Mars sample return

Suppose we want to bring a small sample back from Mars. The Mars rovers mass
around 180 kg. If 10% was reserved for the return rocket it would mass 18
kg.

We need about 5.3 km/s delta V, and I'd guess a solid would be the only way
to go. At a good solid ISP that's around 85% fuel. Cases and motors are
around 5%, so that's 1.8 kg for flight electronics, re-entry shield and
payload.


It would probably be worthwhile bringing 200 grams back, and using 200 g for
re-entry stuff. Leaving 1.4 kg for flight electronics. And power supplies.
And so on.

Is that do-able? You are allowed to change the numbers a bit as long as it
works, and the launch mass is 18 kg.

Any special problems doing it that small?


--
Peter Fairbrother

In article ,
Peter Fairbrother wrote:
It would probably be worthwhile bringing 200 grams back, and using 200 g for
re-entry stuff. Leaving 1.4 kg for flight electronics. And power supplies.
And so on.

Don't forget propulsion. There's no practical way to fly an
interplanetary trajectory to a successful reentry without midcourse
corrections. Especially if the initial boost is done with solids.

Any special problems doing it that small?

Fast answer: it's well outside the state of the art. Not that it's
utterly impossible -- I've done some work on fully-capable planetary
spacecraft weighing only a few kilograms (without relying on exotic future
technology) -- but there's essentially no off-the-shelf hardware or flight
experience to base it on. So it's a major development effort and it
probably wouldn't work the first time.

You're better off investing some of that effort in making the specs less
demanding. For example, fuel it from local resources rather than having
to ship all the fuel along, so a higher fraction of that 18kg can be dry
mass. Or revise the mission design so that a larger part of the rover
mass can go to the return vehicle; 10% is really pretty small. Or have
the sample capsule just enter Mars orbit, to be picked up there by an
orbiting return spacecraft: there is some saving in delta-V, but more
significantly, much of the support hardware for the return trip doesn't
have to be part of the mass-critical capsule.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Peter,

A solid is not the only way to go. You could have microrockets. Think
of the precision with which inkjet printheads deliver ink to where its
desired. Now, consider rocket fuel delivered the same way.

http://www.grc.nasa.gov/WWW/RT1998/5...chneider2.html

According to DARPA, who was quoted in a recent AIAA publication, a
propulsive skin of millions of microrockets on a vehicle is expected
to have a thrust to weight ratio of 1000:1 - which is about 12x better
than the best macrorocket.

This skin would be capable of providing any thrust vector imaginable
on the vehicle with extreme reliabilty and safety. Propellant would
be held in cartridges behind each propulsive tile. This would give
you the control you need to fly the vehicle throughout the return
journey with course corrections and such. The ride would be quiet,
smooth, reliable and safe. With a million rockets some are certain to
fail - but the probability of all of them failing is nil. And
engineers can design them to fail in safer ways if some failure rate
is expected.

You may also reserve more than 18 kg for the return rocket.

Here's a mars lander for example that masses 360 kg with a 16 kg
microrover

http://marsprogram.jpl.nasa.gov/MPF/...et.html#SCCHAR

The polar lander massed 576 kg

http://marsprogram.jpl.nasa.gov/miss...larlander.html

So, here's a way to do a sample return mission from mars;

(1) Send a 500 kg lander to mars with three to six microrovers
(2) Send another 500 kg lander to mars with a 360 kg return rocket
(3) Make sure they land within microrover range of one another.
(4) The microrovers leave one lander, gather interesting things
(5) The microrovers deposit interesting things in return rocket
(6) Return rocket operates and send materials back to Earth

Landing two vehicle within sight of one another and causing them to
interact is a good test of Zubrin's ideas about going to mars. Just
as Gemini allowed us to test the ideas of docking and lunar orbit
rendezvous - a two lander system that coordinated resources would
allow us to test an important aspect of mars refueling.

With an exhaust speed of 4 km/sec - a LOX/LH microrocket array would
require a propellant fraction of 75.3% to achieve 5.6 km/sec you say
is needed. With a 10% structural fraction this leaves 14.7% payload
fraction. A 60 kg starting mass would permit 8.76 kg of payload - if
done in a single stage. The microrocket array may also be used to
actively cool the vehicle during entry, as well as provide small
inputs to the flow around the vehicle providing flight control during
descent. Terminal speed is likely mid sonic range, and some of the
8.76 kg of payload could be propellant to bring the vehicle to a soft
controlled landing right at the lab its required. Maybe 7.5 kg could
be retrieved in this way. The mars return rocket could even be reused
- but it would likely end up in a museum somewhere!

If we do Zubrin's trick of processing the CO2 in Mars to oxygen and
hydrogen to methane, we could return substantially more payload from
Mars with the same mass hardware. But this would require conversion
of sunlight to electricity and waiting until the tanks were topped
off.

