Mach pressure drop or motor burn out..?

Discussion in 'Rocketry Electronics and Software' started by Tim51, Aug 27, 2019.

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  1. Aug 27, 2019 #1

    Tim51

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    Hi All
    So I've just recovered this data from my SLCF after flying a CTI K740 load over the weekend, and I'm looking at that sudden drop in the red (velocity curve) which more or less corresponds to motor burn out for that motor (2.5s) but also the small smaller corresponding little glitch in the magenta (altitude). Is this a Mach pressure drop? The rocket simmed to M 0.97, and an altitude of 7648', but the actual altitude recorded is 7947'. So I'm wondering whether this indicates the rocket passed through Mach 1? Screenshot below. Does anyone have any Mach + experience with the SLCF to compare?
    Thanks!
    Screen Shot 2019-08-27 at 20.47.44.png
     
  2. Aug 27, 2019 #2

    Eric

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    That velocity is showing that you went over 1900 fps. I had a flight earlier in the year That was supposed to go to 10000 ft, and Mach 1.6. But my velocity spike went up to 2600 FPS. And I have the same dip as your flight does. Was your avbay sealed very tightly? I was wondering if I drilled my static portholes a little too large or too small.

    My flight was on an L1000.

    Untitled.jpg
     
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  3. Aug 27, 2019 #3

    Buckeye

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    Don't pay much attention to the +/- spikes in the velocity curve. That is mostly noise caused by numerical differentiation of the altitude. Plot velocity from a simulation to see a better representation of reality.

    The altitude (barometer) will sometimes show a dip/wiggle when passing through Mach 1. I have seen this on Raven, maybe SLCF as well. Can't remember.
     
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  4. Aug 27, 2019 #4

    Tim51

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    Thanks for posting - yes, the av bay was sealed pretty tight.
     
  5. Aug 27, 2019 #5

    Tim51

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    Ok thanks for the explanation- much appreciated.
     
  6. Aug 27, 2019 #6

    UhClem

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    The pressure anomalies are the result of the shock wave travelling along the length of the rocket. (Position varies with velocity.) This wave forms before you reach an overall velocity of Mach=1 because the nose pushing the air aside increases the velocity. You need to worry about this any time you exceed about Mach=0.8.

    So in the ideal case you will see two anomalies. One while velocity is increasing and the other when it is decreasing.
     
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  7. Aug 28, 2019 #7

    Eric

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    Any idea why the two previously posted flight summaries showed velocities nearly double what was simulated?
     
  8. Aug 28, 2019 #8

    jderimig

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    Because velocity derived from differentiating baro readings is notoriously inaccurate, especially if there is a shockwave involved....
     
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  9. Aug 28, 2019 #9

    gtg738w

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    Can you share some info about your av-bay? Length, Dia, number and size of vents, vent placement (top, bottom, middle), altimeter placement? Do you have a sim file for the flight?
     
  10. Aug 28, 2019 #10

    Tim51

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    Sure - see attached pics and OR file (the rocket itself has seem some action and some repairs, hence the extra couplers you'll see on the airframe etc). It's a pretty normative Av bay, 211mm x 98.9mm. This one has 7 flights on it now, no problems. PML phenolic internally reinforced with 200gsm FG. There are 4 x 3mm static ports, spaced around the switch band, but allowing space for the screw switches. Stepped lids, wire holes sealed with epoxy, tie rods sealed with fibre washers and nylon washers, wing nuts on the drogue end, nyloc nuts at the main end. The altimeters are the SLCF and a RRC2+ (the latter actually gave a higher apogee of 7968' for this flight), placed in parallel mid point on the sled, so they're directly under the static port array. Not shown here are the two 9V Duracells usually slung underneath, on the opposite side of the sled, strapped up with doubled zip ties (I habitually remove batteries after each flight). Binder design screw switches, filleted with epoxy. 20190828_073817.jpg 20190828_074946.jpg 20190828_073744.jpg
     

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  11. Aug 28, 2019 #11

    cerving

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    Your data looks just about what you would expect from a mach+ flight. You'll get that "mach dip" in altitude right before mach due to the pressure buildup, then the altitude will come back up until the motor burns out. You can also see a little squiggle around apogee... that's when the drogue fired. The SLCF is reporting that as a pressure anomaly... I'm guessing that it was your backup, since the primary usually doesn't show the drogue squiggle since it takes its readings before it fires the charge. If you have an altimeter with an accelerometer you won't see a mach dip, but you'll see the drogue deployment as the AV bay separates.
     
