pavelcherepan

Great idea. Really like reading your posts, but can't usually comment on anything because you put a lot of thought behind what you write. Still, I wonder whether there is a significant enough necessity to map the Moon at 75 mm resolution? Wouldn't it be better to concentrate resources on preparations for manned Mars mission?

I would try to solve it differently. When you burn 1 mol of CH4 completely you get 1 mol of co2 and 2 mol of co2 when you burn butane. Normal co2 concentration is .4% then you get the first equation you need to solve: x + 2y = (10.88-0.4) Then you're also down on oxygen. You use 2 mol of o2 to completely burn methane and 3.5 mol to completely burn 1 mol of butane. Normal o2 concentration is 22% then: 2x + 3.5y = (22-(100-85.99-10.88)) Solving thaw two equations you get molar concentrations of methane and buthane in original gas mix. I get 1.22 mol of butane and 8.04 mol of methane. Actually, this is all wrong. Nitrogen concentration is 86% meaning that all ratios have changed.

From what I read in the Earth's upper atmosphere move at around 5 km/s with some more energetic particles peaking at ~16 km/s which is well above Earth's escape velocity. So then if you have similar upper atmosphere conditions and a planet with an escape velocity of 16+ km/s hydrogen will not escape very readily and you'll have a gas giant. http://faculty.washington.edu/dcatling/Catling2009_SciAm.pdf http://www.teachastronomy.com/astropedia/article/Velocity-of-Gas-Particles Then we can use the escape velocity formula: [latex]v_e=\sqrt{\frac{2GM}{r}}[/latex], but if we approximate the shape with a sphere then [latex]M=4/3*r^3*\pi*\rho[/latex], hence [latex]v_e=\sqrt{8/3*G*r^2*\pi*\rho}[/latex] Then if we say that our largest possible rocky planet has a average density a bit higher than Earth, say 6000 kg/m3 and has an escape velocity ~15 km/s so that hydrogen can still escape through both Jeans and hydrodynamic escape, then: [latex]15000 = \sqrt{8/3*6.67*10^{-11}*3.14*6000*r^2}[/latex] [latex]\frac{15000}{0.001832}=r[/latex] [latex]r = 8187772.92\, m[/latex] Probably can be slightly bigger depending on how close it's to the star, what the atmosphere thickness is and so on, but probably not by much. EDIT: Also thinking about it a bit more and potentially you can have an extremely large rocky planet (it's just a guess) if it forms close to the parent star and most of the hydrogen gets blown away to the outer regions while the protoplanet is still small.

There would be advantages in having depots on the Moon, but all of these are completely negated by the prohibitive energy costs. Going LEO->surface of the Moon->Low Lunar Orbit requires ~7.8 km/s of delta-v and this is huge amount of energy. LEO to low-Mars orbit requires ~7.2 km/s. In essence if a spacecraft has to fuel up on the surface of the Moon it would have to carry the amount of fuel that can otherwise get it into Mars capture orbit. Lack of gravity only requires some rather simple engineering solutions, presence of gravity on the other hand comes with a lot of delta-v strings attached. I believe it was UDMH and nitrogen teroxide: http://spaceflight.nasa.gov/history/shuttle-mir/spacecraft/s-mir-detailed-main.htm On the surface of the Moon you'd still need insulation, shielding and cooling, depending on the propellant of your choice. As I see it, there wouldn't be a significant difference in micrometeorite danger between surface of the Moon and LEO simply because Moon has no atmosphere so there's nothing to prevent micrometeorites from hitting the depot on the surface. In addition to the two you've mentioned I see a third option. A craft with fuel on board sent in advance of the main launch then later it docks with the main craft and accompanies it on it's trip. That way you don't need to carry empty fuel tanks on the main launch which will increase the launch mass and costs. There will be more launch windows for a craft rendezvousing with the propellant depot in LEO compared to the Moon, because the orbital period of the depot is ~1.5-3 hours compared to 28 days. The lunar launch windows will likely be a bit longer. I can't say which one will come on top, probably LEO is still better here. https://en.wikipedia.org/wiki/Launch_window

Thanks for your link. It was an interesting read, although I might say that it might not be 100% relevant. Obviously helium can't be used as a propellant and none of the propellants used nowadays are ever in superfluid state. I guess, this experiment was more focusing on transferring helium from one spacecraft to another to be used as a coolant. Had a look around internets and, well there's been plenty of small-scale orbital refueling done and in many cases these were not experiments, it was done on a regular basis, for example, soviet Salyut 6 and Mir space stations had attitude control engines that were regularly refueled by Progress spacecraft, then there was Orbital Express test by DARPA and NASA and there are two other missions in the works - both like Orbital Express aimed at extending the service life of satellites. https://en.wikipedia.org/wiki/Salyut_6 http://www.hightechscience.org/mir_podu_refueling_control_panel.htm https://en.wikipedia.org/wiki/Orbital_Express https://en.wikipedia.org/wiki/Space_Infrastructure_Servicing https://en.wikipedia.org/wiki/Mission_Extension_Vehicle So you see, it has been done with liquid fuels many times on relatively small scale, but I can't see that there would be any major issues with scaling it up.

