L3 Build: LOC 5.5" Sandhawk

[krislhull]

This thread is a long time coming for me!

Earlier in June 2018, I started planning my L3 project, and decided I wanted to build a large rocket. I decided to use the Madcow 8" DX3 as a starting point and modify it by adding nearly 40" in length, and using CNCed interlocking fin pockets and centering rings from Upscale CNC.


After taking a step back, and adding up the costs of all of the components, plus a 98mm case and a M1939, I realized that this was going to be an expensive build, and also a large one that would not be easy for me to transport any time soon. So, I took a step back and looked at my options. On the grounds of economy, I already had a LOC 5.5" Sandhawk that I had just started to work on, and had just finished fiberglassing the airframes on. I immediately stopped all work on that project, and wrote a new proposal for an L3 attempt using the Sandhawk. This would benefit me in several ways. First, the total cost of the project would be about half of the DX3, once I bought a 75mm case and a motor. Second, I already had the kit, and third, the wife would be extremely happy I wasn't breaking the bank on one rocket!

After submitting my proposal packet to a local L3CC member, I had a false start and a bunch of what I will call miscommunications between the two of us. One of the issues was that I had already started the build, which is a no go under NAR L3 rules. After it was pointed out to me by Steve Shannon that that was perfectly ok under TRA cert rules, I decided to switch up and do the cert under my TRA membership. After some more communications with my L3CC member, who is also a TAP, and asking Steve to be my second TAP, I have been given the green light to proceed.

The Rocket:
As mentioned, I am using the LOC 5.5" Sandhawk as the basis for the build. I decided to do away with the plastic nosecone, and picked up a Madcow fiberglass nosecone. I also will be using a SC Precision thrust plate with an Aeropack retainer.




The basic components, still with the plastic nosecone


Thrust plate and retainer

As I mentioned above, I had previously fiberglassed the airframe. I used Soler Composites heavy sleeve for the fiberglass.




Airframe section in work.

Of course, I had to do the obligatory dry fit photo of the project after I fiberglassed the tubes...


So, what's next?

I need to start assembly of the motor mount and fin can. I will most likely be fabricating plywood fin pockets for fin mounting, and also need to decide if I want to bevel the fins or not. Then there is also the avionics bay that will need to be built. I have already decided to use my tried and true combination of a MissileWorks RRC3 and a StratologgerCF for my altimeters, as well as the MissileWorks RTx tracker.
For recovery, I have a custom harness from Teddy at OneBadHawk on hand, and I plan on using Rocketman chutes for recovery.

The cert flight is currently planned to use a M1550R, which should see an altitude of right around 8,000ft. One of the requests from my primary TAP is that I fly this on a 75mm L first, and I was already planning on such. So its first flight will be on a L1390G, hopefully at the Fire in the Sky launch on Memorial Day weekend.

 

[rharshberger]

Subscribed!

 

[Steve Shannon]

Subscribed, go figure!

 

[iqsy59]

Intriguing build...subscribed!

 

[DRAGON64]

Love the Sandhawk, it will be interesting to see where you take this. Most definitely looking great right now.

 

[krislhull]

Got a little more prep work done yesterday and this morning. While test fitting the thrustplate, I discovered that the motor mount OD is slightly too big for to fit the plate, so I would need to make a quick modification. after measuring how far the mount will go into the thrust plate, I took a band clamp and placed it that far down the motor mount.


I then used the edge of the clamp as a straight edge to use an exacto knife to cut into the tube and remove about three layers of the cardboard wrap. I then made another test fit, found that it was now the correct size, then wicked thin CA into the freshly exposed tubing for strength.


After letting the CA cure, I went over it with 120 grit sandpaper to smooth it down. I then inserted the motor mount assembly into the airframe to mark off the locations of the fins accurately. As part of the 2017 TRF Secret Santa, I received a very nice set of custom cut fin alignment guides for this project. I still have no idea who my secret santa (although I do have an idea) was that year, but thank you again! These are amazing!



Using the guides to keep the fins aligned, I took a sharpie and marked the motor mount where each fin mates to it. These will be used later to help position the fin pockets. I also took the time to fit the thrust plate to the airframe to see how it will look.

