building Formula 38 - motor mount length challenge

I had picked up a Fiberglass Formula 38 on Labor day sale from Madcow (it's actually a RW model), and finally got time to start putting it together. One thing became clear quickly - the 5" long motor mount was going to be a challenge to mate with 4 3/8" long TTW fins.
Once you add 3/8" on each side for centering rings (down to 4 2/8” of usable MM length already vs. 4 3/8” needed for the fins! ), and another 3/8" for the motor retainer, and another 1/4" MM needs to stick out to allow motor retainer cap to have enough protruding threads to grab, you had run out of real estate on the MM by -11/8”.
And if you tie the shock cord behind the forward centering ring, that takes yet more precious MM length.

The MM really needs to be about an inch (1-1.5&#8221 longer, or 6-6.5” long!
See the enclosed pics for visuals.


As far as I can see, I now have three (4?) options:
1). Cut the fins to make them shorter. Unfortunately, I will need to cut the most from the front (at least 3/4"), as the front centering ring’s position is dictated by how far out the MM has to extend from the end of the rocket to accommodate the motor retainer. In other words, I will have to butcher the front of the fin, and possibly recut the entire forward slope of the fin.
2). Minimize shock cord intrusion (and fin front-side butchery) by abandoning the around-MM loop, and just tie a few knots and epoxy it in place inside the MM. Hopefully that will be enough to keep it in place.
3). Shop for a new and longer MM tube.
4). ???

Any other suggestions?

TIA,
alex f

afadeev said:

I had picked up a Fiberglass Formula 38 on Labor day sale from Madcow (it's actually a RW model), and finally got time to start putting it together. One thing became clear quickly - the 5" long motor [FONT=Lato, Arial, sans-serif]mount was going to be a challenge to mate with 4 3/8" long TTW fins.[/FONT]
Once you add 3/8" on each side for centering rings (down to 4 2/8” of usable MM length already vs. 4 3/8” needed for the fins! ), and another 3/8" for the motor retainer, and another 1/4" MM needs to stick out to allow motor retainer cap to have enough protruding threads to grab, you had run out of real estate on the MM by -11/8”.
And if you tie the shock cord behind the forward centering ring, that takes yet more precious MM length.

The MM really needs to be about an inch (1-1.5&#8221 longer, or 6-6.5” long!
See the enclosed pics for visuals.
View attachment 328502View attachment 328503View attachment 328504

As far as I can see, I now have three (4) options:
1). Cut the fins to make them shorter. Unfortunately, I will need to cut the most from the front (at least 3/4"), as the front centering ring’s position is dictated by how far out the MM has to extend from the end of the rocket to accommodate the motor retainer. In other words, I will have to butcher the front of the find, and possibly recut the entire forward slope of the fin.
2). Minimize shock cord intrusion (and fin front-side butchery) by abandoning the around-MM loop, and just tie a few knots and epoxy it in place inside the MM. Hopefully that will be enough to keep it in place.
3). Shop for a new and longer MM tube.
4). ???

Any other suggestions?

TIA,
alex f

Click to expand...

I would get a longer MMT. Most of the time I run the shock cord down between fins and glue it on. It has never failed for me.

crossfire said:

I would get a longer MMT. Most of the time I run the shock cord down between fins and glue it on. It has never failed for me.

Click to expand...

I hear you on longer MMT, though that will take time to find and ship.
For gluing the shock cord, do you tie it around the MMT circumstance first and then pass through the centering ring cutout, or simply epoxy it longitudinally?

I usually do the latter with HP shock straps, but those are 1/2" - 1" wide.
Formula 38 came with just the fiberglass parts, everything else you have to supply yourself. In my case, I had 2,000 lb strength Kevlar braided line laying around, but it's round, and has limited surface area when laid on the MMT.
OTOH, it will be a mid-power rocket. I may just wrap and exposy some thinner lines around it cross-wise to provide extra bonding surface, and call it a night.

Thanks,
a

How about just notching the root edge of the fins so they fit in against the MMT in the available area, but still fit in the slot of the BT for the full length. You don't need to have the full length of the fin glued to the MMT to have almost all of the strength of through the tube mounting.

+1 on notching the root edge to clear everything. You'll never notice from the outside.

