Little Dog DD Level 1 Build

[Obskur]

First off, I'd like to introduce myself. My name is Reese, and I'm an alcoho- I mean, born again rocketeer. I flew Estes low power as a kid, and this past year I decided to give it another go. Except this time it has taken over my life. After building 3 low power, and two mid power rockets, and sucking 3 of my friends into the hobby I've decided to go bigger, and fiberglass.

I chose the Little Dog, because it will go on a G, and if I plan the construction right, I can swap it between single- and dual-deploy. This project will be a learning experience, but I love a challenge, and I intend to get L1 and learn dual-deployment with it (and maybe L2, but I'll probably have the itch to build another rocket for that). I bought the kit from Randy over at Always Ready Rocketry, who is great to deal with, and I highly recommend him if you're in the market for a Performance Rocketry or Blue Tube kit.

Enough babbling, on with the build!

Parts List, Kit:
A. Fiberglass 5:1 Ogive Nosecone
B. G10 Fiberglass 54mm Nosecone Bulkplate
C. G10 Fiberglass 54mm Airframe Tube 16” Length
D. G10 Fiberglass 1/16” Airframe Bulkplate
E. G10 Fiberglass 1/16” Coupler Bulkplate
F. G10 Fiberglass 54mm Coupler Tube 6.188” Length
G. G10 Fiberglass 54mm Airframe Tube 25” Length
H. G10 Fiberglass 3/32” Fins (3)
I. G10 Fiberglass 1/16” Centering Rings (2)
J. G10 Fiberglass 38mm Motor Mount 8.125” Length

Some of the above parts DO DIFFER from the parts list that is available online: https://www.alwaysreadyrocketry.com/Products/Little%20DogDD-exp.pdf. In particular, the rings are all 1/16" thick instead of 3/32", and the lengths of the inside tubes are slightly longer than advertised. This is actually important later on for this particular build.

Parts I have added so far:
Giant Leap Slimline Tailcone Retainer, 54mm-38mm
1/2" x 3' Tubular Kevlar® shock cord mount
MAS Epoxies FLAG resin and Medium Hardener

As many know, Performance Rocketry builds a solid kit well known for quality. All of the parts shown below are what constitutes the kit, and are all 100% fiberglass, resin, and gel coat.

I've decided to use MAS Epoxies resin system. Its comparable to the system from West System, but it mixes 2:1 instead of 5:1 which is easier to measure out, and the FLAG resin (which means Filleting, Laminating And Gluing) is just a little bit thicker to aid with filleting and bonding. This combo has a pot life of about 15 minutes, and advertises overnight sandability. Also, I had it laying around

My first step was to sand all the parts with 60-80 grit to accept epoxy, then I began by mocking up the tailcone retainer. The slimline retainer epoxies to the motor tube, the end of the booster tube, and the lowest centering ring. It requires about 1" of motor tube to extend past the aft centering ring, and the aft centering ring should be 1/8" inside the airframe tube. I had to change the design slightly due to how short the engine tube is on this kit compared to the length of the fin tabs. I could either notch the fin tabs, use a longer engine tube, or extend the fin slots and move the fins rearward about 3/8." I chose the latter option. Using a longer tube would have been easier, but I'm trying to keep the weight down a little to fly this on a G.

I slid the engine tube into the retainer, until it hit the internal step, slid a centering ring over, and marked a line for that CR. Then, measured the length of the fin tabs, and marked a line for the top centering ring.

I extended the fin slots by hand with a Dremel®. They're not perfect, or pretty, but the slots are long enough to align the fins, and the gaps will be covered by the fillets later.

My next move was to notch the forward centering ring and epoxy the ring and the 1/2" Kevlar® to the engine tube and let it cure overnight.

My hardworking rocket-building experts used West System syringes to measure out small quantities of resin and hardener.

This is where the differences in the dimensions of some of the parts came in handy. The engine tube measured 0.125" oversize, and the rings were 1/32" thinner than spec. The difference added up to 3/16" of extra room for the fins and tailcone to fit. Without the extra, the centering ring would have just barely made it on the tube. I like having a little extra there. I then re-installed the rear CR and tailcone to check that the fins were still going to fit between the rings. A perfect fit!

To be continued...

 

[Gillard]

Nice build so far
love the lego guys.

 

[OverLord]

Welcome back! Build is looking great! "Tis addicting isn't it? LOL

Mike

 

[Obskur]

Next, I epoxied the engine mount into the booster tube. After sanding the inside of the tube around the area where the front centering ring would be, I used a long stick to smear epoxy around the circumference of the tube. The motor tube went in, and I used the motor retainer to push the engine tube in to the correct depth (without gluing the retainer). I left it that way to cure for a few hours, and then removed the retainer to allow access to the fin tabs for internal fillets.

