&quot;Suborbital&quot; refers to a ballistic trajectory that is &quot;close&quot; to an orbital trajectory, so that the projectile follows an unpowered arc, but one that is not wide enough to achieve orbit. The energy required for suborbital trajectories is still in the same neighborhood as orbital flight.<p>The reason your proposal doesn&#x27;t work is that during the portion of the trajectory outside the atmosphere, you either need to be ballistic (and therefore suborbital if you&#x27;re traveling intercontinental) or you need to be constantly directing a portion of your rockets thrust downward.  The latter is fantastically inefficient, which is why planes operate in the atmosphere (where they can use air to convert efficient horizontal thrust to lift) and why suborbital rockets are ballastic for the large majority of their flight.