Super Big Bertha fails spin test

[Jim McNabb]

I slowly and meticulously assembled the Super Big Bertha as the instructions outlined. I'm sure the finished rocket has been built correctly yet when I perform the spin test it fails. I had heard larger rockets can fail the spin test yet still be stable in flight. True or not I don't know. Before I start adding weight to the nose I was wondering if anyone had some insight to this issue. The rocket was configured for flight with engine and recovery system. Thanks.

 

[n3tjm]

I would not worry about it, personally I find the "Spin Test" a lousy way to check sability since I too had perfectly stable rockets want to fly backwards when spun around on a string. SBB and BB are very stable designs.

 

[Antares JS]

Agree with above. The spin test is overrated. I haven't done one since I was 12, and my Fat Boy that had flown perfectly fine before flew backwards. I also own a Super Big Bertha that I built perfectly stock, and it flies just fine.

 

[Scott_650]

If anything the SBB built straight from the box is on the high side of over stable, weather cocking is much more likely than going ballistic. Mine has been modded - stretched a bit with a payload bay, bulkheads, NCR-style cable and kevlar “gorilla” shock cord, papered fins - and it’s been a great flying rocket before and after being modified. The downside to the mods is it’s too heavy with the payload section for calm conditions flying on an adapted E12 - only works without the bay in “shorty” configuration.

 

[cbrarick]

the spin test is your problem.
Always question your results to a test when it should confirm what you know to be reality.

the bigger question is "why would you use a gross test (spin test) when you have much better test (any rocket simulation program)???

 

[SolarYellow]

In the spin test (or swing test), you tie the rocket to the string at the c.g., which is usually somewhat rearward of the longitudinal center of the rocket, due to the mass of the motor and fins. For a rocket "flying" with an alignment tangent to the circle at the string, the airflow is at zero angle of attack only at the point where the string is tied. The portions ahead of the string experience a progressively greater aoa and airspeed in one direction as you move forward from the string, and the portions behind the string experience a progressively greater aoa and airspeed in the opposite direction as you move rearward from the string. The angular progression with distance from the string is non-linear, and the forward portions usually experience more of it, because they extend farther into that non-linear range. So it can be expected that the effect of the non-linearly-increasing aoa and airspeed on the portions ahead of the string may overcome the opposite aoa and airspeed acting on the portions behind the string, and the rocket will appear to fail the test. This is why the swing test is not a good test for long rockets.

 

[rcktnut]

With sim programs spin tests are a thing of the past. They were unreliable anyway to begin with. I have a SBB and it flies just fine built stock.

 

[lakeroadster]

A simulation is a poor substitute for a swing test:

The simulation assumes all data is input correctly.​

The swing test is real world.​

How long was the string? How fast did you spin the rocket? Did the rockets nose hang slightly below level?

 

[Tractionengines]

I have a SBB built with plywood fins and centering rings from Launch Lab Rocketry [Vander-Burn upgrades]. (Definitely more tail weight than stock.) Adding a "laundry shelf" at the coupler, so the chute/harness is in the front body tube, is enough to keep it stable. Has flown fine on G40's.

 

[n3tjm]

No, swing test is not the real world. A wind tunnel would be a better test. Under normal circustrances rockets dont have a circufrical force acting on them in that vector.

There is nothing wrong with simulations as long as you know your data inpute is correct.

 

[Jim McNabb]

the spin test is your problem.
Always question your results to a test when it should confirm what you know to be reality.

the bigger question is "why would you use a gross test (spin test) when you have much better test (any rocket simulation program)???

I'll look into a simulation program. I've never used one before. This is my first "large" MPR.

 

[lakeroadster]

I'll look into a simulation program. I've never used one before. This is my first MPR.

• How long was the string?
• How fast did you spin the rocket?
• Did the rockets nose hang slightly below level?

 

[Jim McNabb]

• How long was the string?
• How fast did you spin the rocket?
• Did the rockets nose hang slightly below level?

The string was about 10ft. I'm not sure exactly how fast I was spinning the rocket, maybe a revolution every 2 to 2.5 seconds. The nose didn't hang below level.

 

[OzHybrid]

Minimum stable flying speed for a model rocket off rail is 30 Mph=44ft/second
10ft radius=20 ft dia=63ft/revolution =1.4 seconds per revolution for 30Mph to get to minimum generally acceptable flying speed.
At 2-2.5 secs per revolution, you're not swinging fast enough to get to stability..... The angle of attack from the diameter will require you to spin even faster. The longer your swinging rope the less this effect will be.
You may achieve a stable flight at a lower speed.
Make sure you have the cg in the correct spot and the tether is tied around the cg and the tether doesn't move from cg.
In swing tests, I've had rockets fly backward for most of the test and then slowly turn around to fly stable once it got above flying speed
Good luck.

 

[NTP2]

I'll look into a simulation program. I've never used one before. This is my first "large" MPR.

(Paging open rocket salesman @neil_w )

 

[Art Upton]

I'll look into a simulation program. I've never used one before. This is my first "large" MPR.

Open Rocket Free here: https://openrocket.info/

 

[Jim McNabb]

Open Rocket Free here: https://openrocket.info/

Thank you

 

[Art Upton]

Thank you

Also , here is the thread that contains the OpenRocket simulator file for that rocket Super Big Bertha.

Makes it easy to start, then you can change the weight to your rocket's build.

https://www.rocketryforum.com/threads/sim-file-for-new-psii-super-big-bertha.145601/

Edit: Seems you also need to change the motor mount back to 29mm , as the maker of the file up graded it.

