New scratch build, the Plasma Fart

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qquake2k

qquake2k

Captain Low-N-Slow
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At the last LUNAR launch, Tony Cooper mentioned that he always thought "Plasma Fart" would be a good name for a rocket. Well, that's all the impetus I needed! My Plasma Fart will be a short, fat, cute thing, and who doesn't like those? The design I came up with is a simple 4" diameter 3FNC, with a 38mm motor mount. I'm going to paint it fluorescent green and yellow, and already have the decal ordered.

screenshot.jpg

plasma_fart8_empty_grn_yel_decal.jpg

cloud1_green_decal.jpg
 
I got the fins cut out and sanded yesterday. I'm using 1/4" birch plywood from Michael's. Love those 40% off coupons! These were probably the easiest fins I've ever cut out.

01_fins.jpg

02_fins.jpg

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The centering rings are 1/8" plywood, also from Michael's. As usual I used a hole saw to cut them out, and trimmed to final size on my wood lathe. I cut the rings with a 4-1/4" hole saw, and the center holes with a 1-5/8".

07_centering_rings.jpg

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I cut the motor tube to length with the bandsaw. Today when I get home I'll work on assembling the motor mount, and getting it installed. Then cut the fin slots and get the fins epoxied on. I'll also need to put 5oz of weight in the nose cone.

10_motor_tube.jpg

11_motor_tube.jpg

14_fins_centering_rings.jpg
 
Looks way overstable, especially for a squat rocket. They tend to need smaller stability margins, since the CP moves less with angle of attack. My 3" squat is perfectly stable with about 2 inches between the CP and CG. Check using 'analyze'->'component analysis' in OpenRocket the movement of the CP with angle of attack: you might not end up needing that nose weight.

My planned 4" squat with big, huge, gigantic fins has 1 caliber of stability, which slightly decreases with angle, but increases with velocity. For some reason, highly swept-back fins shifted the CP forward as the speed increased, but when I made the trailing edge swept forward, the CP shifted backwards with increasing airspeed.
 
CarVac said:
Looks way overstable, especially for a squat rocket. They tend to need smaller stability margins, since the CP moves less with angle of attack. My 3" squat is perfectly stable with about 2 inches between the CP and CG. Check using 'analyze'->'component analysis' in OpenRocket the movement of the CP with angle of attack: you might not end up needing that nose weight.

My planned 4" squat with big, huge, gigantic fins has 1 caliber of stability, which slightly decreases with angle, but increases with velocity. For some reason, highly swept-back fins shifted the CP forward as the speed increased, but when I made the trailing edge swept forward, the CP shifted backwards with increasing airspeed.
Click to expand...

I've heard the term "over stable", but what exactly does it mean? I put nose weight in my last two stubbies, and they seemed fine. My Who's Your Daddy was squirrely off the pad, but that could have been the wind and/or it may have been underpowered.

[YOUTUBE]TgvSxIqZ_kQ[/YOUTUBE]

[YOUTUBE]s5-60Ycse84[/YOUTUBE]

[YOUTUBE]OJ3DkGHhOHQ[/YOUTUBE]
 
Got the rail button bosses (for lack of a better term) and tee nut blocks epoxied to the centering rings. Also got the forward centering ring epoxied to the motor tube.

15_centering_rings.jpg

16_centering_rings.jpg

17_motor_mount.jpg

18_motor_mount.jpg
 
I drilled through the tee nut blocks, and pressed in the tee nuts with the vise.

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And got the motor mount epoxied in. Next will be the fin slots and fins.

28_motor_mount.jpg

29_motor_mount.jpg
 
I have no idea what happened to the tabs. They're about 1/8" too short. The only thing I can think is that Adobe Reader must have been set to reduce when I printed the patterns. Oh well, I've been here before. I'll epoxy pieces of dowel in when I apply the internal fillets. I dry fitted the fins, and I like the look!

30_tab.jpg

31_tab.jpg

32_dry_fit.jpg
 
qquake2k said:
I have no idea what happened to the tabs. They're about 1/8" too short.
Click to expand...
Where have we heard that complaint before? Hmmm...

You're reverting, Jim.

On a related subject, you've been using the 4-1/4" hole saw for your centering rings. By my calipers, the centering rings need to be a bit less than 4" in diameter to fit in a LOC 3.9" tube. How much are removing from your cutouts on the lathe? Think I could get by with 4-1/8" hole saw?

BTW, the 1/8" kevlar from Fuddrucker arrived today. Let me know if you need some for this build.
 
o1d_dude said:
Where have we heard that complaint before? Hmmm...

You're reverting, Jim.

On a related subject, you've been using the 4-1/4" hole saw for your centering rings. By my calipers, the centering rings need to be a bit less than 4" in diameter to fit in a LOC 3.9" tube. How much are removing from your cutouts on the lathe? Think I could get by with 4-1/8" hole saw?

BTW, the 1/8" kevlar from Fuddrucker arrived today. Let me know if you need some for this build.
Click to expand...

The problems I had with tabs in the past was due to math errors on my part. Open Rocket figures tab length automatically. I'm sure I input something wrong.

The ID of a hole saw is about 1/4" smaller than the OD, so a 4-1/8" would be too small. I'm okay on Kevlar for now, but I'd like to see if yours is the same as what I've got.
 
