Accelerometer gaffe in U.S. Finals : LUSENET : Junkyard Wars : One Thread

One thing brought a smile to me during the finals...

It appears both teams' experts gaffed on the accelerometers! Both rockets fired their ejection charges when the rockets were about halfway to the top of their trajectories.

The assumption is that at the top of the rocket's trajectory, there is zero acceleration as the rocket comes to zero velocity before starting to fall back down, and that's when the teams thought the accelerometers would fire the ejection charges for the ostrich eggs.

What they didn't account for was that acceleration actually drops to zero the moment the rocket motor stops firing (about halfway up), then start to decelerate to a stop as it coasts the rest of the way up on momentum alone, so the accelerometers fired the ejection charges about halfway up! DOH! :-)

Reckon they should have programmed the accelerometers to fire the ejection charges after TWO zero-acceleration points were encountered. Oh well..

(Who knew that Newtonian physics could be so much fun outside the college classroom?)


-- Thomas (, January 25, 2001


One thing the RDF editors forgot to put in is a graphic illustration why a machine failed. I'm not so sure the general public will easily understand why the accelerometers failed to fire the ejection charges properly.

(To reply in email replace blort dot invalid with anime dot net)

-- Dan Hollis (goemon@blort.invalid), January 25, 2001.

I agree.. It would take too much air time to explain the whole F=ma business (when there is no more force propelling the rocket, i.e., when the motor shuts off, force from thrust becomes zero, 0=ma, therefore there is no acceleration). The derivation would bore the audience to tears. :-P

Still, I can't but help think that Cathy Rogers might have come up with a pithy way of explaining it. :-)


-- Thomas (, January 25, 2001.

Actually, the accelerometer wouldn't encounter a second zero acceleration point; at engine shut-down, the rocket becomes and remains essentially weightless throughout the rest of its "free fall." (However, there WOULD be an effect at that final change of acceleration at ground level....)

-- Richard Mallery (, January 25, 2001.

Richard, you better check your physics textbook.

There are TWO zero acceleration points during those rocket flights. It is fact and not open to debate.


-- Thomas (, January 25, 2001.

What the heck, I'll spell it out before I head for bed.

There are TWO zero acceleration points during those rocket flights. Keep in mind Sir Isaac Newton's laws of motion and the famous equations, F=ma, Force equals mass times acceleration, and a=vt, acceleration equals velocity times time.


A rocket motor exerts a force called thrust. Since F=ma, the rocket motor force is providing acceleration. What happens when a rocket motor burns out and is no longer providing the Force? Let's substitute F for 0 and see what happens:

F = ma

Substituting Force with 0 (when the rocket motor burns out):

0 = ma

dividing both sides of the equation by m:

0/m = ma/m

0 = a

Acceleration = ZERO!

(More accurately, this zero acceleration point actually happens when the waning rocket motor near the end of its burn puts out just enough force to cancel out gravity force, which is a vector in the opposite direction, but it doesn't change the fact that the zero acceleration point is there).


The rocket motor burns out, but the rocket is STILL going up on sheer momentum! The sole force acting on the rocket now is Gravity. So Gravity will eventually stop the rocket from going up higher. At the apex of the flight when gravity stops the rocket from going up any further, vertical velocity becomes ZERO. Using the equation acceleration = speed x time:


Substituting v (velocity) with zero at the apex of the rocket trajectory:




That's your second ZERO acceleration point.

Mathematical proof, QED.

Thomas (once upon a time a physics TA in college)

-- Thomas (, January 25, 2001.

