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What makes the fliskits Nell stable-Goddard Pedulum rocket Fallacy

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ScrapDaddy

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In 1927, modern rocketry was born, when Robert Goddard launched his Nell into the stratosphere(not really more like 100 feet or somthing around that:D) his rocket was sopposed to prove the pedulum rocket fallacy, which Mark graciously explained in greatly simpiflyed terms why this wouldn't work


"In plain (and simplified) terms, the pendulum theory assumes that gravity will always pull the weighted aft end of the rocket down toward the ground. This will bring the motor back up to a vertical orientation and insure that its line of thrust is perpendicular to the ground (so that the rocket goes straight up). The problem is that if the rocket motor's thrust is powerful enough to lift the rocket up against gravity and launch it into the air, then it is also strong enough to overcome the supposed stabilizing effect of gravity. Also, even if the rocket motor remains in an absolutely vertical orientation, its line of thrust will vary a little bit from perpendicular to the ground. That variance (or misalignment) will cause the thrust to "push" the rocket over. Since the thrust is stronger than the pull of gravity, it can easily overcome any pendulum effect and will keep forcing (torquing) the entire rocket over and turning the line of flight away from vertical until the rocket's trajectory eventually intersects the ground.

The rocket needs some other stabilizing mechanism because the pull of gravity will never be sufficient to keep the rocket aligned vertically in relation to the ground."

As us modern rocketeers know the Nell did not achivemrnt stability during it's flight proving the theory false,but strangly enough 83 years later Jim Flis shock the world(not really this stuff is right up his alley) by creating a stable sport scale model of the Nell, now the question is how did he do it? We all know the basic criteria for stability is CG in front of the CP...... Hopefuly at the end of this ten day period Jim will be willing to share how the stability is achived in his model...... If not I'll google it......... And Jim, if you see this tread please don't respond until 4/7/10..... However if you wish you can tell the TRFers I they are right or wrong :)
 
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The EGE

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It's not terribly complicated.

CG is up front because the heavy motor is up front; everything else is light tubing and thin dowels. CP is aft because of the big conical transition. When you ignore the side dowels - which have very little aerodynamic effect - it's all standard Barrowman stuff.

It'll post a file in a few minutes.
 

The EGE

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Here's a quick and dirty OpenRocket file. The part sizes aren't perfect, and there's a lot of the internal stuff missing.

This simulation says it's only barely stable on a B6, but more stable on a C6. However, that's ignoring the weight of the recovery system and much of the outside tubes and centering rings, so real-life stability is somewhat better.

View attachment NotScaleNell.ork
 

jflis

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I can't respond till 4/7!??!

ARGH!

LOL
 

Micromeister

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In 1927, modern rocketry was born, when Robert Goddard launched his Nell into the stratosphere(not really more like 100 feet or somthing around that:D) his rocket was sopposed to prove the pedulum rocket fallacy, which Mark graciously explained in greatly simpiflyed terms why this wouldn't work


"In plain (and simplified) terms, the pendulum theory assumes that gravity will always pull the weighted aft end of the rocket down toward the ground. This will bring the motor back up to a vertical orientation and insure that its line of thrust is perpendicular to the ground (so that the rocket goes straight up). The problem is that if the rocket motor's thrust is powerful enough to lift the rocket up against gravity and launch it into the air, then it is also strong enough to overcome the supposed stabilizing effect of gravity. Also, even if the rocket motor remains in an absolutely vertical orientation, its line of thrust will vary a little bit from perpendicular to the ground. That variance (or misalignment) will cause the thrust to "push" the rocket over. Since the thrust is stronger than the pull of gravity, it can easily overcome any pendulum effect and will keep forcing (torquing) the entire rocket over and turning the line of flight away from vertical until the rocket's trajectory eventually intersects the ground.

The rocket needs some other stabilizing mechanism because the pull of gravity will never be sufficient to keep the rocket aligned vertically in relation to the ground."

As us modern rocketeers know the Nell did not achivemrnt stability during it's flight proving the theory false,but strangly enough 83 years later Jim Flis shock the world(not really this stuff is right up his alley) by creating a stable sport scale model of the Nell, now the question is how did he do it? We all know the basic criteria for stability is CG in front of the CP...... Hopefuly at the end of this ten day period Jim will be willing to share how the stability is achived in his model...... If not I'll google it......... And Jim, if you see this tread please don't respond until 4/7/10..... However if you wish you can tell the TRFers I they are right or wrong :)
ScrappyDaddy:
Full scale 1926 goddard rocket scale models have been flying on Solid rocket motors for Many DECADES not just Jims nell model. For many years the annaversary of Goddards achievment of first liquid rocket flight was celebrated at Goddard Space Flight Center in Greenbelt Maryland with the perfectly stable launch of a full scale model powered by FSI F100 motors. I can't tell you the exact number flown but it was done more than a decade until the model was destroyed by an accident in the storage building.
Later I built and flew a 1/3 scale, I believe we've seen another very successful full scale version flight at the openning of Naram-49. EGE has explained it here quite well:)
 

jflis

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busy, busy, busy... :)

Ok, my 2 cents....

Goddard's original had many things working against it which include, but is not limited to:
  • He built only one, unlike me who built about a half dozen, for testing purposes.
  • His thrust to weight ratio was much lower than what I had to work with
  • His rocket used much more flexible materials (the fuel and O2 tubes) so that the tank end of the rocket was able to move around a lot more than mine (affecting the orientation of the craft)
  • With regard to "rocket design", Goddard was not familair with a lot of what we, today, take for granted and use in our every day rocket designing (CG/CP, for example)
  • I had the opportunity to fly any one of my proto's many, many times, working out any issues...

Some of the biggest things about the FlisKits Nell that improve the way it flies (vrs the original) are the lighter weight and greater thrust to weight ratio, the stiffness of the faux fuel and O2 tubes, and the fact that the lower portion of the model is one rigid structure (the original fuel and O2 tanks were not directly connected to each other as I do in the model)

My first Goddard Nell rocket was built in the early-mid 1970's (about 35 years ago). Not sure if it was the first or not, but it has been done many times over the years. My personal contribution (I hope :) ) is that I was able to work out a simplified design and detailed set of instructions so that it could be kitted up for others to build. I think it is a first. I have certainly never heard of a flying Nell kit before, but I could be wrong (I have been before, so... LOL)

Hope this helps!
jim
 
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