But directly behind the rocket is where most of the energy goes.
I'd also be curious how the tether would avoid getting cooked by the rocket exhaust. I think a long mast, putting the riders in front of a large mass of fuel, would be much simpler.
But you're right, it is a lot easier to design an ultra-light, super-strong tether than a giant mast.
Two habitats with an engine in the middle spinning around like a top could provide gravity, and absorb the engine impulse. Under no acceleration the three bodies would be co planar, and under acceleration would deform into a cone shape.
That's a smart design! If you spin up to 0.5g and accelerate at 0.5g (which, while not enough for takeoff, is plenty for interplanetary travel) the tether angle would be 45 degrees, more than enough to get out of the exhaust plume.
Another free benefit would be that on a change of acceleration, angular momentum would be conserved. Rotation would increase or decrease to keep the acceleration felt by the occupants approximately constant!
From what I've seen the Project Orion and later designs concluded that was the most efficient design. Imagine a parachute (hemisphere) with the place where the explosion happens in the center and the (heavily shielded) capsule behind it, so fully 50% of the explosion is providing forward thrust. You don't have a single tether passing through the heart of the exhaust, rather you've got several going around the outside (like the frame of a cone tent, with the capsule at the apex). This was apparently easier than designing a rigid "pusher plate" with the right shape to capture a lot of thrust.
Yes that design has been proposed before. The mass savings from using a tensile structure can make up for the slight loss of thrust efficiency. There was a somewhat plausible design in Avatar.
Its possible that the exhaust could be made to disperse without losing much thrust - like a defocussed electron beam. Just so long as its cool enough when it hits the habitat. Which could have an aluminum-foil shield or some such...
I'd also be curious how the tether would avoid getting cooked by the rocket exhaust. I think a long mast, putting the riders in front of a large mass of fuel, would be much simpler.
But you're right, it is a lot easier to design an ultra-light, super-strong tether than a giant mast.