There is a Kerbal Space Program mod [1] that adds this to the game. It is hysterically fun to use because it is insanely unbalanced compared to normal space flight requirements. Want to change your orbit 180deg and fly backwards? No problem. Goto any planet the "direct" route? Sure, why not. Take off from the launch pad so fast you'll think the game broke? Not a problem - The Kerbal's don't worry about a little fallout.

No idea how this would ever actually work in reality for manned spaceflight. For unmanned it seems relatively doable (ignoring the entire cost of getting it off the ground and into space first). But for a manned mission? Being subject to 2-4g is one thing, but to have it happen regularly... like a jackhammer almost? Sounds like a really bad trip. You could not do anything while that engine was firing (not that it'd be easy to do something under 2g constant acceleration) without risk of being in the wrong place/position at the wrong time. I don't know what the long terms effects of repeated, assumingly rapid, 2-4g pulsing is on the human body.... but I'd guess it falls under "can't be anything good" category.

[1] https://umbraspaceindustries.github.io/UmbraSpaceIndustries/