darthgently Posted January 24 Share Posted January 24 1 hour ago, farmerben said: How many times does the Earth rotate in one year? I'm not sure if the answer is 364 or 366. If you had a planet that didn't rotate at all one day would equal one year. Venus is very interesting in this regard. 365.24, thus the leap years every 4 years dropping a day in February. Then some leap seconds thrown in on some schedule Quote Link to comment Share on other sites More sharing options...
tomf Posted January 24 Share Posted January 24 (edited) 2 hours ago, farmerben said: How many times does the Earth rotate in one year? I'm not sure if the answer is 364 or 366. If you had a planet that didn't rotate at all one day would equal one year. Venus is very interesting in this regard. It is 366.25. The sidereal day (compared to fixed stars) is 4 minutes shorter than the mean solar day. Your example of a non rotating planet would have a day the length of the year, but the sun would appear to be travelling in the wrong direction. Edited January 24 by tomf More precision Quote Link to comment Share on other sites More sharing options...
darthgently Posted January 24 Share Posted January 24 56 minutes ago, tomf said: It is 366.25. The sidereal day (compared to fixed stars) is 4 minutes shorter than the mean solar day. Your example of a non rotating planet would have a day the length of the year, but the sun would appear to be travelling in the wrong direction. 365.24 Quote Link to comment Share on other sites More sharing options...
farmerben Posted January 24 Share Posted January 24 Every day you do one rotation plus a little extra. The extras add up to a full rotation in one year. Quote Link to comment Share on other sites More sharing options...
darthgently Posted January 24 Share Posted January 24 26 minutes ago, farmerben said: Every day you do one rotation plus a little extra. The extras add up to a full rotation in one year. Depends on reference frame I suppose. Works either way Quote Link to comment Share on other sites More sharing options...
Jacke Posted January 26 Share Posted January 26 On 1/24/2025 at 6:12 PM, farmerben said: How many times does the Earth rotate in one year? I'm not sure if the answer is 364 or 366. If you had a planet that didn't rotate at all one day would equal one year. Venus is very interesting in this regard. Depends on which "day" and which "year". https://en.wikipedia.org/wiki/Day https://en.wikipedia.org/wiki/Year If you mean mean solar day and Tropical year: > The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition[13] (= 365.24219 d × 86400 s). The length of the tropical year varies a bit over thousands of years because the rate of axial precession is not constant. Quote Link to comment Share on other sites More sharing options...
monophonic Posted January 27 Share Posted January 27 On 1/26/2025 at 7:07 AM, Jacke said: Depends on which "day" and which "year". https://en.wikipedia.org/wiki/Day https://en.wikipedia.org/wiki/Year If you mean mean solar day and Tropical year: > The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds, using the modern definition[13] (= 365.24219 d × 86400 s). The length of the tropical year varies a bit over thousands of years because the rate of axial precession is not constant. "Day" and "rotation" are not synonymous, and use of the latter does imply sidereal day. That is where @farmerbens question becomes interesting. Indeed if the Earth did not rotate at all around its axis, we would have negative one days in a year. Negative to account for the opposite movement of the sun. If Earth was tidally locked to the Sun, so we had no day/night cycle, Earth would rotate one full rotation around its axis in a year. Add to this the 365 and a bit days we observe, and the answer is indeed one more rotations than solar days. Or 366.24219*360°=131 847,1884°. Quote Link to comment Share on other sites More sharing options...
Fizzlebop Smith Posted January 27 Share Posted January 27 I have been trying to envision a Celestial composition that produces a semi-locked planet. I am trying to write a work of fiction where the planet has a day / night side with a terminus at the prime meridian. However... I want the Sun to move in relation to observers on the planet. I want the sun to appear to take an elliptical path through the sky. As you go from terminus to pole .. the sun goes from low on horizon to overhead. The issue is a tidally locked planet has not observable movements for the sun. So I would need an axial Wobble? This is probably a dumb question to those with knowledge of celestial mechanical, but it's driving me a little loopy. I can just wave hands since it's a work of fiction and atmosphere and life would not be present.. but it still drives me up the wall in those moments of boredom. Essentially I need a planet with a variable tilt that is tied to rotation. Was this articulated in a way where it makes sense what I'm asking? If not just write it off as babbling of a GM wanting to record the campaign one day. Quote Link to comment Share on other sites More sharing options...
magnemoe Posted January 27 Share Posted January 27 1 hour ago, Fizzlebop Smith said: I have been trying to envision a Celestial composition that produces a semi-locked planet. I am trying to write a work of fiction where the planet has a day / night side with a terminus at the prime meridian. However... I want the Sun to move in relation to observers on the planet. I want the sun to appear to take an elliptical path through the sky. As you go from terminus to pole .. the sun goes from low on horizon to overhead. The issue is a tidally locked planet has not observable movements for the sun. So I would need an axial Wobble? This is probably a dumb question to those with knowledge of celestial mechanical, but it's driving me a little loopy. I can just wave hands since it's a work of fiction and atmosphere and life would not be present.. but it still drives me up the wall in those moments of boredom. Essentially I need a planet with a variable tilt that is tied to rotation. Was this articulated in a way where it makes sense what I'm asking? If not just write it off as babbling of a GM wanting to record the campaign one day. The moon does tilt a bit relative to earth but its 10% something, having the sun move 30 degree would probably work but I guess you need an decent elliptical orbit to start with, or having an decent sized moon, probably closer but it could be smaller. If planet is pretty young it also helps. Quote Link to comment Share on other sites More sharing options...
