Bryan A. Palaszewski, Aloysius F. Hepp, Michael J. Kulis, and Donald A. Jaworske

Glenn Research Center, Cleveland, Ohio

Aerospace Fuels From Nonpetroleum Raw Materials

NASA/TM—2013-216492

February 2013

AIAA–2013–0591
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Bryan A. Palaszewski, Aloysius F. Hepp, Michael J. Kulis, and Donald A. Jaworske

Glenn Research Center, Cleveland, Ohio

Aerospace Fuels From Nonpetroleum Raw Materials

NASA/TM—2013-216492

February 2013

AIAA–2013–0591

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

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Grapevine, Texas, January 7–10, 2013
Acknowledgments

The following students are acknowledged for their contribution(s) to this work during summer fellowships at NASA Glenn 

Research Center: Pierce Gordon, Karen Guerrero-Medina, Blain McLeod, and Thomaz Santana. We also acknowledge technical 

discussions with, and input from our colleagues, Ms. Diane Linne, Mr. Anthony Colozza, and Dr. Kurt Sacksteder.

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subject to revision as analysis proceeds.
NASA/TM—2013-216492 1

Aerospace Fuels From Nonpetroleum Raw Materials

Bryan A. Palaszewski, Aloysius F. Hepp, Michael J. Kulis, and Donald A. Jaworske

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

Recycling human metabolic and plastic wastes minimizes cost and increases efficiency by reducing 

the need to transport consumables and return trash, respectively, from orbit to support a space station 

crew. If the much larger costs of transporting consumables to the Moon and beyond are taken into 

account, developing waste recycling technologies becomes imperative and possibly mission enabling. 

Reduction of terrestrial waste streams while producing energy and/or valuable raw materials is an 

opportunity being realized by a new generation of visionary entrepreneurs; several relevant technologies 

are briefly compared, contrasted and assessed for space applications. A two-step approach to 

nonpetroleum raw materials utilization is presented; the first step involves production of supply or 

producer gas. This is akin to synthesis gas containing carbon oxides, hydrogen, and simple hydrocarbons. 

The second step involves production of fuel via the Sabatier process, a methanation reaction, or another 

gas-to-liquid technology, typically Fischer-Tropsch processing. Optimization to enhance the fraction of 

product stream relevant to transportation fuels via catalytic (process) development at NASA Glenn 

Research Center is described. Energy utilization is a concern for production of fuels whether for operation 

on the lunar or Martian surface, or beyond. The term “green” relates to not only mitigating excess carbon 

release but also to the efficiency of energy usage. For space, energy usage can be an essential concern. 

Another issue of great concern is minimizing impurities in the product stream(s), especially those that are 

potential health risks and/or could degrade operations through catalyst poisoning or equipment damage; 

technologies being developed to remove heteroatom impurities are discussed. Alternative technologies to 

utilize waste fluids, such as a propulsion option called the resistojet, are discussed. The resistojet is an 

electric propulsion technology with a powered thruster to vaporize and heat a propellant to high 

temperature, hot gases are subsequently passed through a converging-diverging nozzle expanding gases to 

supersonic velocities. A resistojet can accommodate many different fluids, including various reaction 

chamber (by-)products. 

Nomenclature

C1 generic term for simple one-carbon (CO, CO2, CH4) compounds 

CH4 chemical symbol for methane 

CO chemical symbol for carbon monoxide 

CO2 chemical symbol for carbon dioxide 

∆G Gibbs free energy change 

∆H enthalpy change, related to Gibbs free energy change: ∆G = ∆H – T∆S 

F-T Fischer-Tropsch 

F-T(S) Fischer-Tropsch (synthesis)

H2 chemical symbol for molecular hydrogen 

H2O chemical symbol for water

ISRU In Situ Resource Utilization

PAG plasma assisted gasification

Syn-gas synthesis gas: Hydrogen/carbon monoxide gaseous mixture, typical ratio is 2-3:1 

