Lunar H2 and O2 Fuel Processing Project Aldrin-Purdue

Julian Wang | 1

AA | In-Situ Lunar Propellant Production and Processes

I. Introduction

Imagine a gallery where there were Earth-launching rockets, all stood like soldiers standing

attention to their commander. Alongside those rockets, is a small gold-colored rectangle, with

facts about those rockets. One thing that may stand out to the visitor is that the mass of the fuel

relative to the rest of the rocket is large. The mass ratios of Earth-launching rockets are very

high, to the point where most of the rocket is fuel. For a Mars-bound trip like this, a lot of fuel

would be required. Attempting to transport all the fuel to even just LEO seems a bit cost-

prohibitive. Therefore, Lunar in-situ propellant production should be considered as a way to

decrease the initial mass in LEO.

II. Introduction to Lunar H2 and O2 Fuel Processing

The motivation of lunar in-situ H2 and O2 production is to provide access to a long term

sustainable supply of propellant to power the spaceships of tomorrow. This section will assume

the lunar colonists have chosen Shackleton Crater as a colony site and have readily available

liquid water in order to explore the steps behind the H2 and O2 propellant production process.

III. H2 and O2 Propellant Production Thermodynamic Analysis

In this section, we will explore the subsystems necessary to produce H2 and O2 from liquid

water. The H2 and O2 propellant production process is split up into two main subsystems: the

propellant production process and the thermal & power regeneration process. We will in the next

few pages explore the thermodynamics behind it. Throughout the analysis, all processes are

assumed to be 100% isentropic and have zero pressure drops across heat exchangers. While such

assumptions are unrealistic in practice, it will provide a basic understanding what is necessary to

achieve in-situ H2 and O2 propellant production on the Moon.

Lunar H2 and O2 Fuel Processing Project Aldrin-Purdue

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Figure AA.1: The complete H2 and O2 Production Cycle

A. Propellant Production Process

The purpose of the propellant production process is to take in liquid water and output liquid H2

and liquid O2. This system accomplishes with the use of four main subsystems: the PEM

electrolysis, the liquid O2 condenser, the gaseous H2 compressor, and the liquid H2 condenser.

1. PEM Electrolysis

The purpose of the PEM Electrolysis is to convert liquid water into gaseous H2 and O2 via

electrolysis. The liquid water enters at a temperature of 400 K and a pressure of 250 kPa. After

the electrolysis process, a gaseous H2 and O2 mixture exits at a temperature of 400 K and a

pressure of 250 kPa. The electrolysis process will be considered adiabatic. We will be assuming

that all of energy used in the electrolysis will be used direct towards splitting up the water

molecule and not towards raising the average temperature of the output gas mixture. In addition,

we will also assume the electrolysis process to be isobaric. The reasoning is the PEM electrolyze

process is under two phase conditions, therefore the pressure of the H2 and O2 gas mixture must

equal the pressure of the liquid water.

Lunar H2 and O2 Fuel Processing Project Aldrin-Purdue

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The amount of energy required by the electrolysis process can be examined by evaluating the

enthalpy of reaction, shown in the following expression:

H2O → H2(g) +1

2O2(g), ∆H = 286,000

KJ

Kmol (AA. 1)

Therefore in order to electrolyze 1 Kg of water, we require roughly 15,900 KJ. We should also

note, in order to promote favorable electrolysis conditions, we would want to have the liquid

water to be at a high pressure. This is due to the Le Chatelier principle.

Finally the gaseous H2 and O2 will be filtered in order to produce two separate flows of pure H2

and O2.

2. Liquid Oxygen Condenser

The purpose of the liquid oxygen condenser serves two main functions: to liquefy the gaseous

oxygen into liquid oxygen and to provide the helium cycle energy. In this section, we will only

focus on the liquefaction of oxygen.

At a pressure of 250 kPa, the gaseous oxygen will condenser at a temperature of 90 K. We will

accomplish this cooling through the use of super cooled helium. Heat from the gaseous oxygen

will be transferred to the helium until the gaseous oxygen condenses. After this exchange, we

can expect the exiting liquid oxygen be at a temperature of 90 K and at a pressure of 250 kPa.

