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Rules of thumb for rocket mods: fins, cameras, etc

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cbwho

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More experience needed!
As a kid, I loved spaceships or airplane rockets. My Estes NASA X-16 being my favorite (no idea what happened to them after I left home).

Anyhow, I also love experiments. I installed OpenRocket today but I'd also like to learn some quick rules. I prefer building versus sitting in of the computer (as I already do that for work!)

From my recent gap of 3 decades, I don't remember tube fins from the 80s.

I love the idea of adding tube fins to existing 3 and 4 fin rocket cheapo kits as they are instant hyperspace/jet engines. What's a good rule of thumb for the CP CG change when adding more fins? Weigh the additional fins and put those grams as clay in the nose cone? I suspect I need to add the tubes in a balanced nature, with -, X, or + configuration.

I also prefer shorter rockets to longer rockets. If I cut a body tube in half, weigh the "discard" and put those grams as clay in the nose cone?

i like the idea of "wing" fins. Those are 2 fins that stick out at 180 degrees from one another. Does it matter much if there are "extra" *side ways* fins added to an existing design? Do I simply weigh them and add those as grams of clay to the nose cone?

One clever design trick I've noticed is T shaped fins. Instantly more space plane like. If one takes a 4 fin design and move the top and bottom fins out (some surface area)?

I bought a cheap keyring camera that stripped down like the Astrocam. I've taped it to the top of the body tube but it caused the rockets to veer. Is there a way to minimize that? Or simply put in the biggest motor?

How can I add a canopy for the plane? I could paint it on obviously, but it would be neat to carve a balsa one. Will that make it veer?

thanks for the wisdom!
 

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Nytrunner

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I try and put cameras between the CG and CP so that their impact is minimal.

What's a good rule of thumb for the CP CG change when adding more fins? Weigh the additional fins and put those grams as clay in the nose cone? I suspect I need to add the tubes in a balanced nature, with -, X, or + configuration.
As far as CP, the closest youll get to a rule of thumb is the barrowman equations. They are tedious to do by hand which is why so many of us use Rocsim or Openrocket. And even then, they have limitations (like T-fins, canopies, etc....)

The best practice is: determine where the CP is with your fin configuration, and use enough nose weight so that the CG (with motor installed) is safely in front of the CP.

If you really dont like designing them with computers, you can do the old fashioned swing test. Load the motor in, tie a string/rope at the CG, and swing it around. If it flips or spins, its unstable. If it noses into the swing, it should be good.

It really sounds like you're more into Oddroc type builds, and those are hard to simulate/calculate. They rely on experience and sometimes trial and error
 

cbwho

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Nytrunner, thank you for your message. Ok, I have a small scale so I will measure the weight added.

Thank you for the camera advice as well.

I have a copy of the Stone book from the 80s, I'll review it some more.
 

JohnCoker

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Given OpenRocket a try. It's easy to get carried away and enter every little part of your rocket, but all that matters is the exterior shape really. I then override the dry weight and center of gravity (C.G.) with measured values to let the simulator calculate the center of pressure (C.P.) to check stability.
 

cbwho

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My souped up Galaxy Patrol. I papered everything so I didn't need to use a drop of paint. The extra jet engines added 3 grams

I may clear coat it.
 

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cbwho

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Winter here so no spraying so have to resort to brush methods. I have water based clear acrylic spar urethane. The sticky notes hold the color well but the white printer paper gives a bit of the wet look. It's still wet however, will be interesting how it dries.
 

cbwho

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I added 3g of clay to the nose to offset the additional tube fin weight,
did the swing test and with an A motor it did ok. With a C motor, if you started with the swing in reverse, the rocket stayed in reverse.

I took the Estes Loadstar load lifter and with the swing test, the rocket showed to be very stable no matter starting position.

I flew the space patrol on the A engine in 10mph winds and it flew up perfectly straight. With that success, flew up on a C motor and the rocket went very high straight and true. A perfect flight. How reliable is the swing test, is it an issue of swing speed? Unlike the swing test, the C engine was perfect in reality.

*Update* google found: https://apogeerockets.com/education/downloads/Newsletter53.pdf

I was only using a swing length of 4 feet as I didn't want to hit the room walls... However the Loadstar is twice as long as Space Patrol... Mmm.

Update 2: my fault was that I didn't start spinning in the proper direction. This video explains it at the 4 minute mark:
 
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cbwho

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Indeed not an indoor activity!
Out of curiosity, I spun my little red Squadron XWing fighter, I crashed it hard and broke 2 wings off. Ouch. Luckily CA fixed it.

Great video for proper swing test:
I needed to start it oriented correctly.
 
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BABAR

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My experience with the keychain cameras has been they are more weight than drag. I mount them with tape as far forward as I can (usually just tailward of the forward edge of the body tube, but not on nose cone) and have yet to have stability issues PROVIDED THE ADDED WEIGHT DOESN’T significantly reduce rail velocity.
 

scsager

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Get yourself a copy of the Harry Stine Handbook of Model Rocketry, for starters.

Brendan
+1000 for the Handbook.
I recommend to the OP - if you are interested in rocketry and intent on building your own rockets, GET THE BOOK - asap.
Read it cover to cover.
Then read it again.
Seriously.
 

BABAR

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It really is an entertaining read for adults, for kids I’d guess 12 and up, under 12 for a really smart and motivated kid.
 

mooffle

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If you do anything oddroc like with OR keep in mind that sometimes you need to trick the program into simulating an approximation of your design.
Some examples:
Interior tubes do not factor into the CP calculations, only exterior body tubes. So if you have an engine mount tube that stick outs the back of your main tube really far, make sure to add a regular body tube on top and override the mass of one of the parts to zero.
Ring fins are also not currently supported, my suggestion is to take the ring circumference, divide it by the number of supporting fins then make plain rectangular fins.

