Land and sea mines: Countering sea mines (1949 – )


Countering sea mines (1949 – )

The first versions of contact mines originate back to the 14th century (see: Wikipedia). Contact mines are equipped with ‘antennas’; these are protrusions that provide contact with the detonation of the sea mine. Contact mines are held in place by an anchor (e.g. a heavy block of concrete) below the water surface. If a vessel comes into contact with the mine, it will explode.

During the Second World War, influence-triggered sea mines were used at large scale in the battle for the European seas, both by the Allied and German forces.
The ignition of these sea mines could be activated without direct contact. The main triggers by ships are:

  • the magnetic field,
  • the noise emitted in the water, and
  • the disturbance of the water pressure.

Respectively these types of mines are called: magnetic mines, acoustic mines, and pressure-sensitive mines. Initially, the Navy deployed mine sweepers. Since the 1960s, the Navy replaced them with mine hunters. The counter-mine operations vessels are designed in such a way that they disturb as little as possible the three influencing factors such as a wooden, aluminium, or polyester hull and a motor that makes as little noise as possible. One must be able to sail through a minefield for a sweeping operation without running the risk of triggering a mine.

The control of these influence-triggered sea mines was a completely new problem in which the TNO Physics Laboratory became involved in 1949. Subsequent R&D:

  • In 1951, a photoelectric flux meter in an electronic tube was developed which measured the pressure changes on the seabed due to passing ships. These measurements were recorded.
    Photoflux meter to be placed on the seabed
    Photoflux meter to be placed on the seabed (1951

    This recorder was designed to record the field strengths of closed loop magnetic mine sweeper coils by means of FM modulation. The recording speed is 1.5 cm/s. To accelerate the processing of the data, the playback speed is fifty times as fast (75 cm/s).
    This recorder was designed to record the field strengths of closed-loop magnetic mine sweeper coils using FM modulation. The recording speed is 1.5 cm/s. To accelerate the processing of the data, the playback speed is fifty times as fast (75 cm/s).
  • After an initial attempt to determine the effects of closed magnetic coil sweeping by calculations, the construction of an apparatus started in 1954. This apparatus allowed us to use mine sweeping models at a scale of 1: 100. This measuring installation proved to be very good until the moment the computer appeared. In 1962, a method was designed using a three-electrode sweeping coil system for the shallow water minesweepers of the “Van Straelen” class minesweepers. Around 1966, the use of this design made a significant improvement as compared to previous types of equipment. Due to the dimensioning of the used cables and the electrodes – a patent was obtained – it was possible to achieve that the distribution of power flow over the electrodes remained virtually constant under different environmental conditions. That was of great importance for the magnetic ‘environment’ of the ship and thus to the safety of the minesweeper.
  • Technically interesting, but without practical results, were the attempts to develop an acoustic minesweeping device based on the periodic explosion of gas mixtures.

From the outset, it was clear that environmental conditions can play an important role in the effectiveness of counter-sea mine measures. Much effort was therefore devoted to the development of an effective methodology for researching the sea environment and the development of specifically needed measuring equipment. Over the years, fundamental insight was acquired into underwater acoustics. A considerable experimental experience was gathered. The problem of determining the effectiveness of acoustic minesweeping operations, however, continued to be extremely complex.

The papertape reader (1963)
The magnetic tape reader (1963).

The magnetic tape reader above was developed in 1963 as part of a device for determining the active widths of electrodes to detect magnetic sea mines. Analogue and digital calculation techniques were used. This device was never completed because a universal digital calculator became available much earlier than expected. Exceptional about this papertape reader was that four characters were read simultaneously. A complicated function was recorded on the exchangeable telex tape which described the magnetic field generated by every electrode in certain conditions. Other tapes were to be used for all other environmental conditions. One papertape reader was needed per electrode.

It was much easier when magnetic fields had to be generated. The environmental conditions then played an important role when only electrode-based devices were used. In particular, the electrical properties of the seawater and the seabed soil were important. To be able to measure these influences, the Environmental Measurement Installation System (MEINOPA) was designed around 1968.
In 1980, a new compact version was developed: MEINOPA II

Unlike the acoustic and magnetic fields, the pressure field of a sailing ship could only be efficiently generated with a ship or a similar vessel. The development of countermeasures against pressure mines followed its path. With the aid of large numbers of measurements with ships and scale models, safe sailing regulations could be drawn up, which, according to the insights at that time, were very usable. In the context of international standardisation, this method was abandoned because of the spread in the set of parameters.  The development of statistical methods for the analysis of real counter-mine operations was then required. The already available expertise was linked to that of the Operational Research group. Patterns for minesweeping operations were optimised and the (probable) location of sea mines based on wind and flow patterns was statistically determined.

If one of these methods for sea mine detection was better, the usefulness of countermeasures could be increased. Based upon this idea, the building of the new TNO laboratory in  1968 was an opportunity to create a separate sea mine laboratory building constructed with carefully selected a-magnetic materials. An installation in that building with accurately known disturbances of the earth’s magnetic field had a three-axis system of coils. Since 1971, the characteristics of magnetic sea mine detonators have been investigated in detail using this facility.

