The physics aren't bad; the fuel requirements for the same payload for point to point suborbital travel are about the same as a full tank for a 747 from Sydney

The physics aren't bad; the fuel requirements for the same payload for point to point suborbital travel are about the same as a full tank for a 747 from Sydney to Istanbul. You spend a lot of energy getting up to speed, but you have much less drag. The challenge is making it safe enough / rapidly reusable enough. If you're at just suborbital speeds, the temperature from the adiabatic compression on reentry is intense, and it's a technical challenge to handle the heating either non-ablatively, or ablatively but with cheap, safe rapidly re-applicable ablative material.