Much has been made of the problems and expense of developing and building new rounds to meet the USMC NGFS requirements.
A while back, I saw a paper dealing with the De-mil and disposal of various amounts of different explosive materials removed from
older ordnance. The list of explosives included a very large quantity of Explosive-D. It was 1,112 tons (short ton = 2,000 lb). This
was removed from 14,811 rounds. The only ordnance that I know of that would produce this much explosives for so few rounds is
the 16-inch Mark-13 HC round.
This got me to thinking about what could be done with these projectiles.
I thought it might be interesting to see what could be done to extend the range of the Mark-13 HC projectiles and increase their
lethality and thus, avoid the expense of modifying the 16-inch guns or designing and building all new projectiles to meet current
USMC requirements.
What I concluded was this, the recoil system for the 16"/50 Mark 7 gun can handle firing a 1900 lb projectile at 3,553 fps with out
incurring any ill effects. The reason for this is quite simple, A Mark 13 fired at this velocity produces the same recoil impulse as a
Mark 8 fired at 2,500 fps.
This begs the question, how far will a Mark 13 HC projectile fly with this muzzle velocity?
With out making any changes to the projectile, this would give it a range of approximately 67,700 yards or 61,900 meters or 33.4
nautical miles. This doesn't quite reach the minimum threshold for the USMC's near-term requirements but it's a start.
Now the question is, can any easy "fixes" be found that will increase the range of this projectile further with out costing an arm
and a leg. It does little good to make changes to the projectile that would cost more than producing new projectiles.
The Mark 13 is only 64 inches in length. The projectile storage area in the barbette can handle longer projectiles than this.
This brings to mind at least two possibilities for further range extensions.
The easiest method would be to attach a ballistic cap to the nose of the projectile to improve its coefficient of drag or it's "wave"
drag. A 9 caliber radius tangent ogive is approximately 3 calibers in length or 48". The ogive is cut short to allow for a fuse to be
attached at the nose of the projectile. The actual length of the ogive portion of a Mark 13 is 34.9 inches. A 12.5 caliber radius
tangent ogive is approximately 3.5 calibers in length or 56". If we extend the nose with a ballistic cap, such that, at the point
that the ogive is the same the diameter as the fuse well, then projectile length is 72 inches and the radius of the tangent ogive
is 12.5 calibers. This would significantly decrease the projectile drags.
A tangent ogive isn't the least drag form available. A secant ogive of about twice the corresponding tangent ogive in radius has
a much lower drag for the same length of projectile used. Since both tangent and secant ogives have curves purely circular in
nature they are easy to fabricate using any of the standard methods. Neither of these designs are "optimal" but both are easy
to fabricate and the secant ogive is quite good. In fact, using a rough approximation a 25 caliber secant ogive generates about
40% of the wave drag of a 9 caliber radius tangent ogive. Since the Wave drag at these velocities are roughly 50% of total drag,
this would represent about a 30% reduction in total drag. This would seem top be enough to allow this round to achieve the
required 41 nmi near term requirement.
A Von Carmen ogive will out perform both of the other ogive forms if they are all made to the same length in calibers. In this
sense it is ballistically "optimal" and, it has one other very nice feature. It's blunter at its tip than the other two. For unguided
projectile this is irrelevant. If however, a precision guidance kit were to be place ahead of the fuse the extra space would be of
great use. The disadvantage to using a Von Karman Ogive is it is more difficult and therefore more costly to produce.
The other way to increase the range of this round is to reduce its base drag. Like the wave drag, base drag represents about
50% of the projectiles total drag at these velocities. There are two ways to decrease the base drag in common use. They are
boat-tails and base-burn units.
Of the two methods, attaching a base burn unit has the greatest potential for range extension. Range extensions of 25% are
common and higher figures are achievable. The disadvantage to using this method is that it involves using energetic materials.
The new IM regulations require testing be done to ensure compliance. This would add an additional expense that would be better
off spent else where if possible. The addition problem with this method is that only eight inches can be added to the projectiles length.
Adding a boat-tail seems less promising since there is only half a caliber to work with and traditionally boat-tails yield smaller gains
in range, albeit at low cost, than the other methods. Its advantage is, it's cheap to implement.
