Three-Spool Architecture — LP/IP/HP Stages, Shafts and the Combustor

The overview established three rotors, each with its own speed. This article zooms into the cross-section: how many stages each compressor has, what blades the fan carries, what form the combustor takes, how the three turbines each drive their own compressor, and how three shafts nest concentrically without interfering. This is the physical groundwork for stall margin (VSV), speed relationships, vibration sources and overspeed protection later in the chapter.

1. The three-spool module map

Each rotor is a compressor–turbine pair on one shaft, turning independently at its own speed. From AMM 72-30/40/50:

[!note]- The harder a shaft compresses, the fewer turbine stages it needs (integrative synthesis) The fan is a single stage but is driven by a 4-stage LP turbine; the HP compresses across 6 stages yet uses only a 1-stage turbine. The pattern: the more core-ward, higher-pressure-ratio rotor needs fewer turbine stages (the hot, high-pressure gas gives up most of its energy in the first stages). This is a reading of the AMM stage counts, not an FCOM/AMM statement.

2. LP compressor (the fan) — N1

The LP compressor is a one stage rotor with 26 wide-chord type blades which engage in axial dovetail slots... The LP shaft is attached to the disk with a curvic coupling and held with bolts. Behind the coupling there is a roller bearing which keeps the shaft in the correct radial position.

Per AMM 72-30-00 §2.A, the fan is a single stage with 26 wide-chord blades in axial dovetail slots, the LP shaft joined to the disk by a curvic coupling. Speed is sensed mechanically:

At the rear of the roller-bearing inner-race assembly there is a machined phonic wheel. This is used with an electrical pick-up to measure LP compressor speed.

A phonic wheel plus electrical pick-up measures LP speed (N1); the indicating side is covered in Engine Indicating. The single large fan is the bypass thrust producer, and wide-chord blading is a foreign-object-damage and aerodynamic-efficiency feature — relevant to Volcanic Ash / FOD.

3. IP compressor — N2 (with VSV)

The IP compressor is an eight stage assembly that includes the primary items which follow: ‐ the Front Bearing Housing (FBH) ‐ the Variable Stator Vane (VSV) case ‐ the IP compressor rotor ‐ the IP compressor case.

The IP compressor is an 8-stage assembly (AMM 72-30-00 §2.C) and carries the Variable Stator Vane (VSV) case.

[!note]- What the VSV does, and why it sits on the IP (integrative synthesis) Variable Stator Vanes change stator blade angle with operating point, redirecting the flow into each stage to keep compressor stall margin across the speed range. This article only notes its location on the IP compressor; the control law and stall mechanism are in Stall / Surge. The VSV's existence is AMM-72-30 §2.C verbatim; "angles to preserve stall margin" is engineering reasoning.

4. HP compressor — N3

The HP compressor rotor is a six stage assembly of titanium disks welded together as one drum. Adjacent to the rotor blade locations each disk has fins which make interstage seals with the stator vane shrouds.

Per AMM 72-30-00 §2.E, the HP compressor is a 6-stage drum of titanium disks welded together. N3 is the HP-rotor speed — the highest-turning and most critical core-health parameter, shown on the E/WD.

5. Combustion chamber — annular

The combustion chamber is an annular liner assembly. There is an inner combustion liner and an outer combustion liner. The liners contain holes and chutes that are used to mix the air/fuel and also give dilution. Two of the chutes in the outer liner are used to give the locations for the igniter plugs.

The combustor is annular, with inner and outer liners (AMM 72-40-00 §2.C). Note the two chutes in the outer liner for igniter plugs — the hardware basis for the dual ignition channels covered in Ignition.

6. The three turbines

The HP turbine is a one stage disk which is attached to the rear of the compressor drum with bolts.

The IP turbine is also a one stage disk... A stub shaft is connected to the inner race of the turbine roller bearing to hold the turbine in position. The other shaft goes through this and the HP assembly and connects to the IP compressor.

The LP turbine rotor has four disks which are connected together with bolts to make one drum. Each disk has axial firtree root slots around the circumference for the installation of turbine blades.

Per AMM 72-50-00: HP turbine 1 stage, IP turbine 1 stage, LP turbine 4 stages. Crucially, the IP turbine's shaft goes through the HP assembly centre to reach the IP compressor — the verbatim basis for the concentric-shaft topology below.

7. Concentric, nested shafts (integrative synthesis)

Stitching the verbatim facts together — the IP shaft passes through the HP assembly centre (AMM 72-50-00 §2.B) and each turbine drives its associated compressor via a shaft (DSC-70-10-10) — gives the nested layout:

 front ◄──────────────────────────────────────────────────► rear
 ┌Fan (LP,1)┐ ┌IP comp(8)┐ ┌HP comp(6)┐ │comb│ ┌HPt┐┌IPt┐┌─LP turb(4)─┐
 │   N1     │ │   N2     │ │   N3     │ │    │ │ 1 ││ 1 ││            │
 └────┬─────┘ └────┬─────┘ └────┬─────┘ └────┘ └─┬─┘└─┬─┘└─────┬──────┘
      │ LP shaft (innermost, longest, full length) ─────────────┘  N1
           │ IP shaft (through the HP assembly centre) ─────┘       N2
                     │ HP shaft (outer, shortest, mid-engine) ┘     N3

• HP shaft (N3): HP compressor (6) ↔ HP turbine (1); shortest, mid-engine.
• IP shaft (N2): IP compressor (8) ↔ IP turbine (1); passes through the HP assembly centre (AMM 72-50-00 §2.B verbatim).
• LP shaft (N1): fan (1) ↔ LP turbine (4); innermost and longest, running the full length.

Three concentric shafts, each at its own speed — the physical realisation of "three spools" and the reason there are three independent speed parameters.

8. Gas generator fairings

The gas generator fairings give a smooth inner surface to the annulus through which the Low Pressure (LP) compressor air flows. Six fairings are installed, in Zone 2, around the front part.

Six fairings in Zone 2 (AMM 72-22-00 §1) smooth the annulus through which LP-compressor air flows.

Self-test

[!note]- Q1. How many stages do the LP / IP / HP compressors have, and what blades does the fan carry? LP = single stage (26 wide-chord blades) / IP = 8 stages (with VSV) / HP = 6 stages (welded titanium drum).

[!note]- Q2. How many stages do the HP / IP / LP turbines have? HP turbine 1 / IP turbine 1 / LP turbine 4.

[!note]- Q3. How do the three shafts stay concentric without interfering? Nested concentrically: HP shaft shortest (mid-engine), IP shaft through the HP assembly centre, LP shaft innermost and longest. Each turns at its own speed → N1/N2/N3.

[!note]- Q4. What form is the combustor, and where are the igniters? Annular, inner and outer liners; two chutes in the outer liner hold the two igniter plugs.

[!note]- Q5. How is N1 measured? A phonic wheel plus electrical pick-up behind the LP roller bearing measures LP speed = N1.

Key takeaways

References

• AMM 72-30-00 §2.A/C/E — LP fan (single stage, 26 wide-chord, curvic coupling, phonic wheel), IP compressor (8 stages, VSV), HP compressor (6-stage welded titanium drum).
• AMM 72-40-00 §2.C — annular combustor, inner/outer liners, two igniter-plug chutes.
• AMM 72-50-00 §2.A/B/C — HP turbine (1 stage), IP turbine (1 stage, shaft through HP centre), LP turbine (4 stages).
• AMM 72-22-00 §1 — six gas-generator fairings, Zone 2.
• FCOM DSC-70-10-10 — each turbine drives its associated compressor (overview link).

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.