TNO-PML: Ammunition Lifetime

 

Ammunition Lifetime

 
Ammunition has to function continuously reliably and safely during its operational life after purchase. Checking the quality regularly or randomly and predicting the remaining operational lifetime prevents unsafe or unreliable ammunition from being used in operations. Such testing can yield major savings, prevent the ammunition from suddenly becoming unusable, and often ensure that the ammunition can be used for longer than expected.

In the early 1960s, projectiles from New Guinea were examined to see whether storage in tropical conditions degraded the gun ammunition to such an extent that there could be a danger of premature ignition. The quality of ammunition from the Antilles was also examined to determine whether storage in the tropics caused a significant deterioration.

In the 1960s, research into rocket engines started. The maintenance of rocket engines, however, is more complex than that of gun ammunition.
In 1963, the first check on the usability of 5-inch HVAR rockets took place. Many other missiles followed: 2.75-inch FFAR, Nike Hercules, Sidewinder, Patriot, the Standard Missile, AMRAAM, Stinger Basic and the improved FMS and the EURO Stinger.
During the initial period, various lifespan studies such as 5-inch HVAR, 2.75-inch FFAR, and Nike Hercules were carried out. Because every nation faces the same problems, maintenance programs of large weapon systems have become increasingly internationally organised over the years.  

In 1958, the NATO Maintenance & Supply Agency (NAMSA, now NSPA (NATO Support & Procurement Agency in Luxembourg)) was established, which provides international maintenance of all kinds of systems, including expensive missile systems. The Dutch Armed Forces have many systems under contract with NAMSA. This puts ammunition maintenance in a strong international context.

TNO-PML deployed its expertise internationally, also for NAMSA assignments. The first contract for surveillance of the Sidewinder was signed in 1973. At the end of the seventies, the PML tendered for a very large NAMSA contract for which a new indoor missile test stand was built. The contract, however, was lost. However, NAMSA contracts for the AMRAAM and the Stinger followed.

Longevity research also remains an important theme nationally. Before the fall of the Berlin Wall in 1989, the intended use was mainly focused on the climate of West Germany, with actual use limited to exercises. With the emergence of out-area operations by the Armed Forces, ammunition suddenly has to be used in other areas and not just on paper. This means that not only the actual use increases dramatically but that operations will also have to take place in much more extreme climatic conditions. For example, the limited number of flying hours for the Sidewinder on paper suddenly turns out to be a major limitation, while in reality, the number of flying hours can be much greater.

In 1994, a collaboration was established with the Wehrtechnische Dienststelle 91 (WTD91) in Meppen, Germany to jointly investigate the lifespan of the Patriot warhead. This gave a positive outcome for the lifespan of the Patriot warhead. Longevity research was also being done for the Royal Netherlands Navy on the Standard Missile 1. A problem arose during a static test that is part of the surveillance program. During firing and operation, the complete aluminium acoustic protection is ripped off the wall of the missile test stand for over a length of fifty meters due to the heat, the pressure difference and the gunpowder gases. This led to a costly repair. After that, a Standard Missile 1 was successfully tested. However, it was concluded that the lifespan of the missile had been reached. The Royal Netherlands Navy therefore decided to dispose of the missiles.

Restored missile test stand with steel wall cladding with a Standard Missile 1 in the foreground
Restored missile test stand with steel wall cladding with a Standard Missile 1 in the foreground

Sidewinder can be used much longer

Before the fall of the Berlin Wall, Sidewinders of the Royal Netherlands Air Force were only mounted under aircraft wings during major (international) exercises. Normally, the fighter planes flew with mock-ups to prevent the expensive missiles from being used under extreme climate stress (-60 degrees Celsius high in the air) which limited the operational lifespan of the missiles. During the later peacekeeping operations, the Sidewinders had to be hooked up to the combat aircraft. It turned out that the maximum number of flying hours of a Sidewinder was reached relatively quickly, after which they would be unusable (unreliable) according to the supplier. After research by the PML, it turned out that the number of flying hours of the Sidewinder missiles could be drastically higher so that they did not have to be replaced so quickly. This meant a very large saving for the Dutch Defence.

