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  1. #1
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    Descent: How fast is too fast before main chute deploys?

    Asked this in another thread, did not get a response. See attached graph, my IQSY Tomahawk was coming down really fast (over 150 fps) until JLCR released the chute exactly at 500 feet for a nice landing. Weight was 3480g with the I-500 motor (2900g with no motor). Was the 150fps too fast? Is there any rule of thumb on how fast is too fast?
    Thanks,
    Bill

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    L1 3/25/17 H135
    L2 8/12/17 J180

  2. #2
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    150 fps is a little too fast. Puts a lot of stress on rocket when your main deploys for starters. Most of mine come in between 50-85 fps.

    Tim
    L3 NAR 98225

  3. #3
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    Guess the solution is to add a streamer or drogue. The DinoChutes Guide Chute looks interesting but apparently it is only for rockets up to 2 lbs. or so, see https://www.apogeerockets.com/Buildi...in-Guide-Chute.

    L1 3/25/17 H135
    L2 8/12/17 J180

  4. #4
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    You ought to ask Michael or Rick or any one of the other experienced flyers where you normally launch. Iím sure they can help. Brian the motorman is pretty knowledgeable too.
    Tim
    L3 NAR 98225

  5. #5
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    I would try to slow it down by either up-sizing your drogue or adding a drogue. Using a slider ring or another technology/method of retarding how fast your main chute inflates could also be used as a strategy, however I would still use a drogue.
    Michael Pitfield
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  6. #6
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    Mostly it depends on your chute. My brother was a licensed rigger for man-rated chutes and told me our rockets don't deploy at a high enough speed and don't need to be as strong as man-rated chutes. The zigzag stitching on the chutes is fine. I argued with him.

    What he told me was that as long as the chutes have a reasonable shroud to chute attachment, they should be easily strong enough to handle deployments at 100 to 150 mph. Your 150 ft/sec works out to 102 mph. So the opinion is, that should be well within the safe limits for chute deployment. If you want it to descend slower, that is fine, but don't do it because of the chute. The chute will handle it.
    Handeman

    TRA #09903 L3 3/29/2015

    "If you don't use your head, you have to use your feet!" my Dad

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  7. #7
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    Seems to me if it recovered successfully with no damage to rocket, harness or chute then you are doing what is supposed to be done. A little fast for my liking but it seemed to have worked just fine. With the chute already out of the airframe bundled with the JLCR, the chance of zippering should be reduced. Was there a large shock or jerk when the chute opened, high G's? Or did it inflate and just slow the rocket down? Like I say, seems to work fine as long as your equipment can handle it.
    Nothing is foolproof to a sufficiently talented fool.
    TRA #16513
    Level 1: Danger Close ---AT H123W to 1240'--- 29 OCT 2016
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  8. #8
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    I question your altimeter. The altitude and velocity data is so noisy during drogue. The green velocity shows no sign of reaching drogue terminal velocity, even after 20 seconds of fall. Looks like accel-based velocity, and just look at the time lag of 15 seconds to reach terminal main velocity of ~25 fps. Meanwhile, the red altitude shows immediate constant velocity of the main. These curves make no sense to me.

    You didn't mention the diameter, but I have a 4" Madcow Frenzy that also weighs ~3000g which I fly drogueless. Let's assume your rocket and mine have similar drag. My SLCF and Marsa altimeters show nice, smooth altitude (baro) under drogue. Near constant terminal velocity is reached very quickly after apogee. Descent rate (slope of the altitude curve) works out to be 55-60 fps for both altimeters.

    I think your rocket is falling at far less than 150 fps and thus not a problem at all.

  9. #9
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    I agree altimeter data looks very suspicious.

    That said, a useful upper bound on g forces caused by a parachute is (speed before chute / final descent rate under chute)^2. So for example a 100fps descent rate before opening a parachute that will slow rocket to 20fps will be 25Gs of opening shock if the parachute opens very quickly.

