Build Thread: SBR Thor 54

[NateB]

I purchased this kit over the Winter and it is time to start a build for it. This is a 54mm Thor with a 38mm motor mount. The kit is fairly complete for single deploy and includes recovery and a laser cut altimeter mount. Motor retention is up to the user, I'm using an Aero Pack retainer. The body is in two sections and it comes with a 3D printed coupler which also serves as the shock cord mount. The instructions state to attach the nosecone to the payload bay with included rivets and break the rocket at the coupler. The vinyl wrap runs the length of the rocket. Rather than cutting the wrap to allow for single or dual deploy at the coupler, I'm going to epoxy the coupler, booster, and payload together, and eject the nosecone with the Eggtimer Apogee and use a Chute Release for shorter walks. I could probably build a baffle into the coupler, but I'm planning on using plugged motors anyway. After all, a Thor needs to be with Loki. I have a Loki G70 Cocktail for the first flight, but this rocket is begging for a sparky.

Here's the first draft of the sim file which doesn't account for adhesives, paint, and the wrap.

 

[NateB]

First up is assembling the Eggtimer Apogee and the Easy Mount sled.

The tiny battery does not come with any charge, unlike the larger Lipo batteries. With the first Apogee I built, I learned that these batteries don't charge properly with the smart charger I have due to the built in circuit protection in the battery. I picked up the USB charger with this order and everything worked better.



Assembling the Apogee is easy well documented on the Eggtimer site. All of the components except for one are through-hole, so this project shouldn't give anyone too much trouble.



Apogee is complete and powers up through its tests and as expected, beeps that there is no continuity.



Moving on to the Easy Mount. The instructions are pictures are also well documented on the Eggtimer site, so there is no need to repeat each step here. Included are the 3D printed parts, all of the hardware, a bulkhead mounting ring, and a 24mm motor mount to add some protection to the board.





Once assembled, everything fits nicely in place. The Apogee powered up, showed continuity with an e-match, and passed the vacuum test. Total weight for the Apogee, battery, and sled is 30g.



The 24mm motor mount is a snug fit, but it slides on. Punch a hole for access to the screw switch. The other piece is the bulkhead mounting ring and 2 screws to hold it together. I'll get to that later in the build.

 

[NateB]

Moving on to the rocket itself. Starting with the motor mount, mix up some JB Weld, spread a thin layer on the motor mount and the inside of the retainer and slide the retainer on the motor mount, twisting it to spread the epoxy out.



A little epoxy will get pushed out of the top of the retainer and it is just perfect for rear centering ring. Push the centering ring against the retainer and make a small fillet. Clean up any epoxy that gets where you don't want it with a paper towel and a little alcohol. I set this aside to cure and will mark the location for the other center rings later.

 

[NateB]

While the motor mount was curing, I prepped the fins by rounding the leading and trailing edges with the NCR sanding guide.

 

[NateB]

Cardboard and plywood rockets certainly move along faster than fiberglass ones. To finish up the motor mount, first I slid the centering rings on and insert the motor mount into the booster body tube. No glue yet. The nice reference lines were already marked on the tubes from SBR.



I marked the location for the fins through the slots and made sure the centering rings would clear the fin slots once glued into place. For split fin fiberglass kits, I like a 4th centering ring to keep them snug against the fins. A tight fit against the fins is necessary when injecting internal fillets or using fin pockets. With this rocket, doubled dipped wood glue on the fin roots and good external fillets will be all that is needed.



I dry fit the fins and numbered the fin slots. I'll sand the burn marks from the laser cutter off the root edge of the fin tabs and that slight gap should close up giving a perfect fit.



Now that the motor mount is marked, the centering rings are glued into place with small fillets.



The last step for tonight was drilling a few pilot holes for the rail buttons. The forward rail button is located at the CP as calculated by Open Rocket. I also drilled a small hole near the top of the booster tube as a vent to make inserting motors easier.

 

[NateB]

I don't like my rockets to have a bare rear centering ring, and it is a hard spot to paint with the retainer masked off after the build. I brushed some black paint on the rear centering ring before assembly into the booster. I also scuffed up the glassine layer of the motor mount where the fin roots will attach.



