[AR] Re: shuttle SRBs (was Re: Re: Phenolic regression rate)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 6 Feb 2018 23:27:25 -0500 (EST)
On Tue, 6 Feb 2018, William Claybaugh wrote:
a lot of "what if X happens?"
questions are answered "we really hope it doesn't"...
This statement can be made about my wife's Prius. Or *any* human built
system.
At *some* point that answer always starts showing up, as I said, but
*when* matters. Your wife's Prius does have unsurvivable "you'd better
hope it doesn't happen" failure modes, but they're very unlikely ones (or
else class-action lawsuits ensue). The biggest problem with solids is
that almost any significant failure is unsurvivable, and failures are
*not* very unlikely.
Neither automobiles nor aviation was made reasonably safe by eliminating
all failures; that would be impossibly expensive, not to mention probably
impractical. Rather, the systems are fault-tolerant, and plausible
failures are generally survivable. The absence of those characteristics
in solids, especially big solids, is what makes them questionable for
reliable rocketry (e.g., manned flights).
Meanwhile, the solid propellant sep. motors on shuttle flew over 2000
times w/o failure. Hell, the SRB's failed once in 270 flights.
The briefest comparison of those two numbers strongly indicates that these
are two different technologies, with different reliabilities, because the
favorable experience with the former clearly does not carry over to the
latter. Even though reliability of the latter got far more attention and
effort. As I mentioned elsewhere in this discussion, small solids whose
mass ratio isn't critical are easily made quite reliable, by overbuilding
them. That isn't generally considered acceptable for main propulsion.
(Estimates of the failure rate of the SRBs are necessarily imprecise and
arguable, given the limited dataset, but they definitely aren't what you
would expect from the exact same technology that produced a none-in-2000+
result.)
Further, solids are twice as reliable as liquids in the data set of all
space launches since 1980.
As above, any single number quoted for "solids in space launches" is
confusing at least two different technologies -- small simple solids and
big complex ones -- and thus is questionable at best. Especially since
the small simple solids dominate the sample, and appear to be rather more
reliable than the big complex ones.
Moreover, reliability, i.e. failure rate, has to be considered together
with the *consequences* of failure. The same 135 shuttle flights that had
one solid-motor failure also had one liquid-engine failure (STS-51F, also
on Challenger). It messed up the flight plan some and spoiled a few of
the experiments -- which usually isn't even mentioned in the official
histories -- because the orbiter ended up in a slightly lower orbit than
planned. Nobody issues press releases mourning the anniversary of that.
Of course, that was just a malfunctioning sensor, not an actual failure
with an engine spilling flame like on 51L... but on Apollo 6, exactly that
happened in the second stage, and despite some other problems on top of
it, the stage managed to limp into orbit and finish its part of the
mission (to the astonishment of its builders). And then there was
SpaceX's spectacular first-stage engine explosion in 2012, after which...
the rocket quietly kept going and made orbit. When there are enough
engines (with tested control systems, unlike KORD) that the thrust loss
isn't too great, liquid failures are often pretty benign, which is not a
word usually applied to solid failures.
If system X has N% probability of usually-catastrophic failures, and
system Y has 2N% probability of usually-benign failures, it's clear to me
which one I'd prefer to ride.
The technical issue with solids is that they give a rough ride due to
"organ pipe" effects which can be hard on crews.
When they work, yes, that's the technical issue.
Henry
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