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Post by apophys on Nov 18, 2016 18:53:01 GMT
They are indeed using capacitors, and fire rates are indeed very low. So it seems approximately correct.
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Post by wafflestoo on Nov 18, 2016 20:18:06 GMT
They are indeed using capacitors, and fire rates are indeed very low. So it seems approximately correct. And here I thought I'd be able to solve the energy crisis by wrapping my high school physics teacher's coffin in copper wire. So not only are our in-game railguns broken, they are outrageously broken. |
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Post by wafflestoo on Nov 18, 2016 21:08:01 GMT
GOT IT! got it, got it... finally got my head wrapped all the way around it. Now to tie things off with a nice little bow.
Mixing your units, gets me every time.
So, going back to our spherical, non-radiating railgun with a power input of 25 MW ,muzzle energy of 20 MJ, 6 m bbl, and 2 km/s muzzle velocity. The power output on the rails is 3.33 GW, it HAS to be. BUT, the total energy output is still only 20 MJ (it's just put out very, very quickly). SO at 25 MW input power, the capacitors can be recharged in just under a second (not two-and-a-quarter minutes like I'd originally thought) by the very definition of a Watt (Joules / seconds).
So, assuming capacitive discharge is delivering the incredibly high power requirement (and not the generators directly) for the rails then we've come full circle back to where we started from. Power input = Muzzle Energy / Refire Delay.
Thank you for joining me on this ridiculous little ride, please leave your tray tables and seat back in their full upright position. Keep your hands inside the car and remain seated until the ride comes to a full stop.
Thanks you, and good night.
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Post by shiolle on Nov 18, 2016 22:26:15 GMT
I can't get my head around it. I read through a few articles on the railgun the USN is testing and it isn't adding up. They're quoting a power-input of 25 MW, muzzle energy of 20-32 MJ, and a muzzle velocity of 2 km/s. Looking at pictures of it I'm estimating the barrel length to be about 6m. Low-balling the energy output at 20 MJ, With an average velocity of 1000 m/s, the projectile spends about 6ms in the barrel. Power = Energy / time so 20,000,000 J / 0.006 s gives us ~3.33 GW of power... NOT 25 MW. The only thing I can think of is either they are using capacitors to deliver the three DeLoreans worth of power each shot takes and 25 MW is the steady power draw to recover the capacitors between shots (putting its sustained rate-of-fire at one round every two+ minutes) or I am flat wrong about how I am going about things. HELP! When I read your comment about the stock designs being broken too, I basically got the same idea, and ran into the same problems. I think there were different experiments at different power levels, and also some sources cite actual conditions of those experiments and other cite target performance parameters, which I think have not yet been achieved. Here is one page that talks about USN experiment in 2008 and as well as target performance parameters of the complete system: www.navweaps.com/Weapons/WNUS_Rail_Gun.php Barrel length for the test device isn't listed, but the information for the final version is complete. There is also an interesting report from 2012. It's about damage to the rails after continued fire, but the only thing I couldn't find in the document is the mass of their armature + projectile. On the other hand there is a graph on page 89 (it's seems this was a part of a larger review) that shows that the speed of the projectile inside the barrel does not increase linearly. It seems there were sea trials for the railgun in summer of this year. Unfortunately this is not a technical article, and though it lists all the necessary data, it is not clear whether the system also included some capacitors or if the power plant they list was the only source of energy. Finally, there is this document. They've build a much smaller system, but on the plus side, it has a ton of data. So, I've taken the complete system specifications from navweaps, data from WSJ article and data from California Polytechnic State University and try to apply these formula to them. Here is the updated spreadsheet: Attachment Deleted. NLR specification from navweap: - Kinetic energy of the projectile matches that figure in the article.
- If they supplied 150 MJ per shot (8 ms inside the barrel), this means that power draw should have been 18.75 GW.
USN sea trials from WSJ article: - If that power plant was the only power source, it shouldn't have been able to supply two orders of magnitude less power than it had. Either there was a capacitor bank or something is wrong with calculations.