I love oddrocs and with almost every rocket I fly OR cannot sim it. However I've gotten pretty good approximations by doing stuff such as the above.

Canopies (especially on LPR) in my experience don't mess with the flight path much, especially if they are close to the body and follow a smooth curve.
 

cbwho

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Thank you Moofle, that helps.

I was playing with the 4 fin crucible fin layout in OpenRocket. Going from 90 layout to a more X layout. It didn't seem to notice a difference even if I made the degree difference of a few degrees. So OpenRocket doesn't "understand" the importance of a balanced fin arrangement. I wonder how narrow the X can be and still be stable enough?

I have a production 2 T fin rocket, and the T tops are about 1/2 the size of the T bottoms. I suspect this may be "ok" since the farther from the main tube, the more leverage it has to change the orientation. I suspect for the fin pressure, there is the amount of torque it has on the rocket. The necessary fin area should be Newton meters of torque. Obviously calculation of that would need to be done via calculus or an approximation of it. Surely, I'm not the first to suggest this? I am a little surprised that OpenRocket doesn't calculate this.

A square fin will have less leverage in Newton-meters than a delta fin of equal root chord length due to the delta sticking out farther.
 

Nytrunner

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I am a little surprised that OpenRocket doesn't calculate this.
To understand Openrocket's limitations, you need to understand the assumptions included with the Barrowman equatiosn

Openrocket applies the same simple Barrowman equations that can be worked by hand in a closed form solution. If you want a more complex model, try Rocsim. The Rocsim developers took the Barrowman equations, undid some of the assumptions that make them hand solvable, and expanded them to accommodate more complex rocket geometries. (conversely, OR has better accuracy in transonic+ flight regimes than RS)

That being said, you can also make approximations of some structures in Openrocket using ~equivalent fin geometries, but that's an exercise left to the hobbyist.

If I have to simulate a rocket that I can't work in OR, I make a cfd model of it in another program
 

cbwho

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To understand Openrocket's limitations, you need to understand the assumptions included with the Barrowman equatiosn

Openrocket applies the same simple Barrowman equations that can be worked by hand in a closed form solution. If you want a more complex model, try Rocsim. The Rocsim developers took the Barrowman equations, undid some of the assumptions that make them hand solvable, and expanded them to accommodate more complex rocket geometries. (conversely, OR has better accuracy in transonic+ flight regimes than RS)

That being said, you can also make approximations of some structures in Openrocket using ~equivalent fin geometries, but that's an exercise left to the hobbyist.

If I have to simulate a rocket that I can't work in OR, I make a cfd model of it in another program
I appreciate your response. That makes sense.

I suspect that a fin on struts away from the body tube (more distance from the CG) applies more restorative pressure (via torque) than an equivalent sized fin.
Or to put it more realistically, 2 fins of equal area, but one with a short root chord and another with a long root chord, the shorter root chord will perform better - even without a change to the center of gravity - simply because more fin area will be farther from the center of pressure.

Indeed referring the Barrowman logic, he confirmed that, but his equations make the assumption of it being axially symmetric.
 
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Alan R

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I prefer building versus sitting in of the computer ...
I also prefer shorter rockets to longer rockets. If I cut a body tube in half, weigh the "discard" and put those grams as clay in the nose cone?
A couple of things to note. Short stubby rockets can fly with a lower caliber. Less distance between CG and CP. As in, a short rocket will fly fine on a .3 cal versus a long rocket that needs at least 1 cal or better. It's not really the weight that matters here, but it IS something you need to sim.

As for sitting in front of the computer, there is an Open Rocket or Rocksim file for almost all commercial kits. Look on Apogee site or RocketReviews for those. OR will open both file types. If you want to mod those, just download the appropriate file and chop off the body tube, design your own fin, or make whatever change you want. You don't have to model the whole thing, it's already been done.

Tube fins are a bit trickier, but there was a piece somewhere as a science experiment by TVM's daughter that showed that the effectiveness of tube fin compared to a straight fin was proportional to the total fin area. Diameter*pi*L for a tube vs flat area for a straight fin. Or something along those lines anyway. You could do a rough calculation and get pretty close. Tubes have more drag.
 

cbwho

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Alan,
Thank you for your reply.
I appreciate the suggestion to look for pre-existing data files as that will save time considerably. Also the part about the caliber allowance.

I don't remember tube fins from the late 80s. When where they invented? When did they become known?

Who is TVM's daughter ?

I took a tube fin rocket (Razor) and added a BSG Viper look. The rocket flew perfectly straight with no weather cocking.
 

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Alan R

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Who is TVM's daughter ?

I took a tube fin rocket (Razor) and added a BSG Viper look. The rocket flew perfectly straight with no weather cocking.
Double check my off-the-cuff statement about a .3 caliber for a stubby rocket. I think it's closer to .7, but still less than one.
Nice job on that rocket. Beautiful.
Sorry, you're new. TVM is Tim Van Milligan. He owns Apogee rockets.
 

cbwho

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My next rule of thumb rocket, built entirely of left over parts.
It's a cross between an X-Wing and Warthog fighters.
The X is set to 45 degrees.
Passes the balance test as well as swing tests.

I believe it will be stable in yaw and pitch. Soon to test before I decorate more...
 

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