I thought that they gave them altimeters not accelerometers! Maybe I just heard what would be logical. An altimeter would release the nose cone at max altitude and record the highest rocket. Both teams could have recovered unbroken eggs. Then who would have won? I thought the highest rocket factor was going to be used as a tiebreaker? Notice how both teams were asked about how high their rocket would go and how they had given specific answers? If altitude was not a factor in the competition, why did it appear that they had spent time on the calculations? Had the rockets ejected the cones at highest altitude, the velocity would have been zero as opposed to the hundreds of mph that they did eject act. Art Attack's chute may have opened properly at a lower speed. I too thought that the whole mishap should have been explained. Why didn't they edit out the part where Art Attacks' misfires? It played no role in the construction or the out come of the show. Editing it out would have given them a minuet or so to explain why the cones ejected early. The rockets flew great and both teams did a fine job with the challenge, but this has to be the worst planned challenge that I have seen.

-- Mark Richter (, January 25, 2001.

Good expaination of the two acceration nulls. It is probably easiest to look at it starting with positive acceleration as the motor runs, then rapidly switching to negative acceleration (decelleration) when the motor quits. The decelleration will lessen until the velocity is zero, at which point acceleration is also zero.

They should have used dAlt/dt rather than dv/dt so the chute would open when the rate of change of altitude passed through zero.

-- Michael (Canadian P.Eng.) (, January 25, 2001.

Our altimeter stayed intact after the flight. It read, I think something like 1300-1500 ft. The Long brothers altimeter was trashed on the impact. Also the speed from thier rocket blew the light weight styrofoam cone off in the beginning which led to an early deployment of the chute. Our altimeter was supposed to deploy our chute at max height. Instead it didn't deploy the chute until 10 ft before we hit ground zero. If our egg had withstood the crash landing....then our altimeter reading would have been the final word. All in all, the challenge was very enlightening and fun. We had a great time with the Long Bros. and were proud to be in the finals!

-- Duane Flatmo, Art Attack (, January 25, 2001.

I think those were definitely accelerometers..

The only two types of altimeters I've ever seen are:

1) radar altimeters. Which means you need a radar transciever. Those thingies used on the show sure as heck aren't radar altimeters.

2) air pressure altimeters. These require a static pitot probe to detect air pressure, but I don't see it on those devices used on the show either.

So I think it stands to reason that those electronic devices George and Cathy handed out to the team are accelerometers-- You can actually use acceleration values detected by those things to calculate speed, and if you know the speed, you can calculate altitude over time. :-)


-- Thomas (, January 25, 2001.

They make model rocketry instruments which do both accelleration and altitude, both can be bought in one package, and they can trigger the ejection charge by either method. We have never tried launching one in a pumpkin, watermelon or bowling ball with our air cannons, as the accelleration in the barrell is on the order of 600 G's, and we don't think they would survive. We did however launch a camcorder one time, at Raleigh (provided by a repair shop)which didn't film beyond the triggering. It went about 1,000 ft, and recovery was by the bucket method. I think that the motor locked up or the battery came loose from it upon launch. The tape was recovered and respooled for viewing. It would have been intresting to have launched vid cameras instead of eggs for the JYW finals. The surviving camera would win, same as the egg thing. With the RDF budget for the programs, a couple of camcorders would not be a big factor. I think it's a tossup about the Art Attack rocket's ejection as to which factor caused the ejection. Either the big tail section decellerated much faster than the nose due to wind resistance (not too likely, as the nose cone rolled down the side of it on the way up) or that the erratic flight path caused it to wobble off. I am sure that they have been looking at the vid over and over and we will hear what happened on the board soon.

-- Waddy Thompson (, January 25, 2001.

I have to watch it again, but wasn't there a short shot from the rocket pointing towards the launch pad? Are you sure they didn't put one of their radio cams on a rocket? I've seen the tiny ones they've used on most episodes to shoot from the vehicles during the race. No particular reason they couldn't. They use similar cams in many sporting events -- helmet cams, skate cams, whatever.

p.s. Waddy, what is the bucket method of recovery?

-- Michael (Canadian P.Eng.) (, January 25, 2001.

