Saturn V S-IC Fairing
Though most model kits ignore them, the air scoops are a prominent feature of the fairings on the Saturn V first stage. However, they are not as prominent as they would be. In fact, the air scoops as they finally came to be implemented, are largely non-function and not required.
Most Saturn V drawings and illustrations of the '60's and their reissues and reincarnations over the decades do show a fairly significant assemblage of air scoops both on the periphery of the thrust structure of the S-IC stage and on the fairings. As mentioned earlier, it was discovered that the "base heating" on large vehicles was very significant. This was particularly borne out with the Saturn I where there was a plethora of various scoops on and around the base of the elegantly designed thrust structure that almost seemed afterthoughts (for the most part they were...). The attempts to reduce base heating led to the further "uglying up" of the Saturn I thrust structure by four less-than-glamorous snakishly shaped turbine exhaust ducts exiting right through its surface and trailing well away from and behind the vehicle. When the design of the Saturn I thrust structure became more "industrial" with the Block II Saturn I and the Saturn IB, the air scoop design situation became simplified since there was a uniformly circular structure on which to mount the trough-type air scoops.
But the base heating situation wasn't just a matter of hot exhaust products circulating back into the low pressure area at the base of the vehicle. It was gradually realized that the base heating was also contributed to by the radiant heating in the form of all the intense infrared generating hardware and mechanisms of all that was going on at the base of the Saturns in flight. A less obvious third factor was that of acoustic effects on heating. Sound waves, it was discovered, in all their varying intensities and frequencies at the base of rockets serves to manipulate and concentrate heat in strange and unpredictable ways. The most obvious solution, particularly for a vehicle flying in the atmosphere, is to increase the pressure at the base of the vehicle. The most obvious way to do this is to scoop up air from along side the rocket as the vehicle flies through it, and dump the air into the hot, low pressure area behind the vehicle. The additional air adds mass that soaks up heat. That air then moves away and is displaced by more cool air as the vehicle flies along. And, though still air is not a highly effective barrier against radiant heat, moving air is...well, better than still air. Finally, air can act as an effective acoustic barrier, particularly if it is moving.
Knowing that the massive F-1 engines would be prodigious producers of all kinds of energy, particularly heat and noise, designers provided the Saturn V with ample air scooping devices early in the design and production phase of the vehicle.
As NASA engineers got more experience and data with respect to base heating on flights of the Saturn I vehicles as well as other vehicles which were having their own issues with base heating (most notably Atlas and Thor/Delta) it was realized that the airflow and base heating situation for the Saturn V was very complex. One of the great concerns was that almost the entirety of each F-1 engine would be exposed aft of the first stage thrust structure. Quite reasonably, it was expected that many components of this exposed machinery would be delicate. It was discovered fairly early on, that controlling the complex situation at the base of the Saturn V wasn't solvable by the simple matter of dumping huge volumes of air into that space.
Original S-IC designs called for no fewer than twenty four separate air scoops at the base of the S-IC. Eight would be located on the aft edge of the thrust structure between the fairings and there would be 16 scoops on the fairings...four on each fairing. Two scoops would be located to either side of the fin on each fairing at a level just a couple of inches forward of the fin trailing edge. These scoops project into the airflow and direct air directly into the plumbing and combustion chamber area of the outboard F-1 engines. Two more scoops were located to either side of each fairing at the same level as the aft thrust structure firewall and project several inches into the fairings and out several feet around the periphery of the thrust structure. There is a gap between the firewall level scoops to accommodate the launch pad hold down devices and umbilicals.
Of interest to space historians and modelers alike is the S-IC stage at the U.S. Space and Rocket center in Huntsville, AL. Though it may be difficult to see now that the vehicle is housed indoors, there are panels covering the four air scoop cutouts on either side of the fins on each of the fairings on the vehicle. Observing the fairings from the underside, you can also see how the internal structure accommodates the four scoops that were located on the fairings early in the program. These fairings also have the remnants of the air scoop structures that connected the portion of the scoops that were part of the fairings to the portions of the scoops at firewall level.
Chronologically in the design and R&D phases of the development of the air scoop system, it was the two scoops on either side of the fin on the fairing that were the first to be deleted. This was before the fairings for all of the production S-IC stages were manufactured (the fairings were among the first S-IC components to be manufactured). The deletion of these scoops was the result of flight experience with existing vehicles as well as wind tunnel testing of the S-IC stage.
