Gas Turbine World

Garrett Airesearch (Honeywell) Gas Turbines

Garrett Aireasearch are one of the largest and most successful small gas turbine manufacturers in the World.

Garrett GTP30 APU

The Garrett GTP30 is the smallest of the Garrett APU family that was produced in the 1960’s. This small unit is rated between 40-100 HP and is used in stationary generator sets and as an on-board APU (GTP30-92) in the HS125 aircraft. The GTP30-67 model was fitted to a 25Kva USAF generator set known as the EMU12. A number of these EMU12 generator sets have been disposed of and entered the surplus market, this may not meet with the manufacturer Garrett’s (Honeywell’s) approval!

The GTP30 is a simple single shaft unit employing a single radial inflow turbine impeller. The rotor is supported on spring loaded ball bearings mounted outside of the engine in a “canter-leaver” fashion. This clever trick ensures the bearings run in the coolest area possible. The bearings are mounted in a carrier just in front of the compressor wheel and a carbon face seal ensures oil consumption is kept to a minimum. The engine employs a wet-sump gearbox, a hydro-mechanical fuel control system and the combustor is a simple single burner can arrangement. The single can combustor was a trade mark design of Garrett engines in that era. The engine usually drove a AC or DC generator at 8,000 rpm via a single stage reduction gearbox.

This simple electric start engine is a great small and compact engine rivaled by the neat little Solar T62 series of small gas turbines.

These engines have a straight-cut gearbox and can sound quite rough when starting, also the heavy over-hung rotor can be heard to pass through critical whirl speeds as the engine runs up and down.

Garrett GTP30 Specification (GTP30-67)

Garrett GTCP36

A newer and very successful generation medium size auxiliary power unit the GTCP36 series. A popular unit fitted to many medium size business jets and regional commuter jets. This dash 150 example is fitted to the BAE 146 aircraft. Different units were produced, earlier models featured a single can combustor and hydro-mechanical fuel control, this is similar to previous models such as the GTP30. Many later units are found with an annular combustion chamber with multiple atomising burners. Later units also featured and electronic fuel control system. The engine ring gear is fitted with teeth to produce a speed signal in conjunction with a magnetic reluctance probe. The speed signal is processed by an electronic control unit (ECU) which in turns drives an electrically operated metering valve mounted on the fuel pump.

The GTCP36 is more powerful than it’s predecessors rated at around 200HP, the single compressor turns at 60,000 rpm, is more highly loaded and driven from a radial inflow wheel. This unit can make 1LB/s bleed air at a pressure of over 50 PSI. Units are electric start and also fitted with a 400Hz generator. A straight forward electric start unit, surplus ones may be bench run without an ECU but putting it under load and to good use requires the ECU to control it properly.

Garrett GTP70 Gas Turbine

The GTP70 is a single shaft unit built to drive a 60Kw alternator for use on board aircraft. This unit features a two-stage radial flow compressor and a single stage radial inflow turbine. The rated output is 160HP at 40,000 rpm. This early generation gas turbine featured a two-stage compressor that was lightly loaded producing a pressure ratio of just 4:1. The GTP70 is quite a large and heavy engine for it’s modest power output. Off-load a cool running unit producing around 300 degree C exhaust temperature.

Like other Garrett engines a nicely built unit with magnesium alloy reduction gearbox that carries the accessory drives. The GTP70 unit again carries the Garrett trade-mark single can combustor fitted with a simplex burner.

A useful engine for research purposes, as with other Garrett models the single can combustor arrangement facilitates alternative combustion arrangements. It is relatively easy to replace the combustion chamber with a modified version. Such an arrangement has enabled a GTP70 model to be run a gaseous fuel by enlarging the combustor volume and fitting a gas-type burner head.

