Deltas are expendable launch vehicles (ELVs), which means they can only be used once. Each Delta II launch vehicle consists of:
- Stage I: RP-1 and liquid oxygen tanks that feed the Rocketdyne RS-27 main engine for the ascent.
- Solid rocket booster motors: Used to increase thrust during the initial two minutes of flight. The medium-capacity Delta II has nine motors total (six fire on the ground, three in flight); the other models use only three or four.
- Interstage: A spacer between stage I and stage II. The first friction stir welded interstage module was launched in 1999.
- Stage II: Fuel and oxidizer tanks feeding a restartable hypergolic Aerojet AJ10-118K engine that fires one or more times to insert the vehicle-spacecraft stack into low Earth orbit. This propellant mixture is highly corrosive, so once loaded the launch must occur within approximately 37 days, or the stage will have to be refurbished or replaced. This stage also contains the vehicle's "brains", a combined inertial platform and guidance system that controls all flight events.
- Stage III: Optional ATK-Thiokol solid rocket motor provides the majority of the velocity change needed to leave Earth orbit and inject the spacecraft on a trajectory to Mars or other target beyond Earth orbit. It is connected to the spacecraft until it is done firing, and then separates. This stage is spin-stabilized and has no active guidance control; it depends on the second stage for proper orientation prior to Stage II/III separation. It also includes a yo-yo de-spin mechanism to slow the spin before spacecraft release, as many spacecraft cannot handle the high spin rates needed for stability of this stage. Note that some Delta II vehicles are two-stage only, these generally being used for Earth-orbit missions.
- Payload fairing: Thin metal or composite payload fairing (aka "nose cone") to protect the spacecraft during the ascent through Earth's atmosphere.
Read more about this topic: Delta II
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