Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power. The power source is external. Also magsails,

Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power. The power source is external. Also magsails, electric charge sails, gravity assists (which could be seen as a gravity well sail) can have constant acceleration for a period of time. There are these more exotic methods where a drive interacts with its surrounding medium and the system is open, not self-contained. I don’t know if it’s the case that some method of this kind has been discovered. But in theory, you can push yourself off of anything that repels, pull yourself towards anything that attracts, or receive momentum from anything that has it in an open system. Every electric and magnetic charge you come across in space, you could use in theory, if you were yourself appropriately charged in that moment. This could use a lot of power and therefore a lot of work over time, and may not even be efficient. But it would still meet the propellantless criterium. It’s a bit sci-fi, of course. But probably not physically impossible. We humans propel ourselves without a propellant every day when we walk because we operate in an open system where there is a surrounding medium to grasp and pull or push against. Em drive and some other similar stuff were suggesting that they can make a force in a closed system, not through an interaction with a surrounding medium within which the truster exists, nor through expelling own propellant. It suggested a reactionless drive, which is not considered possible. But propellentless is very possible.