  10. #10
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    Quote Originally Posted by Handeman View Post
    Mostly it depends on your chute. My brother was a licensed rigger for man-rated chutes and told me our rockets don't deploy at a high enough speed and don't need to be as strong as man-rated chutes. The zigzag stitching on the chutes is fine. I argued with him.

    What he told me was that as long as the chutes have a reasonable shroud to chute attachment, they should be easily strong enough to handle deployments at 100 to 150 mph. Your 150 ft/sec works out to 102 mph. So the opinion is, that should be well within the safe limits for chute deployment. If you want it to descend slower, that is fine, but don't do it because of the chute. The chute will handle it.
    That's probably right. I once had a drag separation at motor burnout that caused the main chute (FruityChutes elliptical) to open at about 300 mph. The chute was undamaged but there was a long zipper in the Blue Tube airframe.
    NAR Level 2 #96210

  11. #11
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    Quote Originally Posted by billdz View Post
    Asked this in another thread, did not get a response. See attached graph, my IQSY Tomahawk was coming down really fast (over 150 fps) until JLCR released the chute exactly at 500 feet for a nice landing. Weight was 3480g with the I-500 motor (2900g with no motor). Was the 150fps too fast? Is there any rule of thumb on how fast is too fast?
    Thanks,
    Bill
    Hmmm, doing some simple math shows the descent rate was 130fps at most. Using the graph the rocket is at about 2850 at 15 seconds and takes 18 seconds to hit 500 feet. That's about 130fps. A pretty good clip but not beyond reason. The issue is not just whether the chute will survive, but will all the attachment points and other components. On extreme altitude flights I like to try and come down at about 80-100fps to try and minimize drift. But the faster things come down the less controlled they are likely to be.

    I do agree that is some of the noisiest data I have seen in quite a while. But I guess you could say that since the chute deployed ok and there was no damage, then your descent rate wasn't too fast. But I don't think it would make sense to extrapolate that to every rocket in your fleet.


    Tony
    why do people put so much stuff in their sigs?

  12. #12
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    Like Handeman says, it depends on the chute. A TopFlite or similar lightweight chute is fine at 50-70 fps, but 100 fps may be enough to rip a shroud line off since they're only sewn a few inches into the canopy. A premium chute with hefty shroud lines that are sewn around the perimeter or across the canopy of the chute will take your 100 fps descent no problem, and can probably handle a lot more.

  13. #13
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    What causes noisy altimeter data? It does not usually look like that. The rocket was falling faster than any other I've seen although the end result was fine. Not sure if I can count on the same good result next flight. It was a standard 48" rip-stop chute; it did not have shroud lines sewn around the perimeter.

    L1 3/25/17 H135
    L2 8/12/17 J180

  14. #14
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    How do your sampling holes look? Any burrs ? Are they clean and round?
    Tim
    L3 NAR 98225

  15. #15
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    Quote Originally Posted by manixFan View Post
    Hmmm, doing some simple math shows the descent rate was 130fps at most. Using the graph the rocket is at about 2850 at 15 seconds and takes 18 seconds to hit 500 feet. That's about 130fps. A pretty good clip but not beyond reason.

    Tony
    I did the same, trying to measure the averaged slope of the red altitude curve. However, can we trust that noisy altitude that bounces around +/- 200 ft? I think 130fps is beyond reason. The other thread mentioned by the OP is collecting drogueless descent rates. Only large L3 class rockets exceed 100 fps. This rocket is not in that class.

    Comparing to my Frenzy example above, for this rocket to achieve 3X the descent rate, the CdA must be 9X less. Not likely.

    I believe the altimeter data shown is all accelerometer-based. This looks like a TeleMetrum. Other accelerometers stop recording once the apogee event blows, because the data is garbage after that. Can you plot baro data instead? That would be more reliable to calculate drogue velocity.