Magnetic research facility to research sea mines with so-called Helmholtz coils in six windows (one in the floor). With these coils, a magnetic field of adjustable size and direction can be generated in the center of the space of the a-magnetic laboratory.
Magnetic research facility to research sea mines with so-called Helmholtz coils in six windows (one on the floor) and a mine in position. With these coils, a magnetic field of adjustable size and direction can be generated in the centre of the space of the a-magnetic space.

Identical measuring systems were designed for the investigation of the igniters of pressure mines and acoustic mines.

Pressure simulator for research on pressure-activated sea mines (1975)
Pressure simulator for research on pressure-activated sea mines (1975)
Control panel of the pressure simulator for research on pressure-activated sea mines (1975)
The control panel of the pressure simulator for research on pressure-activated sea mines (1975)

In 1975 the laboratory developed a pressure simulator which allowed research on detonators of pressure-activated sea mines under realistic conditions in a simple way. The simulator offered the possibility to superimpose a wrinkle pressure to an adjustable 5 to 50 meter water column static pressure. The wrinkle pressure was adjustable to a maximum frequency of 10 Hertz. Its amplitude could be varied between 5 and 500 millimetres water column. The device is very complex because a very large ratio between the absolute pressure and wrinkle pressure had to be realised.

The counter-mine research group’s efforts were constantly aimed at debunking the statement: “Minesweeping is a science of vague assumptions, based on debatable figures, tasks from inconclusive experiments, performed with instruments or problematic accuracy, by persons or doubtful reliability.


The remote-controlled ignition system for a mine disposal charge (1980)

When a sea mine is found during sea mine hunting operations, the mine will be exploded using a mine disposal charge. Remote ignition of this charge takes at the minehunter, using the onboard ignition control unit.
The method in the early 1980s was as follows: when the sonar located an object that could be a sea mine, a yellow mini-submarine (Poisson Auto Propulsé (PAP)) equipped with a strong lamp and a television camera would be launched into the sea. The payload under the PAP was a sea mine disposal charge. Using the sonar on board a mine hunter (Alkmaar class), the PAP was brought to the object to be inspected. With the television camera, it was determined whether or not the object was a sea mine. If it was a sea mine, the explosive charge was released next to the sea mine. The PAP, freed from the load, then surfaced and was hoisted on board the mine-hunter.

Poisson Auto Propulsé (PAP) - photo Navy museum Den Helder
Poisson Auto Propulsé (PAP) – photo taken at the Navy museum Den Helder

The explosive charge was equipped with a hydrophone (underwater microphone) with an amplifier and a decoder circuit. Only when a specific code was received, ignition occurred. Each explosive charge was provided with its own ‘security’ code. From the moment the load was freed from the PAP, a clock started in the ignition circuit. That clock ensured that the charge could not be activated within the next 15 minutes. At that timespan, the PAP was taken aboard. After 15 minutes, the charge became hot.
The onboard ignition system was pre-set to the same code as the receiver on the mine disposal charge. The device was then switched on and a button was pressed. The preset code was generated and transmitted via the transducer in the sea. When the received code at the charge matched the code of the charge ignition system, the charge ignited. Due to the force of the explosion, the sea mine also exploded. When the charge could not be detonated, the internal clock shortened the ignition system battery thirty minutes after becoming hot, that is 45 minutes after release. Then the ignition of the charge became impossible.

The onboard ignition system
The onboard ignition system

In 1980, after preliminary research by various TNO research groups, a prototype and a series of seven mine destruction charge systems were built according to military requirements. Each system consisted of:

  • the onboard ignition system
  • a transducer with 30 metres of cable
  • a battery charger, and
  • a box to store the complete system.

The system was later manufactured in a slightly modified form by Rheinmetall as the ‘Remotely Controlled Fuze System (RCFS) DM 1001 for Mine Disposal Charge’. Thereafter, the system was used by various NATO member nations.

The ignition device was made up of the following components:

  • a crystal oscillator,
  • a shift register that operated as a divider,
  • a filter, ad
  • an end amplifier.

Code setting switches were used to set the length of the shift register. The length of this register was the dividend used to divide the crystal frequency. That signal was fed to the amplifier via the filter. The transducer was connected to the final amplifier step via a pot core transformer to obtain an optimal adaptation. The oscillator timing signal was also derived from that timing signal. One second generated frequency one, then a second frequency two was generated and then three seconds in which no frequency could be generated. That was to prevent unwanted effects. The power of the final stage was about 5 W. The supply voltage came from two rechargeable nickel-cadmium batteries, 12 V 0.1 Ah.
With full batteries, about 150 transmissions would be possible before the output level became too low. An LED indicated the remaining output power. The batteries could be charged with the battery charger. It supplied a nominal current of 10 mA at temperatures above 10 °C. When the temperature of the batteries became lower, the charge current was reduced (up to 5 mA at -20 °C). A too-high charge current at a low temperature could damage the batteries.
Requirements for the mechanical properties of the system were high. The system had to be portable, easy to operate, need for low maintenance, and above all watertight because it also had to be used on board rafts. All these requirements were met.