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From: (Henry Spencer)
Subject: Re: Apollo: NOVA booster
Date: Tue, 5 Jan 1999 21:03:26 GMT

In article <>,
James A Davis  <> wrote:
>Would you or someone else be good enough to summarize the reasons why
>the Saturn first stages did not have closed loop guidance?  The use of
>fins on the Saturn first stages has always puzzled me.

Careful here.  There's a difference between closed-loop control (which
all the Saturns had at all times) and closed-loop guidance.  Control is
what points the thing in the desired direction; guidance is what tells
it which way to point.

The open-loop part here was that during first-stage burn, the direction
was simply programmed.  There was no attempt to keep the thing on a
specific trajectory, only to keep it pointed in a specific direction which
tilted over as a preprogrammed function of time.  After staging, the
guidance system switched to a closed-loop scheme which tried to zero out
the error in the trajectory, i.e. the difference between the actual
trajectory and the desired one.  This ran until shortly before orbit
insertion, when a third scheme -- essentially a constant pointing
direction, again mostly open-loop -- took over.

So, why were the first and third schemes used?

Well, the first was there because of an overriding issue:  while the
rocket is plowing through thick air at high speed, it *must* *not* try to
fly sideways even slightly.  Particularly at the time of maximum dynamic
pressure, it must point almost exactly "into the wind" at all times, or it
will break up due to excessive aerodynamic loads.  Combining this with
closed-loop guidance is tricky.  Just using a preprogrammed tilt angle,
calculated to keep the nose pointed in the direction of motion, got the
rocket out of the atmosphere adequately, and although winds and other
disturbances did result in trajectory errors, they did not build up to the
point where the second stage couldn't fix it.  So they did things that way
because it was the easiest way to avoid aerodynamic problems.

Some work was done on closed-loop guidance methods smart enough to handle
first-stage flight, but that was aimed at later upgrades (which never

The third scheme was there to solve a different problem.  As the rocket
gets close to orbit insertion, the remaining error in position and
velocity, which the guidance system is trying to zero out, gets quite
small and starts to change size and direction quite rapidly.  Closed-loop
guidance starts to resemble trying to swat a mosquito with a sledgehammer;
it's too easy for small imperfections in guidance or control to start
dominating the problem, resulting in the rocket flailing wildly in an
attempt to cancel out trivial errors.  So right at the end, it's best to
just point the thing in the right direction and hold it there, with the
remaining guidance effort concentrated on timing engine cutoff to give
the lowest remaining error.

>...The Saturn I began
>with no fins in its Block I configuration. Did it have closed loop
>guidance or was it stable despite the lack of fins?

As mentioned above, all the Saturns had closed-loop control, actively
controlling the pointing direction rather than relying on the fins to
stabilize it.  As far as I know, none of them were aerodynamically stable
at any time.  The various sizes of fins on various versions were there not
for full stabilization, but just to reduce the instability a little at the
most crucial time, to make the control system's job a little easier.  (In
fact, it is not clear that the fins on the Saturn V were useful enough to
be worth having, and Boeing at one point suggested deleting them to save
weight.  It might have been done eventually.)
The good old days                   |  Henry Spencer
weren't.                            |      (aka

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