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Second Generation Airbag Landing System for the Orion Crew Module

Lauren S. Shook1, Richard B. Timmers2, and Jon Hinkle3 ILC Dover LP, Frederica, DE, 19946

In support of NASA Langley Research Center’s (LaRC) second phase of development of the Orion land landing system, ILC Dover has designed and built an impact attenuating airbag landing system. This system incorporates lessons learned from the first generation design and testing. Full-scale drop tests using a test article representative of the Orion Crew Module (CM) were performed to investigate the landing system dynamics. The ILC Dover second generation design used six impact attenuation airbag sets placed around the base of the CM. Each airbag set consisted of an outer main bag and an inner anti-bottoming (AB) bag. The primary design change from the first generation airbag landing system was a shift to a design where the airbag set is not directly attached to the crew module. Instead, the bags are held in position by a webbing restraint assembly. This paper will present the overall design of ILC Dover’s second generation impact attenuating airbag system and a summary of preliminary drop test results.

Nomenclature

AB = Anti-Bottoming Bag CM = Crew Module LandIR = Landing Impact Research Facility

I. Introduction

ollowing the retirement of the Space Shuttle fleet, the Orion spacecraft system will be the next manned space transportation vehicle used by NASA. The Orion crew exploration vehicle will return to a booster-type design,

with a crew and service module. Upon return to Earth, the Orion Crew Module (CM) will re-enter the atmosphere, be slowed by a parachute system, and land. An impact attenuating airbag system was under consideration for CM land and water landings. In the event a land landing was selected, an airbag system was evaluated for consideration under both nominal and contingency scenarios.

In support of NASA Langley Research Center’s (LaRC) second phase of development of the Orion land landing system, ILC Dover has designed and built such an impact attenuating airbag landing system. This system incorporates lessons learned from the first generation design and testing. Full-scale drop tests using a test article representative of the Orion crew module were utilized to investigate the landing system dynamics. The ILC Dover second generation design used six impact attenuation airbag sets placed around the base of the CM. Each airbag set consisted of an outer main bag and an inner anti-bottoming bag. The main bag provided the primary landing attenuation, while the AB bag provided a lesser attenuation contribution and a stable protective platform on which the vehicle rested after the landing event was complete. Figure 1 shows the second generation design.

1 Senior Design Engineer, One Moonwalker Road, Frederica, DE 19946, AIAA Member 2 Senior Analysis Engineer, One Moonwalker Road, Frederica, DE 19946, AIAA Member 3 Design Engineer, One Moonwalker Road, Frederica, DE 19946, AIAA Member

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20th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar<BR>4 - 7 May 2009, Seattle, Washington

AIAA 2009-2989

Copyright © 2009 by ILC Dover, LP. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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Figure 1. ILC Dover Second Generation Airbag Landing System.

The primary design change from the first generation airbag landing system was a shift to a design where the

airbag set is not directly attached to the crew module. Instead, the bags are held in position by a webbing restraint assembly. This paper will present the overall design of ILC Dover’s second generation impact attenuating airbag system and a summary of preliminary drop test results.

II. Airbag Design

Airbag landing systems operate by converting the kinetic energy of an object into potential energy in the form of compressed gas. If the gas remains contained, the potential energy is converted back to kinetic energy as a rebound, with the process continuing until the energy is dissipated through various sources, such as internal vents. This type of contained gas airbag system was employed by ILC Dover for several Mars landing missions1, 2, and was appropriate for Mars landings given the unknown terrain conditions and the expectation of a significant horizontal velocity component. An alternative approach is to vent the gas after the airbag is first compressed, allowing the energy to be dissipated to the atmosphere. Venting prevents multiple rebounds of the landing system. If the landing conditions are appropriate and the possibility of rollover is low, only one face of the lander needs to be protected by airbags in a vented system; this results in a smaller overall airbag landing system than a contained gas airbag system. A vented landing system was selected for the first generation Orion airbag landing system based on the system requirements.

