Mars Sample Return |
Mars Sample Return |
Apr 7 2006, 07:32 AM
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#1
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Member Group: Members Posts: 370 Joined: 12-September 05 From: France Member No.: 495 |
Next phase reached in definition of Mars Sample Return mission
http://www.esa.int/esaCP/SEMJAGNFGLE_index_0.html |
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Oct 17 2007, 06:51 PM
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#2
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Junior Member Group: Members Posts: 99 Joined: 17-September 07 Member No.: 3901 |
You're right, nprev, that a solid first stage is very attractive. NASA's reference design concept for a MAV has been a 2-stage solid more or less since about 1999. The good news is that solid rocket motors on the scale of interest (100 kg give or take a factor of 2) are existing technology, and they are a whopping 90 percent propellant. However, there's devil is in the details. The thrust of such small solid motors is way more than is needed. It would reach high speeds while still low in the Mars atmosphere, so there's somewhat more aerodynamic drag than for a liquid MAV. Worse, perhaps, is that the high thrust also requires the directional control system to be larger and heavier than would otherwise be needed, and control must be very responsive (quick) to steer correctly for the 20 seconds or so before the first stage motor burns out.
Solid motors and their payloads are usually spinning when used for space maneuvers. Launching a spinning MAV would require the lander to have a spin table rigidly anchored to the ground so it doesn't start wiggling when the MAV is spun up. The landing orientation cannot be guaranteed, so the launch platform would require tilt adjustments on two axes, and then still be rigid when it starts spinning. How to design such a lander or estimate its weight to compare with other options? A spinning MAV was considered at NASA in 1998-1999 and ruled out. Pressurizing the "heck" out of tanks and leaving the pumps on Mars is not a solution because the high-pressure tanks would be way heavier than pumps. You hit 2 nails on their heads, monitorlizard. 1. There are so many different kinds of rockets and missiles out there, that it is way too easy for the "collective consciousness" to assume that it is possible to just go and buy something that can launch off of Mars. Therefore there has been no NASA (or ESA) money dedicated to aggressive technology development, most likely necessary. 2. Minimum size for avionics is really what determines the smallest MAV. Who wants to make the agonizing decision about how much telemetry to put on board? If it doesn't reach Mars orbit, how much data is needed to know why the multi-billion dollar mission failed (the painful lesson from Mars Observer 1992). Rising through the atmosphere with a helium balloon before launching the rocket would be the ideal way to get off of Venus, if only the balloon could be kept from melting. So for Mars ascent there are several possible solutions, none of which is existing technology. Ideally, some amount of engineering effort (building and testing things) would be affordable for each candidate, to help sort out what makes sense to pursue further. John W. |
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Oct 18 2007, 12:34 AM
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Member Group: Members Posts: 321 Joined: 6-April 06 From: Cape Canaveral Member No.: 734 |
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Oct 19 2007, 01:10 AM
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#4
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Junior Member Group: Members Posts: 99 Joined: 17-September 07 Member No.: 3901 |
But not too heavy for a MAV If the pressure-fed versus pump-fed question is to be debated, then mass budgets need to be compared. Descending down one layer into what determines the mass budget, one key question is how heavy are the tanks relative to the propellant they contain? Is there stored gas on board to push the propellants out, and how heavy are the extra tanks (or extra tank volume) for storing the gas? Regarding the earlier suggestion to fill it up with Mars atmosphere then leave pumps on the ground, note that the Mars atmosphere is carbon dioxide, which is 11 times as heavy as helium. A good rule of thumb is that the best aerospace pressure vessels flying today can contain 1/4 of their own weight in helium. Or to put that a different way, the helium needed weighs only 1/4 of the vessel. If carbon dioxide were to be used instead, it would weigh more than the propellant tanks and gas storage vessel(s) combined. What does this look like on the mass budget? Back in 2001, NASA funded 3 companies to go and design Mars ascent vehicles. The results, made public by AIAA paper number 2002-4318, were: Lockheed-Martin, 268 kg solid rocket TRW, 254 kg gel propellant rocket Boeing, 400 kg pressure-fed liquid bipropellant (hypergolic) These numbers don't count the mass of essential hardware that remains on Mars, just to support the launch. The pressure-fed liquid was the heaviest option, and in recent years, all three have been considered too heavy to do the MSR mission within the Mars science budget. All this leads to the conclusion that it's really a nitty-gritty technology problem that is unlikely to be solved in the usual way by engineering design studies. Before I forget to mention them again, here are two fun-to-read articles that offer an inside view of the people at NASA as they have worked to figure out the MAV. I'm going to mark this spot with a Mars so these refs can be found easily scrolling through here. Reichhardt, "The One-Pound Problem," Air & Space (the Smithsonian's magazine), October-November 1999, page 50. I have read it online at http://www.AirAndSpaceMagazine.com/ASM/Mag.../1999/topp.html but if you can find the original in a library there are some nice artist concepts too. That article was written just before two Mars-bound spacecraft were lost, and years later the conclusions in that article turned out not to be the final answer. The other is a two-part personal perspective by Dr. Mark Adler of JPL, on the Planetary Society blog in September 2006, http://www.planetary.org/blog/article/00000701/ and Part Two the next day at http://www.planetary.org/blog/article/00000703/ John W. |
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