TRF Summer Build Off: Der V-Max

[jqavins]

The more I look, the more I like the geometry of that fin can. I think I'll put a build like that on my unwritten to-do list. Regular motor mount, but that arrangement of 240° partial tube fins. And maybe a four fin (270°) version.

Seeing this drawn, I think I'll call it "Lucky."

 

[Screaminhelo]

THE NEW ROCKET PARTS ARE HERE! THE NEW ROCKET PARTS ARE HERE! I REALLY AM A STEELY EYED MISSILE MAN!

 

[dhbarr]

THE NEW ROCKET PARTS ARE HERE! THE NEW ROCKET PARTS ARE HERE! I REALLY AM A STEELY EYED MISSILE MAN!

Ride out, son of Terra! Drive far that thund'ring jet!

 

[Screaminhelo]

O.K. I now have this big, beautiful chunk of balsa nose cone. I have played around with the idea of sealing balsa with CA but have achieved mixed results. I tried rubbing it in while wearing nitrile gloves and it was going well, until the finger bonded with the surface. I also tried a piece of cotton cloth soaked in CA and that went well...until that too bonded to the surface. The best results that achieved was with my bare finger (I know, not the smartest idea) and the finish wasn't all that bad. My though now is to simply use enough CA to harden the cone to mitigate damage and finish with more familiar techniques. Any suggestions here?


Maybe a review of the wood hardener thread is in order.

 

[neil_w]

For CAing a cone, you hold it by the shoulder (with gloves), tip pointing down, and drizzle the ultra thin CA down the side until the whole surface is saturated. K'Tesh wrote a good tutorial.

I've had problems getting it smooth during sanding, and so for my latest build I put thinned CWF over the sanded CA (suggested by someone around here but I can't remember who). Looks good so far, painting in progress.

 

[Screaminhelo]

For CAing a cone, you hold it by the shoulder (with gloves), tip pointing down, and drizzle the ultra thin CA down the side until the whole surface is saturated. K'Tesh wrote a good tutorial.

I've had problems getting it smooth during sanding, and so for my latest build I put thinned CWF over the sanded CA (suggested by someone around here but I can't remember who). Looks good so far, painting in progress.

That is, essentially, what I plan to do. I have read in a couple of threads of folks actually achieving a smooth finish using CA. I am curious to give it a try if I can find the info again but my primary goal is to add a little durability to the balsa.

 

[jqavins]

I'm a fan of dope. No, it's not as strong or as hard as CA, but it is a whole lot easier to work with and does rather nicely getting the surface hard enough to take a nice smooth finish. Multiple coats can be applied, as each coat is driven a little deeper by the solvent when applying the next. For extra deep penetration, I've sometimes started with dope that's thinned 1:1 with acetone; apply three or four coats of that then switch to undiluted, or even go back and forth to get lots of dope solids and lots of penetration alternating.

 

[Screaminhelo]

I've been playing with fiberglass the last couple of days. The first inlet came out pretty nicely but the second pulled some of my poorly protected paper clay mold with it. Pics in the morning.

 

[Screaminhelo]

Well, I forgot to get a photo of the first one but here is the second inlet

I am just using a squish mold since I don't have a vacuum setup.



Here it is pulled from the mold. Looks pretty good at this point. I do have a little weave to fill but that is no big deal.


Now you see the backside of the new piece. All of the white that you see is from the female side of the mold. I made the mold from paper clay and I was not happy with the initial finish so I laid in a little more clay to smooth things out. I sealed the inside of the mold with clear lacquer once the clay had dried and went to town. The first shot went pretty well but I ended up with some residual epoxy in the mold in spite of the cooking spray that I was using as a release agent. When I got ready to lay up the second inlet, I made sure that the mold was well coated with spray but it was thin enough to push out and allow the epoxy to bond to the mold, resulting in the finish layer to pull out when I pulled it all apart. It is really not that big of a deal, I'll just sand the paper clay off of the piece and re-line the mold to make the last inlet. I should have used something thick like Vaseline as a release agent but the cooking spray was easy.

 

[neil_w]

That first picture looks like surgery.

 

[TangoJuliet]

This is a very interesting design and the discussion on DGS has been very informative. I'm looking forward to seeing your final results. As far as balsa nose cone hardening, I for one can't stand to use thin CA for anything. I much prefer using a Balsa Hardener/Dope for sealing and filling the grain. Then use primer and paint.

 

[rstaff3]

I like the idea of a molded inlet vs. just a hole. Will look very cool.

 

[Screaminhelo]

OK, I haven't fallen off the face of the Earth or anything like that. Life got busy and I haven't gotten around to posting my progress. It has been a bit of a slow go the last few weeks but I have made some good progress.

The MMT is done and will soon be adapted to serve as a baffle as well. All of the major components are test fitted and a few minor adjustments have been identified. I am still working the plan for attachment of the fin can to the airframe, it is a little weak for my taste right now.

Unfortunately, I am on a new network at work and may not be able to post pictures until this evening but I will definitely be soliciting opinions on how best to attach the fin can once I get a little eye candy posted.

 

[Screaminhelo]

Well, I found a way so here it is in its current state.


First, is the initial dry fit. All looks promising but I need to adjust the struts that connect to the fin can to better fit a round tube. I haven't done the cutouts for the inlets yet but that will come soon


Next up is the MMT next to the main body tube. I am planning on plugging the FWD end of the tube and drilling some holes AFT of the FWD centering ring to create a baffle.