WTE waste-to-energy technologies 
NASA/TM—2013-216492 2

Introduction and Background

As NASA moves forward with plans to support human exploration of the solar system, a critical need 

arises to supply basic raw materials such as food, other life support (water and oxygen), energy and 

propellants, and other materials (radiation shielding, clothing, etc.) (Ref. 1). Thus far, NASA has relied on 

materials brought from Earth; this conservative approach, while quite costly, minimizes risk to crews. As 

mankind ventures farther from Earth and for greater periods of time, it becomes imperative to develop 

technologies and mission architectures that utilize local resources such as Lunar regolith or Martian 

atmosphere, referred to as in-situ resource utilization or ISRU (Ref. 2). For a terrestrial analog, efficient 

utilization of raw materials and energy often involves recycling: re-use or recovery of hydrocarbons (or syngas, vide infra) from waste plastics (Ref. 3). These relate to in-flight utilization of waste and trash to produce 

essential materials such as water, fuel(s), and oxygen. Lighter elements such as oxygen, nitrogen, and 

particularly carbon and hydrogen are either not readily available or strongly bonded to metal or metalloid 

atoms in rocks and minerals (especially oxygen) requiring significant energy for extraction (Ref. 4).

We have previously discussed specific technologies (catalysts, reactor technologies, solar energy 

and other green power sources) being developed at NASA Glenn Research Center (GRC) and through 

the efforts of partnering contractors and university collaborators to utilize nonpetroleum raw materials 

(Ref. 5). In this paper, we discuss the problem at hand, recycling waste and trash into useful raw 

materials, from a systems level with a view towards integration of subsystems, energy utilization, 

human factors, mission applicability, and opportunities for insertion of such disruptive technologies, to 

enable specific missions.

The organization that we will adopt is illustrated by Figure 1. The major concerns and issues that we 

will address are broken down into three areas that define the organization of the paper: preprocessing, 

waste and trash breakdown, and re-formation of products. Balance-of-system issues, energy efficiency, 

and related concerns will be addressed in a follow-on section. Insights to be gained from modeling and 

theoretical consideration of components, unit operations, and subsystems will then be discussed. We 

conclude with addressing potentially enabling new approaches and technologies for missions farther from 

Earth.

Figure 1.—Overview of generic process flow diagram for a Trash-to-Supply Gas system. 
NASA/TM—2013-216492 3

Overview of Major Processing Subsystems 

The general flow diagram above (Figure 1) serves as a platform for a discussion of technological road 

blocks and technical solutions. This is a top-level discussion with more in-depth analyses in following 

sections. 

Rationale and Goals for Modeling of Waste Materials Processing

In the Advanced Exploration Systems—Logistics Reduction and Repurposing (LRR) program, 

modeling of various waste conversion processes is underway. Modeling will lead to an optimized flow 

and production rate for each of the processes. With this information, a down-select can be made amongst 

competing conversion methods. Many additional factors have been identified including waste production 

rate, process temperature and pressure (which can affect crew safety and lengthen the processing time), 

the unusable matter that may accumulate in the process components, and the time needed from the 

astronauts for charging and cleaning the process components. The complexity, safety, and the time scales 

for processing the waste materials may have a very strong influence on the selection process. 

Stage 1: Production of Supply Gas and/or 

Hydrocarbons Via Waste-to-Energy Technologies

Plastic solid waste can be recycled or utilized via four major classes of processes, designated as 

primary to quaternary, respectively, as physical and/or chemical breakdown increases. These include

recycling (primary) and mechanical recovery (secondary) that are not discussed further, chemical 

recycling (tertiary), and energy recovery (quaternary) (Ref. 3). Commercial processes including 

incineration (to directly produce electricity) (Ref. 6) and biologically-assisted digestion (Ref. 7) are 

outside of the scope of this discussion and will also not be addressed. 

There have been numerous excellent reviews that summarize and analyze in detail the various 

technologies, products, energy balance, and economics of several mainstream waste-to-energy (WTE) 

technologies. Several example tertiary or quaternary processes include: chemical recycling via pyrolysis 

(thermal or catalytic cracking) to produce mostly higher hydrocarbons; gasification (thermal cracking in 

air and/or steam) to produce CO2 and syn-(thesis) gas (CO and H2) and small amounts of oxygen, water, 

and methane, sometimes called producer gas (Ref. 8); and plasma-assisted gasification (also a quaternary 

process) (Ref. 3) that relies on a very high temperature plasma torch to produce primarily syn-gas.

