magnetic field. As a result, F M decreases. When the total spring force is sufficient to overcome both the  magnetic force and the friction force, or 

  (2) f M S F F F+> the friction plate moves away from the solenoid and reengages the brake rotor. Once motion begins, the  friction plate moves quickly to the engaged position since F M decreases much faster than F S as the air  gap increases. In the case of the failed brake, the condition in equation 2 was not met even though the  solenoid voltage and therefore current was zero. F M was non-zero due to residual magnetism. 

The pull-in and drop-out motions of the brake are critical to the operation of the LPA and must be  assessed for force margin like any other critical depl oyment. The desire was to have a force capability of  at least twice the force needed to move the component s over all conditions of environment and operating  voltage. This equates to a minimum factor of safety (t he ratio of force capability to force required) of 2.0.  The flight brakes were measured to have a pull- in voltage <17 VDC, demonstrating operating margin  from the minimum flight input voltage of 24 VDC. The magnetic force is 20 N at 17 VDC and 37.6 N at 24  VDC compared to a maximum total spring force of 10.45 N and an analytically determined maximum  friction force of 0.013 N, surpassing the minimum desired force factor of safety for the pull-in deployment  by a large amount. Drop-out voltage was measur ed to be <10 VDC, ensuring adequate separation  between the pull-in and drop-out behavior. However no lower threshold on drop-out voltage was defined  to ensure operating margin above zero input voltage.  The lack of an appropriately specified lower bound  for this parameter was an oversight t hat hindered the discovery of brakes with insufficient force margin for  drop-out during acceptance testing. The minimum r equired force factor of safety for the drop-out  deployment was not proven during acc eptance testing. Since the failed LPA clearly did not have sufficient  margin for drop-out, the other flight LPAs, which were already installed on the flight Landers, were  evaluated. The pull-in and drop-out voltages of all L PA brakes were recorded during acceptance testing  and measured again after the failure (Table 1). Although the failed unit had the lowest measured value of  drop-out voltage, SN 003 and 008 also had very low values  for drop-out voltage, raising suspicions that  these two actuators might also hav e insufficient margin for drop-out. A proper specification of drop-out  voltage defining sufficient force margin was needed to properly evaluate the fli ght LPAs and determine if  rework was required. LPAs were switched between the tw o landers, placing the three flight LPAs with the  highest values of drop-out voltage on the first lander being prepared for launch. This allowed preparations  to continue on the most time-critical lander while the LPA assessment continued, maximizing the chance  that the failure could be addressed without impacting either launch. 

Table 1.  LPA Pull-In and Drop-Out Voltage Measurements 

Acceptance Test Values  (VDC) As Remeasured  (VDC)  SN 

(a)Failed Unit  Pull-In Drop-Out Pull-In Drop-Out Type of Unit  001 14.1 2.2 12.3 2.22 Qualification  002 Not avail. Not avail. 12.9 3.25 Flight  003 14.3 1.9 13.6 0.79 Flight  004 12.0 5.0 15.9 3.79 Flight  005 15.5 1.0 Not meas. Not meas. Testbed  006 14.07 2.6 14.1 2.40 Flight  007(a) 12.7 0.6 12.8 -0.3, 0.29 Flight  008 13.7 1.1 11.7 1.25 Flight  009 12.8 1.72 12.2 1.02 Testbed  010 15.1 0.85 Not meas. Not meas. Testbed  011 13.8 2.3 13.7 2.54 Flight Spare 

There is no way to take a direct measurement of force in the brake assembly on the fully assembled LPA,  therefore there is no way to directly verify the force margin for drop-out on each LPA. Since drop-out  voltage is the only easily acquired measurement on the fully assembled LPA, what was needed was a  relationship between drop-out voltage and force so that a minimum force factor of safety of 2.0 could be  guaranteed. A series of tests was performed on a spare brake assembly to determine how drop-out  227