Index Home About Blog
Subject: Re: X33-why it's not SSTO
Date: Oct 16 1996
Newsgroups: (Norman Yarvin) wrote:

>The accelerometers, combined with the gyros, tell you your exact
>position, velocity, and orientation.  This is sufficient for guidance.
>The guidance procedure is: if the rocket is oriented wrong, straighten

Agreed. (Except the exact part ;)

Discussing the part below:

>If it is off the precomputed path, add a slight deviation in the
>right direction to bring it back. 

Strangely enough, in the large launch vehicles that I'm familiar with
(Saturn, STS, and studies of ALS and NLS) the procedure in the 
previous sentence is _not_ followed. When in the atmosphere, the 
vehicle follows a precomputed attitude profile. There is no 
precomputed path that is followed. The reason is that if you 
were to try to follow a path in the presence of dispersions, 
you would have a significant probability of exceeding
load constraints. (Mostly around max-dynamic pressure of course, 
as has been discussed.)

That procedure isn't followed exo-atmospheric either. Once outside
the atmosphere, many close to optimal guidance algorithms exist to
calculate a complete new trajectory to the desired orbit. The 
vehicle follows this trajectory by updating it's attitude profiles 
in closed loop fashion. (A minor point to you, but to me... 
my way means I get paid, your way means another discipline 
gets the job ;)

Anyway, I realize that I'm picking at what to most people 
would seem a small point. But this realization might motivate 
why navigation attitude accuracy is important. Given that the 
trajectory isn't corrected closed loop until perhaps 2.5 minutes 
into the flight. (An NLS number.) 

- Matt

From: (Henry Spencer)
Subject: Re: About the trajectory of the shuttle
Date: Wed, 7 Jul 1999 16:13:11 GMT

In article <>,
Doug Goncz  <> wrote:
>Can anyone here confirm that the gravity turn is the most efficient?

In fact, it isn't, but this fact only becomes important on airless bodies.

In an atmosphere, the gravity turn has an overwhelming advantage which
completely dominates the issue:  it has the rocket always facing straight
into the airflow (that is, the "angle of attack" is zero).  This is very
important for practical rockets, because structure mass grows rapidly if
you require that the rocket be able to fly at a non-zero angle of attack.
(Obviously, you have to allow for *some* non-zero angle, since winds etc.
prevent keeping it precisely at zero throughout... but minimizing it is
very important.  Large rockets typically can take only a few degrees.)

On the Saturn V, for example, angle of attack was carefully held down to
nearly zero through the first-stage burn -- roughly the time needed to
leave the atmosphere -- and then rose rapidly to 10-15 degrees as the
smarter guidance technique used for the second stage took over, in vacuum.

>...Also, chaos theory applies, I think--no matter how carefully the
>initial conditions are set, the result will be less than what is desired unless
>course corrections are made.

Don't go overboard on the chaos theory; it isn't the answer to everything.
Chaos theory deals with situations where small initial deviations grow
*enormously* as time goes on.  That is not the case for a rocket ascent;
in general, errors do have to be corrected, but their growth rate is not
huge.  The Saturn V made no attempt to correct errors until after it left
the atmosphere and shed its first stage, and the shuttle takes much the
same approach.  Black Arrow made no attempt to correct errors at all -- it
simply followed a predefined tilt program -- and it functioned perfectly
well as an orbital launcher (although smarter guidance was on the to-do
list, had the program been continued).
The good old days                   |  Henry Spencer
weren't.                            |      (aka

Index Home About Blog