Knowledge Zone

Glossary

Every term the cards use, in one place. Each one links to the card it belongs to.

Orbits and shapes

Orbit: The closed path a ship falls along around a body when it moves sideways fast enough that the ground curves away beneath it. → Orbit is falling sideways
Periapsis: The lowest point of an orbit, closest to the centre of the body. Around Earth it is called perigee, around the Sun perihelion. → Kepler's ellipses
Apoapsis: The highest point of an orbit, farthest from the body. Apogee around Earth, aphelion around the Sun. → Kepler's ellipses
Semi-major axis: Half the long diameter of the ellipse; the orbit's average size, which alone fixes its period. → Kepler's ellipses
Eccentricity: How stretched an orbit is: 0 is a circle, between 0 and 1 an ellipse, 1 or more means the ship never comes back. → Kepler's ellipses
Orbital period: Time for one full lap. Depends only on the semi-major axis and the body's mass. → Kepler's ellipses
Circular orbit: An orbit with periapsis equal to apoapsis, so speed and altitude stay constant. → Orbit is falling sideways
Hyperbolic trajectory: An unbound path: the ship has more than escape speed and leaves the body for good. → Escape velocity
LEO: Low Earth orbit, roughly 200 to 2 000 km up. The game's parking orbit sits at 400 km, moving at about 7.7 km/s. → Orbit is falling sideways
GEO: Geostationary orbit, 42 164 km from Earth's centre, where one lap takes one day. Used in the game as a burn target. → Kepler's ellipses
Parking orbit: A temporary low orbit reached after launch, where the ship waits for the transfer burn. Success in Phase 1 is a periapsis above 150 km. → Orbit is falling sideways

Motion and energy

State vector: A ship's position and velocity at one instant, relative to the body it orbits. Everything else about the orbit follows from it.
Gravitational parameter (μ): The body's mass times the gravitational constant; the only number gravity needs. Earth: 3.986e14 m³/s².
Two-body problem: Gravity from a single body acting on the ship, with everything else ignored. Exactly solvable and good enough for an arcade.
Propagation: Advancing a state vector forward in time step by step. The game uses a fourth-order Runge-Kutta (RK4) integrator.
Specific orbital energy: Kinetic plus potential energy per kilogram. Negative means bound to the body, zero is escape, positive is a flyby. → Escape velocity
Angular momentum: Position crossed with velocity. Constant in a two-body orbit; only burns with a sideways component change it. → Kepler's ellipses
Escape velocity: The speed at which a ship's energy reaches zero and it never falls back: √2 times circular speed at the same radius. → Escape velocity
Prograde: Along the direction of travel. A prograde burn adds speed and raises the opposite side of the orbit. → Kepler's ellipses
Retrograde: Against the direction of travel. A retrograde burn removes speed and lowers the opposite side of the orbit. → Kepler's ellipses
Radial: Straight away from (radial-out) or towards (radial-in) the body. Rotates the orbit rather than resizing it. → Kepler's ellipses

Rockets and burns

Delta-v (Δv): Change in speed. The currency of spaceflight: every manoeuvre costs some, and a rocket carries a fixed budget. → The rocket equation
Burn: Firing the engine. The game treats burns as impulsive: an instant change of velocity, position unchanged. → Kepler's ellipses
Specific impulse (Isp): Engine efficiency in seconds: how long one kilogram of propellant can produce one kilogram-force of thrust. Higher is better. → The rocket equation
Thrust-to-weight ratio (TWR): Thrust divided by weight. Below 1 the rocket cannot leave the pad. → Thrust against weight
Rocket equation: Tsiolkovsky's formula: Δv = Isp × g₀ × ln(full mass / empty mass). Doubling fuel does not double Δv. → The rocket equation
Staging: Dropping empty tanks and engines in flight so the remaining fuel pushes less dead mass. → Staging
Gravity turn: Tilting gradually from vertical to horizontal so the ship ends up moving sideways at orbital speed. → Orbit is falling sideways
Pitch program: The plan for that tilt: when to start turning and how fast. The two sliders in the launch cockpit. → Orbit is falling sideways
Pitch: The angle between the rocket's thrust and the local vertical: 0° is straight up, 90° is horizontal. → Orbit is falling sideways
Pitch-over: The moment, set by altitude, when the rocket stops flying straight up and starts to tilt. → Orbit is falling sideways
Engine cut-off: Stopping the engine once the orbit is good enough. The game cuts off when the periapsis clears 150 km; the propellant left is the transfer budget. → Orbit is falling sideways
Gravity losses: Δv wasted fighting gravity while climbing slowly. Turning earlier reduces them, drag permitting. → Gravity and drag losses
Drag: Air resistance, proportional to density and speed squared. The atmosphere thins exponentially, halving every ~6 km. → Gravity and drag losses
Scale height: The climb over which air density drops by a factor e (2.7): 8.5 km in the game. Above 200 km there is effectively no air. → Gravity and drag losses

