An important aspect influencing the economic advantage of reusable launch vehicles over their disposable counterparts is the cost associated with the reuse itself. These costs are optimized through design for inspection and maintenance, as well as effective condition monitoring and predictive maintenance. In this context, the fouling of rocket cooling channels due to the thermal decomposition of hydrocarbon fuels, known as pyrolysis, is particularly challenging as cooling channels are difficult to inspect and clean. Carbon depositions resulting from the pyrolysis process act as thermal insulation, due to which wall temperatures can rise and the thermo-mechanical damage to the wall can increase. The current work proposes an approach to the monitoring of cooling channel fouling based on the sensing of decomposition products in the fuel flow. In addition to acting in real-time, this approach provides an early warning of damage that will occur, as it is based on reaction products that precede the deposition of carbon. It is inherently faster than the alternative approach of sensing increases in the wall temperature. The decomposition products are sensed by thermal conductivity gauges and, using a simplified theoretical model, the data is used to infer the mass of deposited carbon in the channel. The approach is tested experimentally for methane-based fuels on Inconel 600, Inconel 625 and Haynes 230. The model is shown to match carbon deposition mass measurements to within approximately the uncertainty of the scale used. Although these results are promising, further testing under more arduous conditions needs to be conducted in future work.

Additive manufacturing (AM) enables the fabrication of complex internal cooling channels for high-temperatureaerospace applications, but the rough as-built surfaces strongly influence flow. Understanding and predicting theseeffects is critical for accurate thermal–hydraulic design of AM-based cooling components. This study investigates therole of surface roughness on pressure drop and friction factor in an AM rectangular channel fabricated from Haynes230, a nickel-based superalloy. The cooling channel, with a hydraulic diameter of 1.995 mm, was tested in a closed-loop nitrogen flow facility at different mass flow rates, corresponding to Reynolds numbers in the range of 104 - 105.The test facility was developed under the MERiT+ project funded by Swedish National Space Agency at KTH Royal Institute of Technology, with industry partners from Siemens Energy and GKN Aerospace. Pressure drops wererecorded using multiple taps located along axial direction, and the Darcy friction factor was determined from localpressure drops. Complementary optical measurements of cross-sections at different streamwise locations wereperformed to assess variations in hydraulic diameter, while white light interferometry was employed to quantify surface roughness parameters, including Sa, Sq ,Ssk, and Sku. Numerical simulations were carried out in ANSYS Fluent usingthe pressure-based steady solver. Simulations were conducted with equivalent sand-grain roughness height, ks, derivedfrom characteristic parameters. Comparison between experimental and numerical results demonstrated consistenttrends in pressure drop and validated the approach for implementing hydraulic roughness in CFD. The data was furtheranalyzed to establish a predictive correlation between measured roughness parameters and equivalent sand-grainroughness. Several log-linear model forms were evaluated, and fitting was performed by minimizing errors in predictedfriction factors using the Haaland equation. The final correlation expresses (ks/Dh) as a function of normalized RMS roughness, and skewness, providing a compact, physically interpretable model. This work demonstrates a combinedexperimental, metrological, and numerical methodology for quantifying and predicting friction factor in AM channels.The proposed correlation, calibrated for Haynes 230 LPBF channels in the turbulent regime, bridges advanced surface characterization with classical fluid mechanics and offers a foundation for more reliable hydraulic design of AM-based cooling systems.

Methane pyrolysis has been a topic of research in both energy technology and aerospace propulsion. Variousaspects such as heat transfer, catalytic materials and chemical kinetics as well as carbon deposition behaviourhave been in focus. However, the temporal behaviour of pyrolysis and carbon deposition has received relativelylittle attention. Considering the effect of maintenance on programme costs, it is important to have sensors withgood time resolution both in the laboratory and in operational systems. Therefore, it is of interest to investigatethe efficacy of real-time sensors that have the potential for integration into systems with real-world applications.Speed of Sound (SoS) transducers and Thermal Conductivity Gauges (TCG) have the potential to produce high-quality measurements at relatively low cost and in a compact form factor; however, they have not yet beendemonstrated in this research area. Therefore, the current work assesses their performance compared to gaschromatography (GC), using a newly developed experimental setup. It is shown, for the conditions tested, thatreal-time and in-flow SoS and TCG sensors can attain measurement quality comparable to that of the GC used;however, this performance is implementation dependent. Specifically, PCB based TCG and Time-of-Flight basedSoS measurements are found to be highly effective in capturing the dynamic characteristics of the methanepyrolysis process. Although the current indication is promising, further work is necessary to test the approachunder representative operational conditions specific to, for example, aerospace propulsion applications.

