The Rocket's Job Ends. The Spacecraft's Is Just Starting.
Modules 1 and 2 covered why space is a hard place to build for, and where things end up once they get there. This module is the groundwork for what actually does the building and the getting-there: a lighter overview here, with the deeper engineering treatment waiting in SSA-02.
Two Different Machines
Casual coverage calls the whole stack "the rocket." Space studies treats them as two separate machines with two separate jobs.
Launch Vehicle
Gets It There
The rocket itself. Its only job is delivering a payload to the right altitude, speed, and trajectory. Most of its own mass — fuel, tanks, boosters — is discarded or burned along the way.
- Job lasts minutes, not years
- Stages are shed or return to Earth
- Falcon 9, Atlas V, Ariane 6, SLS, JAXA's H3
Spacecraft
Does the Job
The actual payload. Whatever it was built to do once it's there — orbit Earth, carry a crew, study a planet, dock with a station — happens after the launch vehicle has already finished its part and separated.
- Mission lasts months to decades
- Has to survive the Module 2 environment
- A comms satellite, a capsule, a probe, a station module
Once separation happens, the launch vehicle's story is over — it's re-entering, discarded, or (increasingly) flying itself back down to be reused. Everything from here is the spacecraft's problem: surviving vacuum and radiation, holding the right orbit, and doing whatever it was actually sent up to do.
Three Systems Every Spacecraft Needs
Regardless of mission, almost every spacecraft is solving the same three problems.
Propulsion
Not just for launch. Once in orbit, propulsion handles station-keeping, attitude changes, and orbit transfers. Chemical thrusters give quick powerful bursts; electric (ion) thrusters sip fuel over much longer burns — a trade-off covered in depth in SSA-02.
Power
Solar panels are the default anywhere near enough to the Sun, with batteries carrying the load through eclipses. Missions heading past Mars — where sunlight gets too weak — often switch to radioisotope generators instead, like Voyager and the Mars rovers.
Guidance & Navigation
How a spacecraft knows where it is and points where it needs to. Star trackers and gyroscopes for orientation, GPS in LEO, ground-based tracking farther out, and reaction wheels to turn the vehicle without burning fuel.
Not All Rockets Lift the Same Load
No single official cutoff exists — agencies and companies draw the lines differently — but a rough shared framework looks like this:
Small Lift — roughly up to 2,000 kg to LEO
Built for a single small satellite and a fast, dedicated launch rather than sharing a ride. Rocket Lab's Electron is the clearest example of this class.
Medium Lift — roughly 2,000–20,000 kg to LEO
The current commercial and government workhorse class. Falcon 9, Soyuz, and Japan's H-IIA all sit here, covering most routine satellite and crew launches.
Heavy Lift — roughly 20,000–50,000 kg to LEO
Reserved for big science payloads and national-security missions that need more mass or more delta-v than a medium-lift vehicle can offer. Falcon Heavy and Delta IV Heavy fall in this range.
Super Heavy Lift — roughly 50,000+ kg to LEO
Built specifically for crewed lunar and Mars ambitions, and for launching the largest single station or habitat modules. NASA's SLS and SpaceX's Starship both target this class.
Not All Spacecraft Do the Same Job
The word "spacecraft" covers wildly different machines. What it's for decides almost everything else about how it's built — including which orbital band from Module 2 it ends up in.
Crewed Spacecraft
Carries People
Life support, abort systems, and return capability are non-negotiable. Missions tend to be shorter than uncrewed ones. Crew Dragon and Soyuz are current examples.
Comms & Earth-Observation Satellites
The Bulk of Everything in Orbit
Imaging satellites tend to sit in LEO for resolution; broadcast and weather satellites sit in GEO to stay fixed over one spot on the ground.
Navigation Satellites
Broadcasts a Clock, Not Much Else
GPS and other GNSS constellations live in MEO. Their entire job is transmitting a precise, synchronized time signal that receivers on the ground triangulate.
Space Probes
No Return Trip
Built to travel to other bodies — the Moon, Mars, the outer planets, asteroids — with mission durations that can stretch decades, like Voyager and JAXA's Hayabusa2 asteroid sample-return mission.
Space Station Modules
Assembled, Not Launched Whole
Long-duration crewed habitats built to dock together in orbit and be resupplied indefinitely rather than replaced — the ISS being the standing example.
Key Terms
- Launch Vehicle
- The rocket itself. Its only job is delivering a payload to the right altitude, speed, and trajectory — a job usually finished within minutes.
- Spacecraft
- The actual payload — whatever the launch vehicle was carrying to do a job once it arrives. Its mission can run for years to decades.
- Payload
- Whatever a launch vehicle is actually carrying — a satellite, a crew capsule, cargo — as distinct from the rocket doing the carrying.
- Delta-v
- A measure of the total "velocity change" a spacecraft or launch vehicle can produce — shorthand for how much maneuvering capability it has, whether that's reaching a higher orbit or changing course.
- Station-Keeping
- The ongoing use of propulsion to correct orbital drift and hold a spacecraft in its intended position — a maintenance job, not a one-time maneuver.
- Reaction Wheel
- A spinning wheel used to rotate a spacecraft's orientation without burning fuel — turning the vehicle by conserving angular momentum instead.
- Radioisotope Thermoelectric Generator (RTG)
- A power source that converts heat from decaying radioactive material into electricity — used where sunlight is too weak for solar panels, as on Voyager and the Mars rovers.
- GNC (Guidance, Navigation & Control)
- The combined systems that tell a spacecraft where it is, where it needs to go, and how to point and move to get there.
For This Module
Public technical fact sheets from SpaceX, Rocket Lab, ULA, and NASA (SLS/Artemis program pages), plus NASA's power-systems and RTG educational materials.
Continue the Sequence
Module 4 covers space operations, commerce, and resources.