Discussion in 'High Power Rocketry (HPR)' started by Ez2cDave, May 26, 2019.
NEUTRAL STABILITY . . .
That's a disaster
I made that motor. It worked great!
Nothing to do with airframe.
That was a great burn!
OK . . .
What, other than Neutral Stability, would have caused the rocket to wander all over the sky and then, later, stabilize, after enough propellant was consumed to move the CG forward ?
I think we were saying " Yeah, I claim the motor but not the airframe. ". That is, the off-balance bit wasn't necessarily decided by the mixers & packers.
Dave , you answered your own question . CG shift from a 100 pound motor.
I already knew the answer and that is was, indeed, a prime case of neutral stability.
My question was directed to "Mclark" who said "nothing to do with the airframe". ( I took that to mean that he was disputing Neutral Stability, as the cause of the incident ).
Anyway, it's the best example of Neutral Stability I have ever seen. Fortunately, it was pointed in a relatively safe direction when it finally stabilized. ( I have no idea where it impacted / landed )
Would someone define Neutral Stability for me please.
I should have written "I had nothing to do with the airframe."
Nose weight or larger fins and this would have been just another P motor flight at BALLS few remember.
A rocket is stable, or has positive stability when the center of gravity (Cg) is ahead of the center of pressure (Cg). If it turns, aerodynamic forces acting through the Cp straighten it.
Neutral stability is when the Cg and Cp are very close together. The rocket doesn’t receive corrective fives and wanders around but not violently. That’s bad.
Negative stability is when the Cp is ahead of the Cg. The rocket will constantly try to reverse direction. That’s really bad.
I guess I still get amazed when people make advanced projects and fail at the basics.
So Neutral stability = ~0 caliper. Thanks
No problem, sir !
BTW - What was the "aftermath" of that flight ?
What did they recover and where ?
Imbedded about half way in the ground 5 miles north. Airframe was mostly carbon so it disintegrated on impact.
The person (shall be nameless) who made the airframe and I have an annual greeting at Balls, "Will you make me a motor?, "NO!"
The question he should be asking is "Will you teach me basic rocketry aerodynamics ? " . . . LOL !
Yeah, how does that even happen? You get to level 3, you get the FAA clearance for a class 3 rocket, and no one seems to know it's not stable?
I would suspect inaccurate estimation of Cp, but at that size it is also more difficult to measure the center of gravity.
Was launched before Class 3 and individual waivers.
With the proliferation of sims, the oversight and documentation required, and the simple due diligence required of that power level....simple mistakes like that are all too common. I see 1 in 3 L3 attempts go bad, usually for simple stuff. Simply no excuse for it.
If you can't do weight and balance correctly, you, your mentor, nor the RSO should be launching that rocket. The RSO's job embodies 'trust but verify'.
The worst thing that I witnessed was last year of a L3 stubby rocket that power pranged right in front if the RSO/LCO table, it was the flyers second HP rocket build ever. I no longer launch with that club.
Still seems to happen with alarming frequency, regardless of power level.
That is exactly what we need to prevent.
As an RSO for many of our clubs launches, I hate that "it meets the requirements, but doesn't feel quite right" feeling. Thats when I get a second opinion.
As someone who I believe to be fairly knowledgeable stated (in another thread) that if the motor was a Bates grain then the CG remains fairly static throughout the burn, I am wondering if what we have here is the CP shifting forward as the rocket goes through transonic/mach and combined with a narrow margin of stability (1caliber or less) that was the cause for the instability.
Good for you, Rich. All of our Prefects, TAPs, and RSOs need to know that their authority allows them to say no if they don’t feel comfortable. And their responsibility is to do just that.
I would like to believe that "every rocket has a pad", but sometimes we just don't have enough setback distance!
There are cases where placing a questionable rocket on a farther pad is safe and instructional, but some rockets should never be flown (or maybe some flyers should be given a chance to fix their problems before flight).
Some years ago at a launch (Potter, or MDRA???) I saw a person setting up an L2/L3 rocket (I forget which, the rocket was sized suitable for either). He was working on the recovery assembly. Honestly nothing was even close to acceptible. If it had deployed, it would have ripped the chutes out. Attachment hardpoints weren't hardpoints, knots used were very bad choices, size of harness material was quite marginal, chutes not packed reasonably for good deployement. He was getting ready for a cert flight. I'd have put the odds of a successful flight at less than 10%. It would have taken a streak of good luck. I'm afraid I stepped in to help a bit. Then others joined in. I think in the end he chose not to cert that day, and may have successfully certed the next day or the next month. That was the right decision.
I think when it comes to L3 cert flights, the person doing the flight should give a short on-field talk and Q&A session with other L3 rocket folks and the RSO or another experienced club rep(s) - with the mentor present of course. But the rocket person should be the one describing design, features, construction methods, sims, procedures, and answering the questions. Not as a grilling - but to demonstrate that they really do understand what they are doing. Consider it a last minute chance to correct errors before they are triggered by a bad flight!
My L3 design and build was publically documented through the process. I also consider that a good idea. There is a lot of experience here that is very happy to help! It is easier to correct things in the earlier stages. But honestly I think there is too much reliance on kits, particularly for L3. That leads to a false sense of security and an over-estimation of skill and experience level.
The part I bolted and italicized is exactly what a TAP or L3CC should be asking the candidate about when it’s not very obvious that the candidate understands what they’re doing.
I’ve had candidates who obviously knew what they were doing. They were typically the folks who had 20 flights or more on five or six different rockets after achieving L2.
There are also candidates who push their parents through certifications. They study everything they can get their hands on and live to fly rockets. They don’t scare me either and when they turn 18 they’re the ones I wouldn’t hesitate to have certify 1-2-3 in a short time.
The ones who scare me are the ones who are trying to certify very quickly without getting exposed to experience building flights. Some are trying to impress people by how quickly they get their certifications and don’t consider the fact that they’re sacrificing experience for speed. Some are under the gun due to an external constraint, such as a collegiate project.
The problem is discerning which is which sometimes. Prefects and TAPs have their hands completely full sometimes and may not know someone’s background or experience level. think almost all of our TAPs are extremely conscientious. It’s not a simple problem with an easy solution. I
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