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Encyclopedia Britannica



GRAVIMETRIC

This article appears in Volume V03, Page 279 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: GRA-GUI
GRAVIMETRIC VOLUME
In the heading of the 6-in. range table we find the description of the charge.
Charge:
weight
  13 lb 4 oz.; gravimetric density 55.01/0.504; nature, cordite, size 30.
So that P=13.25, the G. D.=0504, the upper figure 55.01 de-noting the specific volume of the charge measured in cubic inches per lb, filling the chamber in a state of gas, the product of the two numbers 55.01 and 0.504 being 27.73; and the chamber capacity C =13.25X55.01 =730 cub. in., equivalent to 25.8 in. or 2.15 ft. length of bore, now called the equivalent length of the chamber (E.L.C.).
If the shot was not free to move, the closed chamber pressure due to the explosion of the charge at this G.D. (=0.5) would be nearly 49 tons per sq. in., much too
great
  to be safe.
But the shot advances during the combustion of the cordite, and the
chief
  problem in interior
ballistics
  is to adjust the G.D. of the charge to the'
weight
  of the shot so that the advance of the shot during the combustion of the charge should prevent the maximum pressure from exceeding a safe limit, as shown by the maximum ordinate of the pressure curve CPD in fig. 3.
Suppose this limit is fixed at 16 tons per sq. in., corresponding in Table I. to a G.D., 0.2 ; the powder-gas will now occupy a volume b=: ;C=1825 cub. in., corresponding to an advance of the shot X2.15=3.225 ft.
Assuming an
average
  pressure of 8 tons per sq. in., the shot will have acquired energy 8 X 47rd2X 3.225 = 730 foot-tons, and a velocity about v =1020 f/s, so that the time over the 3.225 ft. at an
average
  velocity 510 f/s is about ooo63 sec.
Comparing this time with the experimental value of the time occupied by the cordite in burning, a start is made for a fresh estimate and a closer approximation.
Assuming, however, that the agreement is close enough for
practical
  requirement, the combustion of the cordite may be considered complete at this stage P, and in the subsequent expansion it is assumed that the gas obeys an adiabatic law in which the pressure varies inversely as some mi power of the volume.
The
work
  done in expanding to infinity from p tons per sq. in.,
V
24-22
20
-
22 20
18
IS
rc
p Cl
N N
IC W
0







16 I6
14-
12 4
12
10 10
8
6 ...._ter... ..... ~.._ ...
4

2 2
00 1 2. 8 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200
at volume b cub. in. is then pb/(mI) inch-tons, or to any volume 13 cub. in. is
(9) mpb1 [ (b\m-IJ J J
It is found experimentally that m=1.2 is a good average value to take for cordite; so now supposing the combustion of the charge of the 6-in. is complete in 0.0063 sec., when p=16 tons per sq. in., b=1825 cub. in., and that the gas expands adiabatically up to the muzzle, where
(so) B 216+25'8=3'75,
b 2.5X25.8
we find the
work
  realized by expansion is 2826 foot-tons, sufficient to increase the velocity from 1020 to 2250 f/s at the muzzle.
This muzzle velocity is about 5 % greater than the 2150 f/s of the range table, so on these considerations we may suppose about lo % of work is lost by friction in the bore; this is expressed by saying that the factor of effect is f =0.9.
The experimental determination of the time of burning under the influence of the varying pressure and density, and the size of the grain, is thus of
great
 
practical
  importance, as thereby it is possible to estimate close limits to the maximum pressure that will be reached in the bore of a gun, and to
design
  the chamber so that the G.D. of the charge may be suitable for the weight and acceleration of the shot. Empirical formulas based on practical experience are employed for an approximation to the result.
A great
change
  has come over interior
ballistics
  in
recent
  years, as the old black gunpowder has been abandoned in artillery after holding the field for six hundred years. It is replaced by modern explosives such as those indicated on fig. 4, capable of giving off a very much larger volume of gas at a greater temperature and pressure, more than threefold as seen on fig. 8, so that the charge may be reduced in proportion, and possessing the military advantage of being nearly smokeless. (See Ex-


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