I540 to Mach 2 - 38mm Flying Case

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CarVac said:
My rocket.

It was always predicted by RASAero to be at least 2 calibers stable up to M4.2, accounting for the consumption of propellant. We made it so that that would be true. However, it would stubbornly go unstable in OpenRocket whenever I simmed it, with a very low altitude estimate that I couldn't understand (since it didn't actually emit the warning about a high angle of attack) until after the flight.

More recently, I redid my simulations and if I either add another fin or add half a degree of fin cant, it remains stable in the OpenRocket simulation.

Just so you know, the OpenRocket simulation is full 6dof in the background, and I am fairly sure based on rudimentary looks at the graphs that it was coning in the simulation much as it did in real life.
Click to expand...

A totally symmetric rocket may be stable in circumstances where the as built rocket is not. If you have not measured and input the 3D CG and CP, the off-axis thrust vector, and any aerodynamic asymmetries of your as built rockets. Without those values, and the equations to evaluate them, I don't see how you can accurately predict the onset of roll-pitch coupling so I believe you should err on the side of stability by a large margin to avoid turning sideways at 30-40kft.

Bob
 
bobkrech said:
A totally symmetric rocket may be stable in circumstances where the as built rocket is not. If you have not measured and input the 3D CG and CP, the off-axis thrust vector, and any aerodynamic asymmetries of your as built rockets. Without those values, and the equations to evaluate them, I don't see how you can accurately predict the onset of roll-pitch coupling so I believe you should err on the side of stability by a large margin to avoid turning sideways at 30-40kft.

Bob
Click to expand...

OpenRocket predicted exactly zero calibers of static stability at top speed, just for the record.

My suspicion is that in the simulation, the slightly off center three fin cp causes rotation of the rocket upon hitting wind shear from the pink noise wind model, and that is enough disturbance to set it coning and scrubbing velocity, even when the simulation has a mathematically perfect rocket.

In real life it could have been that, or any tiny asymmetry (less than you might think thanks to a completely CNC'd fincan and nosecone), or the motor spitting a casting tube as seems to show up in the acceleration traces.
 
SinfulDarkLord said:
Stability excuses, you guys make me laugh. More like not launched properly.

I will say no more.


Alexander Solis
Click to expand...
This makes absolutely zero sense. Launched properly? Am I sticking the igniter in the nosecone? Was my rocket upside down?
Of course you say "I will say no more" like you're hiding some piece of amazing aerodynamic research that we can't or won't comprehend. I think you don't have a valid explanation but just want to feel superior. Why don't you put your money where your mouth is and explain your statement.
bobkrech said:
Young ones. You need to take some advanced coursed in flight dynamics so you can understand what happened on your failed flights. Simulation programs are only as good as the phenomenon included in the simulation, and none of the hobby rocket simulations include roll-pitch coupling which caused the type of failures that occurred on your flights, nor have you collected the necessary data to perform these calculations even if you know how to do them.

If you go back and search TRF for coning, which is roll-pitch coupling, a dynamic stability problem that is not simulated in any hobby simulation, you will find several discussion on the topic and Chuck Rogers, the author of RASAero, discussed this fact in several replies.

Alex, would you like to make any other comments?

Bob
Click to expand...
I've seen RAS deliver some pretty faulty stability calcs and Openrocket give some fairly accurate ones.
Case and point:
BN (as presented above)

Aidan S's Guardian:
Aidan used Rasaero for calculating the entire flight. IIRC it was predicted to be stable to M4. OR predicted instability at M2.7 (burnout velocity). We decided it was risky, but he'd fly it anyway. If it was stable, we knew that OR was overly pessimistic and if it wasn't we'd know that RAS wasn't anywhere close. The rocket was unstable exactly where openrocket predicted. It turned sideways at burnout and we spent a while digging pieces out of the ground.

