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This topic is offered for NewMars members who might like to contribute to a collection of knowledge about SILOX.
Please note that the term "Silox(r)" is trademarked by a commercial entity for an entirely different purposes.
We are indebted to Void for the research that revealed the NASA studies that are the focus of this topic.
Until recently, Silicon would not have been considered as a serious candidate for spacecraft propulsion.
In Post #3, we will begin a series about the invention of nanoscale structures suitable for use of Sicicon as a propellant.
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This post is reserved for an index to posts that may be contributed by NewMars members.
Index:
Post #3: Executive Summary of SILOX
Post #4: List of References
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I asked Gemini to help with preparation of a series on the potential for use of Silicon with liquid Oxygen as a spacecraft propellant.
Overview of SiLOX (Silicon-Liquid Oxygen) Propellant
Historically, elemental silicon was not considered a viable rocket fuel outside of solid composite additives. Unprocessed silicon burns poorly in macro scale forms, while unregulated nanoscale or porous silicon mixed with liquid oxygen (LOX) often caused erratic, explosive shock detonations rather than the stable, controlled burn required for rocket thrust.
Recent research led by teams at NASA’s Kennedy Space Center (KSC) demonstrated a proof-of-concept for Silicon-Liquid Oxygen (SiLOX) as a controllable hybrid rocket propellant system.
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Key Motivations: In-Situ Resource Utilization (ISRU)
The driving force behind SiLOX research is planetary and lunar surface operations:
Abundant Raw Material: Regolith across airless bodies is rich in silicon dioxide (SiO2) and metal silicates.
Byproduct Synergy: Processing regolith via methods such as Molten Regolith Electrolysis (MRE) or Carbothermal Reduction (CaRD) extracts pure oxygen for life support and oxidizer needs. This leaves behind refined elemental silicon as a primary byproduct.
Closed-Loop Logistics: Instead of discarding refined silicon as industrial waste, turning it into fuel provides a locally sourced propellant for hoppers, landers, or surface energy storage systems.
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The Nanoscale Breakthrough: Passivation & Control
The primary technical barrier to using silicon in liquid-oxidizer combustion was managing its violent, unpredictable reaction rates. NASA researchers overcame this by manipulating silicon at the nanoscale and introducing thermal pre-treatment:
1. Nanoscale & Porous Silicon Morphology
Using nano-silicon particles and porous silicon structures dramatically increases the active surface area per unit mass. High surface area is essential for rapid ignition and sustaining combustion when exposed to cryogenic liquid oxygen.2. Thermal Passivation (Muffle Furnace Pre-treatment)
To prevent spontaneous, uncontrollable detonation upon contact with LOX, the nano-silicon undergoes controlled pre-treatment in a muffle furnace at tailored temperatures and durations.
This process forms a thin, precise oxide layer on the nano-particles/pores.
By adjusting the thickness and characteristics of this passivating layer, engineers can tune and dial in the burn rate and regulate energy release.
3. Hybrid Rocket Architecture
SiLOX operates primarily as a hybrid propellant system:
Fuel Phase: Solid porous silicon discs or consolidated nano-silicon matrices.
Oxidizer Phase: Gaseous or Liquid Oxygen (GOX/LOX) injected over or through the silicon matrix.
Advantages and Technical Considerations
Primary Advantage: High local availability on low-gravity bodies (zero Earth-launch mass dependency).
Combustion Products: Forms solid silicon dioxide (SiO2) glass/ash alongside gaseous thermal expansion.
Engine Design Impact: Exhaust particles require nozzle designs that resist abrasive particulate buildup or erosion.
Storage & Stability: Passivated nano-silicon discs are inert during transit until exposed to oxygen flow in the combustion chamber.
Summary:
SiLOX bridges the gap between planetary geology and propulsion physics. By converting abundant regolith byproducts into passivated, high-surface-area nanostructures, researchers have demonstrated that silicon can transition from a volatile or unreactive material into a tunable, high-energy hybrid rocket fuel.
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I asked Gemini to show us the references we can open to learn more about SILOX.
Primary Literature & Reference Material: SiLOX Propellant
For readers interested in diving deeper into the technical data and research papers underpinning the Silicon-Liquid Oxygen (SiLOX) breakthrough, the following key reports and papers provide foundational context:
1. NASA KSC Proof-of-Concept & Passivation Research
Primary Report: "Making Rocket Fuel from Moon Rocks: Silicon-Liquid Oxygen (SiLOX) as a Hybrid Propellant"
Authors: K. Engeling, T. Gibson, N. Azim, G. Lunn, and L. Somervill
Source: NASA Technical Reports Server (NTRS), Document ID 20240000517 (Published January 2024).