Hobbesian Posted February 6 Share Posted February 6 Why was STS-125 launched without a docking port? As in, it had the adapter that is normally used to dock with say the ISS, but it lacked the top docking port, operating exclusivley as an airlock. I'd assume if STS-125 had a docking port, in the event of STS-400 having to go up, Endeavour could have (placing endeavour's docking adapter more in the middle of the bay to maximize clearance) docked with Atlantis instead of the spacewalk crew transfer. I will say that I am familiar with most other reasons why STS-125 didn't. Weight, no real expected use due to the high altitude and low inclination making an ISS rendezvous impossible. But why didn't they add one as a contingency? If I had to guess it was due to there simply not being enough clearance even if Endeavour had her docking adapter moved somewhere to the center. I can't visualize it in my head how such a procedure would look and if there would be point of danger. Likely a fairly stupid question, but that's why it's here. I'm just curious as to why they decided not to go through with this plan, or if it was even considered. STS-125 was planned well in advance, and came after the risks of shuttle free-flying came to light after Columbia. Quote Link to comment Share on other sites More sharing options...
darthgently Posted February 9 Share Posted February 9 (edited) How much fuel is wasted lifting ice mass for many cryofueled rockets? Especially on the smaller rockets like Electron it seems like a spray of deicer shortly before liftoff could be a small win Edited February 9 by darthgently Quote Link to comment Share on other sites More sharing options...
AckSed Posted February 9 Share Posted February 9 Like a lot in rocketry, it depends. On one hand, the ice is added mass. However, the majority of it will probably be on the booster, and the rule of thumb is that 8kg saved on the booster equals 1 kg of payload. On the other hand, the ice acts as partial insulation, sort of like an igloo for the cryogenic fluid. Also, per this StackExchange post, it's a small part of the total mass and the vibration of launch will shake most of it off: https://space.stackexchange.com/questions/10086/how-big-a-weight-problem-is-ice-sticking-to-a-rocket On the gripping hand, LH2 is prone to condensing out nitrogen and oxygen if it is not insulated. The water ice can help here too, but you really don't want ice or LOX forming in the insulation. Quote Link to comment Share on other sites More sharing options...
AckSed Posted February 9 Share Posted February 9 Could you build a partial Lunar orbital launch tower out of sintered regolith, and how tall would it need to be? I was made aware of a recent paper on vacuum-sintered regolith simulant: https://www.sciencedirect.com/science/article/pii/S2095268624000764 tl;dr regolith, if sintered in vacuum at the correct temperature, has a compressive strength that exceeds the finest brick. And if there's three things the Moon has in abundance, it's vacuum, loose regolith and unfiltered sunlight to melt/sinter. It also has an escape velocity of 2,380 m/s. Spoiler I say "tower", but I was thinking more about an enormous three-sided ramp with a sloped ridge that the rails (or mass driver) would run up. How tall could you make it and what reduction in velocity for the mass driver could be achieved? Assume we have a way to lock the bricks mechanically together with the same strength, so it can be treated as one object. Quote Link to comment Share on other sites More sharing options...
darthgently Posted February 9 Share Posted February 9 33 minutes ago, AckSed said: Assume we have a way to lock the bricks mechanically together with the same strength, so it can be treated as one object. Imagine if the brick mold left insets of 1/2” wide by 6” deep and after assembly a laser and applied regolith could sinter the blocks together in the gaps to a significant degree like welded mortar. Or maybe a scanning laser that would heat all the mating surfaces to the sticky point as each brick was moving into place during assembly. Quote Link to comment Share on other sites More sharing options...
darthgently Posted February 9 Share Posted February 9 Better yet, forget bricks. 3D sinter-print the ramp in place monolithically. Literally monolithically. Not figuratively, by gosh Quote Link to comment Share on other sites More sharing options...
