WW II missile technology

From: J. Forster (jfor@onemain.com)
Date: Sat May 05 2001 - 22:34:36 PDT


This is a note on some of the technological problems faced by the German
designers of the V-1 and V-2 in WW II. In hope you find it interesting from a
technology point of view.

The V-1 was the predecessor of a whole series of cruise missiles, which includes
US missiles like the Snark,Matador, and Tomahawk, if memory serves. These birds
fly essentially as airplanes, and have wings and air breathing engines. Their
guidance can be "open" or "closed" loop. In the open loop systems, the missile
is pre-programmed with a flight direction, altitude, and range, and then is at
the mercy of air currents, etc. They navigate essentially be dead reconing. In
the recent systems, the position of the bird is monitored by such techniques as
inertial navigation, terrain mapping radar, and GPS, and the on board computer
adjusts the flight path to accurately hit the target. These are closed loop,
since the bird is guided to it's target, knowing where the target is, relative
to the vehicle.

But, now look at the problem from the perspective of an engineer in the 1940's.
No GPS, no digital computers, no transistors or integrated circuits. That makes
the problem a LOT harder. So what could you do with the available technology?
Well, we know the Germans were able to build effectively an airplane autopilot
to guide the V-1. Two things were controlled, the direction of flight and the
altitude. Some years ago, I looked at the sectioned V-1 in the Science Museum in
South Kensington in London. As I remember, there were tanks of compressed gas
to operate the gyrocompass which controlled the rudder through a servomechanism
to control the flight direction, and to operate the elevators through another
servomechanism to control flight altitude. I think there was also a small
propeller on the nose of the bird to set the range.When that prop had turned a
programmed number of times, the fuel was shut off to the engine and the rudder
was put hard over to cause the bird to spiral in to impact on the target. From
my relative's description, the time between the engine shutoff (it made a loud
buzz.... hence "buzz bomb") until impact was truely terrifying. That was good
enough technology to deliver bombs to a large city like London, but was
essentially useless against point targets. The only way of correcting for errors
in the system was by knowing where a bomb had landed through observer's reports
and adjust the pre-flight settings. The Brits were able to distort the reports,
as mentioned previously, and thereby walk the bombs off target. My grandfather's
land was hit by quite a number of bombs, guided away from London into the
surrounding rural areas.

The V-2 was a ballistic missile, one that followed an essentially ballistic
trajectory once the boost phase was ended. A ballistic trajectory is like the
path followed by the shell from a cannon. There are very difficult problems to
solve here also. During the boost phase, the missile is essentially balanced on
the engine, at the bottom of the vehicle. This is essentially the same problem
as balancing a pole on your hand. It is unstable. Robert Goddard faced these
problems in his early rocketry experiments by putting the fuel tanks down low,
and the engine up top, so the weight of the vehicle was essentially suspended
from the engine, which is a stable configuration. It is stable, but not very
practical. The Germans eventually solved the problem with a feedback control
system using not vacuum tubes, but manetic amplifiers. These same kind of
stability problems plagued the early US space progeam.

In addition to the stabiloity problem, directional control in the early stages
of flight is hard. As with a cannon, the early stages of the projectile's flight
are crucial, because that's when the aiming is done. With the Atlas ICBM, the
vehicle was actively guided by radio signals throught it powered flight. The
trajectory was measured by ground based radars and correction signals were sent
up to the missile by radio. If you remember the James Bond movie "Dr. No", the
reason 007 went to Crab Key (other than Ursela Andress, of course) was to find
and destroy the equipment that was "toppling" the missiles from Cape Canaveral.
This was actually plausible, since the birds were radio controlled during the
boost phase. Now there are course corrections later in flight, and even during
re-entry, I believe, so accuracy is much improved.

What is truly remarkable, to my mind at least, is how these very difficult
problems were solved using the technology of the day. The engineers were able to
build very impressive, and functional, machines with very little. I think their
achievements should be understood and remembered.

-J



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