Rolls-Royce Viper Thermal Failure Investigation
Aero-thermal failure analysis of the central bearing, identifying heat transfer breakdown at the bearing race as the primary failure driver
Led a thermal failure investigation of the Rolls-Royce Viper turbojet’s central bearing as project lead during a research internship at the Indian Air Force Base Repair Depot, Kanpur.
Background
The Rolls-Royce Viper is a single-spool axial turbojet used in the HAL HJT-16 Kiran trainer aircraft. The central bearing supports the compressor-turbine shaft and operates in a high-temperature, high-speed environment where heat management is critical. Recurring thermal failures at the bearing race prompted the investigation.
Failure Mode Analysis
The failure was characterized by:
- Discoloration and micro-cracking at the inner bearing race
- Evidence of lubricant coking near the oil jet entry
- Material softening consistent with sustained overtemperature
The root cause was identified as a breakdown in convective heat transfer at the bearing race, driven by lubricant flow starvation at elevated shaft speeds.
Thermal Analysis
The heat balance at the bearing was modeled as:
\[Q_{\text{gen}} = Q_{\text{conv}} + Q_{\text{cond}}\]Frictional heat generation in the bearing:
\[Q_{\text{gen}} = \mu \cdot F_n \cdot \omega \cdot r\]Convective removal by the oil film:
\[Q_{\text{conv}} = h A_s (T_{\text{race}} - T_{\text{oil}})\]The convective coefficient \(h\) drops sharply when the oil film thickness falls below a critical value, causing \(Q_{\text{conv}} < Q_{\text{gen}}\) and driving the race temperature above the material’s continuous operating limit.
Gas Turbine Thermodynamic Context
The bearing operates in a region of the engine between compressor delivery and turbine entry. The local temperature \(T_{2.5}\) was estimated using isentropic compression relations:
\[T_{2.5} = T_1 \left(\frac{P_{2.5}}{P_1}\right)^{\frac{\gamma-1}{\gamma}}\]At design point conditions (OAT 35°C), \(T_{2.5}\) was computed and used to establish the thermal boundary condition at the bearing housing.
Corrective Thresholds
Operating limits were derived to prevent recurrence:
| Parameter | Limit |
|---|---|
| Maximum bearing race temperature | < 180°C continuous |
| Minimum oil flow rate at max N1 | > 0.8 L/min |
| Maximum shaft speed before enhanced cooling required | 95% N1 |
Pressure and temperature thresholds at the oil supply jet were established as the primary inspectable/monitorable parameters in maintenance procedures.
Engines Studied
During the internship, overhaul and maintenance procedures were studied for:
- Tumansky R-29 — afterburning turbojet (MiG-27)
- Snecma M53 — augmented turbofan (Mirage 2000)
- Rolls-Royce Viper 11/535 — turbojet (Kiran Mk.I/II)
Exposure to the overhaul cycle for these engines — from disassembly through dimensional inspection to reassembly — built the propulsion hardware intuition that directly informs the physics-based simulation models developed in later work.