Using a stick with glue, I applied a glue on the inside of the booster tube for the forward centering ring. I also applied a thin layer of glue to the outside of each centering ring as the motor mount was inserted. There will be plenty more glue used once the fins are inserted to make sure everything is secure.

 

[NateB]

I lightly sanded the root edges of the lower fin set, just enough to take off most of the burn marks from the laser cutter. A final dry fit shows they fit just right.



I applied a bead of wood glue to the root edge of the fin tab and "triple dipped" the fin through the slot making sure to wipe any excess glue into the slot.



The Badass fin guide just fit. I made sure to slide each fin against the bottom of the slots. I'll let them dry over night and move on to the upper fin set when I get off work tomorrow.

 

[NateB]

The upper fins are mounted the same way, only using binder clips and stir sticks to keep the 2 fin sets aligned while the glue sets.

 

[NateB]

This kit uses a 3D printed coupler and nosecone. The instructions state to brush thinned 15 minute epoxy on the inside and outside surfaces of the 3D printed parts to smooth the grooves from the printer and provide some additional strength. The builder is also told to brush some epoxy on the fins to seal them.

I was using US Composites epoxy which is already thin enough. I masked off the body tube to avoid epoxy running where I didn't want it to, but that proved not to be necessary.



Epoxy was brushed on each fin and around the edges. I only wanted a thin layer, just enough to seal the plywood. It was brushed out as smooth as I could get it and I used just enough to give an even wet appearance across the fin.



I did the same with the inside and outside of the nosecone. The epoxy is brushed on and smoothed out to fill the grooves. There is a very thin layer of epoxy on the shoulder of the nosecone as well. I'll be adding a bulkhead to the nosecone for recovery attachment and to hold the Eggtime Apogee.



I also brushed epoxy on the inside of the coupler and on the band on the outside. I'm going to permanently fix the coupler in place, so no epoxy was needed on the shoulders until I'm ready to attach it to the booster and payload tubes.



Once cured, everything will be sanded smooth and I will be able to move on to the external fillets.

 

[bobbyg23]

Looking good.

 

[NateB]

This was my weekend to work, so I didn't have too much time for hobbies. I was able to get more done this evening.

No pictures necessary, but the fins were sanded with 220 grit sandpaper to smooth out the epoxy coating.

Using the Easy Mount pieces as a template, I marked the bulkhead where it needed to be drilled. The outside circle represents the lip on the 3D printed nosecone.





I used my drill press to drill a 1" diameter hole for the Easy Mount sled and holes to accept the hardware and recovery attachment.



The hole for the sled needed a little sanding, but otherwise everything fit perfect.







I sprayed a layer of black paint on the outside of the bulkhead and will epoxy the ring for the Easy Mount once it dries enough to handle.

The bulkhead will then be epoxied onto the nosecone. I haven't worked with 3D printed parts too much. Will epoxy hold the G10 bulkhead to the 3D printed nosecone well enough to handle the shock of deployment or will I need a mechanical attachment as well?

 

[NateB]

The ring for the Easy Mount is simply epoxied into place on the bulkhead. First, I roughed up the surface of the bulkhead and the edge of the adapter ring with 100 grit sandpaper and mixed up a small amount of Rocketpoxy. I chose Rocketpoxy because it is thick enough to not run, has a long enough pot life to make adjustments to the position of the ring, and cures fairly quickly.



A small amount of Rocketpocky on the ring, taking care to not get any on the inside or the threaded inserts.



I used the sled itself and the provided screws to ensure the ring is properly aligned.



I let the epoxy set up some and carefully removed the sled to make sure it was not accidently epoxied into place. I then used the screws to hold the ring into place while the epoxy fully cures. You can see a thin fillet around the outside of the ring as well.

 

[NateB]

To wrap up the nosecone, I tied a loop of Kevlar cord as an attachment point for the shock cord and parachute.



I measured where the switch ends up and drilled a hole through the shoulder of the nosecone to access the screw switch and serve as a vent for the altimeter.



