bthotugigem05 said:
They've had lots of static fires (including in McGregor) but I don't think they've ever done a re-light of the Raptor after a static fire.
My thinking is they'll likely get it to flip back vertical after the belly flop but then have to scuttle into the ocean when the engines don't ignite again.
This is a valid point but I believe it has been discussed/addressed publicly by the company. I think it quite is a bit different with the Raptor vs. the Merlin (the latter is what the F9 uses), especially because the former (on starship) is a full flow engine.
https://www.teslarati.com/spacex-scrubs-final-starhopper-flight-test-elon-musk/Basically, the Raptor uses igniters inside the engine (which are little blowtorches). The Merlin engines use cartridges with chemical igniters.
If/when the cartridges run out - for example, on Mars, if you went there using/depending on a Merlin to get you back...it's not good.
Quote:
Merlin 1D and MVacD both rely on a relatively simple, reliable, cheap, and easy method of chemical ignition, using a duo of pyrophoric materials known as triethylaluminum-triethylborane (TEA-TEB). When mixed, these materials immediately combust, generating an iconic green flash visible during Falcon 9 and Heavy launches, and thus producing the 'spark' needed to start Merlin engines.
Generally speaking, TEA-TEB is an excellent method of igniting rockets, even if it is more of a brute-force, inelegant solution than alternatives. It does, however, bring limitations: every single ignition requires a new 'cartridge' be expended, fundamentally limiting the number of times Merlin 1D (and Merlin Vacuum) engines can be ignited before and after liftoff.
This doesn't even consider the fact that TEA-TEB are extremely complex chemical products that would be next to impossible to produce off of Earth, at least for the indefinite future.
To combat these downsides, SpaceX has designed Raptor with an entirely different method of ignition, known as torch ignition. Technically speaking, Raptor's power, design, and methalox propellant combine to demand more than a relatively common solution, in which spark plugs are used to ignite an engine. Instead, Raptor uses those spark plugs to ignite its ignition sources, what CEO Elon Musk has described as full-up blow torches. Once ignited, those blow torches likely miniature rocket engines using the same methane and oxygen fuel as Raptor then ignite the engine's methane and oxygen preburners before finally igniting those mixed, high-pressure gases in the combustion chamber.
In simple terms, the fact that Raptor is a full-flow staged-combustion (FFSC) engine means that the pressures it must operate under are extreme, verging on unprecedented in large-scale rocketry. Extremely high-pressure gases (on the order of 3,000-10,000+ psi or 200-700+ bar) are just as difficult to reliably ignite, especially if hypergolic solutions (i.e. TEA-TEB) are off the table.
To get an even ignition critical to avoid burn-through, minor explosions, and even catastrophic engine failures Raptor's torch ignition may actually involve a 360-degree ring of spark plug-lit torches around the point of ignition, an undeniably complex solution.
However, as Musk notes, these significant, "finicky" challenges brought on by Raptor's exotic ignition method are motivated by the potential benefits such a solution might bring. Relative to Merlin 1D's TEA-TEB ignition, torch ignition once optimized and matured into a reliable solution will permit an almost unlimited number of Raptor ignitions before, during, and after flight.
Avoiding TEA-TEB and other complex chemical igniters also means that Starship will technically be able to launch to Mars or the Moon, perform injection and landing burns, maybe even hop around the surface, and still be able to return to Earth all without resupply. Such a return voyage would still be predicated on the ability to generate the methane and oxygen propellant needed to fuel Starships, but assuming that challenge can be solved torch-lit Raptors would be ready for such a mission. In the event that, say, something like August 26th's scrub happens to a Starship on Mars, the crew would also be able to get out, inspect Starship's Raptors, and even replace faulty spark plugs if necessary.
