Athena

The primary propulsion for the Athena vehicles is provided by two or three solid stages. The first stage is based on Thiokol’s Castor 120 motor. The Athena II includes a second Castor 120 motor as a second stage. An Orbus 21D motor provides the basis for the second stage on the Athena I, and the third stage on Athena II. To avoid confusion over stage numbering, this stage is generally referred to as the equipment section boost motor (ESBM). The ESBM is topped by the OAM, also called the equipment section, which houses the vehicle avionics and has a small monopropellant liquid-propulsion system. The OAM supports the payload adapter and fairing. The Castor 120 is a commercial SRM developed by Thiokol. It is a derivative of the Peacekeeper ICBM first-stage motor, with modifications for space launch applications. It has the same diameter as the Peacekeeper, but is about 0.5 m (1.7 ft) longer to include more propellant. A slight change to the propellant formulation results in a longer burn time that is more suitable for space launch vehicles. The casing is filament-wound graphite–epoxy composite, and TVC is provided by a new blowdown hydraulic nozzle gimbal system similar to those developed for the Castor IVB and SSLV Taurus first stage. The Pratt & Whitney Orbus 21D motor is derived from the Orbus 21 motors used in the IUS and TOS upper stages for the Space Shuttle and Titan. The primary difference is a carbon–phenolic nozzle exit cone, which is stronger and less expensive but also heavier than the carbon–carbon exit cone it replaces. For future missions Lockheed Martin will upgrade to the Orbus 21G motor, which has a lightweight graphite–epoxy motor casing in place of the current Kevlar case.

The OAM is a small, monopropellant hydrazine stage that provides precise orbit injection as well as attitude control throughout flight. It also contains the avionics bays. The combined propulsion–ACS system is a monopropellant blowdown system, and therefore the thrust decays during flight as the tank pressure decreases. Axial propulsion is provided by four small 220 N (50 lbf) hydrazine thrusters spaced 90 deg apart around the circumference of the stage. The thrusters can be restarted almost indefinitely because they were originally developed for spacecraft attitude control applications. The axial propulsion thrusters and lateral-radial ACS thrusters are identical and are fed from common tankage. The OAM is available with four or six propellant tanks, depending on mission requirements. The hydrazine tanks are mounted in pairs in an egg-crate frame structure inside the stage.

Attitude Control System

Length 6.1 m (20 ft) Primary Diameter 2.3 m (7.6 ft) Mass 535 kg (1180 lbm) Sections 2 Structure Ring-stiffened monocoque Material Aluminum–lithium

Avionics

The Athena avionics are housed in two avionics bays in the OAM. A single Litton LN100 IMU feeds navigational data to the digital flight computer, which controls the vehicle based on flight software output. Commands for attitude control, stage ignition, or separation events are distributed throughout the vehicle by the interlocks package (IP). Data on vehicle performance and environments are collected through a distributed network of instruments and encoders and transmitted over two separate S-band telemetry channels (one analog, one digital). The flight termination system (FTS) has independent batteries and antennas and can terminate thrust on each solid stage by command from the ground or automatically in the event of unplanned stage separation.

Payload Fairing

The standard Athena fairing is the Model 92, so named because its cylindrical section is 92 in. in diameter. The fairing splits into two sections using a zip-tube noncontaminating separation system. The halves are pushed apart with springs and rotate 45 deg on hinges before being released from the rocket. Payload access doors are standard, and RF windows and acoustic or thermal blankets are available options. To speed up launch operations, the payload is mounted onto the launch vehicle adapter and encapsulated into the fairing in an offline processing facility. Courtesy Lockheed Martin Corporation. OAM Attitude Control System Courtesy Lockheed