So, maybe I'll try a three-stager

[JimJarvis50]

I was looking at the EasyMega data for the period after the rocket started tumbling (at 75 seconds) to the point where the sections were blown apart (at 130 seconds). A plot of the yaw, pitch and roll data is attached. Isn't it facinating how regular the motion is for an extended period of time? I've been told that this is pitch-roll reasonance with roll lock-in, but I'll leave it to you rocket scientists to tell me what is really going on here.

Jim

 

[JimJarvis50]

I posted my on-board video on youtube. Enjoy!

Jim

 

[plugger]

HOLY COW JIM! You weren't joking about the pitch-roll resonance when your 3rd stage was approaching apogee. That's INSANE! It also nixes my thoughts about using magnetic apogee detection for apogee events above 100k' AGL. Still, better to know about that without testing, right?

 

[Nytrunner]

Great video cut!

I'm having a hard time picturing the motion of the 3rd stage during that resonance period. Since it lost aero stability, was it just sort of flying upwards at an angle while spinning around (sort of tracing an inverted cone?). I saw the camera would alternate between full earth, full sky, and horizon views.

 

[neil_w]

All I can say is "wow".

 

[JimJarvis50]

HOLY COW JIM! You weren't joking about the pitch-roll resonance when your 3rd stage was approaching apogee. That's INSANE! It also nixes my thoughts about using magnetic apogee detection for apogee events above 100k' AGL. Still, better to know about that without testing, right?

I'm reading a few things about it, but I don't understand it. I guess it is quite common for sounding rockets.

Jim

 

[JimJarvis50]

Great video cut!

I'm having a hard time picturing the motion of the 3rd stage during that resonance period. Since it lost aero stability, was it just sort of flying upwards at an angle while spinning around (sort of tracing an inverted cone?). I saw the camera would alternate between full earth, full sky, and horizon views.

Yes, me too. In some cases, I've read that the motion can be "lunar", with the same side of the rocket pointing outward. I don't think that happened here, but I don't know that for sure.

Jim

 

[Nytrunner]

I don't suppose you can find a reaction wheel both small and strong enough for in the 2nd stage that could assist maintaining orientation once guidance unit separates?

 

[UhClem]

I don't suppose you can find a reaction wheel both small and strong enough for in the 2nd stage that could assist maintaining orientation once guidance unit separates?

The usual fix is to ensure rotation at a high enough rate to avoid the resonance. Which is why "Yo-Yo De-spin" is popular in sounding rockets.

I suppose that the guidance section could spin up the rocket just before it drops. But that assumes that you have an analysis in hand telling you what frequencies to avoid.

And that the roll rate isn't high enough to increase the motor burn rate too much.

 

[Pyropetepete]

Bloody Hell Jim. Congratulations on the flight and hitting 175K plus the Mach 3.3

So what’s next..? Maybe we should get you to try a 4 stager

 

[TheAviator]

Great flight, Jim. That's a really interesting phenomenon on the third stage. For those having a hard time visualizing, I imagine it's subverting like this.

 

[Nytrunner]

Isn't that a behavior affecting objects with 3 dissimilar moments of inertia at each axis?

Since the pitch/yaw moments of inertia are (probably) similar, I have a hard time seeing a rocket like Jim's experiencing the Dzhanibekov effect

 

[boatgeek]

Thanks for posting video. It looks like the rocket was dead on the RasAero sim until 155K feet. Do you think the tumbling made it miss the predicted apogee by 10K feet, or do you have another explanation? On the one hand, tumbling is really draggy, on the other there's not much air that high.

Clearly this means you need a 4-stager for next year!

 

[JimJarvis50]

Thanks for posting video. It looks like the rocket was dead on the RasAero sim until 155K feet. Do you think the tumbling made it miss the predicted apogee by 10K feet, or do you have another explanation? On the one hand, tumbling is really draggy, on the other there's not much air that high.

Clearly this means you need a 4-stager for next year!

I think I will go ahead and make a plot of all of the gps data against the RasAero simulation. I think Chuck is looking at that as well. One thing that will happen is that the effect of drag will become more obvious. Remember, the simulation is a balistic return versus the actual split rocket. I don't want to speculate too much just yet, but I think a plot will be helpful. I do suspect that there was an impact of the coning on the apogee altitude. I'll post that later tonight - but I think I'll leave the 4-stagers to Fred!

Jim

 

[neil_w]

For those having a hard time visualizing, I imagine it's subverting like this.

I would just like to state for the record that that is bonkers.

 

[rfjustin]

Well done sir.

 

[boatgeek]

I think I will go ahead and make a plot of all of the gps data against the RasAero simulation. I think Chuck is looking at that as well. One thing that will happen is that the effect of drag will become more obvious. Remember, the simulation is a balistic return versus the actual split rocket. I don't want to speculate too much just yet, but I think a plot will be helpful. I do suspect that there was an impact of the coning on the apogee altitude. I'll post that later tonight - but I think I'll leave the 4-stagers to Fred!

Jim

Will look forward to that. Do you have good altitude data from other flights so you can estimate the actual descent speed with the split rocket?

