Engine Fire Due To Fuel Leak Led To Prototype Rocket Explosion Explains Musk

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After its fourth high-altitude flight test for the Starship next-generation interplanetary launch vehicle system ended in a dramatic mid-air explosion last week, Space Exploration Technologies Corp. (SpaceX) chief executive officer and chief engineer Mr. Elon Musk has provided more details on the reasons behind the unsuccessful landing. Out of the four prototypes that SpaceX has tested so far, SN11, which was tested last Tuesday was the most dramatic, as debris from the vehicle started hitting the ground soon after it attempted to reignite its Raptor engines, with confused onlookers struggling to get a view amidst a dense Texas fog.

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Soon after the test failure, Musk took to Twitter to explain that one of SN11's three engines had shown problems during the vehicle's ascent phase. This led to inadequate pressure in its combustion chamber, but since SN11 exploded as it started its landing burn, the executive was short on details about the reasons for the failure.

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A Starship test consists of three phases during which the vehicle reaches its testing altitude, flips to a horizontal position and reorients itself vertically to reignite its engines for a landing attempt. So far, SpaceX has managed to successfully land only one prototype, with the rest having insufficient thrust or tank pressure to counter gravity and therefore end up exploding soon after touching the ground.

A new tweet by Musk explains that in its latest test, Starship once again managed to successfully execute two of the three aforementioned phases. However, a fuel leak caused one of the three Raptor engines to catch fire, which in turn ended up damaging Starship's electronic control equipment. This resulted in one of the Raptor engine's turbopump starting with a higher than expected pressure.

In his words:

Ascent phase, transition to horizontal & control during free fall were good.

A (relatively) small CH4 leak led to fire on engine 2 & fried part of avionics, causing hard start attempting landing burn in CH4 turbopump.

This is getting fixed 6 ways to Sunday.

In a rocket engine, the turbopump is the component responsible for ensuring that its fuel and oxidizer flow to its combustion chamber with adequate pressure needed to generate sufficient thrust. A "hard start" as described by Musk likely refers to the pressure inside the Raptor's Methane (CH4) turbopump being higher than the profile set by SpaceX for a landing burn. This, in turn, could have forced fuel with a higher pressure inside the combustion chamber resulting in an anomaly that ultimately led to the SN11 prototype disintegrating during flight.

Judging by the tweet, SpaceX seems to have identified the source of the fuel leak behind the accident, which implies that a similar issue should not cause problems for future test flights.

Given that the next test will involve the company's SN15 prototype with a host of upgrades to the vehicle and its engines, the company might get lucky and successfully land and prevent a prototype from exploding later on. While SpaceX had managed to successfully land SN11's predecessor, that vehicle's landing legs did not successfully deploy which caused it to explode soon after it had placed itself firmly on the ground.