Weather cock or top heavy?

[Richardx]

Rocket is a standard TARC specification (BT80, 3 fins). Stability margin of 1.4 cal. AeroTech F51NT. Thrust to weight ratio is about 9:1.
Pretty windy launch condition (9+ mph with strong gust)
Rocket launches off the pad straight but immediately tilts slightly. Rest of flight is pretty straight.

If the rocket flew into the wind during the coast phase, I can chuck it up to a weather cocking. However, the tilt right out of rail seem odd to me.
Question is, is this a typical weather cocking behavior or something else (like top heavy)?
Posting 3 photos

Thank you!

 

[Richardx]

Similar behavior across 2 launches.
Just to add. When the rocket flew 15g lighter, it flew fine.

 

[lakeroadster]

It is tilting into the wind... correct?

 

[Steve Shannon]

“Top heavy” isn’t a thing except when it causes overstability.
This could be weathercocking or it could be leaving the rail badly, similar to rod whip. How easily do the rail guides slide in the rail? How straight is the motor mount? Is the CG of the rocket in line with the centerline of the motor mount? If it’s offset your thrust will cause the rocket to rotate around the CG when it leaves the rail.

 

[neil_w]

If the rocket flew into the wind during the coast phase, I can chuck it up to a weather cocking. However, the tilt right out of rail seem odd to me.
Question is, is this a typical weather cocking behavior or something else (like top heavy)?

Bending off the rail and then straightening out is absolutely typical weathercocking behavior. However, it could be other things as well, as @Steve Shannon points out above.

 

[Richardx]

It is tilting into the wind... correct?

Not quite. Wind was blowing into the lakebed. Also the smoke trail was pretty calm near the rail. That's why it was hard to rule it as a weather cocking.

 

[rharshberger]

Coning is one possible effect of to much noseweight (top heavy) the tail kinda wiggles in a circular motion as the rocket ascends. Tilting off the rail into the wind which that looks like can be mitigated by A) more velocity off the rail or B) angling the rail with the wind so the rocket weathercocks to a more vertical trajectory, or a bit of both.

 

[Richardx]

“Top heavy” isn’t a thing except when it causes overstability.
This could be weathercocking or it could be leaving the rail badly, similar to rod whip. How easily do the rail guides slide in the rail? How straight is the motor mount? Is the CG of the rocket in line with the centerline of the motor mount? If it’s offset your thrust will cause the rocket to rotate around the CG when it leaves the rail.

The rocket flies dead straight when it is little lighter. (photo from second post) Flew this rocket several launch days before without any problem. We had to change the motor and needed to fly little heavier. As soon as additional weights were added, it started to tilt. Same stability margin, plenty of thrust. scratching my head trying to figure it out.

 

[tfish]

how long is your rail?
What your exit speed off the rail?
what's your fins simi span?

Tony

 

[Steve Shannon]

The rocket flies dead straight when it is little lighter. (photo from second post) Flew this rocket several launch days before without any problem. We had to change the motor and needed to fly little heavier. As soon as additional weights were added, it started to tilt. Same stability margin, plenty of thrust. scratching my head trying to figure it out.

How do you maintain the same stability margin while adding mass?

 

[Richardx]

how long is your rail?
What your exit speed off the rail?
what's your fins simi span?

Tony

8' 1010 rail but 7' clearance
According to sim, the launch velocity is around 31mph
Semi span is about 1x diameter and root cord is around 2x diameter

 

[Richardx]

How do you maintain the same stability margin while adding mass?

Rocket has 2 weight pockets. One forward of Cg and other below the Cg. Near equal distance. by adding equal weights on both location, additional weight can be added without changing the Cg location.

 

[Steve Shannon]

Rocket has 2 weight pockets. One forward of Cg and other below the Cg. Near equal distance. by adding equal weights on both location, additional weight can be added without changing the Cg location.

Clever.

 

[Richardx]

Coning is one possible effect of to much noseweight (top heavy) the tail kinda wiggles in a circular motion as the rocket ascends. Tilting off the rail into the wind which that looks like can be mitigated by A) more velocity off the rail or B) angling the rail with the wind so the rocket weathercocks to a more vertical trajectory, or a bit of both.

My gut feeling is option A. Additional weight might just tip the scale on the thrust. Only 15g difference.
But I don't quite understand what could cause it. Thrust to weight ratio is well above 5:1. It is over 9:1.
At 30mph off the rail, it is coming off fast enough.

 

[Steve Shannon]

My gut feeling is option A. Additional weight might just tip the scale on the thrust. Only 15g difference.
But I don't quite understand what could cause it. Thrust to weight ratio is well above 5:1. It is over 9:1.
At 30mph off the rail, it is coming off fast enough.

30 mph is the bare minimum for velocity off the rail. Try a longer rail or higher thrust motor.

 

[tfish]

8' 1010 rail but 7' clearance
According to sim, the launch velocity is around 31mph
Semi span is about 1x diameter and root cord is around 2x diameter

That all looks in order...

Tony

 

[Richardx]

30 mph is the bare minimum for velocity off the rail. Try a longer rail or higher thrust motor.

That's what I am thinking. Not enough velocity off the rail. Might be ok on the sim but not enough in the real flight.
Thank you very much for taking the time to respond!

 

[Richardx]

Steve mentioned that the top heaviness is irrelevant. Which I agree for the most part of flight. However, during the initial launch, the drag isn't quite enough to stabilize the rocket using the pressure. If so, can't the weight distribution (top or bottom heaviness) influence the rocket's initial behavior?

