Usually it boils down to ablatives. Either your pusher is designed to ablate at a known rate and factored into the propulsion calculations as fuel mass, or you

Usually it boils down to ablatives. Either your pusher is designed to ablate at a known rate and factored into the propulsion calculations as fuel mass, or you are protecting the pusher plate by doing something like injecting synthetic oil that wont spontaneously boil in the vacuum of space through pores in the pusher plate so that the oil becomes a protective layer that boils off instead of eroding the surface of the pusher plate. The other method I've seen is to "go big" and scale things up to the point where the pusher plate is collecting enough force, but spread over a sufficient area that while the pusher can effectively and "safely" capture the plasma debris from the pulses while keeping the flux of high energy photons low enough that it doesn't erode the pusher plate from the radiation flux. The "Medusa" design scales this up to the extreme, putting the explosion ahead of the spacecraft and using a giant spherical "sail" to capture the energy from each explosion, if you simplify Project Orion as "explosion under pogo stick while you stand on it", then the Medusa design is "explosion above your head while you wear a hard hat, holding onto an umbrella with a pole made from elastic bands". Detail https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion#Medusa https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion#Medus... and http://www.projectrho.com/public_html/rocket/enginelist3.php#medusa http://www.projectrho.com/public_html/rocket/enginelist3.php... Since you mention the tests with conventional explosives, I love bringing up the actual tests that were done when people talk dismiss Orion as an impractical idea that wouldn't have worked, seeing the "Hot Rod" fly powered by RDX explosions is usually enough to convince someone the physics of the idea is completely sound. Theres a nice set of videos from the test program here https://www.youtube.com/watch?v=Q8Sv5y6iHUM https://www.youtube.com/watch?v=Q8Sv5y6iHUM which also includes the "Hot Rod" test, which is also the largest remaining object/artefact of the entire Project Orion program of work. The "Hot Rod" test vehicle is housed in a crate (not on display) at the Smithsonian, its part of the National Air and Space Museum Collection https://www.si.edu/object/propulsion-test-vehicle-project-orion:nasm_A19721008000 https://www.si.edu/object/propulsion-test-vehicle-project-or...