I need some help

[flynfrog]

That make me think to the boats that sail on the missisipi in America. They have those sorts of hydro wheels that can be found also on water mills. If the bottom of the water tank is linked to this kind of hydro-wheel, while this weel itself is on the same rigid axe as the car's wheels, it would make the car to go forward with the natural pression of the water tank emptying.

Then, you could add a spring on top of the tank (which should have the shape of a tube), with a tight push rod, so that it would act as a syringe as soon as the spring is released (so of course you need a manualy releasable clip to keep the spring compressed at the begining).

not if it is under water

i think the rules are intened to make them design a vehicle to propel its self under water unless im sill mis reading them

[Crabbia]

Ya, i'm actually getting confused as to what this guy needs to do. I was under the impression that it was in the tank but nowe i look at it again and think that the tank of water must be carried by the car... donno, very confused, rafael, can you try to explain a bit better what u need to do?

[Ciro Pabón]

Under water? Where did Rafael talked of "underwater"? It's a car propelled by a tank of water.

Vyslegend idea is the "most constructible", I think. I'd complement it, by noting that you need an aditional "gravity" valve, like this:

First, all energy is provided by the spring: this means the syringe-tank-tube-nozzle system has to be airtight. Of course, you want the strongest spring and the largest syringe you can construct: you do not want any of the pressure provided by the spring to be lost on its way to the nozzle.

Second: once the syringe "discharges", you need to open a valve on top of tank to use the "rest of the water", feed by gravity. If you don't open this valve, the water won't "fall" anymore, once the spring extends, precisely because the system is airtight. This is the way the pressure of air acts: you do not feel it because it is "all around you", but is fairly high. In an airtight system is not available unless you open the valve I suggest.

The valve is the black thing on top of the tank, which is on top of the dome, the spring and syringe are to the left and the nozzle to the right in this awesome picture I made:



This way, you would have "two-sources" of energy, the mechanical one from the spring and the gravity potential from the tank of water. You can experiment with the nozzle and a waterwheel attached to the wheels as vyslegend and some others have posted.

If they do not allow electrolysis and hydrogen-burning, I do not see what else to do...

You need a "high" chassis for the tank of water. I'd try a geodesic dome for supporting the tank, instead of an arrangement of beams, as you can see "clearly" in my "awesome" drawing. This way you'd get a tall and light structure with an stable base.

A dome is not that hard to build: you only need layouts for constructing the unions. Actually, I think this is an interesting problem for a chassis constructor: most of the weight of the car is on its "roof"! This is why I dare to suggest the dome...

And I am not having anything, as Mikey_s thinks! This is my natural state...

Oh, I almost forgot to baptize this design: I suggest the "Sambodomo"...

[vyselegend]

I think since Raphael used the word "car", it is not a boat or a sub. This is an interesting project, maybe we could be of better help if you give us all the details and rules (min/max lenght, height, weight of the car, quantity of water allowed...)

Second: once the syringe "discharges", you need to open a valve on top of tank to use the "rest of the water", feed by gravity. If you don't open this valve, the water won't "fall" anymore, once the spring extends.

You're right, it need a valve to allow the remaining water to come out. But a valve on the top would be very difficult to install, since the push rod must be air tight, and is movable. Maybe the valve can be installed on the side of the tank, just under where the released spring stop its travel. That would allow the valve to be opened by another mecanical switch activated by the spring reaching its final position.

Or another solution could be to make the tank rotating from a straight position (||) to a diagonal one (//) during the spring action, because the remaining water in a tube is stucked while there is no air on top, but when you incline it, it falls, because of the water floor staying parrallel with the ground while the tube's hole is not, thus freeing the water.

[Ciro Pabón]

Vyslegend: the valve I draw (I edited my post to add an "awesome" drawing), should open by itself once the pressure from the spring ceases (it is a "flap valve").

Or another solution could be to make the tank rotating from a straight position (||) to a diagonal one (//) during the spring action, because the remaining water in a tube is stucked while there is no air on top, but when you incline it, it falls, because of the water floor staying parrallel with the ground while the tube's hole is not, thus freeing the water.

What do you mean? I don't get it, please, explain...Bernoulli's equation is the same, being the floor parallel or perpendicular, I'd say... Anyway, we seem more interested on it than Rafael himself...

