1、Lessons Learned Entry: 1712Lesson Info:a71 Lesson Number: 1712a71 Lesson Date: 2005-12-16a71 Submitting Organization: JPLa71 Submitted by: David Oberhettingera71 POC Name: Robert Manninga71 POC Email: Robert.M.Manningjpl.nasa.gova71 POC Phone: 818-393-7815Subject: If You Dont Understand an Environme
2、nt, Provide Well-Margined Capabilities to Encompass the Worst Case Abstract: Mars Exploration Rover designers responded to a high level of uncertainty regarding Martian winds by providing a set of small, sideways-pointing rockets and adding a capability to directly sense horizontal motion. This rede
3、sign to reduce the risk that the lander would graze Martian terrain during the landing was made only 1.5 years before launch. Assure rigorous assessment of environmental risks, and design to counter the critical risks with substantial margin.Description of Driving Event: The entry and descent of Mar
4、s Exploration Rover (MER), and its landing on Mars, were accomplished with limited knowledge of local environmental conditions. Reference (1) discusses our poor insight into Mars atmospheric density, and touts the flexible MER flight system and mission design features that successfully responded to
5、new, crucial density data received during the latter stages of Mars Encounter. MER designers faced a similar knowledge deficit in regard to Martian wind velocity and effects. The estimate of Martian winds used in the July 1997 entry, descent, and landing (EDL) of Mars Pathfinder was based upon the a
6、ltitude-specific historical record of daily winds and atmospheric pressure at Kennedy Space Center in Florida. This Earth data was considered to be conservative, and it was the most comprehensive dataset available. Seven years later, MER EDL planning benefited from new models of the local effects of
7、 known Mars terrain features on various postulated Martian Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-wind conditions. However, these models had never been validated with actual Martian weather. Figure 1 is a color image of a ground test that sh
8、ows the MER lander suspended by wires from a high bay, with three large propulsion plumes directed downward, and one small propulsion plume shooting out to the side. Clicking on this still image brings up a short video of the same subject from which the still was taken. In the video, the downward RA
9、D rocket burn continues for about one minute. Perhaps halfway through this burn, three sideways TIRS rocket burns initiate in sequenceFigure 1 - RAD and TIRS (small horizontal plume) test stand firing. + View VideoFigure 2 is a color still, taken from a video simulation of the MER Entry, Descent, an
10、d Landing (EDL) sequence, in which the MER lander approaches the surface of Mars under parachute. Clicking on this still image brings up a short video of the same subject from which the still was taken. The full video shows the airbags deploying beneath the lander as it descents through the Martian
11、atmosphere. Before the bridle is cut (allowing the lander to strike the ground and bounce upward), the simulation shows the RAD rockets fire. A careful viewer would also see the shadow of the parachute drifting from left to right during the RAD burn, and then the drift slowingFigure 2 - Animation de
12、picting RAD (and TIRS) firing to decelerate Spirit and correct drift. + View VideoFigure 3 is a grayscale image that resembles a pockmarked lunar surface. A flat surface is punctuated by two large craters, four medium sized craters, and myriad small craters making a slightly textured surface. The la
13、rgest crater is circled by a dark shadow, and dark striations radiate across the surface. The ground appears uneven, but at the altitude the image was taken there is no evidence of rocky groundFigure 3 - Image of the Gusev Crater taken by the DIMES camera as the Spirit lander descended to Mars.If wi
14、nd shear in the lower Martian atmosphere were to tilt the descending lander from the vertical, the concern was that the firing of the Rocket Assisted Descent (RAD) rockets that slow its descent could propel it sideways and cause the lander to graze the terrain at a high lateral speed. Ground drop te
15、sting had demonstrated that the ability of the airbags to survive a rock strike at even a small horizontal velocity was marginal. To counteract such tilt, the MER design evolved early to include a Transverse Impulse Rocket System (TIRS), composed of three additional small rockets mounted on the land
16、er backshell. The Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-three TIRS rockets were designed to fire in any combination needed to right the backshell and reduce the landers horizontal motion during RAD rocket burn (Figure 1). To control TIRS an
17、d to also cancel wind-induced drift, a decision was made only 1.5 years before launch to add a Descent Image Motion Estimating System (DIMES). This camera took a series of three photos that determined the landers horizontal speed in the seconds prior to landing. TIRS/DIMES provided a capability to c
18、ompensate for relatively mild winds, and its criticality to the success of the planned MER landings could not be proven. The MER Opportunity lander did not encounter windy conditions at its Meridiani Planum landing site, so Opportunity did not fire the newly designed TIRS system. However, the MER Sp
19、irit landers DIMES detected strong winds in the Gusev Crater and fired the TIRS. A video simulation (Figure 2), based on actual MER data, looks north and shows the TIRS rockets canceling the ground drift from a westerly wind. Without TIRS and DIMES working in tandem, the lateral motion across the ru
20、gged incline of the Gusev Crater (Figure 3) would likely have torn the airbags and threatened the mission. References: (1) “Provide In-flight Capability to Modify Mission Plans During All Operations,“ Lessons Learned No. 1480, NASA Engineering Network, July 13, 2004. (2) Robert M. Manning, “MER Proj
21、ect: Stealing Success from the Jaws of Failure,“ Video Production No. AVC-2005-200, September 23, 2005. Lesson(s) Learned: Where tests/analyses indicate a high level of uncertainty regarding mission-critical environmental conditions and equipment capabilities, re-design (even fairly late in system d
22、evelopment) may be justified to compensate for conditions exceeding initial design requirements.Recommendation(s): Mitigate the overall risk of planetary missions through (1) rigorous assessment of the major known environmental risk contributors and (2) provision of design capabilities to counter cr
23、itical environmental risks at the upper bounds of their probable severity, with substantial margin.Evidence of Recurrence Control Effectiveness: JPL Preventive Action Notice (PAN) Z88036 was closed on 3/14/06. The recommendation is incorporated into the “Design, Verification/Validation & Ops Princip
24、les for Flight Systems (Design Principles) (D-17868), Rev. 2,“ JPL DocID: 43913, Paragraphs “4.1.3 Design Robustness“ and “4.1.4 Design Margins“Documents Related to Lesson: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-“Design, Verification/Validat
25、ion & Ops Principles for Flight Systems (Design Principles),“ (JPL Document D-17868), Rev. 2, JPL DocID 43913, March 3, 2003, Para. 4.1.3 (Design Robustness)Mission Directorate(s): a71 Sciencea71 Aeronautics ResearchAdditional Key Phrase(s): a71 Additional Categories.Environmenta71 Additional Catego
26、ries.Flight Equipmenta71 Additional Categories.Flight Operationsa71 Additional Categories.Hardwarea71 Additional Categories.Payloadsa71 Additional Categories.Program and Project Managementa71 Additional Categories.Risk Management/Assessmenta71 Additional Categories.SpacecraftAdditional Info: a71 Project: Mars Exploration Rovera71 Year of Occurrence: 2002Approval Info: a71 Approval Date: 2006-03-15a71 Approval Name: dkruhma71 Approval Organization: HQProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
