Why Is There No Talk of Takata’s SSI-20 Side Impact Inflator?

by Kevin Fitzgerald & David Schumann on September 5, 2018

Takata designed and manufactured only one phase stabilized ammonium nitrate (PSAN), side impact inflator, designated SSI-20. There are various applications for this type of inflator, but they are predominately used for thoracic protection in side collisions and are located either in the door panel or buried in the seat. There are millions of SSI-20 airbags on our roads and for some unknown reason, they have escaped the massive recall, most of them the only non-desiccated Takata inflators to do so. This article is meant to awaken NHTSA and the driving public to the hazards associated with this inflator and ask the obvious question, ‘Why is it not being recalled?’ Unfortunately, there is no good answer.

The following excerpt is lifted from a NHTSA Takata investigation report, titled Coordinated Remedy Program Proceeding, document number 11945-102915-v1.

While we have been gathering information in the Coordinated Remedy Proceeding, we have also continued our normal work in our ongoing investigation of the Takata inflators. During that investigation the agency has learned of a new potential safety concern involving an air bag inflator Takata produces that has not been included in any of Takata’s recalls, though some vehicle manufacturers are recalling parts. This additional inflator, called an SSI-20, is primarily used in side air bags and has been added to our investigation.

Since June of this year (2105), both General Motors and Volkswagen have reported SSI-20 rupture incidents to the agency. Currently, General Motors has a recall for 395 model year 2015 vehicles. Those vehicles, all model year 2015, are the Buick Lacrosse, the Cadillac XTS, the Chevrolet Camaro, the Chevrolet Equinox, and the Chevrolet Malibu.

The Volkswagen SSI-20 rupture occurred in a MY15 Tiguan on June 7, 2015. The inflator was produced at Takata’s Frieberg, Germany assembly plant. The GM rupture occurred during a routine lot acceptance test at Takata’s Monclova, Mexico plant, also in 2015. NHTSA decided to group the two catastrophic failures, attributing them to manufacturing errors, and issued a limited recall of only 1100 parts, less than one day’s production spread across the two different assembly plants. Seems light.

During Volkswagen’s root cause investigation, an additional SSI-20 rupture was reported in a lot acceptance test, bringing the total number of failures to three. Since the anomalies all occurred shortly after manufacture, root cause could not be assigned to PSAN degradation in the presence of heat and moisture, although those failures are coming. Another excuse had to be found, and Volkswagen, in cahoots with Takata, pointed to a loss of density in the inflator’s propellant during shipment from Takata’s propellant plant in Moses Lake, Washington to the Freiburg assembly plant. GM jumped on the bandwagon and the limited recall was issued, but this was not root cause. The truth is the SSI-20 is one of the most dangerous inflators Takata produced because on top of its inevitable PSAN aging issue, it possesses a serious design flaw. Buckle up.

SSI-20 Design Description

Figure 1 presents a cross section of the SSI-20 inflator, labeling all the key internal components that will be used to further the discussion.

The Volkswagen and GM versions of the inflator are unique, driven by each automaker’s output requirements and these variations will be discussed in detail later, but let’s first describe how the SSI-20 operates. Upon impact, the vehicle’s electronic control module sends an electrical signal that fires the inflator’s initiator which in turn ignites the 3110 granular booster. The output of the booster spews hot particulate into the 2004 tablet bed and pressurizes the inflator’s internal free volume, providing both the spark and pressure needed to bootstrap combustion. As the 2004 propellant produces gas, it first passes through multiple holes in the propellant sleeve, where it then turns and runs down the annulus between the sleeve and the outer body. When it reaches the baffle, it is turned again and then once more before it exits through the inflator exhaust ports.

A Major Design Flaw

The SSI-20 does not have a filter to trap the particulate produced when its 2004 solid propellant is converted to a gas. Takata’s PSAN is highly efficient, meaning it generates only a fraction of solids upon combustion, about 4%. Since there are only 0.5 moles of propellant to begin with, very little particulate is generated, and it is trapped by the multiple turns the gas takes to exit the inflator, or what is referred to as a ‘torturous path.’ Gas can turn corners, but solids have momentum, and with each turn, they dead-end and plate on the inflator’s interior surfaces. The particulate that does manage to escape is always well within customer specifications. The removal of the filter allowed Takata to reduce the inflator’s diameter to 20 mm, the smallest on the market.

That brings us to the design flaw. The SSI-20’s small diameter may have made for a great sales pitch, but it came with serious consequences that Takata didn’t discover until after the inflator was in production. At such a small a diameter, the output of the initiator and 3110 booster produces a nasty piston effect on the propellant column. This was first discovered after a rupture of a GM SSI-20 during a routine lot acceptance test in Monclova in 2009 that went unreported by Takata and is not included in the three failures discussed above. As part of that failure investigation, inflators produced immediately before and after the anomaly were dissected to look for clues, revealing that up to 30% of the tablets were being cracked during assembly. That sends the initial internal pressure soaring far beyond what the system was designed for. Yes, a manufacturing error, but the investigation uncovered more.