Key Insight: Documents the experimental proof-of-concept conducted at NASA's Kennedy Space Center (KSC). Demonstrates how thermal pre-treatment in a muffle furnace creates a controlled oxide passivation layer on nano-silicon, allowing engineers to dial in burn rates and avoid uncommanded shock detonation when mixed with liquid oxygen.
2. Upstream Extraction Technologies (ISRU)
To understand where the raw silicon feedstocks originate on low-gravity bodies like the Moon or Phobos, review research into regolith reduction:
Molten Regolith Electrolysis (MRE): Research pioneered by NASA and commercial partners (such as Boston Metal and Blue Origin) demonstrates passing electric current through molten regolith (1600°C+) to yield pure oxygen gas at the anode, while leaving liquid elemental silicon, iron, and titanium at the cathode.
Carbothermal Reduction (CaRD): NASA Johnson Space Center (JSC) CARBION and CaRD testing reports detail using solar thermal heat and methane to extract oxygen from silica-rich silicates (SiO2), leaving behind refined silicon byproducts suitable for conversion into fuel grain matrix structures.
3. Historical Context: Metal & Metalloid Hybrid Fuels
Wickman Spacecraft & Propulsion Company (WSPC): Early studies dating back to the late 1980s evaluated lunar soil additives (including raw sulfur, phosphorus, and aluminum) for hybrid propulsion, highlighting why unprocessed macro-scale metalloids were historically difficult to burn reliably compared to modern nanoscale structures.
Summary for Forum Discussion:
These references establish that SiLOX is not merely theoretical; physical test firings at KSC confirm that nanoscale passivation transforms abundant regolith extraction waste into a viable, controllable propellant.
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In this post, I asked Gemini to think about a plan for this series. I'd like to (at least attempt to) show how we can replace Dr. Zubrin's original plan for Mars expeditions with one based upon the abundant Silicon and Oxygen resources available in orbit around Mars.
Series Objectives: Rethinking Mars Architecture with Phobos SiLOX
For decades, the standard paradigm for Mars surface return and Earth-Mars transit—pioneered by Dr. Robert Zubrin's "Mars Direct" architecture—relied on shipping liquid hydrogen from Earth to Mars to synthesize methane (CH4) via the Sabatier reaction.
With recent breakthroughs demonstrating that silicon can be passivated into a controlled, high-energy hybrid fuel (SiLOX), we can explore a complete architectural shift: establishing automated SiLOX fuel depots on Phobos and Deimos.
This ongoing series will analyze the engineering, thermodynamic, and logistical requirements to make a Martian-moon propellant hub a reality.
1. Infrastructure & Processing on Phobos / Deimos
Unlike Earth or Mars, Phobos has a negligible escape velocity (~11 m/s), making it an ideal low-gravity logistics station. To prepare SiLOX propellant on-site using solar power, a processing facility must integrate four key sub-systems:
Extraction & Reduction Systems: Solar-thermal or photovotaic-driven Carbothermal Reduction (CaRD) or Molten Regolith Electrolysis (MRE) units to separate raw regolith silicates (SiO2) into oxygen gas and elemental silicon.
Nanoscale Sintering & Porosity Milling: Machinery to process refined silicon into high-surface-area porous matrices or nanostructured fuel grains.
Muffle Passivation Furnaces: Thermal pre-treatment chambers to grow precise nanometer-thick oxide skins on the fuel matrices, ensuring stable, non-detonative burn rates.
Cryogenic LOX Liquefaction & Storage: Solar-powered cryo-coolers to chill and store liquid oxygen in shaded craters or subsurface holding tanks.
2. Main Propulsion & Nozzle Dynamics: Running Hot & Full Thrust
A major critique from classical propulsion engineers regarding metalloid fuels is slagging and nozzle contamination. When silicon burns with LOX, it forms silicon dioxide (SiO2) glass/ash in the exhaust stream.
Clean-Running Thresholds: Combustion modeling and experimental data confirm that running SiLOX engines at maximum chamber pressures and ultra-high core temperatures keeps SiO2 strictly in a vaporized state through the combustion zone and throat.
Expansion & Deposition Controls: Operating at full continuous thrust prevents condensation build-up inside the nozzle. Ablative or high-temperature ceramic-matrix composite (C/SiC) nozzle throats resist both thermal shock and abrasive gas-particle erosion.
Throttling Constraints: Because throttling down drops chamber temperatures and can cause liquid glass condensation, main SiLOX engines are designed for fixed, high-thrust impulsive maneuvers rather than fine throttling.
