In my preceding article "The Low-CapEx European Launcher", I established the economic and thermodynamic baseline for an accessible, sovereign European micro-launcher (1.5-2.0 ton to LEO). By selecting a Bio-Ethanol/LOX propellant combination, I eliminated RP-1 coking penalties, complied fully with EU Green Deal and REACH guidelines, and introduced an onboard catalytic micro-reformer paired with a PEM fuel cell to break the traditional "Battery Mass Wall."
However, translating ground-level capital efficiency into orbital velocity requires solving the upper-stage packaging and thrust-vectoring problem. Standard vacuum upper stages rely on long, fragile expansion bell nozzles, heavy electromechanical gimbal actuators, and high-pressure downcomer lines.
This article details the second-stage propulsion system designed specifically for my architecture: the Monolithic Cutaway Aerospike Ring Engine with Central Pancake BLDC Pumps. I examine its fluid mechanics, explain why my coaxial nested tank layout uniquely unlocks this geometry, and benchmark its performance against conventional vacuum upper stages.
Engine Architecture & Mechanics
The Geometry: Inverted Annular Cutaway Aerospike
Rather than expanding combustion gases through a long, central conical bell, the second-stage engine uses an annular (ring) combustion chamber firing outward and downward across a short, truncated perimeter expansion ramp.
For a 25 kN vacuum thrust class stage operating at 25-30 bar chamber pressure:
Outer Ring Diameter: 500 mm
Combustion Channel Width: 20 mm
Axial Height: 250 mm (compared to >1.2 m for an equivalent vacuum bell nozzle)
The interior of the 500 mm ring forms an open, hollow void. Because the expansion occurs along the perimeter lip, the central core remains unpressurized and thermally isolated.
Core Integration: Axial-Flux "Pancake" Electric Feed
Standard centrifugal turbopumps are axially long, making them awkward to package inside compact stages. In contrast, axial-flux BLDC electric motors have a naturally flat, high-diameter-to-length "pancake" profile.
Central Placement: The pancake motor, direct-drive impellers, inverter electronics, and the PEM fuel cell stack sit directly inside the hollow center core of the combustion ring.
Thermal Management: Radiation from the inner chamber wall provides the precise 250°C thermal environment required for the catalytic micro-reformer bed, eliminating secondary heating loops.
Short Fluid Paths: Propellants drain directly into the central impellers and exit radially outward into the 360° injector face over distances of just a few centimeters.
Why This Rocket Architecture Unlocks the Ring Engine
A ring engine cannot simply be bolted onto a conventional rocket. It requires a specific structural and thermodynamic environment to function without severe mass penalties. My launcher provides three unique structural enablers:
Distributed Perimeter Load Mechanics (Eliminating Tall Thrust Cones)
In a conventional launcher, a bell-nozzle engine concentrates its total thrust onto a single, high-stress central point (the gimbal dome). Supporting a single point load on a 2.0-meter diameter stage requires a tall, heavy, conical thrust structure extending deep into the lower propellant tank to distribute stress outward to the vehicle skin.
My second stage resolves this by using a 500 mm diameter ring engine attached to a shallow, stiffened toroidal mounting ring on the aft bulkhead:
From Point Load to Ring Load: A 500 mm ring engine distributes 25 kN of thrust continuously along a 1.5-meter circumference, rather than focusing force into a single central node.
Shallow Toroidal Interface: Because the thrust is already broadly distributed across a wide circle, it transfers into the 2.0-meter stage sidewalls via a flat, composite ring flange built into the bottom bulkhead.
Mass & Volume Savings: This completely eliminates the tall internal thrust cone, significantly reducing dry mass and freeing up critical volumetric tank space for additional propellant.
Direct Bulkhead Feeding (Zero Downcomers)
Conventional stacked tanks require a long, heavy, vacuum-insulated downcomer pipe to bring fuel from the upper tank down to the engine. In my nested setup, both liquid oxygen and ethanol exit from the same bottom aft bulkhead. Ethanol drains straight down into the outer regenerative cooling jacket. LOX drains directly into the central impeller core.
Reformer Fuel Cell Synergy
Because ethanol is soot-free and exhibits a high specific heat capacity, it cools the 3D-printed GRCop-42 ring walls without coking. The 1.5% ethanol bleed directed to the micro-reformer generates pure H₂ on demand, powering the PEM fuel cell loop that drives the pancake pumps. Without this specific fuel chemistry, the electric power loop would require heavy lithium-ion batteries, destroying the stage's mass fraction.
Software-Defined Flight Control: 4-Quadrant TVC
Traditional upper stages adjust their thrust vector using heavy electromechanical actuators, flexible high-pressure joints, and structural gimbal bearings. The pancake ring engine replaces all moving mechanical TVC hardware with software-controlled sector-differential throttling:
Segmented Feed: The annular injector ring is split into four independent 90° quadrants, each supplied by an independent inverter channel on the pancake electric pump.
Differential Control: To pitch down, the flight computer increases inverter output to Quadrant 1 (105% speed) while decreasing Quadrant 3 (95% speed). This creates a net moment arm across the 500 mm engine diameter.
Millisecond Response: Because electric motor torque responds to inverter frequency changes in milliseconds—without turbine spool lag—flight control is instantaneous, precise, and completely solid-state.
Comparative Trade Analysis
When evaluating bare engine mass, a ring combustion chamber is slightly heavier than a single bell nozzle due to its larger wetted cooling surface area. However, when evaluated at the integrated stage level, the system yields significant mass and complexity savings.
Manufacturing & Post-Processing Realization
The 500 mm GRCop-42 copper-alloy ring combustor is manufactured as a single monolithic component using Powder Bed Fusion (PBF-LB) on commercial industrial printers (e.g., Velo3D Sapphire XC or SLM 500).
Monolithic Print: Internal 0.8 mm cooling passages, 360° injector ports, and expansion lips are printed simultaneously in a 60-hour build cycle.
Hot Isostatic Pressing (HIP): The raw print undergoes HIP processing at 900°C-950°C and 1,000-1,500 bar in an argon atmosphere. This closes all internal micro-porosities, bringing density to > 99.9% and ensuring the thin-walled cooling channels resist thermal fatigue under cyclic loads.
Finish Machining: Only mating flanges and dynamic seal faces receive CNC turning, keeping subtractive machining hours near zero.
Conclusion
The Pancake Ring Engine is not an isolated mechanical gimmick; it is the logical endpoint of a fully integrated, software-first upper stage. By coupling the chemical advantages of ethanol with nested coaxial tanks and an onboard micro-reformer power loop, I eliminate the three largest sources of dry-mass overhead and development risk in small launch vehicles: turbopumps, mechanical gimbals, and long interstages.
For European spaceports seeking low-CapEx, high-cadence, and environmentally compliant orbital access, this software-defined second stage delivers a robust, highly manufacturable solution built on existing European additive manufacturing infrastructure.
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