A big factor with the nose cone coming off too soon....The hole that went thru our rocket could not be centered because of all the components. The hole was offset to one side. As we progressed closer to the launch time, the wind started kicking up. We tipped the launch pad at a small angle. Not into the wind but with the wind. It seems wrong, but as the wind hits the rocket from the side, we figured it would put more pressure against the widest part near the bottom. Therefore causing it to curve into the wind.... I think our main problemwas this >>> when sitting on the launch pad at the angle, the bulk of the rocket was on the uphill side. We shood have spun it 180 degrees before launching it. The rocket spun slightly as it took off and kept spinning... which spun the cone off before it's time. I was amazed that sucker did as well as it did. It would have been easy to build a typical tube rocket, (which I'm sure all of us junk yard fans have built growing up) but there is alot of satifaction in trying something a little more avant garde. Hats off to James Tucci, expert/ funguy.....

-- Duane Flatmo, Art Attack (, January 25, 2001.

There will NOT be two zero acceleration points -- there will be only one. At any point in the rocket's flight, the acceleration of the rocket will be a(t) = Fr(t)/m - ag, where a(t) is the acceleration of the rocket (which will be a function of time), Fr(t) is the force supplied by the rocket, which will also be a function of time, m is the mass of the rockt, which strictly speaking will also be a function of time as the feul is used up, and ag is the acceleration of gravity, which equals -9.8m/s/s.

When the rocket is launched, Fr/m is greater than ag, and you get liftoff. As Fr/m decreases, eventually it will equal ag. At this point, and only this point, will there be zero acceleration. The rocket may still be firing at this point, but it is just balancing gravity.

What other posters are implying is that there will be another zero acceleration point at the peak of the trip. This is false. Just because there is zero velocity does not mean there is zero acceleration -- it only means the rocket was iniitally moving opposite the direction of gravity. Think of it this way: Throw a ball in the air. As soon as it leaves your hand, gravity is the only force acting on the ball and it will be constantly accelerated at 9.8 m/s/s towards the ground -- there will not be a zero acceleration point until it hits the ground.


-- Doug Williams (dougw@, January 25, 2001.

Hats off to Doug for being the first person to get his freshman (O levels in the UK?) physics correct! My guess is that the accelerometers actually worked correctly, and that they correctly fired when the VELOCITY of the nose cones reached zero. Only the nose cones tore lose from the body of the rockets due to design errors. (if the accelerometers were actually on the body of the rockets, then i'm obviously wrong). My other guess is that the accelerometers are a standard bit of kit for folks into model rocketry and are thus designed to detect zero velocity at max altitude.

-- harold nations (, January 25, 2001.

Art Attack should (as they did) put the rocket on the upwind side of the launch rod. Rockets "weather-cock" because the "fins" are pushed downwind, pointing the "nose" upwind.

A possible explanation for the premature seperations is drag (e.g. air resistance) and burnout stutter. If the nose cone is not firmly secured to the rocket, the motor burnout can cause the nose to be "bumped" up off the rest of the body. If the drag on the nose is less in comparison to its mass than the drag on the rest of the rocket compared to that mass, the nose will slow less quickly (look at shotgun wadding) and pull ahead. Once seperated, they could easily tumble.

Doug did get his high school physics right, but in my college, Basic Physics 102 included the air resistance we neglected in 101. There may be 2, or more, zero acceleration instants in the flight of even a single engine rocket in a non-steady state atmosphere.

In powered flight the acelleration is: F_motor - F_gravity -F_air / M_rocket. As the motor runs out, the acelleration passes through zero on its way to: 0-Fgravity-Fair/Mrocket. Of course, if the motor stutters as it burns out (inevitable with solid fuel), there could be many swings through the zero point. After the engines cut out, the rocket's acceleration is: 0-Fgravity-Fair/Mrocket. But after it reaches max height, Fair reverses, resisting the falling vector. So as the rocket accelerates downward, it will eventually reach terminal velocity, and thus zero acceleration. Finally, with a gusty wind, the actual air resistance can vary radically (a V-cubed term) from instant to instant. This variation could swing acceleration from + to - and back.

