Let's put an end to the "Base Drag Hack"

[lakeroadster]

Maybe so, John ...

But I wonder ...

IF @Buckeye's CFD models -and-or- @Spacedog49Krell's instrumentation studies were to identify and then quantify a normal force acting at the base of a low aspect ratio rocket at a non-zero angle of attack, THEN MAYBE the BDH could be 'baked into' the stability calcs for that class of rocket which otherwise violates the assumptions of the classical Barrowman Equations.

Then we would all be one step closer to "Let's Put an end to the Base Drag Hack" for that class of rockets ...

OTOH, I believe there will always be some rockets where the BDH needs to be applied and in that case, the BDH will continue to work as it now exists because it is after all only a cone appended to the tail of the rocket and the the Normal forces were well defined by Barrowman for that shape.

-- kjh

Why? It's easy enough to add the BDH yourself. This entire discussion is ludicrous. If you have a short fat rocket, take the 30 seconds it takes to add the base cone. If you have a long slender rocket, then don't add it.​

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"Baking it in" means it can't be removed unless you know code. Why would anybody want that? I certainly don't.​

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[Spacedog49Krell]

Then "we" need to stop implying that base drag is a stabilizing force.

Buckeye is doing great CFD simulation. I'll get live data for comparison and in a few months to years we should have an answer. I wish there were more of us questioning the established norm and with the talents to prove which side is correct.

 

[Alan15578]

Buckeye is doing great CFD simulation. I'll get live data for comparison and in a few months to years we should have an answer. I wish there were more of us questioning the established norm and with the talents to prove which side is correct.

Apart from the hack, I hope to see more practical CFD from Buckeye.

I am willing to concede that the hack has some utility. There may even be a stability effect from asymmetric base flow.

I suspect the the pi and/or d in the hack should actually be an input variable, and come from a curve fit plot based on real data, that is perhaps mainly a function of L/D.

My objection is that "we" are promoting a folk lore myth about base drag stability. After one gets to a good school and learn things properly, it is harder to unlearn things that you picked up when you were ignorant.

 

[lakeroadster]

Apart from the hack, I hope to see more practical CFD from Buckeye.

I am willing to concede that the hack has some utility. There may even be a stability effect from asymmetric base flow.

I suspect the the pi and/or d in the hack should actually be an input variable, and come from a curve fit plot based on real data, that is perhaps mainly a function of L/D.

My objection is that "we" are promoting a folk lore myth about base drag stability. After one gets to a good school and learn things properly, it is harder to unlearn things that you picked up when you were ignorant.

I don't think anybody here is saying base drag stability is a "myth", or that anybody is "ignorant".​

​

The questions is the magnitude of the base drag, given the geometry of the rocket itself.​

 

[Jeff Lassahn]

This is 4 degrees AoA nose up in the z-direction. I had to shift the Cp scale a bit to show the non-symmetry developing on the top of the base. Also, the counter-rotating vortices in the wake are becoming of different size.

I will look at some more angles, say 0 to 20 degrees, but I want to change my mesh scheme a bit to accommodate.


View attachment 633390View attachment 633391

So if I'm getting all my signs and vector direction correct, doesn't this simulation show that the force on the base is actually de-stabilizing?

As the nose tilts up, the base forms an asymmetric flow pattern where there's a low pressure region on the positive Z side, and the high pressure central region shifts to the negative Z direction.

So that would mean the positive Z side of the base pulls that side of the rocket backwards, and the negative side pushes forward (or at least pulls back less) which produces a net torque pulling the nose even farther in the positive Z direction.

Or am I getting something backwards?

(and deferring the question of if this is even a realistic model of that part of the rocket given that there's a motor generating gas right in the middle that's not modeled here)

 

[Alan15578]

I don't think anybody here is saying base drag stability is a "myth", or that anybody is "ignorant".​

​

The questions is the magnitude of the base drag, given the geometry of the rocket itself.​

I thought my compact word choice might be too objectionable. So instead of myth, lets just say folk lore explanation. In place of ignorant, lets say not formally educated in a fully accredited aerospace engineering curriculum. And yes, even well intentioned and educated people can disagree, especially in the art of language use.

