Here is a list of some of the most common, but perhaps misunderstood, terms, abbreviations and phrases you might hear during the GOES-R satellite launch. Once you know what they mean, follow the launch on Twitter @NOAASatellites!
Atlas V 541 ELV
This is the expendable launch vehicle (ELV) that will carry GOES-R into space. The term “expendable launch vehicle” means that the vehicle can only be used once. The three numbers in the “541” designation signify a payload fairing, or nose cone, that is approximately 5 meters (16.4 feet) in diameter; four solid-rocket boosters fastened alongside the central common core booster; and a one-engine Centaur upper stage. The two-stage Atlas V 541 launch vehicle, provided by United Launch Alliance (ULA), weighs about 1.17 million pounds and is 191 feet tall.
Atlas V Rocket
GOES-R will be going to space aboard a stacked "two stage” rocket called a ULA Atlas V 541 Expendable Launch Vehicle (ELV). Two stages means that the rocket has two sections, or stages, that are mounted on top of each other. Each stage has its own fuel and its own engine. Each stage is ejected into space once its job is done and all the fuel is spent.
The Atlas V Common Core Booster, or rocket, is the first stage of the Atlas V 541 ELV. This booster will ignite first and is the main engine powering the vehicle’s ascent. This stage carries the spacecraft through the first leg of its journey, ending in a geostationary transfer orbit.
Centaur Upper Stage
This is the second stage of the Atlas V rocket and is considered the vehicle's "brains." It is designed to power the second leg of the satellites trip, placing it into a geostationary orbit. The satellite will then separate from the Centaur Upper Stage and complete the final maneuvers on its own, placing itself in a designated location in geostationary orbit.
Geostationary satellites orbit 22,240 miles above the Earth at speeds equal to Earth’s rotation, allowing them to maintain their position and provide continuous coverage of one location. Although satellites traveling in geostationary orbit appear stationary relative to a point on Earth, they are actually moving at a velocity of 1.91 miles per second!
L- and T-
L- (pronounced "L minus”) refers to the days, hours, and minutes remaining in the scheduled countdown to launch, which occurs at L-0. The “L” stands for launch.
T- (pronounced "T minus”) refers to the time remaining on the official countdown clock. The “T” stands for time. During planned holds in the countdown process (when the countdown clock is intentionally stopped), the T- time also stops. The L- time, however, is synced to the clock on the wall and continues to advance.
Under normal conditions, these countdowns remain in sync. For example, there is a 15 minute hold planned at T-4 minutes, which occurs at L-19 minutes. Once the hold is lifted and the countdown resumes, the clocks will be synced and show 4 minutes remaining.
This refers to the period of time in which a rocket can successfully (and safely) launch while still getting the satellite to its planned orbit.
Launch windows vary in length based on the rocket’s power, the satellite’s planned orbit and the tilt, rotation and orbit of Earth. Launch windows can also be constrained to ensure proper solar conditions (important for powering up the satellite), the availability of required ground antenna services, and to avoid launching into space near other satellites or space debris.
Liftoff denotes the exact moment when the rocket, with the satellite onboard, begins to leave the launch pad under its own power, beginning its journey to space.
MES “Main Engine Start” and MECO “Main Engine Cut-Off”
MES refers to the moment when the Centaur Upper Stage’s main engine begins to fire, or burn. GOES-R will go through three main engine starts on its way to geostationary orbit, designated MES1, MES2 and MES3.
MECO refers to the moment when the Centaur Upper Stage has completed a main engine burn and cuts off, entering a coasting phase. GOES-R will go through three main engine starts and cut-offs on its way to geostationary orbit. They are identified as MECO1, MECO2 and MECO3.
The payload fairing is a specially designed nose cone that, in addition to creating a more aerodynamic profile, encapsulates the satellite, protecting it during the ascent through Earth's atmosphere. Once in space, the fairing is no longer needed and is ejected.
SRB “Solid Rocket Boosters”
These boosters are used to increase main engine thrust during lift-off and help the rocket take GOES-R to its final orbit. GOES-R's Atlas V 541 rocket will utilize four separate SRBs, which will separate from the rocket body once expended.
Occurring only minutes before scheduled liftoff, this is when launch operations are transferred from human control to preprogrammed computer systems. At this point, automated launch sequences initiate. This process can only begin after the flight director deems the launch a "go."