While the epoxy was curing, I went to work on the altimeter bay. I used the basic design of one 1/4" eye bolt at each end, and two pieces of 1/4-20 allthread running the length of the coupler to hold the altimeter sled and keep the bulkheads on.

One of the rocketeers fell into the altimeter bay with the boss watching!

The completed avionics section.

At this point I needed to prepare the fins. I decided to round the leading edge, and bevel the trailing edge. The Lego engineers laid down a tape line 7/16" up from the trailing edge to use as a guide while sanding.

Most of the material was cut off with a Dremel® sanding wheel, and I used a flat medium file to make sure the bevels were flat. They turned out okay for doing it by hand.

The rocket is now ready to mount the fins.

 

[Handeman]

Great build so far. I really have to get me a fiberglass rocket.

The only thing I would have done different, I would have used 3/16" threaded rod in the av-bay, but that's just me.

 

[Obskur]

Once I got the Av-bay together, I realized that the 1/4 hardware was pretty damn heavy. It's not a permanent assembly, other than the holes in the bulkheads I can remove all of the hardware. I might go down to 3/16 for weight reasons, just the holes will be slightly oversize. That shouldn't cause the sled to move any during flight?

Update:

The fins are installed, fillets have been added, rivet and pressure relief holes have been drilled. It's almost ready for paint. Here is a graphic out of RockSim of a paint scheme I'm considering:

I'll have a few more pics up of my progress tomorrow. Thanks for the replies, all!

 

[Handeman]

Once I got the Av-bay together, I realized that the 1/4 hardware was pretty damn heavy. It's not a permanent assembly, other than the holes in the bulkheads I can remove all of the hardware. I might go down to 3/16 for weight reasons, just the holes will be slightly oversize. That shouldn't cause the sled to move any during flight?

If you have the rod attached securely, it shouldn't make any difference. You can always put a piece of tape on the bottom, fill the holes with epoxy, and redrill them.

 

[kandsrockets]

Once I got the Av-bay together, I realized that the 1/4 hardware was pretty damn heavy. It's not a permanent assembly, other than the holes in the bulkheads I can remove all of the hardware. I might go down to 3/16 for weight reasons, just the holes will be slightly oversize. That shouldn't cause the sled to move any during flight?

You can keep the 1/4" just get some alum all thread.

 

[Obskur]

I've gotten quite a bit done since the other day.

First step was to tack the fins into place. To align them, I drew marks on the bottom edge of the airframe at the center of each fin slot, and then on the opposite side of the airframe, so that they were equally spaced at 6 points around the circle. I then set the booster on the rocket stand and rotated it so that a fin slot was facing up, and used a level to assure that the two appropriate lines were oriented perfectly vertical.

I applied a thin layer of epoxy on the edges of the fin slot, on the root edge of the fin and leading edge of the fin tab and slid it in to the slot. I used the level to make sure the fin was perfectly vertical and taped the fin in place to hold it while the epoxy cured. I repeated this with the other two fins.

After the fins cured in place, I used unthickened epoxy to create fillets inside the airframe. I have an odd way of doing this, which just seems to work for me. I laid the booster on the stand, mixed up an appropriate amount of epoxy, and used a syringe to pump it into the fin joints. I rotated the fin can periodically to let the epoxy flow around and fill all of the joints, and run all the way to the forward centering ring. The epoxy cured to leave nice fillets on all 12 corners.

The next day, I applied epoxy to the trailing edge of the fin tabs, and around the circumference of the motor tube and airframe and inserted the rear centering ring. Without epoxy, I installed and removed the tailcone retainer to make sure it would seat fully on the aft end of the airframe.

Next, I masked off the area near the external fin joints in preparation of the visible fillets.

I then mixed up a batch of epoxy and thickened it with colloidal silica (West Systems 406 Adhesive Filler). I slathered it on with a mixing stick, then used a section of PVC pipe to smooth out and shape the fillets. I removed the masking tape and allowed the epoxy mixture to cure overnight.

A shot of the cured fillets:

These were sanded smooth the next day, any dents or gaps filled with Bondo glazing putty. The rocket is now off to the paint booth!

 

[Obskur]

I spent a few days working on the paint job. First up was a thick layer of Rustoleum automotive primer. This was wet sanded with 320 and then I hit it with another coat to fill some imperfections. The second coat was again sanded with 320, 600 and then scuffed with a Scotch-Brite until it almost had a dull shine to it.

This was followed up by 2 coats of Rustoleum Professional gloss white. This paint is a little thick, and dries a little fast but sometimes that's not a bad thing. It's been a very durable paint for me in the past. The paint was allowed to dry 24 hours, then I wet sanded it with 600, and hit it with the Scotch-Brite. It has a very smooth, but dull finish at this point.

