Another Crack at "Finless" Bullet Bill

[BigMacDaddy]

I decided I might take another crack at the Bullet Bill rocket using one of the 4C iced tea canisters. I know there are models out there but I wanted to do one without fins sticking out from the body.

My original design had an open front with a few fins shaped like nose cone and a ring body for lack of a better way to describe it (a ring tail that ran most of the length of the body). This did not model very stable since it had fins basically all the way to the front of the rocket.



This design flew relatively stable (i.e., no skywriting) but basically arched over and crashed into the ground.



I was contemplating using the same design but making it spin to try to stabilize (I actually built a couple of models to try this but never launched). However, after some success w/ a GDS stabilized rocket, I decided to try using GDS to stabilize this. Basically this means a full-sized nosecone, recessed motor, and some gaps for air to get pulled into. I am also canting the support fins so that the rocket may spin (real bullets spin, so....)



I tried to model as best I can in OR and used a massless cone to simulate the base drag on this short stubby rocket. I modeled this as if it had a ring tail so did 6 fins for the tail portion.



Interestingly, when you model this in OR if I drop the fins from the rear it actually seems more stable.

 

[jqavins]

Interestingly, when you model this in OR if I drop the fins from the rear it actually seems more stable.

It looks like that base drag simulating cone is giving you a good CP position, so the fins were just lowering the CG.

The base drag cone is usually applied, as far as I know, to "normal" designs, where there is a more or less solid bottom due to a solid aft centering ring. With GDS, I wonder if base drag works the same way. On the other hand, I wonder if the same cone simulates GDS decently well.

 

[cwbullet]

Interesting flight profile.

 

[GlenP]

Because it looks draggy and maybe a bit heavy, it might be stable during thrust, but possibly not during coast when it might still be near the ground, not a high flier. Spinning would definitely help keep it going straight after the motor burn out. You could maybe make the ring fin slightly larger dia for a bit extra stability during coast, or flare it out like a nozzle/transition, just a bit, if you don’t mind changing the appearance like that. Clever use of GDS.

 

[BigMacDaddy]

Finished building the prototype. 240g and CG is 5mm from bottom of nose cone w/ D12-3 engine loaded. This is the first time I tried modelling 3D printing penny pockets for nose cone weight (pennies are pretty cheap weights but I hate that they are all disorganized when filling a nose cone).

 

[lakeroadster]

I think the issue is / will be the big diameter of the rocket in relation to the size of the motor.

As I understand it, which may be flawed, the motor thrust creates a suction in the body tube extension. That suction pulls outside air through the openings in the body tube. That's what adds the stability to the rocket and is also why that stability goes away when the motor stops thrusting.

So a minimum diameter rocket, or near minimum, is needed for GDS in a model rocket. The motors we use just don't create enough suction if the body tube is too large..

My mind sim says this design will not fly straight.

Have you seen this video? Another large body tube diameter to motor diameter GDS bird. Spoiler alert... go all the way to 37:30 first.

I hope to have adressed these issues, as well as the spinning for stability during coast phase, with my "Thunk!" rocket. My care package from BMS just arrived today, with any luck I can start building it next week.

 

[jqavins]

[P]ennies are pretty cheap weights but I hate that they are all disorganized when filling a nose cone.

Be sure to look for pennies from before 1982. Or you might want to use nickels when you're space limited, since those old pennies are not easy to find in quantity.
[UNNECESSARY GEEKINESS]

	Price (¢)	Mass (g)	Diameter (mm)	Thickness (mm)	Volume (cc)	Density (g/cc)	Monetary Efficiency (g/¢)	Best and Worst
Pre-1982 Penny*	1	3.11	19.05	1.52	0.43	7.18	3.11	Best monetary efficiency, very good density
Post-1982 Penny	1	2.50	19.05	1.52	0.43	5.77	2.50	Worst density
Nickel	5	5.00	21.21	1.95	0.69	7.26	1.00	Very good density, not terrible monetary efficiency
Dime	10	2.27	17.91	1.35	0.34	6.67	0.23
Quarter	25	5.67	24.26	1.75	0.81	7.01	0.23
Dollar (modern)	100	8.10	26.49	2.00	1.10	7.35	0.081	Best density, worst monetary efficiency

* The composition of pennies changed during the 1982 production run.
[/UNNECESSARY GEEKINESS]

But then, these fender washers have essentially the same effective density as the dollar coins (using the whole diameter) with the same OD and better than 12 times the monetary efficiency of the old pennies.

 

[boomtube-mk2]

I'm not completely sure what the objective is with this but what do you think would happen if you placed some forward-facing air-scoops, just below the nosecone, that would funnel airflow into the body tube and over the internal fins and out the back?

