Re: The Mars Landing Approach: Getting Large Payloads to the Surface of the Red Planet

On Jul 20, 5:05 am, jsav...@xxxxxxxxxxxxxxxxxxxxxxxxxx (John Savard)
wrote:
On Wed, 18 Jul 2007 16:33:29 GMT, Sam Wormley <sworml...@xxxxxxxxx>
quoted (in sci.astro.amateur):





July 17th, 2007

http://www.universetoday.com/2007/07/17/the-mars-landing-approach-get...

The Mars Landing Approach: Getting Large Payloads to the Surface of the Red Planet

Some proponents of human missions to Mars say we have the technology
today to send people to the Red Planet. But do we? Rob Manning of the
Jet Propulsion Laboratory discusses the intricacies of entry, descent
and landing and what needs to be done to make humans on Mars a
reality.

There's no comfort in the statistics for missions to Mars. To date
over 60% of the missions have failed. The scientists and engineers of
these undertakings use phrases like "Six Minutes of Terror," and "The
Great Galactic Ghoul" to illustrate their experiences, evidence of
the anxiety that's evoked by sending a robotic spacecraft to Mars
\u2014 even among those who have devoted their careers to the task.
But mention sending a human mission to land on the Red Planet, with
payloads several factors larger than an unmanned spacecraft and the
trepidation among that same group grows even larger. Why?

Nobody knows how to do it.

Surprised? Most people are, says Rob Manning the Chief Engineer for
the Mars Exploration Directorate and presently the only person who
has led teams to land three robotic spacecraft successfully on the
surface of Mars.

"It turns out that most people aren't aware of this problem and very
few have worried about the details of how you get something very
heavy safely to the surface of Mars," said Manning.

He believes many people immediately come to the conclusion that
landing humans on Mars should be easy. After all, humans have landed
successfully on the Moon and we can land our human-carrying vehicles
from space to Earth. And since Mars falls between the Earth and the
Moon in size, and also in the amount of atmosphere it has then the
middle ground of Mars should be easy. "There's the mindset that we
should just be able to connect the dots in between," said Manning.

But as of now, the dots will need to connect across a large abyss.

"We know what the problems are. I like to blame the god of war,"
quipped Manning. "This planet is not friendly or conducive for
landing."

The real problem is the combination of Mars' atmosphere and the size
of spacecraft needed for human missions. So far, our robotic
spacecraft have been small enough to enable at least some success in
reaching the surface safely. But while the Apollo lunar lander
weighed approximately 10 metric tons, a human mission to Mars will
require three to six times that mass, given the restraints of staying
on the planet for a year. Landing a payload that heavy on Mars is
currently impossible, using our existing capabilities. "There's too
much atmosphere on Mars to land heavy vehicles like we do on the
moon, using propulsive technology completely," said Manning, "and
there's too little atmosphere to land like we do on Earth. So, it's
in this ugly, grey zone."

But what about airbags, parachutes, or thrusters that have been used
on the previous successful robotic Mars missions, or a lifting body
vehicle similar to the space shuttle?

None of those will work, either on their own or in combination, to
land payloads of one metric ton and beyond on Mars. This problem
affects not only human missions to the Red Planet, but also larger
robotic missions such as a sample return. "Unfortunately, that's
where we are," said Manning. "Until we come up with a whole new
trick, a whole new system, landing humans on Mars will be an ugly and
scary proposition."

Road Mapping

In 2004 NASA organized a Road Mapping session to discuss the current
capabilities and future problems of landing humans on Mars. Manning
co-chaired this event along with Apollo 17 astronaut Harrison Schmitt
and Claude Graves, who has since passed away, from the Johnson Space
Center. Approximately 50 other people from across NASA, academia and
industry attended the session. "At that time the ability to explain
these problems in a coherent way was not as good," said Manning. "The
entry, descent and landing process is actually made up of people from
many different disciplines. Very few people really understood,
especially for large scale systems, what all of the issues were. At
the Road Mapping session we were able to put them all down and talk
about them."

The major conclusion that came from the session was that no one has
yet figured out how to safely get large masses from speeds of entry
and orbit down to the surface of Mars. "We call it the Supersonic
Transition Problem," said Manning. "Unique to Mars, there is a
velocity-altitude gap below Mach 5. The gap is between the delivery
capability of large entry systems at Mars and the capability of
super-and sub-sonic decelerator technologies to get below the speed
of sound."

