Space Forum / Space Flight / September 2005

Cray74@gmail.com wrote:

You can slow down with a non-throttled prop system, which can be a
relatively cheap, high Isp, high thrust (low gravity loss), low dry
mass solid rocket, but which would obviously not be capable of a soft
landing.  With airbags, the only throttle you need are some pyros to
cut the solids loose at the right time, just like MPF and MER.

An all-propulsive landing could use a single bi-prop system all the way
down, like Apollo.  But there may be an advantage even there to having
two prop systems for two different problems, a) slow down from orbital
velocity as quickly as possible, and b) soft land as softly as
possible.  E.g. a) a high-thrust solid, and b) a lower thrust mono-prop
throttled liquid system.

Someone thought that you have to use a parachute with airbags, which is
not the case.  However the parachute on MPF and MER did have a function
besides just drag that would have to be replaced for a similar
architecture to work on the Moon: attitude control.  The parachute kept
the rockets pointed the right way.  A simple, relatively inexpensive
cold-gas system would do the trick.  (I wouldn't use spin, since you'd
like to do a gravity turn to get in the right altitude/velocity box.)

Heavy, yes.  However I'd maintain that airbags and a solid prop system
are simpler than a throttled liquid system designed to do the whole
job, in the senses of system complexity and testability.  Probably cost
too (though that's gut feel -- I haven't done the cost comparison).

The complexity of maintaining a critical two-way comm link for landing
targeting as Henry suggested should not be underestimated.  Maintaining
comm during the significant dynamics of a landing event is definitely
non-trivial.  Plus you need good, fast cameras with some sort of motion
compensation for vibration, ability to work through the rocket plumes
with corrections for distortion (what is the index of refraction of a
rocket plume?), low latency data flow including compression, etc.  More
complexity ...

Another second-order point is that the RADAR you need for timing the
initiation and deployment of the solids, requiring just crude
altimetry, is simpler than the high-precision velocimeter/altimeter you
need for a soft landing.

mark

A rocket engine would be needed to slow the descent enough to use the
airbags.  For using a lunar rover, exact landing site is not an issue.
I simply wonder if airbags are simple enough that it would make sense
to use them when you still have to use a rocket to initially slow the
descent.
On the moon, do we have enough control over the landing to keep
ourselves from hitting a boulder on descent if we use rockets alone?

Turns out that Luna 9 used a combo rocket/airbag for landing on the
moon.