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The GMC Astro 95 and Astro SS Gas-Turbine Tractors


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The future for gas turbines appeared bright in the 1970s, particularly with the introduction of the Detroit Diesel Allison GT-404-4.

Information on these trucks is extremely limited, a but I’d like to share what I know.

The Astro 95 gas-turbine was powered by the General Motors developed GT-309 gas turbine.

It was equipped with GM’s 34,000 pound "Astro-Aire" rear air suspension, available from 1971 and offering up to 1,000 pounds in weight savings.

Throughout its 18-year production run, the Astro’s grille opening was never enlarged, and the gas-turbine Astro was no exception. GMC made the grille appear larger by using a painted section that matched the pattern of the truck's custom grille.

The truck's chassis equipment, including battery boxes, steps, fuel tanks, suspension, and even wheels, were all painted white, complimented by a chrome front bumper.

The truck’s enlarged dual exhaust stacks were painted white to draw attention to the clean-burning nature of the truck’s gas-turbine powerplant.

Like many GM concept trucks such as the Turbo Titan III, the Astro gas-turbine had its own trailer for display and load testing.

The U.S. Department of Energy (DOE) initiated a program in the mid-1970s at the Detroit Diesel Allison Division of General Motors to design ceramic components into the GT- 404-4 gas turbine truck engine. Testing included powering a truck on highways, city roads and the GM proving grounds, where the engine was exposed to extreme vibrational and shock loading on the Belgian-block and truck-durability road courses. Testing clearly demonstrated that properly designed ceramic components could survive under the most severe conditions for a typical vehicle.

GM said the emergence of low-cost, high-temperature ceramic components could allow the potential of gas turbines to be realized. Operating with ceramic components at turbine inlet temperatures up to 2350 F to 2500 F, GM said the GT-404 engine had the potential to offer fuel economy equal or superior to current diesel engines.

From 1974 to late 1977, Detroit Diesel Allison installed nearly one hundred GT-404 and GT-505 series gas turbines in trucks, inter-city coaches (Greyhound), municipal buses (Baltimore – RTS-II), fire apparatus (American LaFrance), marine and industrial applications to conduct field evaluation. Consolidated Freightways, Greyhound, Ruan Transportation, Terminal Transport, GM Truck and Coach and Freightliner participated.

Later in 1979, an “Astro SS” was built equipped with a General Motors GT-404-4 split-shaft regenerative gas turbine incorporating newly developed high temperature resistant ceramic materials. At 1,750 pounds, the GT-404 was approximately 650 pounds lighter than a comparable Detroit Diesel 8V-71. On a vehicle-installed basis, a 1,000 pound weight savings was realized, once reason being the lack of an engine cooling system.

  • GT-404 300/310/325 horsepower
  • GT-505 390/400 horsepower
  • GT-606 475 horsepower





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Dept. of Energy/Dept. of Transportation

Gas Turbine Transit Bus

Demonstration Program

Program Plan

April 1978

One of the most promising advanced propulsion systems is the gas turbine engine. The gas turbine not only offers potential for reductions in noxious emissions and fuel consumption, but also could provide operators with improvements.

Development of the gas turbine engine as a potential power source for automotive vehicles began in the early 1950's. In1976, only three manufacturers in the United States, Chrysler Corporation, the Detroit Diesel Allison (DDA) Division of General Motors Corporation, and Industrial Turbines International (Garrett, Mack Trucks and KHD) were actively engaged in the development of small (150 HP to 650 HP) gas turbines that could realistically be considered engines for automobile, truck and bus use.

The Chrysler engine evolved as a passenger car engine. In 1962 this engine was extensively field-tested in 50 prototype automobiles. The development of this engine is continuing with U.S. Government financial aid, in areas that can best be described as "advanced technology."

The ITI consortium (Garrett, Mack Trucks and KHD) engine is in the 450 HP to 650hp range and is being designed as an eventual replacement for truck diesel engines. It is in the initial, unproven stages of development and prototype engines will not be available for 2 to 3 years. Production engines could not be available for several years after the prototypes.

