Weight Growth

I found it interesting, though maybe not totally surprising, to read
in a recent AvLeak that the Capsule was something like 3,000 lbs over
weight targets, and that the booster was near it's maximum thrust as
well, which was leading to all sorta of tricks and changes to cut the
capsule weight down.

One was to re-do the entire interstage (sorry, don't have the article
near me to put down the right terms...) thrust structure which holds
the OMS and solar panels and other equipment in an effort to reduce
weight.

To me, it sounded like they were doing a ****-poor job of initial
estimation, and hadn't budgetted for any growth outside their
control.

Which seems ludicrious since the Apollo program had exactly the same
problems and von Braun quietly upped the numbers of the apollo
launchers to address this exact issue.

Is the problem today that they're talking too closely to each other
early on? Or that they're believing each other too much and not
padding the initial numbers enough? Basically, were the capsule
people saying "Yeah, we can do it all with a mass budget of 10,000lbs max"
and the launcher people said "Yeah, we can give you 10,000lbs max" and
they both started AT 10,000 (number grabbed out of thin air)?

Should the launcher people had said "Sure you want 10,000 max, we'll
build for 15,000 since we know you'll go over." Or "Sure you'll get
10,000 no problem. Quickly guys, let's plan for 15,000 but not tell
anyone..."

Or how do you budget properly for weight growth? I read about jet
engines for the A380 and B787 which are supposed to be comming in
around 80,000lbs each, but that GE/Rolls/Pratt have actually run them
at 86,000 already, even though they'll be rated lower.

How hard would it be to just build in a margin and hope to never use
it? Or is the design so damm tight, and the perception of "If we
don't use every last pound of thrust available to launch every last
ounce of mass we can use, we're wasting money?"

I dunno... I just don't have a good feeling about CEV and Ares I at
all.

John

In article ,
John Stoffel wrote:
Which seems ludicrious since the Apollo program had exactly the same
problems and von Braun quietly upped the numbers of the apollo
launchers to address this exact issue.

And even so, it came out pretty marginal -- a lot of sweat got expended on
LM weight reduction just to achieve a system that could do fairly tightly
constrained missions. Apollo margins typically were so tight at launch
that they were technically negative -- it was usually necessary to violate
at least one official flight rule slightly just to achieve lunar orbit.
(They progressively eased up after that, as contingencies that might have
required extra fuel etc. didn't happen and the unused contingency reserves
accumulated.)

Is the problem today that they're talking too closely to each other
early on? Or that they're believing each other too much and not
padding the initial numbers enough?

I think part of the problem is the latter: on Apollo, it worked in NASA's
favor that the Huntsville people had a low opinion of the Houston people
and simply didn't believe pronouncements that "this is positively the last
weight increase".

Also, the NASA centers then had a lot more autonomy. Huntsville in
particular really was an independent empire, which told outsiders as
little as it could and listened to Headquarters only when it felt like it.
So there was more room for a cautious von Braun to quietly slip a large
fudge factor into the payload mass. I suspect that's a lot harder now,
because in the name of being one big happy efficient family, the centers
are being forced to work much more closely with each other.

More subtly, though, von Braun was working with systems that were fully
under his control and were being designed from scratch, so he *could* just
dial up the size as required, within very broad limits. A large part of
the problem here is that NASA drank ATK's "simple safe soon" Kool-Aid, and
bought into a rather marginal launcher concept which made some optimistic
assumptions and didn't have much leeway for trouble.

(Which is not to say that said concept, stupid though it always was,
couldn't have been made to work. But it would have taken just the right
sort of leadership, as opposed to management -- a combination of solid
authority over the entire project, iron-fisted "you WILL make this
approach work within your weight budget" discipline, and the technical
savvy needed to help one subsystem after another find ways around
insoluble-looking weight problems. The real masters of weight discipline,
like Ed Heinemann, considered von Braun rather sloppy about it.)

How hard would it be to just build in a margin and hope to never use
it? Or is the design so damm tight, and the perception of "If we
don't use every last pound of thrust available to launch every last
ounce of mass we can use, we're wasting money?"

There's certainly a large element of that in the traditional spaceflight
culture exemplified by JSC and MSFC.

I dunno... I just don't have a good feeling about CEV and Ares I at all.

Join the club...
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mail to henry at zoo.utoronto.ca instead. |

On May 30, 3:32 pm, John Stoffel wrote:
How hard would it be to just build in a margin and hope to never use
it? Or is the design so damm tight, and the perception of "If we
don't use every last pound of thrust available to launch every last
ounce of mass we can use, we're wasting money?"

I suspect a large part of the problem is that the first stage design
performance is pretty much fixed by the choice of the 'shuttle
derived' SRB. Start trying to improve that and pretty soon no-one will
be able to call it 'shuttle derived' anymore while keeping a straight
face; even today it's debatable.

That means the only parts they can improve are the second stage and
CEV/SM, and the second stage also has some similar constraints (it's
based on shuttle ET tooling, isn't it?).

Mark

wrote in message
ups.com...
On May 30, 3:32 pm, John Stoffel wrote:
How hard would it be to just build in a margin and hope to never use
it? Or is the design so damm tight, and the perception of "If we
don't use every last pound of thrust available to launch every last
ounce of mass we can use, we're wasting money?"

I suspect a large part of the problem is that the first stage design
performance is pretty much fixed by the choice of the 'shuttle
derived' SRB. Start trying to improve that and pretty soon no-one will
be able to call it 'shuttle derived' anymore while keeping a straight
face; even today it's debatable.

That means the only parts they can improve are the second stage and
CEV/SM, and the second stage also has some similar constraints (it's
based on shuttle ET tooling, isn't it?).

They've already "improved" the SRB in the design by adding another SRB
segment to the stack. Also, the thrust curve (thrust versus time plot) for
Ares I will need to be different, which means a slightly different grain
cross section. And then there are the changes needed to put a payload
(second stage) on top versus attaching to the ET on the side. And then
there is the need for a roll control package during SRB firing (the SRB can
vector its thrust, but with one SRB, that doesn't give you roll control).

Is that still "shuttle derived"?

Jeff
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