Plainly put, with our current capabilities, a large, heavy vehicle,
streaking through Mars' thin, volatile atmosphere only has about
ninety seconds to slow from Mach 5 to under Mach 1, change and
re-orient itself from a being a spacecraft to a lander, deploy
parachutes to slow down further, then use thrusters to translate to
the landing site and finally, gently touch down.

No Airbags

When this problem is first presented to people, the most offered
solution, Manning says, is to use airbags, since they have been so
successful for the missions that he has been involved with; the
Pathfinder rover, Sojourner and the two Mars Exploration Rovers
(MER), Spirit and Opportunity.

But engineers feel they have reached the capacity of airbags with
MER. "It's not just the mass or the volume of the airbags, or the
size of the airbags themselves, but it's the mass of the beast inside
the airbags," Manning said. "This is about as big as we can take that
particular design."

In addition, an airbag landing subjects the payload to forces between
10-20 G's. While robots can withstand such force, humans can't. This
doesn't mean airbags will never be used again, only that airbag
landings can't be used for something human or heavy.

Even the 2009 Mars Science Laboratory (MSL) rover, weighing 775
kilograms (versus MER at 175.4 kilograms each) requires an entirely
new landing architecture. Too massive for airbags, the small-car
sized rover will use a landing system dubbed the Sky Crane. "Even
though some people laugh when they first see it, my personal view is
that the Sky Crane is actually the most elegant system we've come up
with yet, and the simplest," said Manning. MSL will use a combination
of a rocket-guided entry with a heat shield, a parachute, then
thrusters to slow the vehicle even more, followed by a crane-like
system that lowers the rover on a cable for a soft landing directly
on its wheels. Depending on the success of the Sky Crane with MSL,
it's likely that this system can be scaled for larger payloads, but
probably not the size needed to land humans on Mars.

Atmospheric Anxiety and Parachute Problems

"The great thing about Earth," said Manning "is the atmosphere."
Returning to Earth and entering the atmosphere at speeds between 7-10
kilometers per second, the space shuttle, Apollo and Soyuz capsules
and the proposed Crew Exploration Vehicle (CEV) will all decelerate
to less than Mach 1 at about twenty kilometers above the ground just
by skimming through Earth's luxuriously thick atmosphere and using a
heat shield. To reach slower speeds needed for landing, either a
parachute is deployed, or in the case of the space shuttle, drag and
lift allow the remainder of the speed to bleed away.

But Mars' atmosphere is only one per cent as dense as Earth's. For
comparison, Mars atmosphere at its thickest is equivalent to Earth's
atmosphere at about 35 kilometers above the surface The air is so
thin that a heavy vehicle like a CEV will basically plummet to the
surface; there's not enough air resistance to slow it down
sufficiently. Parachutes can only be opened at speeds less than Mach
2, and a heavy spacecraft on Mars would never go that slow by using
just a heat shield. "And there are no parachutes that you could use
to slow this vehicle down," said Manning. "That's it. You can't land
a CEV on Mars unless you don't mind it being a crater on the
surface."

That's not good news for the Vision for Space Exploration. Would a
higher lift vehicle like the space shuttle save the day? "Well, on
Mars, when you use a very high lift to weight to drag ratio like the
shuttle," said Manning, "in order to get good deceleration and use
the lift properly, you" need to cut low into the atmosphere. You"
still be going at Mach 2 or 3 fairly close to the ground. If you had
a good control system you could spread out your deceleration to
lengthen the time you are in the air. You" eventually slow down to
under Mach 2 to open a parachute, but you" be too close to the ground
and even an ultra large supersonic parachute would not save you."

Supersonic parachute experts have concluded that to sufficiently slow
a large shuttle-type vehicle on Mars and reach the ground at
reasonable speeds would require a parachute one hundred meters in
diameter.

"That's a good fraction of the Rose Bowl. That's huge," said Manning.
"We believe there's no way to make a 100-meter parachute that can be
opened safely supersonically, not to mention the time it takes to
inflate something that large. You'd be on the ground before it was
fully inflated. It would not be a good outcome."