A. Concept of Operations The first generation Orion airbag system designed by ILC Dover3, 4, 5 used eight discrete airbag sets for

redundancy. A bag within a bag approach was used. In this design, the outer main bag provided the primary landing attenuation, while the inner non-vented anti-bottoming bag provided a lesser attenuation contribution and a stable platform for the vehicle to rest upon after the landing event was complete. The bags were inflated using an active inflation system, where bottled gas and a high flow regulator were used to fill the airbags to the required pressure. This type of system has several advantages, including the ability to maintain airbag pressure against leakage, to accurately control the inflation rate, and to allow the flexibility to control inflation pressures based on landing conditions. The first generation airbags were vented using actively controlled vents. This method uses a pyrotechnic cutter to sever a vent cord at a predetermined internal bag pressure.

For the second generation design, many of the overall airbag design concepts (multiple airbags, active inflation system, active venting) have remained the same as they were in the first generation airbag design. However, the airbags’ attachment configuration has changed in that both the main and AB bags are no longer mounted to the CM structure with a clamping ring. Instead, the second generation airbags are attached to the CM through the use of a webbing restraint net. This new attachment configuration offers several advantages over the first generation clamp ring design, one of which is that the AB bag can now be a continuous bag within the main bag. In addition, it was specified by NASA that for the second generation design the airbag system should consist of only six airbag assemblies to more efficiently interface with the CM structure.

Prior to use, the airbags are stowed between the heat shield and the CM’s primary structure. The airbags are deployed and inflated after the heat shield is jettisoned. A pictorial representation of the airbag portion of the Orion CM landing sequence is shown in Figure 2.

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Figure 2. Concept of Operations of the Airbag Landing Sequence of the Orion CM. Analysis conducted to date on the ILC Dover design shows that the airbags are capable of meeting the Orion landing requirements, not only under normal conditions, but also under adverse off-nominal conditions such as parachute failure, high horizontal winds, and unfavorable vehicle/ground angle combinations.

B. Landing System Overview The choice of airbag shape and size was previously studied under the first generation design effort. It was

determined that in order to safely operate under a two-fault tolerant condition, a multi-bag configuration must be used for the landing system. As seen in the concept of operations, the crew module descends on parachutes. Once the crew module reaches the appropriate altitude and descent rate, the heat shield is jettisoned. After ejection of the heat shield, the underside of the vehicle is defined by the spherical bottom of the crew module primary structure.

Trade studies showed that a ring of airbag sets, with each set consisting of a main and an AB bag, was the preferred airbag configuration. This shape was found to provide a gradual deceleration onset rate, to interface well with the underside of the vehicle, and had the advantage of building on the heritage of previous ILC programs. Drop testing of the first generation airbags validated the dynamic predictions of both the preliminary analysis and the more extensive modeling performed using LS-DYNA.

While the fundamental concept of using multiple vented airbags to attenuate the landing of a vehicle was validated, the airbag attachment method and seam loading of the first generation design were identified as areas needing improvement. Based on these lessons, a revised design approach was planned for the second generation design. This was to replace a single structural restraint layer of coated high strength fabric by separating out its functions into two or three different layers of material. This can be accomplished using several different methods. Inflation gas can be retained by having a separate bladder layer or by coating the restraint fabric. Inflation and friction loads can be supported by a fabric restraint layer (coated or uncoated) or can be transferred into an external webbing net that surrounds the restraint layer.

To address both the airbag attachment and the loading experienced by the airbag seams in the first generation design, the addition of a webbing net assembly along with a restraint layer of coated high strength fabric was chosen for the second generation design. The two-layer webbing net/restraint approach has proven on past applications to have the following advantages: require less packing volume, be easier to pack, be lighter weight, and provide higher load carrying capability. For the second generation airbag landing system, this approach required design of the webbing net, the main airbag attachment to the vehicle, and the restraint layer. Design of the attachment of the AB bag to the vehicle (from inside the main bag) was also needed.