 

[marintse]

Rocket is a standard TARC specification (BT80, 3 fins). Stability margin of 1.4 cal. AeroTech F51NT. Thrust to weight ratio is about 9:1.
Pretty windy launch condition (9+ mph with strong gust)
Rocket launches off the pad straight but immediately tilts slightly. Rest of flight is pretty straight.

If the rocket flew into the wind during the coast phase, I can chuck it up to a weather cocking. However, the tilt right out of rail seem odd to me.
Question is, is this a typical weather cocking behavior or something else (like top heavy)?
Posting 3 photos

Thank you!

Winds can vary significantly at elevations above ground level, in both steady velocity and gusts, especially after motor burnout.

 

[Daddyisabar]

Gosh dern it, Rocket Science is hard! Those atmospheric derivations are a b$tch! We can overcome. We have the technology. Trust in machines. Technology will save us. Give me active control vanes, give me gimbaling, give me electronics to land back on the pad under power!

 

[Steve Shannon]

Steve mentioned that the top heaviness is irrelevant. Which I agree for the most part of flight. However, during the initial launch, the drag isn't quite enough to stabilize the rocket using the pressure. If so, can't the weight distribution (top or bottom heaviness) influence the rocket's initial behavior?

Well, I didn’t really intend to say that top heaviness is irrelevant. I think what I said is that it’s not a thing outside of how it relates to center of mass (gravity). What I meant was that top heaviness is already reflected in the location of the center of gravity related to the center of pressure and that the concepts of stability, instability or over-stability already adequately allow us to anticipate how a rocket will react to airflow. A rocket that is top heavy is likely to be overly stable, making it twitchy, reacting more readily to wind shear.
But you have to be careful how you explain stability and drag. Drag doesn’t stabilize a rocket or at least not that simply. A special kind of drag called Lift does. Aerodynamic forces that increase whenever a rocket veers off course (or the direction of airflow changes) act upon a rocket whenever it starts tilting away from the axis of its thrust. In a stable design those aerodynamic forces force the rocket to steer back into the direction airflow, which actually reduces lift.
The farther apart the center of gravity and the center of pressure are, the greater the lever arm for the torque that steers the rocket back into the airflow. So in that sense, being top heavy does have an effect, but it’s already described as having a large margin of stability.

 

[Scott_650]

Steve mentioned that the top heaviness is irrelevant. Which I agree for the most part of flight. However, during the initial launch, the drag isn't quite enough to stabilize the rocket using the pressure. If so, can't the weight distribution (top or bottom heaviness) influence the rocket's initial behavior?

It’s not exactly drag that stabilizes the rocket, it’s lift from the fins working on the moment arm that pivots around the center of mass. (Whether lift is a kind of drag depends on how you look at it) Being “top heavy” would only have an effect if the velocity drops to the point that the fins no longer produce enough lift to keep the rocket stable along it’s current vector - in which every rocket that’s statically stable, i.e. the center of mass is ahead of the center of pressure, is going to nose over and fall. Then gravity is going to accelerate the rocket back to the surface, typically heavy side down, until the fins start producing lift again, making the rocket stable again (but not in, for our needs, a good way) as it falls - giving you the classic “lawn dart”.

What’s really kind of neat is when after the rocket looses enough upward velocity and starts to fall back to earth it accelerates enough for the fins to work while still tail down - you get that backslide style recovery fall.

Steve Shannon’s post popped up while I was still typing - so this whole post could’ve been three words - what he said

 

[Steve Shannon]

It’s not exactly drag that stabilizes the rocket, it’s lift from the fins working on the moment arm that pivots around the center of mass. (Whether lift is a kind of drag depends on how you look at it) Being “top heavy” would only have an effect if the velocity drops to the point that the fins no longer produce enough lift to keep the rocket stable along it’s current vector - in which every rocket that’s statically stable, i.e. the center of mass is ahead of the center of pressure, is going to nose over and fall. Then gravity is going to accelerate the rocket back to the surface, typically heavy side down, until the fins start producing lift again, making the rocket stable again (but not in, for our needs, a good way) as it falls - giving you the classic “lawn dart”.

What’s really kind of neat is when after the rocket looses enough upward velocity and starts to fall back to earth it accelerates enough for the fins to work while still tail down - you get that backslide style recovery fall.

Steve Shannon’s post popped up while I was still typing - so this whole post could’ve been three words - what he said

I’m just glad I didn’t put my foot in my mouth!

 

[Daddyisabar]

Who said you can never have too much nose weight?

Trust in thrust. Nose weight and thrust solve 90% of your problems and that ain't bad. Excessive nose weight and thrust are mere crutches for poor rocket design. Keep your motor and fins at the back of the rocket. Base drag is your friend.

Such mantras beguile my inner youngling rocket builder.

In all aspects of civil society STABILITY MAINTENANCE IS JOB ONE!

 

[BABAR]

I think a number of rockets don’t quiiiiiiite have enough speed of the rod or rail (still accelerating), start to go unstable immediately so they tilt off vertical, but with the continued acceleration (amplified because now no lug or rail button drag) they BECOME stable, and then fly perfectly straight, but unfortunately off vertical axis.

I think the scariest I have seen was at NSL at Lucerne. High power rocket did a complete loop and more, headed off at a 45 degree angle (fortunately UP) straight as an arrow And away from the spectator line.

 

[lakeroadster]

It’s not exactly drag that stabilizes the rocket...

It depends on the design

Some rockets have no fins at all, yet they fly stable: spools, pyramids, etc. For those designs, it has to be drag that makes them stable, right?