Re: Why is the CEV so heavy?
- From: "Murray Anderson" <murraya@xxxxxxxxxxxx>
- Date: Thu, 26 May 2005 19:13:48 -0400
"Ed Kyle" <edkyle99@xxxxxxxxxxx> wrote in message
news:1117140041.017123.293530@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Tom Cuddihy wrote:
> > This seems like a different enough topic to warrant a new thread.
> > George William Herbert pointed out that the ATK-SAIC reliability study
> > apparently had as one of its founding assumptions that CEV mass is
> > going to be somewhere greater than 30,000 lbs, conveniently just out of
> > reach for single core EELVs:
> >
> > http://www.boom.net/~jake/atk_docs/ATK_Thiokol_SRB.J_2S_PRA.pdf
> >
> > <<the footnotes read:>>
> > [1]Current estimates of the CEV mass requirement indicate a mass of
> > 50,000 pounds. This value
> > is in excess of the indicated single core capability for EELVs such as
> > the Atlas V and Delta IV,
> > which have a capacity of approximately 25,000 pounds.
> > [2] SAIC assumed that all worse case accidents, that is, case burst
> > events, would not be
> > survivable.
> >
> > As Damon Hill said, just why is it going to be so darn heavy? When
> > Scott Horowitz talked at NPS, he alluded to thinking in the astronaut
> > office in 2002 that EELV based designs were not going to work, just
> > because of overall system complexity--comments on that?
>
> NASA's CEV spec called for a 20 metric ton
> (44,000 lb) liftoff mass - which would include
> the escape system. This is roughly the mass of
> an earth-orbital Apollo CSM (Apollo 7, Skylab,
> ASTP), so that must have been NASA's starting
> point for CEV. Recently, Griffin has been
> talking about 30 metric tons (66,000 lbs), but
> that seems to be for lunar missions (and would
> also be in-line with Apollo lunar mission CSM
> masses).
>
> BTW, I have a BIG problem accepting the premise
> of this SAIC study on SRB-J2S reliabilty. It
> says that 1 out of 226 SRBs have failed. It
> then goes on to say that based on this history,
> and on some other figuring done by someone sitting
> at a desk somewhere, the SRB-J2S launch vehicle
> would have an expected "loss of vehicle" failure
> rate of only 1/483. No mention of the fact that
> the world's most proven, most reliable launch
> vehicles, the ones that have actually flown more
> than at least one hundred times, have only managed
> no better than about 1/30 to 1/50 over the long
> haul.
>
> (An aside: I have personally witnessed about 20
> space launches during my lifetime. I saw two of
> those blow apart about a minute after liftoff.
> A third reached an improper orbit. I'm sorry,
> but my personal experience prevents me from
> accepting that 1/483 statistical hogwash.)
>
> The study also assumes, BTW, that an SRB CATO
> would be nonsurvivable, but that it would only
> happen 1 out of every 13,858 launches.
>
> How much did SAIC bill for this nonsense?!
>
> - Ed Kyle
>
Recall Richard Feynman's evaluation of SRB reliability in 1986, available
here http://www.fotuva.org/feynman/challenger-appendix.html, for example. He
says
"An estimate of the reliability of solid rockets was made by the range
safety officer, by studying the experience of all previous rocket flights.
Out of a total of nearly 2,900 flights, 121 failed (1 in 25). This includes,
however, what may be called, early errors, rockets flown for the first few
times in which design errors are discovered and fixed. A more reasonable
figure for the mature rockets might be 1 in 50. With special care in the
selection of parts and in inspection, a figure of below 1 in 100 might be
achieved but 1 in 1,000 is probably not attainable with today's technology.
(Since there are two rockets on the Shuttle, these rocket failure rates must
be doubled to get Shuttle failure rates from Solid Rocket Booster failure.)"
Clearly the 88 successful flights since then count for something, but on the
other hand space launch reliability hasn't increased much since then. A
failure rate estimate of 1/200 - 1/500 could be defensible on this basis. I
believe there have been something like 1200 solids launched on the Delta II
and Delta III since Feynman wrote this, with 1 solid failure, one solid
separation failure, and the Delta III control failure, which is at least
partly solid-related. That's about 1/400, and these are smaller solids with
burn time about half the SRB burn time.
The SAIC did an assessment in 1995 of SRB reliability which gave 1/775 for
the reliability of the SRB pair (see page 91 of document we're discussing),
which is essentially the same as their current estimate, with additional
successful flights rolled in.
Murray Anderson
.
- References:
- Why is the CEV so heavy?
- From: Tom Cuddihy
- Re: Why is the CEV so heavy?
- From: Ed Kyle
- Why is the CEV so heavy?
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