It Really Is Rocket Science

To help everyone better understand the science behind the rocketry, we provide the answers to your most pressing rocket science questions.

Have a question? Email it to info@ilslaunch.com. We’ll answer as many as we can and will feature one question per month here on the site.

Current Question

What are Rocket Fairings?

If you have ever seen a rocket verticalized on the launch pad waiting for liftoff, you can’t miss the rocket fairing (also commonly known at the payload fairing or PLF).  At first glance, the PLF seems to be a pretty simple part of the launch vehicle, but looks can be deceiving.  This external protective shell around the payload isn’t just a place for the launch teams to leave their autographs for the mission – it provides environmental protection for the satellite during final pre-launch operations and the initial stages of flight.

Satellites – the payloads for Proton missions – are complex, delicate mechanical and electronic devices. While they have a wide thermal operating range once on orbit, they must remain in a low humidity, particle-free, stable thermal environment throughout processing operations and liftoff. In addition, many of their mechanisms are built for the reduced gravitational environment of space, and cannot even be actuated on earth.

The Proton has two PLF models to accommodate a variety of payload sizes. The standard PLF is just over 15 meters long and the shorter PLF is about 13 meters long. Both are made up of a cylindrical barrel section and a graduated nose section.  These PLFs have radio-transparent windows, cut-outs for the Breeze M to take on propellants and venting holes for pressurization control, as well as access ports for various checks required after SC encapsulation.  The PLF is installed in two halves around the payload and secured by a series of locks that run along the longitudinal seam of the fairing.  

The PLF body is three layers consisting of two carbon fiber shells surrounding a honeycomb filler of aluminum alloy.  Joints and cut-outs are reinforced using polymeric filler, and additional reinforcement is supplied by using adhesive around the external carbon fiber reinforcements near the fittings.  The PLF contains sensitive instrumentation, and is coated outside with thermal insulation and protected from dust and moisture by rubber seals along the seams.  

Once liftoff occurs, pressure in the pneumatic cylinders holding the six PLF vents shut is released.  Spring-loaded covers open over the vents to relieve pressure under the PLF as conditions on the outside of the PLF rapidly change due to the drop in atmospheric pressure. Dropping from a pressure of 1 atmosphere to almost zero only takes the launch vehicle about 120 seconds and occurs when the LV is about 40 km above the ground.  At that point, the vehicle is above the majority of the Earth’s atmosphere, and if you were on the rocket, you would feel almost no pressure.  

When the rocket reaches an altitude of approximately 125 kilometers at about 340 seconds after liftoff, the PLF is jettisoned. The time for PLF jettison is chosen to assure that the free molecular heat flux density (FMHF) radiated onto the satellite does not exceed 1135 W/m², in addition to the PLF halves fall in a pre-determined drop zone.  Pyroactuators activate, causing a chain reaction that pops the longitudinal locks open along the length of the fairing.  Next, two rods at the base of the PLF called “pneumopushers” start pushing the PLF halves away from the adapter to ensure a clear separation. Eventually, the PLF halves reach such an angle that they fall away from the launch vehicle.