William> You do seem very fond of this “...but failures can be
William> catastrophic” line. However, what happens to the rockets
William> after the escape system activates is *irrelevant* to crew
William> safety.
I don't think so. The escape system is just that, a system to try to
escape from a castrophe and *hope* that the crew/passengers/cargo will
survive without too many issues.
There are some small airplanes (Cirrus) with ballistic chutes to be
used in emergencies, and which have saved some lives, but not all
lives since there's generally a minimum altitude you need to pull the
trigger at. And even then, people have been known to fly into the
ground without ever pulling the chute on those planes.
But no commercial plane has this? And that's because making it work
reliably is so hard and costly and eats into payload so much, that
it's more cost effective to build reliable airframes.
I remember how exciting it was for twin engine ETOPs to go from 60
minutes to 90, then 120, then upto the what, 180 minutes with 240
minutes (four hours!) in special cases.
And this is due to the reliability of the engines, which has taken
many years of work and millions of flight hours to prove. And each
engine used for this has gone through quite a number of flight hours
ahead of this as well. So it's both designed and demonstrated
reliability.
William> We spent many thousands of hours during ESAS looking for any
William> solid rocket failure mode that would create a more difficult
William> escape environment than a liquid rocket; that team identified
William> one only which applies only when the solid rocket is in line
William> with and below the crew capsule. Despite much hand waving by
William> an Air Force employee, there were no identified failure modes
William> that created a more difficult escape environment when the
William> solids were side mounted.
So what were the liquid booster failure modes and how difficult were
they to handle?
William> You also seem to enjoy dismissing the very high reliability
William> of smaller solids as unrelated to larger motors. So let’s
William> agree to be illogical and disregard all the data on smaller
William> solid motors. Let’s further disregard logic and dismiss the
William> fact that solid rockets *should* be more reliable than
William> liquids because they are much simpler.
William> Even if we look only at large solid motors, we find—for the
William> Shuttle SRB’s—a demonstrated reliability of 269/270 (99.6%);
William> if we look only at flights since the cause of failure was
William> fixed we are at 220/220 (100%). And then there are the Ariane
William> 5 large solids, with 194 flights and no failures (100%).
I'm not sure I buy these numbers, because didn't they find quite a
number of shuttle solids with advanced wear at the joints? And as
we've seen on the shuttle, just one small jet of hot gas can really
screw up your day.
But more importantly, if there is *any* failure in the solid booster
case, it's game over. With liquids, and enough redundancy (Falcon 9,
Saturn V, Shuttle) it's been shown that a single liquid engine failure
is surviveable and may not even seriously impact the mission.
I've been in a 747 from London to JFK where we lost an engine about an
hour into the flight. No fuss, no muss, we returned to London where
they tried to fix it. This was because the failure of one engine in
the general case is mostly benign and doesn't take out the entire
vehicle.
But even if you had six solids, could you handle the failure of one
without impacting the vehicle drastically?
William> These reliability’s are far higher than the _demonstrated_
William> reliability of any large western liquid rocket stage.
It's the overall system reliabilty that I'm more worried about. And
being able to test the entire system before I go on it seems eminently
sane to me.
William> Engineering should be done using facts, not fears: the
William> Challenger accident was three decades ago, it is time to let
William> it go.
Sure, and it was really a management failure, which is what most
failures are when you get down to it.
John