LOC Doorknob Airframe Extension and Avbay

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wonderboy

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I have the LOC 7.5" Doorknob and love it. I completed my L2 with it on an Aerotech J350 (low and slow) and also flown it on a K535 which got it moving nicely. I'd just been using single altimeter based deployment and used a chute release, but on the last flight I narrowly escaped disaster from a tangled chute (complicated by the chute release and my poor preparation). The flight landed perfectly fine, but it was a nail biter waiting for the chute to fully inflate, which it did about 50 feet from the ground.

Ever since then I've wanted to make this a true dual deploy. I also have noticed that the overall length of the rocket is a bit short for the diameter (compared to the scale drawings in Peter Alway's Rockets of the World supplement). The overall length built stock from LOC is 64". Given the diameter of this rocket compared to the original, I calculated a 48.4" scale factor. The real rocket is 143" (not including the probe extending from the nose) so a more scale-like length would be 69.2".

So I ordered the LOC 7.5" avbay kit and a fresh section of 7.5" airframe. My plan is to replace the original 12" upper airframe with a section cut to 17.2". I built my kit originally with the coupler between the upper and lower airframe acting as the separation point for chute deployment, so it is going to be a simple matter of building the new 17.2" section of airframe the same way, but with the addition of the avbay.

Here is the complete stack in the original configuration, and with the nose cone and old upper airframe sitting on the bench.
DSC_5652.JPGDSC_5653.JPG

This airframe was fiberglassed when I first built it, so I repeated the process for the new section of airframe (subject of another thread of mine describing the soller composite sock method). I added a coupler to the bottom of the new airframe section to duplicate the original. Inside this, I glued the stiffy coupler to form the shoulders to capture the av-bay. I made this part of the assembly permanent.

Here are the old and the new upper aiframe sections side by side. Of course, I still need to paint this, but Michigan weather has been way too cold lately, even with the torpedo heater running in the garage (can't paint this in the basement).
DSC_5654.JPG

I assembled a Missile Works RRC3, RRC2L, and an RTx/GPS unit on the LOC avbay sled. I used my usual technique of copper pipe and caps for charge wells, but decided I'd recess them through holes in the bulkhead to save space on the parachute sides of the bulkheads.
DSC_5655.JPGDSC_5657.JPGDSC_5658.JPG

I 3d printed a dual screw switch holder/guide with mounting holes from top to bottom as well to allow the switch guide to be used to bolt the assembly to the avbay sled. The screw switches are from Missile Works and the switch guide I made is basically a "borrowed" design from the guides available from them as well (just without being a double and having avbay mounting provisions).
DSC_5660.JPG

The only other thing I did was to make a hardwood guide to epoxy into the inside of the avbay and installed a guide pin on the upper bulkhead so that the screw switches wind up perfectly aligned with the holes in the airframe.
DSC_5661.JPGDSC_5663.JPG

Here it is all stacked up! Just need to paint it once we get a warm snap in the weather. I'll be using the same paint I used on the original build: Rustoleum oil based paint (gloss white and Allis-Chalmers orange) sprayed through an HVLP gun. The horizontal stripes on the original are trim monokote (chrome and black). I'm hoping to fly it at Three Oaks this year on an L1000, which will be my largest motor flown to date.
DSC_5666.JPG
 

pathtouch

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Your AV bay has some great ideas in it! I’m building up my own Door Knob right now too and will steal a few of them. I like the recessed charge wells a lot. I’m keeping the stock upper 12” payload bay for now and it’s a really tight fit for me. This will help a little.

Do you know your typical flight weight and altitude you went on the J350? Looking to calibrate my open rocket file.
 

wonderboy

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My J350 flight was VERY low. The configuration on this flight was a single RRC3 and JL Chute Release. The RRC3 reported 741 feet. My flight weight was 17.2 pounds. Note that my open rocket simulation predicted 990 feet. My flight was pretty much vertical, so I'm not sure what else may have accounted for the discrepancy. Perhaps the base drag from the large airframe diameter was the issue. My K535 flight went over 2000 feet (can't find my log from that flight to know exactly, but it was right around 2000).

Despite the low flight on the J350, everything recovered perfectly. The JL chute release opened almost immediately and the chute inflated fine. This was my L2 flight, and I wanted low and slow... I got it!

On the recessed wells, I used 1/2" copper pipe with caps soldered onto the ends. The hole I drilled was pretty snug, but be sure to rough up the copper (or whatever charge well material you use) real well and also put a good fillet of epoxy on both sides of the bulkhead. I used JB Weld on the copper. I've previous used JB Weld to just glue the well onto the surface of the bulkhead, standing up and it proved to be very strong. I'm sure with fillets on the charge well on both sides of the bulkhead, it will be plenty strong. I just thought of doing it this way since there is SO much open space inside that avbay, I figured might as well put that space to better use and save space in the drogue and main chute compartments.