Here is the MMT fit into the body tube, showing the struts that will connect to the fin can. Here is where I am seeking some advice. The struts are 3/32 basswood and feel a bit flimsy. I have a section of BT-80 coupler that I am planning on cutting strips from that will be slotted to fit over the struts and provide additional gluing area. Any thoughts on this?

 

[Cabernut]

This is starting to look really cool.

IMO, the BT-80 is the flimsy part. The struts should be fine. Might be worth reinforcing a bit so it won't get dented like a pop can.

 

[jqavins]

How about cutting WIDE strips from that coupler, like quarters or it? Glue them in between the struts so the fin can goes over a cycle of skinny strut-coupler panel-skinny strut-etc.

How about treating the wood to strengthen and stiffen it? From this week's PoF, the CA soak we've all done with balsa may or may not work with basswood, but "wood hardener" would and works better than CA anyhow.

How about adding a ring nearish the bottom, with an OD that matches the spacing between the struts at your selected height and an ID only about an inch less? You could maybe even make the OD bigger with notches in the outer edge, which would make it even more stable.

None of these are mutually exclusive, provided the coupler panels don't come down as far as the new ring.

 

[Screaminhelo]

This is starting to look really cool.

IMO, the BT-80 is the flimsy part. The struts should be fine. Might be worth reinforcing a bit so it won't get dented like a pop can.

I already have a lopsided tube fin :wink: I am thinking of using CA or wood hardener on the fin can. I have noticed quite a bit of flex in the struts though. Once they are epoxies to the can though, that flex may be gone. This weekend's task is to finish the baffle end and get the MMT installed with the intakes cut out.

 

[Screaminhelo]

Still plugging along with this thing.

The motor mount now serves as a baffle. It's not as pretty as I had originally envisioned but I decided to go for functional and finished rather than getting bogged down in making it just right and rushing the rest of the build later on.


The MMT is installed in the airframe and things are finally starting to take shape.


I really should have taken a before picture here. A little inattention, coupled with some flimsy fiberglass ended up in a wandering knife and a a really ugly cut line and a large gap at the edge of the intake. I laid on a repair patch to cover the gap and will work it down to make a clean edge.


That's pretty much it for now. My plan is to have the fin can on and a ring fin ready for install by the end of the weekend. If things go well, there may even be a first primer coat.

 

[Screaminhelo]

Sorry, yet another delay for pictures.

The airframe is completed and primed, the ring fin layup is curing and mind sim is working on inlet area calculations.

Since the purpose of the inlets is for air flow rather than stabilization, do I need the full D squared inlet area? My rough calculations come up with roughly 1 1/2 inches of flat plate area. It would be relatively simple to add more area but I'm not certain that it is necessary for my purposes. I'll be able to get a pic or two up tomorrow.

 

[jqavins]

Since the purpose of the inlets is for air flow rather than stabilization, do I need the full D squared inlet area?

Insufficient data. For GD stabalization there is experimental data on which the D² rule is determined. Venting olny to avoid Krushnic is a wholly different case and requires wholly different data. I don't have any such data, which certainly doesn't mean that no one else does. But any GD realted rules are entirely irrelevant.

 

[rstaff3]

How much area to avoid thrust reduction is a good question. I have now made 4 successful GDS rockets and all experienced some level of Krushnic.

 

[jqavins]

How was that determined?

 

[rstaff3]

By getting a few hundred feet from a G in a smallish rocket, for instance. Stable, but lousy performance.

 

[Screaminhelo]

How much area to avoid thrust reduction is a good question. I have now made 4 successful GDS rockets and all experienced some level of Krushnic.

There in lies my puzzle. I am using a 2.6" tube and the induction tube is 4" long. My updated flat plate inlet area estimation is 3.5". According to Mr. Black's figures, I need about 6.75" of inlet area for GDS function to be effective. Since the purpose of the inlets is to avoid the Krushnic effect, would you think that I have enough inlet area for this purpose given the length of the induction tube? Adding more area wouldn't be too difficult, I just have to figure the shape.that I want.

 

[rstaff3]

I can't find it now, but you might compare to the area/tube diameters to the proven Lil Augie. The plans are out there somewhere.

 

[Screaminhelo]

How much area to avoid thrust reduction is a good question. I have now made 4 successful GDS rockets and all experienced some level of Krushnic.

There in lies my puzzle. I am using a 2.6" tube and the induction tube is 4" long. My updated flat plate inlet area estimation is 3.5". According to Mr. Black's figures, I need about 6.75" of inlet area for GDS function to be effective. Since the purpose of the inlets is to avoid the Krushnic effect, would you think that I have enough inlet area for this purpose given the length of the induction tube? Adding more area wouldn't be too difficult, I just have to figure the shape.that I want.

 

[Screaminhelo]

Well, once I get the ring fin secured, it will be down to final finishing and installing the recovery gear.

 

[Flyfalcons]

Looking sweet! It's fun seeing all these cool projects reaching the finish line.

 

[Screaminhelo]

Got some paint on today. The ring fin popped off while I was doing some sanding but a quick layup with 5 min epoxy wrapped around the tips of the tube fins did the trick. The tube fin didn't turn out as circular as I wanted but it is symmetrical enough. I figure that since this is somewhat of a concept demonstrator, I'll just have to be sure to do better the next time around.

 

[Screaminhelo]

I'm getting down to the wire here with work and a small goof during finshing. I'm hoping to get a launch this afternoon but it won't be at it's shiny best. Swing testing tells me to be heads up for this one, the fin height is at about 2" on a 2.6" airframe. Unfortunately, I let the design evolve and didn't engage MindSim until I was past the point of no return.