The NASA Exploration program is investigating the merits of water and land landings for the Crew Exploration Vehicle (CEV). For land landings, four options are under investigation: (1) retrorockets, which fire and slow the CEV before landing; (2) deployable crushable material, which deploys just before and crushes during landing, thus absorbing energy; (3) airbags; and (4) deployable legs, which deploy before landing and contain material that absorbs energy during landing. The NASA Glenn Research Center investigated the effectiveness of the deployable leg concept. Structural models of the deployable leg concept were integrated with the Crew Model (CM), and computational simulations were performed to determine vehicle and component loadings, acceleration levels, and astronaut risk.
The simulations needed to simulate the complex transient dynamic behavior of the CM and the attached deployable legs impacting a landing surface. The deployable leg-CM model consists of a collection of structural parts. The main portion of the vehicle, which consists of the pressure vessel, associated structure, and internal components, is modeled as a rigid part having inertia properties equivalent to the Design Analysis Cycle II CM design. Since this part is modeled as rigid, it exhibits no structural deformation and no structural loadings are computed for it. Although the actual landing surface will be some form of soil that will deform on impact and absorb energy, for this study, the landing surface was assumed to not deform or absorb energy from landing, so it also was modeled as a rigid part. The landing surface was made large enough so that the vehicle would not leave the surface during the range of landing simulations performed in this study.
CM model with deployable legs.
Four sets of deployable legs are attached to the pressure vessel and all are centered about the direction of the horizontal landing velocity. The two front sets are 45° apart, and the two rear sets are 135° apart. The locations were determined from the design of the vehicle underbody and the availability of space and attachment points. Having the two front sets closer together is advantageous for horizontal landing velocities, and having the two rear sets more spread out adds to overall vehicle stability.
The primary and secondary landing leg designs are fundamentally similar to those used for the Apollo Lunar Module: the primary legs are designed for compression only, and the secondary legs are designed for both compression and tension. Both the primary and secondary landing legs have an outer housing that contains crushable material which is designed to provide a constant force regardless of leg displacement. The appropriate crushable material could provide constant deceleration to the CEV and could maintain acceleration levels at or below design criteria. However, the material can only be designed for a single design condition, and performance at off-design conditions will be less than optimal. The length of the legs, the crushable force, and the stoke length were altered several times until an optimal design was obtained.
| Pitch, deg |
Maximum acceleration, g | |||
| Horizontal velocity, ft/sec |
||||
|---|---|---|---|---|
| 0 | 20 | 40 | 60 | |
| -15 (nose down) | 4 | 8 | 8 | 10 |
| 0 | 9 | 8 | 8 | 8 |
| +15 (heel down) | 8 | 7 | 9 | 6 |
| aMaximum of x, y, or z body-fixed position. | ||||
The table depicts the overall effectiveness of the deployable legs, summarizing the maximum accelerations in the x- and y-directions for each of the 12 load cases. The maximum acceleration levels were fairly constant regardless of the loading conditions. For large horizontal landing velocities, the maximum acceleration did not exceed 10g; and for zero horizontal velocity, the maximum acceleration was 9g. Pitch angle also had only a limited effect on the acceleration levels. The maximum acceleration was 10g for a nose-down landing and 9g for a heel-down landing. The results show that the deployable legs effectively accommodated the 12 landing conditions.
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