The effects of surface roughness, e.g. due to additive manufacturing or various wear mechanisms, is an active field of research, in particular for aerospace propulsion. With respect to rocket propulsion, such effects include performance changes of turbopump components and increasing pressure drop in cooling channels. An area that has not yet received significant attention is the effect of surface roughness on the thermo-catalytic decomposition of hydrocarbon fuels in cooling channels of, e.g. rocket engines or hypersonic aero engines. Although the decomposition of methane on rocket engine candidate materials and the resulting carbon depositions has been under active investigation in recent years, research mentioning the effect of surface roughness is sparse at best. The thermal decomposition of fuel, also known as pyrolysis, is a complex chemical process that can be catalyzed by materials such as nickel, iron and copper, which are common elements used in alloys for rocket engines and nozzles. Carbon deposited on, e.g. cooling channel walls has a thermal conductivity of a factor 100-1000 lower than that of copper and therefore effectively acts as a thermal insulator. Due to this, it has the potential to increase wall temperatures and thereby thermo-mechanical damage. The pyrolysis process depends on temperature, flow rate (or residence time), pressure, catalytic material, the area of the material exposed to the flow as well as the history of this material. High surface roughness materials may influence catalytic activity by increasing the number of active sites via surface area as well as the activity of the available catalytic sites. This is especially relevant given the increasing use of additive manufacturing in aerospace industry. The aim of this work is therefore to provide new data on the effects of surface roughness on fuel pyrolysis in components of aerospace propulsion systems. In this work, we focus on the experimental investigation of the effect of surface roughness on methane pyrolysis and the resulting carbon depositions. The candidate materials considered are nickel alloys Haynes 230 and Inconel 625 and are prepared by both additive and subtractive manufacturing methods resulting in a surface roughness range from Ra=0.40μm to Ra=11.24μm. This work is carried out as part of the MERiT+ project, a cooperation between KTH Royal Institute of Technology, GKN Aerospace Sweden and Siemens Energy, which is funded by the Swedish National Space Agency.

The rapid development of reusable launch vehicles (RLVs) has shifted attention towards methane as a rocket fuel, motivated by its balance of storability, performance, and the possibility of in-situ resource utilisation. Compared to kerosene-based fuel (e.g. RP-1), methane burns more cleanly and generates less soot. Compared to liquid hydrogen, it is easier to handle and store while still offering favourable propulsive efficiency and cooling performance compared to RP-1. However, impurities in methane derived from natural gas or biogas—such as ethane, propane, and carbon dioxide—may reduce this fuel’s thermal stability. Under rocket-like thermal conditions, methane can break down and deposit carbon (“coke”) on cooling channel walls. This reduces heat transfer, increases wall temperature, and shortens the lifetime of engines. If non-purified methane is to be used, its stability must be fully understood. This paper presents results from the MERiT project, a collaboration between KTH Royal Institute of Technology and GKN Aerospace. A series of controlled experiments examined the catalytic pyrolysis stability of methane with various impurity contents. Nickel 201 and Inconel 600 samples were heated to 800 °C at 200 kPa(A) while gas mixtures containing methane and different proportions of ethane, propane, and CO₂ were tested at a flow rate of 50 ml/min. Hydrogen production, coke deposition, and visual inspection were used to assess pyrolysis stability. The results demonstrate that propane, with concentrations as low as 2%, leads to a visually observable increase in cooking. Ethane also increases pyrolysis rates, though its effect is more moderate compared to propane. The effects of ethane and propane are both exponential in character across the 0–10% range. CO₂ demonstrated varying effects based on the samples used; however, regardless of the sample material, its effects were limited in increasing coke formation compared to ethane and propane. These findings suggest that while some level of ethane and CO₂ may be tolerated, propane must be removed if methane from natural gas is to be used in regenerative rocket nozzles.

In recent years there has been a growing interest in methane as an alternative rocket fuel due to its favourable specific gravity, storage temperature and thermal stability, in addition to its ability to support In-Situ Resource Utilization. Due to these properties methane supports the ongoing design trend of strategic reduction in system complexity and increase of reusability. The current work presents a first step in addressing the lack of information in open literature on the characteristics of methane under conditions found in rocket nozzle cooling channels, i.e. elevated inflow temperature and a high single sided heat load. A new experimental facility has been established at KTH Royal Institute of Technology in cooperation with GKN Aerospace, and as part of ESA's Future Launcher Preparatory Programme. This facility is shown to enable direct measurement of the Heat Transfer Coefficient (HTC) of methane under a range of conditions, with a limited uncertainty and good repeatability. For inflow temperatures of around 400 K, mass flows up to 15 g/s and pressures up to 3 MPa, it has been observed that the effect of single sided heating results in a significant development of the flow field, which influences the heat transfer in second half of the test section. This development results in an increase of the HTC towards the end of the cooling channel. No significant effect of the pressure on the HTC has been observed under the current experimental conditions.

In recent years there has been a growing interest in methane as an alternative rocket fuel due to its favourable specific gravity, storage temperature and thermal stability in addition to its ability to support In-Situ Resource Utilization. Due to these properties methane supports the ongoing design trend of strategic reduction in system complexity and increase of reusability. The current work presents a first step in addressing the lack of information in open literature on the characteristics of methane under conditions found in rocket nozzle cooling channels i.e. elevated inflow temperature and a high single sided heat load. A new experimental facility has been established at KTH - Royal Institute of Technology in cooperation with GKN Aerospace, and as part of ESA's Future Launcher Preparatory Programme. This facility is shown to enable direct measurement of the Heat Transfer Coefficient (HTC) of methane under a range of conditions, with a limited uncertainty and good repeatability. For inflow temperatures of around 400 K, mass flows up to 15 g/s and pressures up to 30 bar, it has been observed that the effect of single sided heating results in a significant development of the flow field, which influences the heat transfer in second half of the test section. This development results in an increase of the HTC towards the end of the cooling channel. No significant effect of the pressure on the HTC has been observed under the current experimental conditions.