Here’s what happens when you cut off a rocket’s fuel supply

Spoiler alert.

Sep 08, 2021

Okay, I’ll start with the punchline—here’s the video:

We’ll begin this discussion by stating that a lot of things can cause a rocket to explode. You’re taking a ten-story-tall tube of extremely flammable kerosene and liquid oxygen and lighting the bottom of it. That’s a recipe either for disaster or incredible speed. Nowadays, with all the safety and know-how we’ve put into rockets, it’s usually the latter. Not on September 2nd; and not for the Austin-based aerospace startup Firefly. Firefly Aerospace aims to simplify spaceflight and bring low-cost options to both US and international markets. They are considerably behind SpaceX and some other private rocket providers, but with others’ shortfalls, comes their advantage. As other companies learn and share their stories, companies like Firefly can learn in their wake. But there comes a time in every big endeavor that you must face your maker. In this case, our maker is impossible atmospheric conditions and the unrelenting Gods of high-powered rocketry.

Firefly did just this last week, launching their Alpha rocket out of Vandenberg Air Force Base in southern California. As we’ve seen with most of these tests—SpaceX and Blue Origin included—they can very easily go wrong. But tests are exactly where you want them to go wrong. That’s where you learn your lessons for when it actually matters. That being said, what actually went wrong? And what made it blow up? Those are the questions that we’ll attempt to wildly speculate on today.

I’ll start with the one that I know the answer to: why did it blow up? Rockets blow up for one of two reasons: the flame makes it into the gas tank, or someone hits the self-destruct button. The first case really only happens when there is critical structural damage to seals around the tanks. If those fail, there’s an opportunity for backflow and explosions. Contrary to what you might think, it’s very, very hard for this to happen. The pumps that send fuel into the ignition chamber do so with such immense pressure that the fire is pushed out the back before it has the opportunity to move up the wrong way. Additionally, all of these pumps have “check valves”, which only allow flow in one direction. So even if there was backflow, it wouldn’t make it to the fuel tank. All that said, it therefore must’ve been the latter reason: someone on the ground decided to blow it up. [1]

The rocket lost one of its engines about 15 seconds after it lifted off. Without this engine, the rocket could still continue up. After all, they design it such that it has sufficient thrust even with an engine out. So this is what it did, and continued to do, for over a minute. Unfortunately, once the rocket went supersonic, the aerodynamic forces acting on it were too great for only three engines to keep control. As a result, the rocket started to wobble. This is never a good sign. This wobbling eventually got to the point where the flight computers were not able to control the vehicle. Without any control, the rocket could veer off any which way, potentially putting people in harm’s way. As such, flight control made the call to blow it up, which would enable the rocket’s forward momentum over the ocean to carry the debris offshore and away from any people.

And that’s the story… as it’s been reported thus far. We’re going to take it a step further.

The engine went out because it lost fuel pressure, which just means that the engine stopped receiving the necessary kerosene to ignite. Like we learned before, these pumps operate at extremely high pressures. So high, in fact, that normal pumps won’t do. They need to use turbopumps. These are essentially mini jet engines that accelerate the fuel to insanely high speeds. Because of the large amount of flow that must pass through the accompanying valves, they are under a lot of stress. Physically and emotionally. As a result, during the flight, a valve closed unexpectedly and the engine’s fuel supply was cut off.

There are actually a lot of reasons why this might happen. First, it could just be an issue with the valve. Under the immense pressure and heat, it could have just failed. But I don’t buy that. Firefly likely outsources a lot of its generic parts like this to external vendors. This means these valve designs have been tested and used many times before. So I would venture to guess it was a controls issue.

Rockets these days are very complex (as if they ever weren’t) and by that I mean they have a lot of sensors and code. So with these types of issues, more often than not they come down to a sensor giving a false reading, a bug in the code, or just plain incorrect controls logic. The first one is the most likely culprit (I’d like to think those engineers know coding like the back of their hand). If, say, an accelerometer (acceleration) or barometer (air pressure) gave a false reading, the rocket might have thought it was higher up than it actually was, indicating it was the end of the flight and time to shut down the engines. A flow meter could have indicated significant backflow when there was none. This would result in the valve shutting down for safety reasons. Whatever the ultimate reason, I can almost guarantee that it was a “perfect storm” sequence of events. Aerospace companies, even startups, have a safety-first mentality, and most failures come from an obscure path of impossible to predict anomalies. Or it could be as simple as someone forgetting a semicolon in their code. Boy, I wouldn’t want to be them.

As always this was my take on what could have happened based on the information available to me. If you’re interested, make sure you keep up with Firefly as they look into the flight data to determine the cause of the issue.

Thank you for reading this week’s edition of It’s Not Rocket Science. Have a happy Wednesday and don’t forget to subscribe!

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