Full-Flow Staged Combustion Engine Cycle (2D)

Engine Cycle

Every kilogram of fuel and oxidizer is fully gasified in two preburners before it ever reaches the main chamber — nothing is dumped overboard.

Controls

Mixture ratio O/F 3.60

Propellant flow 500 kg/s

Preburner turbine limit 880 K

Live Readouts

Chamber pressure

— MPa

Effective I_sp

— s

Thrust

— kN

Dumped overboard

Turbopump shaft power (both pumps)

— MW

How it works

A rocket turbopump feeds propellant to the chamber far faster than a tank's own pressure could push it, and something has to spin that pump. The cycle architecture is how the engine finds that power:

Gas-generator (legacy) — a small slice of propellant (here ≈4.5%) burns fuel-rich in a separate gas generator, spins the turbines, then is dumped overboard through its own low-expansion exhaust, never reaching the main chamber. Simple, but that flow — and the energy in it — is wasted, and the turbine can only ever see a small fraction of the total mass flow.
Full-flow staged combustion (FFSC) — all the fuel drives a fuel-rich preburner and all the oxidizer drives an oxidizer-rich preburner. Both fully-gasified, still-burning streams then enter the main chamber together at the correct overall mixture ratio, so nothing is thrown away and both turbines see the full mass flow — enabling far higher chamber pressure and cooler, less corrosive turbine gas on each side (fuel-rich gas never touches the oxidizer-rich turbine, and vice versa). This is the cycle used by Raptor and BE-4 on methane/LOX.

Model used here (illustrative, not a specific engine) — identical to the 3D version's physics, verified numerically across the full control range (mixture ratio, flow and turbine limit sliders) to stay finite and positive:

m_fuel = m_total / (1+r),  m_ox = r · m_total / (1+r)     r = O/F mixture ratio
Preburner turbine power:  P_t = f · m_total · cp · (T_limit − T_exit) · η_t
Pump pressure rise:       ΔP  = k_cycle · (T_limit − T_exit)      [k_cycle set by turbine/pump sizing]
Chamber pressure:         Pc  = P_tank + 0.85 · ΔP
Flame temperature:        Tc(r) peaks near r ≈ 3.6 for methalox
Characteristic velocity:  c* = √(γRTc) / [γ(2/(γ+1))^((γ+1)/(2(γ-1)))]
Effective Isp:            (1−f)·(c*·Cf/g0) + f·I_dump          f = 0 for FFSC, ≈4.5% for gas-generator

This 2D view renders the same plumbing as a flat, pannable/zoomable schematic (drag to pan, scroll or pinch to zoom) instead of a 3D scene — twin tanks feed twin pumps, which in FFSC mode cross-feed a fuel-rich and an oxidizer-rich preburner before both hot gas streams merge in the main chamber; in gas-generator mode the pumps instead bypass straight to the chamber while a small trickle spins a separate dump generator that vents overboard, visibly wasted.

FFSC

drag to pan · scroll to zoom

Fuel (CH₄) Oxidizer (LOX) Preburner exhaust → chamber Dumped overboard (GG only)