My Level 3 build log - 7.5" Little John

[conneryc]

Well after 20+ years of waiting, I finally have approval to start my Level 3 build. I'm going to use this thread to manage all of my build pictures for my certification package.

I'm starting with a LOC 7.5" Skinwalker kit, and modifying it to scale length for a Little John. I will replace the kit fins with ones that I cut out on my CNC router. Initial RockSim file shows an altitude of about 12,000 which I think is a bit high as it doesn't take into account the all up weight of the completed build.

 

[loopy]

That altitude seems awfully high to me for that...my Skinwalker in openrocket has a dry weight of 393 ounces, 639 when loaded with an M1350. Your rocket in that image with the M1350 is at 464 ounces. The Skinwalker with the 639 ounce loaded weight of the M1350 sims to just under 7000'...

 

[Hobie1dog]

looking forward to this

 

[conneryc]

That altitude seems awfully high to me for that...my Skinwalker in openrocket has a dry weight of 393 ounces, 639 when loaded with an M1350. Your rocket in that image with the M1350 is at 464 ounces. The Skinwalker with the 639 ounce loaded weight of the M1350 sims to just under 7000'...

I agree, like I said I think that altitude is a bit high. I have a better idea once building commences and I can plug in actual weights. I really hope that 5-7000 is more accurate.

 

[conneryc]

Just adding a link to the Fusion 360 design I'm working on: https://a360.co/42MQsvx

 

[sambatterman]

I love the Little John design. We have a Mad Cow 4" diameter fiberglass model and it just goes and goes - built like a tank. Looking forward to watching the build.

 

[conneryc]

I love the Little John design. We have a Mad Cow 4" diameter fiberglass model and it just goes and goes - built like a tank. Looking forward to watching the build.

I love the design as well. I had the original BSD 4" LJ back in 2001. Sadly never got to launch it. Since I got back in to the hobby, I had to have another one, and found out that Mad Cow had taken over the design, so I bought one of theirs, put it together and launched it on Fathers Day weekend. The Mad Cow kit only comes with the "US ARMY" markings in black. This means you are meant to make the Red/White/Black scheme. However, my previous research shows that the version of the LJ with this fin design was only painted in olive drab. The red/white/black scheme was on the second version of the LJ that was deployed with the rectangular fins. Although, you do see several type of paint schemes on this rocket at different locations where an example is on display.

I think the design works well for any level of certification. I like the 4 fins for increased stability with a fin design that has no part extending below the airframe. So when the rocket touches down the first part to hit "should" be the airframe and not a fin....minimizing the chance of a breakage.

I actually considered do a nearly 1:1 scale version (11.5") for my L3, but finding and old PML nosecone is difficult. RockSim shows that one at this scale on a baby-M wouldn't even reach 2000'. Maybe after I complete my L3 with the 7.5" version I'll go ahead and 3D print a nose cone form for the 11.41" PML tubes and make a fiberglass one, or do a full scale 12" version with sonotubes.

 

[JackC]

Well after 20+ years of waiting, I finally have approval to start my Level 3 build. I'm going to use this thread to manage all of my build pictures for my certification package.

I'm starting with a LOC 7.5" Skinwalker kit, and modifying it to scale length for a Little John. I will replace the kit fins with ones that I cut out on my CNC router. Initial RockSim file shows an altitude of about 12,000 which I think is a bit high as it doesn't take into account the all up weight of the completed build.

View attachment 589335

How much nose weight do you plan to add?

 

[conneryc]

How much nose weight do you plan to add?

It will depend on the final build weight and balance.

 

[Tim51]

Cool project. Never built one, but a while back I did spend some time researching the history of the Little John as a possible scratch build. As you say the earlier 4 fin LJ design makes for an excellent L3 project. Good luck with the build and your L3 cert - following with interest.

 

[Hobie1dog]

I'm going to follow this build thread. I too am hopefully going to build my L3 dream rocket after seeing it 27 years ago.

 

[OzHybrid]

Well after 20+ years of waiting, I finally have approval to start my Level 3 build. I'm going to use this thread to manage all of my build pictures for my certification package.

I'm starting with a LOC 7.5" Skinwalker kit, and modifying it to scale length for a Little John. I will replace the kit fins with ones that I cut out on my CNC router. Initial RockSim file shows an altitude of about 12,000 which I think is a bit high as it doesn't take into account the all up weight of the completed build.

View attachment 589335

Apply the base drag hack to see what altitude it says.
Post the OR file.

