RetroPellant

The word "propellant" is used to describe many different materials used (in many different ways) to create thrust, while (typically) excluding other reaction mass materials, like the air thrust "backwards" to accelerate an aircraft "forwards" in the direction of motion.

Perhaps a different taxonomy for thrust will help us compare existing propulsion systems to speculative systems like the launch loop.

We can define three characteristics of thrust:

1) Internal (i.e. rocket) or external (airplane, submarine, automobile) reaction mass.

2) Internal (i.e. rocket, submarine, automobile) or external (cable car, laser propulsion) energy source.

3) Prograde (i.e. launch rocket, apogee insertion rocket, propeller, automobile engine, adding "forward" speed and energy) or Retrograde (retro-rockets, brakes, parachutes, subtracting speed and energy) thrust direction.

Prograde-retrograde is a bit loosey-goosey ... thrust direction relative to which velocity vector? ... but launching, landing, starting, stopping, is usually well defined for a particular context.

Thus a "zeroth" characteristic of thrust is an adjectival phrase defining the goal ... flight from A to destination B? Landing on Mars? Reentering the atmosphere? While abstract physics is "goal-free", propulsion, whether by space aliens or water-squirting squid, is motivated by an end result.

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