RKLB Stock Slips Overnight: Rocket Lab Tops 400 Archimedes Hot Fires, Neutron Engine Set Enters Production

Neutron remains on track for Q4 pad delivery, but its year-end launch window is narrowing.

• Archimedes has completed more than 400 hot-fire tests across both engine variants.
• Control and durability were confirmed for all major engine components.
• Flight engines will undergo acceptance testing before integration in Virginia.

Shares of Rocket Lab (RKLB) slipped 0.5% overnight late Wednesday after the company said that its Archimedes engine had completed more than 400 hot fires and that the full engine set for Neutron’s maiden launch was now in production.

RKLB stock rose more than 1% on Wednesday to close at $81.17 but is on-track for a weekly decline, down 2% so far this week. 

RKLB Clears Key Neutron Engine Milestone 

“Archimedes has been cookin’,” Rocket Lab said on X, adding that testing had confirmed control and durability across flight hardware for all major engine components. CEO Peter Beck said during this week’s second-quarter earnings call that Archimedes was entering the “final stretch” of testing at NASA’s Stennis Space Center.

The 400-plus hot fires cover both the reusable first-stage engine and the vacuum-optimized second-stage engine. Testing has progressed from power levels, mixture-ratio control and full-duration burns to extended runs and repeated restarts designed to prove durability beyond normal flight requirements. Once qualification is complete, the flight engines will undergo acceptance testing at Stennis before heading to Launch Complex 3 in Virginia for integration.

Rocket Lab Ramps Up Neutron Engine Development 

Rocket Lab completed a full-duration burn of its Archimedes vacuum engine, or AVac, last month, following full-duration burns, thrust-vector-control sweeps and vacuum-engine hot fires across both Stennis test cells in May. AVac produces about 1.2 times the thrust of its first-stage counterpart and features an extended nozzle optimized for space.

The company said in April that Archimedes would power Neutron’s launches and landings, building on its flight-proven Rutherford and Curie propulsion systems. A month earlier, it highlighted the engine’s 3D-printed turbopump housings, thrust chambers, valve housings and structural components.

This progress is supported by four facilities Rocket Lab detailed in February, including its Long Beach engine production complex and side-by-side Archimedes test cells at NASA’s Stennis Space Center.

Neutron Launch Window Narrows 

Archimedes completed its first hot fire in August 2024, reaching 102% power. Each engine can produce up to 165,000 pounds of thrust, while Neutron’s nine-engine first stage is designed to generate about 1.45 million pounds combined. Neutron can carry up to 13,000 kilograms to orbit and is intended to serve commercial constellations and national-security missions.

Rocket Lab said production remains aligned with delivering Neutron to the launch pad in the fourth quarter of 2026, although Beck acknowledged that the window for a year-end debut is narrowing. The company is prioritizing readiness for full-scale production and frequent launches following the rocket’s first flight.

Neutron has already secured dedicated missions from the U.S. Space Force and Kepler Communications. Rocket Lab also reported more than $437 million in recent launch bookings and a record backlog exceeding 90 missions.

How Do Retail Traders Feel About RKLB?

On Stocktwits, retail sentiment for RKLB was ‘extremely bullish’ amid ‘high’ message volume.

One user said, “$RKLB Neuron will happen, just be patient. Peter Beck wants to get it right the first time, and he probably will.”

View this Stocktwits post

Another user said, “$RKLB Pete, is not thinking about the Neutron debut launch....he is thinking about reusability and the next 100 launches.....no point delivering one flawless maiden launch if u can't repeat it time and time again. That's a visionary.”

View this Stocktwits post

RKLB stock has jumped 87% over the past year. 

For updates and corrections, email newsroom[at]stocktwits[dot]com.<