A comparison of relevant technical details as well as energy utilization and the infrastructural 

requirements that determine suitability for a variety of space missions is detailed in Table I and includes 

thermal cracking, flash cracking, steam reformer/Sabatier, and plasma assisted/Fischer-Tropsch (F-T)

(Ref. 9) reactors. The processes previously discussed (Ref. 5) being developed by RES Polyflow (tertiary) 

and Pioneer Astronautics (quaternary) are typical of these technologies that use heat in a variety of 

environments to break down polymers into a product soup. The Pioneer Astronautics process involves an 

integrated Sabatier process to produce methane but requires a hydrogen source. The rather complex 

plasma-assisted gasification process relies on quite high temperatures with a significant balance-ofsystem to recover thermal energies.

While it is difficult to make direct comparisons regarding scale of infrastructure required, a typical 

plasma system requires significant balance-of-system hardware in order to enable self-sustaining 

electrical power (Ref. 9). An energy-efficient system will include reusing otherwise wasted thermal 

energy from stage 1 products (i.e., syn-gas at > 1000 °C) to drive turbines to generate electricity. This 

would then be followed by a F-T operation to convert syn-gas (CO and hydrogen) into a product soup of 

saturated and unsaturated hydrocarbons as well as some oxygenated products such as alcohols of C5 to 

C20 or so with some waxy materials. The balance-of-system technology requirements drive up the 

complexity (and cost) (Ref. 9), and minimize suitability for space applications; in fact, these issues 

eliminate PAG technology from further consideration.
NASA/TM—2013-216492 4

TABLE I.—COMPARISON OF RELEVANT METRICS FOR TERTIARY AND QUATERNARY WTE PROCESSES

Process

(class)

Temperature 

range

Approximate

pressure 

(MPa)

Product 

output

Technical

complexity

Approach to energy 

efficiency or 

utilization

Infrastructure

needs

Thermal 

cracking 

(tertiary)

400 to 450 °C 4 to 6.9 C1-C5 and mostly 

> C6 hydrocarbons Low

Burning C1-C5 

fraction supplies 80% 

energy

Large

Flash cracking 

(tertiary or

quaternary) 

400 to 600 °C 0.1 to 1.0

Flexible: C1-C10 

depending upon T, 

P, catalyst

Medium

Designed to be low 

energy; potential solar 

energy use

Modest 

Steam reforming 

(quaternary) ~850 °C 0.3 to 0.7 Syn-gas (CO + 

H2), CO2, CH4

Medium

Balancing endothermic 

and exothermic 

reactions

Modest

Plasma assisted 

gasification 

(quaternary)

Typically

5,000 °C 0.1 Primarily syn-gas High

Recovery of thermal 

energy from > 1000 °C 

syn-gas stream

Significant

The three remaining technologies to be compared include a chemical recycling technology being 

developed by (among others) an Akron, Ohio-based start-up company (RES Polyflow) (Ref. 5), flash 

cracking or pyrolysis (simple or catalytic) (Ref. 10), and an SBIR-funded steam reforming process 

(Ref. 5). The RES Polyflow pyrolytic process is quite simple but scalability may be an issue (Ref. 5). A 

flash cracking reactor is quite flexible from a process perspective, and is represented by a system 

discussed in the literature; the final product mix can be tailored depending upon the temperature and 

pressure as well as the presence of a catalyst (Ref. 10). The lower energy demand for this process can be 

met by solar thermal and photovoltaic sources, as discussed below. Pioneer Astronautics has combined 

two unit operations that work in tandem to produce methane and oxygen. The process is quite scalable 

and available in 2013 as a prototype unit from a Phase II program. One issue is the need for an external 

hydrogen source for a Sabatier reactor: this would most likely come from splitting water (Ref. 4). Solar 

energy as well as efficient thermal energy utilization will be required if this technology is to be 

considered as a serious flight candidate.

