Re: Minimum speed required for Shuttle Orbiter to enter the atmosphere?

Hello, Jake!

The present shuttle design requires atmospheric braking to descend. It has 
retrofiring early on in its deorbiting sequence, but is not able to use its 
engines later on (not built that way). 

So the speed of the beast is set pretty much by how fast you can dissipate 
kinetic energy as heat from friction with the atmosphere. This is a 
complicated problem, depending not only on speed but attitude of the wings, 
the density of the air at any given altitude, the shuttle's weight (loaded 
or unloaded), etc.

Now, once you're down low enough that the shuttle's wings start to act like 
those of an airplane (i.e. the atmosphere is thick enough), then the concept 
of stall speed comes in. The shuttle, as a delta-wing craft, does not have a 
well defined stall speed, per this NASA webpage:

  http://quest.arc.nasa.gov/space/ask/landing/Orbiter_Stall_Speeds.txt

However, NASA does list the following two values for stall speed in a table.

     Loaded: 1,096 mph 
     Unloaded: 219 mph

These should represent the range of speeds that the shuttle must maintain to 
glide effectively "down low" in the atmosphere (and I don't have exact 
numbers as to where  "down low" begins...perhaps the moderator can find 
someone from NASA to add a codicil to this answer...but I'd guess somewhere 
between 50,000 to 100,000 feet up.).