Good luck with your build (EDIT: I've been following your build thread, it's coming out great!).
 

StreuB1

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Stretching LOC kits is one of the most amazing things that can be done in rocketry. Taking awesome kits and improving on them is one of my favorite things. I have a Minie Magg that I changed to 54mm and stretched and an IROC that I changed to a cluster (38mm w/ 6x29mm radials) and then stretched it as well.

I've been eyeing the Doorknob for over a year and this now has really caught my eye!!!

The submerged charge wells is an AWESOME idea!! I have never seen that before in my life.

Your home made binding posts with the knurled brass thumb nuts is also an amazing idea!! I use real binding posts for electronics but your version is far more robust and uses parts that are readily available from the hardware store. STEALING THIS IDEA!!! :cool:
 

wonderboy

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Thanks! With regard to the recessed charge wells, it is so nice to have the luxury of all that space on the 7.5" avbay. I figured I might as well take advantage of it.

You are correct, those binding posts are completely 100% hardware store available. Here is the rundown. It is all 6-32 hardware, screw, nut, and washer is stainless. There is the brass knurled nut (from the specialty drawers section of the hardware aisle). The screw is a button head (allen) screw 6-32 x 3/4". On the backside (electronics side) I use a solder eyelet for a #6 screw. Assembly order goes like this: Screw, solder eyelet, avbay bulkhead, washer, plain nut, then brass thumb screw. I tighten the plain nut onto the screw to make sure there is a tight connection at the solder eyelet on the backside of the bulkhead.
DSC_5668.JPG

Here is a close-up of the same system on another rocket avbay build in progress (minus the one terminal I just removed for the shot above)
DSC_5669.JPGDSC_5670.JPG

One word of caution using this method: when you wrap your e-match wires around the post (the 6-32 screw) just make sure you don't wrap the wire tightly into the threads of the screw. You want the threads of the screw clear so that you can tighten the brass nut down fully and clamp the wire between the faces of the plain nut and brass nut.
 
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StreuB1

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Thanks! With regard to the recessed charge wells, it is so nice to have the luxury of all that space on the 7.5" avbay. I figured I might as well take advantage of it.

You are correct, those binding posts are completely 100% hardware store available. Here is the rundown. It is all 6-32 hardware, screw, nut, and washer is stainless. There is the brass knurled nut (from the specialty drawers section of the hardware aisle). The screw is a button head (allen) screw 6-32 x 3/4". On the backside (electronics side) I use a solder eyelet for a #6 screw. Assembly order goes like this: Screw, solder eyelet, avbay bulkhead, washer, plain nut, then brass thumb screw. I tighten the plain nut onto the screw to make sure there is a tight connection at the solder eyelet on the backside of the bulkhead.
View attachment 499466

Here is a close-up of the same system on another rocket avbay build in progress (minus the one terminal I just removed for the shot above)
View attachment 499467View attachment 499470

One word of caution using this method: when you wrap your e-match wires around the post (the 6-32 screw) just make sure you don't wrap the wire tightly into the threads of the screw. You want the threads of the screw clear so that you can tighten the brass nut down fully and clamp the wire between the faces of the plain nut and brass nut.
You should make a thread about this, with these pics, and then have the mods sticky it......

This is outstanding.

Only downside is the bare posts being shorted by the recovery quick link. I think that could be easily remedied though with a quick bit of tape over the posts prior to flight.

I am going to integrate this into my WM Demon 150 I am currently building. Thank you! :D
 

thzero

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View attachment 499467View attachment 499470

One word of caution using this method: when you wrap your e-match wires around the post (the 6-32 screw) just make sure you don't wrap the wire tightly into the threads of the screw. You want the threads of the screw clear so that you can tighten the brass nut down fully and clamp the wire between the faces of the plain nut and brass nut.
Same thing I've done too; except I had used 3mm hardware and thumb nuts (can see it either of two my build threads https://www.rocketryforum.com/threads/my-l2-project.167503/ or https://www.rocketryforum.com/threads/hpr-custom-build.162115/). But yup, you want a tight clamp of the wire. Your wiring is a bit tighter and cleaner than mine. :)

Its worked well thus far. I do like the inverted ejection canisters. Wish I had thought or seen it before I built my 5.5" rocket as I do have room. Oh well, on another build, right? :D
 

wonderboy

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Thanks for the positive comments guys! I appreciate the feedback, especially the comment about the quick-link potentially shorting on the terminals. The first time I used this terminal system was on a smaller rocket and I actually used a larks-head (https://en.wikibooks.org/wiki/Knots/Hitch_knots/Lark's_head) to attach the recovery harness to the avbay, so no metal to worry about. I wasn't thinking about this on the Doorknob project, but I am now. I'll have to think through my rigging and see if I can get away with a larks-head here too. If not, I'm thinking that once connected up, wrapping tape around the closed link should work to stop any shorts from happening.