OOOPS! sorry for the length. :)

-- John Calderwood (, January 25, 2001.

OK, I was wrong. Thinking in the second order is something that I haven't had to do much since University. Not to mention that I'm electrical, and acceleration isn't one of those things we looked at too much....

If you think of acceleration in the upwards or downwards direction you will come to the conclusion that the rocket moving down is just a continuation of the downwards acceleration that has been present since the motor shut off.

Glad someone called us on this.

And the answer to the whole thing was that the nose cone "fell" off!

-- Michael (Canadian P. Eng.) (, January 25, 2001.

I got the impression that the timing of the separation charge was determined by the altimeter-and/or-accelerometer as provided to the teams...meaning it was the folks conducting the contest who screwed up, not the teams. Seems unfair...

-- Eric (, January 26, 2001.

Umm you guys are way over my High School educated level of understanding but wouldn't it have been easier to have used a mercury switch? as soon as rocket turned over (Top of flight minus a few feet) the ejection charge would fire.

-- Stephen A. Binion (, January 26, 2001.

I hate to be the one to break the news to you guys but your all wrong! There are actually 4 zero-acceleration points. Then the rocket fires it accelerates at a steady rate up to the point that the rocket stops burning. At that moment the acceleration goes to zero. As the rocket continues to raise it is now decelerating until it reaches its maximum altitude & zero acceleration again. From that point it is accelerating again slowly until it reaches the speed that a falling object of that size & wind resistance can reach (a feather falls much slower then a rock due to wind resistance) Just like the astronauts practice zero gravity in a free falling plane. Therefor it remains at zero acceleration until the rocket hits the ground. Then rapidly decelerates back to zero again. (If the chute did open) LOL I'm not a physics master; I just use common sense. If you have acceleration you also haft to have deceleration with a short period of zero acceleration in between. Also, one of the reasons that the Art Attacks rocket failed was because it was aerodynamically unstable. The bottom of the skirt collapsed after launch from the wind resistance causing it to wobble in flight, throwing off the nose cone.

-- Rick The Rocket (Future Junky) (, January 26, 2001.

Hey "Rick the Rocket". You are right on the money! I have gone over this launch for the past few days with Ken and June. Now seeing the video again, we've determined that when the wind hit the shirt of the rocket bottom from the acceleration ....It actually created 4 crude uneven fins because of the indentation it created. Even a hair off in the size of 1 fin, could cause it to spin which would cause the cone to fall off prematurely. I say 4, because we had 4 long strips of aluminum for the struts which we mounted the aluminum sheeting to. We should have gone ahead and made that round ring like I talked about earlier, at the bottom for strength. Also the charge to deploy the parachute did not work until 10 ft. before it hit the ground. It was set to deploy at the top of the arch just before coming back down....that wasn't seen on TV. We watched the parachute open 10 ft before it hit the gound and knew right there we had scrambled eggs.

-- Duane Flatmo, Art Attack (, January 27, 2001.

Well, Rick the Rocket isn't right about the acceleration of the rocket. Doug still comes closest to being right (other than ignoring the trivial solution of the rocket "instantaneously" being brought to zero velocity upon hitting the ground, which I certainly won't hold against him). The rocket ALWAYS "feels" an acceleration of 1 g toward the ground since the altitude obtained was so low that terminal velocity was never close to being reached. Thus zero net acceleration only comes when the acceleration of gravity is exactly balanced by the rocket thrust in the opposite direction. (one could nit-pick that this happens twice, once immediately before lift-off and once again near burnout, but the first time is of no consequence as liftoff is nearly instantaneous and at very high positive g). If you watch closely, I think the Long Brothers 'chute deployed soon after their nose cone fell off, as i suggested in my original post. My third guess is that Art Attacks 'chute deployed so late purely for aerodynamic and packing reasons, having nothing to do with when their separation charge went off. In retrospect, it's pretty amazing that even one team got a parachute deployed, seeing that they had no practice and that this is often the single hardest thing to do correctly in a model rocket launch. (Though the Long Brothers chute WAS way too small; did you see the way their payload bounced upon landing?). Both teams performed at a very high level throughout the competition, while keeping their senses of humor well intact!

