Planetary Lander

Overview

A planetary lander is a spacecraft built to do one hard thing well: arrive at the surface of a moon or planet intact and upright, then keep working. Reaching the surface is not the same problem as reaching orbit. The vehicle must shed all of its orbital velocity, sense how far it is from a surface it has never touched, pick a spot free of boulders and slopes, and set down gently enough that nothing breaks. Everything in the design serves that descent and the surface mission that follows.

The vehicle is organized around a stiff Primary Structure that carries every other subsystem. Hung on it are the Descent Propulsion that brakes the fall, four Landing Leg legs that absorb the touchdown, the Guidance & Control that flies the trajectory, a Power Subsystem subsystem, the Thermal Control control that keeps electronics alive through the temperature swings, the Communications link back to Earth, and the Science Payload that does the science once the dust settles.

Primary structure

The Primary Structure is a load path more than a box. A central Thrust Tube of carbon fiber takes the concentrated thrust of the descent engines and spreads it into a stack of Honeycomb Deck Panel honeycomb decks. The decks are light aluminum-honeycomb sandwiches that give large flat areas for mounting tanks, avionics, and instruments. A web of titanium Truss Strut members ties the decks and propellant tanks into a frame stiff enough to survive launch vibration without flexing into its own equipment. At four points, Leg Bracket fittings carry the landing loads from the legs back into this frame.

Descent propulsion

The Descent Propulsion turns orbital speed into a controlled fall. Four Descent Engine units burn storable bipropellant, fuel and oxidizer that stay liquid at spacecraft temperatures and ignite on contact, so there is no ignition system to fail at the worst moment. Each engine has a regeneratively cooled Thrust Chamber, a fine Injector to mix the propellants, fast Engine Valve units to start and stop the flow, and a pair of Gimbal Actuator units that tilt the thrust to steer. Because the lander gets lighter as it burns propellant and must hover near touchdown, the engines throttle deeply rather than running at one fixed level.

Propellant comes from a Fuel Tank and an Oxidizer Tank pushed out by helium from a Helium Tank through a Pressure Regulator. A network of Propellant Valve units and a welded Propellant Line Set manifold routes the flow. Twelve small RCS Thruster units handle fine attitude during the coast and descent, when the main engines are off or too coarse for small corrections.

Landing gear and touchdown

Each Landing Leg leg is an energy-absorbing machine. A telescoping Primary Strut carries the vertical impact, and inside it a Crush Cartridge of aluminum honeycomb crushes at a known force, converting the kinetic energy of touchdown into permanent deformation instead of a bounce. Two Secondary Strut braces stabilize the leg against sideways drift, and a wide Footpad keeps the leg from punching into loose regolith. The legs ride folded against the body through launch and cruise; a Deploy Mechanism with a spring, a release actuator, and lock pins swings them out and latches them rigid before landing.

Guidance, power, and thermal

The Guidance & Control subsystem is what separates a soft landing from a crater. A Flight Computer built around a radiation-hardened processor fuses data from the Inertial Measurement Unit, a Landing Radar measuring speed and altitude, a hazard Hazard Lidar that maps the ground for boulders, and Terrain Camera units that match the view to a stored map. A Star Tracker and Reaction Wheel set hold attitude during the long cruise before descent begins.

Power comes from the Power Subsystem subsystem: deployable Solar Array wings charge a lithium Lithium Battery, and a Power Control Unit distributes regulated buses. The Thermal Control subsystem matters more than it looks, because surface temperatures swing by hundreds of degrees between day and night. An MLI Blanket insulates the body, Radiator Panel panels dump waste heat, Heat Pipe runs carry it there, and Survival Heater units cycled by thermostats keep cold-sensitive boxes above their limits through the dark.

Communications and payload

The Communications subsystem keeps the lander in contact. A Transponder drives a steerable High-Gain Antenna dish for high-rate science return and a pair of Low-Gain Antenna antennas for low-rate, wide-angle contact when the dish is not pointed. The Science Payload is the mission's purpose: Surface Camera units document the site, a Spectrometer reads surface composition, an Environment Package logs radiation and dust, and a Sample Arm with a scoop gathers material for analysis. Variants range from small commercial lunar landers carrying a few instruments to large interplanetary landers that double as rover delivery platforms, but the descent problem and its hardware stay the same across all of them.