California Polytechnic State University (mini railgun in the spreadsheet): - Projectile energy matches the figure in the document (211 J vs 200 J in the document)
- Input power was calculated by multiplying voltage by current. It satisfied the criteria for maximum power delivered along the barrel, but that may be due to the fact that the efficiency of this railgun is only 1.25% (so that power supplied was much larger than what would be needed at higher efficiency).
- I tried to calculate minimum required input energy and it doesn't add up. The article says they were using 16kJ capacitor, but I estimated required energy at 118kJ. This is because I used peak current when in reality current changed as the projectile moved across the barrel.
All in all, it seems there are a lot of problems with assumptions in this formula. I.e. projectile does not travel with constant acceleration and the power draw is not constant. |
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Post by wafflestoo on Nov 19, 2016 0:17:04 GMT
Reading through some of those articles makes it sound like they are using something called a 'Compensated Pulsed Alternator' to store energy energy from a "low wattage power source" (if we can call a 25 MW reactor a low wattage power source) and release it in a high wattage spike (unsurprising when you consider that we both have calculated rail power in the GW range as minimums) so it does seem they're using some manner of capacitive storage for the weapon. Personally, I was starting with the projectile kinetic energy and working backwards to set the floor for energy and power usage and drawing conclusions from there. I think what had me tripped up was the labels the game was using, making me assume (incorrectly) that the rails were being powered directly off of the reactor. Then again, that IS how the rail and coilguns behave in-game; as if the rails are powered directly (steady power draw regardless of fire rate) instead of using a capacitive system (power draw rising as fire rate increases)... I'm still sticking to my conclusion that the guns are broken as they are depicted right now  EDIT: Looking over your spreadsheet and I can only draw the conclusion that the only way we'd see energy levels as they are depicted is to use a capacitor of some kind to spike the power output. There's just no other way to deliver that kind of energy to the projectile in the timeframe allowed. |
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Post by thorneel on Nov 19, 2016 1:23:28 GMT
So the reason railguns and coilguns are broken is because the energy consumption doesn't vary with rate of fire.
The rail/coil power consumption slider should be replaced with an energy consumption slider (in J instead of W) and final rail/coil consumption calculated with rate of fire and energy consumption of each firing.
Or did I miss something?
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Post by ross128 on Nov 19, 2016 1:39:35 GMT
Yeah, theoretically you can accelerate a projectile to ludicrous speed with "just" a few megawatts of continuous power, but doing that would require a rail several miles long. To actually get that kind of muzzle energy in a reasonably gun-sized package, you need capacitors (or a similar storage medium capable of really fast discharge) to deliver gigawatt-range spikes.  You see those blue things in the background? Those are the capacitors for the gun. Note the exit door on the right for scale (a rough scale at least, considering this looks like it was taken with a wide-angle lens). |
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Post by dragonkid11 on Nov 19, 2016 2:31:10 GMT
Still slightly annoyed that I can't make railgun that fire a 8kg projectile at any reasonable weight. :c
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Post by Durandal on Nov 19, 2016 7:21:56 GMT
I believe I've developed a highly nuke resistant engine*. I took my standard high-thrust frigate engine, slapped an eleven meter gimbal on it, armored it in a meter of diamond, and maxed out the (also diamond) chamber thickness. Went back and tinkered with the engine a bit and increased the gimbal angle and reaction wheels and voila!    After several volleys of nukes, the actual hull of the ship cracked before the engine died. As you can see, it floated away intact. The cost is fairly steep; 50mc. You can buy a ship for that. But the ability to reliably survive nuclear weapons and retain mobility? Priceless. *Untested against flak and KE weapons yet. *edit* It does mass the ship down pretty heavy though. I may try to find an alternate armor material or try to increase the thrust of the engine. Right now it only has about a tenth of the acceleration of a conventionally engined ship, and it is not able to mount MPD thrusters amongbst the NTR thrusters as I like to do with full capital ships. I've also considered trying a resistojet version for high energy (1GW+) ships. |
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Post by shiolle on Nov 19, 2016 7:54:20 GMT
So the reason railguns and coilguns are broken is because the energy consumption doesn't vary with rate of fire. The rail/coil power consumption slider should be replaced with an energy consumption slider (in J instead of W) and final rail/coil consumption calculated with rate of fire and energy consumption of each firing. Or did I miss something? My conclusion is this (also serves as tldr version): existing sliders can remain there without change, rail and loader power consumption should add and capacitor banks should be modeled for accurate representation of railguns/coilguns, adding a whole lot of new sliders. There are two checks we are doing. - First is for projectile energy and rate of fire. Let's say that a projectile has has 5 MJ of energy by the time it leaves the barrel of a gun, and the gun shoots two projectiles per second. That means that the gun should require at least 10 MW (since watt is joule per second) of power. Projectile energy is defined by the mass of the projectile and muzzle velocity of the gun.