However, the air scoops between the fairings at heat shield level remained a part of the overall design until late in the design and construction of the S-IC before they, too, were deleted. Two design elements contributed to the deletion of the last of the air scoops. As the Saturn I was developed there was the constant push to develop an effective method of creating a barrier between the hot base of the rocket and the delicate equipment within. On the Saturn I (and Saturn IB) vehicles the bulk of the first stage rocket engines is housed inside the vehicle with only the last portion of the engine nozzles protruding. NASA developed stainless steel honeycomb panels that were coated on their aft face with a 1/4" asbestos-containing "plaster" called H-31 that proved very effective at shielding and insulating against conducted and radiant heat as well as the harsh acoustic environment. The Saturn V inherited larger versions of these shields.
But the Saturn V F-1 engines, unlike the Saturn I H-1 engines, are mounted completely exposed outside of the firewall area which is covered by the honeycomb heat shields. It was determined that the engines would need to be shielded from the harsh environment at the base of the vehicle but the design of the vehicle would not allow the bulk of the F-1 engine to be housed inside the thrust structure as were the Saturn I engines. Instead, a "cocoon" for the F-1 engines was designed that completely enclosed the engines in a protective steel shell. Some sources erroneously report this shell as "foil" insulation because of the shell's apparently delicate wrinkled appearance. The lower nozzle areas do contain insulation beneath the shell but the upper shell is a hot nitrogen purged void.
This shell serves two purposes. First, it is robust enough to protect against the harsh thermal and acoustic environment that is generated by and engulfs the engines. Second, it provides a somewhat sealed environment for maintaining a controlled air-conditioned environment around the engines in the minutes before launch when super-cold LOX is circulated into some of the the engine plumbing. (Air-conditioning may conjure the concept of "cooling" the engines in the hot Florida sun but the opposite is true...the "air conditioning" in the form of heated gaseous nitrogen within the shells actually heats the engines and its components to over 300-degrees F to make sure they don't freeze up and are ready for instant action and spin-up during the ignition sequence in the few seconds that the engines go from cold sleep to searing action at ignition.) Encasing the engines in this multi-function shell reduced even more the need to circulate air at the base of the vehicle and therefore the last remaining scoops were deleted from the design.
But what about the air scoops that were part of the final fairing design that flew on all Saturn V vehicles? By the time it was decided to drop the air scoops from the Saturn V all of the contracted fairings had been manufactured and included the air scoop that is visible on the fairings of the Saturn V at the U.S. Space and Rocket Center. The scoops on the manufactured fairings that were slated for the Saturns V vehicles assigned missions were modified to remove the protruding portions of the air scoops leaving the last vestiges of the air scoops apparent on either side of the fairings. The amount of air that these air scoop remnants actually direct under the base of the vehicle is probably negligible. Close examination of the air scoops on the Saturn V at Johnson Space Center show evidence of the post manufacturing modifications to eliminate their protruding portions.
Attaching the Fairings
The method for mounting the fairings to the S-IC thrust structure is robust and ingenious. In effect, the fairings themselves are mounted so that they stand away from the thrust structure skin by about five inches. This provides clearance over the stringers which are three inches tall as well as room for accessing the mounting bolts and fixtures. But the fairings to not attach to or touch the stringers at any point. The fairings mount to the thrust structure skin via inverted "V" shaped structures that straddle the stringers at 14 points along the edges of the fairing and a large semicircular structural forging at Sta 115.5 of the vehicle which attaches via two mounting clevises on the thrust structure. This structure also serves as the mounting point for the heat shield panels under the fairings as well as the primary load-carrying member to which the main fin spar is mounted. Only the fore and mid fairings attach to the thrust structure. The aft fairing does not mount to the thrust structure at any point but is cantilevered from its its forward circumference by bolting to the semicircular ring forging that also mounts the fin main spar.
Once the fairings are bolted in place, a series of panels are attached to the edges of the fairings over the stringers to aerodynamically seal the gap between the fairing and stringers.
Though the fairings extend over the aft end of the S-IC fuel tank by about 18 inches they do not mount to the tank.
At one point there was consideration given to making the S-IC stage recoverable and reusable. The volume under the fairings was the obvious place for the storage of recovery parachutes and auxiliary floatation devices. There was also one concept to eliminate the retro rockets under the fairings and utilize the fairings as giant airbrakes by swinging them open like flower petals on hinges at the thrust structure firewall. They were also considered as places to mounting devices such as ejectable camera pods for recording the events in the engine area (whether this was an environment that cameras and other devices could survive is another question).
Early S-IC fairing with air scoop
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