Garrett GTP70 Specification
Power Output................................…........................................…………………………………………….........160 Bhp
RPM.......................................….……..........................................……………………………………….................40,800
Compressor...............................…............................................…..............Two-stage Centrifugal Impellers
Combustion Chamber...........................................................Reverse Flow Can Type with Single Burner
Turbine.......................................................................................….………………............Radial Inflow Impeller
Starting....................................................................................….……………………………............Electric Starter
Ignition.........................................…..........................................……………………………….…...........High Energy
Fuel Syste……........Gear Pump with Centrifugal Governor, Acceleration and Temperature Controls
Lubrication System..................…….......Return Dry-Sump with engine driven Oil Cooler and Blower
Layout...............................….………...... Single spool with reduction gearbox, output speed 6,000 rpm.
Weight..........................................................................................….…………………………………………..........91 kg
Applications............................................................................................................Airborne APU and GPU

Garrett GTCP85 Four Bearing Engine

GTC85 and CTCP85

The “Gas Turbine Compressor model 85” or the “Gas Turbine Compressor & Power model 85” must be the World’s most common and successful gas turbine compressor for aircraft starting. Many thousands have been built and used both in ground air-start carts and as APUs in aircraft.

The model 85 consists of a two-stage compressor and single stage radial turbine and a single can combustor. The first stage compressor consists of a “Whittle style” back-to-back impeller arrangement and a ring of gross-over elbow ducts to transfer the air in to the second stage. The back-to-back arrangement reduces the overall diameter of the compressor assembly. This assembly most accommodate the compressor wheel itself, the surrounding diffuser and the cross over ducts. Space must also be provided so that the backward facing compressor intake may draw air in around the crossover ducts.

The two-stage compressor loading is such that it will tolerate the differing demands for bleed air without stalling or surging. Other rival gas turbine compressors of similar size and capacity make use of just one, more heavily-loaded compressor stage (e.g. Palouste engine). The lighter compressor loading alleviates the requirement for a regulating air dump valve, consequently the engine runs cooler when the valve is shut.

Several different models of 85 series exist. A unit known as the GTC85 consists of a compressor unit with only a small accessory gearbox and so function only as a compressor unit. A small gearbox carries the engine accessories and a small generator for battery re-charging. The GTC85 series unit was mounted in the USAF starting cart known as the MA1A and not to be confused with the gas turbine powered M1 Abrams tank of similar designation (also powered by a very different Garrett/Honeywell engine).

Garrett GTCP85  Specification
Power Output................................…........................................…………………………………………….........300 Bhp
Air bleed output....................................................................................................................................1LBs/s
RPM.......................................….……..........................................……………………………………….................40,000
Compressor...............................….............................Doubled sided compressor followed by single stage
Combustion Chamber...........................................................Reverse Flow Can Type with Single Burner
Turbine.......................................................................................….………………............Radial Inflow Impeller
Starting....................................................................................….……………………………............Electric Starter
Ignition.........................................…..........................................……………………………….…...........High Energy
Fuel Syste……........Gear Pump with Centrifugal Governor, Acceleration and Temperature Controls
Lubrication System..................…….......Return Dry-Sump with engine driven Oil Cooler and Blower
Layout...............................….………...... Single spool with reduction gearbox, output speed 6,000 rpm.
Weight..........................................................................................….…………………………………………..........91 kg
Applications............................................................................................................Airborne APU and GPU.

Garrett GTCP85 Two Bearing Engine

A later version of the model 85 adopts a modified shaft arrangement. Original 85’s had four main shaft bearings, a newer arrangement makes use of just two. These engine’s also use a larger flat-topped combustion chamber case.

Model 85s were also fitted with 400Hz generators able to provide up to 60 KVa.

There are a number of airborne versions of the GTCP85 engine. Versions of the model 85 are fitted to aircraft such as the C130, Boeing 727, Boeing 737 and were also fitted to the BAC 1-11.

Garrett Airesearch JFS100 

The JFS100 gas turbine starter. A small twin-shaft gas turbine engine that is used as a starter motor for starting much larger aircraft engines. The engine is completely self-contained and itself may be started by electrical power or compressed air. An unusual design featuring an offset accessory gearbox fed with a radial shaft and bevel gear from the main compressor shaft.

A great little engine which may be converted to a jet or even better used as shaft power in a small vehicle.

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