TARC Rocket Weathercocking Issue

[DuckMaster]

Hello, I am new to designing model rockets and while designing one for a TARC challenge, my rocket weathercocks a large amount even though I have a low margin (0.6) and the wind speed in RockSim 10 is only slightly breezy (8-14 mph). The launch guide length that I am using is 8 feet and I am using an E16W-4 Aerotech rocket motor. I have tried make the margin closer to 1, but this increases the amount of weathercocking during flight and going any lower than 0.6 increases the chance of my rocket becoming unstable during flight. The amount of weathercocking makes the altitude of my rocket inconsistent which I need to avoid during a TARC challenge. Any help as to why my rocket is weathercocking is appreciated. Some screenshots of my model rocket as well as the RockSim 10 file are provided.

 

[Rex R]

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[Rex R]

I suspect that your rocket is going to slow off the launch guide. You're going to need a bigger motor and/or fins with more span or both

 

[boatgeek]

8-14 mph is pretty breezy. It's hard not to weathercock in that much wind. Any stability margin below 1.0 on this size and shape of rocket is pretty questionable and more likely to fly unpredictably.

 

[Zeta]

Before you worry too much about weather cocking :
Take a look at your motor choice E16-4 and the "optimal delay" 1.09 - 1.31...
I have not run the numbers on it given your target apogee.... but it seems like not enough time to get up there ... consider plotting the flight and look at where "ejection", and "apogee" are on the graph.. ... what tipped me off was that velocity at deployment is greater than the velocity off the launch rod. Deployment at 36 mph could be pretty rough on a TARC rocket that typically has thin cardboard tube.

Rex has a good idea.... a little more motor and fins that are a little longer... specifically the dimension from the air frame to the tip of the fin.

A stability of 1 would be my preferred minimum for a TARC rocket

 

[Voyager1]

As mentioned above, your stability margin is far too low at 0.65. Aim for 1.5 - 2.0 cal by adding extra forward mass or increase the span of your fins.
Additionally, position your launch lug near the CoG.

 

[Hobie1dog]

to the forum

 

[boatgeek]

As mentioned above, your stability margin is far too low at 0.65. Aim for 1.5 - 2.0 cal by adding extra forward mass or increase the span of your fins.
Additionally, position your launch lug near the CoG.

Increasing fin span is definitely a good idea. A good rule of thumb is that the fin span (body tube to fin tip) should be one rocket diameter. It's hard to do that with a TARC rocket and still end up with less than 1.5-2 calibers of stability.

 

[cls]

yeah, wider fins.

I can't tell from the pictures, did you add a correct amount of mass for the egg, padding, and altimeter?

Use an F motor, like F42.

do all that and check static margin, it should easily be 1.5 to 2

Enjoy the journey!

 

[ThirstyBarbarian]

I agree with the suggestions above. You need more speed off the rail. That means more thrust. Just as an example, instead of an E16, try an E20 or an E30. Those are all E motors and fall into the same impulse range, but the E20 has more thrust, and the E30 even more. They will fly straighter and weathercock less.

Usually TARC rockets fly on F motors, not E. They need that much impulse to reach the target altitude. So I think it’s likely your sim file is off. The rocket’s weight in the sim is probably too light. A typical TARC rocket with payload is too heavy to reach 800’ on an E16. That’s probably why the sim says it leaves the rail with adequate speed but it still weathercocks — it’s not actually getting off the rail as fast as the sim says. And that’s probably also why the sim says the ideal delay is so short — light rockets slow down very fast.

You should weigh the rocket with everything loaded in it, including payload and all recovery gear, but no motor or motor hardware loaded. Then balance it and measure from the tip of the nose to the balance point. Then check your file and make sure the mass and center of gravity in the file match your measurements. If not, override the mass and CG with what you measured. Then run your sims with the overrides and see what you get.

 

[mikec]

Have you actually flown this rocket or are you just asking about simulation results? If it's the latter, while none of this advice is bad, you really won't know what will happen until you get actual flight data.

 

[Ez2cDave]

Not enough fin span, combined with a large diameter airframe, is pretty much a guarantee to "wallow all over the sky". Like a HELLFIRE missile.

Dave F.

 

[DuckMaster]

Thank you all for this information! I am currently working on a new rocket design that takes into consideration all of these tips. I will post it here when I am finished. Also, I have not actually built the rocket in person yet, this is just a design in Rocksim. However, I will be building the new design I am working on in person and it will be flown soon.

 

[Zeta]

Go Duck Go

 

[DuckMaster]

Hello, while creating the new design for my rocket I have found that with a F motor such as the F42T the rocket goes much higher then the projected height of 850 feet (more than 1000 feet in fact). I have increased the mass of the rocket and the size of the fins from my previous design. As many of you have pointed out, most TARC rockets that have a projected altitude of around 800 feet use an F motor and I am wondering if there is a design flaw in my rocket or if there is a bug in my RockSim software. I triple checked the overridden masses of each part on my design to make sure that It matches up with the masses that are listed online. However, I have not created this design in person because I do not want to buy parts just to find out that my software or design was flawed and it does not reach the projected altitude. The weather cocking issue was resolved though . Any help would be much appreciated. The RockSim file as well as some screenshots are shown below:

 

[Ez2cDave]

Fins need to be about 40% - 50% smaller . . .

Dave F.

 

[sriegel]

A trick you can try to mitigate weathercocking is to angle the rail slightly downwind. Yes, downwind.

As your rocket leaves the rail, it is still picking up speed and crosswind causes it to weathercock. Launching a few degrees downwind will cause the weathetcocking to turn you more vertical. The trick is to find the right angle to match the windspeed.

Play with this in the sims to see for yourself.

 

[Zeta]

the team I worked with was using econojet E motors ...
adding weight to the rocket was done with oil clay, so this could be changed as need...ie not all eggs weight the same.

not sure what your budget is there but it is a good idea when you settle in on the motor choice.. get a lot of them and put the motors on the gram scale ... mark each one for it's weight ...

 

[ValorEagle]

Where can you put the oil clay in the rocket if you can't put it in the nose cone?

 

[smstachwick]

Where can you put the oil clay in the rocket if you can't put it in the nose cone?

Why wouldn’t you be able to? Most blow-molded noses are hollow, and you can drill holes in a balsa nose to fill with clay.

More generally for this rocket: A lot of TARC teams at the club I fly with prefer the AeroTech F67W motor. It’s got plenty of thrust to get the rocket off the pad with minimal weathercocking, and it’s not terribly expensive. They also make a “pow!” kind of sound that turns heads during a sport launch, which is fun

Also be sure to secure the eggs firmly in the capsule. If they shift around they’ll pull the rocket off-course.

 

[caveduck]

I think you need fins somewhere in between the ones you started with and the latest iteration.

There is another issue - your model is too short; TARC rules require a 650mm minimum length. That will add some minor amount of mass, and generally increases stability margin a bit.