BSEP Engine 4 — Ethan Sheehan

01

Elsa: Bristol's first LOx engine

Engine 4 keeps the 5 kN, 20 bar architecture of Engines 1 to 3 and swaps nitrous oxide for liquid oxygen. That one change drove the rest. The chamber goes back to Inconel 718 from CP1 aluminium, because an ox-rich shutdown with LOx has to be survivable. The injector becomes an ox-centred coaxial swirler, which let us delete film cooling. O/F 1.6, IPA fuel, 8 s design burn. Built for Race 2 Space 2026, with Engine 1 declared as the backup: chambers and injectors stay interchangeable across every BSEP engine. The long goal is the 9 km liquid category at EuRoC 2027.

CAD render of Engine 4: the isogrid-latticed chamber and nozzle, three-quarter view from the injector end — Elsa: isogrid chamber and nozzle, three-quarter view.

CAD render of the full Engine 4 assembly: bolted injector top plate, isogrid chamber and nozzle — Full assembly: injector stack, isogrid chamber, nozzle.

CAD render of the Engine 4 chamber on its L-bracket test mount with feed plumbing attached — On the L-bracket test mount with feed plumbing.

02

Design point

5.0 kN thrust at 20 bar chamber pressure, 30 bar delivery on both sides. O/F 1.6: 0.828 kg/s of IPA against 1.325 kg/s of LOx. The chamber is 281.0 mm long, 117.8 mm across, with a 48.10 mm throat, expansion ratio 3.81 and contraction ratio 6.00. Regen cooling runs through 48 spiral channels at a constant 35 degrees behind a 0.8 mm hot wall, inside an isogrid shell to save mass. Five identical coax swirl elements, sized by the Bazarov method, ox inside and fuel fed to the lower manifold straight from the regen exits. PTFE cryo face seals rated to minus 200 degrees C, a silicone piston seal, no inter-propellant seals. Ignition by the airborne torch igniter.

03

The injector

The ox-centred layout is the headline. LOx drops straight through the top plate into the inner swirlers; fuel never has to cross the ox manifold, so the seal count falls. Ox in through a 3/4 inch BSPP port at the centre top, fuel in through 1/2 inch BSPP at the regen inlet. The machined parts arrived in August 2026. Read the strip as the stack, top plate down to the body.

Machined top plate exterior with a central hex boss holding the threaded oxidiser inlet and two smaller instrumentation ports — Top plate, ox inlet side.

Flat machined plate face with a bolt pattern and a ring of small drilled ports near the rim — Plate face, bolt pattern and drilled ring.

Machined plate with a central threaded port and a fine spiral finish across its face — Plate with central threaded port.

Plate with five inner oxidiser swirler posts protruding from its face — Oxidiser posts, five elements.

Oxidiser posts, profile.

Injector body face-on with five element bores standing in a counterbore and a ring of small drilled ports on the shoulder — Body, five element bores.

Injector body at an angle showing the depth of the element bores and the drilled ring — Body, angled.

Reverse side of the injector body with five machined element seats inside the bore and a lobed flange with threaded holes — Body, reverse: element seats.

04

Printed

The Inconel 718 chamber came off the LPBF printer in June 2026, brace webs still on. Isogrid lattice, E-4 and the Elsa snowflake embossed on the shell, the same three-quarter framing as the render. Going back to Inconel from CP1 aluminium is what lets a LOx engine survive an off-nominal shutdown, and the chamber stays interchangeable with every other BSEP injector.

05

Documents

The Preliminary Design Report from January 2026 and the Critical Design Report from May 2026. The CDR is the canonical document: parameter tables, injector scheme, seals, test plan, sizing calculations, the isogrid appendix and a STAR-CCM exhaust simulation. The test plan is two full-duration burns, then depth of throttle, then increased chamber pressure. Hot-fire status is pending.

Race 2 Space 2026 Preliminary Design ReportPreliminary Design Report, 24 pp. January 2026.24 pp

Race 2 Space 2026 Critical Design ReportCritical Design Report, 30 pp. May 2026.30 pp