Air-Breathing Propulsion

Why air‑breathing?

• Uses atmospheric $\mathrm{O}{}_2$ as oxidizer
• Avoids carrying oxidizer mass $⟹$ potentially high effective $I_{sp}$ for flight in atmosphere

Families

• Turbojet:
  • all core flow to nozzle
• Turbofan:
  • bypass stream adds thrust
  • high‑BPR improves propulsive efficiency at subsonic
• Ramjet/Scramjet:
  • no compressor/turbine
  • rely on inlet compression
  • scramjet maintains supersonic combustor

Brayton Cycle Basics (Gas Turbine Cycle)

Stations

• 0/2 (inlet)
• 3 (compressor exit)
• 4 (turbine inlet)
• 5 (turbine exit)
• 9 (nozzle exit)

Ideal cycle steps

1. Isentropic compression (inlet+compressor) with pressure ratio ${\pi }_c$.
2. Constant‑pressure heat addition in combustor (set by $T_{t4}$ limit).
3. Isentropic expansion in turbine.
4. Isentropic expansion in nozzle to ambient.

Balances

• Turbine work must drive compressor and accessories
• $$W_t\approx W_c/({\eta }_m)$$

Loss models

• Inlet pressure recovery ${\pi }_d$
• compressor/turbine efficiencies $({\eta }_c,{\eta }_t)$
• combustor pressure loss $\Delta p_c$

Turbojet

Architecture

• Inlet → compressor → combustor → turbine → nozzle

Pros

• High specific thrust at high Mach
• simple fanless architecture

Cons

• High TSFC at subsonic
• loud/hot exhaust

Design levers

• ${\pi }_c$, $T_{t4}$
• variable geometry (IGVs/VSVs)
• afterburner option

Turbofan

Bypass ratio (BPR)

• $$\text{BPR}=\frac{{\dot{m}}_{\text{bypass}}}{{\dot{m}}_{\text{core}}}$$

High‑BPR fans

• Improved propulsive efficiency at subsonic
• lower exhaust velocity closer to flight speed

Trade‑offs

• Diameter/drag
• nacelle weight
• fan tip Mach
• noise regulations

Ramjet & Scramjet

Ramjet

• Subsonic combustor
• requires $M\gtrsim 2$ to self‑sustain
• good for high‑supersonic cruise

Scramjet

• Supersonic combustor
• operates at hypersonic
• mixing/ignition in milliseconds
• intense thermal protection

Inlet criticality

• Shock placement & pressure recovery dictate operability

Inlets & Diffusers

Goals

• Turn freestream into high‑pressure
• low‑distortion flow with minimal total pressure loss

Types

• Subsonic:
  • pitot/scoop
  • avoid separation
• Supersonic
  • external/internal compression with ramps/cones
  • normal/oblique shocks

Metrics

• Pressure recovery ${\pi }_d$
• distortion indices
• bleed & variable geometry for stability

Compressor Maps, Stall & Surge

Map axes

• Corrected flow vs. pressure ratio with speed lines
• surge line bounds stable operation

Controls

• IGVs/VSVs
• bleed valves
• variable area nozzles to keep operating point away from surge

Dynamics

• Transients can cross surge line
• need schedule and FADEC logic

Combustors (Gas Turbine)

Function

• Add heat at near‑constant pressure with adequate residence time for complete combustion and low pattern factor at turbine inlet

Considerations

• Pressure loss
• stability/ignition margin
• emissions (NOx, CO, UHC)
• liner cooling (film/effusion)

Turbine & Work Balance

Role

• Extract just enough work to power compressor + accessories
• keep $T_{t4}$ within material limits

Cooling

• Film and internal cooling
• blade life vs. performance

Afterburners & Nozzles

Afterburner

• Secondary combustor adding fuel in exhaust
• large thrust boost with high TSFC
• used in tactical/high‑Mach segments

Nozzles

• Converging (subsonic), C–D (supersonic)
• variable area to match back‑pressure
• jet area ratio
• over/under‑expansion effects

Matching

• Nozzle area schedules with fan/core flows to maintain stability and specific thrust