, each fin is made up of two pieces, which means the grain can be aligned in each piece to improve strength.

Once the fins are made, laminate them with paper. Trace round the fin onto a piece of paper, twice for each fin. Cut out the shapes, glue one onto each side of each fin, then put the fins between a couple of pieces of wax paper and put something heavy on top of them while the glue dries, e.g. a few heavy books. Paper lamination like this makes the fins a good deal stronger and also gives a better surface finish than raw balsa - you'll save your son a bit of effort with sanding sealer and sandpaper!

 

[Micromeister]

Pipen:
Looks like you guys are will on your way to a fun flying model.
I agree with adrian on fin construction of this type. keeping the grain running with the leading edge for max strength and laiminating with paper or light 67lb cardstock. Another Small (engineering) change might be to suggest giving those long tapered fins a 3/16"or so Flat tip
Hope this helps...Looks like a fun project

 

[jflis]

Oh man, you guys are having a ball

Agreed on the fins. You really have to provide some strength there.

Here is how I would approach them.


Assuming you want 1/8" thick fins:

Take a sheet of 1/16" balsa with the grain running like this: /////
Then another sheet with the grain running like this: \\\\\

Glue them together to create a sort of Balsa Plywood for extra strength.

The technique of laminating them in paper is good and the easiest way I have found is to laminate the balsa sheet stock in paper then cut the fins out of that (that way you are assure of having clean edges where the paper is).

I also agree with having a flat spot on those fin tips.

Another idea (which would add strength and also look cool) would be this:

After the fins are cut out, assuming you put a flat spot on the fin tips, take a strip of 1/8" balsa the same width as the tip of the fin and about 4 times longer. Sand one side rounded then glue this to both sides of each fin tip. This will add some interesting detail to the fin tip while adding good strength as well. If I get a chance I will draw up an image of what I mean when i get home this afternoon.

Bottom line, keep having fun. YOu will both remember this for a long time to come

jim

 

[powderburner]

Maybe these old threads will help explain some of the basics?

https://www.rocketryforum.com/showthread.php?threadid=2053

https://www.rocketryforum.com/showthread.php?threadid=22362

https://www.rocketryforum.com/showthread.php?threadid=18334

 

[Pippen]

Originally posted by sylvie369
That's a heartwarming photo - a boy building a rocket using a coping saw and a vise - no matter how ragged those edges come out.

I think so too. Originally I thought to have him cut it with his dad the next morning but my son was raring to go. I figured with a garage full of tools there had to be a way for him to get to do it on his own.

Thank you all *very* much for your thoughts and suggestions. I knew with this kiddo it would be best to do a little advance work on the fins to prevent fin cutting from being a deal breaker so I made a small balsa fin out of his original design so he could see why it needed some changes. Then we went through several cardstock sizes and shapes of new ones and I made a balsa pattern off the one he was happiest with. You'll see on the picture below he settled on a wider fin with a sturdier wingtip.

I'm still thinking on which method might be best (translation: simplest/least frustrating) to go about addressing the grain alignment. If he went the route of making balsa "plywood" would you still suggest laminating it?

BTW, as soon as I looked at those plans Adrian posted I remember making some other rocket with fins that were pieced together like that. Landed behind my house in a field of tall grass never to be seen again...

 

[adrian]

That picture looks as though the grain is parallel to the root. If so, the tip won't snap as easily, but the entire fin is likely to snap off somewhere near the root.

Making balsa "plywood" means someone needs to cut out at least twice as many fin shapes. And, for it to be effective plywood, half of them will need the grain running across the tip. I'd be surprised if some tips don't get broken off during cutting. Of course, that just means the pieces need to be glued together, which is what must be done anyway if each fin is in several pieces as in "Space Cruiser Excalibur".

I'd still recommend laminating with paper. At the very least, it provides a good surface finish without all the sealing and sanding!

 

[Pippen]

Thanks again--this was all very helpful.

In the end we went with the method that seemed most do-able for him. He couldn't get around those curves using 1/8 inch so we went with the plywood idea plus did the laminate. I heeded your warning on the tips and added a little extra length to the pattern thinking it would give us a little safety cushion if needed and that they could maybe be knocked down to size.