Activities Involved in Developing Improved Gas Turbine Engines

Chrysler Corporation - ERDA Baseline Engine (150 HP) and Upgraded Engine (123 HP) under active development, 2-shaft regenerative

Detroit Diesel Allison - 300 HP, 400 HP and 500 HP, 2-shaft regenerative engine under active development for bus, truck, industrial application

ITI (Garrett/Mack/KHD) - Comprehensive program for truck and industrial applications 450-650hp range

The DDA gas turbine engine has been designed as an alternative for the diesel engines manufactured by that firm, and is currently developed to a state where volume production can be seriously considered. In addition to research and development engines, nearly 100 DDA gas turbines have been field-tested in trucks, transit coaches, intercity coaches, marine craft, and industrial electrical generator sets and air compressors.

Since 1972, test engines in 24 trucks of ten different brands have operated in widely divergent service in all parts of the country. In addition, eight Greyhound inter-city coaches powered by GT-404 gas turbines have operated throughout the country. In the most recent of these applications, the turbine engine has demonstrated greatly improved reliability and fuel consumption rates compared with earlier turbines. In fact, the fuel consumption in the inter-city coaches was nearly competitive with that of the diesel engine.

In the early 1970's, under the Urban Mass Transportation Administration (UMTA) Transbus Engineering Test Program, three DDA gas turbine engines were installed in the Transbus prototype coaches manufactured by the Truck and Coach Division of General Motors. This engine was selected for testing in Transbus because of the gas turbine's apparent advantages and demonstrated potential in heavy trucks. During this program the turbine engine demonstrated its potential as a viable transit coach powerplant by exhibiting the following advantages over the conventional diesel engine in transit coach application:

  • Reduction of installed weight and volume
  • Elimination of cooling radiator, fan, and
  • attendant piping
  • Cleaner exhaust emissions
  • Lower noise level
  • Reduced or low vibration operation
  • Reduced lubricating oil consumption
  • Good reliability
  • Improved vehicle performance for power rating
  • Greater engine braking capability
  • Superior cold weather starting

Fuel consumption demonstrated in the program was higher than contemporary diesel engines. The extended periods of idle and extensive part-load operations, both inherent to transit coach service, accounted for a measure of the high fuel consumption.

However, improvements have been made since these coaches were evaluated and further improvements are scheduled for reducing brake specific fuel consumption (BSFC) in the transit coach duty cycle.

Because of the gas turbine's apparent advantages and demonstrated success in heavy trucks, an in-depth survey of gas turbine engine manufacturers was conducted to determine the suitability of the turbine engine for transit coach application.

The following sections trace the development of the only gas turbine engine found to be sufficiently developed to be considered for near-term volume production and subsequent application to transit coach service.

Gas Turbine Engine Development at Detroit Diesel Allison

For over 20 years, Detroit Diesel Allison both independently and with government assistance has been energetically developing gas turbine engines to compete in many commercial applications with piston diesel engines. Specifically, they have concentrated on applications in large trucks and buses, although field evaluation has included construction equipment, marine equipment, and electrical power generation units.

DDA's goals were to develop a practical, cost-effective turbine engine to meet the requirements of heavy vehicles. This program has resulted in a family of three engines covering the power range from 300 HP to 500 HP (SAE rating): The smallest of these engines is designated GT-404, and engines incorporating the latest design changes with improved performance have the -4 suffix.

The GT-404 gas turbine is a two-shaft, regenerative gas engine featuring a power transfer system. A rigid block assembly, constructed of cast iron, serves as the main structural support member for the engine. It houses, in a modular fashion, the burner, gasifier section, power section, regenerators, and the reduction and accessory drive gearing. The modular design allows easy service and unit replacement of the various sections. The engine controls are electronic and are remotely mounted from the engine block itself. The engine is available with a rated maximum output shaft speed of 2880 rpm.

The GT-404 engine's normal dry weight of 1750 pounds is approximately 650 pounds lighter than a comparable diesel engine, the DDA 8V-71, and its basic size is very similar. On an average vehicle-installed basis, this weight savings advantage is about 1,000 pounds, and the GT-404 can be installed in virtually any vehicle accommodating the 8V-71.

The GT-404-4 will have similar performance but with improved fuel economy and durability. Maximum torque is produced at power turbine (output shaft) stall (0 rpm) condition. With the high torque rise characteristic, fewer number of transmission gear ranges are required-generally five or less in trucks, and four or less in buses. With an automatic transmission, a torque converter is not required and a fluid-coupling can be used. The dynamic braking capability of the engine is equal to the rated power at maximum output shaft speed and is effective in each transmission gear range.

