LOC Doorknob centering ring modification

[samb]

I've had pictures of my build gathering digital dust for some time now. Here is a little mod I did on the centering ring/fin assembly.

The LOC Doorknob appeals to me on many levels; A rocket capable of L2 certification without electronics and a big cardboard/wood/plastic model rocket. The fins have a tab that goes through the air frame wall but does not extend to the motor mount tube. Instead they are secured to the inside of the air frame with a piece of wood fitted through a slot in the fin tab:



This provides a very strong attachment point but I decided I wanted a little extra security. I cut 4 notches in the centering rings at the fin tab locations to provide a little more lateral stability:



The 7.6 inch diameter tube gave me plenty of room to work. I installed the forward centering first to engage the top of the fin tabs followed by the aft ring. The motor mount tube went in after. I made up a third centering ring of 1/4 inch ply to attach to the forward end of the motor tube. I'm very pleased with the result.




https://shop.locprecision.com/product.sc?productId=216

 

[Tim51]

Interesting choice - to me the Door knob also appeals because it's a four fin stubby design rather than 3FNC. Other than the mods you've described above are you building this stock ? Do you have any sim data yet?

Good luck with your L2 cert.

 

[samb]

Interesting choice - to me the Door knob also appeals because it's a four fin stubby design rather than 3FNC. Other than the mods you've described above are you building this stock ?

The only other mod I made was to the shock cord attachment points. I used a u-bolt on the forward centering ring instead of the kit supplied eye bolt and an eye bolt on the nose cone instead of the molded plastic loop.

Do you have any sim data yet?

I just copied the Rocksim file available on RocketReviews since 1999 into OR. https://www.rocketreviews.com/unknown-pk-90-doorknob.html

Good luck with your L2 cert.

Mission already accomplished ! I told you these pictures have been sitting on the ol' hard drive a while. :blush: Aerotech J460T at Hearne Texas.


Andy Berger trying to wrestle it away from me.



Nice shot of the medusa nozzle



photos courtesy of my friends Serpico and CL(VII)

 

[Tim51]

Mission already accomplished ! I told you these pictures have been sitting on the ol' hard drive a while.

Aerotech J460T at Hearne Texas.

Ah - ok - well belated congratulations and thanks for positing anyway - great close up lift off pics there.

 

[manixFan]

Sam,

Great mod to the fins. Otherwise the design relies on the stiffness of the body tube to prevent the fins from flexing. The slotted fin is a cool way to keep weight down by avoiding a huge fin tab all the way to the motor mount - I'd never really noticed that before. With your mod the fins should be nice and sturdy.

I have section of 7.5 body tube and a nosecone laying around, and plenty of good 1/4" Baltic Birch plywood. Now I know what to do with it... but I won't be able to resist a 3" motor mount.

Thanks for the tip,


Tony

 

[crossfire]

Sure looks like Warlock fins?

 

[neond7]

Sure looks like Warlock fins?

I've got the Warlock, Doorknob, and Bruiser. They all have the same fins, the Doorknob has four fins and the others have three.

Interesting idea on locking down your fins. I ended up doing tip to tip fiberglass which locked mine down. ;-)

 

[crossfire]

I didn't know that. Looking at LOC site fins on the DK look different.

 

[samb]

Ah - ok - well belated congratulations and thanks for positing anyway - great close up lift off pics there.

Found my sim data for the J460 flight:



I think most observers agreed that the actual altitude was pretty close. The ejection event occurred before apogee but close enough to avoid a disastrous zipper.

 

[samb]

Sam,

Great mod to the fins. Otherwise the design relies on the stiffness of the body tube to prevent the fins from flexing. The slotted fin is a cool way to keep weight down by avoiding a huge fin tab all the way to the motor mount - I'd never really noticed that before. With your mod the fins should be nice and sturdy.

I have section of 7.5 body tube and a nosecone laying around, and plenty of good 1/4" Baltic Birch plywood. Now I know what to do with it... but I won't be able to resist a 3" motor mount.

Thanks for the tip,


Tony

You're welcome Tony. Simple and effective, I hope. I wanted to break out of my recent spate of Watering Hole/non-rocketry related posts.

 

[samb]

I didn't know that. Looking at LOC site fins on the DK look different.

Yeah, the artists rendering on the LOC site has a different fin profile, maybe an older version of the kit or something closer to the Sandia Doorknob:



BTW lots of great examples of the LOC model here from Neond7, Boosterdude, and Judo

 

[ECayemberg]

I've got the Warlock, Doorknob, and Bruiser. They all have the same fins, the Doorknob has four fins and the others have three.

Interesting idea on locking down your fins. I ended up doing tip to tip fiberglass which locked mine down. ;-)

Hey Jeff,

You have me wondering...my Bruiser and Warlock have slightly different fins. The Warlock fins have 1" less semi-span. My Bruiser is about 10 years old, however, and I know a few of the kits have had different iterations of the fins over time.

Bruiser fin dimensions: root chord: 10.5", span: 8", tip chord: 4.25"
Warloc fin dimensions: root chord: 10.5", span: 7", tip chord: 4.625".

If you don't mind, how does your trio compare?

-Eric-

 

[ECayemberg]

You're welcome Tony. Simple and effective, I hope. I wanted to break out of my recent spate of Watering Hole/non-rocketry related posts.

Sam,

Awesome modification! Like you said, simple and effective; I love it!!!! Great job! :clap:

-Eric-

 

[ThirstyBarbarian]

Interesting mod. I love my Warlock --- how easy it was to build and how light it is for such a big rocket. But I did get some fin damage from a rough landing. Now one fin wobbles in its slot, like the piece of wood that locks the fin in place has torn free from the inside of the airframe. Something to provide more stability might have prevented that.