A second generation airbag set consists of one AB bag within one main bag. The airbag sets are mounted at the bottom of the vehicle, in the space between the primary structure and the main heat shield. The bags are sized and located to provide end-to-end contact interference between adjacent bags. This allows the bags to bear against one another, substantially improving the stiffness of the system in response to sliding loads at landing.

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Each main airbag is a crescent-shaped airbag. This design retains the basic cylindrical shape of the first generation airbags, while allowing the airbag geometry to follow the curved geometry of the CM more closely. The operating pressure of each airbag varies based on its position on the CM. The variation in fill pressures between airbags was a result of a stability study conducted in LS-DYNA. A main airbag has two actively controlled vents. The location of the main bag vents changed between the first and second generation designs. The two vents in the first generation airbags were located on a section of each end cap. It was found during drop testing that in this location the vents could become blocked during the landing with high horizontal velocities, preventing the gas release. Therefore, the location was moved to prevent blockage. The initial selection of two vents, as opposed to a single equivalent area vent, also allows for redundancy should a single vent become blocked during the landing event. Figure 3 shows the main airbag with attached webbing restraint net.

Figure 3. Main Airbag.

An AB bag is a cylindrical airbag with end caps. The design differs from the first generation in that the AB bag

does not contain a relief valve. The AB relief valve was previously used to inflate the main bag. This was done by directly filling the main bag to a small amount of pressure before a commanded valve to the main bag was closed. After that point, only a valve directly into the AB bag was filled, and venting through the AB relief valve into the main bag was responsible for controlling the main bag pressure. Because both the main and AB bags have separate fill ports in the second generation design, there was no need for a relief valve in the AB bag. Figure 4 shows the AB airbag with attached webbing restraint net.

Figure 4. Anti-bottoming (AB) Airbag.

C. Webbing Restraint Design The main and AB webbing restraint assemblies both consist of Vectran webbing sewn together to form a net

around each airbag. Each restraint net is constructed from webbings in both the axial and hoop directions. The hoop webbings have sewn loops on each end that attach to the vehicle with a pin/bracket. The axial webbings are primarily sewn bands in which the webbing terminates back on itself. All of the webbings were sized based on either the circumferential run length around the air bag or the run length around the air bag between attachment

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locations. Once these dimensions were calculated, the length of each of the webbings was undersized by a set percentage. The effect of this undersizing is to force the restraint net to load before the airbag fabric. This places the majority of loading in the restraint net, thus reducing the loads in the airbag fabric. Figure 5 shows the design of the main bag webbing net.

Figure 5. Main Bag Webbing Net Design.

D. Vehicle Interface In the first generation design, the main and AB bags were attached to the vehicle using a clamping ring. For the

second generation airbags, the clamping ring was replaced by the webbing restraint assembly for two purposes. These were to attach each airbag set to the vehicle and to carry the impact loading. Each end of the webbing restraint net terminates with a loop that is passed through both the CM structural mounting surface and an attachment bracket on the opposite side. A pin placed through the loop of webbing is used along with the bracket to retain the webbing net. In the case of the AB webbing net, the webbings must first pass through the wall of the main bag before attaching to the vehicle. Figure 6 shows the method for attaching the airbag webbing net to the vehicle.

Figure 6. Attachment of Webbing Restraint Net.

E. Webbing Load Measurement During the design phase, it was determined that the loads on the webbing restraint net needed to be measured

during drop testing in order to validate the LS-DYNA model of the airbag assembly. An indirect in-line tension

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load measurement method was found to be the best method of measuring these loads. This type of sensor minimizes the instrument profile and is used in the automotive industry to measure seat belt loads in impact tests. The devices selected for the second generation test article were supplied by NovaTech and are pictured in Figure 7.

Figure 7. In-Line Tension Load Cell (NovaTech).

Twelve load cell units were attached to the main bag webbings near the vehicle interface. These were only

attached to the webbings on one of the leading main bags. Since only six load cells could have been used to record the forces on the leading end of a bag, the option existed to move the additional load cells to other bag locations to record areas of interest. Figure 8 shows a single load cell installed on a main bag webbing restraint net.