Aim-9 Sidewinder
Aim-9 Sidewinder

 

Stinger Surveillance

Together with WTD91 and MBDA, various extensive Stinger surveillance programs are being carried out for NAMSA to monitor the quality of the complete missile, so that the system can be deployed safely, reliably and even longer. In the surveillance program, several missiles are taken apart. Critical components are examined and subjected to extensive test programs. For example, TNO investigates the system’s flight and launch engine. The propellant is removed from the rocket engine, after which it is processed into test samples that are chemically and mechanically examined and analysed. In addition to component tests, static and flight tests are also performed to test the functioning of the complete system. All results are analysed and discussed with the various parties. After that, advice for further operational use is issued.

The shoulder-launched Stinger missile system
The shoulder-launched Stinger missile system

 

Technology development regarding the ammunition lifespan

Initially, lifespan research was mainly based on laborious wet-chemical analyses. Samples of explosives, such as gunpowder, are held separately under conditions that mimic actual storage. The samples are then checked for chemical deterioration as well as degradation of mechanical properties. However, this is not yet a prediction. In the second half of the fifties, climatic chambers came into use to accelerate the ageing behaviour of gunpowder by storing it at a higher temperature. With the Arrhenius equation, it is possible to make a certain prediction about the lifespan of ammunition.

However, in addition to temperature, many other factors are important for rocket engines, such as relative humidity and the presence of air (oxygen) that can strongly influence the mechanical properties (e.g. increase in brittleness, which can cause cracking when aging, or debonding can occur). It is also important to functionally test the properties. Where with gunpowder a closed barrel can be used, for rocket propellant it is necessary to go a step further and perform a static thrust test. The thrust of the rocket is measured as a function of the (burning) time.

In the sixties and seventies, a lot of research was done in these fields. In the late 1970s, it became possible to translate all these properties into a missile reliability study and thus a complete surveillance program of a rocket engine can be drawn up, with static test substantiation. Since a rocket motor is the most critical part of a rocket or missile, it is important to be able to carry out such a surveillance program nationally as well.

However, the static testing of a rocket motor requires a large test area that was not available in Rijswijk. It was decided to build an indoor missile test stand at the former Ypenburg airbase, which was put into use in 1981. Inside tests up to the size of an Improved Hawk or Standard Missile could be carried out. The missile test stand provides a strong noise reduction and any unwanted incident during firing remains within the walls of the test stand, which is also a bunker. However, the use of the rocket test stand for static testing of large rocket engines was considerably less intensive than previously envisaged.

Laboratory for ballistics research at Ypenburg
Laboratory for ballistics research at Ypenburg (1981)

Technically, the research has improved even further because calculation programs have also been developed that can calculate the mechanical strength of heavily loaded parts of a rocket engine using finite element methods. Non-destructive methods also provide more insight, whereby the missile does not have to be destroyed. Longevity research received a strong new impulse in the late 1980s, after the fall of the Berlin Wall. This is because the ammunition is no longer safely stored in bunkers in the Netherlands and Germany, but is taken to deployment areas. The ammunition is heavily stressed by shocks and vibrations during handling and transport and then exposed to extreme temperatures. It is now becoming important to be able to monitor the storage conditions of the ammunition over time to estimate the effects of, among other things, high temperature. The development of small sensors in the second half of the nineties made it possible to store all kinds of data, such as relative humidity, temperature and pressure, in ammunition boxes or in storage in data loggers, whereby the actual storage conditions are recorded.

Ongoing research

Major steps have also been taken in recent years in the development of microelectronics, (embedded) sensors, data communication and data processing, which has brought the applicability of these techniques within reach for many applications in the ammunition itself. For example, there are very small embedded sensors that can be used to measure the presence of oxygen or changing mechanical properties in a rocket engine. In principle, this will also make it possible in the future to monitor the quality of individual components and systems. By using these new techniques in combination with specialist knowledge from TNO, it becomes possible to have the current status of an individual system immediately available. Maintenance can therefore be organized more efficiently so that maintenance costs can be reduced.
 

Source

PML Vaarwel (2019) [Dutch]