25/06/2020 | Story

Strapped in – Seatbelt tensioner and airbags: The automotive industry makes use of technologies from the defence industry

Fractions of a second are key in both segments. Just as the cartridge of a firearm contains a propellant powder that accelerates the projectile, many safety systems in cars have propellants that trigger life-saving devices when ignited.

In the event of a collision, the seatbelt's tightening mechanism works in the same way as the acceleration of a projectile when firing a firearm. The seatbelt tensioner is filled with the same chemical propellant based on nitrocellulose. 

When the propellant charge is triggered, gases expand explosively in up to 15 milliseconds, tightening the seatbelt by about 15 centimetres. Without the propellant charge, the seatbelt tensioner would not be triggered and would not therefore be able to provide the vehicle occupants with the required protection. 

The seatbelt tensioner is thus one of the passive safety systems in a car. Rheinmetall Defence also supplies propellant powder for seatbelt tensioners to customers in the automotive industry.

Propellant charge: A cross-industry product

Propellants are traditionally used extensively in shooting and hunting as well as in the defence industry. Rheinmetall supplies propellant powders for high-performance ammunition as well as for rescue systems. For example, propellant powder is used in ejector seats of fighter aircraft: Pilots in their seat can rescue themselves from a crashing aircraft by igniting the propellant charge.

In addition to belt tensioning, the automotive industry is increasingly using propellants in other areas of the vehicle, such as airbag deployment systems.

Airbag deployment system is connected to the seatbelt tensioner and also uses propellant powder


The airbag is triggered by the propellant charge. When the vehicle brakes sharply, the propellant charge ignites, similar to the ignition of the propellant charge of a projectile or a seatbelt tensioner, and inflates the airbag in a fraction of a second. 

The seatbelt tensioner is also connected to the airbag deployment system, such that information such as the vehicle speed is transmitted via the sensors. In the event of a collision, the airbag deployment system processes such information and decides whether just the seatbelt tensioner is triggered or the airbag too.


At an impact of approx. 60 km/h and an upper body weight of 50 kg, a force of around 23 kN (approx. 2300 kg) acts on the shoulders of the occupants (assuming a braking time of 36 ms).


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