The GT-404 engine displays a considerable cost savings, consuming just one quart of lubricating oil per 20,000 to 30,000 miles compared with about 1 quart per 800 miles for a diesel engine in transit coach service . In addition, the turbine engine oil requires changing every 250,000 miles compared with 9,000 miles for the transit coach diesel engine.

The GT-404 engine emits less noise than a comparable diesel engine. Results of an exterior noise test on trucks, conducted to the SAE test standards, demonstrated that the noise level of the GT-404 engine is 11 dBA lower than a standard diesel powered truck. This 11 dBA lower reading represents nearly a 75 percent reduction in sound pressure over a diesel engine. Development effort is continuing toward a still further reduction of the noise level to meet future noise attenuation requirements.

Major pollution elements in transit coaches would be minimized with the GT-404 engine through highly efficient, low-pressure, continuous burning with large amounts of excess air resulting in almost 100 percent combustion.

The gas turbine’s exhaust odor is virtually undetectable, exhibiting only as light kerosene odor a t engine idle, and exhaust smoke is virtually undetectable. And it’s ability to start quickly at low temperature is superior to any conventional diesel powerplant. The GT-404 has demonstrated its ability to start, without aids, in temperatures well below 0º F. However, batteries must be reasonably well-charged and #1 diesel fuel must be used (since #2 diesel fuel begins to gel around +20º F).

The GT-404 does not require a water-based cooling system because it is internally cooled by the excess air passing through the engine and by the lubricating oil which is cooled through a small oil-to-air heat exchanger. The elimination of a water-based cooling system greatly decreases engine maintenance and downtime, thereby reducing a major maintenance cost area in transit coaches.

The gas turbine engine, in general, requires less maintenance than a diesel engine because of fewer wearing parts and almost vibration-free operation. All moving elements in the basic turbine engine are rotary in motion, compared with reciprocating components in diesel engines.

No water hoses or pipes, drive belts or other elements that tend to be unreliable are used. Use of self-cleaning inertial air filters, absence of a liquid cooling system and manifold exhaust system, extended brake life, and expected extended life of components will contribute to reduced maintenance costs and significantly increase vehicle availability.

The fuel system of the GT-404 engine can operate on a wide range of petroleum-based fuels including: #1 diesel, #2 diesel, heating oil, JP fuels, kerosene and gasoline. The diesel fuels are most commonly used because of their higher energy content and current ready availability. Gasoline is the least preferred fuel because the lead additive tends to deposit and foul the turbine, nozzles, and regenerators, thereby reducing engine performance. When synthetic or other fuels become available in quantity, the fuel handling and control system of the GT-404 can be modified to accommodate these fuels.

Detroit Diesel Allison Field Tests

DDA has manufactured approximately 100 GT series gas turbines for field evaluation in trucks, buses, boats, electrical power generation units and other applications. These engines include the GT-404 and GT-505 in both the -2 and -3 versions. Pilot models of the engine began going into service in 1972 for extensive field evaluation. The engines have been tested in 24 trucks from ten manufacturers, eight motor coaches from MCI-Greyhound, coaches from GMC Truck and Coach Division, Transbus prototypes, various watercraft and industrial applications.

Consignment engines were operational with Greyhound on the East Coast and West Coast, Binswanger Trucking in Los Angeles, Freightliner Corporation and Consolidated Freightways in Portland, Acadian Marine Rentals in New Orleans, Terminal Transport in Atlanta, Gardner-Denver in Quincy, Illinois, a Hatteras yacht operating in the waters of New Jersey, GMC Truck and Coach Division of General-Motors in Pontiac, Michigan, and Detroit Diesel Allison in its Indianapolis-based field-test vehicles.

The major effort at DDA in the development of the GT-404 engine has been directed toward heavy trucks. The field experience with turbine engines in transit coaches has been the Transbus prototypes, and several transit coach engineering models assembled by the Truck and Coach Division of General Motors. The most extensive coach experience with turbine engines is in the Greyhound fleet. However, it must be recognized that the duty cycle and service requirements for intercity coaches, such as those operated by Greyhound, are substantially different from transit coaches.

