Figure: In the year 2004, the Huygens Descent Control Sub-System will decelerate and control the descent of the Huygens Probe through Titan's atmosphere.

IN THIS ISSUE

TECHNOLOGY IN REVIEW
Overview of the latest advances in decelerator systems technology.

PARACHUTE PERFORMANCE - 3D SIMULATIONS
Advances in numerical simulation of parachute inflation toward reducing costly, full-scale testing.

DROPPING IN ON TITAN
Design highlights, analytical methods and testing of the Huygens Descent Control Sub-System.

FOREWORD

Do you think "high performance" or "high tech" when you think of parachute? Consider a wadded lump of fabric dumped at 1,100 miles per hour into an atmosphere with a density only half a percent of our own. Now expect that fabric to change shape within a fraction of a second to create drag -- without blowing itself apart -- to slow down a $150 million spacecraft so that the spacecraft can deliver a sophisticated planetary rover to Mars.

How do you begin to model the inflation? How do you select materials that will survive the vacuum of space travel and then function properly the very first time you ever use them? And that doesn't even begin to address the type of parachute you should use and how it integrates with the payload.

These are the kinds of challenges engineers and scientists serving on the AIAA Aerodynamic Decelerator Systems Technical Committee face every day. We put this newsletter together to try and keep you informed of the latest in the high technology world of parachutes. We hope you find it intriguing and informative. We also hope you'll call us if you need help.

SCOPE:
Development and application of aerodynamic deceleration systems and lifting parachutes, pararotators, and inflatable decelerators for aerodynamic deceleration, sustentation, and landing of manner and unmanned vehicles.

CONTACT:
ADS TC Chairman, 1999 Dr. Vance L. Behr
Senior Member of the Technical Staff
Sandia National Laboratories
Unsteady and Reactive Fluid Mechanics, Department 9116
Albuquerque, NM 87185-0836
PHONE: 505-845-8916
FAX: 505-844-4507
E-MAIL: vlbehr@sandia.gov


WEB SITE:
For current information on short courses, conferences, membership, publications, meetings and neat images and videos of parachutes visit our web site.

The address is:

www.engr.uconn.edu/~adstc

The state-of-the-art in aerodynamic decelerator technology has advanced significantly this year. Contributions from fundamental research through large scale development have been reported from all sectors of government, industry and academia, towards a wide range of applications.

Planetary Exploration In the area of planetary exploration, post-flight data analysis has confirmed the excellent performance of the NASA/JPL Mars Pathfinder parachute system. A nearly identical system has been built by Pioneer Aerospace for use on the Mars Surveyor 98 Lander. Lockheed Martin Astronautics was responsible for integration of the Stardust comet sample return capsule. This probe was launched in February, 1999, returning seven years later on a two-stage parachute system. Lockheed Martin has teamed with Pioneer Aerospace and Vertigo to develop the next-generation parafoil Mid-Air-Retrieval System, which will be used for the first time on the Genesis solar particle sample return capsule.

Reusable Launch Vehicle Closer to earth, Irvin Aerospace is developing the landing system for Kistler Aerospace's K-1 Reusable Launch Vehicle. The main parachute test program was concluded with the successful drop of six 156 foot diameter ringsail parachutes (a world record).

Drop testing of the drogue and stabilization parachutes will be completed later this year. Ground impact tests of one-fourth scale airbags is also being completed and results are being used to calibrate analytical models. Figure: Successful drop test of six 156 foot diameter ringsail parachutes for the Kistler Aerospace K-1 Reusable Launch Vehicle.

X-38 Crew Return Vehicle The X-38 performed its first free flight and successful, safe landing under a 5500 ft2 parafoil at NASA Dryden in March, 1998. The second and third flights were completed successfully in February and March of this year. The X-38 is a NASA in-house technology demonstration program established to satisfy the International Space Station requirement for a crew return vehicle to replace Soyuz. The successful drop followed extensive testing of the parafoil based recovery system (Pioneer Aerospace) using a weighted pallet at Yuma Proving Grounds. Met objectives included successful deployment of the drogue/parafoil recovery system from the actual X-38 vehicle, positive parafoil control authority and low landing vertical velocities.