Finally, if the nozzle acts on a waterwheel atached to the rear wheels, you use the energy from the water twice, I think, first as reaction, like in a rocket, and second as rotation of wheels, as in a waterwheel. Am I wrong? I am not sure: you cannot duplicate energy out of nothing, but the water still has kinetic energy. THIS is an interesting question for a physics first year textbook...

[Crabbia]

ok now i'm on the same page, or rather the same book

See tho the abvantage of a tube of as opposed to a tank is that the initial water flow out of the jet will be greater (as torrichelli would have u believe) and thus it will more easily overcome its inertia so u get more acceleration off the line.

What i thought about now is how about a spring that liftsthe whole tank up on guides as the tank gets lighter. This way u could keep a relatively constant flow all the way till the tank empties.

[Rafael de Oliveira]

As I said, the rules are:

1- The car must be moved by a water jet
2- I can´t use any kind of compressed gas or chemical reactions
3- I can use only the pressure of the atmosphere or any kind of mechanical device to pull the water out of the tank.
4- There is no limit about lenght, weight, water volume and all the other things.

Thanks a lot

[Ciro Pabón]

As I said, the rules are:....

Well, the "design" given complies with rules 1, 2 and 3. But the rule number 4.... that's the hard one.

Of course there are limits: they are given by the equations. You should start tinkering around for parts, and once you have the basics and know how much they weight (I mean, tank material, wheels, "syringe-and-spring" or whatever you are going to use to pressurize the tank), maybe we can help a little with the amount of water you should carry...

I'd recommend balsa wood for the chassis instinctively. I think I can design the frame, if you give me the weights of the things you are going to use. In a rocket, all mass is critical (and apparently, in a flying saucer too: have you noticed how thin the aliens are depicted? ).

It would be useful (if this is not the "first season") to know the times posted by previous record holders for the "10 meter drag track".

As an example, let's do a quick check for the "Sambodomo" without spring and with 1 liter of water at 50 cm of height:

Energy available: 9.8 m/s2 * 0.5 m * 1 kg = 4.9 joules

Let's assume 2 kilos for everything except the water (chassis, wheels) and an optimistic 50% efficiency, then your final speed would be:

V = (2E/m)^1/2 = (2 * 4.9 joules * 50% / 3 kg) = 1.3 m/s

This is over 4 kph. I don't expect a heavy aerodynamic design...

Your acceleration would be a meagre:

a = (1.3 m/s)^2 / 2 * 10 m = 0.08 m/s2

No need for "neck training" in this race...

And your time for the run:

t = 1.3 m/s / 0.08 m/s2 = 16 seconds

If you want to go into the "low 10" you need a spring that provides at least an extra 5 or 6 joules (or 10 to 12 before factoring in the losses, if it is true they are 50%).

The acceleration would be fairly constant, nonwithstanding some members opinions, as the drop in the hydraulic head would be fairly low: a liter fits in a cube 10x10x10 cm, so even a cube tank would only diminish the pressure 20% in the entire run (the height of the water would drop from 60 cm to 50 cm). Of course, the shallower the tank, the smaller the drop in the hydraulic pressure. There is no relation between the quantity of water in the tank and the nozzle speed, as some had suggested.

Any gains in the weight of the chassis or in the height of the tank should give you the inverse of the square root of the gain in speed. This is, if you shave your chassis to 0.5 kilos, for a total of 1.5 kilos with water (half the assumed weight), your speed should increase the square root of the inverse of 50%, around 140%, to 1.8 m/s.

The same applies if you double the height of the tank from 0.5 m to 1 m. As the water weights a lot (duhhh..), it is more important (with the numbers given) the height of the tank than the weight of the chassis: you have to reduce the chassis weight to a fourth of the assumed initial value, from 2 kilos to 0.5 kilos, which is a lot, to get the same gain as a doubling of the height will give you. The taller the chassis, the better, even if it weights a little more. I'd guess the best design would be some kind of stayed "brazilian Eiffel Tower", instead of a dome, as I suggested previously.

