[pfdietz]

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.

[m4rtink]

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.

[pfdietz]

And all but rather short ballistic trajectories (well below orbital speed) will come in at a steep angle.

Unless one has seriously variable aerodynamics, the vehicle will have to swerve to nearly horizontal over a distance of about 1 scale height of the atmosphere, which is about 10 km. The exponentially thinning atmosphere goes from "too thin to matter" to "brick wall" over a short distance.

The acceleration for turning is v^2/r; for v = 5000 m/s and r = 10 km this is 250 g.

Acceleration also limits how rapidly one can reenter from beyond Earth orbit. At > LEO velocity, the vehicle has to use (downward) lift to stay in the atmosphere, and if v is too high the required acceleration is too high.

[seabass-labrax]

It would still be possible to fly a predominantly ballistic trajectory, yet use rocket propulsion to decelerate whilst still outside the atmosphere. It would require a huge amount of extra fuel compared to a purely ballistic trajectory, but perhaps still less than achieving a full orbit and de-orbiting again for some destinations.

[pfdietz]

At global distances a fraction orbital trajectory would use less total rocket delta-V.

More practical would be a trajectory that was a series of small suborbital arcs with skipping off the atmosphere (perhaps with some airbreathing propulsion during the skips.) The thermal protection can cool by radiation between the skips.

[seabass-labrax]

That would be very cool to witness! SABRE sounds like it would be an ideal engine for that application.

[phire]

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.

[pfdietz]

No, it's an issue for most arcing trajectories. Lift doesn't help much if you're coming in at a steep angle. Reentry from orbit only works well because the entry is almost flat; there even a little lift helps a lot.