 

[Alan15578]

The exhaust fills the space behind the tube and acts like a fairing, as long as the motor is producing thrust.
For a large diameter tube, this fairing effect is smaller such as a Minni Magg with an "H" motor. So, you get appreciable base drag, and therefore some stability.
When you bump up to a "J" motor the fairing effect is stronger, which will reduce base drag and thus stability.
The ultimate source of the "yeet" are what aerodynamicists call "non-linear aerodynamic effects" which is a fancy way of saying a behavior that is inherently chaotic in nature and does not follow a predictable, equation driven model.

I think what you are describing might be BDE, Big Dawg Effect?

I don't think "yeet" has anything to do with aerodynamic effect. I mean, if you rotate 90 degrees would it be zeet, or even xeet?

 

[Buckeye]

So if I'm getting all my signs and vector direction correct, doesn't this simulation show that the force on the base is actually de-stabilizing?

As the nose tilts up, the base forms an asymmetric flow pattern where there's a low pressure region on the positive Z side, and the high pressure central region shifts to the negative Z direction.

So that would mean the positive Z side of the base pulls that side of the rocket backwards, and the negative side pushes forward (or at least pulls back less) which produces a net torque pulling the nose even farther in the positive Z direction.

Or am I getting something backwards?

(and deferring the question of if this is even a realistic model of that part of the rocket given that there's a motor generating gas right in the middle that's not modeled here)

I see what you are saying. There is more negative pressure on the base top, less negative pressure on the base bottom, which would seem to create a moment that moves the nose up to more angle of attack. Is that de-stabilizing? I guess so? Maybe Barrowman is correct. It is all about normal forces, not axial forces that make the rocket stable. See my annotations.

However, the total y-moment on the rocket is negative, which would push the nose back down?

 

[bill_s]

I don't think anybody here is saying base drag stability is a "myth", or that anybody is "ignorant".​

​

The questions is the magnitude of the base drag, given the geometry of the rocket itself.​

I question that the stabilizing effect has much or anything with the base drag at all. Consider the most popular saucer rocket shape, a flattened cone. If this starts traveling at an angle, the side pitched forward becomes more square on to the air, and the other side less so, producing an asymmetrical drag that stabilizes the rocket. All this happens without the first consideration of the back side of the rocket, of which my intuitive sense can tell you nothing, other than that there will be drag and it's located at the back.

If I open this flattened cone in OR, it shows the CP and CG almost identical, yet we know from experience that a motor can shift the CG rearwards and it will still be stable. Perhaps that's the whole problem, the models just place the CP of the cone(s) too far forward. If you put it on the front of a long rocket, that difference is almost insignificant and a tiny bit more force centered further aft would have the same effect on stability. But on a short rocket this would produce larger effects, appearing more stable, and on a saucer would make it instead of breaking it.

I see what you are saying. There is more negative pressure on the base top, less negative pressure on the base bottom, which would seem to create a moment that moves the nose up to more angle of attack. Is that de-stabilizing? I guess so? Maybe Barrowman is correct. It is all about normal forces, not axial forces that make the rocket stable. See my annotations.

However, the total y-moment on the rocket is negative, which would push the nose back down?

View attachment 633534

Interesting. At the same time, the whole base moves to the side of the CG centerline of movement in a stabilizing manner, how could these always perfectly cancel each other?

I also note the red pressure on the nose cone shifts to the side, producing a normal force but also an asymmetrical drag component that may be stabilizing for a rocket this short, but wouldn't be overall on a long rocket. Again, the effect is that the cone's CP is farther back. Do Barrowman's nosecone models actually lump in a drag effect, using a typical rocket? Is it simply a real measurement, EXCEPT for the CP? NCs definitely have a strong effect on stability, them vs. fins is the whole thing (and tailcones, if any).

 

[Spacedog49Krell]

Those of the Forum that have designed "stick" rockets in OR know that OR does not calculate CP correctly for non-classical rocket designs.

