A Dumb MER question

"Japperm" wrote in message . ..
From what I've read and heard about the MERs, they will stop working after a
few months only because Mars dust will cover top of the solar panels. If
this is the case, why didn't the MERs include a little automated air blower?
It seems like a simple solution. Does any knowledgeable person know the
answer?

Heres another dumb MER question. Id assume that airbag landing
approach will not be suitable for possible future human martian
landings.
So are further developments of such landing technique actually worth
it ? Why not perfect thruster-powered landings instead ? Because
otherwise if and when the need arises we'd have to start from a clean
sheet again.
Does it actually offer any significant mass savings, or are there less
potential failure modes or what ?

-kert

(Kaido Kert) wrote in
om:
Heres another dumb MER question. Id assume that airbag landing
approach will not be suitable for possible future human martian
landings.
So are further developments of such landing technique actually worth
it ? Why not perfect thruster-powered landings instead ? Because
otherwise if and when the need arises we'd have to start from a clean
sheet again.
Does it actually offer any significant mass savings, or are there less
potential failure modes or what ?

-kert


Airbags have a lowish upper mass limit they can handle, the rovers are
close to that limit. Humans would not like the 40G-bounces that go along
with this landing mode.

Why airbags? As compared to thruster-powered landings, you have lots less
failure modes, medium-big mass savings and huge complexity savings. Of
course the few failure modes that airbags do have tend to be lethal and
unpredictable. (things like landing on a sharp rock. the airbag system is
virtually unguided as to final approach, and totally unguided once bouncing
begins)

Why not perfect thruster-powered landings instead?
Been there, done that. We know exactly how to do these in a safe way,
including some backups to handle several of the possible failure modes,
etc. Its just that the whole package of thrusters, surface-sensors, control
mechanisms, etc.. makes for a very heavy package. Not something you can
launch on a mere Delta. The high complexity also leads to longish
development times, nasty budgets, etc.

In article ,
Marvin wrote:
Why airbags? As compared to thruster-powered landings, you have lots less
failure modes...

Not really. The airbag systems have a bunch of failure modes all their
own, including failure of their braking rockets. (Beagle 2 is the only
probe to ever attempt landing on Mars with no rocket braking, unless
possibly the Russians tried it way back when.) It's particularly bad for
the MER landing system, which relies on rocket thrust not just for braking
but also for canceling side motion due to wind, based on real-time
analysis of descent-camera images (!).

medium-big mass savings, and huge complexity savings.

Sorry, simply not true. That's why the MESUR project, which eventually
begat Mars Pathfinder, baselined airbags -- in the *expectation* that the
result would be a simple, robust, lightweight landing system. But that is
*NOT* how it turned out. The airbag systems are, in fact, heavy and
complex compared to rocket landing, and they also constrain you to land in
relatively low areas (they need relatively dense air for their parachutes).

The one real technical advantage airbags have is that in rough terrain,
they have a rather better chance of a safe landing, because they cope much
better with a touchdown point that isn't flat and level.

For MER, they also had a programmatic advantage in that the US currently
has no flight-qualified rocket-landing system, given the lingering worries
about MPL's hardware design. However, adapting the MP airbag system to
handle the heavy rovers was a lot harder than people first expected; the
MER team has been worked to death for the last 18 months or so. Quite
probably it would not have been attempted if folks had known then what
they know now.

Why not perfect thruster-powered landings instead?
Been there, done that. We know exactly how to do these in a safe way,
including some backups to handle several of the possible failure modes,
etc. Its just that the whole package of thrusters, surface-sensors, control
mechanisms, etc.. makes for a very heavy package. Not something you can
launch on a mere Delta.

Nonsense. MPL was launched on a Delta. Phoenix, which is using the copy
of the MPL hardware built for the cancelled 2001 lander, will also launch
on a Delta. As noted above, a rocket landing system is actually lighter
than the airbags for a given payload. It's also simpler (!) and more
versatile, and gives a much gentler ride. The one price you do pay is
that the current systems are not smart enough to notice that they're about
to set down on a boulder that will wreck them.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Kaido Kert wrote:
So are further developments of such landing technique actually worth it ?

Probably not. All future landers are planned to use rocket landing.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Henry Spencer) writes:

Correct. But in that thin air, it'll be a significant effort to get the
velocity up to what's needed to remove fine, well-adhering dust.

I'm thinking that if you design the nozzle right, you might keep the same
momentum-per-cross-sectional-area, or at least keep the loss of same to
acceptable levels.

Keep in mind the thinner atmosphere may cut both ways: there's less
back pressure on the nozzle, and there will be less drag induced
on the compressed stream of, er, air. (For lack of a better term).

--
Phil Fraering
http://newsfromthefridge.typepad.com