 

[OzHybrid]

A stable swing test tells you your rocket IS going to fly stably.( provided your motor has sufficient thrust) An unsuccessful swing test does NOT tell you your rocket IS unstable. It says it might be.
I'd use Open Rocket first. It takes into account the rocket aerodynamics, weight, and thrust of the motor. Allows you to change motors without having to fly it.

If you are in any doubt, you'll get good answers here.

 

[neil_w]

I saw the bat-signal.

For rockets with what I would call “standard” configurations, the sim programs should give you a pretty accurate prediction of stability and flight characteristics. SBB certainly falls into this category. A swing test should not be necessary.

For oddrocs and other non-traditional configurations, sim results are often less reliable, and sometimes complete nonsense. In these situations a swing test might provide useful data.

 

[OzHybrid]

No, swing test is not the real world. A wind tunnel would be a better test. Under normal circustrances rockets dont have a circufrical force acting on them in that vector.

There is nothing wrong with simulations as long as you know your data inpute is correct.

The centripetal force is the force exerted on the rocket by the tether toward the center of rotation. the centrifugal force is the force exerted by mass of the rotating rocket away from the center of the rotation.
In a real rocket flight, any wind acting horizontally would be similar to the centripetal force, but would be pushing from the opposite side. This is why the swing test is a good test of the types of real world forces a rocket is subject to and if the test shows as stable the rocket will be stable in flight.
However, because the forces centripitally create an effective high angle of attack if a small swing diameter is used, the effective angle of attack may be greater than would be seen in any real world rocket flight situation.
You've got to understand what the swing test is actually doing, know how fast your rocket is flying for the test and your angle of attack.

A superficial examination of the swing test says it's an easy test to do. A closer examination shows the test is actually a harder test for the rocket to pass, but if it does pass, it's going to be stable.

 

[SolarYellow]

Minimum stable flying speed for a model rocket off rail is 30 Mph=44ft/second

That rule of thumb is really just to ensure there's some margin vs. cross winds to keep a gentle breeze from making the angle of attack too great and shifting the CP forward so the rocket becomes unstable. With no wind, the only disturbance to the rocket, aerodynamic or otherwise, is rod whip, so you really just need to have enough forward velocity to recover from whatever whip the rod imparts. Which should be near-zero if you're launching from a rail or tower, and not that much if you have a nice, straight, clean rod on an LPR whoosh-pop. Without a cross-wind or other disturbance, a stable rocket should be stable at much lower speeds.

 

[OzHybrid]

That rule of thumb is really just to ensure there's some margin vs. cross winds to keep a gentle breeze from making the angle of attack too great and shifting the CP forward so the rocket becomes unstable. With no wind, the only disturbance to the rocket, aerodynamic or otherwise, is rod whip, so you really just need to have enough forward velocity to recover from whatever whip the rod imparts. Which should be near-zero if you're launching from a rail or tower, and not that much if you have a nice, straight, clean rod on an LPR whoosh-pop. Without a cross-wind or other disturbance, a stable rocket should be stable at much lower speeds.

30 Mph is that reasonable speed for flight. That velocity is WHY the recommended min acceleration is 4-5 G. That amount of acceleration gets you to at least 30 Mph by the time you leave the guide rod for a model rocket. Tripoli rules CAN permit lower limits with RSO approval, but in general terms, thats what and why they exist.
At 0 velocity forward, your rocket is only dynamically stable in the theoretical world, not the real one.

 

[green dragon]

meh - It's a KIT... actually a well known and tested proven design..... not sure why anyone would worry it wasn't stable, unless maybe a J motor in the back or somesuch.
Cp is pretty simple to guestimate on a generic 3 or 4 fin design, eyeball the 'cutout method', find the CG and call it good.

 

[OzHybrid]

meh - It's a KIT... actually a well known and tested proven design..... not sure why anyone would worry it wasn't stable, unless maybe a J motor in the back or somesuch.
Cp is pretty simple to guestimate on a generic 3 or 4 fin design, eyeball the 'cutout method', find the CG and call it good.

It would be really great if all kit manufacturers had the CP either marked on the rocket or specified in the instructions. Rather relying on the good will of others to create the modeling for them in Open Rocket or Rocksim.
Neither the CP or the expected CG seems to be specified for anything on the Estes website. A bit too much glue at the back end of a rocket and you could unknowingly be going into skywriting flight territory.

 

[MikeT]

Mine fly's fine . I did replace the shock cord and upgraded the fins and centering ring to ply. Never even bothered to check CG,

 

[SolarYellow]

It would be really great if all kit manufacturers had the CP either marked on the rocket or specified in the instructions. Rather relying on the good will of others to create the modeling for them in Open Rocket or Rocksim.
Neither the CP or the expected CG seems to be specified for anything on the Estes website. A bit too much glue at the back end of a rocket and you could unknowingly be going into skywriting flight territory.

I've read that all Estes rockets are designed for a minimum of 1.5 calibers. Obviously, longer rockets get more. If that's true, you'd have to really be out of whack in your build to have a problem, assuming you build to the instructions.

 

[rcktnut]

So what is the weight limit of a rocket to perform a swing test? If a sim is good enough for over the limit it's good enough for ones under the limit.

 

[neil_w]

So what is the weight limit of a rocket to perform a swing test? If a sim is good enough for over the limit it's good enough for ones under the limit.

A sim *is* good enough when the design is within the capabilities of the software, regardless of weight.

 

[OzHybrid]

So what is the weight limit of a rocket to perform a swing test? If a sim is good enough for over the limit it's good enough for ones under the limit.

I've done 5Kg. That's a lot of hanging on while you're spinning it and you really want to make sure that if it gets loose it's not going to land anywhere unfortunate. Just wanted to see if it was possible and probably wouldn't do it again.