Build Thread MIM-23 Hawk

[Bruiser]

Since my DIY nose cone lathe is stalled while I figure out some hardware issues I decided it's time to start on what may turn out to be a new collection of rockets for me.

This is an Army Hawk based around a BT-20 body tube. It's designed around the Estes 13mm engine and if all goes well I hope to also build a Little John, a Patriot, and a Nike Smoke along with others.



I purchased an Estes Mini Comanche kit to use for parts. It has enough body tube and engine mounts to build at least two, maybe three, rockets this size. I just need nose cones, that's why I am also building a diy mini lathe

Back when I scratch build R/C airplanes, I would usually start off by building a kit, so that is where I started with this.



The fins are going to be layered with a center core of 1/32 bass sandwiched by 1/16th balsa. Then the fins will be sanded to the correct profile similar to those of an Honest John.

To be continued...

-Bob

 

[Bruiser]

I've been struggling with the fins on this one, more than my last two builds of BT-60 sized rockets. While waiting for the primer to dry again I went to work on the motor mount.

I glued in the engine block so 1/4 inch of the engine would protrude for removal. The rear centering ring is positioned where it is because of the boat tail.



Next I glued the motor mount assembly in the body tube. I left a little of the rear centering ring sticking out to glue the boat tail to.



Next I was able to come up with the pattern for the boat tail after a few tries.



I've never done a boat tail before. I guess I use a little cwf to smooth it all out. I confess that I should have positioned the seam where it would be covered by the conduit that runs up two sides of the rocket but I only just thought of it

-Bob


Tomorrow, it'll be back to the fins

 

[Rex R]

did you make the transition(boat tail) from card stock or...? I've made mine from cardstock using a 'glue behind tab'(not an overlap joint), followed up with ca and glazing/sealing putty then sanded smooth (320 grit).
Rex

 

[Bruiser]

It was from card stock (um, paper plate) and I did use a glue behind tab, but it still "peaked" at the glue joint. So CA it first, then the filler? Thanks for the help.

-Bob

 

[rharshberger]

To help mitigate the "peaking" I use a butt joint with backer strip instead of a lap joint. It helps reduce the peak so that CA and sanding are usually all thats required to make as close to perfect as its going to get.

 

[Rex R]

an old mouse pad and a 1/4 - 5/16" diameter rod help to 'roll' your paper to the right shape.
Rex

 

[Bruiser]

I've been working on the fins but for some reason I am having trouble with these ones. They are balsa with a bass core. I am having to apply cwf several times to fill the grain. I had a separate thread going about cwf and fins. I have two fins done now, one just got sprayed and I can see it will need some cwf touch up and the fourth just got a major coating of cwf. I did a little experiment with the cwf on the first coat that just didn't work out. I used alcohol to thin the cwf in a effort to have it dry faster. Well I think it was too thick to actually get in the grain to fill it. I know it went on a lot thicker. I have decided to try papering the fins on my next rocket. Here's the two that are done:




Argh, maybe not. I just realized that you can see the "line" too well so I need to do a little masking and a quick spray to accent one "face' from the other. Anybody know what to call them? Facets maybe?

-Bob

 

[neil_w]

For what it's worth, you didn't need the through-hole fins here. Those fins have a *lot* of root surface area, and would have been fine, especially with 13mm motors.

The TTW construction certainly doesn't hurt, though, just making an observation.

To help mitigate the "peaking" I use a butt joint with backer strip instead of a lap joint. It helps reduce the peak so that CA and sanding are usually all thats required to make as close to perfect as its going to get.

It's funny, when I've tried using a backer strip I get worse results, because I can't get the curve of the double-layered part to match the rest of the shroud. I must be doing something wrong.

I've never done a boat tail before. I guess I use a little cwf to smooth it all out. I confess that I should have positioned the seam where it would be covered by the conduit that runs up two sides of the rocket but I only just thought of it

Rule #1: whenever possible, position the seam so it'll be underneath something (mine seem to most often find themselves underneath fins)
Rule #2: you can smooth out anything. Don't worry, it'll be fine.

