Acoustic Testing of Hollow Core Fairing Designs

Failure of sensitive electronics and fragile substructures resulting from acoustic loading has been identified as a principal cause of payload degradation during launch. Depending on the mission launch vehicle, the fragilities of the payload and other factors, payload protection can range from an un-insulated payload fairing, to the addition of 3 to 6 inches of various acoustic blankets, to various resonant systems (i.e. Helmholtz resonators) to trap energy of critical frequency. Alliant Tech Systems (ATK) has suggested an innovative method for significantly reducing acoustic propagation through the fairing. They have suggested and hold a patent (Ref. Hopkins, Oyler, Raun, Jessen) on a concept of providing a temporary water jacket during the first tens of seconds of launch, while acoustic noise reflected from the surface of the earth generates maximum acoustic loads. For this concept a hollow sandwich panel construction is filled with water and carried a few thousand feet vertically (out of ground reflections). The jacket is then ported, pressurized and evacuated to lighten the mass of the fairing. Although the water mass is a significant factor in total payload/accommodations mass pre-launch, the actual rocket fuel expended in changing the potential and kinetic energy of the water prior to ejection can be very similar to that required to carry conventional acoustic measures to fairing separation. The benefit of the system is a dramatic improvement in acoustic attenuation at low altitude. The Air Force Research Laboratory (AFRL) has developed a patented structural system (ChamberCore), which consists of axial hollow tubes of composite cores bonded between balanced ply configurations of inner and outer composite skin plies. This configuration lends itself easily to supporting the ATK concept for water filled sandwich panels. In 2001, AFRL tested a payload ground transportation module with 5 cm thick voids in both filled and unfilled ChamberCore configuration. Results indicated potential for up to 12 Db of acoustic mitigation over a wide range of frequencies (100 to 5000 Hz). For the present study the AFRL has built and tested a 60 inch tall x 60 inch diameter ChamberCore cylinder filled with various impedance media to test the influence of baseline hollow response to water-filled, denser saturated CaCl₂ solution-filled and to air-entrained water-filled configurations. Results of these tests clearly demonstrate improved acoustic mitigation for all of the fluid variants. Contrary to anticipated results, the type of fluid media had only a small impact on overall acoustic mitigation. These cylinder test results have led to speculation that the internal structure (ribs) between tubes provides a dominant acoustic path below about 6 dB of acoustic attenuation in the fluid media. To test this hypothesis a series of smaller one-dimensional acoustic transmission tests were conducted with varying amounts internal web structure and varying material stiffness, as well as a matrix of various fill media.