- Second, the projectile only spends a very small amount of time accelerating through the barrel. This time can be calculated from muzzle velocity and barrel length, and is not a function of how quickly a loader can shove new projectiles into the barrel. The formula proposed by wafflestoo assumes that the acceleration is constant, but that's not the case. However, it seems certain that this amount of power is at least an order of magnitude greater than what is required based only on the rate of fire and muzzle energy.
In other words our steady supply of energy may be sufficient (as determined by the first check), but it should be delivered in pulses of huge amplitude (second check). The common way to accomplish that is to use capacitors, but currently they are heavy, bulky and do not react well to damage. I.e. they are a significant part of the whole system; most current railgun and coilgun designs, including stock ones, can't hope to accomplish stated performance without them. The thing with capacitors and batteries is that they undergo a very rapid development at the moment. For example, Tesla claims to have doubled the energy density of their powerpacks between their first and second model. Capacitors receive less attention, but it's still unclear what happens to them in a decade, let alone a century. However for CDE that shouldn't be an issue since it uses only currently demonstrated technology, at the risk of gathering some zeerust prety quickly. So, while there are no capacitors in the game I think we should ignore the second check (for energy delivered during acceleration), but the first check (for projectile kinetic energy and rate of fire) remains relevant. |
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Post by shiolle on Nov 19, 2016 8:00:26 GMT
I believe I've developed a highly nuke resistant engine*. I took my standard high-thrust frigate engine, slapped an eleven meter gimbal on it, armored it in a meter of diamond, and maxed out the (also diamond) chamber thickness. Went back and tinkered with the engine a bit and increased the gimbal angle and reaction wheels and voila! So it looks like the reason engines die so easily is that injector cannot be currently armored. Once it was "swallowed" by the gimbal armor, the engine became resistant to nukes. I had my suspicions, but decided it must be something else because injector never glows after a nuclear strike. |
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Post by Durandal on Nov 19, 2016 8:23:18 GMT
I believe I've developed a highly nuke resistant engine*. I took my standard high-thrust frigate engine, slapped an eleven meter gimbal on it, armored it in a meter of diamond, and maxed out the (also diamond) chamber thickness. Went back and tinkered with the engine a bit and increased the gimbal angle and reaction wheels and voila! So it looks like the reason engines die so easily is that injector cannot be currently armored. Once it was "swallowed" by the gimbal armor, the engine became resistant to nukes. I had my suspicions, but decided it must be something else because injector never glows after a nuclear strike. I believe that's exactly it. The high gimbal speed and wide angle can make it a bit "wobbly" it lives. |
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Post by thorneel on Nov 19, 2016 22:46:03 GMT
The cheapest high-velocity coilgun I managed to make. I use potassium as it is the cheapest material out of those that give good enough velocities.  And a ridiculously long coilgun on the same principle. Impractical as far as I can tell, but who knows?  I haven't checked if they were broken, but they probably are. To unbreak them can probably be easily done by dropping fire rate. From preliminary experiments, it seems putting a small turret on those helps with precisioin at range. |
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Post by dragonkid11 on Nov 20, 2016 5:26:32 GMT
Wait, potassium? Are you sure these coilgun won't just explode when in contact with just about anything? |
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Post by teeth on Nov 20, 2016 5:46:55 GMT
That coilgun is absolutely bananas.
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