Doorknob Build

[Bruiser]

I am officially starting my Doorknob build. It's been sitting on the back burner while I worked thru picking out the electronics for such a rocket as this will be my first two-stage "composite motor powered" rocket. This may take some time, so please bare with me and maybe once in the while throw in a "you go Bob" to help keep me moving forward. I tend to flounder a bit with stuff I haven't done before so some encouragement may be needed

I decided to start with the hard part first, the booster. Of course it's a scratch built booster and it is based on Mr. Alway's drawings, along with the few photos there are of the two stage Doorknobs. BTW, if you have more pictures of this rocket, please share them here


So, back to the body tube. Oh, I should say this is all based on the Estes Pro Series Doorknob kit so the body tube is 3 inches. I cut it to the correct length then I marked the fin openings, the 12 openings in the fin can with a gel pen. Came out really sweet looking. Oh, I should state now that the body tube weighed 2.29 ounces. Next I started the quasi glass process. One leg of Sheer Energy hose, two coats of water based poly, light sanding, coat of Z-poxy, light sanding, second coat of Z-poxy, another sanding, 3rd coat of Z-poxy and one more light sanding. It came out super smooth and it now weighs 3.08 ounces.

Remember those markings I put on with the gel pen? Well they got very blurry after the first application of water based poly I ended up marking them all over again on the other end of the tube.



More to come,
-Bob

 

[Ez2cDave]

https://www.rocketryforum.com/threads/question-about-doorknob-details.164594

https://www.rocketryforum.com/threads/doorknob-scale-info.160750

https://www.rocketryforum.com/threads/doorknob1-vs-doorknob2-variance.157323

https://www.rocketryforum.com/threads/sandia-doorknob.2822

Dave F.

 

[Bruiser]

Lot's of data there Dave but no other pictures of the rocket I posted above. Just the same picture in each thread.

Today I worked a little on the inter stage coupler/booster nose cone. Coming up with design was something new to me. I had a thread about it and pulled some ideas from there. Not only does it couple the booster to the sustainer, it also functions as a "nose cone" for the booster.

Quick explanation of what I mean is the booster fires and up the rocket goes. When the motor burns out, the rocket enters the coast stage and the booster starts to drag separate from the sustainer. The electronics in the sustainer sense the coast and ignite to sustainer motor and it completely separates from the booster. So the booster has been coasting while the motor delay has been ticking down. When the delay is up, the ejection charge fires and poof, the inter stage coupler is pushed out by the parachute just like the nose cone of most other rockets.

First job was to cut out the pieces. Stainless screw eye for the harness that goes into the 1/4 ply bulkhead. Then the piece of body tube that is the upper most part of the booster where the spin rockets attach followed by the coupler. The last two items are stiffeners to help keep the coupler "solid".


Here is what it looks like assembled. This phot clearly shows the cut outs for the sustainer fins. One of the topics discussed in the thread about the coupler was how far should the coupler go into the sustainer. My past experience is with gap staging the Estes Pro Series booster and it goes in the sustainer about 1.25 inches. The consensus on this project was that it needed to go in further than that but I did not want to give up on the ttw fin design on the sustainer. The "work around" was to engineer in cut outs for the fin tabs. I think it's going to work great!


I think that's about it for this week then. Next week will be engine mount time. I've been picking the brains of another Doorknob builder and I just about have a plan for it

Have a great weekend,
-Bob

 

[Ez2cDave]

Lot's of data there Dave but no other pictures of the rocket I posted above. Just the same picture in each thread.

Bob

Bob,

That's because those are the only known pictures of the prototype, unfortunately.

I provided the links to consolodate everything in one location, in order to make searching easier.

Dave F.

 

[John Brohm]

It looks good, Bob. Should be an interesting project.

 

[MNay]

Enclosed my 2-stage LOC Doorknob project.

 

[Bruiser]

That looks bigger than a 3 inch!

-Bob

 

[Ez2cDave]

Enclosed my 2-stage LOC Doorknob project.

Very nice . . . I especially liked the staging photo !

Dave F.

 

[Cape Byron]

Great shot of staging!

 

[Bruiser]

I'm back at it but it looks like I have taken two steps backwards

I purchased some laser cut fins (two sets) for this rocket. First time I have done that, but I thought it'd save some time.

Last week I sanded the leading edges to the scale taper. I noticed the pointy tip on two of the fins warped overnight. I clamped them to a straight edge and carefully applied some thin CA to them. After the CA dried I removed the clamps and they looked good.



So I thought I had dodged the bomb shell until it blew up this morning when I took them back out to apply some finishing resin. One of the fins took a major warp this time.


It's not just the tip, the whole thing has a bow to it. I realize that I may be able to clamp it in some way and let it sit for awhile and maybe it'll straighten out but what's going to happen when I apply the finishing resin to it?

I think what I'll do is apply finishing resin to one of the fins that looks ok and see what happens...