From this point, the liquid oxygen can be stored in tanks for future use.

3. Hydrogen Compressor

Liquid hydrogen has an extremely low boiling point. At 100 kPa, liquid hydrogen has a boiling

point of 20 K. We can increase the boiling point of hydrogen by increasing the pressure.

Therefore to achieve this, we will need to run the gaseous hydrogen through a compressor.

Compressing the hydrogen more will reduce the energy required to liquefy the hydrogen, but

will cost the compressor higher power. However the energy required to liquefy the hydrogen, as

we will examine later, is eventually used to power the power and thermal regenerative system.

Therefore we opt to reduce the compressor power requirement because the energy required to for

liquefaction will be reused later on. With a CPR of 1.4, the exiting hydrogen gas will have a

pressure of 350 kPa and a temperature of 440 K.

4. Liquid Hydrogen Condenser

Lunar H2 and O2 Fuel Processing Project Aldrin-Purdue

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The purpose of the liquid hydrogen condenser serves two main functions: to liquefy the

gaseous hydrogen into liquid hydrogen and to provide the helium cycle energy. In this section,

we will only focus on the liquefaction of hydrogen.

At a pressure of 350 kPa, the gaseous hydrogen will condenser at a temperature of 40 K. We

will accomplish this cooling through the use of super cooled helium. Heat from the gaseous

hydrogen will be transferred to the helium until the gaseous hydrogen condenses. After this

exchange, we can expect the exiting liquid hydrogen be at a temperature of 40 K and at a

pressure of 350 kPa. From this point, the liquid hydrogen can be stored in tanks for future use.

B. Thermal and Power Regeneration Cycle

The purpose of the thermal and power regeneration cycle to cool the hydrogen and oxygen to

extremely low temperatures. The regeneration cycle is comprised of four main subsystems, the

hydrogen liquefier, helium turbine, oxygen liquefier, helium compressor, and radiator.

1. Hydrogen liquefier (Helium Side)

As previously discussed, the liquid water condenser serves two main functions. This section

will focus on the heat transfer between the water vapor and the helium. The helium enters the

liquid water condenser at a temperature of 20 K and a pressure of 800 KPa and exits at a

temperature of 30 K and a pressure of 800 KPa. Due to the second law of thermodynamics, we

will need to outlet temperature of helium to be lower than or equal to the exit temperature of the

liquid water. Thus the exit temperature of the helium was set at a temperature of 400 K, since it

was our max allowable exit temperature. With a known exit temperature value, the inlet

temperature will be determined by power and mass constraints. A higher inlet temperature will

require a higher helium mass flow rate to achieve the same amount of cooling, while reducing

the overall system power requirement more. Biased towards power reduced again, the inlet

temperature of the helium was set at 20 K. The pressure of the helium was set equal to the

pressure of the water vapor since we would ideally want constant pressure heat transfer between

the water vapor and the helium.

2. Helium Turbine

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The purpose of the turbine is to provide power to the helium compressor and to reduce the

pressure of the helium so we can have isobaric heat transfer for the oxygen liquefier. Therefore

the exit condition pressure of the helium turbine will be 250 KPa. Using the equation below, we

can also determine the temperature after the turbine as well:

P1 = P2 ∙ [T2

T1]

−𝛾

𝛾−1 (AA. 3)

The exit temperature of the turbine was found to be 19 K. This temperature is low enough for

the helium to cool the oxygen gas into liquid form.

3. Oxygen liquefier (Helium Side)

The helium enters the liquid water condenser at a temperature of 19 K and a pressure of 250

KPa and exits at a temperature of 33 K and a pressure of 250 KPa. Through the processes of

liquefying the oxygen, the exit temperature of the helium was calculated to be 20.5 K. The

cooling process was again assumed to be isobaric, so the exit pressure of the helium was 250

KPa.