Or would that defeat what is being attempted here?

 

[BigMacDaddy]

Finally got to test this w/ a D12-3 -- flew marginally better than previous version in a bit of a corkscrew (kinda like I imagine Bullet Bill would actually fly)...

Parachute got melted (which is odd since it is outside the buffer tube inside the nose cone -- maybe the heat came through the buffer tube?).

Still cannot write this off as finished / problems solved but getting closer I think... What should I try next? I am thinking E-engine and more nose weight + don't worry about canting the fins (although that might have been what made this corkscrew and kept it up there)..

 

[lakeroadster]

Finally got to test this w/ a D12-3 -- flew marginally better than previous version in a bit of a corkscrew (kinda like I imagine Bullet Bill would actually fly)...

Parachute got melted (which is odd since it is outside the buffer tube inside the nose cone -- maybe the heat came through the buffer tube?).

Still cannot write this off as finished / problems solved but getting closer I think... What should I try next? I am thinking E-engine and more nose weight + don't worry about canting the fins (although that might have been what made this corkscrew and kept it up there)..

Looks like progress. Since we don't know which direction the wind was blowing, did it windcock?

 

[BigMacDaddy]

Looks like progress. Since we don't know which direction the wind was blowing, did it windcock?

It flew into the wind but I am not sure if that is a coincidence or windcocking (also wind was not too bad - around 7mph). There are no real fins to speak of but the rocket is definitely nose heavy. Also motor is pretty far forward in model (goal is to use GDS for stability) Do rockets with light tails or traction motor setups but no fins windcock? I always imagined that fins where what drove windcocking but I guess it is really anything that make the model aerodynamically stable.

 

[lakeroadster]

It flew into the wind but I am not sure if that is a coincidence or windcocking (also wind was not too bad - around 7mph). There are no real fins to speak of but the rocket is definitely nose heavy. Also motor is pretty far forward in model (goal is to use GDS for stability) Do rockets with light tails or traction motor setups but no fins to speak of windcock? I always imagined that fins where what drover windcocking but I guess it is really anything that make the model aerodynamically stable.

Maybe launch it 3 or 4 more times... and see if the flight profile is consistent?

 

[jqavins]

It's anything that makes a rocket aerodynamically stable, bit it's fins that do that. A finless rocket like yours is not aerodynamically stable; gas dynamic stability is the an alternative.

As far as I know, a rocket like yours using GDS shouldn't weathercock. I await being corrected should someone know better.

 

[lakeroadster]

It's anything that makes a rocket aerodynamically stable, bit it's fins that do that. A finless rocket like yours is not aerodynamically stable; gas dynamic stability is the an alternative.

As far as I know, a rocket like yours using GDS shouldn't weathercock. I await being corrected should someone know better.

Every rocket experiences weathercocking, unless there is no wind.

The body and the fins generate an aerodynamic lift force. The lift force generates a torque about the cg which causes the rocket to rotate.

Rocket Weathercocking

 

[BigMacDaddy]

I think that if it relied 100% on GDS than it would be less prone (immune?) to weathercocking (since it would be aerodynamically neutral without engine pulling air through some holes and pushing it out other holes). However, I also make my GDS rockets nose heavy so they would have a tendency to rotate around their CG and point nose towards wind (or direction of motion).

 

[jqavins]

Every rocket experiences weathercocking, unless there is no wind.

The body and the fins generate an aerodynamic lift force. The lift force generates a torque about the cg which causes the rocket to rotate.

Rocket Weathercocking

Baring GDS, a 0FNC is aerodynamically unstable; the body (and nose cone) do indeed create a torque that rotates the rocket around the CG, but in the wrong direction. But of course you know all that.

Weathercocking in a "regular" rocket is a direct consequence stability. Weathercocking is aerodynamically identical to stable recovery from a flight perturbation. That's why "overstable" is a thing; it's synonomous with "over-weathercocky". But you know that too.

A 0FNC, having the opposite of stability, would do the opposite of weathercocking, i.e. it would turn away from the wind, not into it. Or rather, it would do that if it weren't busy skywriting.

Now, I'm not saying that a GDS rocket doesn't weathercock (turn into the wind) but if it does, it's for a different reason. Come to think lf it, since it's basicaly a 0FNC with stabilization by magic, it might even anti-weathercock, turning away from the wind.

 

[lakeroadster]

A 0FNC, having the opposite of stability, would do the opposite of weathercocking, i.e. it would turn away from the wind, not into it

Got any research data to back that up or is this based on @jqavins mindsim file?

My mindsim says maybe into the wind, maybe not, based on the configuration of the rocket.

