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From: Henry Spencer <>
Subject: Re: Why don't big launchers fall at lift-off time ?? (serious question 
	:-& )
Date: Wed, 31 Jul 1996 19:36:59 GMT

>I cannot imagine that big rockets like Ariane 5/Saturn 5/Shuttle could 
>keep in stable attitude during the very start of the lift-off  without 
>external device while the rocket moves still very slowly.

Most large rockets are unstable even at high speeds.  Their control
systems detect and correct attitude disturbances very quickly, so they
look stable.

>Because its speed is very low, possible ailerons would be inefficient...

Large rockets are controlled and steered by altering the direction of
their engine thrust, usually (although not always) by swivelling the
engines themselves. 

>If nothing is done, the 
>rocket would fall like a simple pencil on my finger.

As others have already pointed out, this analogy is incorrect -- the
pencil is highly unstable because the direction of the force on it does
not change as it starts to fall over, but a rocket's thrust turns with
it.  A rocket with properly-aligned engines has no tendency to fall over
at launch, but no particular tendency to stand up straight either; it is
"neutrally stable".  As it picks up speed, it may gain either stability or
instability, depending on the details of the airflow around it.  Most
large rockets, in fact, become unstable as speed increases.  The control
system typically works hardest in the stratosphere, where the rocket is at
high speed in air that is still fairly thick, and where winds are often
strong.  They go back to being neutrally stable as the air thins out and
aerodynamic forces become insignificant.

Fins on a large rocket are seldom big enough to change this overall picture.
When a large rocket has fins at all, typically they are there to help the
control system a little bit in the worst part of the stratosphere.  In some
cases, like the Saturn V, the fins are no help at all during normal ascent,
but they slow down the loss of control in certain emergency situations, and
give escape systems more time to react.
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |

From: Jordin Kare <>
Subject: Re: Why don't big launchers fall at lift-off time ?? (serious question 
	:-& )
Date: 31 Jul 1996 01:05:22 GMT

In article <DvD3AA.1Lq@info.uucp> James Mantle u,
>: Because its speed is very low, possible ailerons would be inefficient,
>: therefore we have to find another solution. If nothing is done, the rocket
>: would fall like a simple pencil on my finger.
>You're right, if nothing is done, the whole stack will teeter over. But 
>just as you can balance a broom on your finger by dynamically moving your 
>finger around under the broom, the main engines are mounted on gimbals. 
>By changing the angle of the exhaust, ever so slightly, the entire stack 
>can stay balanced on top of the engines.

The "broomstick on your finger" analogy for a rocket at liftoff is
common, but wrong.  The force vector from your fingertip is
always vertical, so that the only equilibrium position is when 
the broomstick is exactly vertical; at any other angle, the 
offset between the broomstick's center of gravity (CG) and 
your finger results in an unbalanced force on the broomstick,
making it fall farther from vertical.  Conversely, if you hold
a broomstick at the top, the vertical broomstick is stable (hanging
straight down) and any displacement from vertical results in
a restoring force.

The force vector from a (properly aligned) rocket engine always 
points through the rocket's center of gravity, so the rocket is 
neutrally stable independent of its attitude, vertical or otherwise.

The "pitching moment" on the rocket that must be corrected by
the gimballing of the engines or some other active means comes
primarily from two sources:
	-- Engine misalignment or thrust imbalance
	-- Wind loads

Of course, if the rocket attitude happens not to be vertical, there's 
a horizontal component to the rocket thrust, and the rocket will
happily accelerate sideways.  Since this acceleration is not 
counteracted by gravity, even a modest horizontal acceleration
can make the rocket with a thrust to weight ratio near 1 
move sideways faster than it moves up, usually with embarrassing
consequences.  Thus the control system must generally keep
the rocket *very* close to vertical during the first few seconds of

The fallacious notion that the exhaust 
supports the rocket like a finger supports
a broomstick is what led some early rocket designers (notably Goddard)
to put the rocket nozzle at the front of the rocket (despite the 
resulting need to push propellant through long pipes and to shield
the propellant tanks from the exhaust.  They were trying to
"hang the broomstick from the top" to get stability.  Didn't work....

Jordin ("Why yes, I *am* a rocket scientist") Kare

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