Figure 8. Load Cell Installed on Webbing Net.

III. Airbag Drop Testing

The second generation airbag system was drop tested at the Landing Impact Research Facility (LandIR) at NASA Langley as part of the Orion Landing System Advanced Development Project. The facility is shown in Figure 9.

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Figure 9. Landing Impact Research Facility (LandIR) at NASA Langley.

The six airbag sets were attached around the base of a test article which represented the full-scale CEV Crew

Module. The test article, along with the airbag system and instrumentation, weighed 15,990 lbs. The airbag system installed on the test article is shown in Figure 10.

Figure 10. Second Generation Airbag System Installed on Test Article.

The second generation airbag system was tested under various drop test conditions. The landing conditions were

varied based on the pitch angle of the test article at ground impact and combinations of selected horizontal and vertical velocities. The vertical velocity was 25 ft/sec for all of the drop tests. The horizontal velocities tested were 0, 20 and 40 ft/sec. A more complete discussion of the drop testing and post-test analysis is available in Ref. 6 and 7. A drop test sequence is shown in Figure 11.

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Figure 11. Drop Testing of Second Generation Airbag System.

IV. Conclusion

This report presents the design of a second generation airbag landing system for the Orion Crew Module. This included the design of main and AB airbags, webbing restraint assemblies, and airbag attachment methods. Seven drop tests were conducted using the ILC Dover second generation airbag system. The airbag system performed successfully throughout the testing, even under the challenging 40 ft/sec drop conditions. The system met both x-axis and z-axis translational acceleration landing requirements for all cases. Changes from the first generation single structural restraint layer design to the second generation webbing net design did extremely well, addressing both the airbag attachment method and seam loading issues identified during earlier testing. Only minor damage to the airbags was seen during this phase, typically after repeated drops.

The ILC Dover second generation airbag development program has demonstrated a strong landing system design, building upon many of the concepts brought forward in the first generation airbag testing. Both testing and analysis have shown that the ILC Dover airbag landing system provides excellent performance under severe landing conditions, without resulting in a rollover of the vehicle or exceeding acceleration requirements. The information gathered under this project will provide a strong foundation for future work in this arena.

Acknowledgments

The authors would like to thank Jim Corliss, Keith Johnson, Richard Boitnott, and Steve Sieder at NASA Langley Research Center, as well as the technical staff at the LaRC LandIR facility. We would also like to thank the ILC Dover personnel who supported the on-site testing.

References

1Cadogan, D., Sandy, C., Grahne, M., “Development and Evaluation of the Mars Pathfinder Inflatable Airbag Landing

System”, 49th International Astronautical Congress Paper No. IAF-98-I.6.02, September 28 - October 2, 1998, Melbourne, Australia, also see: Acta Astronautica Vol. 50, No. 10, pp. 633-640, 2002.

2Stein, J., Sandy, C., “Recent Developments in Inflatable Airbag Impact Attenuation Systems for Mars Exploration” Paper No. AAAF-061, Arcachon, France, 2003.

3ILC Dover, LP, “Airbag Landing System Generation 1 Final Report: Design, Analysis, and Testing“, NASA Contractor Report, November 2007.

4Smith, T., Sandy, C., Wilson, D., and Willey, C., “CEV Airbag Landing System Design,” 19th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar, AIAA, Williamsburg, VA, 2007.

5Welch, J., McKinney, J., and Wang, J., “CEV Airbag Landing System Modeling & Simulation,” 19th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar, AIAA, Williamsburg, VA, 2007.

6Timmers, R., Hardy, R., and Welch, J., “Modeling and Simulation of the Second-Generation Orion Crew Module Air Bag Landing System,” 20th AIAA Aerodynamic Decelerator Systems Technology Conference and Seminar, AIAA, Seattle, WA, 2009, (to be published).

7Timmers, R. and Hardy, R., “Second Generation of the Orion Crew Module Air Bag Landing System Modeling and Testing”, NASA Contractor Report, February 2009.