New Guy's Onyx + Minie Magg Build

[luisv]

Interesting idea... looking forward to seeing how it comes out...

[Aksrockets]

Man, why are you watching fox news at 10:46?
Kiddin'

Nice build so far. Rockets look great so far. The Minnie magg looks HUGE. Dwarfs the motor tube!

[Kainam]

Haha, it even dwarfs that TV. That is a 42" TV!

[Kainam]

I got one of those Jolly Logic altimeters to play with. Anyone know what size static pressure ports I need make in these body tubes?

[o1d_dude]

Check your documentation that should have come with the Alt 1.

I usually drill three 1/8" diameter holes in the body tube. Not sure of the exact dimensions called for by Jolly Logic and my Alt 1 paperwork is out in the shop...in a locked toolbox...at the bottom of the stack...and I'm in my PJs...

[Kainam]

I saw that in the documentation, but they don't say anything about body tube volume with relation to port size.... wouldn't that make a difference?

[o1d_dude]

Hard to know. I've used my Alt 1 in a number of different birds ranging from BT-60 to 3" tubes and it's always reported an altitude that seemed within reason.

Given that it's just an altimeter rather than a flight computer controlling deployment, etc., the recommended size is probably adequate for the intended purpose.

[Kainam]

Interesting. The instructions say "atleast 3 1/16" to 1/8" holes,

but this site says 3 7/32" holes for a 5.5" dia tube ~10" long....
http://www.vernk.com/AltimeterPortSizing.htm
but this is for a dedicated avionics bay.....


effective volume would be less with the parachute etc in the body tube....

[luisv]

I would go with what the manual says. If you still question the size holes, reach out to Jolly Logic...

http://www.jollylogic.com/products/altimeterone/

http://www.jollylogic.com/http://www...rGuideV2-0.pdf

[JPVegh]

I've found 3 1/16 inch holes work well but I have yet to launch one in anything bigger than BT-20. I poke the holes with a push pin then enlarge them with a drill bit followed by a little CA and sanding.

[Kainam]

I've found 3 1/16 inch holes work well but I have yet to launch one in anything bigger than BT-20. I poke the holes with a push pin then enlarge them with a drill bit followed by a little CA and sanding.

Good idea!

[o1d_dude]

Best results cosmetically when you poke the holes BEFORE you prime and paint. John Beans (Jolly Logic proprietor) suggest using sewing grommets if you're drilling the holes after primer and paint.

[Kainam]

Yup. The primer that is on the tubes now will get sanded down. I'm just using the body tubes to hold the nose cone and mask the sholders while I prime those.

[DizWolf]

I used 1/8" holes in my Magg for my ALT 2. On a G77-4R it reported 417 feet at 106 MPH. Seems to be about right.

[Kainam]

Good deal, looks like we have a consensus. Holes drilled and CA glue drying.

I emailed John Beans, and discussed sizing of the port holes, and for the Minie Magg, if the parachute bay was empty the formulas he shared with me (and the formulas on the vernk site) say it would need 3 1/4" holes (volume of ~250 cu in). So I took a SWAG and guessed that the parachute and recovery items would take up about half that volume, and calculated that the Magg would need 3 ports of 5/32" diameter.

The Onyx, I calculated a parachute bay volume of 35 1/3 cu in. I didn't deduct any volume for the recovery items on this one since it has such a small volume, and came up with needing 3 ports of 5/64" diameter.

[JPVegh]

Best results cosmetically when you poke the holes BEFORE you prime and paint. John Beans (Jolly Logic proprietor) suggest using sewing grommets if you're drilling the holes after primer and paint.

A very good point and one I hadn't considered. I've only launched the Altimeter one in naked competition models so far.

[John Beans]

The most important factor that determines the proper vent size is the amount of air in the bay after it is loaded with your chute and/or any other “stuff.” If there is not much air, the holes do not have to be very big. If you could somehow eliminate ALL of the free air in the bay, you could have a port that was not much bigger than the pinpoint size of the port on the pressure sensor chip itself. If there is a good deal of air that expands and needs to rush out as the rocket travels to lower pressures, you’ll need larger holes so that the time it takes for the air to escape (which shows up as a time lag between pressure and actual altitude on the way up) doesn’t become too large. Fortunately, the altitudes we care most about (apogee and ejection altitude) occur at the top where (mostly/oftentimes) the rocket is moving slower and the air has had time to equalize. If you looked at a graph of “measured pressure altitude” vs “actual altitude”, you would see that they disagree at first, but then come together at the top as the pressures equalize. And of course, they will agree after ejection, if your altimeter is riding with the chute and is then exposed to open air.

As a guideline, I’d agree with the recommendation of 0.05 square inches of port for every 100 cubic inches of bay. So (for instance) if we had an 11" long bay in a 5.5" diameter body tube and it was empty except for the altimeter (very little volume), you could estimate it as:

V = 11*pi*(5.5/2)^2= 261 cubic inches
A(needed) = 0.05*261/100 = 0.13 square inches
Area of each of the three holes = 0.13/3 = 0.043 square inches
Diameter of each of three holes = 2*sqrt(0.043/pi) = 0.235 (or about ¼”)

If you’re packing a parachute in there, they can correspondingly smaller as you have less than 261 cubic inches of air to use in the calculations. In that case, replace “261” with whatever the actual free air is in the bay and repeat the calcs.

In case it helps.

--John Beans, Jolly Logic

[Kainam]

Thanks John.

