The Apollo CM was selected for lunar return because it was a very conservative design (it was designed around 1959). The business end of the Apollo CM is simply a spherical section. By going with a spherical section, static stability was assurred for all Mach numbers.

There are two big problems with the Apollo CM

1) The Lift-over-drag ratio is poor. The Apollo CM requires impossible entry accuracy when entering the atmosphere at greater than 12 km/sec. The limitations are atmospheric skip off versus excessive deceleration loading (over 20 gravities).

2) Due to the high angle-of-attack, the stagnation point heating gets too close to the vehicle's corner where flow expansion occurs. This actually doubles the heating rate due to convective heating. A sphere-cone or raked conic geometry removes this problem.

People have known this for years but an Apollo CM shape is unusable for a manned Mars return vehicle. If you look at literature from the late 1960s, you'll see that they almost always proposed a weird shape for a manned Mars return vehicle.

Gary