Concordia Researchers Develop Kerosene Gel Fuel That Holds Its Shape at Rest and Flows Under Shear

In rocket and satellite propulsion, a key challenge is balancing storage stability against ease of fuel delivery. Liquid propellants are easy to feed through pipes, and their flow can be finely controlled. But in microgravity or during attitude changes, the fuel can slosh around inside the tank. This sloshing can disrupt a spacecraft's attitude control, and it can also stress the tank or cause gas ingestion that destabilizes fuel delivery. During long-term storage in orbit, preventing fuel leaks from valves and seals is also essential.

Solid propellants, by contrast, do not flow, so there is no need to worry about sloshing or liquid leakage. However, a typical solid rocket motor is hard to fine-tune in thrust once ignited, and it is difficult to restart after burnout. As a candidate that combines the strengths of liquid and solid propellants, researchers have been studying "gel propellants," in which a liquid fuel is turned into a gel. The idea is that the material holds its shape like a solid at rest and flows like a liquid when needed. Conventional gel fuels, however, have been hard to control in viscosity, and pressure loss in piping and poor atomization at the injector have stood in the way of practical use.

Pranay Mudaliar and Associate Professor Jerin John of the Gina Cody School of Engineering and Computer Science at Concordia University in Canada developed a new kerosene-based gel fuel and examined its flow behavior and thermal response in detail. The study was published in the journal Langmuir, appearing online on July 29, 2026, and in Volume 42, Issue 35 (pages 25399–25414). The team gelled kerosene using natural lecithin and a trace of calcium chloride, and clarified how its viscoelasticity changes with shear, temperature and chemical stimuli. Note, however, that the work consisted of fundamental material-property tests. It did not involve combustion tests in a rocket engine or demonstrations in space.

How Worm-Like Micelles Gel Kerosene

The material the team developed consists of kerosene, a hydrocarbon fuel also used in aerospace, to which natural lecithin and a low concentration of calcium chloride (CaCl₂) are added. Lecithin is known as a component of biological membranes, and here it serves as the gelling agent. According to the paper, the resulting gel is combustible overall and optically transparent. This transparency could help in the future with optically observing how fuel flows and deforms inside pipes and injectors.

The key to gelation is the self-assembly of lecithin molecules promoted by calcium chloride. Divalent calcium ions interact with the polar head groups of lecithin, causing the molecules to assemble spontaneously into long, flexible "worm-like micelles." These micelles entangle with one another to form a three-dimensional network that restricts the flow of the kerosene within. In other words, the kerosene itself is not solidified; rather, the assemblies of molecules limit the movement of the liquid.

As the concentration of the gelling agent increases, the material shifts from a state closer to a viscous liquid toward one closer to an elastic-dominated solid. The "gel" here differs from a conventional solid propellant: it is a viscoelastic material with both viscous and elastic properties, and it behaves as a non-Newtonian fluid when flowing. Lecithin, being organic, contributes to combustion, whereas calcium chloride, which promotes network formation, is not itself a fuel component. The team used rheometers and other instruments to examine in detail how this structure responds to external forces and heat.

A 100-Fold Increase in Shear Rate Cuts Viscosity to About 1/100

One important property for making gel propellants practical is shear-thinning: the apparent viscosity falls as the applied shear rate rises. Ideally, the fuel resists flow while at rest in the tank but flows easily through pipes when supplied to the engine. A material with these properties could offer both storage stability and ease of delivery.

In steady shear flow tests, the synthesized kerosene gel showed marked shear-thinning. When the shear rate was raised 100-fold, from 1 s⁻¹ to 100 s⁻¹, the apparent viscosity fell to roughly one-hundredth. The absolute viscosity values before and after the change are not given in the publicly available materials we could confirm. This drop is thought to occur because the entangled worm-like micelles are oriented and rearranged by the shear flow, reducing resistance to flow.

The yield stress, a measure of when the material begins to flow, ranged from 14 to 40 Pa and increased with the concentration of the gelling agent. This means that the denser the micellar network, the greater the stress required to start flow. Being able to tune the yield stress through composition is an important finding for designing both the shape retention and the delivery characteristics of the fuel.

