- Engineers and technicians from NAWCWD China Lake and the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. recently conducted the first demonstration of a new method of performing load tests on large parachutes.
Historically, parachutes have been load-tested by various methods including release from an aircraft, deploying in a wind tunnel, dragging through water, and shooting out of an air cannon. Each type of testing has its own advantages and drawbacks. JPL conducted a review of all of the available test methods and, due to the loading mechanics peculiar to parachutes deploying in a very thin atmosphere, decided that none were appropriate for testing the next generation of Mars lander parachutes.
Having had recent experience with NAWCWD's Supersonic Naval Ordnance Research Track (SNORT) for the supersonic inflatable aerodynamic decelerator (SIAD) test series, they decided on a new approach to parachute testing. A helicopter would lift the parachute into the sky with a rope trailing down to a pulley on the ground. The rope would go through the pulley and attach to a rocket sled. The parachute would be released and inflate as it dropped to the ground. Once the parachute was fully inflated, the rockets would be fired and the sled would tow the parachute down towards the ground. This would impart the desired loads to the parachute in the fully-open shape it would take above Mars, verifying its design.
During the course of two years, China Lake and JPL engineers refined the Parachute Design Verification (PDV) test series. As this was a novel test approach, a myriad of test configuration determinations needed to be made, both from the airdrop perspective as well as the parachute loading perspective. Personnel in the NAWCWD Escape, Parachute and Crashworthy Division spent the first year focusing on the development of the specialized airdrop equipment necessary for this test, including the release platform, load platform, a launch support structure, and a method to attach and release the packed parachute from a helicopter. They then performed an airdrop test to verify the function of the newly-developed equipment, as well as determine the motion characteristics of a 4,000-foot rope suspended by a helicopter. After this very successful test, which indicated that this test method just may work, significant development and fabrication began on the parachute loading apparatus.
JPL determined that the structure that supports the pulley would have to withstand 200,000 pounds of force. To support the pulley, a large tripod was designed with 12 inch-by-12 inch square beams spanning 40 feet. A large funnel to guide the rope to the pulley would be placed on top of the tripod bringing the total height to 19 feet. The tripod assembly was coordinated by Jack Ingle from the Escape, Parachute and Crashworthy Division with support from the NAWCWD Weapons Prototype Division and Weapons Survivability Lab machine shops and personnel.
A concrete foundation was needed to support and anchor the tripod. The final design ended up being composed of two large blocks, one on either side of the track. The west footing block was 15 feet wide, 15 feet long, and 18 feet deep. The east footing was 15 feet wide, 42 feet long, and 18 feet deep. The combined weight of the footings was about 2 million pounds. The tripod was attached to these footings with 52 bolts two inches in diameter.
Another design effort involved making sure there would be little to no slack in the tow rope as the parachute fell and inflated. Slack in the rope could cause snags at the pulley or catastrophic snatch loading on the parachute when the rocket motors fired. A hydraulic winch was designed and built to take up that slack. The winch was capable of pulling with 350 pounds of force at up to 100 mph. Powering the hydraulic pump was a 500 kilowatt generator. Once the rocket sled began moving, an explosive cutter was fired to sever the winch line allowing the sled to pull the parachute toward the ground.
The rocket sled was designed to allow the rocket motors to be tilted up or down to fine-tune the amount of pull force on the parachute. Concrete ballast blocks were placed in the sled to bring the total weight up to 136,000 pounds. This weight was needed to keep the acceleration and deceleration of the sled low while keeping a constant tension on the tow rope. Behind the rocket motors and ballast portion of the sled was a 120 foot long tow bar sled. This was necessary to keep the hot rocket motor plume from melting the tow rope. This record breaking sled train was fabricated by the Weapons Prototype Division machine shop and personnel.
Most important of all was the safety of the helicopter crew. More than a dozen layers of safety were designed into the firing system so that there was no possibility of inadvertently pulling down on the helicopter. Thirteen inhibits, three man-in-the-loop safety switches, 11 live camera feeds, a brand new firing sequencer, and a hand-off from helicopter to winch to rocket sled help ensure aircrew safety.
As the various pieces came together, a series of checkout tests were conducted. A winch test was run to ensure it could reel in the rope fast enough to prevent slack build-up. Next, samples of the tow rope were dragged along the track trough to determine the abrasion strength loss from scraping on the concrete foundation of the track. Various techniques to keep the rope from being damaged were tested. Following these tests, a crane was brought out to simulate the helicopter. Several tests were conducted to see if the pulley/funnel system would work properly at various rope angles. Finally, leading up to the parachute test, several practice runs both with and without the helicopter were conducted to validate the test procedures.
On the morning of the test, Lt. James Hong and Lt. Christopher Webster, of Air Test and Evaluation Squadron 31, flew a China Lake MH-60 search and rescue helicopter from Armitage Airfield and landed at SNORT. Groundcrewman AWS2 Erik Potter ensured the helicopter was promptly connected to the parachute release platform and lifted off again. Upon reaching 4,110 feet, AWS2 Justin Stonebraker released the 110 foot diameter parachute from inside the helicopter. The helicopter exited the SNORT area as the parachute fell and inflated. Three observers in the SNORT Fire Control room each determined that the helicopter was at a safe distance, the parachute had inflated, and the winch was operating properly, and pushed their individual “Go” buttons. This allowed the sled to fire its rocket motors once the tow rope had reached the track and automatically latched to the sled. The motors fired, applying a force of more than 90,000 pounds to the parachute and towing it down about 1,100 feet at a top speed of 60 mph. The parachute slowed and stopped the sled after the motors had burned out. The parachute continued to descend to the ground and was recovered after personnel were cleared into the test area.
Following the test, the parachute team inspected the parachute and provided design improvement recommendations to JPL engineers. An improved design will be tested later in the year. This parachute may eventually be used to land larger rovers on the surface of Mars with better accuracy and at higher altitudes than ever before.
Source: Naval Air Systems Command
Date: Nov 22, 2013