B1091 “A Heavy Core in Falcon 9 Clothing”

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SpaceX’s recent launch involved another successful landing of its Falcon 9 booster, but this time marked a Falcon Heavy center core booster launching and landing as a single-stick Falcon 9 rocket.

The booster, designated B1091, powered the KF-02 mission, deploying 24 satellites for Amazon’s Project Kuiper broadband constellation into low Earth orbit. This event marks a technical first for the company, blending elements of its Falcon 9 and Falcon Heavy programs.

The launch occurred on August 11, 2025, at 8:35 a.m. ET from Space Launch Complex 40 (SLC-40) at Cape Canaveral Space Force Station. Approximately eight minutes after liftoff, B1091 executed a precise landing on the drone ship A Shortfall of Gravitas stationed in the Atlantic Ocean.

At first glance, the launch appeared to be a standard Falcon 9 mission. However, B1091 is no ordinary booster. As revealed by SpaceX Vice President of Falcon and Dragon, Jon Edwards, in a May 2025 post on X, B1091 was originally built as a Falcon Heavy center core.

“B1091 is in fact a Falcon Heavy center core that will fly in a single stick configuration a handful of times before being reconfigured and flying as a Falcon Heavy,” Edwards stated. “This way we get some use out of it while the Heavy customer finishes up the payload while also reducing risk as it will be flight proven.”

Falcon Heavy center cores differ significantly from standard Falcon 9 boosters. Designed to withstand the intense stresses of being flanked by two side boosters, center cores feature thicker tank walls, additional structural supports, reinforced attachment points, and advanced materials like aluminum-lithium alloys with friction stir welding.

These enhancements allow them to handle tripled vibrations, acoustic levels, and lateral forces from 27 Merlin engines firing simultaneously—far beyond what a single Falcon 9 experiences.

In contrast, Falcon Heavy side boosters are nearly identical to Falcon 9 cores, requiring only minor integration hardware and aerodynamic nosecones.

All boosters utilize an “Octaweb” engine mounting structure for nine Merlin 1D engines, but the center core is qualified for much higher loads. This specialized design necessitates a separate production line for center cores, while side boosters can often be repurposed from existing Falcon 9 fleets.

SpaceX has repurposed Falcon Heavy side boosters for Falcon 9 missions in the past, including the inaugural Falcon Heavy flight in 2018, which used boosters previously flown as Falcon 9s.
The primary deterrent has been the added mass from reinforcements, estimated at 10-20% more than a standard Falcon 9 booster. This extra weight reduces fuel efficiency and payload capacity by several tons, making it an issue for most Falcon 9 missions where maximizing orbit delivery is key.

Historically, center cores were either expended on high-energy Heavy launches or held in reserve. Converting one for single-stick use involves modifications to remove or adapt attachment hardware, but the performance penalty had previously outweighed the benefits.

In B1091’s case, SpaceX opted for this approach to utilize the booster while awaiting a delayed Heavy payload, simultaneously flight-proving it for future missions.

During its return, observers noticed evidence of the booster’s heavier build during the KF-02 landing.

The entry burn began at a higher altitude—approximately 4.1 km—compared to typical Falcon 9 missions (around 3.2 km), suggesting a longer burn was needed to decelerate the denser core.
For SpaceX, this innovation demonstrates the versatility of its reusable rocket ecosystem, potentially reducing costs and risks by maximizing hardware utilization. The KF-02 mission itself advances Amazon’s Project Kuiper, pushing the constellation past 100 satellites and closer to providing global broadband coverage.

Ironically, Project Kuiper is seen as a rival to SpaceX’s own Starlink constellation, but SpaceX is happy to take Amazon’s launch contract money with confidence that Starlink is vastly ahead of its rival.

This flight could pave the way for more hybrid uses of its boosters, complementing its fleet-leading vehicles, with B1067 soon to launch its 30th mission, all while the company prepares its superior Starship program for operational flights.

Lead image: Ben Cooper for SpaceX

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