-Bob

 

[user 35280]

I'm back at it but it looks like I have taken two steps backwards

I purchased some laser cut fins (two sets) for this rocket. First time I have done that, but I thought it'd save some time.

Last week I sanded the leading edges to the scale taper. I noticed the pointy tip on two of the fins warped overnight. I clamped them to a straight edge and carefully applied some thin CA to them. After the CA dried I removed the clamps and they looked good.
View attachment 477857


So I thought I had dodged the bomb shell until it blew up this morning when I took them back out to apply some finishing resin. One of the fins took a major warp this time.
View attachment 477856

It's not just the tip, the whole thing has a bow to it. I realize that I may be able to clamp it in some way and let it sit for awhile and maybe it'll straighten out but what's going to happen when I apply the finishing resin to it?

I think what I'll do is apply finishing resin to one of the fins that looks ok and see what happens...

-Bob

Plywood or balsa fins?

 

[Bruiser]

1/8th inch plywood. I bought 2 sets of the fins and centering rings. I thought I read that it was Midwest ply but now when I go back and look it doesn't specify.

-Bob

 

[Bruiser]

I applied the finishing resin to the fins as if it was a stain. I brushed on an coat and then wiped most of it off with a cloth. I'm on the second side now and they seem to be holding their shape. The other thing is that I just looked at the fourth fin and it is mostly flat now. I has just been laying on my desk all day. I have it weighted right now and if it's flat in the morning I'll apply the resin to it as well.

Meanwhile I installed the upper centering ring. Since the booster is a few inches shorter than a regular Doorknob I decided to put the eye bolt in the upper centering ring instead of installing a plate mid-tube. I also decided to make the motor tube 6 inches long instead of the standard 8 inches.


That's it for today,
-Bob

 

[Bruiser]

The saga of the fin goes on. The three fins that I coated with Z-poxy yesterday still look ok. The fourth one has a small warp along the leading edge. I clamped it in place and applied Z-poxy to one side. After it dries I'll flip it and do the same to other side and see what happens.

EZDave has been trying to figure out what is inside those vent holes in the fin can. I can't answer that question but I decided that I need to simulate openings there.

So first I started with the openings. Made by multiple scorings with an X acto knife and number 11 blade. I apllied some thin CA around the edges and filed them to what I thought was a nice rectangle (until I saw this picture)



Next I made the inner structure from some card stock



Next I inserted the inner structure to the correct place in the body tube and glued it in place. I like the 3D effect it gives.



Next I need to work on how I am going to attach the rail buttons. I ordered some 8-32 t-nuts and they came in last night but the bases are huge! They measure 11/32nds, almost 3/4ths of an inch. I am thinking about drilling a hole in a 1/4 thick piece of ply that is the proper size to thread with a 8-32 tap. Then you put thin CA in the hole to harden the wood, run the tap thru, put more thin CA in and run the tap thru once again. That is how the wing hold down plates were made back in my R/C airplane days...

More later,
-Bob

 

[Ez2cDave]

EZDave has been trying to figure out what is inside those vent holes in the fin can. I can't answer that question but I decided that I need to simulate openings there.

The concensus appears to be that the openings permit access to bolts, used to secure the Fin Can in place. I agree with this.

I found some images of the TX-10 / XM-10 / XM-10E1 LACROSSE rocket motor. However, no "mounting hardware" for the motor is documented.

Dave F.









Dave F.

 

[Bruiser]

So I pressed on with the plan for the rail button mounts. After the 1/8th ply was laminated, drilled and tapped I epoxied it inside the body tube. Then I epoxied the aft centering ring in and JB Welded the retainer on. Motor mount done



I moved on to the fin area next. I purchased some printed fin plates for the booster and sustainer. I trial fit one in place and found two issues.

First thing is that they are about 1/32" thick and now the fins do not touch the motor tube. Second thing is that the forward brackets are not twisted and moved closer together so they match the leading edge taper.

So the first problem is not too big of a concern. I'll just trim 1/32" off the fin "shoulders" which should bring the tabs back in contact with the motor mount. That joint is going to be the major glue joint for the fins as the printed fins plates will prevent any kind of fillet on the outside of the body tube. Now I realize I should have left the aft centering ring off until I attached the fins because then I could of made a nice fillet of glue on the inside of the body tube as well. That is what I will do with the sustainer.

The second problem is the spacing of the forward brackets on the printed fin plates. The problem is that all the brackets are spaced 1/8th inch apart for the fins but if you taper the leading edge of the fin (scale) the forward most brackets need to be closer together and slightly twisted.



I am thinking the easiest way to fix this would be to cut the bracket out (red lines), remove some material (blue lines) to allow the twist, glue them in place and fill the gaps with some sort of putty...



I was going to work on the fins some today but I need to get them home so I can remove that 1/32nd of an inch off the root with my bandsaw. That way it will be a nice, straight cut. I have the fins stacked and taped together so they will all come out the same.

-Bob