In the fuel tank of the Falcon 9. Image: SpaceX
Following the surprise explosion of Space Exploration Technologies Corp.’s (SpaceX) Starship SN10 prototype, the company’s CEO and chief engineer, Mr. Elon Musk, took to Twitter to provide details about the accident. SpaceX is in the process of testing upper stage prototypes for its next-generation launch vehicle called Starship – which Mr. Musk hopes will fulfill the company’s fundamental goal of making humanity a multi-planetary species.
Starship prototype had a difficult landing due to tank problems due to previous repair after first prototype crash, Musk explains
The prototype SpaceX tested last week was the third vehicle to successfully take off and perform a complex airborne landing flip maneuver. However, it was the first to land in one piece, while the other two were unable to generate enough thrust to counteract gravity.
Its predecessors, SN8 and SN9, crashed for various reasons. While the latter’s crash was due to a programming decision made by SpaceX – a mistake Musk admitted – the former crashed due to insufficient fuel pressure in the vehicle’s secondary tank at the top. Missile tanks must maintain their internal pressure when the fuel comes out or the engine will not be able to generate stable thrust.
A rendering of a fuel head (secondary) tank, one similar to the one that caused SpaceX’s Starship SN8 to crash while trying to land. Image Credit: Elon Musk / Twitter
Before SN9, SpaceX used autogenous pressure to maintain tank pressure. This includes taking exhaust from Starship’s Raptor engines and feeding it back into the tank to make sure that no vacuum is present.
After the crash, however, the company switched to using Helium to pressurize the tanks, similar to the Falcon 9. Musk confirmed this in a Tweet, but warned that this was not yet a long-term solution. An interior view of the Falcon 9 second stage liquid oxygen tank below shows how this is done. At the bottom of the frame are two black composite wrapped pressure vessels (COPVs) that inject helium into the tank to keep the fuel from drifting off the bottom of the tank in the vacuum of space. Also visible are what engineers describe as ‘baffles’ – something we’ll explain below.
Fuel tank under pressure, baffles may have contributed to insufficient fuel flow to the Raptor engine
The decision to switch to Helium is the main reason SN10 did not land properly. Musk confirmed this in a Tweet yesterday when he explained that some of the helium used to pressurize the tank may have found its way into the Raptor engine. This would have reduced the thrust generated during landing, causing SN10 to land at a speed of 10 m / s.
For a vehicle nearly 12 stories high, the force that causes such speed during a collision with the ground is unlikely to be absorbed by the landing legsHowever, the legs themselves may not have had a chance even if the tanks were operating normally as three of them failed to lock into place after deployment during the landing attempt.
The liquid oxygen tank in the Falcon 9 seconds is visible during a Starlink launch in April last year. Image: SpaceX, edited by Wccftech
It looks like some methane is sloshing into the SN10’s secondary fuel tank, causing some of the helium to end up in the system. Almost all missile tanks (and some others) have structures in them called ‘bulkheads’. These counteract the forces that cause the liquid to move violently in the tank and slosh the percentage.
According to MuskWhile they did manage to keep methane from moving within Starship, they may also have acted as a “bridge” for gaseous methane above the fuel’s liquid surface to enter the engine in the form of bubbles.
Screen channels in missile tanks act as lines for fuel to ensure adequate power for re-ignition of the engine – especially after a reverse maneuver has been performed. Images of the Falcon 9’s first stage tanks have shown the presence of such channels, which cooperate with the second stage pusher to propel fuel to the bottom of the first stage tank for re-ignition.
It’s possible Musk is speculating about bubbles making their way into the Raptor, as he recalls a similar problem that SpaceX faced with the Falcon 1.
SpaceX is currently installing propellants on the launch pad in Boca Chica, where the next Starship prototype – SN11 – is currently awaiting its turn to conduct pre-launch tests. Musk hopes to send an unmanned mission to Mars next year and is also targeting large-scale orbital flights in 2022.