Regards,
Zen
A while back, I saw a paper dealing with the De-mil and disposal of various amounts of different explosive materials removed from
older ordnance. The list of explosives included a very large quantity of Explosive-D. It was 1,112 tons (short ton = 2,000 lb). This
was removed from 14,811 rounds. The only ordnance that I know of that would produce this much explosives for so few rounds is
the 16-inch Mark-13 HC round.
This got me to thinking about what could be done with these projectiles.
I thought it might be interesting to see what could be done to extend the range of the Mark-13 HC projectiles and increase their
lethality and thus, avoid the expense of modifying the 16-inch guns or designing and building all new projectiles to meet current
USMC requirements.
What I concluded was this, the recoil system for the 16"/50 Mark 7 gun can handle firing a 1900 lb projectile at 3,553 fps with out
incurring any ill effects. The reason for this is quite simple, A Mark 13 fired at this velocity produces the same recoil impulse as a
Mark 8 fired at 2,500 fps.
This begs the question, how far will a Mark 13 HC projectile fly with this muzzle velocity?
With out making any changes to the projectile, this would give it a range of approximately 67,700 yards or 61,900 meters or 33.4
nautical miles. This doesn't quite reach the minimum threshold for the USMC's near-term requirements but it's a start.
Now the question is, can any easy "fixes" be found that will increase the range of this projectile further with out costing an arm
and a leg. It does little good to make changes to the projectile that would cost more than producing new projectiles.
The Mark 13 is only 64 inches in length. The projectile storage area in the barbette can handle longer projectiles than this.
This brings to mind at least two possibilities for further range extensions.
The easiest method would be to attach a ballistic cap to the nose of the projectile to improve its coefficient of drag or it's "wave"
drag. A 9 caliber radius tangent ogive is approximately 3 calibers in length or 48". The ogive is cut short to allow for a fuse to be
attached at the nose of the projectile. The actual length of the ogive portion of a Mark 13 is 34.9 inches. A 12.5 caliber radius
tangent ogive is approximately 3.5 calibers in length or 56". If we extend the nose with a ballistic cap, such that, at the point
that the ogive is the same the diameter as the fuse well, then projectile length is 72 inches and the radius of the tangent ogive
is 12.5 calibers. This would significantly decrease the projectile drags.
A tangent ogive isn't the least drag form available. A secant ogive of about twice the corresponding tangent ogive in radius has
a much lower drag for the same length of projectile used. Since both tangent and secant ogives have curves purely circular in
nature they are easy to fabricate using any of the standard methods. Neither of these designs are "optimal" but both are easy
to fabricate and the secant ogive is quite good. In fact, using a rough approximation a 25 caliber secant ogive generates about
40% of the wave drag of a 9 caliber radius tangent ogive. Since the Wave drag at these velocities are roughly 50% of total drag,
this would represent about a 30% reduction in total drag. This would seem top be enough to allow this round to achieve the
required 41 nmi near term requirement.
A Von Carmen ogive will out perform both of the other ogive forms if they are all made to the same length in calibers. In this
sense it is ballistically "optimal" and, it has one other very nice feature. It's blunter at its tip than the other two. For unguided
projectile this is irrelevant. If however, a precision guidance kit were to be place ahead of the fuse the extra space would be of
great use. The disadvantage to using a Von Karman Ogive is it is more difficult and therefore more costly to produce.
The other way to increase the range of this round is to reduce its base drag. Like the wave drag, base drag represents about
50% of the projectiles total drag at these velocities. There are two ways to decrease the base drag in common use. They are
boat-tails and base-burn units.
Of the two methods, attaching a base burn unit has the greatest potential for range extension. Range extensions of 25% are
common and higher figures are achievable. The disadvantage to using this method is that it involves using energetic materials.
The new IM regulations require testing be done to ensure compliance. This would add an additional expense that would be better
off spent else where if possible. The addition problem with this method is that only eight inches can be added to the projectiles length.
Adding a boat-tail seems less promising since there is only half a caliber to work with and traditionally boat-tails yield smaller gains
in range, albeit at low cost, than the other methods. Its advantage is, it's cheap to implement.
Regards,
Zen
