During the development of parachute systems, it is desirable to size the lines correctly, and to pack and deploy the parachute in a manner that produces uniform loading among the lines. Analytical methods would greatly reduce the cost of development; however, test verification is essential. This is extremely difficult to do with small lines, as there may be hundreds of lines in a multi-parachute deployment. The load-indicating sensors developed at JSC are extremely small, do not interfere with packing, do not change the load capacity of the lines, and can be inserted into an existing, assembled parachute system.

This invention is a peak-load measuring device for cords and ropes fabricated with a weave that can define a tube. The tension- indicating device is inserted into the weave, and will indicate the peak load that the cord is subjected to. The size of the sensor is chosen to produce a negligible stress concentration on the cord, so as to preserve its full breaking strength. The sensor consists of a body and a sensing film; deflections of the body are proportional to the external loads applied, which cause an imprint on the film. The film is subsequently removed and will indicate the maximum load experienced by the sensor. The sensor body is made of metal or plastic. Aluminum is the preferred metal, and acetal or similar engineering resin is the preferred plastic.

The body geometry consists of a cylinder, the ends of which are tapered, spherical, or similarly reduced in diameter symmetrically along the axis of the cylinder. The diameter of the cylinder is chosen to be larger than the resting inner diameter of the cord, yet not so large as to create an excessive stress concentration in the cord. This shape allows for self-insertion of the sensor into the cord. The conically tapered ends also smooth the cord transition from its collapsed size to the larger diameter required to surround the cylindrical portion of the sensor.

The sensor has a slot originating from the tip of one of the conical ends, following the cylindrical axis, to a point at least halfway along its length but not completely through the cylindrical portion of the body. The presence of this slot defines two opposing beams with approximately semicircular cross-section. These beams deflect and touch each other as a result of an applied radial compressive load. At relatively low loads, the tips of these beams touch. As the load increases, the distance from the beam root to the point of contact decreases. The pressure-sensitive film is placed within the slot between these beams. Because the load is measured as a distance of contact as evidenced by the pressure-sensitive film, a reference mark is made in the sensor that will be imprinted on the film to allow for accurate measurements.

It is also envisioned that the slot terminates in a hole, the axis of which is aligned with the slot. This reduces the effective moment of inertia of the beams at their base, producing an end condition more like a fixed beam than a cantilevered beam.

This work was done by Robert Wilkes and Satish Reddy of Johnson Space Center. MSC-25396-1