Hi all, I haven't checked the forum for a coupe of weeks, so here is my input to this discussion.
> Todd, What parameters did you build your car to?
I dont know if I initially built it to any particular specification. Our main goals were safety, stability then distance. From a safety perspective, we always had multiple points where the pressure vessels were tethered to the chassis, usually 2 separate points per bottle. The chassis had one point (a riser between the bottles) where the bottles exert force on the chassis, this also enables the bottles to be held in place and not separate from the chassis (assisted by the bottle tethers). Our launcher has always had an abort and a rope activated quick release so it can be launched from a distance. We have always had 4 wheels, may want to limit this as the minimum.
We have only recently added the camera but I think for record purposes onboard camera is a must, realistically they are very cheap these days, so this isnt cost prohibitive. Ground video is also good to show the start location and where it ends up, again most people have a video camera of some description that can be used for this purpose.
I dont know if an upper weight limit is necessary as it usually only slows the car down and it will help the cars stability and not let it off the ground. On the other hand a minimum weight limit may be required as the very light cars have a tendency to get airborne and flip which can be dangerous. However once a car flips or gets airborne it wouldn't qualify as a car so it couldn't break the record.
> 2 runs in opposite direction
I think this is a must, averaging the runs within a time limit ie: 2hrs, will ensure that any slope doesnt unfairly add to the distance or speed. Also it way be worth limiting the difference between the 2 runs to no more than 25%. So for example if one run was 400m down a hill and the second was only 10m back the other way, the average is 205m which is obviously an unfair method of taking a record. So with the 25% rule, if the first run was 10m and the second was more than 14m, then it would be more than 25% difference and the second run would either not count or would only be allowed to be 14m as a maximum.
> Metal Parts
Couplings - I think for this class that this may need more debate. We use aluminium robinson coupling which weigh in at about 10grams. Other plastic tornado type coupling weigh around the same ~8 - 13g. So whether its metal or plastic in the case of an explosion and seperating from the car the plastic and metal parts will still impart the same force if they hit someone. In an explosion the couping always stay attached to parts of the bottle, ie: its usually the bottle that rips/tears starting the explosion, its then a matter of containing the pieces. With water rockets cars we have an advantage over the vertical rockets in that we have a chassis to tether the bottles to. In our car design we have the 2 points per bottle as well as the riser to ensure the bottles doesnt separate from the chassis and fly off. To provide additional safety we have been working on a robinson coupling kevlar tether which tethers each of the actual couplings to the chassis, similar to the way F1 cars tether the wheels to the chassis so they no longer separate from the car in a crash. I believe this is a safer alternative than just mandating non metal parts whilst not mandating tethers. If a tether is mandated then there is no difference whether the coupling is metal or plastic.
> Wind Speed should be measured
I dont think its relevant to adding to distance substantially, the run in the reverse direction would even out any gains anyway. We have launched the car in 50kmh wind and it really didnt make a noticeable difference to distance or speed.
> The Floor type must be categorized
I wasnt sureif you were referring to the floor of the car ie: chassis or the road surface.
If its the floor of the car - I think this should be left open to allow for different designs. As long as the chassis provides sufficient locations to tether the bottles it shouldn't matter its construction. If 2 tethers per bottle is mandated then the chassis design will need to conform to that rule.
If its the road surface then outdoor would stop inside smooth concrete etc as suggested, but maybe a class on grass might be an option, going 100m on grass is a lot more difficult than doing in on tarmac, also no vehicles to annoy you on the grass (although people may be an issue). Maybe a tarmac class and a grass class, possibly a snow/ice class as well ?
> Launching on an active road
Realistically you are going to need to launch on a road if you want to find 150m of straight tarmac. We have tried car parks (too many damn concrete blocks), driveways (not long enough any more) so you can find places to test that are not on a road as such. Although there are plenty of cul-de-sac, dead end roads, industrial areas that can be found on the weekend with little or no traffic. You need to be sensible about where you launch and personnel safety. Most people that do come past us when we are launching are interested and slow down and sometimes stop just to take a look and see what it is we are up to
> Stomp rockets
I dont have any issues with stomp rockets being banned from this competition.
This should be left to the designer as to whether they want active (r/c) or passive (fins) steering. Experience tells me that if you seriously want to go after a distance record you are going to need to steer it. R/C Steering is the obvious solution and really not that difficult to build. I had never done r/c before trying it on our car, so if you can build a rocket car to challenge a record its well within your abilities to build the r/c steering for it as well.
The only method of steering which should not be allowed is attaching the car to a tensioned string/rope and letting it run along the string to keep it straight.
> Method of measurement
The simplest and cheapest is marks on the ground. Take a tape measure and mark every 10m. Although its a good verification to check with google earth, which is pretty accurate. As long as you have a reference point, ie: driveway, bend in the road, gutter grate etc which is visible from google earth as the starting location. If the ground marks + google earth are a close match then this would suffice. When we were going distances of 30-45 m the google earth errors were noticeable, but now that we are around 150m google earth is very accurate as a method of confirming distance.
> Vehicle Track
The width of the car between the wheels (track) is one element of the design which can effect stability. Overcoming thrust steering is the difficult part of which a wider track helps but by no means would it cancel it out. Although making a car 1m long with a track of 3m probably wouldnt be in the spirit of the competition. Perhaps limiting the track width to no more than twice the length of the car may be worth considering.
> How much off course would be considered acceptable
The more you go off course the less the distance is anyway so I would think we should accept cars not going perfectly straight. However when it comes to measuring the run, the measurement needs to be in a straight line from the starting location to the finish location, ie: you dont count the off course metres. Alternatively when the 10m marks are layed out on the road/ground, you can nominate a centreline (ie: you intended direction), then when the car comes to rest the measurement is done along the centreline to a distance no further then where the car comes to rest, ie: you only measure that vector of the distance so all off course metres traveled are not counted.
> Does a car need to run in an intended direction to count
Generally to meet the 2 runs in the opposite direction, cancel the hills and wind assistance you would need to do the runs 180 degrees apart, maybe along the intended centreline as above.
> What if the car makes a 90 degree turn along the run
Its probably going to hit something .. like a gutter .. LOL ...
... seriously though, if we use the centreline ,measuring method above any turns along the course just loose potential distance, so there is no advantage in designing a car that snap turns along the run.
> More then a few degrees off the launched direction would make that run ineligible
Getting a car to go straight is very difficult .. a lot harder than it looks ... if we only allow a few degrees we are not going to get anyone setting any records ..
I think the intended direction centreline measuring described above would be sufficient as any off course run is only loosing potential distance and there is no gain to be had by going off course. ie: Longest distance will be gained by making a perfectly straight run.
> Car being in contact with the ground
I think the car needs to be in contact with the ground 99% of the run. We have seen the cars rear wheel hop off the ground at launch for 1/3 of a second or so, so the rule shouldn't mandate 100% contact but it needs to ensure the car is not a plane with skids
wow .. I know that was a lot so if you read this far well done