Heat Shields and Thrusters

It's not that Mars' atmosphere is useless. Manning explained that
with robotic spacecraft, 99% of the kinetic energy of an incoming
vehicle is taken away using a heat shield in the atmosphere. "It's
not inconceivable that we can design larger, lighter heat shields,"
he said, "but the problem is that right now the heat shield diameter
for a human-capable spacecraft overwhelms any possibility of
launching that vehicle from Earth." Manning added that it would
almost be better if Mars were like the moon, with no atmosphere at
all.

If that were the case, an Apollo-type lunar lander with thrusters
could be used. "But that would cause another problem," said Manning,
"in that for every kilogram of stuff in orbit, it takes twice as much
fuel to get to the surface of Mars as the moon. Everything is twice
as bad since Mars is about twice as big as the moon." That would
entail a large amount of fuel, perhaps over 6 times the payload mass
in fuel, to get human-sized payloads to the surface, all of which
would have to be brought along from Earth. Even on a fictitious
air-less Mars that is not an option.

But using current thruster technology in Mars' real, existing
atmosphere poses aerodynamic problems. "Rocket plumes are notoriously
unstable, dynamic, chaotic systems," said Manning. "Basically flying
into the plume at supersonics speeds, the rocket plume is acting like
a nose cone; a nose cone that's moving around in front of you against
very high dynamic pressure. Even though the atmospheric density is
very low, because the velocity is so high, the forces are really
huge."

Manning likened theses forces to a Category Five hurricane. This
would cause extreme stress, with shaking and twisting that would
likely destroy the vehicle. Therefore using propulsive technology
alone is not an option.

Using thrusters in combination with a heat shield and parachute also
poses challenges. Assuming the vehicle has used some technique to
slow to under Mach 1, using propulsion just in last stages of descent
to gradually adjust the lander's trajectory would enable the vehicle
to arrive very precisely at the desired landing site. "We're looking
at firing thrusters less than 1 kilometer above the ground. Your
parachute has been discarded, and you see that you are perhaps 5
kilometers south of where you want to land," said Manning. "So now
you need the ability to turn the vehicle over sideways to try to get
to your landing spot. But this may be an expensive option, adding a
large tax in fuel to get to the desired landing rendezvous point."

Additionally, on the moon, with no atmosphere or weather, there is
nothing pushing against the vehicle, taking it off target, and a la
Neil Armstrong on Apollo 11, the pilot can "fly out the
uncertainties" as Manning called it, to reach a suitable or desired
landing site. On Mars, however, the large variations in the density
of the atmosphere coupled with high and unpredictable winds conspire
to push vehicles off course. "We need to have ways to fight those
forces or ways to make up for any mis-targeting using the propulsion
system," said Manning. "Right now, we don't have that ability and
we're a long way from making it happen."

Supersonic Decelerators

The best hope on the horizon for making the human enterprise on Mars
possible is a new type of supersonic decelerator that's only on the
drawing board. A few companies are developing a new inflatable
supersonic decelerator called a Hypercone.

Imagine a huge donut with a skin across its surface that girdles the
vehicle and inflates very quickly with gas rockets (like air bags) to
create a conical shape. This would inflate about 10 kilometers above
the ground while the vehicle is traveling at Mach 4 or 5, after peak
heating. The Hypercone would act as an aerodynamic anchor to slow the
vehicle to Mach 1.

Glen Brown, Chief Engineer at Vertigo, Inc. in Lake Elsinore,
California was also a participant in the Mars Road Mapping session.
Brown says Vertigo has been doing extensive analysis of the
Hypercone, including sizing and mass estimates for landers from four
to sixty metric tons. "A high pressure inflatable structure in the
form a of a torus is a logical way to support a membrane in a conical
shape, which is stable and has high drag at high Mach numbers," Brown
said, adding that the structure would likely be made of a coated
fabric such as silicon-Vectran matrix materials. Vertigo is currently
competing for funding from NASA for further research, as the next
step, deployment in a supersonic wind tunnel, is quite expensive.

The structure would need to be about thirty to forty meters in
diameter. The problem here is that large, flexible structures are
notoriously difficult to control. At this point in time there are
also several other unknowns of developing and using a Hypercone.

One train of thought is that if the Hypercone can get the vehicle
under Mach 1, then subsonic parachutes could be used, much like the
ones employed by Apollo, or that the CEV is projected to use to land
on Earth. However, it takes time for the parachutes to inflate, and
subsequently there would only be a matter of seconds of use, allowing
time to shed the parachutes before converting to a propulsive system.