Military Parachute Applications Even closer to earth, the US Air Force Office of Scientific Research with the US Army Soldier Systems Command (Natick) have teamed with industry and academia on the New World Vistas Precision Air Delivery program. This basic research program is exploring low cost technologies to improve high altitude aerial delivery accuracy. The program is integrating advanced wind sensors and models into a computed aerial release point planning tool (Draper Labs) which will communicate with the payload(s). Decelerator technologies include the autonomous control of round parachutes (Boeing/Vertigo) and a ballistic low opening cross system (Paranetics & Parks College of Saint Louis University).

Natick has teamed with Simula to develop a replacement for the T-10 personnel airdrop system. The new system has demonstrated softlanding technology to allow for a faster overall descent with a landing velocity of less than 16ft/sec


Figure: Side view of NASA's X-38, an experimental crew return vehicle.


Figure: First free flight test of vehicle 132 (X-38 prototype with controls) on March 5, 1999.

Photos courtesy of NASA X-38 web site.

Semi-Rigid Wing USBI's 30ft autonomous semi-rigid wing (successfully drop tested over 100 times) is being scaled up to a 58ft version (5000 pound payload) and initial testing of the extraction process has been completed. Natick is also conducting research on new parafoil opening and fabrication technologies, and softlanding technologies. Autonomous parafoil technology is being pursued in many locations including Natick and Germany.

Numerical Simulation High performance computer models that couple the fluid and structural dynamic phenomena of decelerators are being developed by Natick with the Army High Performance Computing Research Center, the University of Connecticut and others. Similar efforts are progressing at Sandia, the French Ministry of Defense and other facilities. A fully-coupled 3D capability has been demonstrated in this area for a wide range of systems. These tools are being validated with wind tunnel testing (Sandia, Parks College of Saint Louis University, and others). The goal: to obtain an airdrop "virtual proving ground".

Richard Benney
U.S. Soldier Systems Center -Natick Soldier Center of Excellence
rbenney@natick-emh2.army.mil

Figure: Experimental semi-rigid wing flight test with 500 pound payload.


Figure: Numerical simulation of streamlines and pressure distribution around a parafoil geometry.

The Army After Next program has identified "improved methods of aerial delivery" as vital to the rapid deployment of warfighters, ammunition, equipment and supplies. Moreover, airdrop of food, medical supplies and shelters for humanitarian relief efforts have increased in demand. The U.S. Army Soldier System Center - Natick's research is focused on developing new technologies to advance Department of Defense airdrop capabilities.

High Performance Computing One aspect of Natick's research is the utilization of high performance computing (HPC) to predict airdrop system performance. All airdrop systems encounter highly complex fluid structure interaction (FSI) phenomena as they deploy, inflate, reach steady state conditions, and ultimately provide a soft landing. Without an accurate representation of the inflated parachute shape, the FSI phenomena make it impossible to predict accurately the pressure distribution on a canopy surface. At the same time, the parachute's shape cannot be determined without an accurate representation of the pressure distribution (and other loadings) acting over its surface.

Therefore, even in terminal descent, the Fluid Dynamics (FD) and Structural Dynamics (SD) are intimately coupled. Add to this the time dependent dynamics associated with all parachute systems and the complexities become apparent.

The high-fidelity modeling approach undertaken involves the numerical coupling of computational FD software and SD software. These coupled FSI models are required to capture the physics of the complex dynamic phenomena associated with all airdrop systems.
Figure: The computational mesh necessary to resolve the flow field around an inflated T-10 parachute.

Simulation Results The SD software is utilized to model the canopy, suspension lines and payload while the FD software is utilized to predict the fluid dynamics (i.e., pressure field, velocity field, etc.). This approach has been successfully demonstrated and validated with the coupling of two axisymmetric finite element codes. These axisymmetric simulations have generated significant interest and support, which have led to the larger collaborative team effort now focused on applying this technology in three dimensions (3D). The current baseline airdrop systems being modeled to test and validate the first generation 3D FSI software tools include a personnel parachute system (the T10 system) and a large cargo parafoil system

Parachutes and airdrop systems have been traditionally developed by time-consuming and costly full-scale testing. The ability to use computer software to model airdrop systems will greatly reduce lifecycle costs, assist in the optimization of new airdrop capabilities and provide an airdrop virtual proving ground. For example, although the T10 parachute has been in service for more than 40 years, there is a technology void in predicting its behavior. A recent example of this technology void surfaced during initial mass assault airdrop testing of the T10 from a new transport aircraft. At the time of these tests, little was known about the T10's ability to withstand a wind gust or a lead aircraft wing tip vortex of a given strength. The FSI models are expected to provide this critically needed data.