I mixed up some more Rocketpoxy and epoxied the bulkhead to the nosecone, making sure the mark for the switch and vent hole line up. I also made a small fillet around the nosecone shoulder and G10 bulkhead and smoothed it out with my finger. I'll sand the shoulder as needed for best fit once everything has time to cure.



The final weight of the nosecone after a coating of epoxy, the bulkhead, and Easy Mount ring is 108g, up from 96g for the bare 3D printed nosecone. The Apogee and Easy Mount sled is another 32g.

 

[NateB]

There is a little prep work to mark and tape the area for the external fillets. I'm using a fondant tool to shape the fillet. First I took a piece of paper and made thick pencil mark to be rubbed off on the body tube and fins.



Then use the same tool you will be using to shape the fillet to transfer the pencil line from the paper to the rocket.





The mark is easy to see, and the process is repeated on each fin.



I roughed up the glassine layer of the body tube and the fins using sandpaper where the fillet will be. Last, I taped along the lines to mask places where I don't want epoxy.

 

[thzero]

Nice tip on the pencil lines. I too use a fondant tools.

 

[NateB]

Nice tip on the pencil lines. I too use a fondant tools.

Thanks. That's a tip I picked up on the forum from one of the videos posted by Ian / badassrocketry. I like it better than using a marker on the shaping tool to transfer lines.

 

[NateB]

Time for the first set of fillets. I mixed up some Rocketpoxy and made sure to have everything I needed ready. The fondant tool, a cup of rubbing alcohol, and some paper towels for cleanup.



I let the epoxy sit for 15 minutes to thicken up and for the air bubbles to work to the surface. Then apply to the rocket. I'm not great at judging how much to mix and apply it liberally. It is easier for me remove excess epoxy and get smooth results by using more than necessary.



I waited another 15 minutes for the fillet to self level and thicken a little more before removing the excess epoxy and shaping the fillet with the fondant tool dipped in alcohol.





Once each fillet it shaped, I waited about 5 minutes before pulling the tape. I then smoothed out the ends and any low spot I could easily see with my finger dipped in alcohol.

 

[kalsow]

Thanks. That's a tip I picked up on the forum from one of the videos posted by Ian / badassrocketry. I like it better than using a marker on the shaping tool to transfer lines.

I could never get markers to work well. I've used pencils like you did, but it reminded me too much of sanding. Now, I used old-fashioned carbon paper! It's cheap and a single sheet will do many rockets. Ask for it at your local office supplies store. The teenagers running the store will look puzzled, but usually there's an older person who will know what you're seeking.

 

[NateB]

Each set of fillets went on the same way and are cured. I'm happy with them so far. I'll decide if I want to fill and smooth any spots after the first layer of primer.



Moving on to the coupler. I'm deviating from the instructions here. Remember from the first post, I didn't want to cut the vinyl graphics to allow for separation at the coupler. It would be easy enough to do and install an altimeter in the coupler for dual deploy if you wish. I will be relying on the Apogee and using plugged motors instead.

The coupler has a hole for the included Kevlar shock cord which is held in place with a knot. I also ran a piece of tape just in front of the knot to fill any gap between the cord and the hole in the coupler for the next step.



I'm sure the knot would hold and the 3D printed coupler is strong enough for its intended use. I thought I would fill the coupler with foam, just to add a little more strength to the part and security to the shock cord. I was also curious if my aging bottles of PML foam were still good. I could do the math to figure the volume of the coupler, but I filled it with water instead. It held 200 ml. I mixed up 20ml of foam, poured it in, and allowed it to rise.



Once the foam was cured, I cut off the excess and sanded the end smooth. The coupler with epoxy and foam now weight 66 grams, a slight increase from 48g bare. I also roughed up the shoulder that will go into the booster to help the epoxy adhere to the plastic.

 

[NateB]

Now wrap up the build portion of this rocket. I mixed up some Rocketpoxy to attach the coupler because it is strong and doesn't run after you apply it. I applied a thin layer of epoxy to the booster side shoulder of the coupler and slid it into place, twisting as I went to spread the epoxy out evenly.



