A Spectacularly Ugly Flight on an AT I211W

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kjhambrick

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So I've been poring over my Blue Raven data all week trying to discover clues as to why "T'Pring's P'Toy" ( aka TP ) flew so poorly on Sunday, May 26 at the Texas Shootout.

I flew an AT RMS 38/480 I211W, Batch 08172311 and the loaded rocket mass was 1152 grams.

The initial motor mass was 484 grams and the final motor mass was 223 grams which gives a propellant mass of 261 grams and a rocket coast mass of 891 grams.

The temperature was 76 F at 09:19 am, relative humidity was 44% and the barometric pressure was 958 mB at the 1382 ft MSL Tripoli North Texas Seymour site.

The temperature and humidity translate to ground level speed of sound at about 1137 ft/sec.

EDIT: The wind was blowing about 12 mph with occasional gusts to 20 mph.

OpenRocket sim'd to about 6200 ft at about 30 Gs acceletation for a maximum transonic velocity of 1035 ft/sec.

TP flew dual deployment mode with the main set for 400 ft AGL using 0.75 grams of BP for both drogue and main and I flew @Brainstormz123 Comspec Tracker attached to the drogue shock cord in a gob of duck tape.

She actually flew to about 7,000 ft and acceleration and maximum velocity were in the same ballpark.

TP flew from an 8 foot 1010 rail without any backing but I don't see where there was any 'rail whip' -- she started coning well after she left the rail:
tp-i211-C40526-liftoff.jpgtp-C40526-i211-liftoff-2.jpg

These are the raw pressure altitude, corrected density altitude and the inertial altitude anong with raw accelerometer data with significant events marked on the plots:
tp-C40526-i211-ipd.pngtp-C40526-i211-accel.png
TP definitely went transonic as seen at the tail of the thrust phase n the (x,y,z ) acceleration -vs- time plot ( marked as 'Drag Event' ) but the coning began well before.

Note the (y,z) acceleration and the 'gyro action' begining as soon as the rocket left the rail ( EDIT: at about 0.25 seconds. )
tp-C40526-i211-accel-with-machnum-2.5-sec.pngtp-C40526-i211-gyro-with-machnum-2.5-sec.png
Note the interesting 'batman cowl' in the x-axis ( roll ) gyro data while TP was transonic ...

This is the roll and tilt data for the same 2.5 seconds:
tp-C40526-i211-angles-with-machnum-2.5-sec-fudged.pngtp-C40526-i211-inertial-with-machnum-2.5-sec.png
I give up ! I've looked for clues as to what caused this particular flight to be so ugly but I can't find anything in the data.

This is actually the 4th flight of TP since I rebuilt her last winter and the other three flights were all normal.

Moreover, she flew numerous times back in the 1990s on everything from F-motors at Fiesta Island to a J350 at Ocotillo where I forgot to arm the AltAcc and she crashed.

I am looking for ideas !

Anyhow ... all I can think of is to fly another I211W in TP to see if there is a systematic issue with the rocket ...

She landed undamaged 4872 ft from the pad in a mesquite tree:
tp-i211-gps-tree.jpg

The Comspec tracker and the Walston receiver worked very well -- I never lost the signal but it was a long hike around all the fences out there.

As it turns out, I could have driven to within 25 feet of the tree without ever leaving the road except to hop a fence !
tp-C40526-i211w-pad-to-tree.png

And finally, @manixFan 'found me' on the way back so I didn't have to walk sll the way back to the launch site.

I want to thank the gang at TNT -- they put on a GREAT launch

I'll be back !

-- kjh

EDIT: forgot to insert inertial data
 
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how did the nozzle look after the flight? I generally like the I211W motor, but sometimes on the single use nozzles there can be uneven erosion that leads to off-centered thrust.
 
how did the nozzle look after the flight? I generally like the I211W motor, but sometimes on the single use nozzles there can be uneven erosion that leads to off-centered thrust.
Good call @pugachu !

I don't see anything obvious except there is an thick asymetrical layer of slag on the inside, converging section of the nozzle:
tp-C40526-outside-nozzle.jpgtp-C40526-i211-inside-nozzle.jpg

Maybe the slag caused issues ???

I also saved the phenolic liner and I plan to check that the nozzle end is square where the aft insulator and the nozzle are installed.

Thanks for the ideas !

-- kjh

EDIT: the phenloic liner is still nice and square on both ends so the nozzle was aligned properly.
 
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Your rocket was playing in the transonic region for almost a full second. You also achieved better performance from the sim. I cannot understand why you would blame the motor for functioning absolutely fine. Put a bigger motor in it and punch it thru the sound barrier. It will fly alot better .
 
Your rocket was playing in the transonic region for almost a full second. You also achieved better performance from the sim. I cannot understand why you would blame the motor for functioning absolutely fine. Put a bigger motor in it and punch it thru the sound barrier. It will fly alot better .
Thanks for looking at my post, @3stoogesrocketry !

I was not blaming the motor -- it did give me quite a bit more performance than I expected -- what more could I ask :)

Watching the flight from the ground, it was very ugly -- there was a lot of wiggle, way down near the ground, just after it left the rail -- well before it went transonic.

I was really surprised when I saw the altitude and velocity reported by the Blue Raven because I expected apogee and max velocity to be lower than the sim because of the wiggling and coning.

I wish I had a decent video but @bad_idea commented on the ugliness of the flight when he saw it and the wiggle and roll is evident in the Blue Raven data and I am trying to learn how to interpret what I see in the Blue Raven flight data.

I am curious why the rocket wiggled without any roll as soon as it left the rail and then started rolling and wiggling ( i.e. coning ) when it went transonic.
tp-C40526-i211-angles-with-machnum-2.5-sec-fudged.png
Note the red tilt and blue roll from liftoff up to 1.210 sec before the rocket went transonic.

There was a lot of 'wiggle' but TP only rolled about one-half rotation up to when she went transonic.

Then the rocket suddenly started rolling at 1.210 sec as it went transonic and then decelerated back down -- it made about 3.5 rotations in 0.85 sec.

Maybe there was a momentary zephyr when the LCO pressed the button ?

I dunno ...

These the raw gyro and the tilt and roll plots from liftoff to apogee:
tp-C40526-i211-gyro-20-sec.pngtp-C40526-i211-angles-20-sec.png
It seems like once it started rolling it never stopped but the coning seemed to diminish.

I've never had a flight computer that records 6-DOF before I started flying the Blue Raven but I did fly this same rocket on an I211W at Turkey Shoot on Nov 27, 1999.

It was a little heavier before I rebuilt it last year ( it had an AltAcc Av-Bay and a different forward main chute bay ) and it never went transonic on that flight.

There was also very little wind that day where the wind was blowing about 13 mph on this most recent flight.

I like your suggestion -- fly a bigger motor :)

I've got an I284W that should take it right thru the transonic region and I planned to fly that next time I've got a launch site and weather conditions for an 8000 ft flight.

Thanks again !

-- kjh

p.s. I do want to fly another I211W to see if this is simply a property of the rocket ( it is a 1.91 inch diameter LOC Vulcanite scale model ) or if it was a random fluke due to the wind and the rail and ???

This is the Drag -vs- Velocity Plot for several AltAcc flights from the 1990s before I rebuilt the rocket and renamed it:
vulet-dr.png
An I284W or a J350W and a few other RMS 38/600 , 38/720 and 38/1080 motors should punch right thru that transonic region ...

All I need is a launch site and good weather :)
 
Can someone explain the "spectacularly ugly" part?
@seth_cooper --

Yes, it is hard to see without a video but the rocket wiggled violently thru the entire thrust phase as soon as it left the rail.

This was the fourth flight and all the other flights with smaller motors were nice and smooth with very little tilt or roll.

-- kjh
 
Great data on a long duration transonic flight. That vehicle would be ideal to study transonic issues like vent hole size, vent placement, and shockwave propagation. Mount pressure sensors and mems microphones starting 1 caliber below the nose cone.

I've seen the "Batman Cowl" roll pattern on other rockets entering and exiting the transonic region. A normal shockwave forms over the fins at about Mach 0.8. The vehicle is not supersonic, but a small region of air over the fins is supersonic forming this shockwave that can result in accelerated roll rates due to small differences between the fins. Is it a 3 or 4 fin design?

The nozzle slag and noisy burn characteristics is the result of the higher aluminum content of White Lightning propellant. I'm working on a possible fix for the buildup of slag on the nozzle. Slag passing through the nozzle throat I can't do much about. I just understand the accelerometer pattern.

Krell
 
there was a lot of wiggle, way down near the ground, just after it left the rail
Sounds like an underdamped oscillation, like a mass on a spring or a pendulum. Solutions could include more aerodynamic damping, like bigger fins located as far back as you can get them. Another solution might be to lower the natural frequency of the system by lengthening the body tube to increase the rotational moment of inertia. Like lengthening a pendulum, it will oscillate slower, and maybe lower amplitude.
 
Great data on a long duration transonic flight.
Thanks Krell.

The more I look at this flight data the more interesting it gets !

However, all I did was fly and recover the rocket and then dump and plot @Adrian A's Blue Raven Data :)

He deserves all the credit for this thread !

Without the Blue Raven I might blame a whippy rail or the motor or maybe even retire TP thinking that the fin can was somehow damaged by one of several crashes she suffered back when I was flying AltAccs :(

That vehicle would be ideal to study transonic issues like vent hole size, vent placement, and shockwave propagation. Mount pressure sensors and mems microphones starting 1 caliber below the nose cone.
Yes, I built her way back when thinking it would be small enough to fly supersonic on 'affordable' motors.

I had planned to eventually fly TP on a few RMS 38/1080 J570W motors back in the day ( the biggest 38 mm AT motor at the time ) but I crashed TP on her 'final' J350W flight at Ocotillo in 2000.

You made me look :)

A MEMS Audio Sensor is an interesting idea for studying sonic phenomena ... I need to ponder that !

I've seen the "Batman Cowl" roll pattern on other rockets entering and exiting the transonic region. A normal shockwave forms over the fins at about Mach 0.8. The vehicle is not supersonic, but a small region of air over the fins is supersonic forming this shockwave that can result in accelerated roll rates due to small differences between the fins. Is it a 3 or 4 fin design?
The LOC Vulcanite is a 3-fin rocket and TP has three diamond airfoil fins ( shaped 1/4 inch plywood skins -over- a 1/32 inch fiberglass plate core ).

These are the dimensions of the three fins:

root chord: 4.00 inch
tip chord .: 1.70
height .....: 2.85
sweep .....: 1.80

root thichness: 0.300 inch
tip thickness .: 0.128
t/c .................: 0.075

At least those were the dimensions when I built her :)

This is a pic of TP's tail with in her Kilz undies in subdued light so you can see the airfoils:
tp-tail-pic.jpg

The nozzle slag and noisy burn characteristics is the result of the higher aluminum content of White Lightning propellant. I'm working on a possible fix for the buildup of slag on the nozzle. Slag passing through the nozzle throat I can't do much about. I just understand the accelerometer pattern.

Krell

Yes, I hadn't thought to 'blame Aerotech' for the ugly flight until @pugachu asked about the nozzle ...

That immediate 11-degree tilt is 'interesting' though..

The wind speed was 13 mph and the rocket left the rail at 117 ft/sec so the initial AoA was about 9.3 degrees and an underdamped system will overshoot the input.
tp-C40526-i211-gyro-0.15-sec.pngtp-C40526-i211-inertial-0.15-sec.png
But as @sr205347d said, that could simply be underdamped oscillation, especially between t=+0.046 sec when TP left the rail and t=+1.069 sec when she passed thru mach 0.8 while the motor was burning. This is the same plot as above but with Mach 0.800 marked on the x-axis at t=+1.069 sec:
tp-C40526-i211-angles-with-machnum-2.5-sec-fudged-mach-0.8.png
Anyhow, thanks all'y'all !

I've learned a lot from your feedback but I am still curious why TP wiggled so hard when she left the rail ...

