What are these sparks jets shown here: Discovery Launch Captured by Multiple Cameras - YouTube
I’d say they are some sort of external igniters but googling didn’t help. Are they mounted on the launch platform or the shuttle itself?
Also what are the small jets of white smoke coming from the side of the rocket nozzle?
The sparks coming out are attached to the platform, they ignite the fuel when it starts coming out of the rocket. It’s like holding a lighter in front of a bottle of hairspray or spray paint.
I was just watching a related video and they mentioned that one of the fuels they use is liquid hydrogen. One of the reasons they use it is because it’s so cold. Before it ignites, they pump it through around the nozzle to cool/freeze it. That’s probably what you’re seeing coming off the bottom in the video you linked to. Whether it’s actual liquid hydrogen coming out or just freezing water vapor from the environment, I’m not sure.
It’s kinda funny that something so high-tech like a space shuttle would start in such a low tech way.
And a follow up question: How do they restart the engines once they are in orbit and ready to return to earth? Do they have on board igniters? Are they using a different kind of fuel that doesn’t require igniters?
The three big main boost engines were only used for liftoff/climb to minimal LEO. For placement in final/higher orbit and for deorbit burn they used the Orbital Maneuvering System engines, the two smaller ones on both sides of the tailfin above the three mains (not to be confused with the Reaction Control System, which were the maneuvering thrusters), which used hypergolic propellants (non cryogenic and no need for a spark igniter) .
This is incorrect. They are called sparklers and are there to ensure any hydrogen leakage burns off instantly instead of accumulating - which could potentially cause problems.
From here:
Nik from Urbana, IL: Just before ignition there seems to be sparks flying at the perimeter of the nozzles. What are those? Thanks.
Leinbach: Those sparks are called our hydrogen burn-off igniters and they are intended to burn free hydrogen. When we start up the engines, there is a little bit of hydrogen that comes out that hasn’t ignited yet when combined with the oxygen in the system. Also, if we do have an on-pad engine shutdown after we’ve started the engines and have to turn them off for some reason, we shut down fuel rich as well meaning that the last bit of fuel that comes out of the engines will be hydrogen. So, those sparklers, that we like to call them, will burn off free hydrogen in the atmosphere rather than let it ignite on its own as it travels up the side of the ship. That’s a safety consideration. It burns hydrogen before it causes us any trouble.
More detail here.
The Shuttle main engines have internal spark ignitors which are used to start combustion in the gas generator preburners and main combustion chamber. As pointed out, the external sparklers are only to burn off excess hydrogen vented during startup.
Despite having internal spark ignitors, the shuttle main engines are not restarted during flight. Orbital manuevering engines use different fuels and oxidizers which spontaneously ignite on contact and don’t need an ignition system.