Ablator Application

There had always been questions about exactly how to apply an ablative coating over the surface of an entire airplane, even one as small as the X-15. Even more questions existed on how to maintain the airplane after applying the coating, and how difficult it would be to refurbish the coating between flights. There appears to have been little actual concern about the effectiveness of the ablator; if it was applied correctly, everyone was relatively sure the concept would work.

As part of its initial contract, Martin Marietta developed a comprehensive procedure for applying the coating, maintaining it, and removing it if necessary. Martin accomplished the first complete application of the ablator in general agreement with the schedule and procedures published earlier. Simply because it represents one of the few attempts to use an ablative coating on an entire airplane, it is appropriate to review the application in detail.-1322

The process began with cleaning the airplane, and Martin admitted the preparatory cleaning was "somewhat overdone" for the first application. Technicians masked all joints, gaps, and openings before the cleaning began to prevent solvent from getting into the airplane. The surface condition of the airplane, with its accumulation of contamination and overabundance of lacquer, necessitated the use of a great deal of solvent during the initial cleaning. Technicians accomplished the final cleaning with powdered cleanser and water using a "water-break-free" test to ascertain when the surface was properly clean. Some areas of the aircraft, especially around fastener heads and skin joints, never did achieve a completely water-break-free condition, and Martin noted that "these areas continually bleed hydraulic fluid or other contamination."[323]

Next, technicians used polyethylene tape to mask all of the seams between panels to keep the ablative material out of the aircraft compartments. The only problem encountered in the initial ablator application was that nobody had anticipated masking the gap between the fixed portion of each vertical stabilizer and the rudders. The installation crew then improvised a solution that was mostly successful. As a means of checking the adequacy of the masking during all phases of ablator operation, technicians placed airborne contamination collectors in nine aircraft compartments before beginning the application process. At the end of the process, quality inspectors from Martin Marietta and NASA checked these collectors and found very little contamination, indicating that the masking worked as expected.-1324

Before turning the airplane over to Martin Marietta, NASA had made a few minor changes to accommodate the ablator installation. The retractable pitot tube (or "alternate pitot" as it was called) was installed, as was a new retaining ring around the ball nose that had a step at its aft end. When the ablator was built up during the application, it would fill up to the top of the step, resulting in a smooth surface.[325]

Next up was installing the molded ablator "details" on the aircraft. This included premolded leading-edge covers made from ESA-3560-NA for the wing and horizontal stabilizers, and covers for various antennas, the canopy leading edge, and the vertical stabilizer leading edge. Although it was not provided as part of the kit, the installation team fabricated a detail for the leading edge of the dummy ramjet instrumentation rake from a spare piece of the vertical stabilizer leading-edge detail.[326]

After technicians glued the details onto the surface of the leading edges, they covered the majority of the airplane with polyethylene sheeting to protect cleaned areas from overspray during the sequential ablator applications. The airplane was broken down into nine distinct areas that technicians would spray in sequence. Technicians installed marker strips (a vinyl foam tape) over the contamination masking and applied a layer of DC93-027 RTV over fastener heads and peripheral gaps of the seldom-removed panels. The installation team then sprayed the MA-25S ablator using a commercial paint spray gun. Controlling the thickness of the ablator was the most significant difficulty encountered during the application process, but the team got much better toward the end as they became more familiar with the deposition characteristics of the material. Some areas, particularly the middle of the wing root and the crown centerline of the fuselage, proved to be too much of a stretch for the technicians standing on the ground. This condition resulted in a "somewhat cheezy" ablator application in those areas, but the layer was deemed adequate to protect the airframe.[327]

Once the entire surface was covered, the next task was to go back and remove the trim marker strips. This proved more difficult than had been expected because the tape was "too thick and possessed too high an adhesive tack." Nevertheless, the team eventually accomplished the task, but decided to use a different tape next time. It was important to avoid disturbing the sealing tape under the marker strips, since it would have to protect the compartments from the effects of the sanding operation still to come.[328]

Applying the MA-25S ablator was more involved than most expected. The airplane had to be scrubbed clean, and then each individual panel had to be taped to ensure ablator did not get into the airplane. The ablator was then sprayed, sanded to a consistent finish, and its depth measured. The amount of time required to coat the relatively small X-15 did not bode well for a large Space Shuttle. (NASA)

The ablator was left to cure at room temperature for a few hours, and then technicians sanded the entire surface to remove overspray and irregularities, and to bring the ablator layer down to within 0.020 inch of its design thickness. This proved to be a very tedious operation. First, the team had to draw grid lines on the airplane to establish precise monitor locations, and a penetrating needle dial gage determined the thickness at each point on the grid. Technicians then sanded the surface. Since this removed the grid lines, they would have to be redrawn and the thickness rechecked. The process continued until the desired thickness was reached. It was evident that there was a need for a better way to establish the grid on the airplane.[329]

When the sanding was finished, the team glued 10 test plugs to the ablator surface and cut through the ablator layer around their periphery. A pull test was performed on the plugs to determine whether the ablator had properly bonded to the skin. The first application successfully passed all of its pull tests. Various "hard point inserts" were then installed around the external tank inboard sway brace attach points and the aircraft jacking points. Inserts of MA-25S-1

material also covered the ram air door in the fuselage nose and the engine compartment fire doors on the aft fuselage.-1330!

MA-25S had a natural pinkish color and somehow this seemed inappropriate for the world’s fastest airplane. Fortunately, the specification called for a layer of Dow Corning DC90-090 RTV over the entire airplane to provide a wear coating and to seal the ablator. The DC90-090 was translucent white and did not completely hide the pink, so NASA asked Martin Marietta to apply an extra coat (or two, in some areas) so that the airplane would have a uniform white finish. This exhausted the available supply of the coating; however, Dow Corning had replaced DC90-090 with a similar product called DC92-007. Martin requested samples of the new product to determine its suitability as a substitute.-333-

At this point, the team applied a limited number of hazard and warning markings to the exterior using standard high-temperature aircraft lacquer paint. The last step was to remove the polyethylene tape that sealed the service panels and install strips of MA-25S-1 around their periphery to provide extra durability during panel removal and replacement. Martin then returned the airplane to the X-15 maintenance crews, who installed instrumentation and prepared it for flight.-1332-