Maybe it was just that 13 mph wind off the rail and coning while she was transonic :)

-- kjh
 
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Sounds like an underdamped oscillation, like a mass on a spring or a pendulum. Solutions could include more aerodynamic damping, like bigger fins located as far back as you can get them. Another solution might be to lower the natural frequency of the system by lengthening the body tube to increase the rotational moment of inertia. Like lengthening a pendulum, it will oscillate slower, and maybe lower amplitude.
Yes, I agree, especially from liftoff until TP went transonic.

See post #10.

Thank you @sr205347d !

-- kjh
 
I agree with you, kjh, that the initial oscillation was the result of AoA winds and with @sr205347d that the oscillations are underdamped.

Keep your flight data files available. I've gone back to 5 year old flights and extracted new information from the data that I had not thought about or did not know how to extract the data from the noise.

Don't go blaming Aerotech for the natural characteristics of White Lightning propellant. If you want a smoother burning no slag motor nozzles fly BlackJack and Black Max propellant motors.

Regarding the MEMS mics, I read several NASA papers on MEMS microphones used to detect shock passage along the skin of sounding rockets. I purchased a few for development and testing. I'm looking for a very small cheap sensor to study fin shocks at subsonic and transonic velocities. A flight with them is still months away.
 
Because rockets. Don't kill yourself trying to figure this out. It happens. Most likely a tiny bit of offset thrust caused this issue. Maybe aliens. It happens from time to time.
 
she started coning
Reminds me of the time, in the early '70s, when I built a rocket for an eggloft competition with tiny little fins. My Barrowman calculations (by hand, of course) said it would be stable. It was, statically.

I didn't win. The lesson for me was that smaller fins might have less drag, but the coning certainly can't improve performance.
 
I agree with you, kjh, that the initial oscillation was the result of AoA winds and with @sr205347d that the oscillations are underdamped.
+1
Keep your flight data files available. I've gone back to 5 year old flights and extracted new information from the data that I had not thought about or did not know how to extract the data from the noise.
Shucks, I still have all my AltAcc data from the 1990s as well as numerous flights where I helped other fliers with their data reduction :)

Don't go blaming Aerotech for the natural characteristics of White Lightning propellant. If you want a smoother burning no slag motor nozzles fly BlackJack and Black Max propellant motors.
+1 ... nope. I love the White Lightning propellant and I will continue to fly it.

I did fly an RMS 38/480 I225FJ last winter. Very nice motor !

Regarding the MEMS mics, I read several NASA papers on MEMS microphones used to detect shock passage along the skin of sounding rockets. I purchased a few for development and testing. I'm looking for a very small cheap sensor to study fin shocks at subsonic and transonic velocities. A flight with them is still months away.
Yes, it sounds like one might have to integrate the sensors with the build to avoid the case where the sensors didn't affect the air flow.

Good luck and please let us know how it goes !

Thanks again !

-- kjh
 
Because rockets. Don't kill yourself trying to figure this out. It happens. Most likely a tiny bit of offset thrust caused this issue. Maybe aliens. It happens from time to time.
+1 for the best answer for 'why was my flight so ugly ?' :)

It was aliens !

Yes, that's my story and I am stickin' to it !!

Seriously though ... 'it happens from time to time' is good enough for me.

That design usually flys beautifully in 48mm, 54mm and 73mm diameter rockets so I am going to try another I211W (*) as soon as I can to see what happens next time she goes just barely transonic

I am a data geek and I actually had a lot of fun trying to relate the Blue Raven data to what I thought I saw when TP left the rail so no worries there :)

Thanks again @bobbyg23 !

-- kjh

(*) - According to OpenRocket, there are actually seven AT RMS 38/xxx motors that should accelerate TP into the same transonic ballpark at Apache Pass this summer. I will definitely fly one-or-more of these motors too:

AT MotorMax V (ft/sec)Altitude (ft)V at 8 ftCase - It (Ns)
I195J1024632812238/600 - 426
I357T1030632813238/360 - 342
I211W1040654611538/480 - 442
H550ST1040529618338/360 - 320 ( Whee )
H999N1068536723938/360 - 320 ( Ludicrous )
I305FJ1070645912538/600 - 452
I268R1080628812038/480 - 420

This is my first attempt at a TRF table ... let's see how it looks :)

EDIT: it even looks OK in my phone. Good job, TRF developers !
 
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