Why are rockets tethered before takeoff

[Sami1999]

Hi there,
Recently i was watching some rocket launches and was wondering why they are tethered before takeoff, after some searching i found out that they are used to keep a constant supply of fuel to replace the part that boils off. But is there any other uses for such tethers?

 

[.Scott]

They also receive electric power from a tether. This ends within minutes of launch. If you listen to the countdown sequence, you will often hear them call out when power is transferred from external power to the spaceships batteries.

Depending on the flight, it could also be used for communications and telemetry.

 

[Sami1999]

Thanks, and do you know the mechanism of disconnecting the tethers?(either the ones for fuels or electricity)

 

[Dullard]

The ground connections (fluid and electrical) are typically made via 'Quick Disconnect' devices. The QDs that I've seen use a simple lanyard which actuates a release mechanism as the swingarm retracts. As you might imagine, some attention is applied to making sure that the lanyard is properly adjusted. It's important to run out of lanyard before you run out of hose.

 

[berkeman]

Recently i was watching some rocket launches and was wondering why they are tethered before takeoff, after some searching i found out that they are used to keep a constant supply of fuel to replace the part that boils off. But is there any other uses for such tethers?

When I first saw your question, I thought you were asking about the Hold Down Arms that are used to mechanically secure the rocket to the pad, and to hold the rocket down during launch until its engines are at full power. Hold Down Arms were invented soon after we started launching larger rockets where the engines take a couple of seconds to reach full power.

https://www.hq.nasa.gov/pao/History/SP-4204/ch13-4.html

https://i.ytimg.com/vi/_itiVWwTIQY/maxresdefault.jpg

 

[Sami1999]

Thanks berkeman but do you know the detach mechanism for the tethers?
like these

 

[berkeman]

Thanks berkeman but do you know the detach mechanism for the tethers?

I can't see your image, but I think @Dullard answered that in Post #4. A Google search on Rocket Quick Disconnects gives lots of good hits. Here is a company that makes them (maybe download their brochure at the bottom of the page):

http://www.cooperindustries.com/content/public/en/wiring_devices/interconnect/products/custom_mil_aero_connectors/umbilical_connectors.html

 

[Nik_2213]

IIRC, there are lightweight magnetic versions in bio-medical use, so connecting surfaces are safely smooth. No sticky-out pins to mash or tethered cable to snarl...

Don't some smart phones now come with 'breakaway' magnetic charger / USB-C cables ?

Industrially, a quick search found Rosenberger MultiMag 15 Range. IIRC, there are other designs. YMMV...

 

[hutchphd]

Hold Down Arms were invented soon after we started launching larger rockets where the engines take a couple of seconds to reach full power.

I've always considered it a mark of Sergei Korolev's genius that the arms for the Soyuz/R-7 booster are "hold up" arms. It is my understanding that they have no active components and are weighted to automatically swing out and away when launch commences...pretty sweet engineering.