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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 |
#2
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Mars sample return
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. | |
#3
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Mars sample return
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? |
#4
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Mars sample return
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? |
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