1、Lessons Learned Entry: 0638Lesson Info:a71 Lesson Number: 0638a71 Lesson Date: 1996-07-31a71 Submitting Organization: HQa71 Submitted by: Claude SmithSubject: DC-XA Clipper Graham Mishap Investigation Board Report Description of Driving Event: Photo of DC-XA with flames engulfing the lower portion o
2、f the craftOn July 31, 1996, at 13:15 MDT, NASA successfully launched and flew the Clipper Graham, DC-XA, vehicle for the fourth time. Following an uneventful takeoff, the Clipper Graham flew for 104 seconds reaching an altitude of 4100 feet and traveling 2800 feet up range before returning to the c
3、oncrete landing pad, successfully completing all test objectives.Ninety-eight seconds into the flight and at approximately 400 feet, the DC-XA computer commanded landing gear deployment. Over the next five seconds, three of the four legs successfully deployed. Four seconds after the gear deploy comm
4、and, landing gears 1 and 4 deployed within one-tenth of a second of each other. Then, landing gear 3 deployed one full second later. Landing gear 2 never deployed. Descending from 400 feet, the spacecraft landed safely on three of its four legs. When the weight-on-gear indication was received at 13:
5、17:27 MDT, the engines terminated as programmed and at 13:17:29 MDT, the vehicle toppled toward the position of landing gear 2.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Upon impact, the vehicle was destroyed in a series of three explosions spac
6、ed over the next 90 seconds. The first explosion at 13:17:30 MDT ignited the composite shell and the avionics rack. At 13:17:40 MDT, ten seconds after the initial explosion, the fire suppression system began dispensing water. A second explosion of liquid oxygen from the aluminum-lithium tank rocked
7、the mishap scene ten seconds after the first explosion. The fire suppression system shut down after the tank ran out of water but before complete fire extinction. Approximately 1 minute after the second explosion, the hydrogen tank exploded. This third explosion scattered the composite material from
8、 the aeroshell and hydrogen tank over the mishap scene.The Clipper Graham DC-XA vehicle was totally destroyed by ground impact and ensuing explosions and fires. The vehicle struck the ground on the corner of the vehicle at the undeployed landing gear 2. The upper two-thirds of the composite aeroshel
9、l, aluminum-lithium liquid oxygen tank, composite liquid hydrogen tank, composite intertank, avionics, nose cone, and parachute recovery systems were destroyed during the three explosions and ensuing fire. Parts of landing gears 3 and 4 were melted in the fire as well. Landing gears 2 and 1 mechanis
10、ms were damaged during the tip over and ensuing fires and explosions. The lower one-third of the vehicle aeroshell, containing the throttlable RL-10 engines and the auxiliary propulsion system, were charred and covered with soot. The RL-10 engines and auxiliary propulsion system were the only items
11、appearing to be recoverable.Videotapes of the flight and still photographs of the wreckage showed that landing gear 2 failed to deploy. This failure was also evident in the helium supply pressure time history. NASA analysts at Kennedy Space Center (KSC) performed a helium pressure decay study and sh
12、owed the loss of helium to be greater than expected after the start of gear deployment. Also, they estimated the diameter of the hole in the pneumatic system that would be required to achieve the observed helium pressure decay rate. This analysis indicated that if the brake line was disconnected dur
13、ing landing gear deployment, the decay rate would be equivalent to that which was recorded by the flight instrumentation. Also, post mishap inspection found the landing gear to be stowed and the pneumatic brake line not connected.Therefore, the primary cause of the vehicle mishap was that the brake
14、line on the helium pneumatic system for landing gear 2 was not connected. This unconnected brake line prevented the brake mechanism from being pressurized to release the brake and resulted in landing gear 2 not extending. The vehicle became unstable upon landing, toppled onto its side, exploded, and
15、 burned.Lesson(s) Learned: Contributing causes of the mishap were as follows:1. Design of the system for gear stowage required technicians to break the integrity of the helium brake line after integrity had been already verified. No other check was conducted to reverify the integrity of the system a
16、fter disconnection and reconnection of the line was completed.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2. Landing gear stowage was never identified as a critical process. No special steps were taken to ensure the readiness of this system for f
17、light.3. During the gear stowage process, there was no record of checking off steps or evidence of cross-checking of work by another person.4. Distraction or interruption of the mechanical technician during gear stowage operations may have contributed to the nonconnection of the brake line.The desig
18、n of the DC-XA vehicle and operational procedures were driven by rapid development and low cost. Accordingly, a minimum number of personnel were involved in operations. Also, design was single string, and there was just one flight test vehicle. There was strong reliance on good people but not a lot
19、of margin for human error afforded by the vehicle preparation process. Rapid Prototyping Guidelines or implementation thereof for the DC-XA may have gone too far in the direction of sacrificing quality and reliability. This rapid prototyping concept should be revisited from an operations perspective
20、.Recommendation(s): 1. Critical procedural steps should be identified during systems design and flagged as critical in vehicle operations procedures. Then, independent verification of all critical steps should be performed during execution of operations procedures.2. NASA should perform a handover d
21、esign review when any program is transferred from another agency.3. The “rapid prototyping“ philosophy was cited as the rationale for employing minimal written procedures. The concept should be revisited from an operations perspective.4. Prelaunch processing documentation and data tapes should be ke
22、pt as historical records for each flight at least until a mission is completed and degree of mission success is understood.5. Up-to-date hazardous materials information should be supplied to the appropriate hazardous response agencies at the start of any flight program in the future.Evidence of Recu
23、rrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71 Aeronautics ResearchProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional Key Phrase(s): a71 Aircrafta71 Emergency Preparednessa7
24、1 Flight Operationsa71 Flight Equipmenta71 Ground Operationsa71 Hardwarea71 Hazardous/Toxic Waste/Materialsa71 Launch Processa71 Logisticsa71 Mishap Reportinga71 Spacecrafta71 Test Articlea71 Test & Verificationa71 TransportationMishap Report References: DC-XA Clipper Graham Mishap Investigation Board ReportAdditional Info: Approval Info: a71 Approval Date: 1999-11-02a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
