In article ,
Michael Smith wrote:
I think it would be better if the airbag was attached to the
spacecraft. Wasn't this the issue with John Glenn's heat shield? That
instruments suggested that an airbag had dislodged the heatshield?

Correct -- *after* reentry, the Mercury heatshield dropped down a few feet
on a fabric skirt, forming an airbag to reduce impact loads, mostly to
cover the case of an emergency land touchdown. (Apollo had its couches
mounted on an internal shock-absorber system for that case, while the
Gemini procedure for a land touchdown was to eject.)

The airbag would only have to be a metre deep or so to significantly
reduce the G load at impact. And they are light, and they have been
known to work: Pathfinder.

Unfortunately, contrary to popular mythology, the Pathfinder airbag system
turned out to be complex and quite heavy -- considerably heavier than
landing rockets. Crushable solids like aluminum honeycomb or balsawood
are actually rather lighter than real airbag systems. This was known in
the early 60s, but it keeps getting forgotten.

Even the Mercury airbag ended up much more complex than people had
expected, quite a difficult design and development problem. It needed
reinforcing straps to keep the skirt from tearing due to side loads, an
internal cable network to prevent the heatshield from banging around too
vigorously due to wave action after water landing, a layer of honeycomb to
protect the hull against the possibility of being hit by a heatshield edge
during touchdown, and crushable honeycomb under the couch for extra safety
margin. And a land touchdown with it would have been a traumatic event,
probably involving severe tumbling. It is very difficult to make such a
system cope gracefully with land touchdown with a substantial horizontal
velocity, i.e. due to wind; that's why it was rejected for Apollo (which
did originally have a land-touchdown requirement).

Mars Pathfinder used airbags not because they're a great landing system
(although there were hopes of that early on), but because they permit safe
unguided landings on very rough terrain. If, that is, the application is
an unmanned probe, which can simply cover itself in airbags and bounce and
roll for long distances before stopping. (Pathfinder bounced and rolled
for 2.5 minutes, covering about a kilometer. The first bounce was 18G.)
Oh, and further complexity had to be added to it for the MERs, because it
doesn't handle winds well, and the MER landings are at a different time of
day when winds are expected to be higher.
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