QUESTION:
Why did you chose to use pyrotechnic devices? Are they very
simple/reliable/light?

ANSWER from Rob Manning:
Pyrotechnically activated devices have been in use on space vehicles since
the beginning of the space age, especially on launch vehicles. They are
reasonably lightweight, simple and remarkably reliable. There are drawbacks,
most notably the "one-shot" nature of these devices and the fact that they
pose potential hazards to those who handle them. On Mars Pathfinder, we
require that 42 of these devices (including several varieties) work in order
for the entry, descent and landing process to be successful. That may sound
like a lot, but it is less than half the number required for a typical
expendable launch vehicle (rocket).

On Mars Pathfinder, we used two sizes of pyro cable cutters which are used
to disconnect electrical and mechanical cables and tubing between the big
pieces of the spacecraft (cruise stage, backshell, heatshield, lander and
rover) just prior to physical separation. We also use pyro separation nuts
to disconnect bolts that hold some of these pieces together. We also utilize
pyrotechnically initiated thermal batteries that are used during the entry
descent and landing process to provide electrical power to other pyro
squibs. Of course the parachute motor, the three solid rocket motors and
the three airbag gas generators (an unusual form of rocket motor) must all
be electrically initiated at just the right time by sending electric current
into squibs (called "NSI's" or NASA standard initiators). There are two
factors that primarily govern the system reliability of pyrotechnic
applications; the reliability of the electrical system which delivers
current to the NSI and the manner in which the device is used (the NSI's
will fire if current is delivered to them - what happens after is up to the
spacecraft designer). We were very fortunate to have responsive pyro device
manufacturers and experienced engineers at JPL who could work the design and
test details for all these pyro applications. We also extensively tested
every application in as close to the flight conditions we could.

--Rob Manning, Mars Pathfinder Chief Flight Engineer