Video. Launch at 0:30. It's hard to tell in the video, but it was definitely unstable.
[video=youtube;jXfAdZ4lTsQ]https://www.youtube.com/watch?v=jXfAdZ4lTsQ[/video]

My "Playin With Fire"
RasAero also predicted my rocket was stable way past mach 4! My rocket was unstable, but it wasn't going anywhere near mach 4.

From a club launch a few years ago. I remember this specifically because it was the first unstable HPR I'd seen. This guy showed up with a 29md with tiny little fins. It didn't look to stable, but he got to fly anyway because he had printed out the RasAero stability predictions. It wasn't stable at all. I'll see if I can dig out the video later.

That's why I don't trust RasAero for stability. I'm not saying it's not good software, it still has plenty of uses, but I won't be using it for stability. No, this information isn't based on my knowledge of flight dynamics. It's repeated experiences with the accuracy of those simulations.

Edit: Sorry for hijacking. That's one beautiful tower!

Alex
 
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bobkrech said:
Young ones. You need to take some advanced coursed in flight dynamics so you can understand what happened on your failed flights. Simulation programs are only as good as the phenomenon included in the simulation, and none of the hobby rocket simulations include roll-pitch coupling which caused the type of failures that occurred on your flights, nor have you collected the necessary data to perform these calculations even if you know how to do them.

If you go back and search TRF for coning, which is roll-pitch coupling, a dynamic stability problem that is not simulated in any hobby simulation, you will find several discussion on the topic and Chuck Rogers, the author of RASAero, discussed this fact in several replies.

Alex, would you like to make any other comments?

Bob
Click to expand...

Mentioned in Open Rocket's technical documentation on page 13.

https://openrocket.sourceforge.net/techdoc.pdf
 
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I picked up the finished launch tower, and it is amazing. My dads friend constructed it - he is an amazing machinist and engineer who builds and sometimes designs prototype machines, not unlike like this (though much more complex) for various companies. The stuff he builds is truly amazing - his words "it pained me to make it this simple". :D He wanted to add gears to make all 3 rails move in and out together with the turn of a single handle.

Sorry for the huge pictures, but they were necessary to do it justice!


14995986518_fae3177b91_b.jpg


15182567445_84e3f02e61_o.jpg


14995896350_8de29d8c94_o.jpg


15182569155_f272dba9aa_o.jpg


15182191492_e067655739_o.jpg


15182192122_ee04d52ddb_o.jpg
 
SinfulDarkLord said:
Stability excuses, you guys make me laugh. More like not launched properly.

I will say no more.


Alexander Solis

Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet
Click to expand...

Your ego is pretty remarkable for somebody that had demonstrated countless times, through both your own admission and your evidentiary posts on this forum, that you're a rookie with little street cred. Nothing at all wrong with that until you start running your mouth, which you've now done twice this week.

If you "roll carbon tubes like a champ," why are you selling your practice/mistake tubes at a pretty steady rate? Your ego is further reflected in the price that you started out asking for these things, something which you've since been called on and quietly corrected.

If your most notable flight achievement to date is a phenolic kit flying on a commercial motor the size of which I burned off as bowl scrapings yesterday when mixing my O motor for Balls, why are you speaking with any kind of authority on extreme flights?

Now, let's be clear here - I'm immune from your unfounded accusation. I flew a full N motor that I made myself, with a composite fincan I made myself, in a composite case, last weekend at Airfest, and it was awesome. Video here: [video=youtube;lV5o9YWWrHo]https://www.youtube.com/watch?v=lV5o9YWWrHo[/video]

You won't catch me saying I mix motors like a champ, or lay up fin cans like a champ, even though this one survived a pretty thorough thrashing from that motor. If you're going to be egotistical, go ahead, you certainly wouldn't be the first, but at least have the backing for that which you're trying to speak with authority about. A guy who's flying Level 1 commercial motors and practicing making carbon tubes wouldn't qualify.
 
Uh, guys, can you un-hijack the thread, please? I am sorry to spoil your fun, but this was a good thread about a really interesting build until people started calling each other names.