In summary, we have briefly compared four WTE processes to assess their suitability for ISRU: 

chemical recycling via pyrolysis or flash cracking, combined steam reformation/Sabatier, and plasma 

assisted gasification followed by (likely) F-T synthesis. Because of the moderate cost, scalability for 

space applications, energy utilization and technology heritage of the Sabatier process, two tertiary and one 

quaternary WTE steam processes appear to have potential for further development. There are still 

balance-of-system issues remaining such as the need for hydrogen to drive the hydrocarbon production 

processes, safety concerns, and integration of the candidate technology into the mission architecture. The 

pyrolysis process, a tertiary chemical recycling methodology is technically quite straightforward but has 

some scaling issues due to the impact of dimensions on process kinetics. Plasma assisted gasification at 

the trade-study analysis level (Ref. 9) appears to present some fundamental scaling issues that preclude 

this technology from being further considered for LRR applications. 

Stage 2: Processing Supply Gas Into Fuels 

A brief consideration of C1 and related chemistry provides a context for beginning a discussion of the 

relevant processing options, systems issues and related technologies that will be addressed below

(Ref. 11). C1 chemistry refers to a number of reactions including those that convert simple carboncontaining gases from “Stage 1” processing of trash and waste (mostly carbon dioxide or carbon 

monoxide) into green and/or ultra-clean aerospace fuels. The enthalpy (∆H) change which is related to

total Gibbs free energy change of the reaction (∆G = ∆H – T∆S) is an indication of whether the 
NASA/TM—2013-216492 5

transformation releases energy (exothermic, negative ∆H) or requires energy input (endothermic, positive 

∆H). 

Depending upon the application or desired products, combining endothermic and exothermic 

reactions minimizes the energy that must be added to the overall system. Several endothermic reactions 

are available to produce synthesis gas (1) or supply gas (3) from methane, for example. Reactions (1) and 

(3) are related by combining reaction (1) with the slightly exothermic water gas shift (WGS) reaction (2). 

These are simplified “model systems” that stand in for carbon-containing waste and/or trash raw materials 

as discussed above.

CH4 + H2O => CO + 3H2 ΔH = +49 kcal/mole (1) 

CO + H2O => CO2 + H2 ΔH = –9 kcal/mol (2) 

CH4 + 2 H2O => CO2 + 4 H2 ΔH = +40 kcal/mol (3) 

In “stage 2,” three exothermic reactions are available to produce hydrocarbons from CO or CO2 and 

hydrogen. The Sabatier process (reaction (4)) is the reverse of reaction (3) and is typically catalyzed by 

Ni, Ru, or Rh on oxide supports; and has been a candidate technology for space habitat air revitalization

(Ref. 12). In fact, a commercial Sabatier reactor was delivered to the International Space Station by 

mission STS-131 on April 5, 2010. Methanation (Refs. 13 and 14) (5) and the thermodynamically-related 

Fischer-Tropsch synthesis (FTS) (Refs. 15 and 16) (6) are also reverse reactions of steam reforming of 

methane (1); these reactions are catalyzed by Ni (methanation only), Co, Fe, and Ru on various oxides. 

All three reactions are quite exothermic and can be used to improve the energy balance in an overall 

system when producing fuels from waste or trash. The recycler/heat exchanger in Figure 1 is a “black 

box” with several unit operations including a series of systems for the transfer of thermal energy from 

exothermic processes in stage 2 to provide heat to stage 1 (Ref. 17). Unlike methanation, F-T catalysis 

produces a range of products, both pure hydrocarbons and oxygenated products, depending upon the 

catalyst and reaction temperature and pressure.

CO2 + 4 H 2 => CH4 + 2 H2O ΔH = –40 kcal/mol (4) 

CO + 3 H 2 => CH4 + H2O ΔH = –49 kcal/mol (5) 

 n CO + (2n + 1) H2 => CnH(2n+2) + n H2O ΔH = –49 kcal/mol (n = 1) (6) 

Catalytic Wet Air Oxidation 

Catalytic wet air oxidation (CWAO) is an attractive method for the treatment of waste streams that are 

too dilute to incinerate (< 40 percent) (Ref. 6) and too concentrated for biological treatment (> 1 percent) 

(Ref. 7). The method involves the oxidation of organic substances in an aqueous suspension by means of 

air at elevated temperatures and pressures. Heterogeneous catalysts are used to oxidize refractory 

compounds in the aqueous phase and produce desired products in the gas phase. Typical conditions range 

from 200 °C and 2 MPa to 320 °C and 20 MPa. 

