"Craig Cocca" wrote in message
ups.com...
This reasoning is somewhat debatable. While it is true that liquid
fueled engines can be shut down in flight, would you really *want* to
shut one down, especially during the early portion of ascent? Losing
multiple liquid engines during the first few of minutes of flight
(which is when the SRB-derrived booster on Ares would be firing) almost
certainly would result in a Loss of Vehicle (in case of the STS), or an
launch tower abort situation (in the case of Ares).

Shutting down the engine short-circuits a failure path which could lead to
an unsurvivable explosion or rapid structural failure. The idea is to
"soften" the environment the spacecraft must escape from. It also pads the
time an abort system has to react before things become real ugly.

Also, a solid rocket booster had considerably fewer moving parts than a
liquid booster, meaning there is less that can go wrong while the
booster is firing.

Simplicity doesn't always equate to reliability. Manufacturers must adhere
to an extremely tough quality assurance regimen to ensure successful
operation - and they don't always stay as strict as they should.

The problem with SRMs is that when things go wrong, they tend to get worse
very fast. A study of solid motor failures done by NASA revealed that the
majority of SRM failures have a sudden onset and extremely rapid propogation
of the failure. That calls into question the ability of an abort system to
react quickly enough to get the crew a safe distance away from the
explosion. Estimates are that an abort system requires approximately two
seconds to identify a failure, process its criticality, send commands for an
abort to execute, and then wait for those systems to operate and separate
the spacecraft to a survivable point outside the booster's blast field.

Perhaps the most glaring shortcoming of the SRBs
that have been used on the shuttle are their segmented design. Since
the boosters are manufactured and refurbished in Promontory, Utah, they
have to be shipped to Florida in segments by railcar. Had politics
dictated that the SRBs be manufactured near KSC when the shuttle was
first designed, we might have ended up with a single segment design
instead, which would've completely eliminated the root design flaw with
the O-rings that led to the Challenger disaster.

Single segments have their own set of problems, particularly when they are
the size of the monolithic boosters STS would've required. Attaining a
uniform insulation coating, propellant pour and inspection along the entire
casing would be very challenging. Cost would quickly become a factor. Ground
handling for such a behemoth would be a complete PITA. Monolithic SRMs as
are used on Atlas V, Delta II & IV are tough enough as it is.

I suspect on Ares that one of the abort scenarios will be for sensorsto
detect a sudden drop in chamber pressure in the SRB.

What about -51L's failure mode? There was no sudden drop in chamber
pressure - it was very small and gradual. A failure like that could fairly
quickly overwhelm the autopilot's ability to control the trajectory, leading
to strucutral failure somewhere in the interstage/second stage/spacecraft
areas.

If such a
situation is detected, then the Crew Escape System will be activated,
removing the CEV from the Ares stack and returning the astronauts and
CSV to Earth. The only other major ascent emergency that has to be
dealt with is loss of cabin pressure, which I suspect will be mitigated
by a return to full pressure suits during launch.

There are lots of major failure modes during ascent involving a multitude of
things, from structural failure to exploding batteries. Cabin pressure loss
is a serious, mission-ending failure, but it doesn't put the crew in
immediate peril the way many other ascent failure modes do. Launch vehicle
system failures tend to be far worse than spacecraft system failures during
ascent.

And, yes, the crews will be wearing full pressure launch & entry suits.
Preliminary work on their design has already started.