After trying it a few ways what we settled on was drawing the pattern on white label paper and sticking it onto the balsa. It gave him a guide for the blade and was easier than cutting through it. Getting around the curves was still hard and he wound up punching holes with the knife to perforate it the first time but by the time he got to the 3rd fin he went after it!

We decided to have him cut three on his own and then for me to use those as patterns to cut the matches. (BTW, the only fin tip broken off in the process of cutting was by yours truly. ). I sanded them up so the edges are smooth and he can glue them up tomorrow (if this looks okay to you experts, that is).

Adhesives we have on hand are white glue, wood glue, Tacky Glue, and some smellier choices which I'd rather avoid.

Here's a picture of him working on his second fin:

 

[Pippen]

Here's the third fin he cut out on his own:

 

[Stefan_Jones]

It is nice to see a parent teaching craftsmanship!

One tool I highly recommend: A sanding block. Less than $3.00 at a "Big Lots!" type discount outlet. Standard sheets of sanding paper are designed to be cut into strips to fit standard blocks.

I mention the block now because they are great for evening out fins. Stack the pieces together, stick "T" pins through them and sand the edges nice and flat and even.

Sanding blocks also make rounding and tapering edges easier.

 

[Pippen]

Originally posted by Stefan_Jones
It is nice to see a parent teaching craftsmanship!

That should probably read "It is nice to see a parent *learning* craftsmanship"

Here's the next picture: gluing the fins together. From this step we to put waxpaper down because newspaper and glue does nasty things to labels. We also learned that you should never leave out a tub of glue, etc. on the kitchen table when you have a curious kitten in the house.

 

[Pippen]

After they dried I trimmed them a bit with a blade then I had him put on a bead of glue around the edges to seal up the labels and fill a little.

He'd had quite enough of fin construction by then so I rigged a setup for him to get those fins on quickly and easily. The base is from a Brio construction set that hadn't been used in ages. I put a fin alignment guide on that so he'd have a place to shoot for in lining up the fin tips. In normal circumstances I definitely wouldn't use epoxy with a kid this young building a simple model but I needed to get him through this step fast and in about ten minutes he was all through. Before we started I talked him through mixing and using epoxy and had alcohol wipes on hand for cleanup just in case.

After he got the fins on I did neurosurgery on my daughter's Ken doll who was in serious danger of losing his head.

 

[jflis]

Oh man, that rocket is looking SHARP!!

Your son may be only 9 years old, but *I* wasn't designing rockets that cool when I was *NINE*! LOL

Tell him he's done a great job so far! But he has a lot more to do to make it flyable! And we expect to see it complete AND see it flying too (right folks!?)

Good job!
jim

 

[Pippen]

The day before yesterday we worked through the swing test. He went from "Can we get on with this?"...

 

[Pippen]

...to "What do I do now?"....

 

[Pippen]

...to "Yi-haaw! This flies like a missle!!!"

 

[jflis]

I've done swing tests many, MANY times, but that is the first time in many years that i've seen a picture of someone else doing it ('specially a kid)

Talk about bringing back memories!

 

[rocketking]

For the swing test to be accurate, the rocket Should be set up in 'ready-to-fly' mode- Shock cord & mount installed, parachute in place, motor and recovery wadding in place, launch lug in place... All the necessities need to be completed to gain an idea of the rocket's stability.

All that said, it's still great to see a smile on a kid's face while working on a rocket! That's great parenting!

JimF was right in his comment on 'making a memory'. That IS mighty important with kids. It can take kids from the place of 'I can't do that' and moves them to the "I CAN do it!" location!

All the best! Continue to keep us posted on the progress!

 

[Pippen]

Originally posted by rocketking
For the swing test to be accurate, the rocket Should be set up in 'ready-to-fly' mode- Shock cord & mount installed, parachute in place, motor and recovery wadding in place, launch lug in place... All the necessities need to be completed to gain an idea of the rocket's stability.

All that said, it's still great to see a smile on a kid's face while working on a rocket! That's great parenting!

JimF was right in his comment on 'making a memory'. That IS mighty important with kids. It can take kids from the place of 'I can't do that' and moves them to the "I CAN do it!" location!