 

[Buckeye]

I question that the stabilizing effect has much or anything with the base drag at all. Consider the most popular saucer rocket shape, a flattened cone. If this starts traveling at an angle, the side pitched forward becomes more square on to the air, and the other side less so, producing an asymmetrical drag that stabilizes the rocket. All this happens without the first consideration of the back side of the rocket, of which my intuitive sense can tell you nothing, other than that there will be drag and it's located at the back.

If I open this flattened cone in OR, it shows the CP and CG almost identical, yet we know from experience that a motor can shift the CG rearwards and it will still be stable. Perhaps that's the whole problem, the models just place the CP of the cone(s) too far forward. If you put it on the front of a long rocket, that difference is almost insignificant and a tiny bit more force centered further aft would have the same effect on stability. But on a short rocket this would produce larger effects, appearing more stable, and on a saucer would make it instead of breaking it.


Interesting. At the same time, the whole base moves to the side of the CG centerline of movement in a stabilizing manner, how could these always perfectly cancel each other?

I also note the red pressure on the nose cone shifts to the side, producing a normal force but also an asymmetrical drag component that may be stabilizing for a rocket this short, but wouldn't be overall on a long rocket. Again, the effect is that the cone's CP is farther back. Do Barrowman's nosecone models actually lump in a drag effect, using a typical rocket? Is it simply a real measurement, EXCEPT for the CP? NCs definitely have a strong effect on stability, them vs. fins is the whole thing (and tailcones, if any).

Now, nosecones are in question along with base drag stabilization! Yes!

This is where the holistic simulation of an entire rocket with CFD is very useful vs. superposition of individual parts.

 

[aerostadt]

Of course, all these tools, OR, RocSim, and CFD are useful. I don't think for most model rocketeers any of these tools are going to go away soon.

 

[Spacedog49Krell]

Now, nosecones are in question along with base drag stabilization! Yes!

This is where the holistic simulation of an entire rocket with CFD is very useful vs. superposition of individual parts.

Your nosecone CFD got me thinking that I need to add sensors to the nosecone for AoA determination.

 

[sr205347d]

Those of the Forum that have designed "stick" rockets in OR know that OR does not calculate CP correctly for non-classical rocket designs.

You mean like bottle rockets?





The ancient Chinese must have had a "long stick hack" for OR.

 

[kjhambrick]

The exhaust fills the space behind the tube and acts like a fairing, as long as the motor is producing thrust.
For a large diameter tube, this fairing effect is smaller such as a Minni Magg with an "H" motor. So, you get appreciable base drag, and therefore some stability.
When you bump up to a "J" motor the fairing effect is stronger, which will reduce base drag and thus stability.
The ultimate source of the "yeet" are what aerodynamicists call "non-linear aerodynamic effects" which is a fancy way of saying a behavior that is inherently chaotic in nature and does not follow a predictable, equation driven model.

Speaking of PFM (*)

And as long as I am clouding the issue

Why do rocket exhaust trails take the form that they do ?

This is yesterday's maiden voyage of 'T'Pring's P'Toy, a 1.9 inch diameter LOC Vulcanite scale model on an H180W:

I wonder if there is an answer in a CFD study ?

-- kjh

(*) PFM - TLA for Pure Fu Magic

 

[Buckeye]

More data.

Here is a CFD angle of attack study on the stubby FatBoy. Included are the Galejs equations (what Barrowman left out) and my CFD analysis.

https://www.rocketryforum.com/threads/cop-simulation-comparison-including-cfd.184722/post-2560157.

The punchline is that the CP moves very little with AOA on stubby rockets. The extra 1.0-caliber rule of thumb is intended to provide sufficient margin for CP movement from crosswinds on 10:1-ish rockets. Since the movement is minimal in this case (about 0.1 calibers at 30 degrees), there is really no reason to worry if the static margin is less than 1.0 caliber, and no reason to add a fake cone to the back of the rocket. Stubby rockets with fins that look like rockets do not need a hack.

The Hack is doubly dangerous. It moves the CP too far back to begin with AND restricts the forward movement of CP under crosswinds.