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Thread: Unlimited Rotational Kinetic Energy Storage?

  1. #21
    First of all, I didnt say it was cheating. Obviously, because the rules permit CO2 systems, its legal. I said that I think they are a bit of a cheat, and I think Ive sufficiently conveyed what I wanted to (CO2 systems enter the arena with immediately usable energy that needs no chemical conversion, and ICE and electric motors dont have that luxury). Its inconsistent. And nowhere did I say that non-CO2 bots *couldnt* create usable energy. Obviously, there are plenty of excellent non-CO2 bots.

    I dont know where you were going with the 15 kg linkage stuff. I was just demonstrating that you dont need 100000rpm to have a useful flywheel.

    As for the energy storage in a gas system, Id have to look into my thermodynamics texts and refresh my memory a bit...or alot.


  2. #22
    Im only guessing about this, but doesnt the energy involved in a CO2 bottle depend on how fast you can get your ram to expand?

    If youd have a buffertank a hundred times larger than than your ram, it would still use up the same amount of gas, yet it would expand a lot faster than a ram without buffertanks.

    The same goes for the valves and the pipes, larger bore would equal much higher energy, yet use the same amount of gas.

    Any thoughts about this?

  3. #23

  4. #24
    @ Mack Oh, an electric motor cant give all the power in an instant?Since when has electricity a slower than lightspeed reaction..If you want more electrical power you just use a bigger motor, bigger wires,better and/or bigger batteries.
    All for lower resistance.
    Fueldriven things, just thesame, if you want more power you burn more fuel in the same time, how you do that, more RPM, bigger motor etc.
    With 0.5L fuel mixed in the right amounts with air and BOEM, the arena has a new pit.
    It is all in the idea behind it.Not in the stored energy
    Compressed gas is just an easier and in most cases cheaper way to get fast useable energy.

    How I got to the 15 kg disklinkage.Easy. Total setup to use the very light and strong enough dry plate coupling from a high power motorbike like a Ducati. You need something to pull that.And no, 4 servos acting together dont sound to reliable.So you need or an electrical system based around a large solenoid or a strong and fast enough motor.Or a Pneumatical system.Even hydraulic are possible(with the system Kos is building even one of the lighter options)
    The original housing is not that handy to use in a robot, so a custom build part is needed.As this cant be cast in hi strength alloy(it can but would cost a little much), you will have to make that in a milled version, and that is always heavier than a Hi tech cast version. The axles from and to the energy storage and weapon must be strong and stiff-so weight is adding.
    To make a long story short. I did work on the idea.

    @ Christian, bigger buffertanks, bigger pipes and bigger rams are a way to use more gas in a faster way, again it is all in the construction.And everything needs to be in thesame league, you can use a 200*200 ram, and dont have a weapon.Try to fill that with a 1/8 bsp hose.

  5. #25
    Christian, as Eddy mentioned, the energy in a compressed bottle is fixed. Power is the rate at which you use the energy (or provide it), so if you consider a fixed amount of energy in a single dose of expanding gas, the faster you can deliver it into your opponent, the more power. If you deliver your dose over 1 hour, its pretty useless. 1/10 of a second is MUCH more effective. However, I think that larger plumbing increases the efficiency of the dose; viscosity and turbulence eat up energy, so if you have more efficient plumbing, you decrease these losses and get more of the energy into your linkage (and into your opponent).

    Mario, your motor is limited to 36V, and I dont know what kind of battery out there that can deliver the amperage required at 36V to give the same energy output as is contained in an entire gas bottle. Heavy duty 12V car batteries can only crank out 500 or so amps, but their illegal (wet cell), and weigh 30kg or so. Sure, electricity travels fast, but its circuitry has resistance which causes heat which melts said circuitry if you try to flow too much current through it. Even then, youd need one helluva motor that could handle that current. For an ICE, youd need to burn the whole tank of gas at once which would probably require an above average car motor. Both the electrical and ICE options therefore require a motor that would eat up most of if not all or more of the allowable bot mass of 100kg. You need no motor for a gas system. You need the linkage, but you also need that for the ICE and electric motors. The bottom line is that in one case, you have to convert the chemical energy to another form which requires an electric or ICE motor, and in the other case, you dont. That basic difference is why I think the rules are inconsistent. But as I mentioned before, its legal, so its probably wise to take advantage of it.

    Again, as for the 15kg linkage stuff, I agree that you must connect the flywheel to the weapon. My point was that you could store enough energy in a flywheel of 1/3 your bots total mass and easily within typical bot dimensions to toss a 100 kg bot twice the length of an average house straight up in the air. It was purely demonstrational. A more practical scenario would be to toss it 1.5 meters in the air 20 separate times. The linkages required to achieve this would be much more practical.

    As for cast vs machined bit, Im almost positive that if the shape of the part can be machined (some shapes cannot be machined but can be cast), then the machined part will be the stronger of the two. Therefore, the machined part could be designed to be even lighter than the cast counterpart. The strongest cast aluminum in my materials book is less than half the strength of 7075 T6 aluminum billet. It might be that if you can get a steel mold of the part vs a lost foam sand casting, you could get the part cast in 7075 T6, but usually, you have to either have TONS of cash to make a one-off in this manner or plan on manufacturing numerous parts that can be sold to offset the cost of making the mold. I had a fun time in college making some styrafoam molds of some pillow blocks for a race car that could be used to sand cast some aluminum parts (you bury the foam part in sand, then pour the molten Al into the foam, the foam evaporates holding the sand in place, and the Al fills the void), but we got lazy (err strapped for time) and ended up machining them instead. Made a cool hot wire styrafoam cutter and everything! But thats a bit off topic ;-)


  6. #26
    Mark, your arguments are still valid. I never disagread on them, but I still am from the opinion that a good build ICE, kinetic energy, or hydraulic powered system can be as effective as any pneumaticaly powered weapon. It is all in the construction and the ideas behind them.