All the best! Continue to keep us posted on the progress!

Hey, you can see below that we had it all covered (except now I see I forgot the wadding). But we did remember stunt double for payload Anakin Skywalker.

Thanks to you and everyone for the encouragement. My son is one amazing kid! He's been checking here with me every day to read the hints and all this other good stuff.

Oh, I meant to ask a question on the swing test. Is what happens during the acceleration period critical? It went well once it was up to speed but in the process of getting there it wasn't flying nosecone forward for the entire period.

 

[jraice]

i dont know much about swing testing but if you dont think itis stable I or someone else on this forum with rocksim could help you out... we would need to know the center of gravity (balancing point) distance from the nose with everything but the motor, the weight of the rocket without a motor and the dimensions (including fin shape)... then we could enter the motor you plan on using, tell you if its stable and how it will perform. I could even tell you the best ejection delay for it.

 

[gpoehlein]

Originally posted by adrian
That picture looks as though the grain is parallel to the root. If so, the tip won't snap as easily, but the entire fin is likely to snap off somewhere near the root.

Making balsa "plywood" means someone needs to cut out at least twice as many fin shapes. And, for it to be effective plywood, half of them will need the grain running across the tip. I'd be surprised if some tips don't get broken off during cutting. Of course, that just means the pieces need to be glued together, which is what must be done anyway if each fin is in several pieces as in "Space Cruiser Excalibur".

I'd still recommend laminating with paper. At the very least, it provides a good surface finish without all the sealing and sanding!

When using balsa ply, one of the pieces is always going to have the grain running the "wrong way" This is OK - remember that with plywood, the final piece is actually more than the sum of it's parts - it will be stronger than one piece of 1/8" balsa BECAUSE the grains are perpendicular to each other.

As for having to cut out twice as many fin shapes, you actually want to laminate the balsa panels together before cutting out the fins. Just spread the glue very thin on one balsa panel and place the other on with the grain going 90 degrees to the first. Place under a heavy book or other weighted object to dry. And you can do 3 layers if you want it to be really strong (and if you can find balsa thin enough to have 3 layers match your final fin thickness).

Greg

 

[Pippen]

Originally posted by jraice
i dont know much about swing testing but if you dont think itis stable I or someone else on this forum with rocksim could help you out... we would need to know the center of gravity (balancing point) distance from the nose with everything but the motor, the weight of the rocket without a motor and the dimensions (including fin shape)... then we could enter the motor you plan on using, tell you if its stable and how it will perform. I could even tell you the best ejection delay for it.

Thanks for the offer! We loaded it back up this morning to double check it and on both trials (first by him, second by me) the same thing happened: the rocket started backwards and then after 2-3 revolutions it flipped forward and did an nice even nose forward flight. I did tape the string to hold it in place.

If you’re still up for doing the Rocksim on it, let’s do that. I’m glad he had the see it, feel it, experience of the swing test first (besides he thought it was a blast and wants to bring out all of our rockets to test now). Since we’re in learning mode anyway, the software will show him another way to get a projection.

Weight of rocket w/ everything but no motor = 2.4 ounces
Distant from CG to top of nosecone = 295 mm
Total length = 503 mm
Outside BT diameter = 25 mm

Max height fins = 101 mm
Max width fins = 42 mm
Width of fin at fin tips = 11mm
Distance from base of rocket to where the bottom of the fins attach = 50 mm
Distance from base of the rocket to where top of fins attach = 90mm
Do you need anything else on those fins?

Motor we plan on using – what’s easiest to find around here are A8-3, B6-4, and C6-5. We tested it on the C6-5.

Thanks again for all the help on this. He's excited to get that baby painted and into the air...but I know, first things first.

Oh, you can see the latest --Fuzzy Tube Syndrome--at the link below.
https://rocketryforum.com/attachment.php?postid=331066

 

[jraice]

ughh i feel bad offering to rocksim it and then saying this... i dont know (and dont even think it is possible) to rocksim a fin like that one. Normally a model like that would take me 5 minutes to do (I mean a model of this size not with this fin design) but I cant find a way to make the fins get longer then the length where they attach to the airframe.