Maybe instead of 4 stages, you could do something simple like an O:O:N 3-stager?

 

[JimJarvis50]

Will look forward to that. Do you have good altitude data from other flights so you can estimate the actual descent speed with the split rocket?

Maybe instead of 4 stages, you could do something simple like an O:O:N 3-stager?

So, here's the graph with the full simulation and all of the gps data (the data that is relevant to compare with the simulation). Remarkable fit, no? The simulation is all turbulent flow, Rogers modified Barrowman and smooth paint. Looks like it fell for a prolonged period at 1,380 ft/s. It also appears to me that there is at least some drag even near apogee.

Jim

 

[pendrin2020]

I really appreciate the level of engineering that went into this. The canards, the fin can, and all the control systems are the kind of stuff people people do as projects to earn their degrees!

That video was really well edited, too!

 

[Leo]

Fantastic!

 

[boatgeek]

I really appreciate the level of engineering that went into this. The canards, the fin can, and all the control systems are the kind of stuff people people do as projects to earn their degrees!

That video was really well edited, too!

+1 I really appreciate the way you talked about problems and how you came to solutions to fix them.

 

[markkoelsch]

+1 I really appreciate the way you talked about problems and how you came to solutions to fix them.

For those that have not been around a long time, Jim has always been really open about how he does things. This includes a two part article that is essentially the best treatise on building a carbon fiber rocket I have seen. I have both parts- when I get home I will link them here as they are a really terrific read.

 

[markkoelsch]

As promised, here is a link to the two articles Jim wrote. Great read. Username and password are both guest. Enjoy.

https://rocketryfiles.com/?p=home&d=Technicalarticles/Jim Jarvis Carbon-Tutorial/

 

[JimJarvis50]

For those that have not been around a long time, Jim has always been really open about how he does things. This includes a two part article that is essentially the best treatise on building a carbon fiber rocket I have seen. I have both parts- when I get home I will link them here as they are a really terrific read.

Mark, thanks for pointing out that I have "revealed" pretty much everything I have done in rocketry. That part of rocketry is important to me. After I wrote the article, I noticed a whole lot more carbon rockets showing up at launches.

I still have the rocket that was made in the first article. It is shorter now, and is no longer minimum diameter (actually, it could fly short 75mm motors), but I have flown it many, many times. Harry (Motorman) Spears pulled it out of a swamp at Hearne a few years ago, so it lives on.

The third stage of the three-stager was made in 2012, after the article. This is the rocket that set the N staged record at LDRS at Black Rock in 2013. Tony Alcocer pulled it off Razorback Mountain - quite a story - amazing how that guy has been involved in so many of my flights.

Jim

 

[tfish]

Tony Alcocer pulled it off Razorback Mountain - quite a story - amazing how that guy has been involved in so many of my flights.

Did you notice I tried to avoid you this year at Balls?

I will admit your rockets are landing much closer and at lower elevations then in years past.

Tony

 

[markkoelsch]

Did you notice I tried to avoid you this year at Balls?

I will admit your rockets are landing much closer and at lower elevations then in years past.

Tony

And Tony, you too have shared a lot of knowledge as well. I have learned from both of you even if I have not flown the large stuff you both have. Thanks.

 

[JimJarvis50]

Did you notice I tried to avoid you this year at Balls?

I will admit your rockets are landing much closer and at lower elevations then in years past.

Tony

You know, you're right!? The liner disappeared, but there was no sticker?

Jim

 

[JimJarvis50]

So, this is of interest probably only to me, but I did figure out the motion that the rocket took when it started "coning". As it turns out, the motion was basically end-over-end in a north/south plane. The time for one end-over-end revolution is 8 seconds. This can be seen in the yaw/pitch graph with a maximum rotation rate of 45°(which, times 8 secods, gives 360°). The rotation rate along the long axis is 180°/sec, so the yaw and pitch rates should be out of phase by 1/2 second, which they are. So, one end-over-end rotation every 8 seconds, with 4 rotations along the long axis during that period. If you look closely at the video, the view repeats every 8 seconds. At the time when this motion started, the rocket was still moving at Mach 1.5. Good thing there wasn't much air at 140,000 feet! I just think this is soooo strange.

Jim

 

[OverTheTop]

Things always tend to tumble around the axis with the highest moment of inertia eventually. You will always see big space stations depicted as rings rotating, providing "gravity" to the outside surface. That form is stable. If it were a long skinny tube spinning along the axis the motion degrades naturally to end-over-end tumble in the absence of any other restoring forces.

https://www.quora.com/When-a-rotating-asteroid-spins-is-this-around-a-principal-axis-of-inertia

 

[Nytrunner]

Fascinating indeed. It reminds me of the failure experienced by the Super Strypi in 2015 (a spin stabilized 3 stage solid motor vehicle for orbital payloads by NASA). The telemetry model they displayed went rather...wonky, but in a fashion similar to what you described. For a sounding rocket like yours, the tumble may not matter, but for orbital insertion it was fatal.

So that's the ticket. Just get it going so high so fast, that by the time it goes aerodynamically unstable, the atmosphere is too thin to matter!