thumb|right|275px|Expander rocket cycle. Expander rocket engine (closed cycle). Heat from the nozzle and combustion chamber powers the fuel and oxidizer pumps.

The expander cycle is a power cycle of a bipropellant rocket engine. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The now heated and gaseous fuel then powers the turbine that drives the engine's fuel and oxidizer pumps before being injected into the combustion chamber and burned.

Because of the necessary phase change, the expander cycle is thrust limited by the square–cube law. When a bell-shaped nozzle is scaled, the nozzle surface area with which to heat the fuel increases as the square of the radius, but the volume of fuel to be heated increases as the cube of the radius. Thus beyond approximately 3000 kN (700,000 lbf) of thrust, there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps.

Expander bleed cycle

thumb|right|250px|Expander bleed cycle. Expander open cycle (Also named coolant tap-off).

This operational cycle is a modification of the traditional expander cycle. In the bleed (or open) cycle, instead of routing all of the heated propellant through the turbine and sending it back to be combusted, only a small portion of the heated propellant is used to drive the turbine and is then bled off, being vented overboard without going through the combustion chamber. The other portion is injected into the combustion chamber. Bleeding off the turbine exhaust allows for a higher turbopump efficiency by decreasing backpressure and maximizing the pressure drop through the turbine. Compared with a standard expander cycle, this allows higher engine thrust at the cost of efficiency by dumping the turbine exhaust. The Mitsubishi LE-9 is the world's first first stage expander bleed cycle engine.

Blue Origin chose the expander bleed cycle for the BE-3U engine used on the upper stage of its New Glenn launch vehicle.

Dual expander

In a similar way that the staged combustion can be implemented separately on the oxidizer and fuel on the full flow cycle, the expander cycle can be implemented on two separate paths as the dual expander cycle. The use of hot gases of the same chemistry as the liquid for the turbine and pump side of the turbopumps eliminates the need for purges and some failure modes. Additionally, when the density of the fuel and oxidizer is significantly different, as it is in the H<sub>2</sub>/LOX case, the optimal turbopump speeds differ so much that they need a gearbox between the fuel and oxidizer pumps.

| 180&nbsp;kN (40,000&nbsp;lbf)

| 88.36&nbsp;kN (19,860&nbsp;lbf)

| 250&nbsp;kN (56,200&nbsp;lbf)

| 68.6&nbsp;kN (15,400&nbsp;lbf)

| 137.2&nbsp;kN (30,840&nbsp;lbf)

| 1471&nbsp;kN (330,000&nbsp;lbf)

|-

! Mixture ratio

| 5.88

|

| 5.8

| 6.0

| 6.0

|

| 5

| 5.9

|-

! Nozzle ratio

| 280

|

| 240

| 80

| 160

|

| 110

| 37

|-

! I<sub>sp</sub>, vacuum (s)

| 462

| 455

| 457

| 442.6

| 455.2

| 470

| 447

| 426

|-

! Chamber pressure (MPa)

| 4.412

|

| 6.1

| 4.1

| 7.0

| 5.9

| 3.58

| 10.0

|-

! LH<sub>2</sub> TP (rpm)

|

|

| 65,000

| 98,180

|

|

| 52,000

|

|-

! LOX TP (rpm)

|

|

|

|

|

|

| 18,000

|

|-

! Length (m)

| 4.14

|

| 4.2

|

|

| 3.358

| 2.79

| 3.8

|-

! Dry mass (kg)

| 277

|

| 280

| 265

|

|

| 285

| 2400

|}

See also

  • Gas-generator cycle
  • Combustion tap-off cycle
  • Staged combustion cycle
  • Pressure-fed engine
  • Electric-pump-fed engine

References

  • Rocket power cycles