Chris Vancil said:

>I was on a panel at a local Sci Fi convention (Norwescon 28)
>and pointed out your objections to the CEV design and got >arguments that you could just aerobrake to a slower speed for
>entry or add more ablative to your heat shield. I think folks didn't
>quite get it or I explained your objections poorly.

First some jargon:

"aerobraking" is done in the upper atmosphere where the atmospheric gas is free molecular (non-continuum). The aerodynamic forces are tiny. The objective of aerobraking is to slowly reduce the apoapsis (maximum altitude) of a planet centered elliptical orbit to eventually circularize the orbit.

"aerocapture" is done in the deep atmosphere where the gas is a continuum. The aerodynamic forces are huge (tons of force). The objective of aerocapture is to transform a hyperbolic entry trajectory into a planet centered elliptical orbit.

An Apollo or Mars return vehicle both use a form of aerocapture where the capture orbit is actually suborbital (the trajectory intersects with the Earth's surface). The Apollo return trajectory from the Moon plunged the Apollo CM into the Earth's atmosphere and then skipped out of the atmosphere into a suborbital trajectory which then brought the CM immediately back into the Earth's atmosphere. The Apollo Guidance Computer (AGC) followed a fairly complicated control law that kept the Apollo CM within its entry corridor without exceeding the maximum allowed G loading. The peak G loading for Apollo 10 was 4.5 G. The AGC controlled the CM purely through roll modulation (The CM always remained at its trim angle-of-attack). The Apollo CM flew a "lifting trajectory" because it had a nonzero Lift-over-Drag ratio (L/D). Entry angle is defined as the angle between the velocity vector and the local horizon. The entry corridor is defined as the difference between the "overshoot entry angle" and the "undershoot entry angle".

The overshoot entry angle is the entry angle where the CM's lift vector is always pointed towards the Earth. The overshoot entry angle is the minimum angle that results in the CM not escaping the Earth's atmosphere into a hyperbolic trajectory (certain slow death for the astronauts).

The undershoot entry angle is the entry angle where the CM's lift vector is always pointed away from the Earth. The undershoot entry angle is the maximum angle (most normal to the local horizon) that results in the CM's peak entry heat flux and dynamic pressure not exceeding the material limit of the CM's heat shield.

The AGC flies the CM along a trajectory that is between the undershoot and overshoot trajectories. The AGC does this by rolling the lift vector back-and-forth between full lift up and full lift down. The overshoot and undershoot trajectories are defined by the entry vehicles's L/D and entry velocity. If the L/D is too small or the entry velocity is too large then the difference between overshoot and undershoot entry angles can become so small that a guided trajectory becomes impossible.

There is an additional aspect to the Apollo entry control law. In theory the Apollo CM could fly an aerocapture trajectory where the apoapsis of the captured orbit was very large. However a large apoapsis would take the CM back through the van Allen belts with the flight crew remaining there for an unacceptable length of time accumulating a large radiation dose. The control law of the AGC during the intial entry into the atmosphere is set to aerocapture to a low suborbital trajectory below the van Allen belts.

>Isn't your primary objection that the reentry corridor is too narrow
>for safe entry using an Apollo style gumdrop?

Yes. As the entry velocity grows larger for return from Mars, the L/D must also grow larger such that the entry corridor is wide enough for the vehicle to remain within the corridor through roll modulation.

>Another person after the panel suggested you stop at EML-1,
>refuel and them return to Earth!

The Mars return vehicle would return to Earth with a huge amount of kinetic energy. The whole idea of return to Earth is to use the vehicle's aerodynamic drag in the Earth's atmosphere to transform that kinetic energy into heat and dump it into the atmosphere. Return to the Moon from Mars means the vehicle must carry extra mass as fuel to kill off that kinetic energy. No practical design would return from Mars directly to the Moon.

Gary

P.S. Read NASA TN D-6725 by Graves and Harpold if you want to learn more about this.