BANG!!

On Sunday 25th July, at the ‘Ladies That Launch’ event at Shakespeare County Raceway, ‘Laffin-Gas’ suffered its first start-line incident.

We were there in support of the very first event dedicated to the women who take part in Drag Racing. We were also using the weekend as an opportunity to train our newest team member, John London, who comes from the world of rocketry and was having his first experience of the world of Drag Racing. As training had gone well on Saturday, we decided to go for a two-rocket run on Sunday, which turned out to be a lot hotter than forecast.

There was a very loud bang, and a lot of gas was released from the car. We exploded one of the four main-throttle valves on opening the main throttle. The debris from the exploding valve was caught by the ballistic protection we had built in around the throttle assembly. While the bang was loud and, therefore, very energetic, the damage was limited to the valve itself and the hoses immediately upstream. No damage was caused either to the rocket or the pressure-vessel. The burst-disc protecting the pressure-vessel was not ruptured, indicating that the energy was dissipated further downstream. No-one, (including myself) was at serious physical risk from this event, but it does require an explanation.

It was the same sort of thing that occasionally happens in any big Nitrous burning car, often resulting in bits of carb and manifold being ejected through the air-scoop on the bonnet. In fact it was nothing like that big! When our car is on the line, everything is so quiet- the bang really stands out. In other cars it gets masked by the roar of the engine.

When we returned to the pits, we invited Paul Satchel (Senior Scrutineer and member of the MSA Drag Racing Committee) to inspect the car. Paul was happy that the safety systems had done their job and that we had acted responsibly.

The explosion was a result of something called ‘Compression Shock’, which happened when very warm gas (it was a hot day), which had built up in the very small space between the safety shut-off and the throttle valve, was exposed to back-pressure from the idling rocket just as the throttle was opened. We were firing two rockets. The one with a safety valve that was tightly sealed, did not have this problem. The volume of gas that did explode was less than 20cc. That is a tiny volume for such a big concussion!

We have built chillers into our race transporter to ensure our gasses and liquids are cold when we fuel up. We use insulating blankets to keep things cold after fueling. We pre-pressurise with Nitrogen Gas to ensure that Nitrous in the vessels and lines is always in its liquid state. But Sunday was a hot day, and a very small amount of N2O in a small section of the system, turned to gas, and was not purged (as the rest of the system is) when the start-up valves were opened.

We have now devised a new protocol to ensure that this section of the system is purged immediately prior to launch. We will also employ a cooling method directly on the throttles in future.

Photo courtesy of Missb Fotos