JoeSchmuckatelli Posted February 11 Share Posted February 11 I think this was asked but I cannot remember the answer and my googlefu is lost in generica... But here goes: If I take a standard diesel engine (like for a tank or military truck) and instead of using the power output for mechanical engagement of the drivetrain use it to generate electricity, and given that electric motors generally have great torque, using electric motors to move the relevant wheels can I get better performance? Mind you - I don't care about emissions. The only thing I care about is getting the vehicle from A to B and range/economy (logistics). ... I'm thinking the answer is 'no' but there have been advancements in electric drive motors lately. Quote Link to comment Share on other sites More sharing options...
razark Posted February 11 Share Posted February 11 2 hours ago, JoeSchmuckatelli said: I'm thinking the answer is 'no' but there have been advancements in electric drive motors lately. Since we've been using it to power trains since the 1930s/'40s (I seem to recall there was some cooperation between railroads and the US Navy submarine development during WWII), there has to be some advantage. Also, because of this forum: "As of 2003, each crawler had 16 traction motors, powered by four 1,000 kW (1,341 hp) generators, in turn driven by two 2,050 kW (2,750 hp) V16 ALCO 251C diesel engines." Quote Link to comment Share on other sites More sharing options...
Mr. Kerbin Posted February 11 Share Posted February 11 (edited) What structures will survive the longest on Earth, and how long? Edited February 11 by Mr. Kerbin Quote Link to comment Share on other sites More sharing options...
razark Posted February 11 Share Posted February 11 https://en.wikipedia.org/wiki/List_of_oldest_extant_buildings Quote Link to comment Share on other sites More sharing options...
Mr. Kerbin Posted February 11 Share Posted February 11 4 minutes ago, razark said: https://en.wikipedia.org/wiki/List_of_oldest_extant_buildings That my friend, is a list of old buildings. But what will survive on a ejected (via brown dwarf, now orbiting), frozen Earth? Quote Link to comment Share on other sites More sharing options...
razark Posted February 11 Share Posted February 11 (edited) 4 minutes ago, Mr. Kerbin said: That my friend, is a list of old buildings. But what will survive on a ejected (via brown dwarf, now orbiting), frozen Earth? Probably one of those. If they're not gone by now, they'll probably be around for a while, unless someone decides to destroy them deliberately. And things newer than them tend to be considered more disposable. Edit: I'd really like to see a good argument for the expenditure of time, money, and effort to bulldoze the Great Pyramid of Giza... Edited February 11 by razark Quote Link to comment Share on other sites More sharing options...
DDE Posted February 11 Share Posted February 11 4 hours ago, JoeSchmuckatelli said: If I take a standard diesel engine (like for a tank or military truck) and instead of using the power output for mechanical engagement of the drivetrain use it to generate electricity, and given that electric motors generally have great torque, using electric motors to move the relevant wheels can I get better performance? Diesel-electric transmission is an established technology - because of the above as well as difficulties in engineering gearboxes of appropriate size. Almost all the huge dump-trucks use it because of the torque at zero RPM, although apparently both the diesel and the electric engines lack performance in high RPM modes. In tanks, you've got Saint Chamond, Tiger(P) and Maus for petrol-powered units utilizing the same concept, alongside experimentals from every tank-building nation under the sun, although cooling and the required amounts of copper seemed to be major obstacles. Quote Link to comment Share on other sites More sharing options...
farmerben Posted February 11 Share Posted February 11 4 hours ago, DDE said: Diesel-electric transmission is an established technology - because of the above as well as difficulties in engineering gearboxes of appropriate size. Almost all the huge dump-trucks use it because of the torque at zero RPM, although apparently both the diesel and the electric engines lack performance in high RPM modes. In tanks, you've got Saint Chamond, Tiger(P) and Maus for petrol-powered units utilizing the same concept, alongside experimentals from every tank-building nation under the sun, although cooling and the required amounts of copper seemed to be major obstacles. Taking the motion of a shaft into electricity and then back into a shaft involves losses. What percentage is lost I'm not sure but 10% is quite significant. It makes the most sense if you stagger the inputs and outputs in time. A hybrid that stops and starts frequently and has good batteries is ideal. If you need to motor continuously for many hours than basic diesel powered shaft is going to be more efficient than hybrid. Quote Link to comment Share on other sites More sharing options...
JoeSchmuckatelli Posted February 11 Share Posted February 11 7 hours ago, razark said: power 1 hour ago, farmerben said: involves losses 5 hours ago, DDE said: stablished technology Thanks fellers. The part about losing efficiency via converting rotary motion into electrical potential and back again makes sense. Did not even think about it. What I was thinking about was whether we could get 90% of the power output but greatly extend the range of operations. Late night thinking about not optimizing for emissions made me think there was opportunity. But now that I think about it, everything involves trade offs. Adding batteries adds weight, requires space and presents new critical infrastructure that needs to be protected and maintained. Trying to cut out (or extend) one part of the log train doesn't make sense if you have to build out another. Still - one of the limitations of modern military vehicles is fuel use and the massive logistics infrastructure required to keep them operating in the field. Quote Link to comment Share on other sites More sharing options...
farmerben Posted February 11 Share Posted February 11 (edited) Diesel fuel has 44 times more energy/kg compared to lithium ion batteries. That is 4400% better. *correction its not that much better because the conversion of thermal power to shaft power is around 40%, whereas electric battery combos are closer to 90% Edited February 11 by farmerben Quote Link to comment Share on other sites More sharing options...
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