As stated in the link I provided: The body of the rocket generates aerodynamic lift forces. The lift force generates a torque about the cg which causes the rocket to rotate.

So if the cg is above the midpoint of the rocket, it will indeed windcock into the wind.

 

[jqavins]

Got any research data to back that up or is this based on @jqavins mindsim file?

Neither. It's based on many sources and explanations by various authoritative sources, and on RS/OR design work. Start with a body tube alone. These programs ignore body lift, and the reported CP will be either be at the middle of the tube or NaN. The nose cone also generates lift, and moves the CP forward. An engine mounted in the aft end dominates the CG, so it is behind the CP; the rocket is unstable.

Fins generate far more lift than the body tube and nose cone, and pull the CP back a lot. If the CP is still forward of the CG, the fins may be expanded, or the CG moved forward by the addition of nose weight.

I don't have research data, nor is this mind sim. This is the most basic of textbook stuff.

So if the cg is above the midpoint of the rocket, it will indeed windcock into the wind.

If the CG is ahead of the CP, yes. The CP will be above the midpoint because of lift generated by the nose cone. But again, this is all basic textbook stuff, but for a little bit of mind sim just here: my mind sim says that the opening where air comes in for the GDS action has negligible effect on the CP, and if that's true then the CP is certainly above the midpoint. Basic textbook.

GDS is not basic. GDS does not work by using lift, but generates torque by a different means. The action of air being drawn into the ports and accelerated out the back actually generates a sort of latteral thrust by means I don't really understand. OK, I don't fully understand it, but I do know that it's not about lift, like fins are. It doesn't move the CP.

If the CG were even further forward then the rocket would be stable without GDS. In other words, if we take it as given that the rocket actually needs GDS to be stable, then it follows the the CG is below the CP. So we have the CP and CG reversed relative to the (textbook) figure you included.

If you'll allow a bit more of my mind sim, it says that with the CP above the CG, and GDS holding the rocket stable despite this, it should anti-weathercock. But, and this is a huge BUT, because I don't fully understand GDS I don't trust my mind sim as far as I can throw an elephant. And, meaning no disrespect - I really mean that - uness you've got a bunch of GDS design and flight experience that I'm not aware of, I don't trust your mind sim any further than I do my own.

Which brings me full circle. I'm not saying it won't weathercock. I'm saying that there is no reason that's apparent to me, nor any reason in the textbook material you've linked to, to expect that it will.

 

[jqavins]

Accidental double post.

 

[lakeroadster]

I don't have research data, nor is thismind sim. This is the most basic textbook stuff.

Alrighty then. I could have sworn I read this somewhere...

I await being corrected should someone know better.

 

[BigMacDaddy]

my mind sim says that the opening where gas come in for the GDS action has negligible effect on the CP, and if that's true then the CP is certainly above the midpoint.

Do you think if I used a smaller gap (or mostly sealed the gap and only left it open at 4 spots around the rocket) that GDS would play a bigger role?

 

[jqavins]

Alrighty then. I could have sworn I read this somewhere...

Please reread my previous post. I accidentally hit the "Post reply" button before it was done, and you posted a response before I had a chance to fix it.

Do you think if I used a smaller gap (or mostly sealed the gap and only left it open at 4 spots around the rocket) that GDS would play a bigger role?

No idea.

 

[lakeroadster]

Which brings me full circle. I'm not saying it won't weathercock. I'm saying that there is no reason that's apparent to me, nor any reason in the textbook material you've linked to, to expect that it will.

So you don't believe the NASA document? Based on that document if the body generates lift, which it does, then it will weathercock. The only question is how much, and which direction.

 

[jqavins]

I don't believe the NASA document is applicable. I don't believe your statement "If the body generates lift... then it will weathercock" is an appropriate message to take from the NASA document. Lots of things generate lift, and most of them don't weathercock. The weathercocking that the NASA document describes applies only to an aerodynamically stable rocket, and this one is aerodynamically unstable.

 

[jqavins]

I did it again, hit Post before I was done. Since you're so qick, I thought I'd better post this to ask you to wair for me to finish.

 

[jqavins]

Here's the rest. Do some mind sim experiemts with me. Take an ordinary, stable, 4FNC rocket and launch it. Hit it with a side gust. It weathercocks, of course.

Now remove it's fins so it's unstable, and launch it from a mile long, perfectly rigid launch rod so it doesn't skywrite. Hit it with the same side gust. What torque will it generate? Because the CG is aft of the CP (that's what makes it unstable) the lift generated by the body will create a torque away from the wind, not into it (that's what being unstable means). So 1) the NASA document doesn't apply, as it is written and illustrated about stable rockets, and 2) it "anti-weathercocks".

