Saturn V S-IC Fairing Evolution
Fairing Evolution
The necessity of aerodynamic fairings to "shield" the outboard engines was established very early in the evolution of the very large vehicle that would eventually become the Saturn V. It was certain that the F-1 engines would be monstrous and when clustered in configurations of two or more engines would definitely extend beyond the extremities of any practical airframe that they would be attached to. In the early '60's there were still many unknowns with respect to aerodynamic flow on large vehicles and how much protection the machinery of the engines would need.
It turns out that aerodynamic protection from the airflow was one of the least of problems with respect to protecting the engines. However, the fairings had a very significant impact on the aerodynamic stability of the vehicle. Wind tunnel testing indicated that smaller fairings or the total lack of fairings was very detrimental to the controllability of the vehicle, particularly at rising angles of attack. The acceptable margin of control without the fairings was very much less than when they were added.
It had become evident with early vehicles with relatively large base diameters, such as the Army Jupiter and the Saturn I that base heating would be a much greater factor than most had anticipated. The very low pressure generated as the vehicle passed through the atmosphere caused a great deal of backflow of the heated gasses being exhausted by the engines. Even dumping of the relatively low volume and low temperature turbine exhaust was a significant problem and many early failures on a variety of vehicles was traced back to backflow of heated gasses back up into the machinery and structures at the aft ends of vehicles. Usually, such problems didn't show up in static testing. It turns out that the amount of heat shielding that would be required was greater than most expected and the fact that large diameter vehicles would require very heavy heat shielding structures became a fact of life.
The very earliest design concepts for most Saturns had a softer "sexier" look to them than the vehicles that eventually evolved. This goes directly to the von Braun design teams who still leaned heavily on the V-2 for design hints. By today's standards...and the prevalent standards of the '50s...the V-2 was far more streamlined than it needed to be, bearing heavy influences of of aircraft design aesthetics rather than the realities of what was required. This design sense found its way into the von Braun-team design influences on the famous spacecraft of the Collier's magazine series and the Disney television shows where the "next generation rocketships" where showcased. This design influence permeated right into the Block I Saturn I designs which had an elaborately sculpted and smoothed finless thrust structure design which was replaced with the more "industrial" chiseled shapes of the Block II Saturn I and all subsequent space launcher designs...with the exception of the Space Shuttle due to its airplane-like aerodynamic requirements.
The first concepts for the Saturn V with its fairings brushed briefly with the "soft" design concept where the fairings had a gently rounded shape and no external stringers. Indeed, the external stringer concept for the various sections of the Saturn V came almost overnight. Within weeks (or even days) when the artists concepts and sketches were developed, sharper more angular shapes took hold. Not only were such shapes easier to design and manufacture but the real practical need for the soft, streamlined shapes just didn't exist beyond the first few seconds of flight.
Realizing that there was considerable volume under the fairings, designers were quick to take advantage of the space. Based on experience with early generation vehicles, there was considerable concern over and attention paid to the separation of the various stages. In several instances there was "recontact" of either upper stages or spacecraft by their lower stages after separation. This was largely due to "residual" thrust remaining in the lower stages after they had shut down. The flow various gasses and propellants were not instantly shut off when the stages separated even though the engines had ceased to fire. This resulted in a slight but significant continuing forward thrust from the expended stages while the upper stage (or spacecraft) was producing no thrust. Further, the first stage separation occurs low enough in the atmosphere so that there is still aerodynamic drag on the upper stage...which actually slows down up to the point that its engines ignite. Combine a slight deceleration of the upper stage with a lesser deceleration of the booster stage and the possibility for recontact is very real.
Realize, that in the case of the Saturn V and other large launch vehicles being planned in the '60's, you have a booster weighing over 300,000 lbs separating from the remaining vehicle which still weighs significantly in excess of a million pounds. You don't want such large masses colliding at even the slightest speed. The most obvious solution is to "put on the brakes" for the burned out stage. This could be done aerodynamically by deploying flaps or some other drag-producing device but even though the vehicle would still be "in the atmosphere" it would be at such an altitude where 99% of the density of the sea level atmosphere is gone, the effect of the deployment of a reasonably sized device would be negligible Even small aero devices would be fairly weighty and complex devices. Another way to put on the brakes would be to fire rockets against the direction of flight. This was the solution NASA designers opted for. Not only that, the upper stage of the first few Saturn V vehicles was equipped with motors that fired to momentarily increase its acceleration immediately after staging.
Various locations for such reverse-firing motors were studied with some of the first considerations being to either tuck them inside the forward skirt of the first stage or externally on the skirt structure. Quickly, the "retro" rockets found a home in the ample space of the unused volume inside the "garage" of the four fairings at the base of the Saturn V first stage (S-IC).
Once it was decided to locate the retro rockets under the fairings, all was not solved. Fortunately, the fairings were located right at the strongest part of the entire vehicle, the thrust structure, so mounting and providing a solid structure to thrust against was almost a non-issue. Initially, the retros (there would be eight...two under each fairing) would fire through ports provided in the fairings. At ignition, the covers to these ports would be blown away and all would be fine. But providing a fairing penetration in the form of these ports was a complexity that wasn't needed. They were also points of failure, particularly with respect to the unpredictable effects of airflow over the ports or localized port heating during ascent. So the retros were moved deeper under the fairings and the fairings would be jettisoned by some kind of mechanism, typically a combination pyrotechnic/spring device, before the retros fired. The fairings, even though they appear small against the bulk of the vehicle, are still very large structures to attempt to separate at several thousand miles per hour. The task was minimized by jettisoning only the portion of the fairings that covered the exhaust area of the rockets. The requirements for the separation mechanism would be simplified and reduced. The ultimate evolution of the fairing with respect to providing for the firing of the retro rockets completely eliminated an active mechanism for the separation of the portion of the fairing over the retros. Quite simply, the overpressure of the retro rockets firing within the confines of the fairing would simply blow away the forward portion of the fairing over the retros at retro ignition.
The fairings and the fins were among the earliest Saturn V components to have their design more or less frozen. they were produced very early in the production cycle of the Saturn V program and became targets of modification and upgrade after they were originally manufactured.
Overall, the fairing consists of the jettisonable forward section ahead of the leading edge of the fin, the mid fairing which houses the retro rockets and the outboard engine gimbal actuator mounting struts and to which the fin is mounted, and the aft fairing which extends beyond the trailing edge of the fin. The two forward fairing sections are primarily aluminum skins riveted over a relatively light aluminum inner structure. The the aft fairing and its internal structure are almost exclusively titanium structures for heat resistance and strength and extend aft of the plane of the thrust structure firewall.
Fully dimensioned drawings of the fairing, including its stringers, air scoops, rivets and visible structure will appear on this site shortly.
Modeling Apollo
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