Tubular composite core stiffens launch vehicle structure
ChamberCore, an alternative to traditional sandwich laminates, also improves vibro-acoustic damping.
By Barry Berenberg, Web Editor/Technical Writer | September 2004
Fairing layup
The VALPE-2 fairing is 1,524 mm/60 inches tall, with a 438-mm/17.25-inch outside diameter at the base. Roughly the lower two-thirds is a straight cylinder; the upper section is a truncated cone. A total of 60 core tubes, running vertically from the base of the cylinder to the top of the cone, are needed to span the circumference. The prepreg layers for the cylindrical and conical portions of each tube were laid up separately, with a splice at the cylindrical/conical transition. Each layer in the cylindrical portion is made with two pieces of prepreg to make up the required length, and each layer in the conical section is made with only one piece; the splices were two layers each at +45°/-45°, but were laid up as four individual pieces on each side of the core.
Source: Delta Velocity
The VALPE-2 vehicle is ready for launch.
Achieving proper alignment of all the layers was difficult. First, the splice pieces were laid up onto the core mandrel. Next, the +45°/-45° layers were laid up flat so they could be applied in a single operation. The plies were then placed on a table against a straight edge, the core mandrel aligned over the plies using reference marks, and the mandrel pressed onto the plies for temporary adhesion. This entire assembly was next carried to a wrapping tool -- actually the same tool that had been used to cast the core mandrels. The layup assembly was pressed into the tool, which forced the prepreg tight against the mandrel and formed sharp corners. The remaining prepreg was pressed by hand against the top of the mandrel, with the overlap seam holding the prepreg in place. Many of the core mandrels cracked before or during the layup process, but the prepreg held everything together. The outer release paper was left on to prevent adhesion during storage.
After all of the cores had been wrapped, layup of the fairing itself was fairly simple. The inner facesheet was laid up directly on the tool, which was placed vertically so the cores could stick to the inner facesheet without falling off. A ply of release film was placed over the facesheet temporarily, so the cores could be positioned before being stuck to the facesheet. Groups of six cores were pressed together on a table, then placed onto the fairing. After each group was positioned properly, the release film was pulled out from behind the cores. Finally, the outer facesheet was laid up over the cores. The entire assembly was covered with a silicone sheet to help smooth the outer surface, then vacuum bagged and autoclave-cured at 121°C (250°F) for two hours.
Flight assembly
After cure, the fairing was shipped back to Delta Velocity's Virginia facility for final processing. The entire fairing was placed horizontally in a water bath to soften the cores. As the Aquacore material softened, the fairing was rotated, and high pressure water was sprayed into the upper cores to remove the material. Softening went slowly because only the ends of the cores were exposed to the water. After most of the Aquacore was dissolved, some material was left on the composite. "We attached a wire shotgun-cleaning brush to a drill and ran it through the cores," said McCrary. "That got rid of most of the loose material." The resin did penetrate into the Aquacore and could only be removed by picking at it with a knife. Ballast was to be added to the fairing anyway, in order to meet flight requirements, so the remaining Aquacore was just left in place.
Source: Delta Velocity
Launch of the VALPE-2 vehicle.
To attach the fairing to the vehicle and nosecone, complex machined, heavy aluminum rings had to be bonded onto both ends of the structure. "We were designing a composite structure to replace an existing metal structure," Padavano explains. "We had to match the existing interfaces, which were not optimal for a composite or sandwich structure." The fairing shell was placed on a rotary table (rotab) for the bonding operation. A "Christmas tree" jig located and aligned the metal rings on the shell. The Loctite (Henkel Loctite, Rocky Hill, Conn., U.S.A.) adhesive required a 0.15 mm to 0.30 mm (0.006 inch to 0.012 inch) bondline, but the fairing outside diameter was about 10 percent smaller than expected -- apparently the Aquacore material had shrunk during drying or compressed slightly during cure. To get back to the required bondline, a couple of layers of sacrificial fiberglass were laid up on either end of the fairing and then turned down on the rotab to the proper diameter.
Despite the use of the silicone rubber sheet during cure, the square cores resulted in a faceted surface, requiring some filling and sanding to smooth out rough spots. Furthermore, occasional divots in the Aquacore cores translated through to the surface and were filled in with a mixture of ground cork and epoxy. A perfectly smooth surface, however, was not required, because a 2.3 mm (0.09 inch) thick layer of cork was bonded and vacuum bagged onto the outer composite surface for thermal protection. Finally, the fairing was painted with a white conductive paint for static dissipation.
The successful launch of VALPE-2 proved the flight-worthiness of ChamberCore structures. "The goal was not so much to develop the structure," said Padavano. "Douglas Aircraft did that in the 1950s. Our main goal was to figure out a cost-effective method for building ChamberCore launch vehicle structures." The follow-up Scorpius launch will give Delta Velocity an opportunity to further refine the manufacturing method while also demonstrating ChamberCore on a larger structure.
Lessons learned
The core mandrels for the Scorpius fairing have already been cast, this time using a washaway salt-based material from
TI International Ltd. (North Chicago, Ill., U.S.A.). "The Aquacore material was not the optimum choice for our long, skinny mandrels. The TI International material has a better surface finish and is much denser, providing the strength necessary to withstand handling loads," explains Padavano.
"Moisture absorption is still an issue, though," McCrary notes. "If we leave the mandrels out in humid air, they warp and crack." Immediately after casting, therefore, the mandrels are sealed in a bag with desiccant. The salt material also dissolves faster if the water is heated to at least 57°C/135°F. Total washout time will probably remain the same, however, because the Scorpius cores are much longer than the VALPE-2 cores.
The most significant changes are being made in the layup process. For the VALPE-2 fairing, cutting the prepreg took three days and wrapping the cores took another three. The same steps for Scorpius would take more than twice the time: not only are the cores longer, but there also are twice as many. This time around, however, Mentis Sciences (Manchester, N.H., U.S.A.) will overbraid the cores with an ATK Composites (Clearfield, Utah, U.S.A.) TCR low-tack towpreg. Much simpler than hand cutting and wrapping, the braiding process also will eliminate the longitudinal seam on each core, resulting in greater tube integrity and strength. The facesheets will be laid up from a TCR high-tack prepreg fabric, further simplifying the layup process. Later fairings may have wound facesheets, resulting in even greater time savings.
These improvements in the manufacturing process are expected to reduce the cost of ChamberCore structures, making them more attractive as alternatives to traditional sandwich structures. Improvements in core uniformity and the overall layup will reduce structural weight and stiffness. These features make ChamberCore an attractive launch vehicle structure, even without the added acoustic benefits. Future experiments will further explore mass loading and Helmholtz resonators for acoustic attenuation.