-- harold nations (, January 28, 2001.

Harold is right on target with his response. The first of the zero net acceleration points would occur moments after engine ignition, with the remainder occuring during the engine burn out period. Which begs the question, why choose this means of parachute deployment? Actually, I understand that when using accelerometers for chute deployment, one calculates the anticipated time period for gravity to reduce your final velocity to zero, and the accelerometer is used to initiate this time delay...or something to this effect. But I wonder why some sort of simple velocity measuring device wasn't used. Perhaps that could consist of a Junkyard Wars challenge...Design and employ an effective, compact velocity monitoring device. I cannot but help picture an itty bitty windmill type thing...connected to a simple it spinning? Yes=no chute deployment, No=chute deployment. Not a grand enough project for the show I think. How about...Design and test a highly portable tactical nuclear device. Yes...quite exciting...I'd watch that show most certainly. Particularly if Cathy misplaced her attire and George happened to get left at ground apologies, I digress.

-- JT (, January 28, 2001.

The "Bucket Method" means taking a bucket downrange, and picking up all the little pieces. My son Robert noticed in the launch cam shot that the skirt on the "Art Attack" rocket bent in on one side, so the theory about the skirt folding and causing the wild flight is correct. It was evident that the rocket was still accellerating upward at ejection, as the nose cone tumbled down the side of the rocket body. The propeller stopping or pidot tube idea of ejection would not work if the rocket happens to fly in an arched trajectory. One of the most dependable methods to fire the ejection charge is a slow fuse ignited by the motor. This method is not the best for altitude however, as it is difficult to know how long it will be from liftoff to appogee of the flight on an untested vehicle.

-- Waddy Thompson (, January 29, 2001.

Well Herald who are we kidding? Your statement "since the altitude obtained was so low that terminal velocity was never close to being reached" ,Do you really think that a Styrofoam none cone wouldn't reach its maximum velocity in the 1000 ft it fell? Come on now! Get real! The wind drag on something that light even with an egg in it would be enough to keep it pretty slow. I'd say it reached its max. velocity in about 200 ft. As for Art Attacks Flying Bucket nose cone it would have been about the same, A little more weight but more wind resistance also. I stick to my theory as the only correct one here . So nener, nener ROFLMAO Just kiddin no hate mail please P.S. Duane your judgement is impeccable though, smart guy, but did you read my other post where I said your rocket was as aerodynamic as a Hippo in a toto. :)

"Rick The Rocket" future Junkyard Wars contestant.

-- Rick The Rocket (, January 29, 2001.

How many noticed the fire extinguisher can protecting the "Long Bros." payload? Heavy, but effective armor. My take on the "Zero G" argument is that 0 G means no appearant weigth in any direction. This would only occur at two points, first when the force of the rocket motor drops in thrust to the point where the force from the motor and the air drag on the rocket are balanced at negative 32ft/sec/sec,(in the vertical plane) and the second time when the rocket turns the corner at the apogee, when the air drag is minimum, provided the trajectory is exactly vertical at that point. At all other points of the flight, there will be appearant accelleration of the payload (in one direction or another,in relation to the rest of the vehicle,)providing either actual or induced G's on the payload. With a hyperbolic flight, there would be only one 0 G crossing, on the way up, as air drag on the front of the vehicle would always be pushing the vehicle against the payload after that point. The "Vomit Comet," NASA's 0 G. test jet, has to accellerate downward from the top an arched flight path to achieve sustained 0 G. inside for a few seconds. That's my take on the accellerometer and "0 G. crossings" (notice the term is "accellerometer," not "gravity meter".) It's a good discussion going on here. The ejection accellerometers for model rocketry have a timer which you set before launching, to delay the ejection charge firing for the amount of time past the "0 G" point that you figure that the vehicle will be coasting upward. The object is to seperate the payload from the rocket body at the point of minimum air drag, and hopefully at the best possible altitude. They close a switch, sending power to the igniter after both conditions are met, the 0 crossing, and the timer delay. They also make altimeters and small still cameras and streaming video cameras for model rocketry. For more, search for "Tripoli Model Rocketry".