Catalytic methods such as wet air oxidation or photocatalysis (Ref. 5) may serve as the basis for a 

system of waste and trash processing, or could be used as part of a suite of supporting or auxiliary 

technologies to boost the hydrogen content of various gas streams, or in using energy-efficient methods to 

reduce the volume and/or increase the utilization of solid or by-product streams. In CWAO, a supported 

metal such as Ru serves as the catalyst for the decomposition of polymers and produces mostly CO2 and 

H2, and offers additional beneficial functionality. 

GRC is currently investigating CWAO as a method to produce methane from waste simulant using 

various catalysts in a one step process. Kulis et al., have demonstrated that significant amounts of 

methane can be produced using CWAO of polyethylene terephthalate with Ru as a catalyst (Ref. 18). In 
NASA/TM—2013-216492 6

addition, concentrated solar energy is being investigated as a thermal source, and photocatalysis is being 

investigated as a supplementary method that would be used to split water in order to produce additional 

hydrogen for methane production. Though CO2 formation and departure from the catalytic surface is 

crucial, the kinetics may be dictated in part by such factors as surface tension, surface roughness, and 

gravity, where a practical understanding of bubble formation and departure may guide design of the 

CWAO process under reduced gravity conditions. 

Balance-of-System Concerns and Resistojet Technology for Gas Utilization

There are a variety of issues related to the integration of the re-use technologies with the rest of the 

spacecraft. This section addresses some of those concerns and highlights some GRC capabilities, 

facilities, and expertise in this area. 

Energy Efficiency Issues

Energy must be considered at a system level, where heat is added to the system for driving the 

endothermic reactions and heat is harvested from the exothermic reactions for improving overall system 

efficiency. Overall heat rejection must also be considered. Electric power derived from spacecraft 

photovoltaics is the most likely source of process energy, including the electrical energy needed for 

running pumps and splitting water. In certain cases there may be merit in considering solar thermal to 

augment the process energy, particularly for the stage 1 endothermic reforming reactions ((1) and (3)). 

Given a Sun-tracking requirement, solar thermal augmentation may best be suited for treatment facilities 

that are anchored to the lunar or Martian surface rather than part of a spacecraft rotating about its axis for 

thermal control. Numerous studies have been completed on the various types of solar thermal concepts 

available for possible use, ranging from rigid structures having facets with high quality optical surfaces 

and concentration ratios of the order of 8000:1, to concepts that tout light weight inflatable structures, 

light pipes, and trough systems with lesser efficiency and lesser concentration ratios (Refs. 19 and 20). 

Perhaps the ideal concept is one where a Sun-tracking trough style solar concentrator anchored to the 

lunar surface heats the carbon-containing waste and/or trash raw materials in a carrier stream of oxidizer, 

optimized such that the residence time in the trough enables the heating of the mixture to the desired 

temperature prior to entering a reaction chamber. Small diameter thick walled tubing coated with a high 

absorptance coating and located at the focus of the trough would enable high pressure high temperature 

operation, particularly for the CWAO process. 

Initial research is now underway to consider solar thermal augmentation for process energy and 

selected on-Sun testing has been performed on a test article that simulates a stainless steel tube at the 

focus of a high efficiency concentrator. The concentrator utilized here is of a Cassegrainian design and is 

described in detail elsewhere (Ref. 21). The concentrator was located on a platform equipped with Suntracking hardware. The test article consisted of a thick-walled stainless steel tube which offers the 

advantage of pressure containment, however, thermal conductivity is a problem. The use of a copper 

jacket to distribute the heat around the perimeter of the tube was considered here, and the stainless steel 

tube was purposely surrounded by a press-fit copper jacket. The test article was instrumented with 

thermocouples and was purposely surrounded by multilayer insulation to minimize heat loss. In practice, 

the tubing would be sized to accommodate the flow of carbon-containing waste and trash, and the mass 

rate of flow along with any endothermic reactions would serve to draw heat away from the region 

illuminated by the concentrator. Here, the test article here was open to the air. The copper jacket was 

painted black, to enhance solar absorption and Figure 2 summarizes the fabrication of the test article, 

reveals the concentrated sunlight as a spot on the surface of the black paint, and shows an excerpt of the 

on-Sun heating data. 
NASA/TM—2013-216492 7

Figure 2.—Copper-jacketed stainless steel test article, (a) painted, instrumented, and enclosed in

insulation, (b) under concentrated sunlight (the glare is removed by viewing through a filter), and (c) an 

excerpt of the on-Sun data. Note: the majority of the energy impinging on the copper is within a small 

spot. 

