Observant viewers of orbital rocket launches would have noticed that the exhaust plumes generated by the launch vehicle on the launchpad expand horizontally before billowing up into the air. It’s a result of the flame trench.
A flame trench is a long deep channel situated below the launchpad of an orbital rocket and forms part of the flame trench-deflector system. It is used to deflect the flames generated by the first-stage rocket boosters away from the rocket to protect the vehicle from the heat and energy generated.
The heat and energy generated by the exhaust plumes of a rocket’s engines escaping its nozzles at supersonic speeds during launch are enough to lift a launch vehicle with an average weight of 1063 metric tons (2.34 million pounds) off a launchpad and into Space.
However, the amount of thrust generated by its first-stage boosters, along with the sound it produces during liftoff, is not only capable of damaging the launchpad and service structure but also cripple or even completely destroying the launch vehicle.
To deal with the acoustic energy produced by the rocket’s boosters (generating sounds in excess of 200 decibels), a sound suppression system is used by modern launch facilities.
(Learn more about the sound suppression system used during a rocket launch, how it works, and the role the prominent water towers at a launch site play in supporting this system in this article.)
To deal with the pressure and heat generated by the flames escaping the engine nozzles at supersonic speeds, the flame trench plays a crucial role, as the following section illustrates.
Purpose Of A Flame Trench
As mentioned in the introduction, a flame trench forms part of the flame trench-deflector system. It is used to deflect the flames generated by the first-stage rocket boosters away from the rocket to protect the vehicle from the heat and energy generated.
A Saturn V rocket’s five F1 engines generated 34.5 million newtons or 7.6 million pounds of thrust during launch. In comparison, the 9 Merlin engines of a modern Falcon 9 rocket generate more than 7.6 million newtons or 1.7 million pounds of thrust.
The heat and energy generated by this amount of thrust will severely damage both the launchpad and rocket if it were to hit and be reflected directly off the launchpad onto the vehicle and launch structure.
As a result, the flames from a rocket’s first-stage boosters travel through openings in the launchpad onto a flame deflector situated in the flame trench, which runs underneath the launch structure and extends well beyond the launchpad itself.
The flame deflector redirects the flames away from the launchpad and rocket and flows horizontally along the length of the flame trench before exiting the structure at a safe distance, where it expands into the billowing clouds of smoke one typically observes.
Although they are primarily used at orbital rocket launch facilities, flame trench-deflector systems are also used at rocket testing facilities like Stennis Space Center (SSC), situated in Hancock County, Mississippi, USA.
The flame trenches of all launch facilities across the world have the same purpose and, as a result, have a similar makeup, as the following section will describe.
Make-Up Of A Flame Trench
The large flame trenches used at Launch Complexes 39A and 39B at Kennedy Space Center, Florida, during the Space Shuttle era, are probably one of the best and most visible examples of how a flame trench works and how effective flame trench-deflector systems are.
Using them as an example will help one to better understand the makeup or structure of a typical flame trench-deflector system:
The trench itself is a long deep channel that runs the entire length of the launchpad and extends well beyond the length of the launch area itself, with exit points situated outside the launchpad perimeter and also facing away from them.
The flame trench at Launch Complex 39B at Kennedy Space Center, which was used during the Space Shuttle Program, was 150 meters (490 feet) long, 18 meters (58 feet) wide, and 13 meters (42 feet) deep.
The main flame deflector was situated inside the trench directly underneath the rocket boosters of an orbital launch vehicle on the launchpad. It was approximately 11.6 meters (38 feet) high, 17.5 meters (57 feet) wide, and 22 meters (72 feet) long.
A typical deflector typically has a concave shape to angle and deflect the hot exhaust gases blowing down into the trench, where they run horizontally before exiting the trench at a safe distance from the launchpad and rocket.
However, in the case of the deflector used in the Space Shuttle flame trench, the deflector was in the shape of an inverted V with both sides concave shaped, as both sides of the deflector were used for a shuttle launch.
Since both the space shuttle’s main engines, as well as its solid rocket boosters, ignited at the same during launch, the exhaust flames from the orbiter (shuttle) and solid boosters needed to be deflected into different directions since they were offset from one another.
The orbiter’s exhaust gases were deflected through the southern part of the flame trench, while the solid rocket booster’s exhaust gases were deflected through the northern section of the flame trench.
The flame trench was made from concrete and covered with refractory, heat-resistant bricks, while the flame deflector consisted of a strong steel structure covered with a 127-millimeter (five-inch) layer of high-temperature concrete-like material (known as Fondue Fyre).
(Since the Shuttle Program’s retirement, the flame trench and deflector were modified & refurbished to accommodate the new Space Launch System, whose main engines and solid rocket boosters are positioned inline & don’t need to be deflected into different directions.)
Flame trenches vary in size and shape from one launch facility to another, but their primary makeup remains the same, as they all serve the same purpose – to carry the hot gases from an orbital rocket safely away from the launchpad and vehicle.
The flame trench-deflector system is one of the unsung heroes of a successful orbital rocket launch. Although relatively simple in design, the system has proven its effectiveness over time by safely deflecting the hot gases from a rocket’s boosters away from the launchpad.
As this article illustrated, alongside the sound suppression system, a launch site’s flame trench forms a crucial part of keeping an orbital launch vehicle, launchpad, and support structure safe during a launch event.
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