{
  "type": "line",
  "title": "Viscosity Drop of Kerosene Gel with Increasing Shear Rate",
  "unit": "Relative viscosity (value at 1 s⁻¹ = 100)",
  "categories": ["1 s⁻¹", "10 s⁻¹", "100 s⁻¹"],
  "series": [
    { "name": "Relative viscosity", "values": [100, 10, 1] }
  ],
  "caption": "As shear rate increases from 1 to 100 s⁻¹, apparent viscosity drops to about 1/100. The value at 10 s⁻¹ is a conceptual interpolation to illustrate the trend, not a measured value",
  "source": "Langmuir (DOI: 10.1021/acs.langmuir.6c01168)"
}

The team also ran oscillatory tests to probe the response to small deformations. In the linear viscoelastic region, where strain amplitude is below 1%, the elastic response exceeded the viscous response. In a comparison using the K₉₀ gel, a sample with 90% kerosene content, the storage modulus G′ of the gel clearly exceeded the loss modulus G″. The storage modulus indicates the tendency to store energy during deformation and return to the original shape, while the loss modulus indicates viscous behavior that dissipates energy. By contrast, in liquid kerosene without the gelling agent, G″ exceeded G′, so liquid behavior dominated.

In addition, large amplitude oscillatory shear (LAOS) tests, which probe the response to large deformations, separated the elastic and viscous contributions for analysis. The results showed that as strain increased, the stress and strain response waveforms within each oscillation cycle became progressively distorted. This is thought to reflect the gradual breakdown of the micellar network under large deformation. The work is also significant because it captured not just a drop in viscosity but how the internal structure of the gel changes under external force.

These results suggest that resistance could be kept low when the gel fuel is pumped through pipes or sprayed from an injector. However, confirming a viscosity drop on a rheometer is a separate matter from ensuring proper "atomization," in which fuel breaks into fine droplets in a combustion chamber. This study did not verify spray patterns, droplet size distributions or combustion efficiency with an actual injector. How much benefit shear-thinning brings in a real propulsion system will need to be established through future tests under conditions closer to those of actual hardware.

How Heating and Ethanol Liquefy the Gel

Beyond shear, the team also examined how the gel's state changes with temperature and chemical stimuli. In temperature sweep tests, which measured viscoelasticity while varying temperature, the material's response changed substantially in the range of 59 to 68 °C. At low temperatures, the storage modulus G′ exceeded the loss modulus G″, but as the sample was heated the relationship reversed and G″ became dominant. This means the gel moved from an elastic, solid-like state to a more fluid, liquid-like state.

The 59–68 °C range, however, is not a single melting point common to all formulations. The transition temperature varies with the concentration and composition of the gelling agent, and samples with denser networks tended to stay gelled up to higher temperatures. As temperature rises, thermal motion of molecules is thought to weaken the interaction between calcium ions and lecithin, making it harder to maintain the worm-like micelle network. Exploiting this thermal response could make it possible to control fuel fluidity through temperature.

A distinctive response to chemical stimuli was also confirmed. When a few drops of ethanol were added to the gel, the material liquefied almost instantly. This is explained by polar ethanol molecules affecting the self-assembled state of the lecithin and breaking down the micellar network that had been holding the kerosene. The finding shows that additives can dramatically change the gel's fluidity without applying strong mechanical shear.

Amplitude sweep tests included in the paper's supplementary data also confirmed that, in the sample after ethanol addition, G″ exceeded G′, so the viscous response dominated. Such chemical liquefaction could lead to a new way of controlling fuel delivery. However, adding ethanol would also change the fuel composition, so its effects on combustion characteristics and the feed system would have to be studied before any application in a real propulsion system.

The main properties confirmed in this study, and what each result means for propulsion engineering, are summarized below. The figures are based on the paper and Concordia University's official announcement. Note that the potential applications to propulsion systems are inferred from material properties and do not mean that real-world performance has been confirmed.

It is important to note that these are physicochemical responses confirmed in laboratory samples. A mechanism in which heaters are installed in real fuel tanks or piping, or in which ethanol is injected to control flow, has not been completed. For liquefaction by heating in particular, the publicly available data we could confirm do not make clear whether the original uniform gel structure fully recovers after cooling, or whether the same properties are maintained through repeated heating and cooling. Verifying this reversibility and durability is essential if temperature is to be used to switch the flow state repeatedly.

Over a Year of Storage Stability, and the Gap to Space Operations

One expected advantage of gel propellants is stable fuel storage over long periods. According to Concordia University, the newly developed gel showed no visible phase separation after being stored in a sealed container for more than a year. The rheological finding that elastic behavior dominated under small deformations also supports the idea that the gel tends to hold its shape when left undisturbed. Such properties are attractive for the propulsion systems of satellites and probes that require long-term storage.

However, long-term stability in a sealed container on the ground does not guarantee storage performance in space. Rocket launches subject fuel to strong vibration, acoustic loads and acceleration, and in orbit one must consider temperature changes from sunlight conditions and radiation. The inside of a fuel tank is not directly exposed to the external vacuum or temperature swings, but the environment the fuel experiences still varies with the spacecraft's thermal control and shielding. The stability of a sample left undisturbed on the ground is not enough to judge whether it can withstand such combined loads.

