Atlas V 551: ULA's Versatile Rocket With Common Core Booster, SRBs, Centaur Upper Stage

The Atlas V 551 configuration is a specific variant of the Atlas V launch vehicle, designed to provide enhanced performance for heavy payloads. The "551" designation indicates that it uses a 5-meter(m) diameter payload fairing, five solid rocket boosters (SRBs), and a single-engine Centaur upper stage.

The first stage of the Atlas V 551 consists of the Common Core Booster (CCB) and the five GEM 63 solid rocket boosters(SRBs). The CCB provides the initial thrust for liftoff. The SRBs are jettisoned after their fuel is exhausted, typically around 150 seconds into the flight.

The CCB is 3.8 m in diameter and 32.5 m in length. The booster's tanks are structurally rigid and constructed of isogrid aluminum barrels, spun-formed aluminum domes and intertank skirts. Booster propulsion is provided by the RD-180 engine system (a single engine with two thrust chambers).

The RD-180 burns RP-1 (Rocket Propellant-1 or highly purified kerosene) and liquid oxygen and delivers 860,200 lbs (3.83 mega-Newtons) of thrust at sea level. Five solid rocket boosters (SRBs) generate the additional power required at liftoff, with each providing 371,550 lbs (1.6 mega-Newtons) of thrust. The Centaur avionics system, provides guidance, flight control and vehicle sequencing functions during the booster and Centaur phases of flight.

The SRBs initially were the Aerojet Rocketdyne AJ-60A SRBs, but they have been phased out in favor of Northrop Grumman's GEM 63 (Graphite-Epoxy Motor) boosters, which provide additional thrust and improved performance.

Each GEM 63 booster provides an additional 1.27 MN (290,000 lbf) of thrust for 94 seconds. These boosters are used in several configurations of the Atlas V, including the 551 variant, and are designed to improve the overall performance and reliability of the launch vehicle.

The second stage (or Centaur upper stage) of the Atlas V 551 is 3 m in diameter and 12.6 m in length, and powered by a single RL10A-4-2 engine, which burns liquid hydrogen and liquid oxygen.

The RL10A-4-2 engine is a turbopump-fed engine known for its high efficiency and reliability, with a specific impulse (Isp) of 451 seconds in a vacuum. The engine is capable of multiple restarts, allowing the Centaur stage to perform complex maneuvers to place the payload into the desired orbit.

The Centaur upper stage propellant tanks are pressure-stabilized and constructed of corrosion-resistant stainless steel. Centaur is a cryogenic vehicle, and its RL10A-4-2 engine produces 22,600 lbs (100.5 kilo-Newtons) of thrust. The cryogenic tanks are insulated with a combination of helium-purged blankets, radiation shields and spray-on foam insulation (SOFI). The Centaur forward adapter (CFA) provides structural mountings for the fault-tolerant avionics system and structural and electrical interfaces with the spacecraft.

Centaur is responsible for the final phase of the ascent. After the first stage is jettisoned, the Centaur stage ignites its RL10A-4-2 engine to continue the ascent. The Centaur stage can perform multiple burns to achieve the desired orbit, depending on the mission requirements, before placing the payload into its final orbit.

The Atlas V 551 uses a 5.4-meter diameter payload fairing (PLF), which encapsulates both the Centaur and payload during ascent through the atmosphere.

The PLF is a sandwich composite structure made with a vented aluminum-honeycomb core and graphite-epoxy face sheets. The bisector (two-piece shell) PLF. The vehicle's height with the 5-meter medium PLF is approximately 62.5 m.

A typical mission payload launch sequence includes,

1. Liftoff: The Atlas V 551 begins its ascent with the ignition of the RD-180 engine and the five GEM 63 SRBs. The vehicle lifts off from the launch pad and begins its climb through the atmosphere.

2. SRB Separation: After approximately 150 seconds of flight, the GEM 63 SRBs are jettisoned, as their fuel is exhausted. The CCB continues to burn until it is also jettisoned, typically around 250 seconds into the flight.

3. Centaur Ignition: After the first stage is jettisoned, the Centaur upper stage ignites its RL10A-4-2 engine to continue the ascent. The Centaur stage may perform multiple burns to achieve the desired orbit.

4. Payload Deployment: Once the Centaur stage has reached the desired orbit, the payload is deployed. The Centaur stage may perform a final burn to ensure the payload is placed into the correct orbit.

Though not reusable and less efficient than SpaceX Faclcon 9, the Atlas V 551 is a versatile and reliable launch vehicle, capable of delivering a wide range of payloads into various orbits for its customers which include government agencies like NASA and private companies.