Pegasus XL Launch Vehicle

Photo: NASA
Photo: NASA

The Pegasus XL Launch Vehicle is a light-weight lift vehicle operated by Orbital Sciences Corporation. It can deliver Payloads of up to 443 Kilograms into Low Earth Orbit.

The Rocket is launched from the L-1011Stargazer Aircraft which makes Pegasus a high flexibility launch system as it is not depending on a fixed Launch Location and associated ground weather.

Also, launching from an Aircraft enables the vehicle to reach a variety of Orbits with different inclinations including nearly equatorial orbits. To date, Pegasus has made 40 Launches, 35 of which were successes and 2 partial failures. Pegasus made its first flight in 1990 delivering two satellites to orbit.

The vehicle is a three-stage, solid propellant rocket with the option of a fourth, liquid fueled stage. The Launch System is a low cost option for smaller payloads not requiring precision injections.

Photo: Orbital Science
Photo: Orbital Science


Pegasus Specifications

Type Pegasus XL
Height 16.9m
Diameter 1.27m
Launch Mass 23,130kg
Stages 3
Stage 1 Orion 50S XL
Stage 2 Orion 50 XL
Stage 3 Orion 38
Stage 4 – Optional HAPS
Mass to LEO 443kg
Launch Cost ~$11 Million (1994)
Image: Orbital Sciences Corporation
Image: Orbital Sciences Corporation
Photo: NASA
Photo: NASA

Type Orion 50S XL
Inert Mass 1,369kg
Diameter 1.28m
Length 10.27m
Propellant Solid
Propellant Mass 15,014kg
Guidance Open Loop
Propulsion Orion 50S XL
Thrust (Vacuum) 726kN
Impulse 295s
Average Pressure 1,090psia
Burn Time 68.6sec
Attitude Control Fin Actuators

First Stage

At T-0 the Launch Vehicle is released by the L-1011 Aircraft and free-falls for five seconds before igniting its Orion 50S Solid Rocket Motor – the first Stage of the Vehicle.

It burns for about 69 seconds before shutting down at an altitude of about 61 Kilometers. During first stage flight, the Rocket is controlled by its fin actuators that provide pitch control.

The Orion 50S does not have Thrust Vector Control Capabilities.

A Delta Wing provides some lift and supports a pitch maneuver that is initiated shortly after ignition. After first stage cutoff, the vehicle holds on to the stage for several more seconds before stage separation occurs. The Wings and Fins are jettisoned with the first stage.

Photo: NASA
Photo: NASA


Type Orion 50 XL
Diameter 1.28m
Length 3.07m
Inert Mass 391kg
Launch Mass 4,306kg
Propellant Solid QDL-1 (HTPB)
Propellant Mass 3,915kg
Guidance Closed Loop PEG
Max Thrust 194.4kN
Avg Thrust 158kN
Impulse 289s
Nozzle Diameter 861mm
Burn Time 71sec
Average Pressure 70.3bar
Attitude control Electromechanical TVC
Cold Gas Reaction Control System

Second Stage

The second stage of the Pegasus rocket is an Orion 50 XL manufactured by Alliant Techsystems, ATK. It is 3.07 meters long and 1.28 meters in diameter with a launch mass of 4,306 Kilograms.

The motor is loaded with 3,915 Kilograms of QDL-1 HTPB-based solid propellant operating at a pressure of 70 bar. Orion 50 XL provides a maximum thrust of 194.4 Kilonewtons and an average thrust of 158 Kilonewtons.

The third stage is equipped with a high-expansion nozzle for operation in vacuum.

The nozzle is gimbaled using an electromechanical Thrust Vector Control System for pitch and yaw control. A Nitrogen Cold Gas Attitude Control System is used for roll control and attitude control during coast phases. Orion 50 XL has a burn time of 71 seconds.

Photo: NASA
Photo: NASA


Type Orion 38
Diameter 0.97m
Length 1.34m
Inert Mass 102.1kg
Launch Mass 872.3kg
Propellant Solid QDL-1 (HTPB)
Propellant Mass 770.2kg
Guidance Closed Loop PEG
Max Thrust 36.9kN
Avg Thrust 32.7kN
Impulse 287s
Nozzle Diameter 526mm
Burn Time 66.8s
Average Pressure 45.5bar
Attitude control Electromechanical TVC +/-5°
Cold Gas Reaction Control System

Third Stage

Serving as a third stage on Pegasus is an ATK Orion 38 solid rocket motor which measures 1.34 meters in length and 0.97 meters in diameter.

