Mars Entry - Not So Fast !!!

With all respect to my esteemed friend Gerald Norley, aerobraking in the Mars atmosphere is quite difficult.

The problem is not the speed itself, it is the small radius and weak gravity of Mars. Apollo entered at 11.2 km/s, essentially escape velocity.

If it had entered too high up, the drag would have been insufficient for aerocapture, and it would have gone into an elliptical orbit, perhaps a slow one. The Apollo capsule had already shed the service module; the resources in the capsule would not have lasted long enough to keep the astronauts alive.

If it had entered at too steep an angle, it would have come down into the deep atmosphere with too high a velocity. The "sudden" impact would have killed the astronauts and smashed the capsule to bits.

Instead, Apollo aimed for a thin slice of atmosphere where the drag was "just right". Just right for what?

The problem was that staying in that thin atmosphere required following a circular arc at approximately the same altitude (and radius) across a considerable stretch of sky. At the onset of reentry, that required an additional "gee" of centripedal acceleration towards the center of the Earth. The only way to get that was with negative lift, with the sphere/cone shaped spacecraft cutting through the thin air and deflecting it upward, while slowing down.

The wings of an ordinary airplane creates quite a bit of lift; for every newton of drag, perhaps 10 newtons of lift, depending on the wing design and the speed. This is a lift to drag ratio ( L/D) of 10. By adding thrust with a propeller or jet or rocket, the drag can be counteracted and the lift maintained indefinitely. However, at Mach 35 orbital speeds, the wings of an ordinary airplane would be burned off or shredded to fragments. The space shuttle had strong narrow wings enclosed in heat-resistant tiles; it had a lift-to-drag ratio of about 1. A flying brick. However, those wings were heavy, and to were designed for entry speeds of less than 8 kilometers per second; if the shuttle had somehow flown in from the Moon or high orbit at 11 kilometers per second, the wings would not be strong or insulated enough to survive.

The Apollo command module had a lift-to-drag ratio of 0.3. To provide an additional gee of downwards lift, it needed 1 gee / L/D or 1 gee / 0.3 or 3.3 gees of drag to make 1 gee of downwards lift. In addition, it needed some "lift" force for sideways maneuvering; although the recovery fleet was stationed where the capsule was targeted to land, a storm a few hours before entry would require both the fleet and the capsule to divert to a new landing zone. Typically the fleet would move downrange, and the capsule would briefly "bounce" out of the atmosphere to extend the range a bit, but in some circumstances it might need to deflect sideways as well. This capability was important for Apollo; for Mars missions it was essential to land in a precise target zone.

Mars entry missions copy Apollo, but with an important difference; they come in faster than escape velocity. The entry velocity from, say, Deimos is close to Mars escape velocity,