I recently bought 2 Formula 38s and ran into the same thing. I just ordered a 30” long 29mm tube from Madcow and cut two new motor mount tubes that are 6” long.

I’ve been meaning to email them the suggestion, but they really should make the motor mount tube longer.

+1 on notching the root edge to clear everything. You'll never notice from the outside.

Click to expand...

+1 again.

Tonimus said:

+1 on notching the root edge to clear everything. You'll never notice from the outside.

Click to expand...

Challenge is that you can't just notch the root edge (I might have to do that a bit anyway). Motor retainer requires protrusion of the MMT out the back, which positions the front edge of the MMT at about 3/4" distance from the front tip of the fin. That means I would end up cutting off the nose of the fin. That would be neither elegant, nor aerodynamic.

A cleaner alternative is to more the front centering ring of the MMT back about 1", then cut notches in the fin further back where the depth of the fin will support notches. I'm thinking of doing that right now.
See pic below for illustration.


The right way to solve this is to get a longer MMT tube. I just heard back from MadCow/RW that they will send me a 7" tube in the mail.
If I have the patience (and other projects), I will wait for that to arrive. If not, I will fool around with the idea above first.

a

GrouchoDuke said:

I recently bought 2 Formula 38s and ran into the same thing. I just ordered a 30” long 29mm tube from Madcow and cut two new motor mount tubes that are 6” long.

I’ve been meaning to email them the suggestion, but they really should make the motor mount tube longer.

Click to expand...

Agree completely.
MMT is just too short for the Formula 38 fins, and way too short if you intend to use a motor retainer of any kind.
6-6.5" length would have been perfect vs. 5.0" provided.

I've just exchanged a few emails with Madcow. MC is aware of the problem with this and a few other original RW kits, and is working on updating RW pipeline. If you get an old inventory product, email them (calling goes to VM), and they might send you a longer MMT to make you whole. I should have a 7"-er in the mail heading my way today.

YMMV,
a

Forgive the crap drawing (I only have paint at the moment), but I mean notch the root, but just barely. Leave all of the fin up where the slot is.

Was reading this build thread because I also have a Madcow/RW Formula kit but mine is a 54 rather than the 38.

The issue of an uncomfortably short motor tube prompted to rush out into the shop and open up the sealed kit bag.

Turns out the motor tube is a confortable 7" long.

~sigh of relief~

OverTheTop said:

+1 again.

Click to expand...

+1. Easiest answer.

Sent from my Moto G (5) Plus using Rocketry Forum mobile app

Tonimus said:

Forgive the crap drawing (I only have paint at the moment), but I mean notch the root, but just barely. Leave all of the fin up where the slot is.

Click to expand...

Nice drawing, BTW, better than anything I could have thrown together in a few minutes.

I ended up doing something very similar, see pic below:


For others reading and having concerns about MC's Formula 38 kit - it's a great and attractively priced fiberglass only package, sans everything, including instructions.

If you are comfortable putting all the pieces together, including those you will need to source yourself (shock cord, chute, eye-bolt, rail buttons, motor retainer, etc), making small adjustments to the fins is no big deal. Either that, or ask for a longer (6.5-7.0") MMT from MC, and they will make sure you get it.

I started this thread because I wasn't sure if my guess that the MMT is just too short was right, or if I was missing something obvious. MC confirmed that this was a RW's inherited design deficiency that they are correcting. The workaround left me with the front MMT centering ring ~1" back from the front edge of the MMT, but I don't think it's much of a concern. The fiberglass MMT is rock solid, and will be further reinforced by epoxied TTW fin filets. I don't see it moving any with the centering rings 5" apart instead of ~5.75" that I could have otherwise had (or even 6" if I had forgone motor retainer).

The biggest attraction of Formula 38 kit for me are the beautiful colored (pick your own color!) fiberglass tubes:
https://www.madcowrocketry.com/formula-38/
I've been spending more time painting, sanding, and re-painting the rockets than the time it takes me to both built and fly them. I don't enjoy paintwork all THAT much.
With this colored fiberglass tubes, I should be ready to fly after just one coat of clear, with zero additional paintwork post future hard landings!

a

Hmmm, this thread has piqued my interest in the 38. Thx for sharing.

Peace,
Tobor.