The cracked tablets essentially act as additional booster material, so another experiment, called a snuff test, was conducted to determine the effect of added ignition material on the propellant column. Large holes were drilled in the inflator’s propellant sleeve and body to let the initial gases escape, preventing combustion from bootstrapping. The event quenches. Once deployed, the inflator can be disassembled, and the condition of the propellant inspected. A good snuff test would show the tablets intact, demonstrating a controlled burn surface area and a predictable, consistent output. A bad snuff test would show the tablets in pieces, leading to an uncontrolled combustion process and variable output. The test results were frightening. A baseline evaluation with no cracked tablets was conducted first, and the heat and pressure from the ignition event was so drastic it transformed the tablet column into a single slug that covered most of the propellant sleeve exits holes, as shown in Figure 2. There were no tablets to inspect. With most of the holes blocked in a baseline test, adding cracked tablets only amplified the problem and occasionally led to rupture. There was just no margin for error, something Takata was prone to.

Volkswagen vs. GM Designs

Now let’s look at how this design flaw impacts the Volkswagen and GM variants differently due to their unique configurations. The inflators differ as follows:

1. The Volkswagen SSI-20 uses a 3/16” diameter X 0.090” thick propellant tablet while the GM inflator uses a thinner 3/16” diameter X 0.060” thick tablet.
2. The Volkswagen inflator’s propellant sleeve has a greater flow area for gas to initially escape than the GM design.
3. The ballistic control point, or where combustion is choked, for the Volkswagen SSI-20 is located at the baffle. The GM choke point is at the propellant sleeve, the reason why it has less flow area.
4. The Volkswagen design does not contain desiccant. The GM version uses Zeolite desiccant.

Why do these differences matter? The Volkswagen’s thicker tablets and greater propellant sleeve flow area provide a more robust system to combat the ignition’s piston load, keeping initial pressures under better control. GM’s thinner tablets and smaller flow area succumbs to the ignition forces and if the propellant sleeve blockage is severe enough, the initial pressure can spiral out of control and lead to disaster. Cracked tablets increase this risk. This is not to say the Volkswagen design is immune as evidenced by its two reported ruptures, one of which occurred in the field. In fact, when Monclova reported the investigation results to Germany, they conducted their own lot acceptance test review and found several instances where the propellant sleeve failed, but the outer body didn’t rupture. The more robust system kept the internal pressure for ramping to an outer body breech and the anomalies went undetected. Since the inflator was already in production, Takata just accepted split propellant sleeves as a de facto part of the design. Yes, you read that correctly.

It is important to note the Takata assembly line in Freiberg, Germany included an inspection for the propellant column height prior to the booster cup insertion, so they never had to contend with the aggravation of cracked tablets, but sleeves split nonetheless. After the GM lot acceptance test rupture in 2009, Monclova added the same inspection to their process, begging the question why the two plants were not in sync on such a critical characteristic. Regardless both designs went into production with a serious design flaw that removed margin and was never fixed, GM’s much worse than Volkswagen’s.

Now add PSAN’s aging issue and it can be seen why this is one of the most dangerous inflators Takata produced. Volkswagen’s SSI-20 might be more robust, but without desiccant it will inevitably yield to heat and humidity. GM will argue that with desiccant their design should not be recalled, but desiccant doesn’t resolve its critical design flaw or Takata’s quality deficiencies. The SSI-20 rupture Takata did report to GM in 2015 was six years after Monclova instituted the height check to prevent cracked tablets, so that fixed nothing. It’s just a dangerous design and both variants need to come off our roads.

Finally, Takata struggled to maintain density on the thinner 0.060” thick tablet used in the GM version. It is just a difficult geometry to manufacture and propellant that starts at less than maximum density ages at an accelerated rate. NHTSA should issue a limited recall of both SSI-20 variants, dissect them, inspect their tablet densities and then build the aged tablets into a heavy wall test fixture that can measure the following three pressures simultaneously. The results will mandate immediate action.

1. Booster cavity pressure
2. Pressure inside the propellant sleeve
3. Pressure outside the propellant sleeve

Final Remarks

If there is any doubt to the veracity of these claims, request a copy of the GM SSI-20 failure investigation from Takata as well as the Freiburg studies. The GM report was entered into Takata’s document control system and shared with executive management, but it was buried, just like SSI-20 airbags in your seats. Replacing them will come at an astronomical cost which, in our opinion, is the real reason they are not under recall. Inaction is not an option.