2. Curtain airbag system fundamentals

A curtain airbag (CAB) is a long, pillow-shaped restraint that deploys downward from the roof rail to cover the side glazing and upper side structure. Unlike a driver frontal airbag, which is a compact, high-pressure cushion fired into a fixed steering-wheel module, a CAB is a distributed pressure vessel that must fill a long, curved, and highly variable cavity between the occupant and the side interior. This geometry is the root of nearly every engineering decision in a CAB program: the cushion must be long enough to cover all seating rows, thin enough to package inside the headliner, stiff enough to stay in position during a rollover, and soft enough to limit head and neck loading during a side impact.

Design Drivers

Four drivers dominate CAB design, and they frequently conflict:

• Occupant coverage. The cushion must span the side windows and the B-pillar region for every occupant size and seating position. Coverage is usually defined by an occupant packaging envelope derived from the seating reference point (SRP) and the head center of gravity (CG) of the 5th-percentile female through the 95th-percentile male.
• Deployment timing. For a side impact, the struck-side occupant moves almost immediately; the CAB must reach the gap between the head and the glazing within roughly 10–20 ms of the impact event. This drives inflator output and cushion volume.
• Retention. In a rollover, the cushion must remain inflated for several seconds and resist the occupant's head pushing outward against the glazing. This drives the use of a pressure-holding cushion with tethers and a slow-leak inflator, or a hybrid inflator with a sustained output.
• Out-of-position (OOP) and occupant interaction. A deploying CAB can strike a child or a small occupant seated close to the side structure. The cushion must be tuned so that the deployment force does not exceed injury thresholds, which often means lower initial pressure and a softer, more compliant cushion.

These drivers are not independent. Increasing inflator output to meet timing reduces OOP performance; increasing cushion volume to improve coverage increases the required gas mass and packaging space. The design process is therefore a constrained optimization, and the MADYMO model is the tool that lets engineers explore that trade space before hardware exists.

Packaging Constraints

The CAB is packaged in the roof rail, between the headliner and the body-in-white (BIW) side structure. The available package space is typically a narrow, curved channel roughly 30–60 mm wide and 40–80 mm deep, running from the A-pillar to the C- or D-pillar. Within this channel, the folded cushion, the inflator, the fill tube (if used), and the mounting brackets must all fit.

Key packaging constraints include:

• Fold pattern. The cushion is folded into a compact, elongated bundle. Common patterns are the accordion fold, the roll fold, and hybrid folds. The fold pattern determines the deployment trajectory and the initial unfolding sequence.
• Inflator location. The inflator is usually mounted at the rear of the roof rail (C- or D-pillar) or at the B-pillar. Its position affects fill time and the pressure distribution along the cushion.
• Trim and headliner interaction. The deploying cushion must push past the headliner and any trim pieces without being snagged or redirected. The headliner is often designed with a tear seam or a frangible section to allow controlled deployment.
• Mounting and reaction. The brackets and the roof rail must withstand the reaction force of the deploying cushion and the occupant loading without excessive deflection.

In a MADYMO model, packaging is represented by the initial folded geometry, the material properties of the cushion fabric, and the contact definitions between the cushion and the surrounding trim. The fold pattern is often simplified into a set of initial conditions or a pre-stressed membrane, but for accurate deployment timing, a detailed folded mesh or a validated folding model is preferred.

Inflation Strategies

CAB inflators fall into three main categories:

The inflation strategy is defined by the mass flow rate and gas temperature history delivered to the cushion. In MADYMO, this is typically input as a time-dependent mass flow curve (often called a GASFLOW or INFLATOR definition) and a gas temperature curve. The cushion pressure is then computed from the ideal gas law and the energy balance, accounting for heat transfer to the fabric and leakage through seams and vents.

A simplified example of an inflator input for a side-impact CAB might be:

INFLATOR
  ID = 1
  TYPE = PYRO
  MASS_FLOW = TABLE
    TIME(ms)  MASS_FLOW(g/ms)
    0.0       0.0
    2.0       12.0
    5.0       18.0
    10.0      8.0
    20.0      0.0
  END
  GAS_TEMP = 1000 K
END

This is a teaching example, not a validated dataset. The actual curve is calibrated against tank tests and sled tests. The key point is that the inflator output must be matched to the cushion volume and the venting strategy. A cushion with large vents will require a higher mass flow to maintain pressure; a cushion with small vents will hold pressure longer but may be too stiff for OOP.

Regulatory and Consumer Test Requirements

CAB performance is evaluated against both regulatory and consumer tests. The main regulatory requirements are FMVSS 226 (ejection mitigation) in the United States and UN ECE R135 (side impact) in Europe. FMVSS 226 requires that the CAB prevent the occupant's head from passing through the side window opening during a rollover or side impact; the test uses a linear impactor or a full vehicle rollover. UN ECE R135 specifies a side impact test with a moving deformable barrier.

Consumer tests, such as Euro NCAP and US NCAP, add more severe side impact and pole tests. Euro NCAP's side pole test, for example, drives the vehicle sideways into a rigid pole at 32 km/h, which produces a very localized loading on the CAB. These tests often drive the need for a pole-specific cushion design with additional tethers or a thicker cushion in the head contact area.

System-Level View

The following diagram summarizes the relationships between the design drivers, packaging constraints, inflation strategy, and test requirements. It is intended as a mental map for the modeling work that follows.

The diagram shows that the design drivers and packaging constraints feed into the inflation strategy, which is then evaluated against the test requirements. The MADYMO model sits at the center of this loop: it takes the folded geometry, the inflator output, and the contact definitions, and predicts the cushion deployment and occupant response. The validation feedback from physical tests then refines the model, which in turn informs the next design iteration.

What This Means for Modeling

When you build a CAB model in MADYMO, you are not just creating a geometric representation of a cushion. You are encoding a set of engineering decisions: the coverage envelope, the fold pattern, the inflator mass flow, the venting area, and the contact stiffness of the surrounding trim. Each of these has a direct counterpart in the physical system, and each must be traceable to a test requirement or a design constraint.

A common mistake is to treat the CAB as a simple pressure vessel and ignore the interaction with the headliner and the seat. In reality, the headliner can redirect the deploying cushion, and the seat can limit the occupant's lateral motion. Both effects are significant in side impact and rollover, and both must be represented in the model if the results are to be meaningful.

In the next section, we will move from these fundamentals to the specific MADYMO modeling techniques for CABs, including the folded mesh, the gas flow definition, and the contact setup. The goal is to translate the system-level understanding developed here into a working model that can be used for design and validation.