Is it farther or further?
1610s, "expulsion," noun of action formed from propuls-, past participle stem of Latin propellere "to propel" (see propel). Meaning "act of driving forward, propulsive force" first attested 1799.
propulsion pro·pul·sion (prə-pŭl'shən)n.
A driving or propelling force.
The leaning or falling forward characteristic of the festination of parkinsonism.