Just curious, why make the fin tabs shorter then the fin root? It would be easier just to make large rectangles. No worrying about tab length, no extra cutting...

An overstable rocket is when the Cg is far infront of the CG. Overstable rockets wont go crazy on you (loopty loops, skywriting etc.), they just tend to weathercock more. Nothing to worry about...

EDIT: oh yeah, nice work so far. You are a rocket building MACHINE. I'm a big fan of stubby rockets!

Alex
 
Last edited:
While Aksrockets touched on it, I think I might be able to clear up the "overstable" issue a little more. Basically, aerodynamic forces are going to act on the CP. But the rocket is going to pivot around the CG. The greater the distance between those two points, the greater the lever arm. So, things like gusty winds and low velocity as the rocket leaves the rail or rod can result in the squirrely flight you experienced. That is the condition you have when your rocket is overstable; you will get a stable flight but it might not be in the direction you wanted! Hope that made sense. John.
 
qquake2k said:
The ID of a hole saw is about 1/4" smaller than the OD, so a 4-1/8" would be too small. I'm okay on Kevlar for now, but I'd like to see if yours is the same as what I've got.
Click to expand...
Good to know about the ID vs OD on the big hole saws. I'm remember Dave Carver saying something about filing down the set of the teeth on the hole saw blade for that very reason.

I'll bring the kevlar out to Snow Ranch so you can take a look at it.
 
o1d_dude said:
Good to know about the ID vs OD on the big hole saws. I'm remember Dave Carver saying something about filing down the set of the teeth on the hole saw blade for that very reason.

I'll bring the kevlar out to Snow Ranch so you can take a look at it.
Click to expand...

If you look at the teeth on a hole saw, you'll see that they're offset in both directions. That makes the kerf about 1/8", just like a table saw blade. But you have to double it since it's cutting on essentially both sides of the hole. That's why the ID is 1/4" less than the OD.
 
jungmann1 said:
While Aksrockets touched on it, I think I might be able to clear up the "overstable" issue a little more. Basically, aerodynamic forces are going to act on the CP. But the rocket is going to pivot around the CG. The greater the distance between those two points, the greater the lever arm. So, things like gusty winds and low velocity as the rocket leaves the rail or rod can result in the squirrely flight you experienced. That is the condition you have when your rocket is overstable; you will get a stable flight but it might not be in the direction you wanted! Hope that made sense. John.
Click to expand...

You know, the whole CG/CP thing is something that I never got a grasp on, even when I was younger. I guess I need to study on it so I can figure it out.
 
Aksrockets said:
Just curious, why make the fin tabs shorter then the fin root? It would be easier just to make large rectangles. No worrying about tab length, no extra cutting...

An overstable rocket is when the Cg is far infront of the CG. Overstable rockets wont go crazy on you (loopty loops, skywriting etc.), they just tend to weathercock more. Nothing to worry about...

EDIT: oh yeah, nice work so far. You are a rocket building MACHINE. I'm a big fan of stubby rockets!

Alex
Click to expand...

Well, I'd still need to have a notch on the bottom of the fin to clear the aft centering ring. I guess I just prefer this style of tabs. Not really sure why. That's the way the tabs were on my LOC-IV and Hyperloc 835 kits.

08_loc_iv_fins.jpg

build32.jpg
 
Careful, in some circles this might be considered a biological weapon. Is there going to be a stealth version?
 
You could also add a third centering ring near the top to butt up the fin against. It does give another surface to which to bond the fins.

OTOH, instead of the fin popping off, it might break which would make repairs significantly more difficult.
 
Those corners do look mighty vulnerable. Are you going to tip-to-tip fiberglass them?

My huge-finned rocket which I mentioned above is gonna try to circumvent that problem by reducing landing stresses and by adding strength. The fins, which will be 13" root chord and 8" tall, will be tapered forward at the back edge to ease landing loads. They'll be made of carbon fiber on foam core for insane stiffness and ultra light weight. Then I'll tip-to-tip them in fiberglass, and then add some awesome very-wide-weave CF that will look really cool. That will go just on the flat fin surfaces, not tip-to-tip, for fin stiffness at the near-Mach speeds it will reach.
 
CarVac said:
Those corners do look mighty vulnerable. Are you going to tip-to-tip fiberglass them?

My huge-finned rocket which I mentioned above is gonna try to circumvent that problem by reducing landing stresses and by adding strength. The fins, which will be 13" root chord and 8" tall, will be tapered forward at the back edge to ease landing loads. They'll be made of carbon fiber on foam core for insane stiffness and ultra light weight. Then I'll tip-to-tip them in fiberglass, and then add some awesome very-wide-weave CF that will look really cool. That will go just on the flat fin surfaces, not tip-to-tip, for fin stiffness at the near-Mach speeds it will reach.
Click to expand...

No, I'm not going to fiberglass them. They're less vulnerable than my Who's Your Daddy's fins. I'll just take my chances. They are 1/4" plywood, so they're pretty strong.
 
And worked on the internal and external fillets. I used pieces of a bamboo skewer to take up the spaces between the fin tabs and motor tube. As usual, I used Titebond molding glue for the external fillets.

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