Ares Launch Abort Systems Block I & II

The above NASA document shows the difference
between the Block I, called PA-1 here [Pad Abort],
and the Block II design with the curvy bullet shaped
fairing.

The Mercury capsule flew with the abort tower
attached directly to the capsule. With Apollo, the
designers added the BPS, Boost Protective Shroud.
I've seen it called BPC, Boost Protective Cover too.
This was a large metal conical cover over the capsule.
The BPSserved double duty; as extra protection for
the capsuleduring an emergency seperation [possibly
allowing a reduction in capsule weight], and the BPS
acts as a stabilizer when the tower motors are fired.

When NASA designed the Launch Abort System for
the Orion capsule, they logically started with the
Apollo system, with a few changes to make it more
compact. Compact usually translates to weight savings.
The obvious change was the deletion of the open struts
that seperate the abort motor nozzles from the BPS.  
In the Orion system this space is taken up by the
seperation motor itself, which is mounted upside down.  
More about that little kluge at a later date.

This all sounds fine and dandy so far.
The other pic is a wind tunnel model of the Ares 1
with visible shock waves. This is a common wind tunnel
technique for researching different designs. As you
can plainly see the biggest shock wave is generated by
the Orion BPC. NASA says they have no problem with
controlling the Ares 1, but any hobby rocketeer can
tell that aint very stable. Not stable at all.
Saturn Apollo didn't have this problem. The Apollo
capsule and BPS were much smaller than Orion and, as
I recall, the biggest shock wave producer on the Saturn
was the reducer, between the 2nd and 3rd stage, which
is further back on the airframe. OTOH; the 1st stage of
the Ares 1 is smaller in diameter than the 2nd stage and
capsule, making the situation even worse.  Natural
stability is no big requirement for modern rocket systems
with computer guidance systems and gimballing nozzles,
but the Big Stick is pushing the envelope here.

Enter the Block II BPS.
The modern BPS's are made of composites instead of
metal, which is sensible, but the Block II will  certainly
weigh a fair bit more than the Block I.  With it's bullet
shape, the Block II drag reduction will be huge and
therefore controllability margins will be enhanced.

Ares 1X, the 1st test flight of the Ares 1 stack will
use a Block 1 BPS. Hopefully later tests will include
Block II or even MLAS.