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

-- Thomas (trh1@cris.com), January 25, 2001

Answers

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

-- Thomas (trh1@cris.com), 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 (rmal42@aol.com), 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

-- Thomas (trh1@cris.com), 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.

THE FIRST ZERO ACCELERATION POINT:

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).

Okay, now the SECOND ZERO ACCELERATION POINT:

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:

a=vt

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

a=0t

so...

a=0!

That's your second ZERO acceleration point.

Mathematical proof, QED.

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

-- Thomas (trh1@cris.com), 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 (mark@troyjaycees.org), 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.) (michael@mks-tech.com), 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 (flatmo@humboldt1.com), 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

-- Thomas (trh1@cris.com), 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 (cthomp3851@aol.com), 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.) (michael@mks-tech.com), 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 (flatmo@humboldt1.com), 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]=-

-- Doug Williams (dougw@127.0.0.1), 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 (nations_ha@yahoo.com), 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 (jkc@ftel.net), 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.) (michael@mks-tech.com), 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 (earsjohnson@usa.net), 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 (Stephenbinion@hotmail.com), 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) (hoodoo2@povn.com), January 26, 2001.