Sent from my iPhone using Rocketry Forum
 
wfcook said:
Uh, guys, can you un-hijack the thread, please? I am sorry to spoil your fun, but this was a good thread about a really interesting build until people started calling each other names.


Sent from my iPhone using Rocketry Forum
Click to expand...

No name calling here. I was merely implying the fact that the program is at no fault here. The fault is in the person alone who, probably did not put all the valuable information to the program and therefore concludes to invalid flight simulations.

My apologies if it sounded harsh, but literally in no way should one blame the program.


Alexander Solis

Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet
 
SinfulDarkLord said:
No name calling here. I was merely implying the fact that the program is at no fault here. The fault is in the person alone who, probably did not put all the valuable information to the program and therefore concludes to invalid flight simulations.

My apologies if it sounded harsh, but literally in no way should one blame the program.


Alexander Solis

Level 1 - Mariah 54 - CTI-I100 Red Lightning Longburn - 6,345 Feet
Click to expand...

I wouldn't say that the program is at fault, just that it's not trustworthy. There's a difference.

Why the flight failed? It's my (and CCotner's) fault for consciously ignoring the ZERO CALIBERS OF STABILITY that OpenRocket predicted, and instead trusting the long successful track record of RASAero which is an excellent sim program in most every other way (except for its not running on Linux).

And then we came here to caution Coleman not to trust it completely either.
 
RocketHunter said:
I picked up the finished launch tower, and it is amazing. My dads friend constructed it - he is an amazing machinist and engineer who builds and sometimes designs prototype machines, not unlike like this (though much more complex) for various companies. The stuff he builds is truly amazing - his words "it pained me to make it this simple". :D He wanted to add gears to make all 3 rails move in and out together with the turn of a single handle.

Sorry for the huge pictures, but they were necessary to do it justice!
Click to expand...

By the way: You can click on the "View all sizes" and then select the appropriate size, right-click on the image, and then insert it with [ img ]url.url[ /img ] so that you don't overwhelm people. Those pictures fill my 4K screen, which I appreciate, but most people...would not appreciate it.

P.S. Bring the tower to the next launch so I can fondle it.
 
CarVac said:
By the way: You can click on the "View all sizes" and then select the appropriate size, right-click on the image, and then insert it with [ img ]url.url[ /img ] so that you don't overwhelm people. Those pictures fill my 4K screen, which I appreciate, but most people...would not appreciate it.

P.S. Bring the tower to the next launch so I can fondle it.
Click to expand...

Ok thanks for the tip, will do in the future. If it fits in the car with everything else I'm bringing, I will!
 
Rockethunter. I do have one concern, but it may be be bad lighting in the photos. Is there a step between the casing and the fin can? Almost looks like the nose cone and fin can tube size is a couple mm larger than the center section.

-Hans
 
HHaase said:
Rockethunter. I do have one concern, but it may be be bad lighting in the photos. Is there a step between the casing and the fin can? Almost looks like the nose cone and fin can tube size is a couple mm larger than the center section.

-Hans
Click to expand...

It does look that way. It's a "flying case" after all.

The only way to really make a gentle taper onto a fincan without an impractically thin edge is with 'chevrons' to sweep the leading edge, thereby letting you use a blunter (and stronger) edge.
 
HHaase said:
Rockethunter. I do have one concern, but it may be be bad lighting in the photos. Is there a step between the casing and the fin can? Almost looks like the nose cone and fin can tube size is a couple mm larger than the center section.

-Hans
Click to expand...

In post #28 you can see the steps in the attached picture of the OpenRocket design.
 
CarVac said:
It does look that way. It's a "flying case" after all.

The only way to really make a gentle taper onto a fincan without an impractically thin edge is with 'chevrons' to sweep the leading edge, thereby letting you use a blunter (and stronger) edge.
Click to expand...

