QUESTION:
How did you determine the exact air pressure of the air bags for inflation?
Also, what type of gases will be used?  
What were some of the variables you needed to take into consideration when 
determining pressure differences?

ANSWER from Robert Manning on January 31, 1997:
Those are very good questions. 

That question turned out to be big challenge for us. At first, we visualized 
our landing system as an "impact attenuator"; something that would protect the
lander (and rover) from a vertical fall from as high as 55 meters above the
Martian ground. With this view, the airbags had to be pressurized so that the
lander would not make contact with a half meter high rock when impacting the
ground vertically at 20 meters per second. This means that the deceleration
"stroke" (the amount the airbags get squished on impact) had to leave one half
meter remaining so a half meter tall big rock wouldn't make contact with the
lander's structure. For the size of the bags we had originally designed (four
interconneted bags, one on each of the four petals with three large lobes on
each bag), it turned out that we needed about two and a half pounds per square
inch (psi) of pressure (17 kilo pascals in metric).

It wasn't too long that we determined that the dynamics of the rocket 
firing and bridle cutting seconds before impact made for a very 
different kind of landing.  Instead of hitting the ground with a high 
VERTICAL velocity, it was more likely that we would hit the ground 
with a much slower vertical velocity (more like 10 meters per second) 
but with a much higher HORIZONTAL velocity (more like 25 meters per second). 
This means that our airbags had to perform much more like aircraft landing 
gear tires! Because of this news (and lots of airbag drop tests inside a 
huge vacuum chamber), we redesigned the bags to add many more layers of 
rock "abrasion" fabric (of course this made them a lot heavier), and we 
changed the shape and size (to reduce the amount of mass that we just added) 
and reduced the pressure inside to only 1 psi (6.8 k pascals). That is why 
the final bag design now has six lobes per bag rather than three in the 
original design you may sometimes see in the pictures.

By the way, the gas used to inflate the bags is really rocket exhaust! 
It comes out of three gas generators mounted outside each of the side 
petals. They inflate the 50 cubic meters of bag volume in about a quarter 
second (!) and they continue to provide pressure for about a minute 
afterward. The gas is yucky stuff made out of nitrogen, carbon, water, 
and lots of nasty chemicals (acids) you wouldn't want to breath. It 
mostly all stays inside the bags as soot. The inside of the bags get 
real dirty! When we occasionally have to climb inside a "fired" bag, we 
have to use a respirator.