Very good catch though!
 

StreuB1

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Thanks for the positive comments guys! I appreciate the feedback, especially the comment about the quick-link potentially shorting on the terminals. The first time I used this terminal system was on a smaller rocket and I actually used a larks-head (https://en.wikibooks.org/wiki/Knots/Hitch_knots/Lark's_head) to attach the recovery harness to the avbay, so no metal to worry about. I wasn't thinking about this on the Doorknob project, but I am now. I'll have to think through my rigging and see if I can get away with a larks-head here too. If not, I'm thinking that once connected up, wrapping tape around the closed link should work to stop any shorts from happening.

Very good catch though!
We're all here to help each other!!

BTW, your L1 rocket was the same as mine, same motor as well!
 

KC3KNM

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Thanks for the positive comments guys! I appreciate the feedback, especially the comment about the quick-link potentially shorting on the terminals. The first time I used this terminal system was on a smaller rocket and I actually used a larks-head (https://en.wikibooks.org/wiki/Knots/Hitch_knots/Lark's_head) to attach the recovery harness to the avbay, so no metal to worry about. I wasn't thinking about this on the Doorknob project, but I am now. I'll have to think through my rigging and see if I can get away with a larks-head here too. If not, I'm thinking that once connected up, wrapping tape around the closed link should work to stop any shorts from happening.

Very good catch though!
Might be able to source some vinyl caps the right size to slip over the terminals?

41250psku.jpg
 

StreuB1

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Might be able to source some vinyl caps the right size to slip over the terminals?

View attachment 499545
Not a bad idea.....

One thing that I do on some of my rockets is I buy a 4x4 nomex blanket from Wildman and I wrap it around my quick links and then zip tie in place. That way its not flopping around and beating things up.....like the edges of my fiberglass. Something like this, though more expensive (slightly?) might be another solution to insulating the quick link from the binding posts.

TBH, I like the binding posts far too much to pass on them due to a simple thing like shorting them out on the quick link. There are many simple solutions that have all identified. Now to find the best one for your particular solution and run with it.

I do like the vinyl caps though.....

Here is the drawing of a #6-32 brass thumb nut and here is a nice vinyl cap from Amazon.....


McMaster has these caps as well but the shortest they carry is 2"!!!! STRANGE!!

McMaster sells a smaller one for foodservice use. Price is cheaper than Amazon but.....shipping.





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wonderboy

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I like the vinyl cap idea. I'm gonna see what I can find at the local hardware store. I can see an advantage to having a long cap when it comes time to remove them from the terminals. If the fit is nice and snug (like I'm thinking I'd want it to be), having a lot some protrusion past the terminal will let you easily pinch just the cap to pull it off (without also gripping the brass nut inside).

This also has the benefit of keeping the terminals a little cleaner than being fully exposed to the ejection charge.
 

StreuB1

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Indeed. The add-on affect of keeping the terminals clean is quite good as corroded terminals can be the cause of non-deploys.
 

wonderboy

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I just took my lunch break and went to the hardware store. I found these:
20220113_111356.jpg20220113_111403.jpg20220113_114521.jpg

They are snug, but not too hard to push on. They remove without too much difficulty either. I think these are the winners for me. Only downside is they're 0.43 each. I know, it's not that much, but I'll bet you can get 100 of 'em on Amazon for 5 bucks. At any rate, I bought enough for my two avbays (both redundant, so 8 per avbay) plus a few spares. These seem snug enough that they shouldn't pop off during flight. Only a real test will tell! Can't wait for spring.

Thanks again for the great collaboration here.
 

cvanc

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First, congratulations on your Level 2! The Doorknob is a great rocket for those low and slow flights, excellent choice.

Secondly, you have some really nice ideas on that avbay and some impressive workmanship too. Love the charge wells. Looking at your ring lugs it appears they were not crimped very well (or at all)? Also they don't look like there's any solder on them?

In any case well done and have fun 👍
 

wonderboy

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Thanks for the compliments!

