Turbojet vs turbofan — what is the difference?

A turbojet exhausts only core gas at high velocity, suited for supersonic flight. A turbofan surrounds the core with a large fan. High-bypass turbofans (BPR 5–12) are quieter and more fuel-efficient, dominating modern commercial aviation.

What does TSFC measure?

Thrust Specific Fuel Consumption (TSFC) is fuel mass flow per unit thrust, in g/(kN·s). Modern high-bypass turbofans achieve ~14–16 g/(kN·s), compared to ~30–40 for 1960s turbojets.

Why does turbine inlet temperature matter so much?

Higher TIT (T3) increases net work output and thermal efficiency. Modern engines push T3 to 1700–2000 K using single-crystal superalloys, thermal barrier coatings, and internal cooling.

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Aerospace Engineering

Jet Engine Performance Calculator

Q: What is the Brayton cycle?

The Brayton cycle is the ideal thermodynamic cycle for gas turbines and jet engines. It consists of isentropic compression, constant-pressure combustion, isentropic expansion, and constant-pressure heat rejection. Ideal thermal efficiency is η = 1 − πc^(−(γ−1)/γ), increasing with pressure ratio.

Analyze turbojet and turbofan Brayton cycles. Set pressure ratio, turbine inlet temperature, bypass ratio, and efficiencies to compute thermal efficiency, specific thrust, and TSFC with a live T-s diagram.

Engine Configuration

Engine Type

Bypass Ratio BPR5.0

Pressure Ratio πc30

Turbine Inlet Temp T3 (K)1600

Compressor Efficiency ηc0.88

Turbine Efficiency ηt0.90

Flight Mach Number0.85

Results

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Thermal Efficiency (%)

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Propulsive Efficiency (%)

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Specific Thrust (N·s/kg)

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TSFC (g/kN·s)

Brayton Cycle

η_th = 1 − πc^{−(γ−1)/γ}

States: 1(intake)→2(compressor)→3(combustor)→4(turbine)

γ = 1.4 (air), cp = 1005 J/(kg·K)

T-s Diagram (Brayton Cycle)

What is the Brayton Cycle in Jet Engines?

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What exactly is the Brayton cycle? I see it mentioned everywhere for jet engines.

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Basically, it's the ideal thermodynamic model for how a gas turbine engine works. Think of it as a four-step process: suck in air, squeeze it, burn fuel to heat it, and then blast it out to create thrust. In practice, the "squeeze" step is key—that's the pressure ratio you set with the slider in the simulator.

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Wait, really? So the pressure ratio is the main thing that controls efficiency? What about the turbine inlet temperature?

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Great question! For the *ideal* cycle, yes, efficiency depends only on pressure ratio. But in the real world, material limits and component inefficiencies come into play. That's where the Turbine Inlet Temp (T3) and the efficiency sliders (ηc, ηt) matter. Try setting a high pressure ratio but a low compressor efficiency in the tool—you'll see the actual performance drop.

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Okay, that makes sense. But what's the point of the "Bypass Ratio" control? It seems like a whole different thing.

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It's the defining feature of a turbofan! A common case is a modern airliner engine. A high bypass ratio means most of the air is just accelerated by the big fan and bypasses the core (the Brayton cycle part). This gives more thrust at lower fuel consumption. Switch the "Engine Type" in the simulator and watch how the Specific Thrust and SFC change dramatically.

Physical Model & Key Equations

The ideal Brayton cycle thermal efficiency depends solely on the pressure ratio across the compressor and the specific heat ratio of air.

$$\eta_{th,ideal}= 1 - \pi_c^{-(\gamma-1)/\gamma}$$

Where $\pi_c$ is the compressor pressure ratio (P₂/P₁) and $\gamma$ (gamma ≈ 1.4) is the ratio of specific heats for air. This shows that, ideally, squeezing the air more (higher πc) leads to higher efficiency.

In reality, components aren't perfect. The actual work and temperature changes account for compressor and turbine isentropic efficiencies.

$$T_{2s}= T_1 \cdot \pi_c^{(\gamma-1)/\gamma}$$ $$T_2 = T_1 + \frac{(T_{2s} - T_1)}{\eta_c}$$

Here, $T_{2s}$ is the ideal (isentropic) exit temperature, $T_2$ is the actual hotter exit temperature, and $\eta_c$ is the compressor efficiency. A similar equation applies for the turbine. Lower efficiency means more work is wasted as heat, reducing net thrust.