Makes sense, though strength wasn't what I was worried about in this instance. I'm just curious about how the transsonics are going to play out that close to the leading edge of the fins, and if it will create a shock cone that temporarily inhibits airflow over the fins. But I'm only going by what I remember from college 15 years ago, you guys have the more recent and relevant practical experience here, so if you guys aren't worried than I'm comfortable as well. I'm learning as I go here, so I hope you don't mind the questions.

-Hans
 
RocketHunter said:
I picked up the finished launch tower, and it is amazing. My dads friend constructed it - he is an amazing machinist and engineer who builds and sometimes designs prototype machines, not unlike like this (though much more complex) for various companies. The stuff he builds is truly amazing - his words "it pained me to make it this simple". :D He wanted to add gears to make all 3 rails move in and out together with the turn of a single handle.

Sorry for the huge pictures, but they were necessary to do it justice!


14995986518_fae3177b91_b.jpg


15182567445_84e3f02e61_o.jpg


14995896350_8de29d8c94_o.jpg


15182569155_f272dba9aa_o.jpg


15182191492_e067655739_o.jpg


15182192122_ee04d52ddb_o.jpg
Click to expand...

WOW. That is an awesome tower! Will this be at LDRS next year and will it permit a 54mm rocket? If so, can I collaborate with you on my project to use your tower?

Thanks!
 
Can someone with more experience with supersonic flow explain what happens to lift with shocks bouncing around from the leading edge of a fincan? The leading edge itself should obviously contribute to some pressure, but does it change (take away) the pressure on the fin itself? If the answer is simply "depends", then never mind.
 
RocketHunter said:
Ok thanks for the tip, will do in the future. If it fits in the car with everything else I'm bringing, I will!
Click to expand...

One thing I will check if I get the opportunity to see it is how resistant to slag gumming up the articulation. The tower CCotner built was fine from things like an I1299, L2300, and N5800, but it was most plugged up by a J570. Maybe you should just avoid using White Lightning in it.
 
bobkrech said:
Young ones. You need to take some advanced coursed in flight dynamics so you can understand what happened on your failed flights.

Bob
Click to expand...

I don't qualify as a young one, but Bob could you recommend a good text on this subject? Something I could pick up used on Amazon perhaps? Good thread, I'd like to formally study this.

- Matt
 
I'm interested in how the upper body tube and recovery harness is attached to the motor case, can you post pics? That's a great looking rocket design.
 
patelldp said:
WOW. That is an awesome tower! Will this be at LDRS next year and will it permit a 54mm rocket? If so, can I collaborate with you on my project to use your tower?

Thanks!
Click to expand...

Thank you! The tower will accommodate rockets from 24mm to 3'', possibly even 4'' if the fins aren't to big. For bigger motors the structure itself is plenty strong, but I would probably have to upgrade the legs, or add guy-wires to the center or top for better stability.

LDRS will be hosted by URRF next year, correct? If so then there is a possibility I would go. At this point I cant say for sure if I can, but
in 5-6 months I should be able to know for sure. If it works out, you are welcome to use it!
 
matthewdlaudato said:
I don't qualify as a young one, but Bob could you recommend a good text on this subject? Something I could pick up used on Amazon perhaps? Good thread, I'd like to formally study this.

- Matt
Click to expand...

Google "MIL-HDBK-762(MI)" or "DESIGN OF AERODYNAMICALLY STABILIZED FREE ROCKETS" It's a 16 Mb download but a good place to start. Do not pay for it, it can be found and downloaded free from several websites. Search "coning" in the .pdf

Also Google "Coning" and "roll-pitch coupling" for more info.

Bob
 
CarVac said:
Can someone with more experience with supersonic flow explain what happens to lift with shocks bouncing around from the leading edge of a fincan? The leading edge itself should obviously contribute to some pressure, but does it change (take away) the pressure on the fin itself? If the answer is simply "depends", then never mind.
Click to expand...
Behind a shock the gas temperature increases reducing the local Mach number and the local gas density. Under these flow conditions, the lift and drag should decrease as they are both density dependent, and lift and drag should decrease further as they are proportional to Mach Number squared, so I believe the shock heating should decrease both lift and drag stabilization of the fins.