  16. #16
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    Quote Originally Posted by Buckeye View Post
    I did the same, trying to measure the averaged slope of the red altitude curve. However, can we trust that noisy altitude that bounces around +/- 200 ft? I think 130fps is beyond reason. The other thread mentioned by the OP is collecting drogueless descent rates. Only large L3 class rockets exceed 100 fps. This rocket is not in that class....<snipped>...
    Several points:

    * I chose 15 sec and 34 sec because they are outside of the noisy data
    * a quick sim shows the altitude to be reasonable
    * the altitude shows about 500 ft at the expected JL deploy
    * The descent under main shows about 26 fps using the same method which is a reasonable descent rate

    So the altitude numbers seem good and simple math of (altitude drop/time) provides a rate of about 130fps. I'm not trying to interpolate data from the slope. Please tell me how my method is incorrect - drop in altitude divided by time during drop. At BALLS I flew a 38mm MD on a K627 that I brought down without a drogue and it had a descent rate of about 110 fps from 15,000 to main at 1,100'. Certainly not a class 3 rocket.

    I admit 130fps is very fast but unless the time scale is off the numbers don't lie.


    Tony
    why do people put so much stuff in their sigs?

  17. #17
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    Quote Originally Posted by manixFan View Post
    Several points:

    * I chose 15 sec and 34 sec because they are outside of the noisy data
    * a quick sim shows the altitude to be reasonable
    * the altitude shows about 500 ft at the expected JL deploy
    * The descent under main shows about 26 fps using the same method which is a reasonable descent rate

    So the altitude numbers seem good and simple math of (altitude drop/time) provides a rate of about 130fps. I'm not trying to interpolate data from the slope. Please tell me how my method is incorrect - drop in altitude divided by time during drop. At BALLS I flew a 38mm MD on a K627 that I brought down without a drogue and it had a descent rate of about 110 fps from 15,000 to main at 1,100'. Certainly not a class 3 rocket.

    I admit 130fps is very fast but unless the time scale is off the numbers don't lie.


    Tony
    Nothing wrong with your method on the provided data. I did the same and got the same answer. All the altimeter data during drogue looks bizarre, so I am questioning the input to our simple averaging calculation.

    I meant the only >100 fps input to the other thread came on a L3 project. Sure, 38MD HPR has a nice high ratio of m/CdA, giving a higher terminal velocity. Your 15,000' flight has some fairly low air density at play, also.

    If the OP could give the diameter of the rocket and some indication of the fall (flat spin vs. nose down ballistic) that would help compare to other observed descent rates.

  18. #18
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    It's a 3" Mad Cow IQSY Tomahawk. The nose was not pointing down when I saw it.

    L1 3/25/17 H135
    L2 8/12/17 J180

  19. #19
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    Tim is probably right, there was blockage at the vent holes, that must have caused the noise

    L1 3/25/17 H135
    L2 8/12/17 J180

  20. #20
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    Guess the solution is to add a streamer or drogue. The DinoChutes Guide Chute looks interesting but apparently it is only for rockets up to 2 lbs. or so, see....
    Our kids used our 36" Guide Chute on their 20 pound SLI rocket and got descent rates around 80fps. It kept all the recovery gear nice in line. I think rockets weighing less than that would probably bring the descent rate to a more reasonable level. In any event, the Guide Chute works with rockets weighing much more than 2 pounds.

  21. #21
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    @PatB - Thanks for that, I'm thinking about getting a Guide Chute
    @Tim - "How do your sampling holes look? Any burrs ? Are they clean and round?" "Tim is probably right, there was blockage at the vent holes, that must have caused the noise."
    After further thought, I'm not sure the vent holes were the problem. Almost all of the noise occurred after ejection. Before then, the lines look pretty clean. Then the noise largely stops after the Chute Release deployed. Perhaps something about the rapid descent caused the noise?

