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.​

​

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

​

​

 

[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.

 

[BABAR]

Warlock swing test

Next launch will be from the far pad.

Edit: crud, Thirsty beat me to it.

 

[ksaves2]

View attachment 644545

Next launch will be from the far pad.

Edit: crud, Thirsty beat me to it.

One way to make crushed nuts!! 'Scuse me I just had to say. Kurt

 

[Daddyisabar]

The CP in front of the CG is a reality for a given geometry. It's up to the designer to remedy the situation, or to abandon the design as unstable.​

​

Thus is the journey of an odd roc designer. A long and skinny 3/4FNC designer will never experience this.​

Oh great Master, taking the oddroc road less traveled is perilous. Push the envelope too far and the given geometry will result in the words "Inherently unstable" written on the permanently recorded flight card. Unforgivable. Demoted to scum and villainy. Beware the Dark Arts. Where are your rebel hacks now? Properly simmed 3-4FNC, commence primary ignition. You may fire when ready

 

[Daddyisabar]

You mean like bottle rockets?

View attachment 633709

View attachment 633710

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

Ancient Chinese Secrets! Calgon, take me away!

 

[RobertH3]

Oh great Master, taking the oddroc road less traveled is perilous. Push the envelope too far and the given geometry will result in the words "Inherently unstable" written on the permanently recorded flight card. Unforgivable. Demoted to scum and villainy. Beware the Dark Arts. Where are your rebel hacks now? Properly simmed 3-4FNC, commence primary ignition. You may fire when ready

View attachment 644634

LEMOI! or, lovin' every minute of it!
Great original. Did it work?

 

[Daddyisabar]

LEMOI! or, lovin' every minute of it!
Great original. Did it work?

Does a computer simulation of an egg with no fins prove stability without a ton of hacks? Egg geometry is very unforgiving.

A collosal failure. All motors lit but it began cartwheeling 10 to 20 feet off the rod and landed next to the pad spinning away and ejecting on the ground. Lots of gasps and giggles from the crowd. Thankfully the regular RSO was in Europe that week or I would have been in deep doo doo. He had a Zero tolerance policy for unstable oddrocs.

STAY AWAY FROM HOBBY LOBBY PLASTIC HOLIDAY EGGS! Danger Danger Danger! Egg rockets need big, traditional fins!

 

[lakeroadster]

Here's a bit of the data I've archived that pertain to Base Drag... or Form Drag

Rocket Aerodynamics NASA Glenn Research Center

We can also think of drag as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the rocket and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating, or adding up the local pressure times the surface area around the entire body. The base area of a model rocket produces form drag.​

Rocket and Space Technology

Form drag, or pressure drag, is caused by the air that is flowing over the body. The separation of air creates turbulence and results in pockets of low and high pressure that leaves a wake behind the body (thus the name pressure drag). This opposes forward motion and is a component of total drag. Since this drag is due to the shape, or form of the body, it is called form drag. Streamlining an aircraft, rocket or reentry vehicle will reduce form drag, and parts of a vehicle that do not lend themselves to streamlining are enclosed in covers called fairings that have a streamlined shape.​

Optimal design in terms of minimum drag depends heavily on the velocity in question. For rockets spending most of their flight at subsonic speeds, the nosecones should be rounded and they should have boat tails.​

Lakeroadster Note:​

When I think about this I recall the huge fairing they built for the space shuttle when they carried it on top of the Boeing 747.​

​

Drag Coefficient Predictions​

Base Drag is a contributor to Pressure Drag, and is attributed to the blunt aftend of the rocket. Base Drag can be a significant contributor to the rocket’s overall drag during power-offf light (after engine burnout).​

 

[bad_idea]

Here's a bit of the data I've archived that pertain to Base Drag... or Form Drag

Rocket Aerodynamics NASA Glenn Research Center

We can also think of drag as aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the rocket and is called form drag. As air flows around a body, the local velocity and pressure are changed. Since pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, a varying pressure distribution will produce a force on the body. We can determine the magnitude of the force by integrating, or adding up the local pressure times the surface area around the entire body. The base area of a model rocket produces form drag.​

