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Post by n2maniac on Nov 22, 2016 3:52:29 GMT
So, has anyone else noticed some weird efficiencies on resistojets?  I know there is some thrust power than can be contributed by simply letting the propellant escape out to vacuum from its stored condition, but this should be minor at 4km/s exhaust velocity. This thing is 500% efficient! Has anyone been able to abuse this usefully? |
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Post by cuddlefish on Nov 24, 2016 0:51:52 GMT
I believe that's from the dissociation of the propellant at the extreme temperatures - you'll notice the effect is particularly pronounced with things like hydrocarbons, and requires certain minimum temperatures to appear.
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Post by n2maniac on Nov 24, 2016 1:44:02 GMT
I believe that's from the dissociation of the propellant at the extreme temperatures - you'll notice the effect is particularly pronounced with things like hydrocarbons, and requires certain minimum temperatures to appear. A good thought, but not sure I buy that. Dissociation absorbs energy to break those bonds, which is properly shown in the efficiency loss present on the MHDs. It is also still over 100% efficient on noble gasses. |
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Post by apophys on Nov 24, 2016 2:23:48 GMT
Decane can get around 82,500% efficient. EDIT: Whoops, I was off by an order of magnitude, it's only 8,250%. Thanks leerooooooy . Still oddly high. |
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Post by n2maniac on Nov 24, 2016 4:48:40 GMT
82500% is... actually better than a NTR.
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Post by n2maniac on Nov 24, 2016 6:57:12 GMT
I believe that's from the dissociation of the propellant at the extreme temperatures - you'll notice the effect is particularly pronounced with things like hydrocarbons, and requires certain minimum temperatures to appear. A good thought, but not sure I buy that. Dissociation absorbs energy to break those bonds, which is properly shown in the efficiency loss present on the MHDs. It is also still over 100% efficient on noble gasses. Okay, I do observe what you describe. That shouldn't happen, but it definitely does here: Modifying pump speed with constant coil power to just see its effect on thrust power sees a very large jump upward (11x) as soon as a threshold is hit, seeing dissociation jump from 3% to 99% from 634K to 641K. Yea, something is tied to dissociation in a weird way. The temperature and flow rate don't jump at the chamber, but temperature does jump downstream of it (going 5x colder past that threshold in the extreme case I am looking at). Did a few calculations: Input power: 0.1GW Thrust power, 3.5% dissociated: 1.13GW Thrust power, 99.6% dissociated: 12.0GW Heat of formation for 4.3t/s of decane (calc): 9.06GW So that is a hell of a coincidence. It should also take about 9GW more electrical power to dissociate that fuel flow rate. Okay, so any other points? Heating liquid decane from 300K to 634K at that flow rate needs... 3.2GW??? Heating it as a gas will need similar as well. Heat of vaporization? Okay, temperature dependent, but on the order of 1.3GW. I am consistently missing how 0.1GW input power does any of this, and how the magic 11x comes to be. Resistojet under test: Well, time to see how this can be abused. |
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Post by goduranus on Nov 24, 2016 9:50:05 GMT
Maybe there an exponent error or something like with coilguns in game?
Maybe this affects every velocity calculation in the game?
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Post by qswitched on Nov 24, 2016 23:50:31 GMT
The "over 100% efficiency" is actually correct behavior for resistojets, because it's not technically over efficiency. All propellant tanks are lightly pressurized, and simply letting it exhaust without adding additional power can yield low but feasible exhaust velocities (<1 km/s). This is how cold gas thrusters work. Resistojets add extra temperature to increase the exit velocity, but a resistojet could technically work fine with 0 W of power.
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Post by goduranus on Nov 25, 2016 6:57:41 GMT
But thrust power being 10 times the energy input?
Even if the thrusters were taking pressure energy from the tank, just releasing the pressure without putting enough heat back in would freeze the propellant tank and the thrust would disappear when the tank temperature gets too low?
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Post by n2maniac on Nov 25, 2016 9:43:54 GMT
The "over 100% efficiency" is actually correct behavior for resistojets, because it's not technically over efficiency. All propellant tanks are lightly pressurized, and simply letting it exhaust without adding additional power can yield low but feasible exhaust velocities (<1 km/s). This is how cold gas thrusters work. Resistojets add extra temperature to increase the exit velocity, but a resistojet could technically work fine with 0 W of power. Roughly agree: "I know there is some thrust power than can be contributed by simply letting the propellant escape out to vacuum from its stored condition, but this should be minor at 4km/s exhaust velocity. This thing is 500% efficient!" Presumably there is an internal energy stored with the propellant that is factored in. Is it stored at a high enough temperature to be significant with that chamber temperature? |
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Post by qswitched on Nov 26, 2016 22:21:59 GMT
The "over 100% efficiency" is actually correct behavior for resistojets, because it's not technically over efficiency. All propellant tanks are lightly pressurized, and simply letting it exhaust without adding additional power can yield low but feasible exhaust velocities (<1 km/s). This is how cold gas thrusters work. Resistojets add extra temperature to increase the exit velocity, but a resistojet could technically work fine with 0 W of power. Roughly agree: "I know there is some thrust power than can be contributed by simply letting the propellant escape out to vacuum from its stored condition, but this should be minor at 4km/s exhaust velocity. This thing is 500% efficient!" Presumably there is an internal energy stored with the propellant that is factored in. Is it stored at a high enough temperature to be significant with that chamber temperature? Yes, the propellant storage temperature is also another factor. It's usually insignificant when your engine temp is 3000 K like with NTRs, but when your chamber temp is low enough like some resistojets are, it can be your primary energy source. |
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Post by leerooooooy on Nov 27, 2016 15:43:27 GMT
Roughly agree: "I know there is some thrust power than can be contributed by simply letting the propellant escape out to vacuum from its stored condition, but this should be minor at 4km/s exhaust velocity. This thing is 500% efficient!" Presumably there is an internal energy stored with the propellant that is factored in. Is it stored at a high enough temperature to be significant with that chamber temperature? Yes, the propellant storage temperature is also another factor. It's usually insignificant when your engine temp is 3000 K like with NTRs, but when your chamber temp is low enough like some resistojets are, it can be your primary energy source. Are you sure things are working properly? Because 8000% efficiency is a TAD silly  And this is 0W thrust  It does not look like the cold gas energy is anywhere near enough to explain what happens here |
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Post by ross128 on Nov 27, 2016 16:18:57 GMT
Hmm, as far as I know Decane doesn't have an exothermic decomposition either.
I wonder what a hydrogen peroxide resistojet would do.
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Post by n2maniac on Nov 27, 2016 18:07:35 GMT
Hmm, as far as I know Decane doesn't have an exothermic decomposition either. I wonder what a hydrogen peroxide resistojet would do. It does not. Excellent question! It seems that hydrogen peroxide has a very high decomposition temperature ingame (~3000K?), but nitromethane is another good one that decomposes much colder (1080K). Its behavior matches decane's pretty closely, but with a different gain in thrust power below and above the decomposition temperature (7x and 20x, respectively). |
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