The problem here (I'd say here is where the fluid dynamics thing finally appears!) is that the hydraulic losses on the tubing might be important. The speed of a rocket depends on the speed of the exhaust (water) on the exit and its pressure. But this speed and pressure depends on the losses on the tubing and the water head....

I'd say you have to take it from there, even if, as you can notice from the length of my post (nothing new), I am on vacation and alone with my cat at home...

Finally, your nozzle: the rate of flow should be high enough to empty the tank before the finish line. In my example, you need 1.000 cc in 16 seconds (let's say 10 for safety). So you need a flow of 100 cc/second, if you are going to use all your fuel. If you don't, your car will be slower than assumed.

Another solution would be a car that has the rotating tank that vyslegend suggests, but with a lenght of ten meters. As soon as the tank rotates by the virtue of its jet (and gravity), its tip would reach the finish line... I would call this design the "Flexi-tank".

A "cleverer" solution (I love to ruin rules) would be the "catapulted car" that you can easily imagine. Nothing in the rules states that you have to carry the water to the finish line: a jet (more like a dump of water) flows out of the car, charges the "trebuchet" and.... hey, this is not cheating, I'm learning from Ferrari...

[vyselegend]

What do you mean? I don't get it, please, explain...Bernoulli's equation is the same, being the floor parallel or perpendicular, I'd say...

Obviously you're by far wiser than me, especially for physics, so I won't argue too much.

Still, I clearly remember at school there was a limit in the inclination of an upside down tube filled with water to keep the water stuck inside, but I assume it depends of the tube volume/ water volume ratio. If you incline the tube so much that it's nearly horizontal, the water flows off the tube because air is allowed to enter. That is when the water distribute itself on the lenght of the tube (and no longer filling the width), it stop acting as a stopper.

Understand I have nothing against that dear ol' Bernoulli guy... but his equation is no longer of use whenever air is allowed to pass, isn't it?

[Ciro Pabón]

I clearly remember at school there was a limit in the inclination of an upside down tube filled with water to keep the water stuck inside, but I assume it depends of the tube volume/ water volume ratio.

Bernoulli still applies, I'd say. And you are right: the experiment works the way you say. What is holding the water is the air pressure outside the bottle, because there is no air inside the bottle.

If you incline the bottle (or shake it, for example) you allow the air that is pressing on the bottom of the water to enter. When this first bubble enters, the pressure inside the bottle is equal to that of the exterior, and water flows.

It goes "glurg, glurg" because, as the "first" water exits, the volume of the that "first" bubble of air increases. Again, the pressure inside is less than outside. This causes a new bubble to enter and the cycle repeats. For water to go out, you need air to go in.

This is not what we want in Rafael's car: we want to use the tube and nozzle only for the water to exit, with maximum pressure, not being stopped by bubbles of air entering the opposite way. Actually, if the nozzle is narrow enough, it will happen the same as in your experiment with the bottle at school: no water will come out.

This is why we suggest to open a valve at the top, once it ends the "extra-pressure" of the syringe you and mikey_s propose. If you use the propeller that mikey_s, flynfrog and Crabbia thought about, then you should have an open tank, without any valves needed. Or you could try the tree solutions you have thought of: pressurizer (with a valve to release water once it ends working), a propeller and a gravity feed.

I believe, also, that some school physics (that could be wrong) does not make me any wiser than you...

[flynfrog]

i like the flexi barell idea a few 100 gallon drums and a marble get them about 30 feet off the ground i belive that gives you 3 atm i could be wrong i kinda forget water presure heighth right now

then a nifty on off valve like a kinked hose shuld push the marble pretty far or at least untill your water flow becomes to distrupted

how long is the race agin

[Crabbia]

well i can help with the maximum exit speed to avoid turbulent flow so u don't 'waste' any energy.

i calculated the reynold's number. If your intial pressure head is even half a meter you'll go into turbulent flow. For this reason Rafael, make sure that ur exit tubing is rigid, maybe even with a little a stonger wall than the braided green garden hose.

Also where gonna have to over design this a little for this reason. alot of the energy Ciro has calculated for us will be converted into the turbulent flow of your jet. to limit the effects of this may i suggest that whatever design u decide on Rafeal, make sure the tube leading to the jet exit is as large as practically possible and shorten the diameter of this tube to make a jet at the very end of the water jet exit.

