Neutron development has progressed in the background while Rocket Lab continues regular launches of its small-lift Electron rocket. The vehicle is now advancing toward a possible debut later this year. The recent second-stage engine test paves the way for integration with Neutron in the coming months.
The burn took place at Rocket Lab’s Archimedes Test Complex, housed at NASA’s Stennis Space Center in Mississippi. The full-duration test aimed to simulate flight-like requirements and lasted just under 5.5 minutes. “What a thing of beauty,” the company said in its announcement.
Neutron’s first stage is equipped with eight Archimedes engines, which together provide nearly 1.5 million pounds of thrust at liftoff. The output of each is comparable to that of each Merlin 1D engine on SpaceX’s Falcon 9. Like the Falcon 9, Neutron is designed for partial reusability, with its first stage capable of returning for landing back at its launch site or on droneship vehicles stationed at sea.
The second stage is powered by a single vacuum-optimized Archimedes, known as the AVac. It features an engine bell that stretches about eight feet, or 2.5 meters, taller than the first-stage engines and produces 1.2 times the thrust of its first-stage counterparts in a vacuum. For the recent test, Rocket Lab installed a shorter sea-level variant skirt on the AVac to compensate for its grounded altitude, which can expose the full-length nozzle to flow separation and instability. “Stub skirts are used to anchor our engineers’ analysis for how the engine will perform with the full nozzle in vacuum conditions,” the company said.
Neutron’s second stage is unique compared to other rockets. Rather than jettisoning protective fairings completely, Neutron’s fairing halves separate like a clam shell in a system the company has dubbed the “Hungry Hippo.” The second stage emerges from the open fairing jaws to provide its payload a final push into orbit.
Rocket Lab had targeted late 2025 for Neutron’s debut but pushed that date to the first half of 2026 as the rocket remained far from being ready for launch. The timeline suffered another blow in January, when the main stage tank ruptured during a pressure test at the company’s Wallops, Virginia, launch facility. Rocket Lab founder and CEO Peter Beck has repeatedly said the Neutron team is focused on reaching orbit when the vehicle is ready, not on meeting an arbitrary target date.
The full-duration burn represents a concrete step in preparing the Archimedes engine for Neutron flight. The test allows Rocket Lab engineers to validate their analysis of how the vacuum-optimized engine will perform with its full nozzle once in space.
Next, Rocket Lab plans to integrate the second-stage Archimedes with Neutron in the coming months as the program moves toward a possible first flight later this year.