The white was supposed to just be a base coat, but I like the white so much, I am thinking about keeping it. I'm just afraid it will go out of sight too easily considering its size and altitude potential.

I've also acquired an Aerotech 29/240 with the Reload Adapter System (so I can run the 120, 180 and 240 N*s reloads), and black nylon 2-56 x 1/4" nylon screws to hold it together. On the way I have removable rivets, some 9/16" tubular nylon, and a 36" chute.

Anyone have experience with this rocket on a G motor? According to RockSim its looks like it might be too heavy for a G79W in DD configuration (54oz without engine), but if I leave the payload section off its about 1.03 margin of stability on the G79. I don't know if I can handle its first flight being a certification flight!

 

[tsai]

Anyone have experience with this rocket on a G motor? According to RockSim its looks like it might be too heavy for a G79W in DD configuration (54oz without engine), but if I leave the payload section off its about 1.03 margin of stability on the G79. I don't know if I can handle its first flight being a certification flight!

I've seen one fly on G motors without the payload. No issues stock, but you've got the tailcone, which means the motor is another inch or so to the rear. Another option would be to use 29mm G motors, which would have higher average thrust because they have more grains.

CTI makes a G118 and a G125 in 29mm-3g configs. Slightly less Ns, but slightly higher average is the G131. Also, any of the VMax loads would work just fine.

On the Aerotech side, there's the brand new G138 for the 29/40-120 hobbyline case.

Cheers,
- Ken

 

[GRIFFIN]

Are the lego guys done... I could use some help finishing up a few projects...They are Union right? (looked like it- one working- one watching- one drinking coffee)

 

[Diosces]

Nice build! Gotta love those hard working Lego guys.

We have a trio of Little Dogs (one with DD option) that we fly regularly on CTI 38 mm G 69 skids, G 79 smokies. Without the payload your rocket should be very stable with those.

Good luck on first flight

 

[Obskur]

So perhaps I should look at dumping the 29/240 case and go with a Pro38 3g and use the spacers?

 

[TheAviator]

So perhaps I should look at dumping the 29/240 case and go with a Pro38 3g and use the spacers?

No reason to dump the case and spacers, Aerotech has some awesome motors as well. I'm sure it will fly well without the DD on either a G79 or G71. (The G71 has a VERY regressive burn, so it actually peaks higher than the G79; 118N initial peak vs. 92N initial peak.) Just go with the short delay.

Just as an FYI, spacers are NOT interchangeable between systems. Aerotech spacers must be used with Aerotech motors, and CTI spacers must be used with CTI motors.

 

[Obskur]

I think my worry on the short version is stability, not thrust. Like tsai said, the the engine is moved back 1 inch so it takes away from the margin of stability some. I haven't weighed it or measured the CG yet in either configuration because I don't have all the pieces.

I do like that the Pro38 3g would let the DD version fly on a G, H or I though...

 

[MarkH]

A stability margin of 1.0 is just a rule of thumb. You could go below a stability margin of 1 and it will still fly straight. It's not like you cross over to 0.99 and the rocket starts doing loops. The closer the CG gets to the CP the more likely you'll have problems, but I would feel comfortable with a 0.75 margin as long winds aren't real strong.

Is a Pro 38 3G with spacers any lighter than an AT 29/240 with spacers? If it is I wouldn't think it'd be enough to make a significant difference.

You could go with Aerotech single use 29mm G motors for test flights which would be lighter still.

I've been considering picking up a Little Dog. Thanks for posting pics. Nice job.

 

[Handeman]

I think my worry on the short version is stability, not thrust. Like tsai said, the the engine is moved back 1 inch so it takes away from the margin of stability some. I haven't weighed it or measured the CG yet in either configuration because I don't have all the pieces.

I do like that the Pro38 3g would let the DD version fly on a G, H or I though...

You do have the CP marked on the rocket, right? Just check the CG in relation to it before each flight. If it looks too close, Tie/tape a fishing sinker to the shock cored right below the nose cone or to the nose cone itself.

 

[Obskur]

You know, I thought about using a t-nut to put in the nosecone bulkhead so I could remove the eyebolt and put some washers on it for weight. Maybe ill see how much lead it would take.

 

[Obskur]

Here is a rocksim shot of the final paint job I applied to the rocket. I'll try to get some pics up later with the tailcone and rail buttons finally installed. The white was nice, but too boring so I sprayed some Rustoleum Cobalt Blue Metallic on the booster, and some silver with black stripes on the payload section.

I'll get real pics up later on.

 

[Obskur]

Okay, the rocket is basically complete at this point. I received my recovery components from K and S Rockets and got everything hooked up. This parachute is the semi-hemispherical hi-vis model, 36" diameter. It's very well constructed and I'm looking forward to seeing this bad boy in action. The shock cords are 9/16" tubular nylon with loops sewn on the ends. It's all connected with 3/16" quick links. Not pictured is the 9x9" Nomex chute protector.

