NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

Unrestrained Separation Nuts May Damage or Hinder Flight Hardware


Description of Driving Event:

In a complex rover separation event after a Mars landing, pyrotechnic devices are used to release the Mars Exploration Rover (MER) Warm Electronics Box (WEB) from the lander basepetal. If the separation does not occur, the rover does not release and cannot escape the lander. The WEB design called for each separation NUT to be free and un-restrained after release, without considering any possible impact damage. The 8 “sep nuts” on each rover were fired successfully during system test. Each was rapidly expelled from the basepetal to strike a rock protection cup located 7 mm beneath the sep NUT.

In testing of both MER1 and MER2, the impact resulted in severe damage to the PYRO cables attached to the bottom of the nuts. The PYRO cabling was damaged sufficiently to cause multiple shorts to chassis that could have over-stressed the PYRO system electronics. Redesign efforts to apply padding to the sep NUT backshells were complicated by the configuration of the adjacent hardware.

Lessons Learned:

Unrestrained pyrotechnic separation nuts may cause mission-critical failures by impacting and damaging PYRO cabling, or by rebounding to interfere with spacecraft mechanisms.


  1. Design release mechanisms to retain spent pyrotechnic separation nuts.
  2. Whether the separation nuts are restrained or unrestrained, take measures to ensure that the released nuts cannot impact and damage adjacent hardware/wiring or interfere with spacecraft deployment or separation.

refer to [D] description


  1. Jet Propulsion Laboratory Problem/Failure Report No. Z78969, January 9, 2003.\PFOCPROD\PROJECTS\WEBALL\ALL_PVB.INI~s3
  2. ”WEB Separation PYRO Cable Protection Redesign Test Results,” JPL document D-25477, February 3, 2003.

Source: NASA Public Lessons Learned Database -|Pyro|nut

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