Sea mine

F-35 shown obsolete on previous posts

   The desirable characteristic of a sea mine is its ability to actually strike a ship.  One way to accomplish this is to make the mine actually hunt the target ship.

   One way of achieving this goal is to build an inflatable mine.  The mine would come folded in a bag, C.  The bag would reduce the loss of plasticizer, chemicals added to the plastic material to keep it flexible,
which can be leached out with exposure to water.  The bag will also help to hide the deflated mine.  For a sand bottom, the bag would have sand glued onto its surface to mimic the bottom; for a mud bottom, the bag would be coated with foam rubber which would look like mud.  The bag would have a bleed hole to force any air out when it descends to the bottom.  When the mine inflates, the bag would rupture and the mine would emerge.
   To inflate the mine, nitrogen under pressure would be stored in tanks.
   If the cone diameter is 1.5 m and the tail is made with a length to diameter ratio of 6, then the volume of the mine would be 1/2 X 4/3 X pi X 0.75 X 0.75 X 0.75 (1/2 of a sphere) for the rounded top + 1/3 X pi X 0.75 X 0.75 X 6 X 1.5 for the cone = 0.88 + 5.3 = 6.2 cubic meters, m3, for a lifting force of 6.2 metric tonnes
X gravity.  At a depth of 1000 m, the pressure is 100 atmospheres or 10 Mega pascals.  Pressure cylinders or balls are built commercially to withstand at least 400 atmospheres.  Allowing 500 atmospheres pressure,
50 000 000 Mega-pascals, there would need to be 1.24 m3 of gas to inflate at 1000 m.  A ball of 1m diameter would have a cross section of 0.78 m2 for a pressure of 50 000 000 X 0.78 = 39 000 000 Newtons.  Aluminum could withstand 300 Mega pascals or more.  The circumference of the ball would be 3.14 m for 39 000 000 / 3.14 = 12 500 000 N/m.  That would require a thickness of 12 500 000 / 300 000 000 = 0.042 m.  The area of the ball would be 4 X pi X 0.5 X 0.5 = 3.14 m2.  X 0.042 = 0.133 m3, or, with aluminum weighing 2.5 tonnes / m3, a weight of 0.333 tonnes.  The volume would be about 0.5 m3, so 3 would be needed to inflate.  3 would be external to the mine for initial inflation, another 1 would be carried inside the mine for additional equalization.  The 3 external would be placed in the bag.
   The mine would be inflated and the bag ruptured and discarded.  The initial trigger would be acoustic and the mine would track acoustically.  Upon inflation, a heat source would raise the internal temperature to the curing point of the plastic, making it hard and the mine rigid.  There is a danger that the heating could damage the electronics, so they would have to be insulated and the interior ball would release nitrogen to coll the electronics and to make up the losses in nitrogen.  The curing temperature would be about 80 C, or 350 K.  The sea water would be about 0 C, so about 1/4 of the gas would expand and be released through a pressure equalizing valve at the back end of the mine.  The nitrogen used to cool the electronics would replace this as the mine cools.  As the mine rises, additional gas would be vented as their is a small limit to the differential pressure form inside to outside that the mine wall could withstand.  The plastic curing would not be perfect, but it would only need to be good enough to allow maneuvering. The inflating balls would be cut loose once inflating has been completed.
    The maximum moment sustainable at the ring of the top curve would be; pi X 0.75 X 0.75 X thickness of skin X maximum allowable force of skin.  Allowing 7 Mega pascals for strength, a fairly low value and a skin thickness of 0.005 m; maximum moment would be 60 000 N-m.  If control surfaces are 8 m from the ring,  a maximum control force of 7 500 N could be obtained.  However, thin walled buckling would greatly reduce this.  If the surface of the mine is not made smooth, but with a series of curved ridges as at D, the point of thin walled buckling would be delayed.  There appears to be enough strength for maneuvering.
   The skin area would be; 2 X pi X 0.75 X 0.75 + pi X 0.75 X 0.75 X 1/2 X 9 = 3.5 + 7.7 = 11.2 m2.  For a plastic density of 1.6 tonnes / m3 and a thickness of 0.005 m = 0.09 tonnes.  Allowing 0.5 tonnes for explosives + 0.33 for internal nitrogen ball + 0.09 = 0.92 or maybe 1 tonne all up.  With 6.2 tonnes of lift there is a net of 5.2 tonnes or the mine would have a thrust to weight ratio of 1 at an angle of about 12% from the horizontal, allowing it to chase down ships.  This is for near surface.  At 6.2 m^3, there would be about 280 standard volumes, each standard volume is 22.4 liters.  The molecular weight of nitrogen is 28, therefore the nitrogen would mass 8 kg for each atmosphere.  At 1000 m, ther would be 100 atmospheres or 800 kg nitrogen mass.
   The terminal guidance would be optical, attacking the mass of the ship.  The conical shape would cause some focusing of the blast shock waves off of the water cone and upwards into the hull of the ship, B.  The cone shape is also highly streamlined and should allow speeds of 50 knots,
   The mine would probably require a minimum depth of water of at least 200 m to function.
   The mine would appear to be debris when lying in the bag, making it difficult to detect.  The nitrogen balls could have plastic outer coverings to disguise their shapes for incresed difficulty in identification.