After pulling it back apart, I used a long piece of aluminum angle to fully mark off the motor mount for the fins.



So, next step is to locate the middle centering ring on the motor mount, then I need to pick up some .125: or .25" plywood and fabricate the sides for the fin pockets. If I cannot make them work, my backup plan is to install the fins conventionally, and reinforce the joints with fiberglass.

 

[rharshberger]

Kris, you can use balsa or basswood for the sides of the fin pocket, all you are doing is containing the epoxy to allow it to self fillet, saying that I used some pieces of poplar on my Cherokee-D upscale iirc (because I had it laying around, just about any scrap material will do). Just did fin pockets on the NucSledge and used US Composites 150 Structural with 1/32" milled FG mixed in and the sides of the fin pockets were balsa.

 

[krislhull]

Kris, you can use balsa or basswood for the sides of the fin pocket, all you are doing is containing the epoxy to allow it to self fillet, saying that I used some pieces of poplar on my Cherokee-D upscale iirc (because I had it laying around, just about any scrap material will do). Just did fin pockets on the NucSledge and used US Composites 150 Structural with 1/32" milled FG mixed in and the sides of the fin pockets were balsa.

I actually forgot all about basswood... I dont have any scrap material laying around other than 3/4 ply, so I will have to go out and buy something regardless. I guess I will have to see what they have in the correct lengths at the store!

 

[3stoogesrocketry]

I would personally j

Kris, you can use balsa or basswood for the sides of the fin pocket, all you are doing is containing the epoxy to allow it to self fillet, saying that I used some pieces of poplar on my Cherokee-D upscale iirc (because I had it laying around, just about any scrap material will do). Just did fin pockets on the NucSledge and used US Composites 150 Structural with 1/32" milled FG mixed in and the sides of the fin pockets were balsa.

How do the pockets add any strength ? It seems to me that all you are doing is making the joint weaker . If you are pushing the fin back and forth , the glue will act like a door stop and keep it stiff. If you pull straight out on the fin , you are relying on the shear strength of your glue to ( in your case ) balsa wood. I would use the tried and true method of 4 internal fillets per quadrant ( 16 total ) and the 8 outter fillets . I would personally worry more about the rear tips of the fins . I have been flying sandhawk for a long time . If they come down with a sideways movement , the fins flex and like to destroy the ends of the fillets and can/will break the wood tips.

 

[rharshberger]

I would personally j


How do the pockets add any strength ? It seems to me that all you are doing is making the joint weaker . If you are pushing the fin back and forth , the glue will act like a door stop and keep it stiff. If you pull straight out on the fin , you are relying on the shear strength of your glue to ( in your case ) balsa wood. I would use the tried and true method of 4 internal fillets per quadrant ( 16 total ) and the 8 outter fillets . I would personally worry more about the rear tips of the fins . I have been flying sandhawk for a long time . If they come down with a sideways movement , the fins flex and like to destroy the ends of the fillets and can/will break the wood tips.

They are not any weaker or stronger than the regular fillet technique, just easier. I can do the pockets out side the airframe insert the motor tube with CR's into airframe, put epoxy in pocket and insert fin, causing the epoxy to squeeze out from the fin forming the fillet between the fin and the airframe and the fin and the motor tube, and it doesn't require trying to manipulate epoxy into a difficult to reach location. When properly done fin pockets use no more or less epoxy than traditional internal fillets.

I have yet to see on of my rockets (or others that I know of) built with fin pockets pop a fin, snap on off externally yes, pop out of the airframe on a bad landing or recovery failure, no. As for the fin tips, well that's why swept back fins aren't recommended for cert flights they are an easy point to break, I bet Kris is planning a larger than nneded chute for that reason, and it would take really bad luck to hit anything hard at the sod farm or FITS launch sites.

 

[crossfire]

Have you test fitted MC fiberglass cone? Just wondering how it fits?

 

[BradMilkomeda]

It looks like you still have the glassine later on the motor tube. Are you planning on removing it for a better bond?

 

[krislhull]

Have you test fitted MC fiberglass cone? Just wondering how it fits?

The nosecone fits great! No issues what so ever with it.