Getting to Mars

Hohmann transfer: The cheapest two-burn route between two circular orbits: one burn to stretch the orbit out, one to round it off at the far side. → The Hohmann transfer
Transfer time: Half a lap of the transfer ellipse. Earth to Mars: about 259 days. → The Hohmann transfer
Synodic period: Time between two consecutive alignments of Earth and Mars: about 780 days, or 26 months. → Launch windows
Phase angle: The angle by which Mars must lead Earth at departure (~44°) so both reach the far side of the transfer together. → Launch windows
Launch window: The dates around the right phase angle. Launching outside it costs extra Δv or misses Mars. → Launch windows
AU: Astronomical unit, the average Earth to Sun distance: 149 597 871 km. Mars orbits at 1.52 AU. → The Hohmann transfer
Heliocentric: Measured relative to the Sun. Once the ship escapes Earth, its state vector is heliocentric. → The Hohmann transfer
Gravity assist: Stealing momentum from a moving body by flying past it. The game offers a Moon flyby as a bonus: pass behind the Moon to gain speed, in front to lose it. → The Moon flyby
Sphere of influence: The region around a body where its pull beats its parent's; the Moon's reaches 66 000 km. A flyby is what happens inside it. → The Moon flyby
Patched conics: Stitching one two-body orbit to the next at the sphere of influence: Earth's conic outside, the Moon's hyperbola inside, Earth's again after. → The Moon flyby
Hyperbolic excess speed (v∞): The speed a ship keeps relative to a body far from it. A flyby changes its direction, never its size. → The Moon flyby
Turn angle: How far a flyby bends the ship's path, from the closest approach and v∞. Closer and slower bends more: 1 km/s at 1 000 km over the Moon turns 80°. → The Moon flyby
Mid-course correction: A small burn during the cruise to fix a small error. The later it is made, the more it costs. → The mid-course correction

Arriving

Capture burn: A retrograde burn at Mars that drops the ship below escape speed so Mars keeps it. → Capture burn or aerobraking
Aerobraking: Skimming the upper atmosphere to shed speed without fuel. Cheaper than a capture burn, riskier if too deep. → Capture burn or aerobraking
Retro-burn: Firing against the direction of fall to slow down for landing. Needed on Mars because its air is too thin for parachutes alone. → Landing on Mars
Entry periapsis: How low through Mars' air the arriving ship aims. A few kilometres decide whether it is captured, bounces off or burns up. → Capture burn or aerobraking
Aerobraking corridor: The band of entry periapsis that captures the ship without destroying it: too deep cooks the shield, too shallow does not slow it enough. → Capture burn or aerobraking
Ballistic coefficient: Mass divided by drag coefficient times frontal area (kg/m²). Low means light and broad, which brakes early and gently; high ploughs on through. → Capture burn or aerobraking
Mars' scale height: The same idea as Earth's, but 11.1 km instead of 8.5, and starting from a sixtieth of the density. Aiming 11 km lower meets e times more air. → Capture burn or aerobraking
Heat flux: How fast the shield is heated, which grows with the cube of speed. It is what sets the bottom of the corridor. → Capture burn or aerobraking
Oberth effect: A burn is worth more the faster the ship is already going, so a capture burn is made at the closest point to Mars and nowhere else. → Capture burn or aerobraking
Safe landing speed: The touchdown speed a lander survives cleanly, 5 m/s here. Up to 20 m/s it lands and loses points; above that it breaks. → Landing on Mars

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