Scratch built L3 - maybe this turns into a build log?

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Dustin Lobner

Dustin Lobner

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Hi everyone,

Some background on me - I started out flying HPR in 8th grade through the fantastic Rockets for Schools program in Sheboygan, WI. Got my L1 cert my senior year of high school, L2 freshman year of college. That was during the ATF lawsuit...money, time, and a LEUP in college were in short supply so I backed out of the hobby. I've finally got to a point in my life where I think going for an L3 is a possibility, so I'm going to do it. I'm getting married next summer, so I'm going to have to cashflow this project without a lot of cash to flow, so this might end up being a 2021 launch.

As I had lapsed my TRA membership, I had to refly both my L1 and L2 certs. L1 was August 24, L2 was September 7, both flown with the same rocket. Flew both with the LARS NAR group out of Sheboygan, WI and had a blast. Jeff and Kevin there have been amazingly helpful.

L2.jpg

I've started doodling around in RockSim with some ideas on an L3. I started looking at some of the L3 projects here and have become concerned I might be thinking too "big", though there is a purpose behind that.

What I've doodled up is a 7.5" dia rocket that is taller than 12 feet (see attached RockSim screenshot). Based on other builds, I'm going to guess that it's going to come in around 60-80 pounds maybe? Right now construction is PML phenolic with a couple 4oz layers of glass because that's what I know and am comfortable with. Also includes their piston ejection system.

2nd idea.PNG

The goal of this configuration (7.5" diameter, heavy) is to keep the altitude of the flight low. Where I will likely be doing this flight (Bong State Rec Area in Wisconsin) is one of the "less good" places to recover a rocket and only has a 10k ceiling, so low and slow is the order of the day.

First question of the day - yes, this is a big/heavy rocket, intentionally. Am I over doing it? I don't have any experience in this impulse range (obviously), so I'm asking for other's experience. Looked over half a dozen L3 builds here and this one is bigger than all of them.

I'll be making contact with a couple of TRA TAP members next Saturday, but trying to get my ducks, if not in a row, at least in the same pond before then. Thanks everyone!

Dustin
NAR 107803
TRA 9503
 
I love a big heavy L3 project. I built a 7.5" Fiberglass IRIS when I turned 18 that was intended to be my L3 project, and it never panned out. The logistics of flying a big rocket (and the cost of building an all glass one) sort of turned me away. However, I wouldn't say you are over-doing it. Just build it as you are comfortable, consult with your TAPS and as long as everything lines up in everyone's eyes between you and your TAPs it'll work out great!

Braden
 
Welcome back and welcome to the forum!

Looks like you've carefully thought out the initial steps and I'm sure the TAPS will help you hammer out the bumps.
 
Hey Dustin;

It took me two different rockets to finally get my L3. They were both big heavy beasts. I had to bail out on the first design as it always broke a fin on landing. The drawbacks to the big rockets are the logistics of transport and construction. Getting everything positioned in a manner as to allow a construction operation can be surprisingly challenging and time consuming. On the plus side, the big rockets present a more visually pleasing flight in that you actually get to see it, and recovery is usually less challenging. Bot of my rockets weighed in at about sixty pounds, all up. Flying on Baby Ms, all of the flight was visible and I certainly did not need a tracker for recovery.
The rocket I was successful with, the Heartbreaker Too is on the Forum. The thread is tiled 'My L3 rocket is complete'
Jim
 
Hi Dustin,

Glad to see you're back in the HPR gang and flying with the LARS group. Nice photos from the recent Silica launch!

A couple of random suggestions from someone fairly familiar to flying in your area...

Bong isn't so bad (and it's a lot better in Spring!). Due to the weeds, water hazards, bugs, and such, my personal favorite time for flying there is January through June...with the launches in Jan being a bit cool, but the water is typically frozen, and the June launches the wild outdoor jungle of Bong is just starting to come to life. Somewhere in between is probably the most ideal.

7.5" airframe is the great size for L3 motors at Bong. All the way up to full M's will generally stay under the 10k (ASL) waiver. You can get away with a 5.5" or 6" airframe with small M's, such as 75-4 and 75-5 grain motors as another option.

