The choice of trajectory when humans are on board is also limited because you need to limit the maximum Gs. There are 4 general ways to deal with the heat.
Variable lift to get onto optimum glid path reduces peak acceleration.
Our reentry simulations indicate we can keep the SSTT under 900 degrees C all the time. This is because the rocket is suborbital, small, and lightweight. Compared to shuttle reentry we will have much lower temperatures (about 900 C compared to 1650 C) and for much shorter time (about 2 min compared to 20 min).
Reentry from a tether generates less heat, for shorter periods than reentry from orbital speed. This makes design for reentry much easier for an SSTT than from full orbit.
The following graphs are designed to illustrate the differences between reentry from a tether and reentry from orbital speed. The data comes from simulation examples 51 and 52 in the SpaceTethers.com simulator. There are 4 views of the 2 data-sets, for a total of 8 graphs.
The graphs each have 8 curves for 8 blunt reentry vehicles. The y-axes shows the altitude. In the first 6 graphs the x-axes show the speed relative to the air, in the last 2 graphs the x-axes shows the time. The vehicles all have the same weight (10,000 Kg) and the same L/D of 0.5. The difference between the vehicles is in the amount of drag area they have. Thedrag areas shown are 640, 320, 160, 80, 40, 20, 10, and 5 sq-meters. In each graph the 640 sq-meter area vehicle is the top curve, as it slows down at a higher altitude. The other vehicles are in descending order according to drag area below the 640 sq-meter vehicle.
There are good heat-shield materials and methods, but designing a reliable reusable reentry vehicle is a hard problem. The idea of transpiration cooling looks good.
The larger area reentry vehicles in the graphs could only be done with some rather novel method. Something like a big parachute with an inflated rim so it would work at high altitudes. But this would not be the blunt shape that these graphs assume, so the edges of the parachute would be hotter than shown here. Still, there are some high temperature cloths so maybe this is possible.