1. Field of the Invention
The invention relates to a deployable rigid system for managing crash energy. More particularly, the invention relates to an expandable cellular structure for absorbing impact energy.
2. Discussion of Background Information
Airbags are a common device for absorbing crash energy. Many vehicles such as automobiles use airbags internally, and space vehicles such as the Mars Pathfinder have used them externally. Such airbags have a variety of drawbacks.
Airbags are ill-equipped to deal with shear forces due to impact. As a result, airbags are generally limited to uses where the direction of impact is known prior to installation. To improve shear stability, large airbags are often designed with several compartments and gas outlets. However, absent optimal conditions airbags are likely to momentarily store energy before releasing it in a trampoline-like effect.
Airbags are also sensitive to the surface area and shape of the impacting object. Objects with small impact profiles are likely to have less kinetic energy dissipated by the airbag. That is, if an impacting object contacts only a small portion of an airbag's area, the airbag will likely deform in such a way that does not provide sufficient energy absorption. Moreover, an airbag can rupture and fail when hit by an object having sharp edges or projections.
Airbags require specialized servicing before they can be redeployed. When an airbag deploys, it typically expends a small explosive charge. This charge must be replaced by a specialist. Specialized servicing is also required to repack the airbag itself after it has been deployed.
Airbags are also limited to single collision protection. The gas pressure in an airbag can generally protect from at most one collision before it is lost. For example, an airbag typically expends its gas pressure upon a first impact and cannot dissipate energy from a second impact if it follows a first impact moments later.
There is also no simple way to tailor impact resistance throughout the energy-absorbing stroke of an airbag. The deformation path of an energy absorber after it is impacted by an object is known as its “crush-stroke” or “energy-absorbing stroke.” At each point throughout the crush-stroke, an energy-absorbing device provides mechanical resistance (the “crush-load”), which typically dissipates an impacting object's kinetic energy by deformation, fracturing, and as heat. Airbags are usually not suitable for providing customized levels of mechanical resistance (or “load tailoring”) throughout their effective energy-absorbing stroke.
Finally, airbags have relatively tight performance parameters in that a single airbag usually cannot perform effectively in a multitude of different situations. Airbags suitable for adults can (and have) caused injuries to children.
Another type of crash energy management device is a semirigid foam-filled bag. A device mixes a two-part polymer liquid and injects it under pressure into a flexible container. The mixture hardens after a time interval, usually several seconds, and provides some amount of crash protection for an aircraft so equipped. A drawback of these devices is that semirigid foam-filled bags offer negligible impact protection until the foam is sufficiently firm. Typically, there is a lag time between the initial mixing and when the mixture hardens. Therefore, semirigid foam-filled bags are typically restricted to applications where there is sufficient notice of an impending impact. Semirigid foam-filled bags are not reusable.
Semirigid foam-filled bags also do not allow for varying crush-loads throughout their energy-absorbing stroke. Because the foam used in such bags is generally homogenous, it is likely to provide the same amount of impact resistance throughout. Hence, semirigid foam-filled bags typically do not admit load tailoring.
Fixed nondeployable cellular structures such as honeycombs have been used for absorbing impact energy. However, fixed nondeployable cellular structures cannot be stowed in a space smaller than their intrinsic structure allows. That is, fixed nondeployable cellular structures occupy the same amount of space whether or not they are positioned for use. Therefore, applications of fixed nondeployable cellular structures are limited by the space available to house them.
U.S. Pat. No. 2,973,172 to Bixby (“Bixby”) describes deployable paper cellular structures. Bixby discloses a landing decelerator for articles dropped in aerial delivery systems. The landing decelerator is disclosed as using expandable honeycomb structures constructed from paper.
Bixby's device is limited by its construction. Paper's in-plane isotropy places limits on the total amount of energy absorbed. Paper also places limitations on the force required to deploy Bixby's device. Because Bixby does not disclose that the cell walls and cell joints (i.e., the weld between the cells) are constructed of different materials, changing the cell wall characteristics to provide greater crush resistance would correspondingly inhibit deployment or increase the force required to deploy Bixby's device. Such increased deployment force would likely slow deployment speed.