I see you mention glassing the airframe a bit. Good call! Since you're a Sheboygan-ite, may I suggest using Loc/Precision airframe for the project? Both phenolic and cardboard 7.5" airframe benefit from light glassing, so perhaps consider supporting the in town player (Plymouth), and save yourself a ton of money. If you need help glassing Loc tubes, I know somebody just north of you that does a lot of glass work on Loc cardboard! ;) On that note, just flew glassed Loc 7.5" tube to 19,575' 1.5 weeks ago! It can handle it!

Definitely get some more practice with electronic based deployment of L2 rockets in the meantime and find a pair of TAPs that fit your style.

I'll see you on the field; whether at Bong, Silica, Princeton, etc. Shoot me a pm if you're looking for a local launch this weekend as well...
 
Thanks for the replies everyone!


Eric - glad to hear you're not too bummed out about Bong...I remember launching there in high school and college, it wasn't terrible but I admittedly didn't have any experience elsewhere. Maybe I'll aim for March of 2021 then...

Regarding LOC, I actually went and visited them over lunch one day with a rocket-minded coworker. Keeping in mind, their place is not a store front...Basically, we knocked on the door, they asked who we are, and we basically said "we like rockets, can we come talk?" They invited us in, we talked with a couple of guys there and they seemed like awesome guys. I was planning on running their parts for this build, but then found out they don't have phenolic tubing in stock in 7.5" - I emailed them, and they don't have it presently. I guess I had written off their cardboard material, which they do offer in 7.5" (and when I heard back from them, they never mentioned the cardboard tubing?). So, if I glass it, cardboard will work OK? My plan was 2 or 3 layers of the glass sock material from Giant Leap. My only other complaint is that their motor mount centering rings are .25" and PML's are .50" - I can just double them up in the end though, whatever.
 
I've been looking at other's builds and most have threaded rods running the length of the motor mount. Do I need these? And why?

Thanks!!

Dustin
 
It's to transmit the recovery forces down the entire motor mount unless I'm mistaken. For the cases where the harness is ties off to hardware on the forward centering ring.

If the harness is epoxies to the motor mount tube, this is less necessary.
 
Got it, thanks! The way this is sketched up right now I'd use them then.

I have it basically mocked up in RockSim. One of the things that I kinda ended up doing in RockSim because I didn't know when or where to stop was the inside of the rocket is solid coupler tube, so basically this thing is double walled. Also put in LOC's "stiffy" coupler stiffener for both the Av bay and the coupler on the top of the fin can/motor mount, so triple walled in a couple of critical areas, lol. Maybe I should name the rocket "Gratuitous Excess"? :D

I'm taking this opportunity to teach myself some Solidworks basics too, so that's been fun.
 
Dustin Lobner said:
Got it, thanks! The way this is sketched up right now I'd use them then.

I have it basically mocked up in RockSim. One of the things that I kinda ended up doing in RockSim because I didn't know when or where to stop was the inside of the rocket is solid coupler tube, so basically this thing is double walled. Also put in LOC's "stiffy" coupler stiffener for both the Av bay and the coupler on the top of the fin can/motor mount, so triple walled in a couple of critical areas, lol. Maybe I should name the rocket "Gratuitous Excess"? :D

I'm taking this opportunity to teach myself some Solidworks basics too, so that's been fun.
Click to expand...

Most components are stronger than you think.

Build it to fly, not to crash. Save weight and complexity and trust your materials.
 
Cameron Anderson said:
Most components are stronger than you think.

Build it to fly, not to crash. Save weight and complexity and trust your materials.
Click to expand...

Point taken. The thinking here is that I want to keep the flight fairly low for easier recovery where I'd be launching (5k feet), so for a 7.5" rocket I'm looking at 60-80 pounds maybe from what I have gathered with RockSim. I can either add weight for the sake of weight, or I can put it to use and strengthen the rocket.
 
Dustin Lobner said:
Point taken. The thinking here is that I want to keep the flight fairly low for easier recovery where I'd be launching (5k feet), so for a 7.5" rocket I'm looking at 60-80 pounds maybe from what I have gathered with RockSim. I can either add weight for the sake of weight, or I can put it to use and strengthen the rocket.
Click to expand...

If you need weight, then yes, build strong.

