Trajectory B/C

[starpug]

I am working on expanding the Trajectory Wiki so that it's more helpful then the one we had last year.

[starpug]

Trajectory Wiki
Ok I've expanded the Trajectory Wiki. It is now something that you C people should look I think it will be a good way to help you get started.

[haven chuck]

If you are using more than one bungee cord, and don't use them all in every launch, make sure you rotate them out so that no one bungee is stretched more than the others and also, right before the competition, check your measurements to make sure that the bungees (after being used for a lot of launches) aren't stretched enough to change your launch much.
P.S. When i say "stretched" i mean longer or easier to pull than they were at one point.

[zorbak5044]

i'm thinking about doing trajectory next year? does anyone from Div B have any ideas about how the device should work, like a catapult or slingshot, or any tips on how to build one

i didnt do it but my freind did it and got 2nd. she had a bazzoka like thing

[captbilly]

Think about how high your projectile will go. Two years ago the best trebuchet at Nationals ended up hitting the ceiling on 2 out of three shots. If trajectory is done indoors where you live you will need to keep the firing angle relatively low so you don't hit a ceiling. Consistancy is obviously the key, it doesn't make any difference at all in trajectory how for your projectile can go, only that it will go exactly as far as you intend. To get enough data points to be competitive at Nationals you will need to fire the thing over a thousand times. As someone said earlier in this thread, the winning trajectory device last year hit the nail at the center of the target on three out of the four shots. Keep in mind that the judges will probably say the ball hit the target at the center of the crater left in the sand, even though the crater doesn't form directly under the impact point (due to the angle of impact). You need to measure your impact point the same way the judges will (to the center of the crater in sand).

You also need to plan for the elevation of the target not being exactly what the judge (or the rulesbook) says it is. At our regional and state competition the targets were set up outdoors on somewhat uneven terrain. The event coordinator measured the height of the target from the grass under the target, not the elevation relative to the launch site. We had to do some quick and rough calculations of the actual height of the target vs. the launcher to get an accurate first shot. At states the event coordinator used an elevated tagret height that was not an interval of 10cm as per the rules, so we had no chart that we could use. So be prepared with a plan of how to deal with inaccurate target heights and distances as well as wind. I believe that this year you can only use a ping pong ball, tennis ball, hacky sack or racket ball (rules aren't final of course) so make sure you account for aerodynamics in your charts.

Of course most of these suggestions won't matter if you have a device that has a CEP (circular error probability) of half a meter. There is little reason to have charts with data points every 10 centimeters if your device can't consistantly send the ball into a 10cm circle. Even at Nationals, many of the trajectory devices had impact point spreads of 20+cm. Remember that the tie breaker is the distance between the first and second shots at the same target so even if you somehow manage to get one good shot at each target you could still lose to a team with a more consistant device.

[starpug]

I agree completely. CONSISTENCY IS EVERYTHING or almost everything. After consistency it's practice. A team with a device that is reasonably consistent and accurate shoot down to the mm could easily lose to a less accurate but just as consistent team that practiced twice as much. So spending all your time building an extremely accurate device isn't everything. Infact a device that is consistent to within 10 cm that's taken 50 total shots could probably be beaten with a device that's consistent within 15 or 20 cm but has been shot 100 times. We took at least 150 shots with our device probably more.

[captbilly]

If you truely understand the physics involved (trajectory and aerodyanamics of a sphere) then you don't actually need to practice very much (once the machine is working properly). I got to watch the winning trajectory team at Nationals this year and they could literally hit a nail on the head, every time. The CEP of that device was probably around 1cm, and based on the accuracy of their first and succeeding shots, the charts they had were also accurate to about 1cm. They has perfect shots on 3 out of four tries and the one shot that didn't hit the nail, went just over the top. I spoke to the boy and girl who did the event and they said that they only had to do practice shots for every half meter distance, and only at zero, 0.5 and 1 meter elevation, perhaps 50-100 shots, to make charts for every 1cm distance and elevation. From what I saw they were able to make their shots with incredible precision with that little bit of calibration.

On the other hand, if your competition is outdoors on uneven terrain and the wind is blowing, there is really no amount of preparation that will enable you to accurately calibrate your device to account for the unpredicatable variables in an outdoor environment. Although the same team that won at Nationals also had 2 out of 3 perfect bullseyes at an outdoor competition(they didn't even shoot the 4th shot because the knew they had won by their third shot. They said that at the State competition the terrain was so uneven that they simply had to guess at the actual elevation of the targets on the first shot, but after that they were able to calculate the correction, which essentially told them the actual height of the target. When I asked them what was special about their trajectory machine, they said that it was absolutely consistent, so they didn't need to average out many shots to get a good setting. They did mention that at first they had some issues with getting the ball to release cleanly, but once those were resolved they were able to calibrate the thing in a few hours.

No amount of time spent calibrating will get a device that has a CEP of 50cm, to regularly hit a 1cm spot, even if you have 2 tries at it. I saw trajectory devices at all levels of competition that probably had CEPs as large as the entire target. Many teams were thrilled just to hit both targets once. The sad thing is that some of these people got very good scores, due more to luck than skill, since you only the best shot counts. It would be nice to have the rules changed so that the shots were averaged together rather than only counting the best. I believe that one year Storm the Castle was scored by averaging two out of three shots. Still not much statistical significance, but lots better than only counting the best shot. A team with very consistent device that they didn't know how to use could certainly lose to a team with a poor device, and unfortunately it is even possible (though unlikely) that a thoroughly prepared team with a very consistent device could lose to a lucky team with a piece of junk.

[starpug]

You do have a valid point captbilly if you truely understand the principles then you do not need to do much in the way of practicing for every level. But from my experiance you shouldn't spend all your time building a perfect device. We did so much practice that we were almost robots when it came to the competition. We had practiced out all likely problems and worse case scenerios and even some unlikely ones. So yes building a device is very important but if you spend to much time doing that and don't have enough time to be fimiliar with your device and ready for problems during the competition. Arden the Nats champs were DQ'd at regionals because they weren't as use to the routine of fireing their device as we were and so they left a hatch open breaking the size rules. But at the time their device wasn't as good as it was at nats so we probably still would have won but they would have gotten 2nd probably. I like the way it's scored because it allows someone who's dealing with outside factors such as slope and such to adjust their device to accomadate those factors on the second shot after seeing how much they missed by.

[rocketman1555]

You do have a valid point captbilly if you truely understand the principles then you do not need to do much in the way of practicing for every level. But from my experiance you shouldn't spend all your time building a perfect device. We did so much practice that we were almost robots when it came to the competition. We had practiced out all likely problems and worse case scenerios and even some unlikely ones. So yes building a device is very important but if you spend to much time doing that and don't have enough time to be fimiliar with your device and ready for problems during the competition. Arden the Nats champs were DQ'd at regionals because they weren't as use to the routine of fireing their device as we were and so they left a hatch open breaking the size rules. But at the time their device wasn't as good as it was at nats so we probably still would have won but they would have gotten 2nd probably. I like the way it's scored because it allows someone who's dealing with outside factors such as slope and such to adjust their device to accomadate those factors on the second shot after seeing how much they missed by.

i personally like practicing the possible problems, especially when u do one like the arm breaks, or for robots, the wheels fall off

[captbilly]

With the 08-09 rules it is could be a bit tricky to hit the far target without hitting the elevated target on the way down. You may have to launch at a quite high angle, which could make hitting the ceiling a problem. I hope they put more thought into the ceiling height than they did at Storm the Castle at 2007 Nationals. Just make sure you have a plan to deal with a low ceiling and a high target.