"You" also need to use thrusters," said Manning. "You're falling 10
times faster because the density of Mars' atmosphere is 100 times
less than Earth's. That means that you can't just land with
parachutes and touch the ground. You" break people's bones, if not
the hardware. So you need to transition from a parachute system to an
Apollo-like lunar legged lander sometime before you get to the
ground."

Manning believes that those who are immersed in these matters, like
himself, see the various problems fighting each other. "It's hard to
get your brain around all these problems because all the pieces
connect in complex ways," he said. "It's very hard to see the right
answer in your mind's eye."

The additional issues of creating new lightweight but strong shapes
and structures, with the ability to come apart and transform from one
stage to another at just the right time means developing a rapid-fire
Rube Goldberg-like contraption.

"The honest truth of the matter," said Manning, "is that we don't
have a standard canonical form, a standard configuration of systems
that allows us to get to the ground, with the right size that
balances the forces, the loads, the people, and allows us to do all
the transformation that needs to be done in the very small amount of
time that we have to land."

Other Options and Issues

Another alternative discussed at the 2004 Mars Road Mapping session
was the space elevator.

"Mars is really begging for a space elevator," said Manning. "I think
it has great potential. That would solve a lot of problems, and Mars
would be an excellent platform to try it." But Manning admitted that
the technology needed to suspend a space elevator has not yet been
invented. The issues with space elevator technology may be vast, even
compared with the challenges of landing.

Despite these known obstacles, there are few at NASA currently
spending any quality time working on any of the issues of landing
humans on Mars.

Manning explained, "NASA does not yet have the resources to solve
this problem and also develop the CEV, complete the International
Space Station and do the lunar landing systems development at the
same time. But NASA knows that this is on its plate of things to do
in the future and is just beginning to get a handle on the needed
technology developments. I try to go out of my way to tell this story
because I'm encouraging young aeronautical engineering students,
particularly graduate students, to start working on this problem on
their own. There is no doubt in my mind that with their help, we can
figure out how to make reliable human-scale landing systems work on
Mars."

While there is much interest throughout NASA and the space sector to
try to tackle these issues in the ensuing years, technology also
needs a few more years to catch up to our dreams of landing humans on
Mars.

And this story, like all good engineering stories, will inevitably
read like a good detective novel with technical twist and turns,
scientific intrigue, and high adventure on another world.

Doing some checking, I find that the mass of a Mercury capsule when in
orbit (that is, exclusive of the escape rocket) was 1,354 kilograms, and
the mass of the Viking lander on the surface of Mars was 600 kilograms.

Of course, while the Viking lander wasn't bounced around like the
Sojourner rover, it may have experienced g forces on the way down that
would be excessive for an astronaut. So the conclusion that only a
factor of two is there to worry about may not be quite true.

If we realize that there's no need for all the astronauts, and
everything they need, to be landed in *one* craft, though, this makes it
seem like the problems will not be insuperable.

And, even if there's a severe fuel penalty in slowing to the Martian
speed of sound with rockets *before* entering the atmosphere, since that
is an option, that makes landing people on Mars expensive, not
impossible.

Given that the Zubrin "Mars Direct" plan gets us from the *fourth* power
to the *square* of the total mass to payload mass ratio, that the ratio
might be a bit on the big side for a Martian landing seems to be a small
problem compared to those plans for landing on Mars originally faced.

John Savardhttp://www.quadibloc.com/index.html- Hide quoted text -

- Show quoted text -

The Mars atmosphere of less than 10 mb isn't hardly an atmosphere for
other than falling feathers.

They'll simply need a fully operational fly-by-rocket lander, similar
to the ones still in R&D for getting items safely onto our moon. Thus
far, those best of R&D prototypes have summarily failed their field
testing, as in hardly reliable enough for deploying robotics and
otherwise entirely unreliable for accommodating any crew of us frail
humans, not to mention their lack of incorporating shield against
solar, cosmic and the unavoidable local anticathode worth of our
moon's gamma and hard-Xrays, plus the double IR/FIR dosage by day
that's just downright hot, or in the case of Mars lacking in solar or
local IR/FIR being downright cold.
- Brad Guth

.

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