## Info

hydrocarbon

From hydrocarbons to hydrogen, the Brayton cycle heat addition with Carnot losses equals the air kinetic energy between 2,160m/s and 2,351 m/s (7,087 ft/s to 7,713 ft/s). From hydrocarbons to hydrogen, the Brayton cycle heat addition with Carnot and non-uniform combustion losses equals the air kinetic energy between 2,196 m/s and 2,019 m/s (6,623 ft/s to 7,208 ft/s). So, for any speed above these speeds, the air kinetic energy is greater than the fuel combustion energy addition to the air stream. Second Law available energy losses make the problem a bigger problem because they limit the actual heat energy added to the air to less than the maximum values in equation sets (4.2) and (4.3). For hydrocarbons there is a range in the heat of combustion, so there is a ±0.18 range on the value in the denominator. There is a practical limit to the combustion energy's ability to offset internal flow and frictional losses that can be determined from first principles. At that point the air-breathing propulsion system can no longer accelerate the vehicle.

If we look at the other energy losses added to the Carnot loss, we see how much greater the air stream kinetic energy is compared to the fuel addition energy. This is what limits the application of airbreathing propulsion to space launchers. In terms of practical operational engines, the maximum flight speed is probably about 14,000 ft/s and perhaps as much 18,000 ft/s for research engines. The latter figure is one-half the specific kinetic energy (energy per unit mass) required to achieve orbit. So, to achieve orbital speed with an airbreather propulsion system, a rocket for final speed in the trajectory and space operations is required.

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