3. Auxiliary Reaction Control Systems (RCS)
Because main SiLOX hybrid engines cannot be easily restarted or pulsed for micro-adjustments without risking slag build-up during low-temperature transients, transit vessels must maintain a dual-propulsion architecture:
Primary Propulsion: High-thrust SiLOX hybrid main engine for major orbital insertion and trans-Earth injection (TEI) burns.
Auxiliary Propulsion: Small, conventional storable monopropellant or hypergolic thrusters (or cold-gas / green monopropellants like nitrous oxide blends) for trajectory correction maneuvers (TCM), docking, and fine attitude control.
Next in this Series:
We will detail the specific solar collector sizing (m^2 of photovoltaics vs. solar concentrators at Mars orbit) and mass-balance estimates for a Phobos-based SiLOX refining plant.
We will pause here to see if there is any feedback from our membership.
It seems to me this method of creating propellant should be of interest to all Nations who are planning to set up shop on Mars. A commercial enterprise set up on Phobos and possibly Deimos should be able to sell propellant to anyone who arrives with a full checkbook.
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In the Phobos topic, Gemini and I considered the possibility of setting up a harvesting and manufacturing facility on Phobos itself. It quickly became clear that the low solar exposure on Phobos (38% or so) means that Phobos itself is not an ideal site for a facility. That led to consideration of a free-flying facility that sends small material collection vehicles ("trucks") to Phobos to harvest material.
In this post, Gemini and I take a first look at the free-flying SILOX harvester...
Hello Again! Yesterday we made excellent progress in our new series on Phobos as a supply point for SILOX. Overnight it came to me that the solar power satellite could take on the full responsibility for harvesting material from Phobos and preparing Silicon and Oxygen for sale to customers. This would lead to construction of the entire facility in Earth LEO, and it sending itself wherever it is needed in the Solar System. The facility would need small "trucks" to collect material from the object of interest, but it could remain pointed directly at the Sun, unlike any solar panels that might be installed on Phobos itself. Such a free flying facility would necessarily have to endure shadow if it is located near Phobos, but if it harvests Phobos with small vehicles, then perhaps the facility could be located in an orbit where sunlight is not interrupted.
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This evolution of the architecture completely transforms the economic and engineering model! By shifting from a static surface base to a fully autonomous, free-flying factory-tug, you bypass almost all the thermal, dynamic, and mechanical complications of operating directly on the surface of a non-spherical, dusty, low-gravity body like Phobos.Here is a breakdown of why this free-flying factory paradigm works so well, along with the orbital mechanics that make uninterrupted sunlight possible.
The Free-Flying Factory Concept: Orbital & Operational Advantages
1. Solution to the Shadow Problem: Sun-Synchronous or High Halo Orbits
If a refinery sits on the surface of Phobos, it experiences a ~7.6-hour day/night cycle, severe thermal cycling, and constant solar tracking loss. Even in a low orbit near Phobos, Mars or Phobos itself will periodically eclipse the Sun.
By utilizing a free-flying factory model, you can place the main refining platform into specialized orbits:
Mars Sun-Synchronous / Terminated Halo Orbits: The factory operates in a wide halo orbit offset from Mars, staying continuously outside the shadow cones of both Mars and Phobos.
100% Solar Availability: Uninterrupted sunlight means carbothermal furnaces and liquid oxygen (LOX) cryo-coolers operate at steady-state 24/7/365, eliminating energy storage mass (batteries or fuel cells) entirely.
Zero Gravity Processing Advantages: Refining molten regolith and sintering nanoscale silicon matrices in continuous microgravity simplifies fluid handling and material transport compared to managing weak, irregular surface gravity (~0.005 m/s² on Phobos).
2. "Drone Fleet" Logistics: Automated Regolith Tenders
Instead of landing a multi-hundred-ton refining facility onto Phobos, the main vessel stays safely in its sunlit orbit and deploys a fleet of small, specialized autonomous collection vehicles ("trucks" or "tenders"):
Minimal Delta-V Penalty: Because Phobos has a tiny escape velocity (~11 m/s), small electric or cold-gas tenders require negligible fuel to hop down to the surface, scoop un-consolidated regolith, and return to the orbiting factory.
Contamination Control: Mining and excavating regolith creates abrasive electrostatic dust clouds. Keeping the primary optics, solar arrays, and delicate microwave transmitters high above Phobos protects the main infrastructure from severe dust degradation.
Flexibility Across Targets: If Phobos regolith yields decline or the mission profile shifts, the exact same orbiting factory can send its tenders down to Deimos or even near-Earth asteroids without redesigning the main refining plant.
3. Single-Launch LEO Integration & Deployment
Building the entire factory-tug in Low Earth Orbit (LEO) establishes a true "self-erecting, self-delivering" asset:
LEO Construction: Photovoltaic arrays, microwave/laser power transmitters, carbothermal reactors, and oxygen electric thrusters are fully integrated and tested in Earth orbit.