It weighs 872.3 Kilograms when loaded with 770.2 Kilograms of QDL-1 propellant. Orion 38 operates at a pressure of 45.5 bar to create an average thrust of 32.7 Kilonewtons, peaking at 36.9 Kilonewtons.

It also uses an electromechanical Thrust Vector Control System to gimbal the engine nozzle by up to 5 degrees for pitch and yaw control.

Roll control during burns and attitude control during coast phases and for spacecraft separation is provided by a Nitrogen Cold Gas Attitude Control System.

The fourth stage has a burn time of 66.8 seconds that is followed by a coast phase and re-orientation for spacecraft separation.

Photo: NASA
Photo: NASA

Optional HAPS

Diameter 0.97m
Length 0.30m
Propellant Hydrazine
Propellant Mass 72kg
Propulsion 3 x MR-107
Catalyst LCH227/202
MR-107 Thrust 109 – 296N (222N nominal)
Mass 740g
Length 22cm
Diameter 6.6cm
Prop Feed Pressure 8.2 – 27.8bar
Chamber Pressure 4.2 – 11.2bar
Specific Impulse 229 – 232kg
Flow Rate 49 – 131 g/s
Expansion Ratio 20.7
Burn Time 131 + 110s (2 Burns)
Attitude Control Cold Gas RCS (N2)
Pegasus can be outfitted with a HAPS (Hydrazine Auxiliary Propulsion System) to act as fifth stage for precise injections. It is a compact structure with a diameter of 97 centimeters and a length of about 0.3 meters. It is equipped with two spherical tanks holding Hydrazine monopropellant. Each tank holds 36 Kilograms of propellant. HAPS is equipped with a total of three MR-107N thrusters (Super HAPS uses 12 engines). Each of these engines provides a nominal thrust of 222 Newtons with a throttle capability of 109 to 296 Newtons. With the three engines at nominal throttle, HAPS has a total thrust of 666 Newtons – 68 Kilogram-force. 

MR-107N operates at a propellant feed pressure of 8.2 to 27.8 bar and a chamber pressure of 4.2 to 11.2 bar to create a specific impulse of 229 to 232 seconds. The engine ingests 49 to 131 grams of fuel per second depending on the thrust setting. It has a expansion ratio of 20.7. MR-107N weighs 740 grams, measuring 22 centimeters in length and 6.6 centimeters in diameter. It uses a Moog Single Seat Valve.
Usually, HAPS performs two burns that are separated by a coast phase of about half an orbit to allow HAPS to perform a precise insertion and refine the vehicle’s orbit to match the target parameters.HAPS is equipped with a Cold Gas Attitude Control System for three-axis control. Differential firings of the three engines can also be used for attitude maneuvers during propulsive flight phases.
MR-107N - Image: Aerojet
MR-107N – Image: Aerojet

Payload Fairing

The Payload Fairing is positioned on top of the stacked vehicle and its integrated payloads. It protects satellites or other spacecraft against aerodynamic, thermal and acoustic environments that the vehicle experiences during atmospheric flight. When the launcher has left the atmosphere, the fairing is jettisoned by pyrotechnically initiated systems. Separating the fairing as early as possible increases ascent performance. Payload Fairing design limits Payload Volume.

Photo: NASA
Photo: NASA


Payload Adapters

Payload Adapters interface with the vehicle and the payload and are the only attachment point of the payload on the Launcher. They house equipment that is needed for Spacecraft Separation and ensure that the satellite or spacecraft is secured during powered flight. Off-the-shelf and custom adapters are available to customers to accommodate a variety of payloads.

Typical Ascent Profile
Time Event Altitude
T-0 Pegasus Release 11.89km
T+0:00:05 Orion 50S Ignition 11.79km
T+0:00:08 Pitch Maneuver
T+0:01:17 First Stage Burnout 54.25km
T+0:01:30 First Stage Separation
T+0:01:32 Orion 50 Ingition 71.63km
T+0:02:08 Payload Fairing Separation 115.21km
T+0:02:45 Second Stage Burnout 180.14km
T+0:08:55 Second Stage Separation
T+0:09:06 Orion 38 Ignition 620.79km
T+0:10:14 Third Stage Burnout 629.68km
T+0:13:14 Spacecraft Separation
End of Pegasus Mission
L-1011 Returns to Departure Point