[Kainam]

Got a bit more done on the rockets...

I decided 2 coats of thinned wood filler was enough, and that I wasn't going to wait to order some Deft spray wood sealer.

Made a couple fin alignment templates out of hobby foam board to align the fins and help hold them while the glue was drying. These also came in handy when I needed to lay the rocket on its side without putting any weight on the fins. With the template made, I used 2 ton epoxy to glue the fins to the body tube.

I also finished up primering the nose cones. On the 3rd or 4th coat of primer with wet sanding between, the mold lines are no longer visible.




Fillets applied to the underside of the top centering ring, and installed the bottom rail button weld nut. Also got the fin lock tabs in place.

On one of the fins, I neglected to do a test fit prior to installation and glueing. It had some excess wood filler that dried in the slot where the lock tab goes, and it was a major PITA to get the tab slid into place.

I had to use needle nose vice grips and a variety of colorful words to get it into place. Finally I chose the right combination of words and got it slid in place.





I used the nosecone as sort of a gauge to determine how far down on the body tube I needed to drill the static pressure ports. The angle is kind of deceptive on this shot. I placed the ports at the vertex of the V that the nose cone shoulder makes, and placed them inline with where the fins go further down the tube.




I decided to try out some Duplicolor Mirage paint on the Onyx. I ordered some Purple/Green for the nose cone and fins:


And some Ice White for the body tube.

[luisv]

Almost there and nice looking boat...

[Kainam]

Not mine, just a pic I found to illustrate the colors.

I just hope the Onyx turns out that nice.

[luisv]

Ah... Gotcha...

[Kainam]

Well, I went ahead and attempted using epoxy for the fillets. The shape of them turned out ok I think, but I had some problems when I tried to primer over the fillets. As the primer dried, it started to crack.

So I sanded down the fillets some and shaped them a bit more thinking that the epoxy might have been too smooth for it to stick, and wet sanded the body tube a little....





On the second coat of primer, it mostly went better on the fillets, but I have one that still cracked....






I need to find out what is causing this, and what I need to do to correct and repair this.

[qquake2k]

Well, I went ahead and attempted using epoxy for the fillets. The shape of them turned out ok I think, but I had some problems when I tried to primer over the fillets. As the primer dried, it started to crack.

So I sanded down the fillets some and shaped them a bit more thinking that the epoxy might have been too smooth for it to stick, and wet sanded the body tube a little....

On the second coat of primer, it mostly went better on the fillets, but I have one that still cracked....

I need to find out what is causing this, and what I need to do to correct and repair this.

Looks like the problems I've had with paint in the past. Never had it happen on the fillets, though.

[DizWolf]

I've had the same issue over epoxy with Rustoeleum filler/primer. Seems to happen when I lay it on thick and it cracks as it dries. Also appears unpredictable. One fillet will crack, another coated the same way will not. One coat cracks, the next over the same area will not. My guess is the way it dries, and it's not getting a good grip on the epoxy so it slides as it contracts.

I don't worry about it too much, I just sand and recoat until it stops. I haven't had it transfer through to a final coat yet.

Paint is indeed the devil.

[patelldp]

My guess is the way it dries, and it's not getting a good grip on the epoxy so it slides as it contracts.

This may be a good explanation for this.

Is there also a chance that this is a chemical issue? My thinking is that the epoxy could have been measured incorrectly, allowing for either excess resin or excess hardener that didn't react and cure. This could then have an adverse effect on the paint and potentially cause it to crack.

My advice would be to wipe down any cured epoxy with a dab of acetone on paper towel.

I'm not entirely educated on the reaction epoxy undergoes, but I could foresee this as some explanation.

[dasein668]

It could be due to amine blush which is typical with some epoxies. The West that I used on my boat tended to have this problem. A quick wash with a scotchbrite pad and water cleans it up. I'd also suggest not painting or priming unsanded epoxy regardless of blush. It may work ok with some paints, but others may have trouble bonding to an unkeyed surface. Finish grit should be fine. Again, that's my boat building experience not rocketry specific!

[Kainam]

I've had the same issue over epoxy with Rustoeleum filler/primer. Seems to happen when I lay it on thick and it cracks as it dries.

You might be onto something here. It is indeed Rustoleum (Automobile) primer.

It seems to be stuck down good, just cracked.....


I will sand down the cracked area and give it a light shot of primer this afternoon.

I don't worry about it too much, I just sand and recoat until it stops. I haven't had it transfer through to a final coat yet.

This is good to know! Thanks.

[DizWolf]

some take multiple coats. Much of the cause of this photo was filling spirals, but I had some cracking exactly like you showed on some early coats on this Optima.


Final coating came out ok. I used Testors One Coat Lacquer.


I also had the problem on my Executioner, which was 4 minute high temp epoxy (it was on hand) and Rustoleum primer. Covered that with some random floro green, had to sand the first coat and re apply. Had a phoenix show the same initial cracking, clean finish with duplicolor black. Not the cleanest fillets ever (was my first time with epoxy) but they painted OK-

[DizWolf]

It could be due to amine blush which is typical with some epoxies. The West that I used on my boat tended to have this problem. A quick wash with a scotchbrite pad and water cleans it up. I'd also suggest not painting or priming unsanded epoxy regardless of blush.

This sounds logical. especially in small batch mixing. It would explain why one fin shows it and another doesn't.

Sanding it down all the time seems a good solution.