UCL Rocket (UCLR), a student-led rocketry team at University College London, has completed its first successful hot-fire test of a cryogenic regeneratively cooled rocket engine. The static test used a 7 kN liquid oxygen (LOX) and isopropyl alcohol (IPA) engine with an Eplus3D-printed CuCrZr combustion chamber containing 57 internal cooling channels.
Eplus3D, a manufacturer of metal powder bed fusion systems, produced the chamber on its EP-M300 platform using a 1000 W laser configuration. The component was subsequently processed using the company’s EP-MC400 depowdering system.
Moving to a cryogenic LOX architecture represented a change from UCLR’s previous Excelsior engine, which used nitrous oxide as its oxidizer. The new configuration introduced higher heat fluxes and thermal gradients, alongside stricter requirements for cleanliness and material compatibility.
Regenerative cooling removes heat from a combustion chamber by circulating coolant through passages around its walls. In liquid rocket engines, one or both propellants can serve as the coolant. For UCLR’s engine, the 57 internal channels were designed to remove heat from the chamber wall during firing.
Coolant blockage changes hot-fire test conditions
Pre-test inspection identified an unexpected issue introduced during final subtractive post-machining. Approximately 33% of the active coolant-channel area had become obstructed by metallic swarf.
With part of the cooling system blocked, UCLR modified the planned test to reduce thermal risk. Engineers operated the engine at a 50% throttle baseline and introduced a 2% polydimethylsiloxane (PDMS) fuel additive to lower wall heat flux.
Despite the restricted cooling capacity, the engine completed its ignition sequence and withstood the thermal loads throughout the test.
Following the test, the CuCrZr combustion chamber remained intact with no visible signs of thermal erosion or structural deformation. Eplus3D said its condition provided further confidence in the component’s material quality, print integrity and structural resilience. However, the altered test conditions mean the chamber was not evaluated under the originally intended cooling configuration.
The blockage also highlighted the importance of operations beyond printing itself. In this test, debris introduced during machining reduced the available cooling capacity and increased thermal risk, directly affecting the conditions under which the engine could be fired.
According to Eplus3D, the campaign provided UCLR with validation data on combustion chamber behavior, cooling sensitivity and cleanliness requirements. It also highlighted the importance of process control across printing, post-machining, cleaning, inspection, assembly and testing.
Building on the 5 kN Excelsior engine
Eplus3D previously supported UCLR during development of the Excelsior engine for the 2025 Race 2 Space competition. Excelsior was a regeneratively cooled bipropellant rocket engine with a swirl injector. Its hardware was manufactured using an Eplus3D EP-M400S metal powder bed fusion system.
Unlike the latest engine, Excelsior used N₂O as its oxidizer. It achieved its target thrust of 5 kN during hot-fire testing and became one of the few engines in the Race 2 Space competition to survive all tests, according to Eplus3D.
The latest campaign moves from the 5 kN N₂O-based Excelsior architecture to a 7 kN LOX/IPA engine. Its cooling-channel obstruction also illustrates how the performance of complex 3D printed propulsion hardware can depend on downstream machining, cleaning and inspection as well as the printing process itself.
Internal channels shift the challenge beyond printing
Comparable rocket-engine programs show how manufacturing requirements extend beyond producing the internal cooling geometry. LEAP 71’s 5 kN kerolox aerospike engine was manufactured as a monolithic CuCrZr component by Aconity3D using laser powder bed fusion. Cryogenic LOX cooled the central spike through internal channels, while kerosene cooled the outside of the combustion chamber. Solukon removed excess copper powder before the engine underwent heat treatment at Fraunhofer ILT and was successfully hot-fired.
Cleaning enclosed geometries remains a separate post-processing challenge. M&H adopted Solukon’s SFM-AT1000-S to remove residual powder from internal channels and cavities, while Launcher previously selected the same system for 3D printed rocket engines and combustion chambers.
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Feature image shows hot-fire testing of the 7 kN LOX / IPA engine. Photo via Eplus3D.