The eight GT-404 turbine powered Greyhound coaches, four MC-7s and four MC-8s, have logged well over one million miles since mid-1975 and several have been refitted with -3 engines. These particular -3 engines were capable of operating at 0.51 BSFC as compared with 0.54 BSFC of the -2 engines used in the Transbus prototypes. The last of the -3 engines with the latest burner improvements, and other developments, lowered the BSFC to 0.45. Even without the latest engine modifications, the fuel penalty sustained by Greyhound has averaged slightly over 1 mpg for the turbine engine compared with the diesel. The gas turbine engine also virtually eliminated engine overheating and other cooling system-related problems, which account for 50 percent of Greyhound's road failures with diesel engines. Brake life on the turbine powered Greyhound coaches was extended by more than 50 percent due to the engine's regenerative braking system. Elimination of engine vibration-induced cracks in refrigerant lines and fittings improved air conditioning system reliability in turbine powered coaches, and elimination of engine vibrations and reduction of the powerplant weight improved coach structural integrity. The largest number of engine problems Greyhound experienced with the turbine engines were with the various electrical controls.

Eleven Allison GT 404-4 gas turbine engines, and five HT740CT and six V730CT Allison automatic transmissions, were supplied to Greyhound for testing in 1981.

The turbine engine in the early years of production is projected to cost 20 to 25 percent more than the 8V-71 diesel engine. The projected production cost of a turbine engined transit coach, however, is projected to be $70,000 in 1976 dollars, or only 2.9 percent more than $68,224 for the typical diesel-powered coach currently in production.

DDA Engine Survey Conclusions

The operational cost impact analysis indicates that transit coaches equipped with the production improvement 'gas turbine engines (GT-404-4 PI) will be essentially equal in total operating cost to current diesel-powered transit coaches, without including any added value for:

  • BSFC performance gain potential beyond 1983
  • Multi-fuel capability
  • Ability to accommodate future synthetic or
  • alternative fuels
  • Improved operational performance
  • Conformance with environmental standards
  • Perceived environmental improvement
  • Reduced noise
  • Reduced gaseous emissions
  • Elimination of exhaust odor and smoke.

The tangible and intangible value/benefit of these attributes can, based on currently proposed regulations, lower the operating cost of a turbine-powered transit bus by well over 1 cent/mile.

If the current government regulatory trends continue into the 1980's, the above factors will become increasingly important and realistic monetary value could be assigned to these benefits offered by the gas turbine engine. The current higher fuel consumption of the gas turbine could continue to be a disadvantage of 'the engine, particularly if the cost of fuel continues to escalate at a faster rate than other costs. The multi-fuel capability and the ability to burn less expensive lower grade middle-distillate fuels may offset this disadvantage, especially if non-petroleum based fuels become readily available.

Beyond 1983, the fuel consumption of the gas turbine engine will continue to improve relative to the diesel engine. Increasing emission controls for diesel engines will result in a more complex engine, with escalating cost and declining fuel economy, while development of improved technologies in all gas turbine component areas indicate that gas turbine fuel economy will continue to improve. In fact, by the mid-1980's gas turbine fuel economy should equal that of the diesel engine at most speed/load ranges (with the exception of idle).

The major factors indicated by the cost impact analysis are the improved reliability and reduced maintenance costs of the gas turbine engine, offsetting the increased fuel consumption. The turbine engine used in the analysis was the DDA GT-404-4 PI model. This improved engine, with a rated MTBO of 10,000 hours, is tentatively scheduled for

volume production in 1983, which is the earliest reasonable date to expect widespread use of turbine engines in transit coaches.

The procurement cost of the turbine engine, as stated previously, will initially be about 20 percent higher than the contemporary diesel engine. As the production volume of the engine is increased, the cost of the engine should be reduced and may be comparable with the diesel engine. Cost of the diesel, moreover, may be adversely affected by additional equipment required as a result of environmental regulations. The anticipated fuel consumption improvements will be attained by increasing the TIT and possibly variable inlet geometry. The higher TIT will necessitate incorporating improved materials in the nozzles and turbine blades and may also require blade cooling. These changes and the additional complexity of the variable inlet mechanism will increase the cost of the engine somewhat, but it must be presumed that the competitive nature of the engine market will maintain the selling price of the gas turbine within a range commensurate with the advantages it has to offer for vehicular power.