Here is a shot of the completed rocket. Only things missing are some shear pins and rivets, but the tailcone has been installed with JB Weld, and the rail buttons attached at the centering rings.

Per the posts above, I'm swapping out the Aerotech 29/240 set (which is now up for sale, brand new and unused) and going with the Cesaroni Pro38 setup. This will allow me to use the G115 WT or G185 Vmax for the first flight, and use the same casing for H and I flights. I'd like to put this thing up in 2 weeks but my upcoming work schedule might put it off until October.

Below is a fairly inclusive list of all of the parts, supplies, and tools I needed to complete this project. This should be a guide to other people thinking about building one of these types of kits for the first time, and not a complete list of items you might need.

Complete parts and supplies list:

Kit:
---Performance Rocketry Little Dog Dual Deploy (Always Ready Rocketry)

Engine Retainer:
---Giant Leap Rocketry Slimline 54-38 Tailcone Retainer (GLR)

Shock Cord Mount:
---1/2" Tubular Kevlar (GLR)

Shock Cord:
---9/16" Tubular Nylon, 20ft and 10ft (K and S Rockets)

Parachute:
---36" Semi-Hemispherical ripstop nylon (K and S Rockets)
---9x9" Nomex cloth parachute protector

Hardware:
---#10 plated steel allthread, nuts and washers for avbay (Lowe's)
---1/4" plated steel eye bolts, nuts and washers for shock cord mounting (Lowe's)
---3/16" plated steel quick-links for chute and shock cord attachment (Lowe's)
---#2-56 black nylon machine screw (McMaster-Carr)
---5/32" black plastic removable rivet (Public Missiles)
---Low drag delrin rail buttons for 1" rail (GLR)

Adhesives:
---MAS Flag epoxy and Medium hardener for construction (Local marine supply)
---West System 406 Colloidal Silica for fillets (local marine supply)
---JB Weld for engine retainer and rail buttons (Lowe's)
---West System 807 Syringes (local marine supply)
---1/2" disposable bristle brush (local marine supply)
---Denatured Alcohol (Lowe's)
---Nitrile Gloves (Home Depot)
---Wooden coffee stirrers (free at Starbucks)

Paint:
---Rustoleum Automotive primer, gray (Lowe's)
---Rustoleum Professional Gloss White (Lowe's)
---Rustoleum Cobalt Blue Metallic (Lowe's)
---Rustoleum Silver Metallic (Lowe's)
---3M 60, 80, 220 sand paper (local marine supply)
---3M 320, 600, 800, 1000 wet sand paper (local marine supply)
---Ultra-Fine scuffing pad (Pep Boy's)
---3/4" black vinyl striping tape (local marine supply)
---Turtle Wax rubbing and polishing compounds (Walmart)
---Meguiars Gold Class paste wax (Walmart)

Tools:
---X-Acto Knife
---Various screwdrivers
---Dremel
---Various wrenches
---Vice Grips
---Cordless drill and various drill bits

Thanks to the vendors listed above for the great service and quality products that helped this project come together.

 

[blackjack2564]

You know, I thought about using a t-nut to put in the nosecone bulkhead so I could remove the eyebolt and put some washers on it for weight. Maybe ill see how much lead it would take.

I have built/helped several of these for myself/others. You should be OK with flying it in the single deploy mode.

I fly mine regularly on G-64-7's 29mm and G-67-7's 38mm weight is 2.25 lbs.

This is due to adding a coupler and new section of airframe to the fincan after forgetting the BP and lawndarting it. [ Duh....:bangpan:]
So unless you have an abnormal amount of epoxy in the fincan, the tailcone should not be a problem.

You can always do what I do and go the old fashioned route. Prep it for flight, tie some shockcord around the CG and see how close you are to the CP.

If needed: the simple way out is to just add quicklink to the NC eyebolt till you get the stability where you want it.

Time to fly DD just remove the extra quicklink

Nice job on the build....colors look great! I used the Cobalt Blue myself on many projects including the 4 in DarkStar. Love the way it goes on and the glossy finish.

 

[gdiscenza]

Per the posts above, I'm swapping out the Aerotech 29/240 set (which is now up for sale, brand new and unused) and going with the Cesaroni Pro38 setup.

Where?

How Much?

G.D.

 

[Obskur]

Thanks for the info, blackjack2564. I weighed it (accurate only to 50g + eyeball on an analog scale) and measured the CG. I plugged it all into RockSim and it looks like it might work okay with the 29, but I think the Pro38 with higher thrust will work for me better, especially being a HPR newb.

Gdiscenza, PM sent.