Re: Briefing on SRB based CEV at NPS with Scott Horowitz

Tom Cuddihy <tom.cuddihy@xxxxxxxxx> wrote:
>George William Herbert wrote:
>> But the general form of the accident could repeat,
>> if a large chunk of propellant grain breaks off and
>> chokes the gas flow somewhere in the casing.
>> And is always by the nature of the vehicle a risk
>> for any solid (and, any hybrid, though their
>> oxidizer injection nature and transient energy
>> potential in the casing at any time reduce the
>> magnitude of energy release in potential failures
>> significantly).
>
>This statement is objectively true--but it either misses the point or
>intentionally muddies the waters when it comes to overall vehicle
>reliability and safety. It's akin to the reasoning used to ban
>nukes,"solids are dangerous!" like "nuclear power is dangerous!", never
>mind the actual reliability numbers.

I am not trying to muddy the waters; this is not a theoretical
discussion for me, and a number of systems with solid rockets
somewhere involved are potential launchers for the
Venturer Aerospace capsule.

>The fact is that the SRBs so far have a better overall safety record
>than any liquid booster available.

As pointed out subsequently, the statistics on this have barely
enough test cases to actually determine the relative hard
statistical differences between for example Delta II core,
Proton core, and SRB over the last 25 years. To first order,
they're statistically comparable.

>The fact that large solids are
>capable of having explosive failures is uncontested--but also
>completely irrelevant unless considered with the reliability numbers.

No, the failure probabilities of minor/soft, major/hard, and
catastrophic explosive failure of the launch vehicle for each
type need to be separately analyzed.

The impact of, say, an engine shutdown on a Delta-IV first
stage well after Max-Q is not comparable to the impact of
a common mode guidance system failure like the first
Ariane-5 flight where the vehicle ended up flying sideways
to the airflow around Max-Q.

Solids can have catastrophic failures without having an
explosion or motor case failure... a Shuttle SRB that
lost its thrust vectoring would be no better than that
Ariane-5 accident.

>Liquid boosters are prone to explosive failures, albeit by a different
>mechanism.

Hasn't happened to any modern liquid booster. Hitting the pad
after liftoff is a fairly rare failure condition and is a
special case of the general 0/0 abort/escape condition.

>The large number of SRB flights results in reliability
>numbers with a statistically small confidence interval compared to a
>comparable liquid booster, very few of which have launched often enough
>to provide adequate reliability data. Even with large confidence
>intervals for liquids, my hunch is that the SRB would still come out
>ahead.

I disagree, based on more detailed looks at the numbers.

What pains me to no end is that *all* the available modern
vehicles have insufficient flight history to have high
confidence in reliability as demonstrated... Times of
change in the industry have their downside, too.

This is not to slander any of the vehicles I'm looking at;
they are all fine on paper and/or to the extent that they
have flown. But tens and hundreds of successful flights
is a level of confidence that no amount of engineering
and simulation can meet.


-george william herbert
gherbert@xxxxxxxxx

.

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