    L1 3/25/17 H135
    L2 8/12/17 J180

  22. #22
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    Quote Originally Posted by billdz View Post
    @PatB - Thanks for that, I'm thinking about getting a Guide Chute
    @Tim - "How do your sampling holes look? Any burrs ? Are they clean and round?" "Tim is probably right, there was blockage at the vent holes, that must have caused the noise."
    After further thought, I'm not sure the vent holes were the problem. Almost all of the noise occurred after ejection. Before then, the lines look pretty clean. Then the noise largely stops after the Chute Release deployed. Perhaps something about the rapid descent caused the noise?
    Wild guess- thing with vent holes flapping around? That would make some amount of sense, at least to me- during ascent it's attached to the airframe, and once the main is out it's going slower so it's less likely to flop around.
    NAR #104043

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  23. #23
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    Hmmm, had not seen the guide chute before. It looks interesting - it is a good idea to control the arrangement of the parts as they descend. I don't like streamers as they seem to always want to get tangled up so I've just used nothing or a very small drogue. But I can see how a 'reduced' drag chute would help with arrangement yet not slow things down too much.

    billdz - I don't think you ever mentioned what altimeter you used? Can you fill us in?


    Tony
    why do people put so much stuff in their sigs?

  24. #24
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    Quote Originally Posted by manixFan View Post
    billdz - I don't think you ever mentioned what altimeter you used? Can you fill us in?
    Tony
    The screen shot looks like an Altus Metrum plotter, but yes please let us know for sure. Also, are the velocity and altitude from baro or accel data? I thought accel at first, but that is not the standard approach after apogee deployment occurs.

  25. #25
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    Yes, it is a TeleMetrum. The device has a baro and an accel, but I do not know where the data came from.

    L1 3/25/17 H135
    L2 8/12/17 J180

  26. #26
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    Does the Telemetrum usually provide cleaner data?

  27. #27
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    Yes, see attached.
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    L1 3/25/17 H135
    L2 8/12/17 J180

  28. #28
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    Quote Originally Posted by billdz View Post
    Yes, see attached.
    Is this the same rocket and with similar descent mass? This data looks much better and shows drogue rate of 60-75 ft/s.

    I still don't understand the velocity curve during the main descent. Really wish we knew the source - baro or accel or mixed?

  29. #29
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    No, it is a totally different rocket.

    "Really wish we knew the source - baro or accel or mixed?"
    Is there a way to determine this? Keith P or Bdale at AltusMetrum could tell us.

    L1 3/25/17 H135
    L2 8/12/17 J180

  30. #30
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    Late to this thread, but a couple of notes I could add.

    The velocity graph isn't useful much past burnout. For my products, I use the accelerometer for max speed at that point, but only as event detection after that. Certainly not for velocity on descent, or even apogee.

    Once you appreciate how little pressure difference there is in a foot of altitude, or even 10 feet, you'd understand why buffetting on the way down leads to so much pressure noise during tumbling and on chute. Wind is evidence of a pressure gradient, and as you can imagine there's a lot of relative wind during a tumble. Kind of unavoidable, though you can filter it (as AltimeterThree does to calculate descent rates pre- and post-chute opening).

    Terminal velocity of this flight was reached really quickly once the rocket arced over and began descending, probably by t = 20-22 seconds. That's when the slope of the baro altitude descent seems to have reached its final slope prior to chute release. As I'm sure some of you are tired of hearing me say, terminal velocity happens really quickly. The rocket is speeding up only for a brief moment at the start of the descent. After that it's just falling at a steady rate.

    The relationships between rocket geometry and weight and descent rates are complicated. All of us have had a rocket that fell "flat" and rather slowly, and others that come down more vertically, and quite quickly. My own pet theory is that more attention should be paid to the BALANCE of the post-ejection parts and their Cg/Cp characteristics to promote whatever falling configuration you desire. For some folks, that's an organized, controlled vertical descent where the shock cord is stretched vertically using a drogue and nothing can get wrapped (though it may twist). For others, it's a more horizontal tumble. In any case, do what billdz did: measure!


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