Richard Benney
U.S. Soldier Systems Center -Natick Soldier Center of Excellence
rbenney@natick-emh2.army.mil

Figure: Complex wake structure of a T-10 parachute system predicted using computational fluid dynamics.
Figure: Regions of high stress on a T-10 parachute predicted using three-dimensional fluid structure interaction software.

October 15, 1997, the Cassini-Huygens spacecraft was launched on its mission to explore the Saturnian system. The mission is a joint NASA-European Space Agency (ESA) venture. NASA is responsible for the Cassini orbiter that will explore the Saturnian system extensively, looking at Saturn itself and at the planet's ring system and moons.T he European contribution is the Huygens Probe that will investigate the atmospheric composition and surface characteristics of Titan, Saturn's largest moon.
Figure: Artist's concept of Huygens in the atmosphere of Titan. Photo compliments of ESA Huygens web site.

Why Titan ? Titan is unique in the solar system in that it is the only moon with a significant atmosphere. At the moon's surface atmospheric density reaches approximately five times the air density at Earth's surface. The significance of the Huygens mission to the scientific community stems from preliminary studies that indicate that the frozen moon harbours gaseous and liquid hydrocarbons in a predominantly nitrogen atmosphere. This is thought to correspond to the conditions that prevailed in the early Earth atmosphere. One reason for the failure of this primordial climate to evolve more complex molecules may have been the slower rate of photochemical reaction resulting from the cryogenic temperatures encountered in the atmosphere. Thus, information from the Huygens probe may help to inform scientists on the evolution of the DNA molecule. The Huygens Descent Control Sub-System (DCSS), designed by Martin-Baker Aircraft Co. Ltd in the UK, will decelerate and control the descent of the Huygens probe through the Titan's atmosphere. The sub-system has been developed under a contract from Aérospatiale (Division Systèmes Stratégiques et Spatiaux), the prime industrial contractor for the probe on behalf of the European Space Agency.

DCSS Function
The orbiter/probe combination will take just under 7 years to reach Saturn. As the orbiter approaches Titan the probe is ejected towards the moon. Twenty-two days later the probe enters the Titan atmosphere and begins to decelerate. During entry an aeroshell and back cover protect the probe against aero-kinetic heating, the plasma around the probe reaching a maximum temperature of 12,000°C. These must both be removed from the probe by the DCSS before scientific experiments can commence.

After the probe has passed through the upper reaches of Titan atmosphere and decelerated to approximately Mach 1.5 at 164 km altitude, the mortar is fired. This ejects the pilot chute through the back cover. The pilot chute deploys and inflates in the supersonic wake of the probe. Two seconds later the back cover is jettisoned. The back cover, attached to the pilot chute by a three-leg bridle, is pulled away from the probe, extracting the large main parachute from the parachute container with a lanyard. After deployment and inflation, the main parachute stabilises and decelerates the probe.

It is necessary to deploy the parachute system in the supersonic regime since the probe becomes aerodynamically unstable at transonic velocity. After thirty seconds, when the probe has decelerated to Mach 0.5, the aeroshell is released and falls away. The probe radio link to the orbiter is established and the scientific experiments are initiated. The probe continues to decelerate until a steady rate of descent is reached.

To maximise acquisition of data within the constraints of the power budget and up-link, the probe must arrive at the surface of the moon 2¼ hours after mortar initiation. Unfortunately, the size of parachute necessary to stabilise the probe during transonic deceleration and to separate the probe from the aeroshell would result in an excessive descent time. Therefore, after 15 minutes, three pyrotechnic parachute jettison mechanisms detach the main parachute bridle from the probe.

The separating main parachute deploys a small stabilising drogue, sized to achieve the required descent time. During the final descent to the surface under the stabilising drogue probe stability is critical. The probe attitude must be maintained at less than 10° to the vertical in order to avoid loss of up-link. Pitch rate must be less than 6°/s to prevent blurring of the images produced by the onboard camera. The drogue is therefore attached to the probe by a three-leg bridle design to provide optimum stability and to minimise gust response. Additionally, during the descent, the probe must rotate slowly at a defined rate to permit the camera to obtain the required data. The motion is driven by vanes on the probe. A low-friction swivel is incorporated in the drogue bridle to prevent random parachute rotation from modifying the probe spin rate.

