It cannot do so without burning fuel. The SABRE concept is designed to burn liquid hydrogen, using oxygen from the atmosphere during its initial air-breathing phase and carrying liquid oxygen for its later rocket phase. That could avoid fossil-derived fuel at the engine, but the available sources do not establish how its hydrogen would be produced or the system’s lifecycle emissions. SABRE is also a proposed engine, not a demonstrated low-cost space service.
What is SABRE?
SABRE (Synergetic Air-Breathing Rocket Engine) is a proposed combined-cycle engine associated with UK company Reaction Engines. It is intended for a spaceplane or reusable launcher, combining air-breathing propulsion with rocket propulsion in one engine concept. It is more accurate to call it a combined air-breathing rocket engine than simply a jet engine.
The idea is to use atmospheric oxygen while the vehicle is moving through the atmosphere, then switch to onboard oxygen for the climb beyond the useful air-breathing part of the flight. ESA described the concept in its 2019 overview.
How would it reach space?
Air-breathing phase
In the proposed first phase, SABRE takes oxygen from incoming atmospheric air and burns it with onboard liquid hydrogen. ESA describes the air-breathing portion as operating to about Mach 5, with a transition at approximately 25 km altitude. These are concept figures, not demonstrated flight limits.
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At very high speed, the vehicle’s motion compresses incoming air in the engine intake. Johann Steelant, ESA technical officer in charge of the General Support Technology Programme activity, explained: “You don’t need a rotating compressor as in classical aero-engines, but the air is rammed by speed into the air intake, it’s rammed in through just the sheer force of flying at those speeds.” ESA discussed the engine’s development in its October 2019 update.
Rocket phase
As the vehicle climbs out of the atmosphere, the engine is designed to stop relying on air for oxygen and instead burn liquid hydrogen with onboard liquid oxygen. Carrying its own oxidizer lets a rocket operate where there is too little atmospheric oxygen for air-breathing combustion. The UK government’s 2022 SABRE programme evaluation describes the proposed fuel and oxidizer modes.
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| Phase | Fuel | Oxygen source |
|---|---|---|
| Air-breathing | Onboard liquid hydrogen | Atmospheric air |
| Rocket | Onboard liquid hydrogen | Onboard liquid oxygen |
Does “without fossil fuels” mean zero emissions?
No. The concept specifies hydrogen as fuel, rather than a conventional fossil fuel such as kerosene. But “hydrogen-powered” does not by itself establish that the fuel is fossil-free: that depends on how the hydrogen is produced. The cited sources do not specify a production pathway or provide lifecycle emissions figures, so they do not support calling SABRE zero-carbon or emissions-free.
The rocket phase also uses liquid oxygen as an oxidizer. Oxygen is not the fuel; it enables the hydrogen to burn when the vehicle is no longer drawing oxygen from the atmosphere.
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Why might it be described as low-cost?
The cost case is a possibility, not a proven price. ESA has presented reusable vehicles, reduced vehicle mass and more frequent launches as potential advantages of air-breathing propulsion. Its 2020 ESA-funded study examined a future launcher architecture and converged on a two-stage reusable concept; it was not a commercial launch-cost demonstration. The study did not establish a ticket price or a verified percentage reduction in launch costs. See ESA’s account of the system study.
That study considered one- and two-stage possibilities. A two-stage architecture divides the climb between vehicles or stages, while a single-stage design would attempt the journey with one vehicle. The study’s preferred architecture was a design outcome, not evidence that a launcher had been built or that its operating economics had been validated.
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What has been demonstrated, and what remains a proposal?
ESA reported that a preliminary design review for a test version had been completed in 2019. Mark Ford, then heading ESA’s Propulsion Engineering section, said: “It confirms the test version of this revolutionary new class of engine is ready for implementation.” That was a development-review statement about a test version, not confirmation of an operational flight engine. ESA also reported a precooler milestone in 2019; neither milestone establishes that SABRE has flown to space.
The UK Space Agency awarded Reaction Engines a grant with a ceiling of £50 million in 2015, as recorded in the government’s 2022 evaluation. That is historical programme funding, not the engine’s current cost, a commercial price, or proof of launch savings.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsWhat is SABRE’s status now?
Reaction Engines Limited entered administration on 31 October 2024. Companies House records the company as “In Administration,” and its filing history includes administrator progress reports filed in June 2026. PwC also states that joint administrators were appointed on 31 October 2024. These records establish the company’s administration; they do not establish whether SABRE work, intellectual property or assets transferred to another organisation or resumed elsewhere. The status of the company should therefore not be confused with confirmation that the engine is either continuing or definitively cancelled.
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