Real-World Applications

Military Fighter Jets (Low Bypass Turbojets/Turbofans): These prioritize high specific thrust (thrust per unit of airflow) for speed and maneuverability. They use moderate pressure ratios and very high turbine inlet temperatures (T3), which you can simulate by setting Bypass Ratio to 0 or 1 and pushing the T3 slider high.

Commercial Airliners (High Bypass Turbofans): The goal is fuel economy and low noise. Engines like the GE9X on the 777X use bypass ratios above 10:1. In the simulator, set a high Bypass Ratio and observe how Specific Fuel Consumption (SFC) drops significantly compared to a turbojet.

Unmanned Aerial Vehicles (UAVs) & Missiles: Small turbojets or turbofans often operate at fixed, optimized conditions. Engineers use these calculations to balance compressor pressure ratio and weight for a given mission range, tweaking the efficiency parameters you see in the tool.

Engine Health Monitoring & Diagnostics: CAE software and onboard sensors track actual engine performance (e.g., exhaust gas temperature) against the ideal Brayton cycle model. Deviations in expected efficiency for a given pressure ratio can signal compressor fouling or turbine degradation.

Common Misconceptions and Points to Note

When you start experimenting with this tool, there are a few key points you should be aware of. First, there's the misconception that "infinite compression ratio leads to near 100% efficiency". While the equations suggest this, in reality, the compressor outlet temperature becomes excessively high, reducing the amount of heat that can be added by combustion. For instance, setting the compression ratio to 60 might show a thermal efficiency over 70% in the tool, but even with a combustion temperature of 1700K, the actual temperature rise becomes small, and thrust output plateaus. In real engines, material strength and cost are limiting factors, and a compression ratio around 40 is close to the current technological limit.

Next, remember that you shouldn't optimize parameters in isolation. For example, setting both "combustion temperature to max" and "bypass ratio to max" would require an impractically large fan in reality, negated by weight and drag. Try setting "bypass ratio 15, combustion temperature 2200K" in the tool. The specific thrust is indeed high, but this represents an extreme and structurally infeasible "paper engine" due to the severe imbalance between the fan and core. In practice, the first step is to consider the "trade-offs" between parameters based on mission requirements like flight Mach number and airframe size.

Related Engineering Fields

The parameters handled by this simulator are actually a gateway to the vast world of CAE. For example, behind the simple goal of increasing the "combustor outlet temperature" lies a deep collaboration between "Computational Fluid Dynamics (CFD)" and "Materials Engineering". Designing the complex internal cooling passages of turbine blades requires super-precise 3D fluid analysis. Furthermore, to increase the "compression ratio", you need airfoil design based on "Turbomachinery Engineering" to ensure each compressor stage blade efficiently compresses the air, coupled with "Structural Mechanics (FEM analysis)" to evaluate vibrations and fatigue from high-speed rotation.

Also, note that changing the "bypass ratio" alters the engine's diameter and weight. This directly impacts the overall aircraft design, i.e., it's a problem of "Aerodynamic-Structural Integrated Optimization for the entire aircraft". A large fan provides good takeoff thrust but increases drag at high speeds. To solve this trade-off, a methodology called "Multidisciplinary Design Optimization (MDO)" is used, which couples engine performance calculations with aircraft aerodynamic analysis. NovaSolver itself is the core performance model that forms the "kernel" of such complex optimizations.

For Further Learning

Once you're comfortable with the tool, the next step is to delve into "why those equations hold true." I recommend a three-step approach. First, manually trace the thermodynamic "Brayton Cycle" using a T-s diagram. Please take the time to use a calculator with compression ratios $$\pi_c$$ of 8 and 15 to physically experience how the ideal thermal efficiency changes. This will help you develop an intuition for the tool's calculation results.

Next, learn about the parts the tool treats as a "black box". Specifically, the concepts of "isentropic efficiency" for the compressor and turbine, and "pressure loss" in the combustor. Consider how performance degrades when these efficiencies, assumed to be 100% in the ideal cycle, drop to, say, 85%. This is the first step towards more realistic "non-ideal cycle analysis".

Finally, aim to connect the tool's outputs—"specific thrust" and "TSFC"—to "airframe and mission" evaluation metrics. For example, for the same thrust, an engine with higher specific thrust requires less fuel weight. This reduces takeoff weight, allowing for a smaller airframe... Understanding this chain reaction (the "snowball effect") means you're already thinking like a capable systems engineer.