Started by Michael_Rolls, November 23, 2018, 08:07:56 am
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Quote from: PDR on November 26, 2018, 20:23:37 pmThe energy required to produce the thrust would be similar in either case, but the ion electrode array would be draggy.It's difficult to do "slow" at high altitude using wing-lift because the air density is so low. Air density at 50,000 feet is only 15% of the density of the thick soup we wade through at sea level, so an aeroplane that flies at 20mph to maintain height at sea level needs to fly at over 130mph to do the same at 50,000 feet. Fortunately the drag is lower as well, but it does funny things to reynolds numbers and pitches of props.PDR
Quote from: PDR on November 27, 2018, 13:34:25 pmMars is an interesting problem. The atmosphere is very thin even at the surface - about 0.6% ISO sea-level density (ie roughly the same as our atmosphere at ~110,000 feet)*, and the temperature varies from around -20C to -100C. The low density causes stall speeds to be much higher, while the low temperature causes the speed of sound (and the onset of compressibility effects) to be much lower. The net result is that pretty well any aeroplane design ends up treatiung a fine line between stall buffet and mach buffet trhoughout the whole flight - a genuine "coffin corner" in the flight envelope. Even if you can keep it subsonic the aeroplane would need to travel much faster, and that's problematic. Lift and drag increase with the square of airspeed, so if you double the airspeed you get four times the lift. But you also get four times the drag, which means that you need eight times the power to do it. And so it all starts stacking up against you. We (the industry, ESA, NASA etc) have looked at "Mars Plane" projects many times, but we haven't seen an answer so far. PDR* That's the only real technical blooper in the film "the Martian" - the atmosphere is so thin that even gale-force storms wouldn't produce the dynamic pressure required to lob an antenna array (the accident at the start of the film). But it's just the one - the three other minor bloopers can be forgiven in the name of dramatic license
Quote from: flynn on November 29, 2018, 16:07:22 pmMaybe you could argue that it wasn't just the wind that dislodged the dish but the combined effect of the wind and the mass of sand that was being blown along with the wind?
Quote from: PDR on November 29, 2018, 19:08:33 pmThe Martian wind wouldn't lift that much sand for essentially the same reason.PDR
Quote from: PDR on November 30, 2018, 11:59:36 amThey do, but the sand is a fine dust that doesn't significantly increase the air density, so it doesn't add the extra pressure/force needed to throw a large antenna (which was the original question). The dust is like flour - it's so fine that it gets into shaft bearings and gears and bungs them up (technical term) - this has been the most common cause of final terminal failure in mars exploration vehicles IIRC.PDR
Quote from: PDR on December 01, 2018, 07:13:19 amAre you calling me a pirate?PDR
Quote from: itsme on December 01, 2018, 08:59:33 amhe was talking to the parrot
Quote from: Phil_G on December 02, 2018, 11:01:15 am"The Martian" is a very technically satisfying read, and those who only saw the film have missed out, To me, Marks detailed thought processes were the essential theme of the story and most of that was lost.
Quote from: PDR on December 02, 2018, 12:02:39 pmAbsolutely agree. When I bought the bluray Sainsburys were giving away the the book with it, and I'm very glad they did because I wouldn't have thought of buying it afterwards. It's not the greatest literary work ever written (it was his first attempt at a book), but it's a fascinating read. The story is longer and richer (the journey from the Hab to the MAV that takes 5 mins of screen time is a third of the book), and a lot of the blanks are filled in (like why the Hab exploded).Don't get me wrong - I love the film*, but reading the book makes the film experience even better. Do both - they complement eachother.PDR* I'm part of a group who are trying to get the guvmint to negotiate a license so that the film and the book can be incorporated into the national curriculum for kids aged about 13-14 to foster interest in STEM subjects as a career.
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