← All work ENGINE 02 — 2025

BSEP Engine 2

Codename Snow White

Lightweight CP1 chamber with a coaxial-swirl injector: the first use of the Al-Fe-Zr alloy in a liquid rocket engine.

RolePropulsion Lead

Engine 2 on the AEL thrust stand at T+3.0 s with a purple plume streaming from the nozzle, sequence timer and firing number visible

Burn

5.0 s full duration

Average thrust

1.59 kN

Chamber mass

~3x lighter

An alloy's first flame

Engine 2 kept Engine 1's cycle and design point, 5 kN at 20 bar on IPA and nitrous oxide at an O/F of 3, and changed what it was made of. The chamber moved from Inconel 718 to CP-1, an Al-Fe-Zr aluminium, cutting chamber mass roughly threefold. It was the first time CP-1 had been used in a liquid rocket engine. On top sat a new coaxial-swirl injector plate for better mixing, efficiency and a route to throttling. The chamber was printed on the AMCM machine through the MTC beam-shaping project; the swirl elements were validated on dedicated test prints first. I was Propulsion Lead.

CP-1 chamber fresh off the AMCM printer, BSEP logo embossed.

The finished flight-weight aluminium chamber held in one hand — Flight-weight chamber held one-handed. That was the point of CP-1.

Build plate carrying small coaxial-swirl element test prints alongside tensile bars — Coaxial-swirl element test prints and tensile bars on the build plate.

Making it

After printing, the chamber had to come off its build plate: a CNC operation on the baseplate. That clip is the one to watch, because it is where the swarf got in. Low-pressure water flow checks passed, since the water flowed around the chips rather than flushing them, so we took the engine to Airborne Engineering at Westcott for Race 2 Space 2025 believing the channels were clean. Installation on the AEL stand, thermocouple wiring and pre-fire marking-up went to plan.

The baseplate CNC op. This is the operation that introduced the swarf.

Engine 2 bolted to the thrust stand in front of the Race 2 Space sponsor board — Installed on the AEL thrust stand, Race 2 Space backdrop.

Team members lifting the engine into position on the test stand — Installing the engine on the stand.

Ethan writing on the test stand during pre-fire preparation — Marking up the stand during pre-fire prep.

Wide view of the test cell with thermocouple wiring running to the engine on the stand — Thermocouple wiring on the stand, Airborne Engineering test cell.

The maiden fire

Run 20250701-002, 1 July 2025. The torch igniter lit cleanly by T+0.73 s. Then the fuel collapsed: swarf from the regen channels was driven into the injector and IPA flow fell to about zero, leaving a weak yellow plume at around 1.5 kN. With no fuel moving, all three cooling modes failed at once, film holes blocked, regen coolant stagnant and boiling, no PDMS deposition, while the chamber ran extremely ox-rich. At T+2.12 s the plume turned purple: aluminium burning through the wall as the first cooling channel burst, then more. From T+4.16 s the IPA flow stabilised, the engine settled, and it completed the full 5 s duration.

Means over 1 to 5 s: 1.59 kN thrust against 5 kN design, peak 3.78 kN; chamber pressure 7.02 bar against 20; Isp 114 s. Delivery pressure sat near 43 bar with the chamber at 7, the blocked-channel signature. The design point was never reached. The burn-phases plot is interactive: scrub it and watch the fuel starve, the channels burst, and the engine keep running.

The burn from the cell camera, cut from the 4K60 master. Purple means the plume is carrying chamber wall.

GoPro angle A, from the cell doors.

GoPro angle B, from outside the cell.

Thrust and propellant mass flows against time for the 1 July 2025 burn, with five phases banded: ignition, low fuel, burnthrough, high fuel, shutdown — Thrust and mass flows with the five phases banded. Watch the IPA flow starve at T+1 s and the thrust climb again after T+4 s.

Delivery pressures around 43 bar plotted against a chamber pressure of about 7 bar over the burn — Delivery ~43 bar against chamber ~7 bar: the blocked-channel signature.

Full three-panel instrument detail, rebuilt from the raw AEL DAQ.

Teardown

Root cause: machining swarf retained in the regenerative cooling channels, introduced when the baseplate was machined off after printing. Five major channel bursts, plus smaller cracks and bulges in almost every other channel, all sat at one elevation, exactly where the swirl elements sit. The throat had melted. There was no PDMS deposition anywhere. The injector came off full of swarf, piled in the fuel collection manifold and the annular gallery, but was undamaged beyond its O-rings. The water flow checks had flowed around the swarf rather than flushing it.

Looking up the nozzle at ruptured cooling channels with metal swarf spilling from the openings — Looking up the nozzle: burst cooling channels, swarf escaping through the openings.

Close-up of cracks running around the outer chamber wall at the height of the cooling channels — Chamber wall cracks at the channel elevation.

View down the chamber bore from the injector end showing bulged and popped channels and melt streaks at the throat — Down the bore from the injector end: bulged and popped channels, throat melt streaks.

Injector plate just removed from the chamber, its face covered in metal swarf — Injector plate straight off the chamber, swarf everywhere.

Opened injector with a pile of swarf sitting in the fuel collection manifold — Inside the injector: swarf piled in the fuel collection manifold.

Swirl-element side of the injector with swarf lodged in the annular gallery — Swirl-element side of the injector, swarf in the annular gallery.

The injector survived

The chamber performed admirably given what it was fed. The coaxial-swirl injector survived and is reusable: its measured N₂O fan angle was about 93°, close to the swirler design prediction, which made a burnthrough the first validation of the coaxial swirl design, under worst-case conditions. I presented the results and the failure investigation at the Race 2 Space 2025 tent. The lessons went straight into Engine 3 and Engine 4.

Ethan standing beside Engine 2 on the thrust stand in a PROPULSION E SHEEHAN team shirt — With Engine 2 on the stand. AEL, Westcott, July 2025.

Candid photo of Ethan at the Airborne Engineering site beside a Warning Sharks sign — At the AEL site.

Ethan presenting Engine 2 results to a seated audience in the Race 2 Space marquee — Presenting the Engine 2 results at the Race 2 Space 2025 tent.

Presenting at Race 2 Space 2025.

BSEP Engine 1