Eli5: why do space rockets have all the cables still connected during the first seconds of lift off?
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Thing is, the rocket launch is a complex and multi-step procedure. Most of the rockets are liquid-fuel — to launch one, you need to pump the fuel inside, like you would do with your car.
And here is the catch — you can’t transport the full rocket from the assembly point to the launchpad — it has to be fueled at the launchpad, hours before launch. And in case of launch abort, de-fueled the same way. So, almost all the time, up to t-00:00:10 — t-00:00:01 the rocket is connected to the ground fuel transfer system (imagine it like a gas pump) via the retractable fuel supply masts. Furthermore — the second compound of rocket fuel, the oxidizer (we are flying to space, no air there), is under excessive pressure — and the fuel transfer system is used to safely drain it almost up to the launch moment.
The third, and final mast connects rocket to ground power supply. For obvious reasons — not to drain rocket batteries and keep a ground signal line — the power cable is also connected to the rocket all the time up to t-00:00:10 — t-00:00:01. The rocket switches to it’s own power supply seconds before ignition.
Also, this is the reason a lot of rocket weapons are solid-fuel. Unlike liquid-fuel rockets they are full with fuel (solid substance, for example gun powder) from the factory, and sometimes all the launch procedure is «switch the rocket on, arm the rocket, aim, press the launch button».
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