Glossary — S
39 terms
Spacecraft
A vehicle designed to operate in space, including satellites, probes, and crewed vehicles.
Example: The spacecraft separated from the upper stage after reaching orbit.
Specific impulse
A measure of propulsion efficiency, equal to thrust per unit weight-flow rate of propellant, expressed in seconds.
Example: Hydrogen engines achieve a higher specific impulse than kerosene engines.
Satellite
An object placed in orbit around a planet or star, whether natural or artificial.
Example: The communications satellite began relaying signals within hours.
Spacecraft bus
The structural and support platform of a satellite that hosts the payload.
Example: The satellite bus supplies power, attitude control, and communications.
SSO
Sun-Synchronous Orbit: a near-polar orbit whose plane precesses to keep a nearly constant angle with the Sun, so lighting is uniform each pass.
Example: Earth-observation satellites use sun-synchronous orbit to image the ground under consistent sunlight.
Solid propellant
A pre-mixed propellant cast into a rigid grain that burns once ignited.
Example: Solid propellant boosters provide high thrust but cannot be throttled or restarted.
Sample return
A mission that collects material and brings it back to Earth.
Example: The sample return mission carried lunar soil back to Earth.
Solar array
Panels of photovoltaic cells that convert sunlight into electrical power.
Example: The solar array deployed and began charging the batteries.
Solar panel
A photovoltaic module that generates power from sunlight.
Example: Each solar panel tracks the Sun for maximum output.
Semi-major axis
Half the longest diameter of an orbit, which determines its period and orbital energy.
Example: Kepler's third law relates a satellite's period to its semi-major axis.
Staging
Separating spent stages to shed dead weight during ascent.
Example: Staging occurred two minutes after liftoff.
Stage
An independently powered section of a rocket jettisoned when spent.
Example: The second stage burned for six minutes to reach orbit.
Star tracker
An optical sensor that identifies star fields to determine orientation.
Example: The star tracker provided precise attitude knowledge after separation.
Second stage
The stage that continues from staging to orbital insertion.
Example: The second stage performed the final circularization burn.
Skip reentry
A reentry that bounces off the atmosphere to shed energy in steps.
Example: A skip reentry spreads heating over a longer path.
State vector
The set of variables describing position and velocity at a time.
Example: The guidance computer propagated the state vector forward.
Sun sensor
A sensor that detects the direction of the Sun for coarse orientation.
Example: Sun sensors helped the spacecraft find a power-safe attitude.
Servicing mission
A mission whose purpose is to repair or upgrade existing hardware.
Example: A servicing mission replaced the telescope's failed instruments.
Staged combustion cycle
A cycle where preburner exhaust is injected into the main chamber, maximizing efficiency.
Example: Staged combustion engines burn propellant almost completely in the main chamber.
SECO
Second Engine Cutoff: the shutdown of the second-stage engine, marking insertion.
Example: SECO put the vehicle into its initial parking orbit.
Supersonic
Faster than the local speed of sound.
Example: Supersonic flight produced shock waves around the vehicle.
Shock wave
A steep pressure wave formed when a body exceeds the speed of sound.
Example: A bow shock formed ahead of the reentering capsule.
Science mission
A mission driven by scientific investigation.
Example: The science mission studied the planet's magnetic field.
Scrub
Cancelling or postponing a launch attempt before liftoff.
Example: High upper-level winds forced a scrub of the attempt.
Stagnation point
The point on a body where airflow comes to rest, with peak heating.
Example: The stagnation point experienced the highest temperatures.
Specific thrust
Thrust per unit propellant mass-flow rate, related to exhaust velocity.
Example: Higher exhaust velocity raises the specific thrust of the engine.
Spacesuit
A wearable life-support system protecting an astronaut in vacuum.
Example: The spacesuit maintained pressure and temperature during the spacewalk.
Suborbital mission
A flight reaching space without completing an orbit.
Example: The suborbital mission provided minutes of weightlessness.
Sky survey
A mission mapping large regions of the sky.
Example: The sky survey charted millions of galaxies.
Splashdown
A water landing of a returning spacecraft.
Example: The capsule splashed down in the Atlantic Ocean.
Static fire
A ground test that runs an engine while the vehicle is held in place.
Example: The company completed a static fire before the launch attempt.
Software-in-the-loop
Testing control software against a simulated plant.
Example: Software-in-the-loop tests validated the landing logic.
Stratosphere
The stable atmospheric layer above the troposphere, holding the ozone layer.
Example: Weather balloons reach the stratosphere.
Solar sail
A large reflective sheet that uses sunlight pressure for propulsion.
Example: The solar sail gently accelerated from photon pressure alone.
Space tourism
Commercial flights carrying private passengers to space.
Example: Space tourism offered civilians a brief orbital stay.
Station-keeping
Small periodic burns used to hold position within a designated orbital slot.
Example: North-south station-keeping counters the Moon's pull on geostationary satellites.
Satellite constellation
A group of satellites working together to provide coverage.
Example: The internet constellation spans thousands of satellites in low orbit.
Stability
A vehicle's tendency to return to steady flight after a disturbance.
Example: The design ensured stability margins across the flight envelope.
Science instrument
A payload device that performs measurements or observations.
Example: The science instrument mapped surface composition in fine detail.