Launch pad vs in-flight Ignition question

[killium]

When the shuttle is on the launch pad, a couple of second before the ignition we can see a lot of big sparks beeing thrown under the motors. I've been told that those are there to ensure a correct ignition. I have three question about that:<br /><br />Is the proper ignition the right answer to this ?<br /><br />If yes, how does the ignition still reliable once in space (no external sparks over there) ?<br /><br />Isn't there a system (electrodes or ?) that is entirely reliable for ignition inside each motor<br /><br />If no then, how does the ignition works ?<br /><br />thanks in advance <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> </div>

 

[aaron38]

Once in space there is still a big external spark, the white hot exaust exiting the engine. Once it's lit, it stays lit as long as the fuel keeps flowing.<br /><br />Are you asking about restarting the engines? I don't believe the SSME's can be restarted once shut down right? But once in space the ET is gone so there's no fuel to burn anyway.

 

[CalliArcale]

The SSMEs are not designed to be restartable in flight, but there have certainly been similar engines that can be restarted. The Saturn V's second and third stage carried such engines, and the S-IVB restarted its single engine in orbit to boost the vehicle onto the translunar trajectory.<br /><br />However, those sparks you see before ignition are NOT lighting the engines. It's a reasonable mistake to make, but what they're really doing is ensuring safety. Prior to ignition, propellant is allowed to flow through the system, so hydrogen will be leaking out under the engines. Hydrogen is very flammable, and if it accumulates prior to ignition, the results could be messy. So the pyrotechnic flares (PICs, I think they're called) burn off the excess hydrogen before it can accumulate. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

Cool. <img src="/images/icons/wink.gif" /><br /><br />I have a copy of "The Mighty Saturns: Saturn V" that includes fascinating footage from the *inside* of one of the tanks. (I believe it's the kerosene tank from the first stage, but I may be misremembering. I'll have to check.) It's really neat watching the propellant level drop over the course of several minutes. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>