A review paper on the rheology and stability of gel fuels, published in the journal Aerospace in 2025, also discusses the importance of distinguishing stability at rest from stability in operating environments where external forces act. Because gel fuels are non-Newtonian fluids, vibration and shear can change their internal structure and thus their flow characteristics. It will be necessary to confirm whether the micellar network survives vibration tests that simulate an actual launch, and whether "syneresis," in which the solvent kerosene separates from the gel during long-term storage, occurs. Whether the original properties are retained after temperature changes and repeated mechanical loading will also be an important evaluation item.

Likewise, the suppression of sloshing and the reduction of fuel leakage remain, at this stage, effects expected from material properties. It has been confirmed that the gel is close to a solid at rest, but how much liquid-surface movement it suppresses in an actual fuel tank has not been measured. In addition, as the yield stress of 14–40 Pa indicates, flow begins once sufficient external force is applied. It is therefore necessary to quantitatively evaluate how much flow suppression can be obtained against acceleration at launch and disturbances accompanying attitude changes.

Another major hurdle to practical use is fuel delivery to the combustion chamber and atomization. As the review paper noted above points out, gel fuels with yield stress and viscoelasticity can cause large pressure losses in an injector depending on flow conditions. When injected, they may also fail to break up uniformly into fine droplets like ordinary liquid fuel, instead forming string-like structures (ligaments) in which droplets remain connected, or agglomerates. Such poor atomization can hinder mixing of fuel and oxidizer, leading to lower combustion efficiency, combustion instability and residual unburned material.

For this kerosene gel, whether such problems actually occur is unknown, and its spray and combustion characteristics have yet to be clarified. Concordia University also stated in its announcement that tests with actual hardware are needed to judge performance in spacecraft propulsion systems. This study did not measure specific impulse (I_sp), combustion efficiency, ignitability or throttling capability, and the safety and reliability required for aerospace use have not been demonstrated. Toward practical use, validation will need to proceed from the material property data obtained here to injection tests, combustion tests and environmental durability tests.

Validation Needed Before It Can Be Used as Rocket Fuel

This study showed that natural lecithin and calcium chloride can give kerosene distinctive viscoelastic properties. In particular, the combination of shape retention at rest and a large viscosity drop under shear is an important result for the material design of gel propellants. At the same time, properties obtained in the laboratory will not necessarily be maintained under the operating conditions of a rocket engine. Practical use will require validation from at least three angles: flow characteristics, environmental durability and effects on combustion.

First is the response to the high shear rates that occur in real injectors. The roughly 100-fold viscosity reduction reported here applies to the range in which the shear rate rises from 1 s⁻¹ to 100 s⁻¹. In actual injection and atomization, however, shear rates as high as roughly 10⁴ to 10⁶ s⁻¹ can arise depending on the flow path and operating conditions. Whether shear-thinning persists in that regime, and whether the micellar structure returns to its original state after changing substantially, cannot be judged from these results alone. In particular, it must be clarified how such structural changes affect droplet formation after injection and the mixing of fuel and oxidizer.

Next is whether the gel can withstand thermal and mechanical loads from launch through long-term operation in orbit. It must be confirmed whether a gel with a yield stress of 14–40 Pa can maintain sufficient shape retention under rocket vibration and acceleration. And while liquefaction at 59–68 °C might be used for flow control, it could also undermine storage stability if the fuel temperature rises unintentionally. It will be important to examine compatibility with spacecraft thermal design and how material properties change under repeated temperature cycling.

In addition, the effect of the gelling agents on combustion must be verified. Calcium chloride, though present in trace amounts, is a nonvolatile inorganic substance, and it must be confirmed that components produced after combustion do not deposit on injector or nozzle surfaces. For elements such as the phosphorus contained in lecithin, the characteristics of combustion products and exhaust, and compatibility with combustion chamber wall materials, also need to be evaluated. This does not mean problems were found in this study; these are items to be verified when incorporating the gel into a real propulsion system.

The results provide basic data for understanding the flow characteristics of kerosene gel and its response to external stimuli. Still, a large gap remains between a material having desirable properties and its delivering sufficient performance as rocket fuel. The next steps will be to measure droplet size and spray patterns in single-injector spray tests, and to directly evaluate ignition delay, flame propagation and combustion efficiency in combustion tests. Whether the property of being stable at rest and flowing readily when needed can be reproduced in an actual propulsion system will determine whether this new gel fuel reaches practical use.