Bare Necessities: N5800 C-Star Flying Case

[scatsob]

My expertise is in aircraft but I am pretty sure the same principles apply to rocket aerodynamics. When a wing, or fin, moves into the supersonic regime the center of pressure moves from the 1/4 chord to the 1/2 chord. In aircraft this leads to a phenomena called "mach tuck" where there is a sudden nose down pitching moment and you have to have enough trim authority to account for it. So in rockets this would actually lead to more stability as the center of pressure of the fins would move aft. So what happens to the Cp of the rest of the rocket to move the Cp forward?

 

[CarVac]

My expertise is in aircraft but I am pretty sure the same principles apply to rocket aerodynamics. When a wing, or fin, moves into the supersonic regime the center of pressure moves from the 1/4 chord to the 1/2 chord. In aircraft this leads to a phenomena called "mach tuck" where there is a sudden nose down pitching moment and you have to have enough trim authority to account for it. So in rockets this would actually lead to more stability as the center of pressure of the fins would move aft. So what happens to the Cp of the rest of the rocket to move the Cp forward?

As you get much faster (Mach 3+), the pressure on the nose cone increases significantly relative to that of any other part of the rocket, thus moving the CP forward.

We tried to minimize this by using an extremely slender, 7.2:1 conical rather than a 5:1 VK ogive, but that evidently wasn't quite enough.

 

[grouch]

As you get much faster (Mach 3+), the pressure on the nose cone increases significantly relative to that of any other part of the rocket, thus moving the CP forward.

We tried to minimize this by using an extremely slender, 7.2:1 conical rather than a 5:1 VK ogive, but that evidently wasn't quite enough.

Ah, that makes sense. Thanks for that explanation.

 

[scatsob]

Wouldn't the longer nose cone actually increase pressure more because of the increased surface area that the oncoming air has to act on? Just a thought. I need to go back and reread my notes on supersonic flow. All the simulations I ran back in the day gave a 4:1 conical the altitude advantage and it moved the Cp aft. These are cool problems to work out, just sucks when you loose expensive hardware. Are you guys going to try again?

As you get much faster (Mach 3+), the pressure on the nose cone increases significantly relative to that of any other part of the rocket, thus moving the CP forward.

We tried to minimize this by using an extremely slender, 7.2:1 conical rather than a 5:1 VK ogive, but that evidently wasn't quite enough.

 

[CarVac]

Wouldn't the longer nose cone actually increase pressure more because of the increased surface area that the oncoming air has to act on? Just a thought. I need to go back and reread my notes on supersonic flow. All the simulations I ran back in the day gave a 4:1 conical the altitude advantage and it moved the Cp aft. These are cool problems to work out, just sucks when you loose expensive hardware. Are you guys going to try again?

The longer nosecone is less steep, which affects CP more than the increased surface area. Remember, the frontal area is still the same.

We are not planning to try again in the near future (< 5 years). Not unless we happen upon some disposable income, perhaps.

 

[REK]

Im later going to respond to that sentiment once I get home to my laptop.

 

[REK]

The longer nosecone is less steep, which affects CP more than the increased surface area. Remember, the frontal area is still the same.

We are not planning to try again in the near future (< 5 years). Not unless we happen upon some disposable income, perhaps.

If the longer nose cone is less steep and affects CP more, then don't you think a shorter nose cone would affect CP less?

 

[CarVac]

If the longer nose cone is less steep and affects CP more, then don't you think a shorter nose cone would affect CP less?

You are misinterpreting my statement of "affecting".

The fact that the cone is less steep is more influential in the backward direction than the increased surface area is influential in the forward direction. Or so the RASAero simulations indicated.

 

[CCotner]

Let me try to clarify-it's a situation where changing the length of the nosecone changes two things-the surface area and the pointyness, to use a technical term i just made up. Lengthening the nosecone makes it more pointy, which, at a given mach, seems to move the CP back; it also increases the surface area, which logically would move the CP forward at that same mach. RASAero strongly implies that the pointiness benefit outweighs the detriment of the increased surface area.

 

[REK]

Let me try to clarify-it's a situation where changing the length of the nosecone changes two things-the surface area and the pointyness, to use a technical term i just made up. Lengthening the nosecone makes it more pointy, which, at a given mach, seems to move the CP back; it also increases the surface area, which logically would move the CP forward at that same mach. RASAero strongly implies that the pointiness benefit outweighs the detriment of the increased surface area.