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Ford Motor Company also experimented with gas turbine powered inter-city coaches in cooperation with Continental Trailways. In Detroit, Ford installed a gas turbine in a 1969 Model 05 Eagle operated by American Bus Lines (unit 29511). In 1970, it went into regular service running from New York City to Los Angeles via St. Louis.

Unlike the Greyhound gas turbine buses where were paid for under grants from the US Dept. of Energy (Greyhound just supplied the MCI buses), the Trailways gas turbine Eagle was entirely privately funded by Continental and Ford.

The Continental-Ford project went better than expected. The initial engine actually proved too powerful, and a smaller gas turbine was subsequently installed.

How often do you see the signage "Turbine Power By Ford" on the rear of a Trailways bus?



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In 1976, Allison began development of a generator powered by the GT-404 gas turbine to supply power to the radar set and engagement control station of the U.S. Army Patriot Missile System. The program’s goal included placement of two 150 kW generator sets providing 100 percent backup in a single container to be carried on an Army 5-ton truck. Other goals included minimizing fuel consumption by the use of twin, rotating ceramic-disk regenerators and developing a reliable, multi-fuel capability without adjustment.

In 1978, Allison began the design, development and construction of five military specification gas turbine-powered generator sets. The completed generator sets were tested at the Aberdeen, Belvoir, Elgin and White Sands facilities with these results:

  • Fuel consumption was reduced from 48 to 16 gallons per hour as compared with previous generators.
  • A 0.1 percent frequency stability at rated load was obtained.
  • Free-shaft starting to minus 50 F was accomplished without heaters.
  • Multi-fuel capability was demonstrated on diesel, JP and gasoline.
  • All reliability requirements were met.
  • Sound level standards of less than 90 dBA were met.

In December 1981, Allison delivered an initial order of 200 generator sets to the U.S. Army. Thru 1997, over 2,000 GT-404 powered generator sets were delivered to-date for the Patriot system, which was employed during the Gulf War.

These generators have logged over 1 million hours of operation without any major problems.

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Well Kevin,

That is quite the bit of history. Hats off to you for typing all this out for all of us to learn more heavy truck technology history.

If memory serves, Ford installed at least 1 in a W series COE & quickly learned to point the exhaust up as the engine idling burned & melted the asphalt road way.

International also performed tests & evaluations of a turbine powered Transtar 2 COE, the unit had a distinctive solid panel covering up the grille.

In the 1960's Kenworth also experimented with a turbine powered 524 conventional. The press releases touted a 200 pound turbine powered the unit.


Richard Mark

Owner / Master Model Maker

Industrial Model Design

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GMC big truck's!!! Once a great company with great class 8 trucks!!!....................... Then came Volvo and BANG!!! all gone now..White motors as well!!.......Mack???


The writing is on the wall, In fact it has been there before the ink was dry the day V took ownership of Mack........ :pat::pat:

Richard Mark

Owner / Master Model Maker

Industrial Model Design

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Caterpillar tested a few gas turbine powered on-highway trucks.

My understanding is the CAT gas turbines were for off-highway dump trucks and construction machinery.

Noel Penny Turbines (NPT) of Coventry, UK designed, developed and built 350 horsepower gas turbine engines for Caterpillar, for off-highway applications.

CAT built an experimental model 621 turbine scraper in 1968, a model 992 turbine rubber tired loader in 1973, and I heard they also built a gas turbine grader.

Penny had been managing director of Leyland Gas Turbines, which had developed two-shaft gas turbine engines with regenerative heat exchangers from 1968 to 1974 for experimental Leyland gas turbine trucks.

Leyland Gas Turbines was the successor to Rover Gas Turbines, a small UK company that had worked with Sir Frank Whittle during World War II on England’s first jet engines. Rover was the earliest company to consider gas turbines for vehicles and built the world’s first gas turbine car in 1950.

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Caterpillar had at least two (2) on-highway tractors in the early 70s. They along with John Deere also experimented with gas turbines in off-highway equipment. CAT had a dedicated "turbine R&D building" at the Techical Center in Mossville.


PRR Country and Charter member of the "Mack Pack"

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