The manned space capsule model
The biggest trouble with tossing a person into space with a trebuchet are the crushing G-forces. We're not sure what exactly they'd be like, but they should be pretty high. Also, in the event of an 'abort' (the capsule doesn't gain escape velocity) it must be durable enough to survive impact. In order to get around these problems, it is obvious that our capsule must be built using durable new space-age technologies, as well as containing a cushion for our passenger to help absorb the stresses of launch.
A reinforced capsule such as the one pictured above would do the trick; the passenger would float in the middle of the capsule, suspended in an oxygenated liquid.
In James Cameron's movie The Abyss, suspending an individual in such an oxygenated liquid allowed them to survive the crushing depths of the ocean's floor. Astronauts flying on the arm of our trebuchet will probably have to do the same; not only that, but they will have to be suspended in the middle of this liquid instead of strapped to a solid surface (such as a seat) that they could be pressed back against and crushed against.
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