If the first trick worked, with a rover lander combo, you could send
another return rocket to take yet more stuff back from Mars. So, if
the rovers had a life that spanned several years, you could send a
return rocket to pick up stuff every synodic period. In this way you
could return 3 to 4 kg per year from mars over an extended period.
This would allow you to analyze the samples returned, adjust your
search and pickup pattern, and return new samples in response to
earlier data.

A decade in the martian mines would be an interesting space career,
and provide tons of information.

This could be coordinated with other rovers around the planet, as well
as martian airplanes powered by sunlight, similar to Helios here on
Earth, to provide close in high resolution imagery and atmospheric
sampling.

Anyone sending a microphone anytime soon?

Peter,

Here's some more cool stuff about micro-rockets;

http://olliver.family.gen.nz/launchpad/pocketrocket.jpg
http://www.me.berkeley.edu/mrcl/pictures/rockexp.gif
http://www.me.berkeley.edu/mrcl/rockets.html
http://www.me.berkeley.edu/mrcl/pictures/rocknozl.gif

this one ^^^^^^^^ is regeneratively cooled, which is a trip!

http://mems.egr.duke.edu/ASASlab/rocket.htm

now this one uses micro-mechanical devices to control the noise of
rockets. Which is really cool. One would think a high speed jet
would necessarily be noisy (I do) but how far can noise be *reduced*?
That's an interesting question - and raises the possibility that we
could have really quiet lifting devices.

This is important in a number of applications. Lifting things off
mars very gently with a large array of micro-rockets is one of these.

http://clifton.mech.northwestern.edu...crorockets.gif


But, micro-rockets made of deeply etched silicon can also be SOLIDS!
Which brings us back to your original point

http://www-bsac.eecs.berkeley.edu/pr...rorockets.html

Interesting thing about this is that on the scale of these rockets,
variations typical of larger rockets need not exist. Again,
controllability is very high. A compact, reliable, safe, and portable
propulsion module should be possible.

A 360 kg return vehicle, with micro-solids, under computer control,
operating at 265 sec Isp ~2.5 km/sec Ve, implies a propellant fraction
requirement of 89.3% - assuming I recalled correctly that you needed
5.6 km/sec terminal velocity requirement.

Assuming a structural fraction of 15% and three stages of operation -

(1) Mars lift off
(2) Mars orbit
(3) Trans Earth Injection

With a small amount of solid propellants - microrockets - built into a
propusive skin to provide course correction and attitude control.

We can compute (without doing any variational optimization as to stage
sizes) that a 53% propellant fraction is needed in each stage. So
with our 15% structural fraction budget we can compute the following
series of sizes;

MASS KG Element
360.00 Vehicle total

189.38 S1 propellant
54.00 S1 structure
243.38 S1 total

61.35 S2 propellant
17.49 S2 structure
78.84 S2 total

19.87 S3 propellant
5.67 S3 structure
25.54 S3 total

12.24 payload returned from surface of mars

At 1.35 grams per cc - we can even figure the approximate propellant
volume of each stage;

S1 - 140.28 liters
S2 - 45.44 liters
S2 - 14.72 liters

If each of these are spheres they would have a diameter as follows

S1 - 64.46 cm
S2 - 44.27 cm
S3 - 30.09 cm

Since the rocket nozzles can be smaller than the diameter of a human
hair, we have some very interesting configuration possibilities. But,
the whole thing need not be any larger than a meter across.

Of course, the return vehicle would be aerodynamically shaped, like a
lifting body from the 1960s...

http://airpower.callihan.cc/images/x...x24a-usafm.jpg

So, a cylindrical sort of propulsion unit on the tail end of an
aerodynamic carrier is possible. The whole thing would lay on its
side, awaiting micro-rovers from a nearby lander to load it up. Then,
once loaded, it would be brought to vertical position, and launched.
The first stage would re-enter downrange from the launch point. The
second stage would take the vehicle to mars orbit. It would then
relight - and initiate trans-Earth injection, but separate as it burnt
out - entering a highly elliptical mars orbit. At apo-[mars?] it
would adjust its orbit slightly to crash into mars at pre-[mars?] The
third stage would inject the vehicle to an interplanetary orbit and
separate. The aerodyamic stage, massing 12 kg, and containing perhaps
2 kg of propellant - would find its way back to Earth, re-enter and
land bearing a 7 kg payload of mars materials.

Kewl.














Peter Fairbrother wrote in message ...



Suppose we want to bring a small sample back from Mars. The Mars rovers mass
around 180 kg. If 10% was reserved for the return rocket it would mass 18
kg.

We need about 5.3 km/s delta V, and I'd guess a solid would be the only way
to go. At a good solid ISP that's around 85% fuel. Cases and motors are
around 5%, so that's 1.8 kg for flight electronics, re-entry shield and
payload.


It would probably be worthwhile bringing 200 grams back, and using 200 g for
re-entry stuff. Leaving 1.4 kg for flight electronics. And power supplies.
And so on.

Is that do-able? You are allowed to change the numbers a bit as long as it
works, and the launch mass is 18 kg.

Any special problems doing it that small?