With regard to the solder lugs, you are correct that I didn't crimp at all (I don't like crimping this style), but they are definitely soldered. I'm careful with the solder so as not to wick too much back into the wire away from the lug, as this stiffens the wire and cuts flexibility. I do have the lug pretty much solid full of solder though. Looking back at my pictures, it does look like there isn't good wetting of the solder to the lug on the eyelet side. I'm gonna go back and reflow those. I know that the wires are pretty solid in there, but I definitely need to make sure that those joints aren't cold. Thanks for the great catch!
 

wonderboy

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On an earlier topic, I realized I took one pic on my phone of how the alignment pin sits in the hardwood guide before gluing the hardwood part into the avbay. I'm not sure if my pics made sense up above, but hopefully if there was any confusion this pic should clear it up.
20211230_235449.jpg
 

cvanc

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Yeah they look pretty dry to me, I'd add a little more solder and wet it out. Need to see those fillets 👍

But honestly the best approach here is "learn to like crimping this style" because crimping really is the better answer.
 

StreuB1

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<----- Senior Engineer speaking from experience; this is what I do for a living.


Soldering crimp connectors creates a fracture point where the solder meets the conductor. Vibration will cause work hardening of the copper at the solder/conductor intersection and then it will fail via fatigue.

If you are going to use crimp connectors, get a pair of proper crimpers and crimp.

These are the crimpers you want. Buy them and use them. Don't solder those connections.

:)

 

wonderboy

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I really don't like crimping unless it is this style of crimp:
20220113_152427.jpg

My job involves LOTS of connections to electronics/actuators/sensors and wiring. I've definitely been taught the lessons you mention.

Just so it's clear, I 100% agree with you that soldering a crimp connection is a bad idea.

However, I also hate crimping the style of lugs that the hardware store had (edit: especially when my wire gauge doesn't fill the barrel significantly). They don't have the style like I show above where you get a tidy electrical crimp plus an insulation crimp. I really hate the style where you just put a dimple in the barrel. My mental tradeoff was to solder, but effectively immobilize the free end by passing through the avbay sled close to those connections (or using cable ties and wire clamps). In this way, the wire won't be flexed significantly. Honestly, I would expect that soldered connection that I made to outlive the rocket given this constraint. I agree it's something to keep an eye on and not the best method. If I can find new eyelets with the crimp style I show above, I'll use those, but ruin my run of success at finding everything I need at the hardware store. 🤣
 
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StreuB1

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I will be working on the AV bay for my WM Demon 150 this weekend and will be implementing the brass thumbscrew binding post idea with the vinyl caps as well as looking into the submerged charge wells.

Thank you again for this thread and bringing your ideas to everyone's attention! Again, I think you should make a new thread where you outline this and as ask a mod to sticky it. I think it shows high merit for design, especially since it uses off the shelf and easily obtainable parts.

:D
 

Hyak

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Wonderboy: I am just letting you know LOC owes you a commission

I have been considering the Doorknob for a while, but after reading your post on the mods/with great photos, I logged on to LOC and promptly bought the kit and needed additions for the scale mod!

LOC owes you! :)

(Its not hard to get me to buy from LOC. Got my L1 on them many years ago! They were good then, and better now!

Anyway, thanks for the great write up and the unintentional kick in the pants to buy one!
 

wonderboy

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Hyak, That's awesome! I am also a huge fan of the LOC rockets, even though I currently only own the one. I'm eyeing several others. Funny enough, I did write a review of their Doorknob kit to use on their website (which got me a % off coupon) so I'm trying to sell their rockets any chance I get. 😁

One note: it does seem like they may have updated their kit since I ordered. Watching pathtouch's build thread here: (Link) it seems that the kit no longer includes the removable nose weight system which WAS included when I ordered mine. Not a big deal, but I kinda liked the versatility of the RNWS option.

Be sure to post up or private message if you run into any trouble with your build.
 

Hyak

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Will do!

It will be sometime next week when I receive it as I just ordered it about 15 minutes before I posted on the forum, but I am excited! (I will be referencing this thread with your great write up and phots as well!)

LOC has always been, at least in my opinion, really solid, good quality As I may have mentioned, I did my very first HPR L1 Cert. flight on a LOC back in the early 90's.

Anyway, thanks! I am sure I'll be in touch!
 

pathtouch

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Hyak, That's awesome! I am also a huge fan of the LOC rockets, even though I currently only own the one. I'm eyeing several others. Funny enough, I did write a review of their Doorknob kit to use on their website (which got me a % off coupon) so I'm trying to sell their rockets any chance I get. 😁

One note: it does seem like they may have updated their kit since I ordered. Watching pathtouch's build thread here: (Link) it seems that the kit no longer includes the removable nose weight system which WAS included when I ordered mine. Not a big deal, but I kinda liked the versatility of the RNWS option.

Be sure to post up or private message if you run into any trouble with your build.
I’ve been talking to a guy on Facebook recently and he said he got the RNWS with his! He also ordered it a few weeks after I put mine in so who knows. I think it’s worth calling them to confirm whether or not it’s a standard feature or not. I wasn’t particularly interested in it so I never followed up.
 
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