That is why I am saying a more shorter nose cone would have less surface area and therefore would affect CP less from moving forward and less drag force would be applied to the nose cone. Thus keeping your rocket stable on the flight. I don't see how having a more pointier nose cone would help in any way. Unless if your rocket had traveled at a perfect 90 degree vertical angle I am certain there would have been no issue with a longer more pointier nose cone. However, the burnt nose cone on one side says it all that it traveled a few degrees sideways. The surface area on the nose cone starting from the tip to the joint where it meets up with the airframe is superior versus your tiny fins. The results end up in the rocket pivoting on its center of gravity and therefore causing your rocket to lose its course and corkscrew.

 

[New Ocean]

Is there not a benefit to a long nosecone with a heavy metal tip moving the CG way forward?

 

[REK]

Is there not a benefit to a long nosecone with a heavy metal tip moving the CG way forward?

It only increases the stability more, but their tiny fins make it that you can use whatever length and it will not drop less than 2 margins in stability. You can even remove the nose cone and it will still be overstable. In fact removing the nose cone makes it jump to a margin of 6 since the center of pressure is all by the fins. In the case of their flight the longer nose cone would have proven to reduce drag and made the airflow flow smooth on their rocket. However, this only applies if their rocket had flown straight up and the airflow would have been equal on both sides of the nose cone. However, their rocket went at an angle of attack and therefore one side of the nose cone was taking the most drag and the other side did not. Just by looking at the burnt up nose cone on one side is proof of that.

 

[scatsob]

Is there a chance the fins were a little off? Did the rocket nutate on the way up? It's hard to see from the video.

 

[CarVac]

Is there a chance the fins were a little off? Did the rocket nutate on the way up? It's hard to see from the video.

No chance the fins were off, because they were machined with the fincan on an indexing head.

I don't know what you mean by nutate: if it's not precessing somehow (something which requires extremely significant spin rates), there's no way it would exhibit nutation.


This does, however, make me wonder if the chances of success would have been higher with canted fins.

 

[CCotner]

...but their tiny fins make it that you can use whatever length and it will not drop less than 2 margins in stability. You can even remove the nose cone and it will still be overstable. In fact removing the nose cone makes it jump to a margin of 6 since the center of pressure is all by the fins. In the case of their flight the longer nose cone would have proven to reduce drag and made the airflow flow smooth on their rocket...

um... no, I'm sorry. Let me try to explain again.

Stability margin at maximum velocity, Mach 4, is what we were concerned about. At M=4, the stabilizing effect of the long pointy nosecone (reducing the trend to shift the CP forward at high mach) greatly outweighed the destabilizing effect of the long pointy nosecone (the surface area problem). This also held true at non-zero angles of attack, at least according to RASAero. What the airstream sees of the nosecone doesn't change very much at all in the first few degrees or AOA, but what the airstream sees of the fins changes dramatically, and the rocket is more stable at 5 degrees angle of attack than it is at 0, by almost 2.5 Cal.

This tendency to become more stable rapidly as AOA moves is a contributing factor to dynamic instability, where the rocket over-corrects from deviations and starts to pitch back and forth. Combined with pitch-roll coupling, it can lead to phenomena like coning and corkscrewing, which appears is part of the puzzle of the failure.

The rocket does not become more stable when you remove the nosecone entirely-that is the simulation tool being confused because of the way it estimates aerodynamic properties. the Cp would (probably) move radically forward because of the drag of that blunt face.

 

[CarVac]

um... no, I'm sorry. Let me try to explain again.

Stability margin at maximum velocity, Mach 4, is what we were concerned about. At M=4, the stabilizing effect of the long pointy nosecone (reducing the trend to shift the CP forward at high mach) greatly outweighed the destabilizing effect of the long pointy nosecone (the surface area problem). This also held true at non-zero angles of attack, at least according to RASAero. What the airstream sees of the nosecone doesn't change very much at all in the first few degrees or AOA, but what the airstream sees of the fins changes dramatically, and the rocket is more stable at 5 degrees angle of attack than it is at 0, by almost 2.5 Cal.

This tendency to become more stable rapidly as AOA moves is a contributing factor to dynamic instability, where the rocket over-corrects from deviations and starts to pitch back and forth. Combined with pitch-roll coupling, it can lead to phenomena like coning and corkscrewing, which appears is part of the puzzle of the failure.

The rocket does not become more stable when you remove the nosecone entirely-that is the simulation tool being confused because of the way it estimates aerodynamic properties. the Cp would (probably) move radically forward because of the drag of that blunt face.

Fixed that for you.

 

[scatsob]

Oops, meant to say corkscrew, not nutate. Got my funky flight characteristics mixed up. I think what you are running into is one of the fundamental properties of engineering: compromise. You are trying to maximize the altitude, but if in that endeavor you have to add a little more drag and loose a little altitude to get a successful flight then that's what you have to do . I have a good cartoon on this, I will try to dig it up.

 

[REK]

um... no, I'm sorry. Let me try to explain again.

