Rocket Upper Stage

Overview

An upper stage does the precision work of a launch. The booster below it is a brute-force machine that climbs out of the atmosphere; the upper stage then adds most of the orbital velocity, coasts, restarts, and places the spacecraft on its target orbit to within fractions of a degree and a few kilometres. The design brief is therefore different from a booster's: vacuum-optimized performance, multiple in-space restarts, hours of battery-powered autonomy, and the cleanliness demanded by the payload riding centimetres above the Payload Adapter & Separation.

This stage burns liquid hydrogen and liquid oxygen. Hydrogen gives the highest specific impulse of any practical chemical propellant, 465 s here, at the cost of a 20 K storage temperature and a density of only 71 kg/m³, which is why the LH2 Tank dwarfs the LOX Tank despite holding a sixth of the propellant mass.

The expander cycle

The Vacuum Engine is an expander-cycle engine, the gentlest of the pump-fed cycles. There is no gas generator and no preburner: liquid hydrogen pumped through the milled cooling channels of the Thrust Chamber both keeps the copper liner from melting and absorbs enough heat to emerge as warm gas, which then spins the Turbine Wheel before being injected and burned. Every joule of turbine drive energy ends up in the chamber, and turbine inlet temperature is a few hundred kelvin rather than a thousand, so the engine starts softly and restarts reliably, up to a dozen times, lit each time by the Spark-Torch Igniter torches. The cycle's physics cap its thrust (chamber surface area grows slower than volume), which is exactly why expander engines live on upper stages and not boosters.

The LH2 Turbopump spins near 50,000 rpm; the LOX Turbopump is geared down from it because dense oxygen needs far less pump speed. The purged Turbopump Shaft Seal stacks between them are among the most safety-critical parts on the stage, hydrogen and oxygen meeting along a shaft is a loss-of-mission event. Thrust is steered ±4° by the TVC Actuator pair and trimmed by the Turbine Bypass Valve, which bleeds hydrogen around the turbine to modulate pump speed.

Cryogenic tankage

The Propellant Tank Assembly are welded aluminium-lithium, separated by a Common Bulkhead: a double-wall dome with an evacuated honeycomb core that holds a 70 K gradient across a few centimetres while saving the mass and length of a separate intertank section. Spray-On Foam Insulation limits boil-off and stops air from liquefying against the hydrogen wall on the pad. Anti-Slosh Baffle rings damp the propellant modes that would otherwise feed back into guidance.

In zero-g coast, propellant floats. Before each restart the Settling Thruster pair fires to settle liquid over the sumps, and the Pressurization Regulator holds ullage near 3 bar from the Helium COPV set, stored inside the LOX tank, where cold multiplies helium storage density. During burns the Autogenous Pressurization Line taps warmed vapour from the engine so each tank pressurizes itself. Between burns the Vent & Relief Valve relieves boil-off, often through aft-facing ducts so even venting produces useful settling thrust.

Brains of the vehicle

The Avionics Equipment Ring at the forward end is the launch vehicle's only brain, the booster takes commands from here. Redundant Flight Computer units vote on guidance solutions from the Inertial Navigation Unit cluster, with the GNSS Receiver bounding inertial drift across long coasts. The independent Flight Termination System answers only to range safety. The Cold-Gas Thruster set holds attitude with clean nitrogen pulses, cold gas is chosen deliberately, since combustion products would condense on the payload.

Flight sequence

After booster burnout, retro motors back the spent stage away as the Linear Separation Charge severs the Interstage Shell that had enclosed the fragile Nozzle Extension. The engine chills down, ignites, and burns to a parking orbit; a coast and second burn shape the transfer; then the Separation Clamp Band releases and the Separation Spring set pushes the spacecraft off at half a metre per second with negligible tip-off. A final brief burn deorbits the stage or lifts it to a graveyard orbit, ending a working life measured in hours on the Avionics Battery supply.