Rocket and Space Technology

Form drag, or pressure drag, is caused by the air that is flowing over the body. The separation of air creates turbulence and results in pockets of low and high pressure that leaves a wake behind the body (thus the name pressure drag). This opposes forward motion and is a component of total drag. Since this drag is due to the shape, or form of the body, it is called form drag. Streamlining an aircraft, rocket or reentry vehicle will reduce form drag, and parts of a vehicle that do not lend themselves to streamlining are enclosed in covers called fairings that have a streamlined shape.​

Optimal design in terms of minimum drag depends heavily on the velocity in question. For rockets spending most of their flight at subsonic speeds, the nosecones should be rounded and they should have boat tails.​

Lakeroadster Note:​

When I think about this I recall the huge fairing they built for the space shuttle when they carried it on top of the Boeing 747.​

View attachment 649045​

Drag Coefficient Predictions​

Base Drag is a contributor to Pressure Drag, and is attributed to the blunt aftend of the rocket. Base Drag can be a significant contributor to the rocket’s overall drag during power-offf light (after engine burnout).​

While this is data in the sense that it is information, it isn't data as the word "data" is used in science or engineering, which is probably the disconnect we have in the other thread, where when you said "data" I thought you meant a collection of analyzable numbers.

There's also nothing in any of what you quoted on the stabilizing effect of base drag, which is what the hack is supposed to represent. If I cover a rocket with carpet, I will have greatly increased its drag without changing its stability. Is the effect of base drag similar or different from that case?

As I said in the other thread, the degree to which base drag contributes to dynamic stability is not clear to me. I would welcome sources on that aspect of base drag.

 

[lakeroadster]

While this is data in the sense that it is information, it isn't data as the word "data" is used in science or engineering, which is probably the disconnect we have in the other thread, where when you said "data" I thought you meant a collection of analyzable numbers.

There's also nothing in any of what you quoted on the stabilizing effect of base drag, which is what the hack is supposed to represent. If I cover a rocket with carpet, I will have greatly increased its drag without changing its stability. Is the effect of base drag similar or different from that case?

As I said in the other thread, the degree to which base drag contributes to dynamic stability is not clear to me. I would welcome sources on that aspect of base drag.

Drag Coefficient Predictions​

Base Drag is a contributor to Pressure Drag, and is attributed to the blunt aftend of the rocket. Base Drag can be a significant contributor to the rocket’s overall drag during power-off flight (after engine burnout).​

 

[bad_idea]

Drag Coefficient Predictions​

Base Drag is a contributor to Pressure Drag, and is attributed to the blunt aftend of the rocket. Base Drag can be a significant contributor to the rocket’s overall drag during power-off flight (after engine burnout).​

Drag does not necessary contribute to dynamic stability, as I indicated with my carpet example. It's a subject I would like to understand better, but your source does not mention stability at all. (perhaps it does, albeit not directly). Edit: on further review, I don't think it does:

We will limit our discussion to Zero Lift Drag or Parasite Drag, which is the total rocket drag
independent of lift. This occurs at zero Angle of Attack for rockets.

 

[lakeroadster]

Drag does not necessary contribute to dynamic stability, as I indicated with my carpet example. It's a subject I would like to understand better, but your source does not mention stability at all. (perhaps it does, albeit not directly).

It's on page 10 of the above referenced article.​

Drag Coefficient Predictions​

 

[bad_idea]

It's on page 10 of the above referenced article.​

That is where base drag is discussed, yes. Where is it discussed in the context of stability? As my edit above says, the author specifically states he is only examining the case of zero angle of attack.

 

[lakeroadster]

That is where base drag is discussed, yes. Where is it discussed in the context of stability? As my edit above says, the author specifically states he is only examining the case of zero angle of attack.

Base Drag pulls the CP back....​

 

[bad_idea]

Base Drag pulls the CP back....​

The degree to which that is true and significant - and the degree to which it's true and significant at increasing angles of attack - would be interesting to know. Would love sources.