Jet engines: how they work and what comes next

By the Vrulum team. Research checked 24 October 2026.

Jet engines create thrust by accelerating gas backwards. Most modern airliners use turbofans, which combine a gas turbine core with a fan and a separate flow of air around that core (NASA: thrust; NASA: turbofans). The basic technology is established; the open question is how much more efficiently it can propel an aircraft.

How the engine works

The gas turbine core follows a sequence: compress air, add heat through combustion, then extract energy as the hot gas expands through a turbine. The turbine drives the compressor on the same shaft. NASA describes the Brayton cycle as the thermodynamic cycle used in all gas turbine engines and used to predict their performance (NASA: Brayton cycle).

In a turbofan, some intake air enters the core and some bypasses it. The fan is driven by an additional turbine on an additional shaft, in what NASA calls a two-spool design. Bypass ratio means bypass airflow divided by core airflow; NASA notes it is a mass flow ratio, not a thrust ratio (NASA: turbofans).

For a simple, single-exit engine, NASA gives the thrust equation as:

F = (ṁV)e − (ṁV)0 + Ae(pe − p0)

Here F is thrust, ṁ is mass flow, V is velocity, A is exit area and p is pressure. Subscripts e and 0 denote exit and free-stream conditions. The pressure term matters when exit and ambient pressures differ (NASA: thrust equation).

The equation shows two routes to thrust: move a large airflow through a modest speed increase, or accelerate a smaller flow much more strongly. NASA associates the first approach with propeller aircraft and high-bypass turbofans, and the second with turbojets, which make the exit velocity very much greater than the incoming velocity (same source).

Main engine families

Type	How it delivers propulsion	Important distinction
Turbojet	Accelerates exhaust from a turbine core	Emphasises a high exhaust speed (NASA).
Turbofan	Produces thrust from core and bypass flows	Used by most modern airliners; low-bypass versions also serve fighter aircraft (NASA).
Turboprop	A gas turbine drives a propeller through gearing	Most thrust comes from the propeller; used on lower-speed aircraft (NASA).
Turboshaft	A gas turbine drives a shaft connected to another device	NASA mentions helicopters among the uses (NASA).
Ramjet	Forward motion compresses incoming air with no mechanical compressor	Cannot produce static thrust and needs another means of acceleration (NASA).

What is available already

Geared turbofans are commercial products. Pratt & Whitney lists its GTF family for the Airbus A320neo family, Airbus A220 and Embraer E-Jets E2. The manufacturer's efficiency figures need careful reading: the page gives "up to 20% per trip" for the A320neo and "up to 25% per seat" for the A220 and E2, both against undefined prior-generation aircraft. Savings per trip and savings per seat are different measures and cannot be treated as interchangeable engine improvements (Pratt & Whitney).

What comes next

UltraFan: manufacturer-reported ground demonstration. Rolls-Royce says the first phase of testing of its UltraFan 80 demonstrator took place in 2023, and that it reached 85,000 lbf of thrust in November 2023 (about 378 kN, converted by Vrulum). It reports testing with 100% sustainable aviation fuel. Its stated efficiency ambitions compare against two different baselines: 25% over first-generation Trent engines and 10% over the Trent XWB. These figures should not be presented as equivalent comparisons or as certified airline service results (Rolls-Royce).

Smaller UltraFan: proposed. The same company page says UltraFan 30 is planned for ground testing in 2028 and targets a 20% fuel-burn improvement relative to current in-service engines. This is a manufacturer plan, with no airline entry-into-service date given on that page (Rolls-Royce).

This article does not cover the CFM RISE programme. Its manufacturer pages could not be opened during checking, so none of its claims could be verified.

Lower fuel consumption does not establish zero-emission flight

The European Union Aviation Safety Agency (EASA) says sustainable aviation fuel (SAF) emission reductions are mainly achieved in production, and that lifecycle assessment results depend on many variables. Combustion emissions are effectively the same as those of fossil fuels, and the page notes substantial uncertainty over non-CO₂ climate effects (EASA). A successful 100% SAF test therefore does not, on its own, establish zero exhaust emissions or approval for every aircraft and fuel combination.

How far away?

There is no single arrival date. Geared turbofans are already commercial. UltraFan remains a demonstrator programme in the reviewed material, and its page establishes no UK passenger-service launch.

For a UK reader choosing flights now, these development targets do not establish a reason to wait for a particular engine. Useful next signals are completed flight demonstrations, independently inspectable performance evidence, certification, aircraft selection and actual airline service. This is an editorial evidence checklist, not a prediction that those events will occur on schedule.

Research scope: a sourced introduction and one civil-engine development programme. This is not an exhaustive review of military propulsion, maintenance practice or certification requirements.