DCSS Design Highlights
This mission requires parachutes to meet requirements generally outside the envelope of terrestrial operation. The pilot and main parachutes must inflate at supersonic Mach number but low dynamic pressure. The designs selected must be compatible with the rigorous stability requirements for the probe. Moreover, the textile materials must sustain strict cleansing procedures, long term vacuum storage, cryogenic temperatures, radiation and a 14 year lifetime without significant degradation.

Prediction of parachute inflation loads and drag performance in a terrestrial environment is still in its infancy. Extrapolation to an extra-terrestrial atmosphere and 1/7 Earth gravity necessitated the development of new models of parachute behaviour. A simulation of parachute inflation has been developed together with a method to predict the drag coefficient. After trade studies the disk-gap-band parachute was selected for all stages, largely based on previous space heritage.

The impossibility of testing the parachute system in the correct environment led to an extensive test campaign. Wind tunnel testing at the 13 ft x 9 ft subsonic tunnel at the Defence Research Agency, Bedford and the 16 ft x 16 ft transonic tunnel at Arnold Engineering Development Center in Tennessee yielded a comprehensive aerodynamic database for disk-gap band parachute. Full scale drop testing, using a specially developed instrumented vehicle, verified full scale subsonic performance and structural strength.

Dr. J. Stephen Lingard, Chief Systems Engineer
Martin-Baker Aircraft Co. Ltd.
Middlesex, England
slingard@martin-baker.co.uk
Figure: Disk-gap-band parachute in wind tunnel.

The CEAS/AIAA 15th Aerodynamic Decelerator Systems Technology Conference and Seminar will take place from 8-11 June 1999, at the Hotel Sofitel in Toulouse, the aerospace capital of France. Registration for the conference includes a pass to THE 1999 PARIS AIR SHOW which begins on Sunday, 13 June. The technology conference will focus on recent advances in parachutes and aerodynamic decelerator system technologies. For the second time in six years, this conference is being held outside the United States in order to encourage international participation. The conference is sponsored jointly by the Confederation of European Aerospace Societies (CEAS) and the AIAA and will be run by the CEAS host society, the Association Aéronautique et Astronautique de France (AAAF).

Parachute Materials Technology is the subject of the one-day seminar scheduled for 8 June. The seminar, presented by a panel of internationally recognized speakers, will be divided into two sessions. The morning sessions will focus on advances in methods for modeling and characterizing woven textile performance, including dynamic stress-strain characteristics during high-speed inflation and the effects of aging and environmental exposure. The afternoon will be devoted to a review of recent developments in high-performance materials, bonding and joining methods, and test techniques. The seminar includes a tour of the CAP, the French Paratroopers Research Center. The faculty includes:

• Prof. Chris Pastore, Philadelphia College of Textiles and Science
• Profs. Sam Hudson and Tim Clapp, North Carolina State University
• M. Christophe Mondage, French Paratroopers Research Center

The three-day conference begins on 9 June and includes over 60 papers from around the world. Plenary session presentations include:

• Evolution of the Ringsail Parachute (Delurgio)
• Mars Pathfinder Parachute System Performance (Witkowski)
• Parachute Opening Shock (Wolf)
• An Overview of the X-38 Prototype Crew Return Vehicle (Stein, Machin, and Muratore)
• Evaluation of the Ariane-5 Booster Recovery System (Bos and Offerman)
• The Atmospheric Re-entry Demonstrator (Leveugle and Reinhard)
• Session topics include Design and Development, Simulation, Systems, Gliding Parachutes, Aerodynamics, and Testing.

Activities surrounding the seminar and conference include a reception at the Toulouse capitol hosted by the mayor, and an awards banquet at the magnificent Hotel Dieu alongside the historic Pont Neuf (pictured above).

Registration information is available on the conference web site at http://www.engr.uconn.edu/~adstc or by contacting the conference chairman, Dr. Dean Jorgensen, at 860-528-0092, x213 or djorgensen@pioneeraero.com.