 

[pathtouch]

Apply the base drag hack to see what altitude it says.
Post the OR file.

I found altitude was more accurate on my 7.5” Doorknob without the base drag hack. Flown up to 4,500 ish in an L1090 so far.

 

[OzHybrid]

I found altitude was more accurate on my 7.5” Doorknob without the base drag hack. Flown up to 4,500 ish in an L1090 so far.

What surface finish did you have selected for the base drag hack and the other components? Post the OR file.

 

[conneryc]

Well the kit has arrived, so it's time to start building. Need to pick up some sheets of Baltic birch to cut out the new fins on the CNC.

 

[Hobie1dog]

Well the kit has arrived, so it's time to start building. Need to pick up some sheets of Baltic birch to cut out the new fins on the CNC.View attachment 590559

nice build shop

 

[conneryc]

nice build shop

Thanks, it's my woodworking shop and home office.

 

[pathtouch]

What surface finish did you have selected for the base drag hack and the other components? Post the OR file.

Sorry I took so long to respond, had the week off for July 4th and was not near my computer.

Here are two files, one with the base drag cone hack, one without, all regular paint finish in Open Rocket and weights and Center of Gravity (CG) overridden for actual values. I have actually flown this configuration with an L1090 to about 4,525 feet averaging my altimeters.

With the cone it predicts 3,673 feet. Without the cone it predicts 4,533.

What I usually use now for an Open Rocket File is more complicated. I keep the cone in so the center of pressure (CP) is in a more accurate location, but override the Cd until it matches flight data. In this case a Cd of 0.35 matches my L1090 flight data. This way I can check stability quickly and still get accurate apogee predictions.

Any updates on the build?!

 

[conneryc]

Any updates on the build?!

The only update is that I have designed the new fins in Fusion 360 and have my cut file ready for the CNC. Going to get some regular 3/8" ply to do some test cuts while I wait for my order of baltic birch to arrive.

 

[Theory]

cool project!

 

[conneryc]

The only 3/8" ply that Lowes had was garbage, so I used some 1/2" (0.47) 7 layer ply that I had left over from building drawers for my shop cabinets. Adjusted the cut file for the new thickness and cut out the first test fin. The only adjustment that I need to make is increase the width of the middle centering ring slot by .01".

 

[Tim51]

Really nice fin profile you've got there!

 

[OzHybrid]

Where does the extra fin weight put your CG?

 

[pathtouch]

For the next time, do you have a Rockler nearby? They tend to have smaller quantities of cabinet grade plywood in stock.

Those things will be stout!

 

[conneryc]

Where does the extra fin weight put your CG?

Won't know the true CG until construction is complete. As I do the assembly, I will update my RockSim file with the true component weight.

 

[conneryc]

For the next time, do you have a Rockler nearby? They tend to have smaller quantities of cabinet grade plywood in stock.

Those things will be stout!

The closest Rockler is an 1.5 hours from my house as I live out in the country. It's only about 20 minutes from my office though. That's where I buy my hardwoods from for other woodworking projects. For the 3/8" Baltic Birch, I got it from MakerStock (which arrived yesterday). So I can get started on cutting out the actual fins and additional centering rings.

 

[OzHybrid]

Won't know the true CG until construction is complete. As I do the assembly, I will update my RockSim file with the true component weight.

No, but you could throw the new thickness of the fins into the existing ORK and see how interesting it's going to make things for you. Might be better to get the 3/8 ply than add the extra weight at the wrong end.

 

[conneryc]

No, but you could throw the new thickness of the fins into the existing ORK and see how interesting it's going to make things for you. Might be better to get the 3/8 ply than add the extra weight at the wrong end.

I'm not sure what you are getting at....the actual fins will be 3/8" baltic birch ply. I just did a test on the CNC of the design using the .47" ply that I had laying around. That way I didn't waste the 3/8" doing a test where I found that I needed to adjust a setting.

 

[OzHybrid]

I'm not sure what you are getting at....the actual fins will be 3/8" baltic birch ply. I just did a test on the CNC of the design using the .47" ply that I had laying around. That way I didn't waste the 3/8" doing a test where I found that I needed to adjust a setting.

Ahhh. I see. That'll be better then...

 

[conneryc]

Got all 4 fins cut out on the CNC and mocked them up using the kits centering rings. Now on to making the centering rings that I will use (3/8" as opposed to the 1/4" that came with the kit).