AA_In_Situ_Lunar_Propellant_Production_And_Processes

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.

Julian Wang | 2
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.

Julian Wang | 3
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
2
O2(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

Julian Wang | 4
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

Julian Wang | 5
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.

Julian Wang | 6
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.

Eiji Shibata | 8
𝑐 𝑝 = 𝑘0 + 𝑘1 𝑇−
1
2 + 𝑘2 𝑇−2
+ 𝑘3 𝑇−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.

Eiji Shibata | 9
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

Eiji Shibata | 10
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.
𝐶𝑎𝐴𝑙2 𝑆𝑖2 𝑂8(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)
𝐹𝑒2 𝑆𝑖𝑂4(𝑠) + 4𝐹2(𝑔) ⇨ 2𝐹𝑒𝐹2(𝑙) + 𝑆𝑖𝐹4(𝑔) + 2𝑂2(𝑔)
𝑀𝑔2 𝑆𝑖𝑂4(𝑠) + 4𝐹2(𝑔) ⇨ 2𝑀𝑔𝐹2(𝑙) + 𝑆𝑖𝐹4(𝑔) + 2𝑂2(𝑔)
(𝐴. 1)

Eiji Shibata | 11
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.
𝐸̅ = ∑ [(% 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛)𝑐 𝑝 𝛥𝑇]
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.

Eiji Shibata | 12
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) ⇨ 2𝐾2 𝑂(𝑙)
𝐶𝑎𝐹2(𝑙) + 𝐾2 𝑂(𝑙) ⇨ 2𝐾𝐹(𝑙) + 2𝐶𝑎𝑂(𝑠)
𝑀𝑔𝐹2(𝑙) + 𝐾2 𝑂(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.

Eiji Shibata | 13
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

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