4. Helium Compressor

The purpose of the helium compressor is to raise the pressure of the helium back to 800 KPa so

we can have constant pressure conditions for the hydrogen liquefier. Since we know the pressure

ratio between the inlet and outlet condition, we can the following equation to determine the exit

temperature:

T2 = T1 ∙ CPR(𝛾−1)

𝛾 (AA. 4)

The exit temperature was calculated to be 33 K. However this temperature is too high to for the

hydrogen liquefier therefore we need to cool down the helium down.

5. Helium Radiator

Since the helium will face a temperature raise across the compressor, we need to cool the

helium down before the helium enters the hydrogen liquefier. So a solution to cool the helium

from 33 K to 20 K is with the use of a radiator. A radiator would prove extremely useful since

Shackleton Crater has permanently shadowed areas, so the ambient radiative temperature will be

extremely low.

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C. Propellant Production Power Requirements

There will be two main sources of power consumption: the PEM electrolysis and the hydrogen

compressor. The following table details the energy consumption of each of the major power

consuming subsystem as percent of total energy required.

Table AA.1: Major Power Consuming Subsystem as Percent of Total Energy Required

Major Power Consuming Subsystems Percent of Total Energy Required

PEM Electrolysis ~ 0.99%

Hydrogen Compressor ~ 0.016%

From Table AA.1, we can see the energy consumption from the PEM electrolysis represents

nearly all of the energy consumption for the entire propellant production power requirements..

Therefore further studies should be made on how to reduce the power requirement for splitting

the water into hydrogen and oxygen.

Overall, using the water extraction and processing system design above, we will need 1603 KJ

of energy for every 1 Kg of water. The power requirement for the propellant production system

can be easily calculated by factoring in the time frame in which we will be required to produce

the water.

Lunar Silicon Processing Project Aldrin-Purdue

Eiji Shibata | 7

IV. Introduction to Lunar Silicon Processing

One great way to minimize initial mass in LEO is by getting resources from places aside from

the Earth. In-situ lunar resource processing allows us to shift some of the mass from LEO to

GEO or L1, greatly decreasing the amount of propellant we would need. One of the materials

we could harvest from the surface of the Moon is solid silicon.

V. Lunar Regolith

We first identified the amount of silicon that is possibly available from the lunar surface. To

do that, we looked at the composition of the lunar regolith.

Like terrestrial soil, lunar regolith is varying, with the exact composition depending on the

location. We looked at some of main minerals of the regolith, which were ilmenite, anorthite,

fayalite, forsterite, and enstatite.

Table A.1 details the mineral properties. We picked these minerals because they were the

most common ones in Shackleton Crater [3].

Table A.1: Some of the minerals from the lunar regolith have their regolith displayed

here; the percentages are approximations.

Mineral Chemical Formula % of Regolith

Ilmenite FeTiO3 20

Anorthite CaAl2Si2O8 40

Enstatite MgSiO3 15

Fayalite Fe2SiO4 10

Forsterite Mg2SiO4 15

For heat capacity of the minerals, we recognized that the value would not be constant as

temperature rose, so we opted to use empirical relations during the calculation of energy

needed. Table A.2 shows the values that eq. A.1 [1,4] takes in.

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𝑐𝑝 = 𝑘0 + 𝑘1𝑇−12 + 𝑘2𝑇−2 + 𝑘3𝑇−3 (𝐴. 1)

Table A.2: The empirical relations for heat capacity are displayed.

Mineral k0 k1 k2 k3

Ilmenite 164.47 -0.09905 -5.092*10-5 -4.875*10-7

Anorthite 439.37 -0.37341 0 -31.702*10-7

Enstatite 139.96 -0.0497 -44.002*10-5 53.571*10-7

Fayalite 248.93 -0.19239 0 -13.91*10-7

Forsterite 238.64 -0.20013 0 -11.624*10-7

VI. Regolith Processing

The process that is detailed in this report comes from a report by Geoffrey A. Landis [2]. The

major products that come from the process are diatomic oxygen and solid silicon, with pure

metals and metal oxides being byproducts. A diagram of this entire process is shown in fig.

A.1.

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Figure A.1: The lunar regolith processing system, with mass inputs and outputs.