Bob
 
bobkrech said:
Behind a shock the gas temperature increases reducing the local Mach number and the local gas density. Under these flow conditions, the lift and drag should decrease as they are both density dependent, and lift and drag should decrease further as they are proportional to Mach Number squared, so I believe the shock heating should decrease both lift and drag stabilization of the fins.

Bob
Click to expand...

Lift/drag coefficient decreases with Mach above supersonic, but the forces actually generally increase since they are a function of the total velocity squared.
 
 
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bobkrech said:
https://en.wikipedia.org/wiki/Drag-resistant_aerospike

https://en.wikipedia.org/wiki/Supercavitation

https://en.wikipedia.org/wiki/Underwater_firearm

Shock phenomena are irreversible discontinuities that do not follows your generalizations.

Bob
Click to expand...

Very interesting. The putative range increase from the aerospike on the trident is amazing. I had always assumed it was a relatively minor effect, trying to squeeze out another 0.5-1% more range, but 550 Km is real distance.
 
bobkrech said:
https://en.wikipedia.org/wiki/Drag-resistant_aerospike

https://en.wikipedia.org/wiki/Supercavitation

https://en.wikipedia.org/wiki/Underwater_firearm

Shock phenomena are irreversible discontinuities that do not follows your generalizations.

Bob
Click to expand...

Cavitation is a liquid phenomenon wherein the liquid undergoes a phase change due to the local pressure being less than the vapor pressure of the substance. There is no phase for the gaseous medium that a rocket flies through to change to and thus reduce drag.

Aerospikes create a shock up front so that high-Mach drag effects are not experienced along the entire length of the rocket. This would be a bow shock, which reduces the Mach number pretty significantly--into the subsonic regime near the axis of the shock. Shocks at the leading edge of the fin can would be primarily oblique shocks, which do not reduce the Mach number nearly as much as a bow shock. In fact, in many cases, the Mach number behind an oblique shock is still supersonic! Shocks are high-drag phenomena, so shocks at the leading edge of the fin can lead to a drag penalty which is not reasonably compensated for by a drag reduction behind the shocks.
 
In the case of the aerospike, the shock is generated by a disk the size of a pie plate, which is substantially smaller than the cross-section of the rocket. The fact is that the shock generated by the aerospike disk raises the air temperature and reduces the air density in front of the missile body substantially reducing effective mach number developed by the rocket. Since the drag is proportional to density and Mach number squared, and the power to maintain a given velocity is proportional to density and Mach number cubed, the reduction in drag greatly increases the missile velocity and thus the range of the weapon system.

The Navy's Fact Sheet states that the missile drag is reduced by 50% due to the Trident aerospike. https://www.navy.mil/navydata/fact_display.asp?cid=2200&tid=1400&ct=2

Bob
 
bobkrech said:
In the case of the aerospike, the shock is generated by a disk the size of a pie plate, which is substantially smaller than the cross-section of the rocket. The fact is that the shock generated by the aerospike disk raises the air temperature and reduces the air density in front of the missile body substantially reducing effective mach number developed by the rocket. Since the drag is proportional to density and Mach number squared, and the power to maintain a given velocity is proportional to density and Mach number cubed, the reduction in drag greatly increases the missile velocity and thus the range of the weapon system.

The Navy's Fact Sheet states that the missile drag is reduced by 50% due to the Trident aerospike. https://www.navy.mil/navydata/fact_display.asp?cid=2200&tid=1400&ct=2

Bob
Click to expand...

Drag is actually proportional to density times velocity squared OR static pressure times Mach number squared (for compressible flows). Static pressure actually increases behind a shock.

The takeaway here is that an aerospike creates a bow shock in front of the rocket, reducing the flow around the entire rocket into the subsonic regime, which is certainly better for drag. Shocks at the leading edge of the fins do not change the Mach number around the whole rocket, just a small area near the fins, so the rest of the rocket is subjected to high-Mach (high drag) flow.
 

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