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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Sep 19 2007, 12:07 AM
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#2
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Junior Member Group: Members Posts: 99 Joined: 17-September 07 Member No.: 3901 |
Here are some comments about "how to get off of Mars" for an affordable sample return mission. We really are talking about a miniature launch vehicle. Mars ascent is far more difficult than any rocket maneuver ever done except earth launch, while Mars ascent needs to be done with about one thousandth the mass of earth launch vehicles.
A Mars ascent vehicle needs to be about 75 percent propellant, e.g. 4200 m/s at 310 seconds Isp. If the rocket stage without payload is 80 percent propellant, then the whole vehicle has to weigh 16 times the payload (12 parts propellant, 3 parts stage hardware, and 1 part payload has the 75% and 80% ratios). If the rocket stage can be as good as 90 percent propellant, then the whole vehicle weighs only 6 times the payload (4.5 parts propellant, 0.5 parts stage hardware, 1 part payload). Much better. For example, a 20 kg Mars ascent payload means that 80-percent rocket stage technology results in 320 kg launching off Mars, while 90-percent technology needs only 120 kg. That's likely the difference between "not possible" and "possible," given forseeable sizes for Mars landers. I believe the latter can actually be done, if the avionics and batteries can be squeezed into the 20-kg payload allocation -- the rocket engineer perspective on what constitutes payload . So the scale (and therefore cost) of the entire Mars sample return mission depends very strongly on the relative masses of propellant and stage hardware, which in turn is limited by the strength of metal and the difficulty of miniaturization. Whole stages of earth launch vehicles are 90 percent, but there is no precedent for achieving such high numbers in the 1-ton range, let alone on a 100-kg scale. While the above analysis assumes one stage, and multiple stages make it easier in theory, the miniaturization challenge is even more difficult for an upper stage. Existing flight-qualified solid rocket motors on the scale of interest (~100 kg) are about 90 percent propellant, so it is very tempting to think the problem is solved. However, it is necessary to add directional control. The extra parts could easily over-burden a Mars ascent vehicle. A useful technology development effort might be to build and test-fly a series of small solid rocket stages, all the while working to reduce the auxiliary weight. For liquid propellants, entirely new custom hardware would have to be developed, because liquid propulsion parts used on satellites and spacecraft are too large and heavy. One possibility for reducing hardware weight is to use a pump-fed engine like launch vehicles do. The principle is to reduce tank weight by making the walls thinner (low pressure), while making the engine more compact (and less massive) by running it at high pressure. The organizations that build spacecraft propulsion systems have not been asked to design rockets completely from scratch since about 1970 (perhaps a few exceptions), and launch vehicle organizations only build big things. A learning curve should be expected. A bit of good news is that building a Mars ascent vehicle promises to be a very exciting project to inspire the next generation. John W. |
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Oct 16 2007, 11:09 AM
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#3
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Member Group: Members Posts: 321 Joined: 6-April 06 From: Cape Canaveral Member No.: 734 |
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