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  12. Aug 28, 2019 #12

    Tim51

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    Thanks for posting Cris. On this flight the SLCF was - putatively - the primary, with the RRC2+ dip switches set to fire the drogue as back up at apogee +1s as backup, but I also retained the motor eject charge as there was no need to remove it. So I'm thinking the apogee squiggle might have been the motor charge firing. My reasoning is based on this graph (below) from a much slower flight 2 years ago (J357 5G 38mm) back when I flew just one altimeter + motor eject, showing that same apogee squiggle. Screen Shot 2017-09-07 at 11.41.38.png

    Either way I certainly would like you to be correct on this because as you might have inferred it would confirm my first Mach + flight, and it was a nice one at a beautiful launch site (Fairlie Moor Rocket Site, Scotland). The onboard video shows the rocket nicely levelling at nose over before the apogee event. On the other hand if there's doubt lingering in the TRF hive mind I suppose I'll have to just go back next year and do more flights with bigger motors...*sigh* : P

    Screen Shot 2019-08-28 at 17.29.48.png

    Screen Shot 2019-08-28 at 17.30.28.png
     
  13. Aug 28, 2019 #13

    cerving

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    OK, with two deployment controllers you'll be better off removing the BP from the motor. Most of the time, the delay is too short, which is what happened here. Both altimeters are picking up the drogue event, which implies that it was external to the altimeters. For the actual mach velocity, you can adjust for temperature, launch site altitude, and actual barometric pressure, but with your uncorrected velocity at around 2,000 fps I think it's pretty safe to say that you broke mach.
     
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  14. Aug 28, 2019 #14

    Buckeye

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    Why so much focus on the "apogee squiggle?" This is always the case in every flight. After the first apogee event fires, all bets are off for any smooth data. There are pressure leaks, violent accelerations, tumbling airframes, and back up events: all affecting the baro measurements.

    Take your 0.97M OpenRocket simulation, adjust speed of sound for temperature if needed, round to 1.0M, and call it good. You at least tickled Mach 1 if not broken it outright. Brag to your friends! They don't know any better.
     
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  15. Aug 28, 2019 #15

    Tim51

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    Thanks to both Buckeye and Cris for an informative discussion, and everyone else who contributed. Following Buckeye's suggestion, I've just researched the recorded weather conditions, airpressure, ASL etc of the area of the launch site on Saturday, and factored these in to a revised OR sim. It suggests the total velocity fell short of Mach, by a significant margin - the revised sim puts the fastest total velocity at 733.6mph, so hopefully next time... File attached.
     

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  16. Aug 28, 2019 #16

    boatgeek

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    How does your sim altitude match up with measured? If it's not spot on, then you can tweak the drag a little with surface finish changes to get the altitude dialed in. Then check for Mach.

    On another topic entirely, how close are you to the wind turbines? I've flown near rocket-eating trees, but never rocket-eating turbines!
     
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  17. Aug 28, 2019 #17

    Buckeye

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    The data file says Mach 0.97 at 2.309 s. That is pretty close in my book.

    Be careful. OpenRocket gets wacky when you enter custom launch site elevation and non-standard day temperature and pressure (like giving me higher altitude at sea level than at 5280 ft!). The data file has ~60 columns of data and not one of them is air density?!?!?!? This tells me that something is seriously screwed up with the atmospheric model in Open Rocket.
     
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  18. Aug 29, 2019 #18

    Tim51

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    Thanks - I'll have another look at that tonight when I'm back from work. Those wind turbines are a long way from the launch site, up on the braeside on the far side of a reservoir. They look closer (and possibly more of a threat) than they are in that picture probably due to the wide angle optic. Even though the prevailing winds aloft were in that same general direction, the rocket's landing location was not even close. Even the highest flight of the week (23,200') landed safely away from them.
     
  19. Aug 29, 2019 #19

    jderimig

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    Mach speed is defined by airspeed not ground speed. If you simmed just under Mach , groundspeed, you may broke Mach 1. You altimeter data suggests you did.
     
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  20. Aug 29, 2019 #20

    Tim51

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    Thanks for this - understood. And thanks to you and to everyone who contributed their knowledge and expertise on this. I'm happy enough on two accounts - (1) at the very least this was clearly a 'near Mach experience' and (2) my scratch built airframe handled it all robustly and smoothly. I understand that to get truly supersonic (my evential goal) I need to aim for M1.2 or so. This flight experience has been an invaluable step towards that.
     

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