Standing several stories high, a rocket seems to be finely balanced on its launchpad, supported by the launch tower. But looks can be deceiving, as launch providers ensure their vehicles stay securely upright before launch.

An orbital rocket is primarily held upright on a launchpad utilizing hold-down clamps or explosive bolts that secure the vehicle to the launch platform before liftoff. Rockets are designed to stand balanced under their own weight, but they need to stay secured until their thrusters reach full power.

The sheer size of modern launch vehicles often makes smaller details fade into the background. Or the camera needs to zoom out so far to fit most of the rocket into a frame that they are almost impossible to notice.

This is precisely the case with the mechanisms that are the subject of this article. From a distance, a rocket seems to be balancing on its first stage thrusters alone, and in some cases, with not even a launch tower connected to the vehicle.

Although this is partially true, simply making a rocket as balanced as possible to stay upright by itself without any additional safety measures is extremely dangerous for a vehicle of which at least 85% of its mass consists of highly combustible fuel.

As a result, launch facilities have to ensure rockets stay secured on the launchpad before liftoff. One might associate the traditional launch tower with fulfilling this role, but as the following section will illustrate, this is not the primary role of these support structures.

Saturn V
A common misconception exists that it is a rocket’s launch umbilical tower that secures and holds the vehicle upright before launch. This image of a Saturn V rocket next to its service structure illustrates why it is so easy to make this association.

Why A Launch Tower Provides Insufficient Support To Keep A Rocket Secured

A common misconception exists that a rocket is primarily held upright by the launch tower. Although part of its function is to help stabilize the vehicle, as was the case with the Saturn V and with the SLS launch vehicles on the crawler-transporters, it is not its main function.

(During the time the crawler-transporter was used to carry the Space Shuttle to the launchpad, the Mobile Launcher Platform on top of the vehicle did not even have a launch tower, also known as the Launch Umbilical Tower, which stayed at the launch site.)

Space Shuttle Atlantis
A crawler-transporter carries the Space Shuttle Atlantis on top of the mobile launch platform to the launchpad without any visible support structure. It is held in place by eight explosive bolts (frangible nuts), with four placed at the base of each solid rocket booster. The fixed launch umbilical tower can be seen in the background.

It is sometimes more aptly known as the service structure of an orbital launch vehicle while standing on the launchpad, which is exactly what its primary function is. It provides access for groundcrews to service the launch vehicle and prepare it for liftoff.

This includes connecting fuel lines and pressure hoses and making electrical connections (collectively known as umbilicals) to the rocket. The structure is also crucial for performing final inspections and troubleshooting before the vehicle is cleared for launch.

(Apart from solid rocket boosters, all orbital rockets are only fueled once they are on the launchpad several hours before launch. To learn more about the different fuel rockets use and their characteristics, you can read the full article here.)

During crewed missions, the launch tower also houses the crew access arm. This “mobile tunnel” swings out to connect the capsule at the top of the launch vehicle to the support structure, providing access for astronauts/cosmonauts to enter the spacecraft.

What is clear, though, is that a launch or umbilical tower’s primary purpose is not to secure or hold a rocket upright. Its position alongside the vehicle also compromises its ability to provide structurally balanced support.

(If one observes different rocket launch events, some orbital rockets stand seemingly without any kind of support structure on the launchpad, sometimes for up to more than an hour ahead of liftoff.)

Antares Rocket
While also being held down by clamps, an Antares rocket stands on the launchpad with very little visible structural support.

If a launch or umbilical tower is not the primary means of holding a rocket upright on a launchpad, specific mechanisms need to be put in place to ensure rockets stay secured before liftoff, and as the following section will illustrate, this is not only to keep them upright.

How Hold-Down Clamps And Explosive Bolts Keep A Rocket Secured And Upright

What secures & keeps an orbital rocket upright on a launchpad are specially designed hold-down clamps or explosive bolts. This is the way it has been done since the Apollo Program of the 1960s and 70s and is still implemented on modern rockets like the Falcon Heavy.

Saturn V Hold Down Arms
The hold-down clamps (or arms) of the Saturn V rocket. A completed unit can be seen on the left next to an adult male, illustrating the scale of the mechanism. The four clamps can be seen while being installed on the launch platform in the image on the right.

The Saturn V used four hold-down clamps (or hold-down arms), weighing over 18 metric tons each, to hold the vehicle down before & during engine ignition. Modern orbital rockets like the Falcon 9, Delta IV Heavy, and Atlas V still use exactly the same type of mechanism.

The clamps connect to specially designed hardpoints at the bottom of the launch vehicle and securely hold the rocket in place on the launchpad. But these clamps need to be strong enough not only to keep the vehicle secured but also to fulfill a crucial additional function.

A slow-motion video of the Saturn V rocket lifting off shows how the vehicle’s hold-down arms keep it secured until full thrust is registered before releasing. One clamp can be seen to the vehicle’s left, while another is partially hidden behind one of the tail service masts. The other two clamps are obscured by the launch vehicle.

As soon as the countdown reaches T-0 and the rocket’s main thrusters ignite, the hold-down clamps do not release the vehicle until sensors detect that the engines have achieved full thrust and a balanced power output.

As a result, hold-down clamps need to be strong enough and provide enough pressure to hold a rocket in place as well as withstand the full thrust generated by the launch vehicle for a limited period until full power is achieved or an emergency shutdown is necessitated.

For example, each of the four clamps holding the Saturn V rocket in place had to withstand a vertical thrust of 725 800 kg (1.6 million pounds). Fortunately, the sheer mass of the vehicle during launch (2 965 000 kg or 6 537 000 pounds) provided crucial assistance with the load.

SpaceX Falcon 9 Hold Down Clamps
Two of the four hold-down clamps securing a Falcon 9 rocket can be seen above and to the vehicle’s right while it is still in a horizontal position.

In the case of SpaceX’s Falcon 9 rocket, the vehicle is held in place for up to 3 seconds after T-0 while its thrusters reach full power. During hot fire testing, the clamps can keep the vehicle stationary for 8-15 seconds while the engines are firing at full thrust.

(Hot or static fire tests take place when the engine’s ability to perform at its maximum output is tested while the launch vehicle remains stationary.)

Sometimes, explosive bolts are used in place of hold-down clamps. This mechanism was used during the Space Shuttle Program. (More specifically, it was the frangible nuts around the bolts that got separated by an explosive charge along a predetermined break plane.)

This video, shot during one of the Space Shuttle launches, shows the explosive bolts (frangible nuts) around the solid rocket boosters detonating before releasing the vehicle.

An explosive charge inside the nut was detonated by an electrical current. This separated the unit and freed the launch vehicle to lift off once, as is the case with hold-down clamps, full thrust was registered by the computer controlling the engines.

Conclusion

Looks can be deceiving. As seen during the earlier years of the Apollo and Space Shuttle Programs, the launch vehicles looked to be securely attached to their launch towers, without which it seemed they would be unable to stay upright.

But as this article illustrated, the primary function of a launch or umbilical tower is to provide access to and service an orbital rocket before liftoff and not secure it to the launchpad or keep it upright.

Although they are mostly hidden away from view during a launch, they are the hold-down clamps or explosive bolts that secure a launch vehicle to the launchpad and keep the rocket in position until it is ready for liftoff.

This is the way orbital launch vehicles were always secured to the launchpad in the past, and the same principles and mechanisms are used in modern rockets today.

This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.

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