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This topic is inspired by the presence of a working engineer in the forum membership.
We have a number of engineers and a ** real ** scientist or two, but in recent times posts by engineers have occurred most frequently.
This topic is specifically inspired by a suggestion/proposal by Calliban, in a topic about activities on the Moon.
What I would like to see in this topic are specifications for various devices/pieces of equipment/entire infrastructure that would employ principles of thermodynamics to insure successful operation in a wide variety of space (Solar System) environments.
Humans (Russians first, then the US, then the European Union, then China, and in recent times, Japan, India and the UAE) have deployed working machinery on a variety of Solar System objects, as well as free-flying in space.
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This post is intended to stimulate creative thinking, including design of machinery able to work on the Moon and other locations away from Earth.
The specific focus of ** this ** post is the proposition (made by Calliban in November of 2022) that a practical moon rover/worker might be constructed to use an internal combustion engine of some kind, with hydrogen and oxygen carried along in pressure tanks.
The hypothesis presented by Calliban (as I understand his post on the subject) is that hot/warm liquids departing an internal combustion engine of some kind would be available to maintain cabin temperature at a comfortable level for human (as is done on Earth) and ** also ** to heat metal parts that might become brittle or turgid in the lunar cold.
This vision should lead directly to physical designs for fabrication on Earth for testing in suitable chambers, and for deployment to the Moon if they are found suitable.
A significant detail of Calliban's proposal (as I remember it) is to employ water as an energy storage medium (a battery in effect) by accepting electrical power from a source (such as a nuclear reactor or solar panel array) to electrolyze water into hydrogen and oxygen. This design makes use of the ** inefficiency ** of the internal combustion engine, to deliver heat to the body of the vehicle via fluids. This is in contrast to the efficiency of solid state equipment, which generates little heat and thus cannot assist with cabin heating.
This topic is available for proposals to employ thermal activity in a great number of applications.
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Alternatively, since the reactor is so hot and has to dump waste heat somewhere in order to generate electrical power, you store that otherwise wasted thermal power output in molten salt or molten metal, instead of radiating it away into space, thereby saving the reactor's electrical output for other uses, such as life support equipment. The molten salt then heats a supercritical CO2 working fluid that powers a small closed-loop gas turbine, producing mechanical or electrical power output. The powered vehicle's molten salt tank and radiator array are heavy, but the sCO2 turbine is very light and compact, and on the moon or Mars, gravity is much less of an issue.
Liquid Sodium metal could also serve as both the thermal power storage medium and working fluid for a turbine, eliminating the need for a highly pressurized gas. Power density in liquid metal systems is fairly high, especially at the temperatures produced by KiloPower. KiloPower also uses NaK / molten metal coolant.
The M1 tank has 21.2hp/t of vehicle weight, and provides excellent cross-country mobility. A lunar vehicle would only need 3.53hp/t for equivalent performance. A 10t vehicle, therefore requires 35.3hp to move at least as well as the M1 Abrams. Some of the largest common-use semi-truck tractors weigh around 11t for comparison purposes.
If none of that is workable, then commercially available Sodium-Nickel Chloride thermal batteries can provide 100Wh/kg. 1 hour of full output is 26.3kWh, so 175.5kg at 150Wh/kg, so 8 hours (210.4kWh total) running at full output means 2,304kg of battery weight, or 384kg in lunar gravity. These batteries operate at 250C to 350C, IIRC, so they could also warm the rest of the vehicle using waste heat, especially rubber tires.
FIAMM's ST523 Sodium-Nickel Chloride product stores 23,500Wh and weighs 256kg, with 620V at nominal output, 450V min and 641V max, and max charge voltage is 700V. It's designed for 4,500 cycles at 80% DoD. Each brick is 566mm wide, 388.6mm tall, and 965mm overall length (only 862mm for the battery itself, the rest is for the control electronics module).
The M1 Abrams manages 45km/hr in rough terrain, so a "light" wheeled vehicle should do just as well if the terrain is relatively flat, which translates into about 360km of range between recharges, in a 100% off-road environment. That's actually pretty good.
I was thinking 8 of those 35 inch diameter Airframes Alaska Bushwheels, operating at no more than 7 to 8psi, with heavy tread and extra Kevlar (Bushwheels are already made with multiple layers of embedded Kevlar to resist puncture from rocks), filled with Nitrogen, and those are able to roll over very large and even sharp rocks when aircraft as heavy as the Pilatus Porter or DeHavilland Beaver land on river beds in Alaska and Canada. The hub itself would supply heat to the tires to keep them warm at all times. Each side could have 2 pairs of rocker bogies, similar to the Mars rovers. This is very well within the load bearing capabilities of the tires, and each tire is only supporting 208kg if the vehicle weighs 10t. A Pilatus Porter at MTOW is 2,800kg and those same tires have no difficulty supporting that amount of weight here on Earth, albeit there are not a lot of volcanic rock field landings (that I'm aware of) in PC-6s. If the tires don't work out, then the backup plan is woven wire wheels.