I build small, thin, and light to go high and fast. I have a good friend who uses 10" sonotube and 1x2's. Different objectives, that's all. But all still fun.
 
Cameron Anderson said:
If you need weight, then yes, build strong.

I build small, thin, and light to go high and fast. I have a good friend who uses 10" sonotube and 1x2's. Different objectives, that's all. But all still fun.
Click to expand...

But all still fun indeed! Some day I'll have to build/fly one to 25k or something just for the challenge of it, but that's not the goal here.

<rambling story>
You made a comment before about trusting materials - I'm actually a Materials Engineer, so I'm right there with you. I was running a tensile test in our lab, piece of stainless steel that was square in cross section, maybe 0.1"x 0.1". I had multiple engineering co-ops (interns) in the room at the time, had them take bets for what the failure load would be. I think the highest anyone guessed was 100 pounds. It broke at give or take 1300 pounds if I recall correctly. When I then explained maybe three or four of these tiny things could pick their car off of the ground, their eyes shot out of the faces pretty well. I seem to remember Ford or Chevy running an add talking about how strong the bolts holding the bed to their truck were, and that they could pick the truck off of the ground by just the bolts holding the bed on. If you do the math, making those bolts out of dried bubblegum would've been close to being sufficient. Good times.
</rambling story>
 
Dustin Lobner said:
But all still fun indeed! Some day I'll have to build/fly one to 25k or something just for the challenge of it, but that's not the goal here.

<rambling story>
You made a comment before about trusting materials - I'm actually a Materials Engineer, so I'm right there with you. I was running a tensile test in our lab, piece of stainless steel that was square in cross section, maybe 0.1"x 0.1". I had multiple engineering co-ops (interns) in the room at the time, had them take bets for what the failure load would be. I think the highest anyone guessed was 100 pounds. It broke at give or take 1300 pounds if I recall correctly. When I then explained maybe three or four of these tiny things could pick their car off of the ground, their eyes shot out of the faces pretty well. I seem to remember Ford or Chevy running an add talking about how strong the bolts holding the bed to their truck were, and that they could pick the truck off of the ground by just the bolts holding the bed on. If you do the math, making those bolts out of dried bubblegum would've been close to being sufficient. Good times.
</rambling story>
Click to expand...

I love a good materials discussion.

Nothing makes me happier than watching confused people stand over their wreck of a rocket after improperly tensioned allthread fails. Or people who use 5,000 pound test tubular nylon with a 500 pound eye bolt in a 3/16 plywood bulkplate without washers.

Or epoxy...I love a good epoxy debate.
 
If you need low and slow , call pml and get a 11.5 inch cone , and 2 48 inch long tubes. Build it just like a standard rocket. Coupler in the middle housing your av bay . If mental math is correct , you can build a upscale loc bruiser for those exact components. A baby M ( M1297) at around 40 pounds loaded should net you about 2500 to 3000 feet .
 
Dustin Lobner said:
Hi everyone,

Some background on me - I started out flying HPR in 8th grade through the fantastic Rockets for Schools program in Sheboygan, WI. Got my L1 cert my senior year of high school, L2 freshman year of college. That was during the ATF lawsuit...money, time, and a LEUP in college were in short supply so I backed out of the hobby. I've finally got to a point in my life where I think going for an L3 is a possibility, so I'm going to do it. I'm getting married next summer, so I'm going to have to cashflow this project without a lot of cash to flow, so this might end up being a 2021 launch.

As I had lapsed my TRA membership, I had to refly both my L1 and L2 certs. L1 was August 24, L2 was September 7, both flown with the same rocket. Flew both with the LARS NAR group out of Sheboygan, WI and had a blast. Jeff and Kevin there have been amazingly helpful.

View attachment 393205

I've started doodling around in RockSim with some ideas on an L3. I started looking at some of the L3 projects here and have become concerned I might be thinking too "big", though there is a purpose behind that.

What I've doodled up is a 7.5" dia rocket that is taller than 12 feet (see attached RockSim screenshot). Based on other builds, I'm going to guess that it's going to come in around 60-80 pounds maybe? Right now construction is PML phenolic with a couple 4oz layers of glass because that's what I know and am comfortable with. Also includes their piston ejection system.

