Marcus said:

>It has been reported that Discovery banks in descent in
>order to help reduce speed.
>
>Now to reduce speed I reckon you can do two things:
>increase ballistic coefficient or reduce vertical speed
>downwards to the point where the potential energy lost
>is less than the kinetic energy lost per time period due
>to friction (by increasing lift).
>
>The only explanation I can give is that the banking is
>done to increase ballistic coefficient by presenting a
>smaller crossection to the direction of airflow in order
>to reduce the thermal stress on the heatshield - but not
>to reduce speed.

Ballistic coefficient [NASA definition} = Mass / (Cd [reference area])

For hypersonic entry you want to maximize bluntness (minimize ballistic coefficient). This minimizes the heat load on the thermal protection system (tps). Minimized ballistic coefficient is normally achieved by maximizing the drag coefficient, Cd.

For manned hypersonic entry you want to keep deceleration below a critical value due to human limitations. For Mars entry that value is 4 gravities. For lunar return Earth entry that value is 10 gravities. The Space Shuttle is a special case because of how the crew is seated (I don't know what the peak allowable deceleration is but it's probably around 3 gravities).

During hypersonic entry, almost all hypersonic vehicles fly at a single trim angle-of-attack for a given Mach number. The exception to this rule is with MaRVs (Manueverable Reentry Vehicles) but they're a special case. The only way a typical hypersonic vehicle can change its trajectory is through "roll modulation". The term "roll modulation" is actually a misnomer because hypersonic vehicles actually employ bank modulation to control their trajectory. The Space Shuttle flies at a very high angle-of-attack (I think(?) about 40 deg. but I could be wrong). If this were for subsonic flight, the Shuttle would be in a deep stall but stall is a meaningless concept when flying at hypersonic Mach numbers. The Shuttle's velocity entry angle and lift-over-drag ratio are determined such that the crew does not exceed its maximum allowable decelleration while a minimum cross range capability is achieved. Cross range capability was a critical parameter that determined the Shuttle's aerodynamics. As the Shuttle descends, it performs S-turns to bleed off kinetic energy while remaining within its entry corridor. The entry corridor is defined by two entry angles, i.e. the overshoot angle and the undershoot angle. Exceeding the overshoot angle results in atmospheric skip out. Going below the undershoot angle exceeds the maximum allowable G loading and peak allowable heat flux and dynamic pressure on the thermal protection system (TPS). The Shuttle flies at high angle-of-attack until it drops below hypersonic velocities (about Mach 6). The Shuttle then reduces its angle-of-attack until the flow reattaches on the wing and it's no longer stalled. The Shuttle then lands as a supersonic glider following more conventional aerodynamics.

Gary