Conclusions from the on-Sun testing are summarized here, briefly. The test article had been on-Sun

for several minutes, though the day was partly cloudy and the excerpt shows the Sun beginning to 

impinge on the test article again after being cloud-covered. Thermocouples 1 and 2 were located on one

side of the copper jacket, 5 and 6 were on the opposite side (6 malfunctioned), 3 and 4 were on the back, 

and 7 and 8 were on the exterior of the stainless steel tube some distance from the copper jacket. 

Thermocouple 9 was located inside the stainless steel pipe. Under ideal system-level operation, the solar 

thermal augmentation would ideally reach steady state, such that the energy entering the system from the 

impinging sunlight just equals the amount of energy departing the illuminated region in the form of mass 

flow and endothermic reactions. The results from the on-Sun testing revealed 1) the copper jacket was 

only partially successful at distributing the heat around the tubing and 2) the interior temperature lagged 

behind due to the thick-walled nature of the stainless steel. As can be seen, it takes many tens of minutes 

for the test article to heat under the influence of the concentrated sunlight. All these observations suggest 

that the ideal system will need an adequate solar tracking system as a minimum, a trough concentrator is 

likely preferred in order to avert the need for distributing heat utilizing a copper jacket, and perhaps the 

most important need is a well anchored solar concentrator. 

The shuttling of excess heat from exothermic reactions to power endothermic reactions may be 

possible utilizing conventional heat pipes or heat exchangers designed to deliver such energy from one 

location to another. However, a temperature gradient is needed in such systems, the processing 

temperatures may limit such a concept, and losses are inevitable. Heat pumps are available to shuttle 

excess heat from one location to another, with the added feature of increasing the temperature at the 

outlet. Of course, heat pumps represent an electric load on the system and must be incorporated into any 

energy balance study. Perhaps the ideal concept is one where sufficient electrical power is provided to the 

reactor into which reactants are provided to overcome the sensible heat of the reactor mass and the energy 

needed to initiate stage 1 endothermic reactions during transient start up operations, and electrical power 

is throttled back as new solar thermal heat sources are brought on line during steady state operation. 

Repeated transient and steady state operation should be expected for either batch or continuous 

processing, though far fewer cycles would be anticipated for a continuous reactor. 

(a)

(b)

(c)
NASA/TM—2013-216492 8

Methods of Handling Heteroatom Impurities

Upon consideration of the recycling of trash and waste materials, a technological hurdle that must be 

surmounted is separation and storage of impurities. In our previous paper, we discussed specific 

technologies being addressed by several co-authoring organizations to remove metals, nitrogen, sulfur, 

and halogens present in packaging materials and human wastes (Ref. 5). If not removed, these materials 

pose potential health hazards to the crew and destructive impact on various components and subsystems 

in the closed waste and trash reprocessing system by material corrosion and catalyst poisoning.

There are basically two approaches to the removal of impurity compounds (oxides, hydrides, sulfides, 

halocarbons): methods involving little or no power that consist of adsorbing and long-term storage of 

impurities, possibly including recharging or re-activation of sorbant materials, or active methods 

involving solar, electrical and/or thermal energy to decompose impurities to simpler species, one or more 

of which can be re-used such as water, oxygen, or hydrogen. Table II summarizes and compares several 

methods to recycle a variety of impurity materials from gas streams of both types.

Of the six methods compared in Table II, the first two methods are the most problematic owing to 

potential mechanical or biological impact on the environment, particularly a closed system. They are also 

difficult to integrate into a processing system (Figure 1). The four methods that primarily rely on catalytic 

processes and/or adsorbant materials use lower amounts of a likely limited power supply and could be 

more readily integrated into a recycling function in a processing system. Utilization of solar energy 

directly or indirectly through electricity generated by photovoltaics would also have advantages in the 

energy balance as discussed above. 