Tobor said:

Hmmm, this thread has piqued my interest in the 38. Thx for sharing.

Click to expand...

She is almost done:





These pics don't do the rocket justice.
The body is translucent, and looks great in sunlight.
The only cosmetic bummer is the black front MMT centering ring. Once I epoxied it in, it started showing through a bit.

External fin fillets have cured (my first experiment with colored epoxy may benefit from 400 grit sanding).
Now just need to do internal fillets, seal the rear MMT centering ring, and JB weld the motor retainer.

Just need to sim what size chute to put in there.
24" will be a safe bet, but I wonder if I could go smaller for shorter walks, given fiberglass tube and fins.

a

P.S.: Sorry about focus issues. I'll dig up DSLR when everything is finished. For now, it's just cell phone pics.

Looks good - getting close!

Looks pretty nice !!

Nice build. If I was in time, my alternative options would have been:
1. Build it MD instead.
2. Half notch the CR and Fin root (could do this on either the front or rear CR or even both). This would be extremely strong.

Built is complete.

Pics below naked:


...and after clear coating:


3 light coats of clear added exactly 4 grams of weight, bringing it up to 389g.
MC/RW claims the rocket should weight ~1 lb / 454 grams, so I am 15% underweight. Not sure why.
That's really atypical for me, as I usually overdose on epoxy fillets (external and double-internal).

Couple notes to self, and anyone else who cares:

1. External fin fillets in black (fin color) are not as cool looking as body-colored fin fillets. In the future, I'm ordering more Rocketpoxy pigments to match the body tube colors.
2. Sanding black Riocketpoxy fin fillets turns them grey and matte. Spraying clear coat on them returns them to the original shiny black color.
3. All fiberglass Formula 38 is built like a tank. I simm-ed recovering it on anything from a streamer (80 ft/s), to 14" chute (30 ft/s), to 18" chute (24 ft/s). Went with the 14" chute because I had one laying around, and because I have no doubts about rocket landing without any damage @30 ft/sec. No need for CR with 14" chute also makes for fewer things to worry about as well.
4. I added both rail buttons and 1/4" lug so that I can launch both off the rail with the local clubs, and off my portable launcher on a whim.

Next up:
- adding nose-cone mount for the Eggfinder. This can wait until after after the first flight, as anything other than H/I/J's will keep it within sight (sub 3K feet). On I205, it is simm-ed to hit 7,500".
- MC Fire Flyer built.

a

Looks good! The sims I'm running have it well over 4k on a big G (OpenRocket, 425g, smooth paint).



The two I'm working on with my kids are going to get Eggfinder Minis in the nose. I'm planning 18" chutes in mine. We'll start out with F motors so the kids can see the whole flights, but they'll get big Gs and maybe Hs soon!

GrouchoDuke said:

Looks good! The sims I'm running have it well over 4k on a big G (OpenRocket, 425g, smooth paint).

View attachment 329519

Click to expand...

Here are my sims.
I'm getting 3400 feet on G80 vs. your 4200.
I can't account for the difference with changing the weight alone...
...:confused2:

View attachment 329536

Here are my sim parameters:
View attachment 329537

GrouchoDuke said:

The two I'm working on with my kids are going to get Eggfinder Minis in the nose.

Click to expand...

I'm installing mine in Chris Bender's 3D printed nose cone tray:
https://www.rocketryforum.com/showthread.php?141570-EggFinder-(full-sized)-3D-printed-sled&highlight=Chris+Bender

GrouchoDuke said:

I'm planning 18" chutes in mine. We'll start out with F motors so the kids can see the whole flights, but they'll get big Gs and maybe Hs soon!

Click to expand...

18" chute was my plan B, but I would have had to fly it with CR.
I may still go that rout, but I also had a nice 14" nylon chute laying around looking for a rocket, and this seamed like a viable, if a bit aggressive, pairing.

a

afadeev said:

Here are my sims.
I'm getting 3400 feet on G80 vs. your 4200.
I can't account for the difference with changing the weight alone...
...:confused2:

View attachment 329536

Here are my sim parameters:
View attachment 329537




I'm installing mine in Chris Bender's 3D printed nose cone tray:
https://www.rocketryforum.com/showthread.php?141570-EggFinder-(full-sized)-3D-printed-sled&highlight=Chris+Bender



18" chute was my plan B, but I would have had to fly it with CR.
I may still go that rout, but I also had a nice 14" nylon chute laying around looking for a rocket, and this seamed like a viable, if a bit aggressive, pairing.

a

Click to expand...

