The M198 Howitzer - Bps

Unit 4:
Projectiles
The M198 Howitzer
Inquiry
Physics
www.inquiryphysics.org
M198 155mm
Howitzer
Used since 1979 by US Army
and Marines; soon to be
replaced by a lighter unit
Photo from Field Artillery Journal, Ft. Sill, Oklahoma
M198 Howitzer: Description
•
•
•
•
15,800 pound gun
155 mm diameter barrel
Cost: $527,000
Various shells (including
nuclear); we’ll look at the
most common one
M198 Howitzer: Firing
M198 Howitzer: Shell
HE M107 shell:
• High-explosive
• Weighs 95 pounds
• 50 m destructive radius
• Gun has max. range of
24,000 m (14.9 miles) and
max. rate of 4/minute with this
shell
M198 Howitzer: Gun Crew
• Gun crew uses firing
tables to set angle and
muzzle velocity
• Angle adjusts from 4°
below horizon to 72°
above
• 11 standard velocities, set
by number of gunpowder
bags and/or type of power
grain; muzzle speed for
this shell ranges from
208 m/s to 684 m/s
(465 mi/h to 1,530 mi/h)
M198 Howitzer: Firing Table
M198 Howitzer: Firing Table
1)
Range in meters
2)
Elevation in mils (6400 mils = 360°)
3)
Fuze setting for a level (graze) burst at standard
range
4)
Changes to decrease the height of the burst
level by 10 meters
5)
How much the range changes for each 1 mil
change in elevation
6)
Amount of elevation change to achieve a
change in range of 4x the probable error
7)
Time of flight in seconds
8)
Drift correction in mils due to shell spin; US
guns have right-hand rifling that makes shells
drift right
9)
Correction for each knot of crosswind (1 knot =
1.15 mi/h)
M198 Howitzer: Firing Table
1) Range in meters
10-19) RANGE CORRECTIONS FOR:
10-11) 1 m/s decrease or increase in
muzzle speed
12-13) 1 knot headwind or tailwind
14-15) 1% decrease or increase in air
temperature from US Standard
Atmosphere
16-17) 1% decrease or increase in air
density from US Standard
Atmosphere
18-19) 1 “square” decrease or increase
in projectile weight
M198 Howitzer:
Theory vs. Reality
In the BHS AP Physics B
course we derive this
formula for range over level
ground:
How well does that formula work with the real
gun, firing a shell at 376 m/s?
1. Formula says max. range
is at sin(2θ)=1, so at 45°
1. Firing table shows max. range
is at 44.4°
2. Formula says max. range
is 14,400 m
2. Firing table shows max.
range is really 9,874 m due
to air resistance, etc.
M198 Howitzer:
Theory vs. Reality
Our “vacuum” range
formula:
The chart shows
range vs. θ for the
formula (dashed
line) versus the
real gun’s firing
table (solid line):
Calculating Firing Tables
Obviously firing tables are
crucial to using such
weapons. Computers
have always been used to
calculate the complex
differential equations.
For centuries, “computers”
were people. Human
computers date back to an
effort in 1757 to calculate
an orbit of Halley’s comet.
70 such “computers”
calculated firing tables for
the U.S. in World War I.
In 1940 the “computer” was
the woman, not the machine
Calculating Firing Tables
During World War II, the
army built ENIAC, the first
general-purpose electronic
digital computer to
compute firing tables.
Occupying 1800 square feet,
ENIAC—the Electrical Numerical
Integrator and Computer—
weighed 30 tons, used
160,000 W of power, and had
17,500 vacuum tubes.
Calculations that took a human
computer 12 hours to perform
were done in 30 seconds by
ENIAC. It was also used for
atomic bomb calculations.
Calculating Firing Tables
ENIAC
Calculating Firing Tables
ENIAC was less powerful than a
modern-day hand-held
calculator.
Computers use many, many
switches to do their work;
ENIAC’s switches were the
17,500 vacuum tubes.
The hot tubes burned out so fast
that it could only operate about
6 hours before needing repairs.
Old vacuum tubes and
resistors did the switching
Calculating Firing Tables
Today, vacuum tubes are
still used in microwave ovens.
Microwave magnetron
(sliced open)
In computers they have been
replaced by transistors. A
modern-day chip has millions to
billions of transistors.
Hundreds of modern transistors
will fit into a single red blood cell.
All source materials are being used under “fair use” provisions of
the Copyright Act for educational purposes.
Primary Source:
Day, Michael A. & Walker, Martin H. (1993, March). Experimenting with the
National Guard: Field Artillery Gunnery. The Physics Teacher (31)3, pp. 136-143.
Inquiry
Physics
www.inquiryphysics.org
This presentation is Creative Commons
licensed for free distribution for
non-commercial use when attributed to
Granger Meador