Stability margin at maximum velocity, Mach 4, is what we were concerned about. At M=4, the stabilizing effect of the long pointy nosecone (reducing the trend to shift the CP forward at high mach) greatly outweighed the destabilizing effect of the long pointy nosecone (the surface area problem). This also held true at non-zero angles of attack, at least according to RASAero. What the airstream sees of the nosecone doesn't change very much at all in the first few degrees or AOA, but what the airstream sees of the fins changes dramatically, and the rocket is more stable at 5 degrees angle of attack than it is at 0, by almost 2.5 Cal.

This tendency to become more stable rapidly as AOA moves is a contributing factor to dynamic instability, where the rocket over-corrects from deviations and starts to pitch back and forth. Combined with pitch-roll coupling, it can lead to phenomena like coning and corkscrewing, which appears is part of the puzzle of the failure.

The rocket does not become more stable when you remove the nosecone entirely-that is the simulation tool being confused because of the way it estimates aerodynamic properties. the Cp would (probably) move radically forward because of the drag of that blunt face.

That is why I mentioned that the longer more pointer nose cone would have no problem at a non-zero angle of attack and of course RASAero very much agrees. However, your rocket had to travel at an angle of attack (who knows why... that's where I blame the motor). If the rocket had a nice stability of 2.5 Cal at Mach 4. Then ask yourself why did it not continue to fly straight, but instead corkscrewed? This is where you are not understanding that no matter how stable the rocket is the forces acting upon the nose cone and fins at an angle of attack will cause the rocket to pivot under its center of gravity depending on which one has a greater surface area and is taking the greatest amount of force. In this situation the nose cone is the one that has more force being applied onto it. The 4:1 ratio scatsob mention before would have been a more sufficient, which is shorter meaning less surface area and thus less affecting CP from moving forward too greatly. Therefore the forces would be balanced and would keep your rocket from pivoting at Mach 4 and would have remained flying straight instead of corkscrewing.

 

[scatsob]

The post above reminded me of something. Another thing to consider is asymmetric thrust. I saw a perfectly stable and properly built minimum diameter 4" rocket skywrite because a large chunk of the nozzle came off one side. Perhaps something similar happened on a smaller scale and if the Cp and Cg were close then it would affect flight drastically. Not saying that's what happened but a possibility.

 

[CarVac]

That is why I mentioned that the longer more pointer nose cone would have no problem at a non-zero angle of attack and of course RASAero very much agrees. However, your rocket had to travel at an angle of attack (who knows why... that's where I blame the motor). If the rocket had a nice stability of 2.5 Cal at Mach 4. Then ask yourself why did it not continue to fly straight, but instead corkscrewed? This is where you are not understanding that no matter how stable the rocket is the forces acting upon the nose cone and fins at an angle of attack will cause the rocket to pivot under its center of gravity depending on which one has a greater surface area and is taking the greatest amount of force. In this situation the nose cone is the one that has more force being applied onto it. The 4:1 ratio scatsob mention before would have been a more sufficient, which is shorter meaning less surface area and thus less affecting CP from moving forward too greatly. Therefore the forces would be balanced and would keep your rocket from pivoting at Mach 4 and would have remained flying straight instead of corkscrewing.

Even if a rocket has static stability, there exist modes of oscillation, like coning, which lead to instability.

I'm fairly sure that if the rocket had a shorter nosecone, it would have gone unstable sooner, because the CP would have moved forward MORE.

 

[CCotner]

Even if a rocket has static stability, there exist modes of oscillation, like coning, which lead to instability.

I'm fairly sure that if the rocket had a shorter nosecone, it would have gone unstable sooner, because the CP would have moved forward MORE.

Yes, this is what I was trying to explain, again: a shorter nosecone would have been unstable in even the simple static definition-it would have over-ended (or tried to-pretty hard to with 5kN of force on one end), rather than kept going straight, as the CP shifted forward at high mach. The long nosecone allowed it to keep going straight, until it hit an unexpected AOA and spin-the 2.7s event-at which point the rocket entered a flight envelope where, while it was stable in the static definition, the rocket overcorrected and veered back and forth around 'straight', pivoting around the CG like you say, and while on average it went straight (just like it was statically stable) the actual flight path was corkscrewing.

The problem might be linked to an inadequate definition of stability. Static Stability simply means the CP is behind the CG, and so when the rocket is not pointed the direction it is traveling (i.e. has a non-zero AOA), it experiences a corrective torque that rotates it back toward being straight. A 'statically stable' rocket can still skywrite for a number of reasons, such as thrust assymetry, overcorrection, pitch-roll coupling, to name a few.

 

[scatsob]

Found it. I hope you guys are having fun, that is what this is all about .