The process uses heat and diatomic fluorine to break apart the minerals in the regolith furnace

(1). Gaseous products go to condensers, which are used to separate out the different gases, while

the liquid products go to a reduction furnace (2). The silicon tetrafluoride is sent to the plasma

chamber as a liquid, where the silicon and fluorines are separated (5). Liquid potassium in the

reduction furnace is used to separate out some of the fluorine from the metals, outputting pure

metals (3). The rest of the fluorides, along with oxygen, go on to the oxidation furnace, where

metal oxides are output and potassium salts are sent to the crucible (4). At the crucible, the salts

are separated into solid potassium and diatomic fluorine (6). Diatomic fluorine goes to a holding

tank, which is where the regolith furnace gets its fluorine (7).

For this study, we looked at the stages where silicon is present in some form.

Specifically, we looked at the regolith furnace, the titanium tetrafluoride condenser, silicon

tetrafluoride condenser, and the plasma chamber. This subsystem had inputs of diatomic

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fluorine and lunar regolith, and outputs of solid silicon, diatomic oxygen and fluorine, and metal

fluorides. Figure A.2 shows this subsystem. We assumed that there would be no heat losses,

both during each process and between each process. Furthermore, we assumed stoichiometric

situations for the input and output of each part. All calculations were done at atmospheric

pressure.

Figure A.2: The subsystem that considers all steps that have silicon in it.

A. Regolith Furnace

The regolith furnace is where the lunar regolith first enters. For this study, we considered the

regolith entering in at 88 K, with the composition that was detailed in section II. However, the

regolith could come in at 330 K, as dry regolith from the water processing cycle. The

stoichiometric chemical equations for the minerals are seen in eq. A.1.

𝐶𝑎𝐴𝑙2𝑆𝑖2𝑂8(𝑠) + 8𝐹2(𝑔) ⇨ 𝐶𝑎𝐴𝑙𝐹5(𝑙) + 𝐴𝑙𝐹3(𝑙) + 2𝑆𝑖𝐹4(𝑔) + 4𝑂2(𝑔)

𝐹𝑒𝑇𝑖𝑂3(𝑠) + 3𝐹2(𝑔) ⇨ 𝐹𝑒𝐹2(𝑙) + 𝑇𝑖𝐹4(𝑔) + 1.5𝑂2(𝑔)

𝑀𝑔𝑆𝑖𝑂3(𝑠) + 3𝐹2(𝑔) ⇨ 𝑀𝑔𝐹2(𝑙) + 𝑆𝑖𝐹4(𝑔) + 1.5𝑂2(𝑔)

𝐹𝑒2𝑆𝑖𝑂4(𝑠) + 4𝐹2(𝑔) ⇨ 2𝐹𝑒𝐹2(𝑙) + 𝑆𝑖𝐹4(𝑔) + 2𝑂2(𝑔)

𝑀𝑔2𝑆𝑖𝑂4(𝑠) + 4𝐹2(𝑔) ⇨ 2𝑀𝑔𝐹2(𝑙) + 𝑆𝑖𝐹4(𝑔) + 2𝑂2(𝑔)

(𝐴. 1)

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The regolith is heated up to 770 K, which causes the silicon and titanium tetrafluorides to

become gaseous, while keeping the other metal fluorides liquid. Diatomic fluorine and oxygen

also come out as gases at that temperature. The gaseous products go on to the condensers, while

the liquid ones go to the reduction chamber.

The thermal energy required for the mixture, on a molar basis, is 4.566 MJ/mol. This was

calculated using eq. A.2.

�̅� = ∑ [(%𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛)𝑐𝑝𝛥𝑇]

𝑚𝑖𝑛𝑒𝑟𝑎𝑙𝑠

+ ∑ [(%𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛)𝛥ℎ]

𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠

(𝐴. 2)

B. Condensers

The condensers take in the gaseous products from the regolith furnace. There are four sets of

condensers. Each condenser uses a radiator towards space to cool the products inside. The first

condenser is at 520 K, which allows the titanium tetrafluoride to become liquid. That is sent to

the reduction furnace. Silicon tetrafluoride is condensed at 175 K and sent to the plasma

chamber. Diatomic oxygen is liquefied at 90 K in the third, and fluorine at 85 K in the fourth.

