Eli5: why do space rockets have all the cables still connected during the first seconds of lift off?


Eli5: why do space rockets have all the cables still connected during the first seconds of lift off?

In: 3479

In the event of an engine failure or other type of abort at liftoff they still want the cables and hoses attached so that they can safely shut it down and pump out the fuel rendering it safe for people to enter the launch ramp. Thes aborts can happen right at liftoff so the cables needs to be attached during the liftoff and then disconnect right after this.

A funny accident was with the Redstone-Mercury 1 launch where there was actually a fault with the length of the cables and the sequence they were disconnected from the rocket which falsely triggerd an abort when the rocket was only inches off the pad. The rocket did land again without damage. But since the cables were disconnected there was no way to control the rocket from a distance. It was therefore sitting on the launch pad full of fuel pressurized in its tanks and nobody could do anything about it. To make matters worse the capsule, sensing the engine shutdown and the short freefall ran through the rest of the launch and landing sequency including disconnecting from the rocket and deploying its parachute, and senisng a problem with the main parachute it deployed the spare parachute as well. So now there was a space rocket full of fuel on the launch pad and two dangling parachutes from its top that could catch the wind at any time, and the weather forecasts predicted winds.

So, Elon wasn’t the first to land a rocket upright?

I know I sound dumb but what are you referring to?

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».

Because the computer makes sure the engines are all running properly before it will release the hold down clamps. Lots of launches are scrubbed after engine startup but before lift off because the engines arnt running at 100%