Regular viewers of rocket launches streamed online by companies like SpaceX and Rocket Lab may have seen two objects falling away from the rocket shortly after second-stage ignition. They are called payload fairings.
A rocket payload fairing forms the nose cone of an orbital launch vehicle and is part of the structural system of a rocket. Its primary purpose is to protect the vehicle’s payload from the air pressure and heat generated by atmospheric drag during the initial stages of its ascent.
A modern orbital rocket accelerates to Mach 1 (the speed of sound) in just under one minute. Approximately 30 Seconds later, it is traveling at more than twice the speed of sound (which equates to approximately 2 447 km/h or 1 522 mph at sea level.)
When a vehicle is traveling at supersonic speeds within our atmosphere, it experiences an immense amount of pressure and heat generated by the drag of the dense air present in the lower part of the Earth’s atmosphere.
To illustrate this point, NASA compares this force with that of what a flat dinner plate would experience at sea level facing 805 km/h or 500 mph winds. That is approximately 1000 pounds per square foot of pressure.
Any unprotected payload the rocket is carrying at the front of the vehicle will be severely damaged, if not completely destroyed, by these forces.
As the upcoming sections will illustrate, this is where a rocket’s payload fairing comes in.
What Is A Rocket Payload Fairing?
As mentioned in the introduction, an orbital rocket’s payload fairing forms the nose cone of an orbital launch vehicle and is part of the structural system of a rocket. Since it protects the payload, some may argue that it also forms part of a rocket’s payload system.
(An orbital rocket consists of 4 primary systems or parts. Learn more about these systems, what they are, as well as their function and characteristics in this article.)
Its primary purpose is to protect the vehicle’s payload from the air pressure and heat generated by atmospheric drag during the initial stages of its ascent. In some cases, it also encloses and protects the rocket’s upper stage (as is the case with the Atlas V rocket).
It also protects the payload against the large amount of acoustic energy released by the rocket’s first-stage boosters during launch. During liftoff, a rocket generates up to 200 decibels of noise, enough to damage the launch structure and the launch vehicle itself.
(Learn more about how loud a rocket can get, the resulting damage it can cause, as well as the measures that are put in place to protect a rocket and launch structure from the acoustic energy in this article.)
Although they come in different shapes and sizes, a typical payload fairing is cylindrical in shape with a cone-shaped nose. This helps to make the vehicle more aerodynamic and reduce the amount of drag (and resulting friction and heat buildup) the rocket experience.
Although many payload fairings are the same diameter as the core stage of a rocket, some larger payloads require fairings with a much larger diameter than that of the launch vehicle itself. This results in the “unbalanced” to “top-heavy” look some orbital rockets display.
Payload fairings are made from strong, lightweight materials to withstand the pressure and heat generated by the atmospheric drag but are also light enough to save mass.
Typically a payload fairing consists of a strong and relatively thick aluminum honeycomb core, with carbon composite materials, which make up the skin, applied to the outside of the structure to further strengthen and stiffen it.
Most payload fairings also make use of acoustic blankets fixed to the inside of the shell to protect the payload from the rocket’s sound energy, while insulators are applied to the outside of the fairing to protect the cargo against excessive heat buildup.
Most payload fairings are of the clamshell variety (clamshell fairings) since they consist of two halves that seem to open and separate in a very similar fashion to that of the two shells surrounding a sea clam.
How A Payload Fairing Works
Some launch vehicle manufacturers, like SpaceX, manufacture their own payload fairings, while others rely on third-party manufacturers. For example, Ariane Space and United Launch Alliance (ULA) make use of specialist aerospace provider RUAG to supply their fairings.
After payload fairings are manufactured, they are typically shipped to the various launch providers’ assembly facilities, along with all other components used by an orbital launch vehicle for a particular launch event.
After the payload is joined with the rocket’s upper stage via a payload adapter in a clean room (an area that is kept free of dust and other possible contaminants), the payload fairing is fixed to the upper stage adapter and enclosed around the payload in the same area.
After assembly, the launch vehicle is rolled out to the launchpad as a complete unit, with all the rocket stages, payload, and fairings already joined and ready for liftoff.
For the initial stage of the rocket’s ascend, the payload fairing stays in place and protects the vehicle payload from the pressure and heat generated by the atmospheric drag, as well as the acoustic energy generated by the first-stage boosters.
However, shortly after the first stage separates from the rest of the vehicle and the second stage rocket booster ignites, the vehicle has already reached the upper atmosphere where there is very little air still present.
As a result, the drag and friction experienced in the lower part of the atmosphere have been reduced to almost zero, and the payload no longer needs to be protected by the payload fairings from external forces.
At this point, at an altitude of approximately 100 km or 62 miles, the payload fairings are separated and safely ejected away from the payload and the rest of the vehicle. This is typically accomplished with the assistance of pyrotechnic devices like frangible bolts.
The vehicle is traveling almost parallel to the Earth’s surface at this stage to gain velocity, and the two halves of a typical clamshell payload fairing are separated by initiating the pyrotechnic-based separation system, pushing them clear of the vehicle.
(In most cases, the payload fairing separates in a horizontal fashion, with two halves falling away to the left and right of the launch vehicle, which ensures that both experience equal external forces and they fall away and react in a similar way.)
Payload fairings are ejected to allow devices onboard the payload system to communicate with ground stations clearly. More importantly, allowing them to fall away saves precious mass, which is the same reason the first-stage boosters are ejected after they are spent.
In the past, payload farings were considered expendable and either burned up in the atmosphere or fell into the ocean after separating from the vehicle. However, with the push towards reusability to save costs, fairings are increasingly being recovered for reuse.
For example, SpaceX uses parachutes on the fairings that deploy after re-entry into the atmosphere and uses a ship outfitted with a large net to catch the payload fairings. The fairings are then refurbished for use on a later orbital launch.
(With a cost of approximately five million dollars per fairing per launch, according to SpaceX CEO Elon Musk, it is hardly surprising that this amount of effort is put into safely retrieving and reusing payload fairings.)
As this article clearly illustrated, payload fairings play a crucial role in any orbital rocket launch and are not merely there to cover the front (and, in some cases, the vehicle’s upper stage.) It plays a crucial role in protecting the expensive and sometimes very fragile payload.
They not only protect the payload from the effects of atmospheric drag experienced in the dense air of the lower atmosphere but also shields it from the amount of acoustic energy produced by the rocket’s first-stage boosters during launch.
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.