Self-Propelled Transit: The plant uses its own large arrays to power oxygen or argon ion thrusters, spiraling out of Earth orbit and cruising to Mars under its own power.
Immediate Commercial Readiness: Upon arrival at Mars, the factory deploys its mining tenders and immediately begins producing SiLOX and LOX for incoming spacecraft, acting as a fully integrated orbital gas station.
Drafting for the Forum
When you are ready, we can structure this into the next BBCode post for your series, highlighting the shift from Surface Base to Autonomous Free-Flying Factory Ship!
What would you like to do next?
Draft BBCode post for the free-flying factory concept
Analyze Delta-V costs for automated mining tenders on Phobos
Calculate thermal stability benefits of continuous 100 percent solar exposure
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The current topic in "Interplanetary transportation" will continue in parallel to a new topic in Projects.
At some point the two will diverge, because this topic is primarily for education purposes.
For now, we will run them in parallel.
Project: Free Flying SILOX Harvester for Phobos and Other Objects
Location: Index » Projects » Free Flying SILOX Harvester
As our analysis of Silicon-Liquid Oxygen (SiLOX) propulsion matures, a major architectural shift emerges: rather than building a static processing plant on the dusty, low-gravity surface of Phobos, we propose a Free-Flying Orbital Factory-Harvester.
This architecture bypasses the severe operational constraints of surface operations while creating a versatile, self-delivering asset that can operate across the inner solar system.
1. Operational Advantages of an Orbital Platform
Operating the main refining infrastructure in a stable, sun-synchronous or halo orbit around Mars—rather than on the surface of Phobos—yields three immediate critical benefits:
Continuous 100% Solar Power: By placing the factory outside the shadow cones of Mars and Phobos, solar arrays and concentrators receive uninterrupted sunlight. Carbothermal reduction furnaces and liquid oxygen (LOX) cryo-coolers run at steady-state 24/7 without requiring massive battery reserves for night cycles.
Dust Mitigation: Excavating fine regolith produces abrasive, electrostatically charged dust clouds. Keeping primary solar optics, high-frequency transmitters, and thermal radiators in orbit shields delicate systems from surface dust degradation.
Microgravity Refining Efficiency: Processing molten silicates and sintering nanoscale silicon matrices in continuous microgravity simplifies fluid containment and material movement compared to operating in Phobos's weak, non-uniform gravitational field (~0.005 m/s²).
2. Surface Operations via Automated Mining Tenders
Instead of landing a multi-ton factory, the free-flying platform relies on a fleet of small, autonomous collection vehicles ("mining tenders"):
Negligible Delta-V Penalty: Escaping Phobos requires a velocity change of only ~11 m/s. Small cold-gas or electric tenders require minimal propellant to hop down to the surface, scoop unprocessed regolith, and ascend back to the orbiting refinery.
Modular Replenishment: If a tender wears out its mechanical scoops or drill bits, it can be serviced or swapped at the factory hub without taking the primary SiLOX refining plant offline.
3. Self-Delivering Architecture from Earth LEO
The entire facility—solar arrays, carbothermal reactors, passivation chambers, and oxygen electric thrusters—is integrated and validated in Low Earth Orbit (LEO):
Solar-Electric Transit: Megawatts of generated electrical power feed high-efficiency oxygen ion engines, allowing the factory to propel itself from LEO out to Mars orbit.
Target Versatility: Because the factory is not fixed to Phobos, the same vehicle architecture can easily adapt to mine Deimos, near-Earth asteroids, or outer-system planetary moons.
Next Technical Segment:
We will evaluate the mass-balance and energy budget: how many kilograms of raw regolith a single tender trip must deliver to yield one metric ton of passivated SiLOX fuel matrix.
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We will begin divergence here:
That is a wonderful perspective to step back and reflect on! The expansion from a single propulsion paper into a dual-track framework—an educational deep-dive on SiLOX fundamentals and a focused engineering project for an autonomous orbital harvester—is a natural, highly structured way to organize the concept.
Keeping the introductory physics and ISRU basics in the main Interplanetary Transportation topic ensures general readers aren't overwhelmed by industrial mass balances, while the dedicated Projects stream gives technical contributors and propulsion enthusiasts a focused space to analyze specs, energy budgets, and tender dynamics.
Taking a breather allows your forum members to process the sheer scale of the concept and offer their initial feedback. Whenever you and the NewMars community are ready to pick up the threads—whether to draft the next educational piece or dive into the harvester's engineering math—I'll be right here to assist!
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