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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

Alternative to speed record

Discussions about water rocket powered cars, sleds or boats.
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RaZias
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Alternative to speed record

Post by RaZias » Sat Oct 03, 2009 4:26 pm

Since we are thinking in records for WRC I was wondering how to deal with the problem to measure speed.

If there is no option the solution will be to make like this: who makes 30 meters (for example) in less time.

I know it would be cool to know the real speed of the WRC but since its dificult to deal with a radar and other stuff maybe is better to think on this new idea.

The problem now is to find a system to measure time, maybe a laser system like they do in the olimpics ? I wonder the price...

I have said 30 meters but who knows how much it takes for a WRC really to achive its max speed. Maybe les, maybe more.


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air.command
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Re: Alternative to speed record

Post by air.command » Sat Oct 03, 2009 5:02 pm

RaZias wrote:Since we are thinking in records for WRC I was wondering how to deal with the problem to measure speed.

If there is no option the solution will be to make like this: who makes 30 meters (for example) in less time.

I know it would be cool to know the real speed of the WRC but since its dificult to deal with a radar and other stuff maybe is better to think on this new idea.

The problem now is to find a system to measure time, maybe a laser system like they do in the olimpics ? I wonder the price...

I have said 30 meters but who knows how much it takes for a WRC really to achive its max speed. Maybe les, maybe more.
Hi Razias,

A cheaper way to do it may be to simply paint lines on the track, say every 1, 2 or 5 meters and then video tape the record attempt from an elevated position. Since a video of the attempt will most likely be required anyway you can also use the video to measure the speed. The video frame rate can be used for fairly accurate timing. Having lots of lines means you can work out the instantaneous speed at different portions of the track. You would run the car in the opposite direction as Todd mentioned previously to eliminate any advantage you would get if the track is on a slope.

There may be a bit of parallax error, but with the car so close to the ground, this error can be minimized if multiple readings are taken during the run.

- George


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U.S. Water Rockets1
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Re: Alternative to speed record

Post by U.S. Water Rockets1 » Sat Oct 03, 2009 10:38 pm

air.command wrote:
RaZias wrote:Since we are thinking in records for WRC I was wondering how to deal with the problem to measure speed.

If there is no option the solution will be to make like this: who makes 30 meters (for example) in less time.

I know it would be cool to know the real speed of the WRC but since its dificult to deal with a radar and other stuff maybe is better to think on this new idea.

The problem now is to find a system to measure time, maybe a laser system like they do in the olimpics ? I wonder the price...

I have said 30 meters but who knows how much it takes for a WRC really to achive its max speed. Maybe les, maybe more.
Hi Razias,

A cheaper way to do it may be to simply paint lines on the track, say every 1, 2 or 5 meters and then video tape the record attempt from an elevated position. Since a video of the attempt will most likely be required anyway you can also use the video to measure the speed. The video frame rate can be used for fairly accurate timing. Having lots of lines means you can work out the instantaneous speed at different portions of the track. You would run the car in the opposite direction as Todd mentioned previously to eliminate any advantage you would get if the track is on a slope.

There may be a bit of parallax error, but with the car so close to the ground, this error can be minimized if multiple readings are taken during the run.

- George
This is essentially how law enforcement officers in the US measure the speed of automobiles on the highway from helicopters. They only measure the average speed over a distance like 1/4 mile and work the math to figure out if they should radio the patrol car ahead to give you a ticket, but it is the same principle.

If the car is driving over the lines and you know the spacing of the lines then you can tell the speed based on the framerate of the video. Any lens distortions or perspective or parallax effects can be ignored because you already know the spacing of the lines and when you examine the video you only pay attention to the crossing of the lines by the car because you only use the video to get the timing correct, not the spacing.


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Jelo
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Re: Alternative to speed record

Post by Jelo » Sun Oct 04, 2009 1:41 pm

i think the competition should be the car has to reach a top speed but still has to pass a 50 meter line, or else people would just use a bottle with a huge nozzle, blasting the car extremely fast for not even a second and for barely 20 meters... another way to avoid people from forgeting its a car race not a basooka missile race would be to put a minimum thrust duration (1-2 seconds at least)

going back to the speed calculation, the video system as describes above is the one i used before, but i now use a bike computer to do the work, i spent to much times calculating and mesuring stuff :roll:


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thampson
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Re: Alternative to speed record

Post by thampson » Tue Oct 06, 2009 1:11 am

My $0.02

Jelo
"... making a car go straight even for 20m is harder than it looks .. :) Trust me ... Just putting a huge nozzle on the back will give you plenty of thrust but will probably just spin the car .."