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Alternatively, since the reactor is so hot and has to dump waste heat somewhere in order to generate electrical power, you store that otherwise wasted thermal power output in molten salt or molten metal, instead of radiating it away into space, thereby saving the reactor's electrical output for other uses, such as life support equipment. The molten salt then heats a supercritical CO2 working fluid that powers a small closed-loop gas turbine, producing mechanical or electrical power output. The powered vehicle's molten salt tank and radiator array are heavy, but the sCO2 turbine is very light and compact, and on the moon or Mars, gravity is much less of an issue.
Liquid Sodium metal could also serve as both the thermal power storage medium and working fluid for a turbine, eliminating the need for a highly pressurized gas. Power density in liquid metal systems is fairly high, especially at the temperatures produced by KiloPower. KiloPower also uses NaK / molten metal coolant.
Liquid sodium would be an excellent heat transfer fluid, thanks its high heat capacity, low viscosity and density and good thermal conductivity. I don't think it would work as a power cycle fluid, because even at 500°C, the vapour pressure is <1KPa. A closed cycle S-CO2 option has plenty of power density, even if heat exchangers are needed. The low vapour pressure and low viscosity of sodium would make it a good choice heat transfer liquid to transfer heat from a reactor sodium pool through heating coils in a vehicle molten salt tank. There is no air to worry about on the moon. We could in fact drop liquid sodium into a chute on top of the vehicle, let it drain through the coils in the salt tank before dropping into a drain under the vehicle. Vapour pressure is so low that it won't explode in the vacuum. Sodium melting point is 97°C.
Battery powered vehicles are a much easier concept to develop. Energy density is low and there won't be much spare energy to heat vehicle components. But maybe we could use a seperate, electrically heated molten salt tank for that purpose.
Last edited by Calliban (2022-11-17 17:14:46)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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Liquid Metal Energy Storage:
First-of-its-Kind Experiment with Liquid Metals in Thermocline Energy Storage at Karlsruhe
Nuclear has to be flown to the moon, at great cost, but we want to use that nuclear power to eventually make solar power on the moon, and later on Mars:
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Liquid Metal Energy Storage:
First-of-its-Kind Experiment with Liquid Metals in Thermocline Energy Storage at Karlsruhe
Nuclear has to be flown to the moon, at great cost, but we want to use that nuclear power to eventually make solar power on the moon, and later on Mars:
I looked up the stats for sodium. Heat of fusion 113KJ/litre. Specific heat = 1.2KJ/kg.K. A 785K temperature range between melting at 98°C and boiling at 883°C. Each kg of sodium between melting and boiling point, could absorb 1MJ of heat. Between 383°C and 883°C, the sodium will absorb 600KJ/kg. Liquid sodium has the really important advantage of having excellent chemical compatability with steel.
https://material-properties.org/Sodium- … expansion/
Last edited by Calliban (2022-11-17 18:54:28)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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We dump the otherwise wasted (radiated into space) 30kWt from KiloPower into liquid Sodium metal. KiloPower is designed to operate at 800C / 1173K during normal full power operation (generating 40kWt and 10kWe). On the shaded side of our lunar rover, ambient temperature plummets to -157C/117K.
Sodium metal has a specific heat capacity of 1.256kJ/kg.K, and our temperature delta for our heat exchanger is 1,056K. Each kg of Sodium metal, heated to 1173K, can then store 1,326.336kJ, or 368.426Wh/kg. We only get half of that back out using our simple cycle sCO2 gas turbine, or 184.213Wh/kg of Sodium metal. At full power output, our 8-wheeled rover requires 26.323kW, so total power requirement is 210.584kW. Sodium is 970kg/m^3, so that means our molten Sodium metal storage tank requires 1.179m^3 of volume and contains about 1,143kg of Sodium. Sodium-Nickel Chloride batteries would require 1.676m^3, excluding control electronics, and weigh 2,304kg, so we're about twice as good on gravimetric energy density and 30% better on volumetric energy density, without all the electronic gadgetry.
Edit: It appears I'm overly-optimistic in our operable temperature range (Sodium would freeze solid at 117K), so we need more Sodium metal. We're probably not much better than the batteries in practice, but no control electronics is a highly desirable bonus feature.
Last edited by kbd512 (2022-11-17 19:21:16)
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Much of this covers similar ground that used a kilowatt reactor for the mars 1 ft vehicle but would be designed differently as there is no atmosphere to act as a carrier of the moisture that would be released by heating the ground.
Without testing of a closed baking oven to see what is also out gassed as a function of heating we may need to send something different in the end run to perform that regolith processing for water.
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What was the "Mars 1 ft vehicle"?
Was this the ice drilling pathfinder project?
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here is that topic as I was going to fix that in the above post Mars Water regolith soils 1 foot depth only
Mars does not have the extreme of the moon but changing how it is taken from the regolith might be different.
It is basically a modified track drive digger that moved the soil to a hopper that feed the tray heating assembly with fresh soil to bake to release the water, with-it other volatiles and co2 to be sucked into a holding tank.
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