TABLE II.—COMPARISON OF METHODS FOR SEPARATION OF IMPURITIES FROM GAS STREAMS

Process Energy Requirements Impurity(ies) Products Comment(s) Literature

references

Centrifugal 

> 30,000 rpm

Electrical—large H2S and CO2 in 

CH4

H2S and CO2

removed

Not suitable for space 

station

22

Biofilters in aqueous media: 

bacterial, wood, textile, 

sand, or combination

Electrical—minimal NH3 (and H2S) 

in air

NH3 (and 

H2S) 

removed

Must be crew biocompatible23 to 25

Filtering by zeolites Thermal—minimal to 

reactivate

Volatile organic 

compounds 

(VOCs) in air

VOCs 

removed

May require heating to 

reactivate active 

adsorbants

26

Oxide catalyst & 

Metal/zeolite filter

Thermal – moderate for 

Temp > 400 °C

Carbon, VOCs, 

NOx, and CO in 

air

CO2 and N2 Useful process for 

recycling by-products

27

Photocatalysis 

TiO2/aluminum silicate 

fiber

Solar energy with UV 

photons 

SO2, NO and 

Hg0 in air

Pollutants 

removed

Passive relies on 

ambient solar 

irradiation

28

Catalytic H2S splitting by 

solar thermal, 

photochemical, 

electrochemical, or 

combination of methods

Solar energy to provide 

photons, heat, or 

electricity; geared 

towards renewable 

energy usage 

H2S H2 + sulfur Very flexible family of 

methods; details 

dependent upon 

catalyst and available 

energy

29 and 30

Resistojet Propulsion Option

As part of the LRR program, the production of fluids (liquids and gases) from the waste materials aboard 

a space vehicle may be many and varied. These fluids can be used in a propulsion option called the 

resistojet. The resistojet is a form of electric propulsion known as an electrothermal thruster, where power is 

provided to the thruster to vaporize and heat the propellant to high temperature, and then the hot gas is 

passed through a converging-diverging deLaval nozzle, expanding the gas to supersonic velocities (Refs. 31

to 37). The heating of the fluid provides much higher exhaust velocity (or specific impulse) than if the fluid 

were used in the cold gas mode without heating. The resistojet can accommodate many different fluids.
NASA/TM—2013-216492 9

Figure 3.—Diagram of resistojet propulsion technology from 1970s (Refs. 31 and 32). 

1970s Resistojet Research

During the study phase of several space station concepts, the idea of using the waste fluids from the 

space station was considered. Resistojet propulsion was investigated and the extensive design showed 

some benefits and drawbacks. Figure 3 shows an overall configuration of the resistojet system. The 

integration of the propulsion system with the station layout of the components is noted. The design model 

of a space station resistojet system uses environmental control life support system (ECLSS)-produced 

biowaste gases (CO2 and CH4) as propellants (Refs. 31 and 32) on the station. The gases are used 

separately, and water is employed as a propellant supplement. The system minimizes resupply 

requirements, furnishes a useful method of biowaste disposal, minimizes contamination, and permits 

near-zero acceleration, reducing the thruster disturbances on the space station. 

The resistojets have a thrust level of 0.1 Newton (0.025 lb-f) and are operated in a high-duty-cycle 

mode (25 to 80 percent) for space station orbit-keeping and control moment gyroscope (CMG) 

desaturation. The thrusters are mounted in modules. Four modules are located at each end of the space 

station, and the gas storage tanks are housed in the pressurizable forward compartment (Ref. 31). 

The major components of the system are compression pumps, heat exchangers, accumulators, 

propellant tankage, thrusters, and the necessary valves and switches for control and checkout. The system 

weighs 259 kg (570 lb-m), occupies a volume of 2.78 m3

 (100 ft3), and requires 100 to 400 W of electrical 

power (Ref. 31).

The space station biowaste resistojet propulsion system was required to operate automatically, with 

little or no crew participation except for maintenance and repair, etc. Thus, system control was required to 

acquire stabilization and attitude control (S&AC) data, calculate impulse requirements, determine 

propellant utilization and provide the necessary operating commands; typically once per orbit. 