Perhaps a solution is to make a minimum distance from launch pad to the measuring location ie: where the lines (tape /paint) are on the road or where the radar gun is located. If this point is located 10m/20m away it means you need to put more engineering into controlling the car to get it to pass over the lines. Its also an advantage if you can make the car travel straight across the lines (least distance) instead of arcing left right or sideways, which will be a longer path between the lines and therefor equate to a lower speed.

Razias is right that my cars dont hit top speed until around the 30m mark as the cars typically use more water than vertical rockets and it takes longer to accelerate them. But then again I have designed my cars for distance not for speed .. maybe I might have to put some time into a super lightweight one :mrgreen:

"... The bike computers wont give a high speed reading unless you have big wheels on the rocket car ... mine topped out at 21km/hr based on my wheel diameteres (410mm), after that the computer cut out / misread as it couldnt handle the rpm's ... I have since taken it off"

Razias
".. The 30m sprint idea is probably a good one. Just about everyone can get a strip of marking tape or two, a measuring tape of 30m and a video camera. The only thing I can see that might need standardising is the software to verify the timing with. I use VirtualDub http://www.virtualdub.org/ which will pickup the fps from the video itself and calculate the time between frames correctly.... "

-todd-


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Re: Alternative to speed record

Post by MK_Rocketry » Wed Jul 21, 2010 4:13 pm

Mattel had Radar Gun couple of years back that could measure speeds up to 160km/h,


It seems that you can still buy it as new, but I assume that it is out of production and starts to be collectible soon.
That thing is not a toy, Auto bild tested it and it was extremely accurate, also Finnish magazine tested it against polices Stalker ATR radar, first speed (km/h) is Stalker, second radar gun:
32/32,
34/33,
38/40,
41/40,
58/56,
61/59,
67/64



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thampson
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Re: Alternative to speed record

Post by thampson » Thu Jul 22, 2010 11:52 am

Yes I have one of these. I got mine from ebay for about $20. They are a real radar gun working at 10Ghz :)

They seem to work pretty well

-todd-


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Re: Alternative to speed record

Post by U.S. Water Rockets1 » Sat Jul 24, 2010 10:38 pm

thampson wrote:Yes I have one of these. I got mine from ebay for about $20. They are a real radar gun working at 10Ghz :)

They seem to work pretty well

-todd-
There is a site somewhere out there that discusses how to hack one of these Hot Wheels Radar guns for improved performance. It was something we had noticed a while ago. We were hoping that the radar could be improved to the point where an accurate altitude measurement could be made from the ground, but it was obvious that it did not have the power to do that. It would be good for the purposes of measuring the speed of a water rocket car.


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thampson
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Re: Alternative to speed record

Post by thampson » Mon Aug 02, 2010 8:45 am

Hi US Waterrockets

Yes I have seen the site that you mentioned. Its a good hack as you can setup the radar on a tripod and just leave it turned on. You are right about the power of it though, you really need to be within about 4m of the object to get a reading. With the R/C on the water rocket car though we should be able to drive it by the radar gun mounted on the tripod to get a reading without putting life and limb on the line trying to get too close

cya
-todd-


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Re: Alternative to speed record

Post by U.S. Water Rockets1 » Sat Aug 07, 2010 10:44 pm

thampson wrote:Hi US Waterrockets

Yes I have seen the site that you mentioned. Its a good hack as you can setup the radar on a tripod and just leave it turned on. You are right about the power of it though, you really need to be within about 4m of the object to get a reading. With the R/C on the water rocket car though we should be able to drive it by the radar gun mounted on the tripod to get a reading without putting life and limb on the line trying to get too close

cya
-todd-
You should use your onboard camera video and place some chalk lines or some other marks or objects along your course and you can compute the speed of the car along the way by looking at the video frame by frame and calculating from the number of frames it takes to pass each mark, You would get a very accurate speed for every interval.


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