    Batteries, according to my info, the Hawker Cyclon 8Ah battery, allowed in RW as they are Gelcells, can crank out over 600A.The 25Ah version reaches the 2000A mark.The motors to handle this current.Any motor can, if it doesnt take seconds.But my bet would be the pancake style motors, or a 6V carstarter from an old volkswagen beetle.
    I wonder how much amps Betas axe draws, that needs an Albright 280 relay, a brand know to be capable of withstand 500Amps and more(the biggest they have can switch 1500A).The example how electrical power beats pneumatical.Beta is the only hammerbot up to now that dented Tough As Nails hard enough to make a real impression.

    For excample, how does a hydraulic set up sounds. Inside 6 kg, flowrate 40l/min and a peak pressure of 3000 psi.And the electrical motor doesnt drop lower than 75% efficiency.Oh, and the 6 kg include the safetyvalve.With 5 kg more you have a ram and valves to operate a Razer powerlevel like weapon.A smaller version is being build by Kos, to be used in a featherweight.

    Now pneumatical systems. It takes more than big tubing and large valves and rams to use the complete content of a bottle in 1 go.Especialy when using CO2. So I repeat the question. How much energy is there in a bottle with a volume of 3L and 55 bar pressure?It is normaly 165L on 1 bar.
    For example Gravitys ram, an 100 bore 180 stroke uses 77.75L in a fill action.How do we get more than 10 very usefull actions out of that system?Oh, that robot uses 2 bottles that size, so theoreticaly 330L on board.
    Just a question, and maybe your answer is also a reason to know that pneumatical systems have own problems and enginering behind them.
    Other excample Tough As Nails, using a 100 bore 354 stroke ram, with only 1 bottle , can have 3 or 4 useable damaging actions, and at least 6 good grabbing actions.How do we do it?

    The cast vs machined bit.The machined parts will be stronger, but heavier, as you will leave metal on spaces it isnt needed.Mountings to put the piece in the mill or lathe,ridges to prevent big deformations. I know this from years of practical machining.Including hi pressure aluminium casting.Also, consider the total amount of time needed to replicate a cast piece with machining,and even using handtools as a Dremel.

    The evaporating foam idea sounds fun, but dangerous, the vapours will try to escape the mold.And not very quality oriented, as gasbubbles will be cought in the solidifying ali.And Off topic, it is nice to hear alternative ideas.

  7. #27

  8. #28

  9. #29
    Mario, the problem with the energy of the gas cylinder is that you need to also factor in the temperature. We know the gas temperature in the bottle is roughly room temperature in the beginning, but the vented gas cools as does the remaining gas in the tank after the first use. If you know the density, temperature, and pressure of the gas, you know its internal energy per unit mass. So we know the initial energy, but not the final. I think that some assumptions can be made about the expansion process (perhaps adiabatic) that might give a good ballpark estimate of the new gas temperature both in the vented gas and the remaining gas (ignore conduction of heat into/out of the gas), but my thermo skills are completely shrouded in cob webs. Ill try to ask some workmates whose thremo is fresh in mind. Im almost positive that a fairly accurate calculation of the work done on the piston by the gas can be made.

    I agree that if you cant machine the same geometry, you dont have an option but to make the part another way. Thats why I said if the shape of the part can be machined. There are plenty of shapes that can be machined, but Im not sure what the part looks like in the Ducati clutch. I was leaning toward an alternative design using some off-the-shelf or slightly modified components (not a clutch).

    The lost foam sand casting bit is not something I invented. Many auto motor blocks are cast this way. If you look at a Saturn block, it looks like silver styrafoam (you can see the individual beads from the styrafoam in the actual block surface texture). The styrafoam vapor escapes through the sand, not through the metal. Its pretty easy to do if you have the ability to do green sand casting and dont need optimal material properties. Agreed, all of the problems with sand casting still exist.


  10. #30
    Mario: using PV=nRT gives you 0.556m^3 of CO2 at 1 bar from 1.1kg, which is equivalent to 56kJ of work done on the atmosphere. The most energy you can actually extract from the gas is 28kJ as a ram with only 1 bar absolute pressure will do no useful work, you need 1 bar gauge pressure which leaves you with only 0.278m^3. This is if the gas is expanded slowly and is allowed to reach ambient temperature. If large buffer tanks are used I think this is approximately correct. [a 30C drop in temp only drops the energy by 10%]
    1.1kg of NiCads would give you 286kJ. 500ml of petrol could give 7600kJ!

    Mack:I dont think you could use up your whole CO2 supply in 1/10second. I reckon it would take more than 3s to completely vent a fire extinguisher and even then you would loose a lot of energy to friction/heating effects. This leaves you with about 8.5kW/kg. [or about 2kW/kg taking into account mass of the entire system]
    Probably the ultimate electrical system would be 3 packs of Nicads and a LEM200 weighing about 6kg, giving out 6kW. obviously this is 1kW/kg
    A RC plane engine can put out about 2HP peak and weighs about 1kg. giving around 1.8kW/kg.
    Now the above engine/motor needs a transmission, but belt drive to a disc weapon doesnt weigh much.
    A steel disc can store about 180kJ/kg before it explodes; a CFRP one can do better at 720kJ/kg, but these are both very high tech, scientifically designed discs with expensive bearings run in a vacuum. I cant find figures for the sort of power you can extract from them but ultimately as Mario suggests it isnt going to be a light solution.
    So all in all I dont think its right to say CO2 systems have an unfair advantage.
    Personally I think ICE is the way to go for weapon power but they all have their advantages and disadvantages.

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