Before the engines started but after the Auxiliary Power Units (which provide hydraulic power) were started, the engines were gimbaled to ensure that the thrust vector control system was working properly.
At T minus 4 minutes, the fuel system purge begins. It is followed at
T minus 3 minutes 25 seconds by the beginning of the engine gimbal
SRB ignition can occur only when a manual lock pin from each SRB safe
and arm device has been removed. The ground crew removes the pin
during prelaunch activities. At T minus five minutes, the SRB safe and
arm device is rotated to the arm position. The solid rocket motor
ignition commands are issued when the three SSMEs are at or ...
For the Space Shuttle Main Engine, four checks were done to set the "Ignition Confirmed" condition.
The High Pressure Fuel Turbopump shaft speed was checked against a minimum limit.
The Main Combustion Chamber pressure was checked twice, once against a minimum limit, and once to ensure it was between an upper and lower bound.
The Antiflood Valve was checked ...
I think this has to be evaluated for each propellant combination and situation individually. I was involved in a horizontal ground test program at AeroAstro in the mid 90's for a LOX/RP-1 (kerosene) engine that was designed to be LOX lead to start. After a very short burn was computer aborted due to an accidental trigger of the "high thrust" alarm, the ...
In order to start the engine, you first have to start the gas generator to run the turbo pump. This means getting a flame up past the turbine and igniting the mixture in the gas generator combustion chamber. This seems unlikely.
Assuming the gas generator is somehow running but the main combustion chamber hasn't ignited, the propellant leaving the nozzle ...
How do you confirm ignition in the combustion chamber of a rocket?
Sometimes, you don't. Pressure sensors, flow sensors, and such are yet another device that can fail. Moreover, what if nothing can be done / needs to be done if ignition fails to occur? In the case of the Shuttle (Organic Marble's answer), all three main engines were needed for launch. ...
Boil-off of methane is piped to a Flare Stack and burned off so it won't be vented unburned into the atmosphere.
You can see a good picture of the flarestack on the right in the image of Starhopper fueling below:
(Image credit: NasaSpaceflight.com (bocachicagal)
This difference was tracked by the Space Shuttle Program.
Each flight had a Propellant Inventory prepared for it. Here is an example.
The major prelaunch times tracked are
LOADED (blue arrow)
ENGINE START COMMAND (green arrow)
SRB IGNITION COMMAND (red arrow)
The mass at SRB IGNITION COMMAND is the mass that was important for ascent performance. (Note: ...
Perhaps there's some confusion in the terminology? At least for the Space Shuttle Main Engine, the Augmented Spark Igniters didn't fire for the whole burn.
The igniters turn off after 4.4 seconds while the ignition
flame continues in order to prevent intermittent and possibly
damaging blowback from the main combustion area. This also ...
The design shown in the patent application doesn't look typical of common IC engines; I'll guess it was an idea someone had that may or may not be a real improvement. Engines I am familiar with have no such pre-chamber; the spark fires in the "main" combustion chamber, but I don't think that's relevant to the question.
The NASA device looks like it's ...