On the ground, you can push against the ground to move (e.g. automobile tires). In the water you can push against the water to move (e.g. speedboat). In the air you can push against the air to move (e.g. airplane).
But in a vacuum (e.g. space), there is nothing to push against. You have to carry everything with you, pusher and pushee.
That's why rockets are useful. It's not just that we understand them -- they work in a vacuum. If you're talking about propulsion that "overcomes Earth's gravity", it has to work in space, otherwise you're just re-inventing the car, boat, or airplane.
But in space you get the bonus of Newton’s 3rd law. Maneuvering thrusters are basically fire extinguishers with attitude, by pushing “stuff” out it applies opposite thrust. Also in space you can use gravity to gain speed.
Part of the idea of getting away from rockets is actually what you put out. Most of that rocket ride it’s in AIR. Why not use that.
And no you’re way of getting out of the gravity well doesn’t HAVE to work in space. You can have more than 1 system. functionally we do that already, they’re just different rockets (when we need a rocket in space, which we don’t always). But there’s launch rockets and in space rockets.
The big problem with rockets is they have this terrible math. You need X amount of propellant to get the payload into space. But of course that propellant has weight, and needs something to contain and aim it which adds more weight. So now you need more propellant. Which adds more weight. More propellant. More weight. You just keep repeating that until you finally get to 0 on the math. If you look at all of our classic space rockets the vast majority of the propellant they use is to launch propellant, not payload.
Think of the classic 3 stage Saturn. Basically (and yes I am over simplifying) the 3rd stage is what we needed to launch the capsule, stage 2 is for launching stage 3, and stage 1 is for launching stage 2. And THAT is the basic problem with rockets. They are grossly inefficient brute force engines that waste a lot of energy in fire and weight.
The space crowd has been wanting to get away from rockets for decades. But all the alternatives are expensive and untried and nobody ever wants to fund them. We can barely get funding for tried and crappy. Forget invent a new technology that works on the chalkboard but nobody has built it.
Not much force, but enough to be useful for some purposes in long term space flight.
Obviously this technique is a result of physics, not some ridiculous "physics doesn't apply" scheme.
“But in a vacuum (e.g. space), there is nothing to push against. You have to carry everything with you, pusher and pushes.”
Can you measure a change in momentum in a vacuum? How about velocity?
Even if the space between celestial objects is a vacuum, there are objects to push against and to provide a frame of reference to measure velocity (which is relative to the frame of reference).
A laser can project a beam from a spacecraft and will change the craft’s momentum when the beam hits some other object. The momentum of both the craft and the other object will be changed slightly even though the laser beam has no mass.
For a craft to be propelled by an energy field without “pushing” against some other object, it would require being able to dynamically alter the curvature of space the way gravity and acceleration do (statically). This would probably rely on a relationship between electromagnetism and gravity that is not currently understood by modern science.
Could a switchable gravity “circuit” operate without violating the laws of conservation of momentum and mass?
Perhaps. If space is curved, time will also be dilated. I would expect this curvature and dilation to be symmetrically mirrored when the circuit is turned off. I think this is essentially what happens with radio waves. The change in the electrical field through a wire induces a magnetic field at a right angle to it but in the path of least resistance.
In a static case (i.e. DC), the path of the magnetic field follows the right-hand rule and forms a field around the electrical field. But in alternating current, the direction of the magnetic field is constantly reversing which causes the magnetic lines of flux to be momentarily perpendicular to the electrical field (with the magnetic field alternating polarity just like the electrical field).
Whether the invention claimed here is real, I think the idea is feasible without breaking the laws of physics. Sometimes inventions and technologies are discovered without fully understanding the laws of nature being exploited.
But these are extraordinary claims requiring extraordinary proof.