Unless a suborbital trip is nearly at orbital velocity, it will involve a high, arcing trajectory. This will make the deceleration at the end unacceptably (lethally) high for all but short arcs. Some of the Mercury suborbital missions involved deceleration of 15 gees, if I recall correctly.
That was only an issue because they were fired pretty much straight up; They only went 500km down range.
You can also reduce peek deceleration forces by using aerodynamic lift to stretch out the reentry over a longer period.
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If the capsule/rocketplane has some lift & preferably steerable aerosurfaces then you can compensate the purely ballistic deceleration somewhat.
But yeah, if it is going down almost vertically then this will not be enough.
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