I ran across an article on a variation on impact fusion called projectile fusion. A relatively large (1cm) projectile is smashed into a target at 14,500 mph. The specific shape of the projectile and target temporarily create extreme pressures and temperatures that has created some laboratory detectable fusion reaction. Railgun type accelerators and other complex gear in the UK experiments.
How long before someone thinks to try these experiments in orbit? Retrograde projectiles could easily have a closure rate of 36,000 mph with a prograde target. far beyond the velocities they currently have available on the ground. And far more massive projectiles. Assuming it works, Fusion Orion/Medusa for deep space propulsion without a lot of radioactive mass on board?
Assuming it works, how long before it gets weaponized? Or configured to move asteroids with a large number of small yield devices of even smaller mass?
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There is a world of difference between ‘laboratory detectable fusion’ and ‘positive energy return’. Like every other inertial confinement method the kinetic energy required will be several orders of magnitude greater than the nuclear energy released.
14,500 mph sounds like they are using a light gas gun. https://en.wikipedia.org/wiki/Light-gas_gun
Accuracy is a major difficulty for retrograde interception. Such a system would rightly be classed as an anti-satellite weapon. Larger projectiles would produce more debris.
Simply firing the projectiles out the back of the rocket would be a more efficient engine. Still not very efficient, but better than projectile fusion.
If you don’t mind ignoring details like energy cost, search for “relativistic ravioli”. I think you’d enjoy it. Cheers.