Also don't go smaller than 75mm diameter for your water jet exit as any smaller and your reynold's numers will start to get way too high.

May i suggets something, donno how the others might feel about it but how about to seperate jets on each side of the cars centreline. I'm thinking this would supply twice the force. oviously it would use up twice the amount of water but the benefit of this is that the enerygy used up in the turbulent flow of the jet could be minimised and it would alos provide a better balanced force for the car.

Ciro, is ther any way i can get a drawing onto the webso i can link it?

[Ciro Pabón]

Ciro, is ther any way i can get a drawing onto the webso i can link it?

Sure. I just opened an account at geocities for my pictures.

1. Go here (or your favorite site, I haven't tried MySpace yet, I confess)
2. Hit "Sign Up", fill the form, hit "Accept" and finally confirm by answering the mail they send you (I think, not sure about the mail).

Once you have your account:

1. Sign in with your username and password (is the same page of the sign up).
2. Hit the tab "Manage", then the link "File Manager", next the link "Open file manager", finally, the button "Upload files"
3. Hit one of the "Uploads" buttons you'll see, and browse for your file on your PC, finally hit "OK". When it is done it will appear a message confirming your upload: open the file manager again and it will show you the list of files you loaded. Hit the "View" link in front of the name of your file: you should see your file in all its glory.

Note: filenames cannot have spaces and must be jpg, gif, tiff, bmp or some other "approved" format.

4. Finally, as with any image, copy the adress of the image on geocities and paste it on your thread with the tag img.

[Becker4]

i think ciro might have suggested something similar to this, so bear with me if i am re-stating the obvious.
in thinking about this, i wonder what the simplest solution is. it seems like designing a device which carries water the entire way, and releases it gradually to create a "jet" (it seems like this is what is being suggested, forgive me if i am wrong) might not be it. now, i have the word jet in quotes because it doesn't seem as though what constitutes a jet was outlined in the rules. I am wondering if instead of a water jet, a waterfall might be something to look at - the car only has enough structure to carry the water for a short amount of time, just before the beginning of the race. when the lights go out, all water is released at once.
as far as i can see:
benefits:
simpler car structure, especially concerning the construction of a device which can harness the gravitational potential energy of water
ablitiy to unleash a relatively large amount of energy compared to other solutions
when the car is accelarated, almost all water will already be out, overcoming the greatest obsticle to this type of project (weight as opposed to air). in another solution where water is carried and released over time producing a jet, at the time of greatest energy release (due to most water weight pushing out of the jet) the car weighs the most, as it still has all its water on board. by the time the car has expended its load and is light, there is much less weight pushing the water out of the nozzle, or whatever is being used. if all water can be separated at once or almost at once, there will the the most force acting on the lightest car (theoretically).
challenges:
creating a car which can bear, even more, use, the energy created by all the water dropping at once.
creating a car which will track predictably when accelarated so fast, and doesnt spin out of control.
optimising a structure which can hold the water for a short time and not break, release it all at once, and still be light.
creating a device which can use a lot of water energy all at once.
getting acceptable conversion of gravitational potential water energy into kinetic energy, accelarating the car.

my initial concept car:

wheels are out wide on outriggers. the middle structure resembles a triangle pointing up when viewed from the front or rear, extruded along the length of the car. the water is held at the top, in a large tank with a bottom that can be rapidly opened. under it is simply a ramp, shaped like 1/4 of a circle, pointing up towards the bottom of the bucket, curving down and point out the back of the car. the structure could be aluminum tubing, basically a truss with a tank on top, wheels at the corners, and a curving ramp in the middle. it seems like this could be made quite light, and stable enough, and still hold quite a bit of water for power. i think its greatest advantage is that when all water is released and pushes around the ramp, powering the car forward, it will already be at its lighest, maximizing accelaration. also, getting power to the wheels through some kind of drivetrain is bypassed. i wouldnt recommend this if it was a distance competition, but it sounds like it only has to go 10 meters as fast as possible.

be gentle, im just a lowly architecture student

[flynfrog]

i would see what the rules state

if the tank does not have to be carried by the car get a ping pong ball and a verry large drum of water and pour it down the track