Why do SpaceX rockets' re-entries seem to burn (as in heat the hull) less than the space shuttle ? Is it because of less friction surface ? Thanks !
The Space Shuttle re-enters from full orbital velocity.
The Falcon 9 first stage cuts off after only 2 minutes of flight. Much more comparable to the SRB flight times. Velocity and height are much lower.
They do a reentry burn to slow down as they enter the atmosphere.
All this suffices to keep the heating down to the level below the melting point of the Aluminum hull (The material it is mostly made of).
The answer of geoffc is only partially true, as some parts of the Falcon 9 are forged out of titanium to sustain more heat than aluminium.
And considering material science the heating has to be kept even lower than to the level of the melting point to avoid that the aluminium alloys undergo a phase change of the crystal structure.