The Allison Model 250, known as the Rolls-Royce M250, (US military designations T63 and T703) is a successful turboshaft engine family, originally developed by the Allison Engine Company in the early 1960s. The Model 250 has been produced by Rolls-Royce since it acquired Allison in 1995.
Allison adopted a reverse airflow engine configuration for the Model 250: although air enters the intake/compression system in the conventional fashion, the compressed air leaving the centrifugal compressor diffuser is ducted rearwards around the turbine system, before being turned through 180 degrees at entry to the combustor; the combustion products expand through the two stage HP turbine, which is connected, via the HP shaft, to the compression system, before expanding through the two stage power turbine; the exhaust gases then turn through 90 degrees to exit the engine in a radial direction; a stub shaft connects the power turbine to a compact reduction gearbox, located inboard, between the centrifugal compressor and the exhaust/power turbine system.

One of the latest versions of the Model 250 is the -C40, which has a centrifugal compressor pulling a pressure ratio of 9.2:1, at an airflow of 6.1 lb/s (2.8 kg/s), and developing, at the shaft, 715 hp (533 kW).

Some of the earlier versions have axial compressor stages mounted on the HP shaft to supercharge a relatively low pressure ratio centrifugal compressor. The -C20R is typical, pulling an overall pressure ratio of 8.0:1, at an airflow of 4.0 lb/s (1.8 kg/s), with a power output, at the shaft, of 450 hp (340 kW).

The Model 250 powers a large number of helicopters, small aircraft and even a motorcycle (MTT Turbine Superbike). As a result, nearly 30,000 Model 250 engines have been produced, making the Model 250 one of the highest-selling engines made by Rolls-Royce.

Variants:
250-B15
250-B15A
250-B15C
250-B17
250-B17B
250-B17C
250-B17D
250-B17F
250-B17F/1
250-B17F/2
250-C18
250-C18A
250-C20
250-C20B
250-C20F
250-C20J
250-C20R
250-C20R/1
250-C20R/2
250-C20R/4
250-C20S
250-C20W
250-C22B
250-C28
250-C28B
250-C28C
250-C30
250-C30G
250-C30G/2
250-C30M
250-C30P
250-C30R
250-C30R/3
250-C30R/3M
250-C30S
250-C30U
250-C34
250-C40B
250-C47B
250-C47E
250-C47M
T63-A-5
T63-A-5A
T63-A-700
T63-A-720
T703-AD-700
Applications:
Fixed-wing
Beechcraft Bonanza {Prop-jet conversions}
Britten-Norman BN-2T Turbine Islander
Extra EA-500
GAF Nomad
Gippsland GA10
Partenavia AP.68TP variants - Spartacus & Viator
Soloy Cessna 206 turbine conversion
Rotary-wing
Agusta A109A
Bell 206B/L/LT
Bell 407
Bell 222SP
Bell 230
Bell 430
Bell OH-58 Kiowa
Bell YOH-4
Boeing AH-6
Cicaré CH-14 Enstrom 480
Eurocopter AS350 Soloy AS350RM AllStar
Eurocopter AS355F
Fairchild Hiller FH-1100
Hughes OH-6 Cayuse
MBB Bo 105
MD Helicopters MD 500
MD Helicopters MD 600
MD Helicopters MH-6 Little Bird
Northrop Grumman MQ-8 Fire Scout
PZL Kania
PZL SW-4
Schweizer 330/330SP
Schweizer S-333
Sikorsky S-76
Kamov Ka-226
Others
Loral GZ-22, non-rigid airship
MTT Turbine Superbike, motorcycle
Specifications:
T63-A-5
Type: Turboshaft
Length: 40.5 in (1,029 mm)
Diameter: 22.5 in (572 mm)
Dry weight: 136 lb (62 kg) dry
Compressor: 6-stage axial + 1-stage Centrifugal compressors
Combustors: Single can combustion chamber
Turbine: 2-stage axial gas generator power turbine + 2-stage axial free-power output turbine
Fuel type: JP-4 Aviation kerosene (alternatively JP-1 or JP-5)
Oil system: pressure spray/splash, dry sump
Maximum power output: 250 hp (190 kW) for Take-off, sea level 59 °F (15 °C)
Overall pressure ratio: 6.2:1
Turbine inlet temperature: 1,805 °F (985 °C)
Specific fuel consumption: 0.77 lb/hp-hr (0.468 kg/kW-hr) at 90% power cruise rating
Power-to-weight ratio: 1.838 hp/lb (3.022 kW/kg)
C20B/F/J
Weight: 73 kg
Airflow rate: 1.230 kg/s
Pressure ratio: 6.2 :1
Takeoff power: 420 WPS
Continuous power: 317 WPS
Height: 571 mm
Width: 483 mm
Length: 986 mm
C20R
Weight: 78 kg
Airflow rate: 1.733 kg/s
Pressure ratio: 7.9 :1
Takeoff power: 450 WPS
Continuous power: 380 WPS
Height: 589 mm
Width: 528 mm
Length: 986 mm
B17D
Weight: 90 kg
Airflow rate: 1.624 kg/s
Pressure ratio: 7.2 :1
Takeoff power: 420 WPS
Continuous power: 370 WPS
B17F
Weight: 93 kg
Airflow rate: 1.733 kg/s
Pressure ratio: 7.9 :1
Takeoff power: 450 WPS
Continuous power: 380 WPS
C30M
Weight: 115 kg
Airflow rate: 2.54 kg/s
Pressure ratio: 8.6 :1
Takeoff power: 650 WPS
Continuous power: 540 WPS
Height: 638 mm
Width: 555 mm
Length: 1041 mm
C30S
Weight: 114 kg
Airflow rate: 254 kg/s
Pressure ratio: 8.6 :1
Takeoff power: 650 WPS
Continuous power: 501 WPS
Height: 638 mm
Width: 555 mm
Length: 1041 mm
C30P
Weight: 114 kg
Airflow rate: 2.54 kg/s
Pressure ratio: 8.6 :1
Takeoff power: 650 WPS
Continuous power: 501 WPS
Height: 638 mm
Width: 555 mm
Length: 1041 mm
C40B
Weight: 127 kg
Airflow rate: 2.77 kg/s
Pressure ratio: 9.2 :1
Takeoff power: 715 WPS
Continuous power: 613 WPS
Height: 638 mm
Width: 555 mm
Length: 1040 mm
C47B/M
Weight: 124 kg
Airflow rate: 2.77 kg/s
Pressure ratio: 9.2 :1
Takeoff power: 650 WPS
Continuous power: 600 WPS
Height: 638 mm
Width: 555 mm
Length: 1040 mm