The oxygen is pumped towards the oxidation furnace, while the fluorine is pumped to the

fluorine tank.

C. Plasma Chamber

The plasma chamber takes in the liquid silicon tetrafluorine from the condenser at 175 K. It

is then heated up to 570 K, making it gaseous. Electrical energy is put into the gas, separating

the silicon and fluorine. With a bond energy of 541 kJ/mol [2], the total energy that is required

for this step is 4.328 MJ/mol of silicon.

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D. Reduction and Oxidation Furnaces

The reduction furnace takes in the liquefied metal fluorides from the regolith furnace and

titanium tetrafluorine condenser, and melted potassium from the potassium tank. These

reactants are at 770 K. The reactions that go on are seen in the stoichiometric equations in eq.

D.1.

𝑇𝑖𝐹4(𝑙) + 4𝐾(𝑙) ⇨ 𝑇𝑖(𝑠) + 4𝐾𝐹(𝑙)

𝐹𝑒𝐹3(𝑙) + 3𝐾(𝑙) ⇨ 𝐹𝑒(𝑠) + 3𝐾𝐹(𝑙)

𝐴𝑙𝐹3(𝑙) + 3𝐾(𝑙) ⇨ 𝐴𝑙(𝑠) + 3𝐾𝐹(𝑙)

(𝐷. 1)

The calcium and magnesium fluorides do not break apart in the reduction furnace. The

metals exit the system at this point, which could be used as a way to harvest titanium, iron, and

aluminum on the Moon.

The oxidation furnace takes in liquid oxygen from the oxygen condenser and the products of

the reduction furnace. Reactants are heated up to 790 K, which allow the stoichiometric

conditions in eq. D.2 to occur.

4𝐾(𝑙) + 𝑂2(𝑙) ⇨ 2𝐾2𝑂(𝑙)

𝐶𝑎𝐹2(𝑙) + 𝐾2𝑂(𝑙) ⇨ 2𝐾𝐹(𝑙) + 2𝐶𝑎𝑂(𝑠)

𝑀𝑔𝐹2(𝑙) + 𝐾2𝑂(𝑙) ⇨ 2𝐾𝐹(𝑙) + 2𝑀𝑔𝑂(𝑠)

(𝐷. 2)

The metal oxides exit the system at this point. They could stay as byproducts of the system,

or have the oxygen be used for potential fuel production. Potassium salts in the latter two

reactions are sent to the crucible, where they are electrolyzed at 950 K to make solid potassium

and diatomic fluorine. The metallic potassium gets sent back to the reduction furnace, while the

fluorine gas goes to a tank for later use.

E. Conclusion

Silicon can be harvested from the Moon, along with several other chemicals. The process

used mainly consisted of a furnace heating and separating the chemicals. Silicon tetrafluorine is

condensed and then put into the plasma chamber, where it is broken apart to create solid silicon.

This silicon could potentially be used for solar cell production or silane fuel production.

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Overall, the energy requirements for were fairly low. Assuming a power plant with an output

of 40 kW, a mole of silicon can be produced approximately every 3.706 minutes. However,

there were several approximations made, and several steps were not fully calculated. For future

work, the reduction and oxidation chambers and the crucible need energy values calculated.

References:

[1] Navrotsky, A., Hon, R., Weill, D. F., and Henry, D. J., “Thermochemistry of glasses

and liquids in the systems,” Geochimica et Cosmochimica, Vol. 44, 1980

[2] Landis, G. A., “Materials refining on the Moon,” Acta Astronautica, Vol. 60, 2007

[3] Seboldt, W., Lingner, S., et al, “Lunar Oxygen Extraction Using Fluorine,”

[4] Berman, R. G., and Brown, T. H., “Heat Capacity of minerals in the system,”

Contributions to Mineralogy and Petrology, 1985