The Imperial Courier is powered by two multi-cycle Bussard ramjet
engines, designated BRE(MC)3 that also serve as power generators and
one takeoff turbofan, designated TF90.
Specifications -
BRE(MC)3:
Traction (atmosphere, sea level 1013 mbar): 15,220
kN nominal
Traction (vacuum): 39,840 kN nominal
Minimum H2 flux for RAFE operation: 580 g/sec
Minimum H2 flux for CR operation: 5.5 kg/sec
Max. exhaust speed: +0.027 c
Max. exhaust temperature: 1.77*10E9 Kelvin
Magnetic ramscoop absorbed power: 550 GW FE,CR mode,
1,02 TW RAFE,FB mode
Exhaust linear accelerator absorbed power: 33 TW nominal
Fusion reactor rated output power: 36,5 TW
Nominal output power @ 25 kV / 500 kHz : 1,22 TW
Specifications -
TF90:
Geometry: 1 low pressure + 5 high pressure compressor,
2 low pressure + 1 high pressure variable geometry turbine
Fan diameter: 8 m
Spool speeds: 3300 RPM fan, 7500 RPM compressor, 22,500
RPM turbine @ 100% thrust
Traction (atmosphere, sea level 1013 mbar): 16,800
kN nominal
Minimum operationg atmospheric pressure: 230 mbar
In order to achieve the tremendous speeds required, the BRE engine can work in 4 different modes:
Fusion engine (FE) operation: in this mode it works like a "normal" multi-cycle starship engine: H2 from the tanks is fused into He and then into progressively heavier elements until iron is obtained. The tremendous amount of power generated is used to accelerate the iron plasma to very high speeds and create thrust in accordance to Newton's third law. While operating in this mode, the ramscoop can be turned on to collect interstellar hydrogen and refill the tanks. This mode is used for low cruising speeds and low accelerations.
Ram-augmented fusion engine (RAFE) operation: in this mode, part of the power output from the fusion reactor is used to power the ramscoop at high wattage and ionize every atom and space dust particle in front then collect it. The resulting intake flux has a much higher mass flow and is comprised of many elements besides H2 and as such it isn't routed to the fuel tanks. Instead it is mixed with the exhaust iron plasma from the reactor, cooling it. The mix will then be again accelerated at high speeds by the linear accelerator and thanks to the increased mass flow, greater thrust will be produced. This is the primary operation mode for the BRE engine.
Catalytic ramjet (CR) operation: for speeds above 1000 km/sec a fusion reaction using the scooped H2 can be sustained, given the average density of interstellar hydrogen in our galactic arm. However, fusing H2 nucleii is much more ineficient and requires higher temperatures than the readily avaiable "hydrogen fuel" - deuterium. By using a catalyst to enlarge the reaction cross-section (increase the likeness of two H2 nucleii fusing at a given temperature), namely carbon, it is possible to actually achieve self-sustained fusion and produce power. This power is used to accelerate the fusion exhaust plasma (iron) as before and generate thrust. This operation mode is used when cruising at high speeds.
Fusion brake (FB) operation: collected H2 will be fused without the catalyst. The reaction requiring external energy to be sustained, and the only energy avaiable being the kinetic energy of the ship, this will create drag and allow deceleration from relativistic speeds.
The engine management computers (located inside the fuel tanks for protection against radiation and strong magnetic fields associated with ramjet operation) automatically decide which operating mode is best suited to current conditions. The switch between modes is totally transparent to the pilot.
The fusion plume is magnetically steerable ±90 degrees, cutting the need for maneouvering thrusters.
The TF 90 engine is used for takeoff and landings from planetary surfaces as the BRE engine exhaust is too hot for the launching pads. Hydrogen from the fuel tanks is used as a combustible. A liquid oxigen tank enables use even on worlds without an oxygen atmosphere, at reduced engine performance though. It is located on the belly of the Courier and is used only to lift the ship vertically to about 500 meters, then the fusion engines are safe to use. For starports with runways, some Courier models are equipped with wheels on the main undercarriage enabling them to do a running takeoff with their fusion engine.