-- Waddy Thompson (, January 29, 2001.

I've drawn up a nice graph of altitude/velocity/acceleration which I'll drop on my server at home tonight. It will let everyone visualize the acceleration/velocity nulls rather than keep arguing about them.

Email me if you want a copy now, or have access to a server to drop it on.

-- Michael (Canadian P.Eng.) (, January 29, 2001.

For those who prefer curves to points:

This is as accurate as I could get freehand by eye... It is fairly good except the alignment of the terminal velocity points.

-- Michael (Canadian P. Eng. (, January 29, 2001.

darn, try again. My mistake:

-- Michael (Canadian P. Eng. (, January 29, 2001.

Ok guys, Here is the real dope on this. The accelerometers/altimers that were used in the final are small computers that intergrate the acceleration to produce the velocity. There also is a Barometric altimer on the unit that is used to provide a check and backup for the acclerometer. Now the cone design is inherently stable by itself as long as it holds it shape. (Ever see a shuttlecock go unstable?)The skirt definatly need more support (20/20 hindsight)because it was deformed by the vacuum created during liftoff. This caused the spin. However, the seperation of the payload was potential that was planned for. The egg payload was self contained. The accelerometer, Egg, Ejection charge and recovery system was contained in the cone. I always thought that the skirt would have more drag than the payload and would drag seperate at burnout. Unfortunatly, the cameras followed the largest piece of the rocket and you did not get to see the nose continue to peak altitude (1,150 ft). The real reason for the egg breaking was that for some reason the altimeter decided to deploy the chute at about 20 to 10 ft off the ground. The altimeter should have deployed the chute at peak altitude. The rocket community have been using these altimeters for a while and for the most part they are very reliable. As for the Long Brothers there rocket also drag seperated at burnout but at a very high speed. They tore out the altimeter and shredded the foam nose cone. They however, had a unbelieveably strong chute and a good egg support that protected the egg during the 9 g opening shock of the chute.

-- James Tucci (, January 29, 2001.

Actually I was a little disappointed that neither team attempted to put together a 2 stage device. I have done a bit of model rocketry and they are not too difficult. There were certianly plenty of engines available.

About the zero accelleration question. The conventional take is that there was only one.

It takes a force to cause an accelleration, and after the engine burns out, the only force on the rocket is gravity and it is continuous all the way through the apex of flight until it hit the ground. You cant have accelleration with out force and the only force on the rocket after the motor shut down was gravity. And unless someone turned off gravity for a while, the accelleration was constant and negative the entire ballistic phase of flight.

If we get down to the real guts of the question, there was air resistance, crosswinds and plenty of accellerations all over the place....but I digress.

-- Logan Mizar (, January 30, 2001.

I see a resident expert has already stated my thoughts. You can measure altitude using an accelerometer if you know the initial velocity and altitude. Integrate Acceleration to get Velocity, Integrate Velocity to get Displacement (Altitude).

I also see my DNS is acting up, so you may get two blank images rather than just one!

-- Michael (Canadian P.Eng.) (, January 30, 2001.

Nice graph Michael! Check out This site gives you just about everything you could want to know about these "rocket data collectors" Has nice graphs of actual flight data. I don't know if these are the same brand that were used on the show, but they must be simular. Makes me want to get back into rocketry just so I can play around with these flight recorders.

-- Mark Richter (, January 31, 2001.

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