I wiped the excess off and tried to push as much as I could into the gap between the band of the couple and the body tube.



While this was setting up, I moved on to the payload section. I measured the location for the hole to access the altimeter switch, first by eyeballing it and then confirmed with a ruler. I made sure to place the hole opposite of the rail guides so it can be reached after the rocket is on the rail.




The hole is drilled and I verified that everything lined up. I drilled a 1/8" vent hole in the payload bay, in line with the rail guides. I also used some CA to reinforce the cardboard around the access hole and the forward end.



The coupler isn't fully cured to the booster section, but it has set up enough to proceed. Same as before, I roughed up the payload side shoulder of the coupler and spread some Rocketpoxy on it.



Payload body tube is twisted on the coupler and the marks lined up so the access hole is away from the rail guides. The excess epoxy is wiped off and the rocket is set aside for fully cure.

 

[NateB]

The coupler is now cured and the rocket could be flown. The weight so far with recovery and electronics is 706g.

With the small payload bay according to the BP charge calculator, I only need a tiny charge of 0.16g. That seems light to me. I was going to try 0.5g first which is even smaller than what is included with the motor.

 

[NateB]

I tested a 0.2g ejection charge.



The result was as I expected.

 

[NateB]

I've been trying to get a few layers of primer on while working around my work schedule and the weather.

The fin can is the most important part. This section will be painted yellow once the primer is dry.



The rest of the rocket will be covered with a vinyl wrap. The gap between the coupler and the body tubes is still visible and I have a fuzzy section on the body tube. I'm not too worried about it. I'll sand it smooth, but the wrap will cover most of it.

 

[NateB]

I thought the primer was dry enough to start the color coats, but I noticed some pinholes form while spraying the color on last night. It didn't look as bad today after the paint has dried, but I still have some sanding to do before the next layer goes on. The humidity should be down this week, so I hope the paint behaves better.

 

[NateB]

I've been in the paint, wait, sand cycle, so I haven't had anything worth updating lately. I did another ground test of the ejection charge today. As I guessed, 0.5g of 4fg ejects the nosecone and recovery bundle with a satisfying pop and wiff of freshly burnt gunpowder. For ease in the field I might just use the charge included with the motor, but I know it will be plenty. The vinyl wrap will becoming up next, wish me luck.

 

[NateB]

Moving on to the decals. First, I drew a reference line on the body tube to line up the wrap. Remember, the tube was already marked out of the bag, but the line was covered since I primed the whole rocket.



Next, I tried to give a little curve to the vinyl by wrapping it around the rocket with the backing sheet still in place.



As I started to peel away the wrap, one edge wasn't scored all the way. No worries, this is an easy fix.



Drats, I can't get the wrap to lay straight. Fortunately it is easy to peel off until you burnish it down. So far, this is the most frustrating part of this build. The wrap on my Lil'Thor didn't give me any problems. I'm going to come back to this part.

The other decals are applied. I drilled the location for the forward rail button on the CP to make it easy to find.

 

[NateB]

Lesson for tonight? I f...... hate vinyl wraps and I'm never going to use one again.

I lined up the vinyl and made a tape hinge to keep the wrap aligned while I peeled off the backing. So far, so good.



Wrapped the vinyl around, holding as much even tension as I could.



Now for the ugly part. The recommended paint color is close to the wrap, but still a pretty noticeable difference. I could live with it, except...



The bubbles. Working from the center out, one bubble disappears and 2 more appear somewhere else. I can poke pin holes to let the air out, but would still have wrinkles.



Any suggestions to even everything out before I rip the wrap off and paint it?

 

[JasonB]

You can try poking a pin hole in it… but it looks to big to smooth out and not wrinkle…

I dont do wraps or large decal over a curve for that reason…

My goblin i painted on the stripe

 

[cwbullet]

I love this rorket. Nice paint jiob.

 

[NateB]

The repaint on my Thor is coming along. Without my paint of choice being easily available, I think it is time to let this last coat cure and work on decals and a clear coat for protection.