 

[Bruiser]

The tabs on the fins are there more for alignment more than strength. My bi-focals trick me into making things look true from one angle and then off true if I shift my view just a little. I usually take them off to do close up work. The distance I'll be working on the fins at while gluing them is to far to see with my glasses off so the tabs will help

I am going to apply some CA to the transition to stiffen it up some and then determine the next step.

-Bob

 

[kuririn]

Bruiser,

Here's a website that will generate a shroud/nozzle/transition when you input the parameters and send you a pdf to print. Very useful for scratch building.

https://www.delorie.com/rockets/transitions.html

 

[Cody Webster]

Looking good! I really like this design.

 

[neil_w]

The tabs on the fins are there more for alignment more than strength. My bi-focals trick me into making things look true from one angle and then off true if I shift my view just a little. I usually take them off to do close up work. The distance I'll be working on the fins at while gluing them is to far to see with my glasses off so the tabs will help

I like this answer so much I'm going to adopt this on my Alcubierre build. I have two veeeeeeery long wings to attach, and am going to put small TTW tabs on them just to help with alignment. Thank you sir.

 

[Bruiser]

I had a little set back this morning. I was planning on gluing the conduits to the side of the body tube. I was reviewing my pictures when it occurred to me again that either my fins looked to large or my body tube is too short. So I compared my model to the drawing and determined that my body tube was .375 of an inch short. I started to agonize over what to do, leave it alone or make it longer... since it's a small rocket and the cg is close to the cp as designed, requiring nose weight I realized that leaving it shorter was a step in the wrong direction. I cut off a section so I could use a coupler to join in the new corrected length body tube without having the coupler interfere with the shoulder of the nose cone.




I attached the new length of body tube with the coupler and Elmer's Carpenter's Wood Glue followed by an application of CWF.




All this for 3/8ths of an inch. But I think it was the wise thing to do. It would have been over an inch if I was building a BT-60 size rocket. The coupler will also add some weight to the nose which should help with moving the CG in the correct direction.

-Bob

 

[kuririn]

Good save!

 

[Bruiser]

Thank you! I'm not sure what happened as I was working off the drawing that I measured it against. All I can think of is that I measured from the wrong lines when I cut the body tube.

I have the conduits on and have been working the bead of glue for a fillet on them and the two fins that are glued on. It's pretty tough getting my finger in there between the fin and the conduit. I tried a q-tip but it allowed the glue to squeeze out "above it" on the fin and the radius of the fillet was small.



-Bob

 

[Bruiser]

I have the other fins mounted and I did get some good fillets done using a combination of fingers and Q-Tips. I also have the rocket primered. It took several tries at spraying, sanding, filling with cwf, sanding and primering again to get it too a level that seems acceptable. Of coarse that may change once the gloss white gets sprayed on




After I did the last prime, I noticed that I had not attached the launch lug yet so I scrapped/sanded off the primer where I needed to glue it and stuck it on with some CA followed by fillets of wood glue. I let it sit overnight planning to spray some primer over it this morning but I see there are some voids in the fillet from when the glue dried so I need to touch them up and let them dry before I spray. I was wanting to get white on it today but I think that'll be pushed until tomorrow. I learned on my last rocket not to rush the painting with the 2X paint. Plan is to spray white tomorrow and then the balck on Monday. That will give the white plenty of time before I tape it off and spray the black.

-Bob

 

[Bruiser]

I did spray the rocket white on Friday but there were some issues with the quality of the finish. I thought I wiped it off very well but the was some dust or sand on it afterwards. I spray in a metal connex container and it's possible the dust devil could have come by and caused the problems. I sanded it and repainted today. It came out much better. Now I have to wait until Wednesday to tape off the white so I can spray the fins and conduits black.



-Bob