Furthermore, the interface with the ECLSS (propellant supply) and the high duty cycle and usage (25 to 

80 percent each orbit) makes operational control significantly different and more complex than 

conventional systems (Ref. 31). 
NASA/TM—2013-216492 10

1980s Resistojet Research

As space station concepts moved toward reality, many propulsion options were considered. Resistojet 

propulsion seemed a natural choice in that many gases were produced by the many systems on the 

planned Space Station Freedom. A detailed set of all waste gases and fluids were identified: from life 

support systems, cooling systems, and cryogenic experiment gases the resistojet could use those fluids for 

space station orbital reboost (Ref. 33). Extensive engineering model testing was conducted and detailed 

requirements were developed for resistojet propulsion for Space Station Freedom. Figure 4 depicts the 

orbital replacement units (ORUs) for the resistojet propulsion system. Figure 5 shows the resistojet 

engineering model tested by NASA for the space station. Extensive testing of the engineering model 

engine with the various waste gases helped identify the platinum and yttria alloys needed to survive the 

oxidizing and reducing environments (Refs. 34 to 37). 

Figure 4.—Resistojet orbital replacement unit (ORU, Ref. 33). 

Figure 5.—Resistojet thruster engineering development model (Ref. 34). 
NASA/TM—2013-216492 11

Figure 6.—Resistojet propulsion performance, experimental 

data (Ref. 35). 

Figure 6 shows the numerous bio-waste propellants that the resistojet can use. Their engine specific 

impulse and other performance parameters (heater power level, thrust levels, etc.) are noted. The cold gas 

specific impulse for the hydrogen resistojet is approximately 380 sec at a power level of 329 W. In 

comparison, the cold gas hydrogen performance (with no heater power) is 250 sec (Ref. 35). 

Gateway Propulsion Needs

While the Gateway vehicle is still at the preliminary design stage, several options have been discussed. 

Figure 7 depicts a configuration and provides a mass summary of two such preliminary designs for 60 and 

500 day stays in space. These mass estimates show all of the vehicle subsystems but not the required 

propulsion system masses. Based on the final mission design, additional masses for the propulsion system 

dry mass (tankage, feed systems, etc.), the required propellant mass and other vehicle interfaces will be 

required. 

LRR and Resistojet Propulsion 

While the planned human space vehicles are not as ambitious as large space stations, the propulsion 

system use of the waste gases on such future vehicles can still be advantageous. Aboard the Gateway 

(Earth-Moon libration point station) or other space vehicle a heat melt compactor (HMC), or other waste 

processing technologies, operated by the astronauts would process the waste materials. The fluids that 

may be produced would be water, methane, methanol, oxygen, etc. Past resistojet propulsion studies and 

experiments have shown that the resistojet can use a wide range of fluids and also mixtures of those 

fluids. By eliminating the need to separate specific fluids, the use of the waste material fluid products can 

be simplified significantly. 

ASTERISK INDICATES COLDGAS SPECIFIC IMPULSE.
NASA/TM—2013-216492 12

Figure 7.—Notional design for Gateway vehicles (Ref. 38). 

Conclusion

Plastic solid waste can be recycled or utilized via four major classes of processes with increasing 

physical and/or chemical breakdown. Relevant to this discussion are chemical recycling (tertiary) and 

energy recovery (quaternary). Depending upon desired products and mission, combining endothermic and 

exothermic reactions minimizes energy that must be added to the overall system. A technological hurdle 

that must be surmounted is separation and storage of impurities. On future space vehicles, an array of 

potential waste fluids has been identified. These fluids may be produced using innovative processes such 

as heat melt compactors, catalysts, reactor technologies, solar energy, and other green power sources, for 

solid waste processing. Other than fluids for life support and cooling, many propulsion requirements can 

be met with the more energetic fluids. The delta-V and other propulsion requirements can use the 

captured waste fluids effectively in a low-thrust rocket engine such as a resistojet. This form of electric

propulsion can accommodate many types of fluids or mixtures of fluids. While there are many processes 

to consider, the final selection will be based on a combination of efficiency, complexity, astronaut safety, 

and consumption of the astronauts’ valuable time in space.

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