[AR] Re: Testing Scaling Law
- From: Ed LeBouthillier <codemonky@xxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 11 Nov 2016 18:24:06 -0700
On Fri, 2016-11-11 at 20:00 -0500, Norman Yarvin wrote:
In general yes, but in this case I doubt it: if you're wondering
"what size test stand should I build?", then that's what you should
be asking, not what the scaling laws are.
(Of course to that question the immediate answer is "Big enough for
your engine, of course. Are you wondering what size engine you
should
build?")
I'm more interested in estimating the scope of a project through to
completion.
Scaling laws are dull, but they can provide big-picture insight into
how things work. I've been doing a lot of reading about scaling laws
lately and so I've been thinking about lots of things in terms of them.
Then, I've been asking myself how they might be useful in understanding
the scope of rocket efforts.
Here's the gist of what I've been thinking: There have been many, many
rocket projects over the past decade and a half that I've watched and
few of them have made it to completion (some have, though. Hurray!).
But, my question to myself has been whether it has been a lack of
understanding of the scale and scope of the projects or whether it's
just a result of starting projects without knowing where they would end
up. There's nothing wrong with starting a project that can't be
completed if you know up front that you don't have the resources. In
that case, it might be a challenge to see how far you want to get. What
we do for our own amusement is merely subject to our own standards.
But, when we're tasked with doing projects on a professional scale, I
would think that the standards are different. If we're contracted to do
a project of a certain scale, then we should be able to apply methods
to reliably estimate the scope and price of a project so that we can
deliver it as if we intended it to be that way :-)
Historically, I remember a project that I had to estimate. I spent a
lot of time breaking things down to very small details and then
summarized all of the little steps into one final answer. When I looked
at that answer, I knew it was too small. I merely multiplied the result
by 4X and we started the project with that estimate. Suprisingly, that
wild guess was right and I looked like a hero for correctly estimating
the cost and time of a project. But, when you really think about it, it
really came down to some unknown gut feeling of the scope of the
project rather than all of the estimation I did.
Can we do better than merely "gut feeling" estimates? I'm pretty sure
that the answer is "yes." I know that there are cost estimating systems
and I wished I knew how to use them better. But is there an "off the
cuff" way of at least estimating to within less than an order of
magnitude? I guess maybe the answer is "experience." But ways that
would help people better estimate without that experience (or use the
experiences of "the ancients" to guide them) would be better.
It's the opposite of that: even when you have two configurations that
are numerically close, such as 8 inch diameter versus 8.5 inch, the
size increase might be quite expensive. On the other hand, when you
back off and look at the graph of cost versus size from a great
distance, you might be able to fit some sort of curve to the points
without looking too silly.
Thanks...
I guess, even given two different rockets with similar performance
values, but built using different technologies might have very
different scopes.
Other related posts:
