Engineering topics

Davis, B. Alan

Publications and source records attributed to Davis, B. Alan.

4 records

NASA NTRS2019

Heat-Cleaned Nextel in MMOD Shielding

Meteoroid and orbital debris (MMOD) shielding can include NextelTM ceramic cloth in the outer layers of the shielding to enhance MMOD breakup. The Nextel fabric can contain size (or sizing) which aids in manufacture of the fabric. Sizing is a starch, oil or waxy material that is added to the rovings and yarns to protect the fibers from being cut or broken during the fabric manufacturing process and by later handling. For spacecraft applications, sizing is removed by heat-cleaning to reduce/eliminate off-gassing during vacuum operations. After the sizing is removed, the fibers in the woven fabric are prone to breakage during handling which reduces fabric strength. Because heat-cleaned Nextel tends to shed fibers that can be irritating to workers, the usual practice for hypervelocity impact tests is to use Nextel with sizing. The reduced strength of heat-cleaned Nextel does not typically effect the performance of MMOD shields with Nextel used in outer layers of the shield, because the density and areal density of the ceramic fibers in the fabric control MMOD breakup (not fabric strength). This paper provides data demonstrating that hypervelocity impact protection performance is not adversely altered for shields containing heat-cleaned Nextel compared to Nextel with sizing.

Christiansen, Eric L.↗

NASA NTRS2011

Burst Pressure Failure of Titanium Tanks Damaged by Secondary Plumes from Hypervelocity Impacts on Aluminum Shields

Metallic pressure tanks used in space missions are inherently vulnerable to hypervelocity impacts from micrometeoroids and orbital debris; thereby knowledge of impact damage and its effect on the tank integrity is crucial to a spacecraft risk assessment. This paper describes tests that have been performed to assess the effects of hypervelocity impact (HVI) damage on Titanium alloy (Ti-6Al-4V) pressure vessels burst pressure and characteristics. The tests consisted of a pair of HVI impact tests on water-filled Ti-6Al-4V tanks (water being used as a surrogate to the actual propellant) and subsequent burst tests as well as a burst test on an undamaged control tank. The tanks were placed behind Aluminum (Al) shields and then each was impacted with a 7 km/s projectile. The resulting impact debris plumes partially penetrated the Ti-6Al-4V tank surfaces resulting in a distribution of craters. During the burst tests, the tank that failed at a lower burst pressure did appear to have the failure initiating at a crater site with observed spall cracks. A fracture mechanics analysis showed that the tanks failure at the impact location may have been due to a spall crack that formed upon impact of a fragmentation on the Titanium surface. This result was corroborated with a finite element analysis from calculated Von-Mises and hoop stresses.

Nahra, Henry↗

NASA NTRS2011

Orion Spacecraft MMOD Protection Design and Assessment

A first-principals, semi-empirical ballistic performance model has been developed for porous ceramics. Lightweight thermal protection systems protect the crew and vehicle of orbital and exo-orbital missions from the intense heat of atmospheric reentry. To maintain low launch weights these materials are their own protection from space hazards like orbital debris and meteoroids. A ballistic performance model is described here that models the performance under a variety of impact conditions. Using the model described here relative to an energy scaled model results in a significantly reduced prediction of full penetration of this material at ISS orbital parameters.

Miller, Joshua E.↗

NASA NTRS2009

Hypervelocity Impacts on ISS Handrails and Evaluation of Alternative Materials to Prevent Extravehicular Mobility Unit (EMU) Glove Damage During EVA

During post-flight processing of STS-116, damage to crewmember Robert Curbeam's Phase VI Glove Thermal Micrometeoroid Garment was discovered. This damage consisted of: loss of RTV-157 palm pads on the thumb area on the right glove, a 0.75 inch cut in the Vectran adjacent to the seam and thumb pad (single event cut), constituting the worst glove damage ever recorded for the U.S. space program. The underlying bladder and restraint were found not be damaged by this event. Evaluation of glove damage found that the outer Vectran fibers were sliced as a result of contact with a sharp edge or pinch point rather than general wear or abrasion (commonly observed on the RTV pads). Damage to gloves was also noted on STS-118 and STS-120. One potential source of EMU glove damages are sharp crater lips on external handrails, generated by micrometeoroid and orbital debris (MMOD) impacts. In this paper, the results of a hypervelocity impact (HVI) test program on representative and actual ISS handrails are presented. These tests were performed in order to characterize impact damage profiles on ISS handrails and evaluate alternatives for limiting risk to future missions. It was determined that both penetrating and non-penetrating MMOD impacts on aluminum and steel ISS handrails are capable of generating protruding crater profiles which exceed the heights required for EMU glove abrasion risk by an order of magnitude. Testing demonstrated that flexible overwraps attached to the outside of existing handrails are capable of limiting contact between hazardous crater formations and crewmember gloves during extravehicular activity (EVA). Additionally, replacing metallic handrails with high strength, low ductility, fiber reinforced composite materials would limit the formation of protruding crater lips on new ISS modules.

Ryan, Shannon↗