Now, add the ports on the sides and set the engine position and do the other things that make GDS work. None of that stuff changes the CP (if I'm right that the ports' effect on the body lift are insignificant). The rocket has been made stable, but not be aerodynamic lift. GDS works completely differently. So the CP is still forward of the CG, and the NASA document still doesn't apply. Will the rocket turn into the wind? Away from the wind?

 

[lakeroadster]

Here's the rest. Do some mind sim experiemts with me. Take an ordinary, stable, 4FNC rocket and launch it. Hit it with a side gust. It weathercocks, of course.

Now remove it's fins so it's unstable, and launch it from a mile long, perfectly rigid launch rod so it doesn't skywrite. Hit it with the same side gust. What torque will it generate? Because the CG is aft of the CP (that's what makes it unstable) the lift generated by the body will create a torque away from the wind, not into it (that's what being unstable means). So 1) the NASA document doesn't apply, as it is written and illustrated about stable rockets, and 2) it "anti-weathercocks".

Now, add the ports on the sides and set the engine position and do the other things that make GDS work. None of that stuff changes the CP (if I'm right that the ports' effect on the body lift are insignificant). The rocket has been made stable, but not be aerodynamic lift. GDS works completely differently. So the CP is still forward of the CG, and the NASA document still doesn't apply. Will the rocket turn into the wind? Away from the wind?

You totally lost me on the mile long launch rod concept.

Why are you saying the NASA document doesn't apply if the CP is forward of the CG? The forces it discusses are still there. Just look at the forces, and what they do. Of course the diagram doesn't address GDS, but that doesn't mean the forces aren't still there.

The farther the CP is from the CG, the worse the rocket will be affected by wind. And a GDS rocket typically has a wide span between the CP and CG... due to it's finless design.

 

[BigMacDaddy]

Here's the rest. Do some mind sim experiemts with me. Take an ordinary, stable, 4FNC rocket and launch it. Hit it with a side gust. It weathercocks, of course.

Now remove it's fins so it's unstable, and launch it from a mile long, perfectly rigid launch rod so it doesn't skywrite. Hit it with the same side gust. What torque will it generate? Because the CG is aft of the CP (that's what makes it unstable) the lift generated by the body will create a torque away from the wind, not into it (that's what being unstable means). So 1) the NASA document doesn't apply, as it is written and illustrated about stable rockets, and 2) it "anti-weathercocks".

Now, add the ports on the sides and set the engine position and do the other things that make GDS work. None of that stuff changes the CP (if I'm right that the ports' effect on the body lift are insignificant). The rocket has been made stable, but not be aerodynamic lift. GDS works completely differently. So the CP is still forward of the CG, and the NASA document still doesn't apply. Will the rocket turn into the wind? Away from the wind?

In your scenario it would seem that if CG is forward of the midpoint of the body then the finless rocket would cock into the wind. I don't think you can ignore the CP impact of the body tube when dealing with lateral air movement.

 

[jqavins]

You totally lost me on the mile long launch rod concept.

Why are you saying the NASA document doesn't apply if the CP is forward of the CG? The forces it discusses are still there. Just look at the forces, and what they do. Of course the diagram doesn't address GDS, but that doesn't mean the forces aren't still there.

The farther the CP is from the CG, the worse the rocket will be affected by wind. And a GDS rocket typically has a wide span between the CP and CG... due to it's finless design.

Perhaps we have a problem of vocabulary. "Weathercocking", as I have always heard the term used, is the tendency of a stable rocket to turn into the wind, or upwind if you prefer. Turning downwind isn't called weathercocking. With the CP above the CG, it will make a turn induced by the side gust, turning downwind, and not called weathercocking.

Never mind the mile long launch rod if that's a problem. Real unstable rockets tumble and skywrite and both scare and entertain the crowd. So it's pointless to discuss their wind turning. So I wanted to sustain the rocket's forward or mostly forward motion despite it's being unstable. I proposed a mile long launch rod. I could have said it goes straight by magic (but not GDS magic since that was the next case).

 

[lakeroadster]

Perhaps we have a problem of vocabulary. "Weathercocking", as I have always heard the term used, is the tendency of a stable rocket to turn into the wind, or upwind if you prefer. Turning downwind isn't called weathercocking. With the CP above the CG, it will make a turn induced by the side gust, turning downwind, and not called weathercocking.

I don't care if it's cocking into the wind, or cocking downwind. I'm just trying to determine if the wind is a part of the poor flight profile, and understand why.

Maybe a couple more flights will help to clarify...

I have a vested interest. If I can get back out into the barn, I can build the lanch silo for Thunk! and see if he's G-D stable.

First I have to buy a mile long launch tube.. or at least a 6 foot piece of pvc.