You are a better man than I if you are going to visually track that rocket to 3k and back. I hope the grass is short.

Good luck on flight, and recovery. Disregard my tracker paranoia...I was scarred by an Estes Hi Flyer once.

Cl(VII) said:

You are a better man than I if you are going to visually track that rocket to 3k and back. I hope the grass is short.

Click to expand...

You are right, 3K may be borderline. I haven't had any trouble upto 2-2.5K with tracking powder and mid-power rockets.
But if there is cloud cover, all bets are off.

Flying re-loadable motors, I am simmed to get upto 2.2K on G54 and 3.3K on G64.
F35 and F39 are 1.3K and 1.5K, respectively. Those would be my starting points.

Cl(VII) said:

Good luck on flight, and recovery. Disregard my tracker paranoia...I was scarred by an Estes Hi Flyer once.

Click to expand...

Ha - I lost Hi-Flyer once as well! Saw it going up, never saw it coming down.
That's why I've all but given up on flying sub-24mm rockets - they are just too small and too easy to loose.

Kids, on the other hand, can fly and loose whatever they built...

a

P.S.: Either way, the f38 is getting an Eggfinder mini as soon as it, and your sled arrive in the mail!

The two I'm working on with my kids are getting close. The only epoxy left is the nosecone coupler. We'll do that tomorrow. After that, it's nosecone avbay stuff & recovery bits. We're going to paint them, but not until after their maiden flights.



I went with 6" motor mount tubes, but I wish I had gone with at least another 0.5". One of my kits worked fine at 6.0", but the other had slightly bigger fins and it needed a little bit longer MMT. I just trimmed the forward fin roots on that rocket.

EDIT:

afadeev said:

Here are my sims.
I'm getting 3400 feet on G80 vs. your 4200.
I can't account for the difference with changing the weight alone...
...:confused2:

Click to expand...

Yeah, I don't know either. I changed my elevation from 2800 to 0 and that lowered it to 4000. My model could be messed up. Here's my OpenRocket file: https://www.dropbox.com/s/9c8pubobz02530b/Formula38.ork.zip?dl=0

GrouchoDuke said:

Yeah, I don't know either. I changed my elevation from 2800 to 0 and that lowered it to 4000. My model could be messed up. Here's my OpenRocket file: https://www.dropbox.com/s/9c8pubobz02530b/Formula38.ork.zip?dl=0

Click to expand...

Mystery solved - we were referring to two separate G80 motors!
I was using 100 N-sec G80 in OR, while your .ork file has G80NBT with 136 N-sec total impulse.

After briefly searching online, it looks like your 136.6 N-sec is the engine spec that is currently on the market, while the G80 I had picked is either an OR engine database error, or something that may had been deprecated by AT. Thurscurve.org lists four (4) G80 entries, but it's hard to tell which one is still available for sale:
https://www.thrustcurve.org/motorsearch.jsp

If I sim your .ork on my phantom 100 N-sec G80, I get 3525 foot flight, which is very close to my sim result of 3411, with the delta attributable to difference in rocket weights.

a

There it is!

afadeev said:

Mystery solved - we were referring to two separate G80 motors!
I was using 100 N-sec G80 in OR, while your .ork file has G80NBT with 136 N-sec total impulse.

After briefly searching online, it looks like your 136.6 N-sec is the engine spec that is currently on the market, while the G80 I had picked is either an OR engine database error, or something that may had been deprecated by AT. Thurscurve.org lists four (4) G80 entries, but it's hard to tell which one is still available for sale:
https://www.thrustcurve.org/motorsearch.jsp

If I sim your .ork on my phantom 100 N-sec G80, I get 3525 foot flight, which is very close to my sim result of 3411, with the delta attributable to difference in rocket weights.

a

Click to expand...

I had this motor mixup quandary in my apogee aspire sims - the NBT curve from thrustcurve.org matches the curve posted on my vendor's site for the g80 they're selling today...