View attachment 393206

The goal of this configuration (7.5" diameter, heavy) is to keep the altitude of the flight low. Where I will likely be doing this flight (Bong State Rec Area in Wisconsin) is one of the "less good" places to recover a rocket and only has a 10k ceiling, so low and slow is the order of the day.

First question of the day - yes, this is a big/heavy rocket, intentionally. Am I over doing it? I don't have any experience in this impulse range (obviously), so I'm asking for other's experience. Looked over half a dozen L3 builds here and this one is bigger than all of them.

I'll be making contact with a couple of TRA TAP members next Saturday, but trying to get my ducks, if not in a row, at least in the same pond before then. Thanks everyone!

Dustin
NAR 107803
TRA 9503
Click to expand...
P.M. sent...
 
Cameron Anderson said:
Nothing makes me happier than watching confused people stand over their wreck of a rocket after improperly tensioned allthread fails.
Click to expand...


Aight, now I'm confused. What the heck kind of rocket were they building where the tensioning of their threaded rod was a structurally critical factor....?
 
I saw a fin can several years ago that was built so the cardboard airframe would just be slid over the top. The airframe was totally removable to access the fun can and the electronics around the motor (for airstarts). It used 4 long pieces of what had to be 4-40 allthread intended to support both thrust and the yank of recovery. I think there were only 3 centering rings. 54mm motor lit and crumpled the airframe like it wrapping paper.

I remember allthread reinforcement being all the rage and never understood why.
 
Cameron Anderson said:
I love a good materials discussion.

Or epoxy...I love a good epoxy debate.
Click to expand...

Lol - In my short time on the forum, I've already seen a good collection.

In my first job, I spent a fair bit of time working with a guy who had a PhD in Adhesives (or Chem E and his thesis was in adhesives?). He told me that, when bonding aluminum, he would rather bond to a properly cleaned mirror finish as compared to something that was sanded down and given a quick wipe down. I ran the lab that had a 60k pound tensile machine, so I tested it. And he was right, and by like double the bond strength, not even close. The absolute best thing that we could do was to clean it as perfectly as we could, and then put epoxy on the face and then sand the face through the epoxy - it would take off the aluminum oxide layer on the surface of the aluminum and not allow it to re-form because it was shielded from atmospheric oxygen via the epoxy. If I recall though it was like 5% higher than the method of "just clean it well", aka not worth the effort.

Dustin
 
3stoogesrocketry said:
If you need low and slow , call pml and get a 11.5 inch cone , and 2 48 inch long tubes. Build it just like a standard rocket. Coupler in the middle housing your av bay . If mental math is correct , you can build a upscale loc bruiser for those exact components. A baby M ( M1297) at around 40 pounds loaded should net you about 2500 to 3000 feet .
Click to expand...

Nice! I'll talk it over with my TAPs. I was hoping to use LOC - they're about 15 minutes from where I work, so it's easy/convenient to stop by and pick up stuff as I need it.
 
Dustin Lobner said:
I think the highest anyone guessed was 100 pounds. It broke at give or take 1300 pounds if I recall correctly.
Click to expand...
Have you thought about titanium bike spokes? Each has a breaking load of around 280kg. The cold-rolled threads actually work-harden the material. I have used them in a couple of projects. Find a bike shop with the rolling machine and they will make them to the lengths you require.

Cameron Anderson said:
4-40 allthread intended to support both thrust and the yank of recovery.
Click to expand...

All thread in compression? Interesting. Can't say I have seen it.

You might want to consider taking the motor thrust to a mount at the top of the motor. That's what I generally do, leaving a gap between the "thrust ring" of the motor and the rear of the motor mount tube. That has the advantage of putting the entire tail section in tension, which is where composites work best. Have a look at my L3 thread, where I think I discussed it : https://forum.ausrocketry.com/viewtopic.php?f=6&t=4612
FYI my L3 documentation pack is stored here: https://www.rocketryforum.com/threa...-links-no-third-party-comments-please.137458/
 
Yeah - all-thread in compression doesn't seem like a good idea.

Had not thought of bike spokes. Interesting idea...I'm not worried about this rocket being too heavy though, so probably going with all-thread. Neat idea though, I'll file it away for future use.

Thanks everyone!
 

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