[ExI] ai class at stanford

spike spike66 at att.net
Thu Aug 25 20:21:33 UTC 2011

>... On Behalf Of Adrian Tymes
Subject: Re: [ExI] ai class at stanford

On Thu, Aug 25, 2011 at 2:50 AM, Emlyn <emlynoregan at gmail.com> wrote:
>> Spreadsheets are fine for temporary/initial/interim solutions, but 
>> long term, they promote crazy spaghetti code, write-only code. It's 
>> un-analyzable, un-maintainable.

>...Agreed.  Problem is, if you have a decade-plus of code in spreadsheets,
it can be really hard (nearly impossible) to make the case to management
that it's time to spend a lot of not-immediately-productive time porting it
to something better - especially if, as in Spike's case, they are personally
invested in the spreadsheets.

This all brings back pleasant memories from my misspent early career.  We
had a pile of Fortran code which calculates rocket ascent trajectories,
which I ported over to a spreadsheet because I found it easier to set up
iterative studies, and also so I could run it at home.  That Fortran code is
dead now, but the spreadsheet lives on.  

An early example from 1991 is this: assuming away the atmosphere, is the
optimal rocket ascent a gravity turn?

If you know what is a gravity turn, skip to the next paragraph.  A gravity
turn is where you have a rocket set to blast into orbit from the surface of
a planet which rotates west to east as the earth does.  It is tilted east as
a slight angle from vertical, perhaps a milliradian, so that in the first
time increment it rises but also tilts slightly more east, then the next
increment slightly more, such that just as it attains orbit velocity it is
flying horizontal.

Answer: on an airless planet, the gravity turn is the optimal ascent.
Closed form solutions are possible.

OK, now assume an earthlike atmosphere, but perfectly still, no wind.  Is a
gravity turn still optimal?  Answer: no.  
Can you prove it in closed form?  Answer: no, not on my best day in my
fondest dream.
Can you show it with a spreadsheet?  Answer: if I am sufficiently clever,
Spike, are you sufficiently clever, and feeling enormous satisfaction with
this whole discussion?  Answer: definitely.  {8^D

So interested was I in that question, I took the thing home and had my Apple
Mac2 grind on it for several days.  Fun little project.  That actually
predated Excel macros.

I still have that atmosphere model.  Keith, that is the sheet I used to
discover that minor error in your friend Herr Doktor's Neptune paper from
1970, remind me his name?


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