It looks like you still have the glassine later on the motor tube. Are you planning on removing it for a better bond?

That is the current plan.

 

[krislhull]

As for the fin tips, well that's why swept back fins aren't recommended for cert flights they are an easy point to break, I bet Kris is planning a larger than needed chute for that reason, and it would take really bad luke to hit anything hard at the sod farm or FITS launch sites.

Rich is correct. I am worried about the integrity of the fins, and as such, I am planning on using a larger chute than I need. Per the descent charts on Rocketman’s site, a 14ft chute would be optimal for this bird, assuming my estimated weight of 35-40 pounds is correct in the end. As such, I am planning on using a 16ft chute, a little overkill, but it should bring the rocket down at a nice gentle roughly 14fps. Plus, as Rich mentioned, my primary field for my cert flight should be either freshly plowed, or have a nice cushion of sod.

 

[krislhull]

Well, progress has been slightly slower than I would like, but since I am still recovering from the case of hand foot and mouth that my daughter graciously gave me for Christmas, as well as dealing with laryngitis this past week, I havent gotten much done!

Today I decided I needed to get some work done on the build, so I went out to the local hobby shop to see what they had in terms of wood to use for the fin pockets. In the end, I walked out with 1/4" x 1" balsa strips that I cut down to the length of the fin tabs.


I then measured out the location of the middle centering ring from the end of the motor mount tube, and prepped it to remove the glassine layer. I used a band clamp to act as a guide to lightly score the layer with an exacto blade in teh areas I wanted to remove the glassine, and then started to peel it off.



Once that was done, I tacked the centering ring in place with a couple of dabs of 5 minute epoxyn and set about removing glassine from where the fins will sit. Once that once done, I tacked the fist set of fin pockets in place using CA





Once the first pockets were tacked in place, I fit tested the motor mount into the booster section to see how it will all look.



Next up, I need to tack the rest of the fin pockets in place, then go around all of the joints with epoxy to permanently secure them in place. I will be using US Composites 150 epoxy with chopped fiber mixed in. Once the top and middle centering rings are locked into place with the same epoxy, it will be time to insert the motor mount into the booster for good.

 

[rharshberger]

Looks like your cruising along even with being sick.

 

[Steve Shannon]

Kris,
Do you epoxy the fin pocket sideboards just to the motor mount tube or at the top to the airframe?

 

[krislhull]

Steve,
The sideboards have not been epoxied in place yet, only tacked in place with CA to make sure I have the positioning correct. Once I have all four sets tacked in place, I will go through and thoroughly epoxy them all to the mount and centering ring with US Composites structural epoxy mixed with milled fiber. Once the whole assembly in installed in the airframe, and the fins are in place, I will run a bead of epoxy in the gap between the sideboards and the inner wall of the airframe.

 

[Steve Shannon]

They will be strong. I think when you insert the root of the fin into the pocket, epoxy will ooze up to the airframe anyway, locking everything in place.

 

[Steve Shannon]

Also, be sure some of your pictures show you doing the work. That’s a requirement. You’re doing a nice job.

 

[rharshberger]

Kris, the epoxy will push out of the pocket when you insert the fin, make sure the mmt side is sealed and the airframe side will take care of itself, also make sure you have enough epoxy in the pocket that it squeezes out, that insures the pocket is full and secure once cured.

Steve, this method was recommended to me by Kent Newman who was my L3CC. And I have used it for the vast majority of my HPR builds and many MPR builds. The only way to remove a fin done this way I have seen is to break a pretty large section of airframe out with it, fins will not break loose at the mmt they tend to snap off right at the external fillet.

 

[Steve Shannon]

Kris, the epoxy will push out of the pocket when you insert the fin, make sure the mmt side is sealed and the airframe side will take care of itself, also make sure you have enough epoxy in the pocket that it squeezes out, that insures the pocket is full and secure once cured.

Steve, this method was recommended to me by Kent Newman who was my L3CC. And I have used it for the vast majority of my HPR builds and many MPR builds. The only way to remove a fin done this way I have seen is to break a pretty large section of airframe out with it, fins will not break loose at the mmt they tend to snap off right at the external fillet.

Thanks, Rich, I agree, very strong. I haven’t used it myself, but I certainly have no problems with it. You explained better what I was getting at with my “ooze” comment. If Kris had to go in there and apply epoxy to the tops of the sideboards to seal them to the airframe he might as well be doing internal fillets, but if the epoxy just runs over and locks everything together, that’s easier.
I think it would be easier yet if a person started with a 3/8 inch wide piece of balsa and cut a dado in the center just larger than the thickness of the fin, then epoxied the channel to the motor mount tube. That would avoid the \_/ shape caused by using discrete slats.

 

[krislhull]

The motor I will be using on the shakedown flight of this rocket, a L1390G, arrived today among a larger order I placed with Balsa Machining a few months ago. This will be my largest motor to date when I fly it later this year!



I finished tacking the boards for the fin pockets in place today, and I realized that I inadvertently cut the boards 0.2" too short. So I cut a few 0.2" strips off the scrap balsa I had, and used CA to secure them in place on the ends of the boards. Once they are fully epoxied in place, it will be on seamless bond.



After finishing tacking the boards in, and fixing my small mistake, I did another dry fit of the motor assembly in the booster section. I need to make an adjustment on one of the side boards, but that is all. Later this week, I will start epoxing it all together!

 

[krislhull]

Over the past four days, I have been applying the internal epoxy fillets to the fin pocket boards. Using US Composites 150 epoxy mixed with milled fiberglass, I did one bay per night.






Today, after I applied the epoxy to the final two side boards, I set about starting to fabricate the attachment points for the rail buttons. On my previous high power builds, such as my 4" ARCAS and Aerobee 150, I would epoxy a plywood tab to the top and bottom centering ring, then dry fit the motor mount and drill the holes for the buttons, then pull the assembly back out and install threaded inserts into the tabs. For this build, I picked up a 1 inch strip of .25" plywood, and cut it into two pieces to fit in between the two sets of centering rings. Once they are epoxied into place, I will then slide the mount assembly into the booster airframe, drill the holes, then pull the assembly back out and install the threaded inserts before I fully install the motor mount into the airframe.



I also need to figure out a spacer system for the thrust plate, since it will not butt up against the aft centering ring. To go about this, I took the same plywood and cut three spacers that will be epoxied onto the aft centering ring, and then they will be drilled for the thrust plate mounting screws, and the threaded wood inserts used on the rail buttons will again be used. The design of the thrust plate makes it so that it can be bolted on and off a rocket, but I think this one will be permanently attached. Final installation of the plate will be with screws and JB Weld.

 

[Theory]

Really like this build with the thrust plate. Good call there!!!

 

[krislhull]

Thanks. I've been eyeing the SC Precision thrust plates for a while, and I figured this would be the perfect project to try one out on. I am very pleased with how it fits, and looks, on the project.

This morning I went finished epoxying the centering rings in place using Rocketpoxy. I then epoxied the strips for mounting the rail buttons in place.



Then this afternoon, when the epoxy was cured to a hard stage, I inserted the mount into the airframe, and drilled the holes for the rail buttons. Once I pulled the motor mount back out, I enlarged the holes to the proper size for the threaded inserts, and installed them.




Once the inserts were installed, it was time to slide the whole assembly back into the airframe and test fit it with the rail buttons installed.



A total of three 1515 rail buttons will be used on this project, with the third sitting just forward of the forward centering ring, so that one will be installed once the motor mount and forward coupler are permanently epoxied into place.

 

[rharshberger]

Coming along nicely Kris.

 

[krislhull]

Coming along nicely Kris.

Thanks, Rich. At this rate, I might be doing the shakedown flight in April or May at the Sod farm if I can get my cases in time!

 

[Rocketjunkie]

I've been building LOC kits for 30 years. Even 5 minute epoxy is stronger than the plywood and paper parts. The base materials break before there's an epoxy failure.
BTW, here's my 7.5" Goblin (32 lb. prepped but no motor) on a L1390G (44" drogue, 15 ft flare chute main.)

 

[Steve Shannon]

Very nice build, Kris. You’re doing a great job of anticipating things. The backing for the rail buttons and the spacers for the thrust plate were nice. I’m not a fan of thrust plates from a function perspective, but they look nice.