Thanks Enthalpy! I was hoping you'd join the discussion. I totally agree with all your points, but then there is the question - if it's really such a good idea and it's relatively easy to accomplish, why after 55+ years of space exploration still no one uses propellant depots, nor are there any plans to create any (at least not as far as I know)?

As far as I know there exists a hypothetical boson that mediates gravity - graviton. Like a photon it's thought to have a 0 rest mass and thus can only travel at c. It hasn't been observed yet, just like gravitational waves.

Unfortunately, you can't travel faster than light and I haven't heard of any attempts of solution to this issue. You can travel in a reasonable amount of time in your own frame of reference, but it will still take a long-long time in an Earth FoR. There is a solution though. Rather than creating sci-fi universe based on the premise that super-luminous space travel exist as most game designers and book writers do, you can try and actually have a think what would a space-faring civilisation look like (socially, economically etc) if they hadn't been able to solve the travel time issue. And given how popular Kerbal Space Program is, this MMOKSP might actually be worth doing.

What do you mean by final project? Are you writing a Master's thesis? If that's the case, I'm really surprised that with 6 months to go you still don't know what your topic is. The way I'm used to is that people tend to work on some topic for the entire duration of their Master's degree, not in the last 6 months. With so little time it's probably best to chose a much narrower topic so that at least you'd have enough time to study it properly.

Could you please paraphrase? It's really hard to make sense of in the current state.

Then it probably should be in Speculations. I believe it's spelled deja vu.

1. If it works as a light source then photons do escape from the device so you can't store them. 2. "Magnifies and increases photons" - care to explain how this is meant to work? 3. Why graphene? What's so special about graphene for the purpose you're planning to use it for?

First of all, the space is big, like really-really big and it's so much harder to hit something than not to. Even within the main asteroid belt distances between objects large enough to be spotted by current technologies (several meters) can be in the range of a million kilometers or more. So with that in mind there's not much danger at all that you're going to crash into stuff. At the same time asteroids do get routinely observed mostly using either visible or infra-red part of spectrum. Radars are not very useful for this purpose because of their wave length. You can only use radars to locate larger objects, but then you can use visible light and get more precision.

Some questions I had in the time spent away from this discussion: a) Does God have free will? By your logic MonDie, if omnimalevolent Satan eventually loses free will, shouldn't omnibenevolent God lose free will too? b) If God loses free will, would he be in the position to save Satan from eternal torture, provided that the latter keeps his free will? c) What would happen if both of them eventually lose free will? d) Given the probabilistic nature of free will as discussed in post #8 and given a time frame of eternity, is it possible that Satan will have changed his mind infinitely many times and then ~50% of the time he would be in Heavens with angels and the other 50% in Hell. 50% of eternity is still an eternity, so mathematically speaking it is very much possible for Satan to be tortured eternally even if he keeps free will and God is still omnibenevolent. - b) and c) and d) could all result in Satan being eternally tortured regardless of him keeping or losing free will and while the God is still omnibenevolent. e) Coming from the Problem of Evil side - if God is omnipotent and omnibenevolent why can't he stop evils that Satan does? Is it at all possible that Satan, too, is omnipotent?

I say we give him a tinfoil hat!

People tend to think that walking is easy, because you have been walking pretty much your entire life and it's all automatic in your head, you don't need ot think about it, but my 11-month old son can prove that learning to walk is hard. Think about it, at stage 1 you're stable with two feet on the ground and your body vertical. Then you lift one leg up. At this moment you center of mass moves slightly forward because you have the weight of your leg in front of you. Next, you tilt your body slightly forward to move even more off-balance. After that you "fall" forwards only stopping your fall with your front leg. Now you need to bring your other leg from behind you. If you have your body in vertical position then when you lift your rear leg the center of mass will move backwards and you'll fall back. So then you need to keep leaning forwards, bring your rear leg under your body, while at the same time moving your torso back into vertical position. And this all was just for one step. For all of this you need plenty of sensors and software to control orientation. If you design a robot that will just lift legs and plant them back down it will simply march on one spot and I trust that's not the result you're looking for. Here's a link for a DIY instruction to create a bipedal robot: http://www.societyofrobots.com/sor_biped_engine.shtml More DIY instructables: http://www.instructables.com/id/BiPed-robot-V-3/ http://www.instructables.com/id/Make-A-Simple-Bidepal-Humanoid-Robot/

Construction materials won't be the main part of the price tag - electronics and software will be. Also that will depend on the degree of autonomy that you want these robots to have. For example, if you want them to be totally autonomous as opposed to being controlled from laptop via Wi-Fi, you'll need a full set of hardware on board - a motherboard, CPU, RAM and some sort of solid-state storage (because they will fall a lot which is not good for conventional HDD's). You'll need software that will at least be capable of making the robot walk on two feet without falling over (which is not an easy task) and have some image recognition capabilities so that it can recognise the shape of other robots to be able to fight them.

Not quite. There's no need to put depots along the path of the rocket. Just at LEO or some place nearby so that we can lift lighter, empty rocket into space, fuel it up there and send it on it's way. A cost difference in launch costs between a launcher that can lift 70 tonnes and a 100 tonnes is huge, based on the numbers I've seen. At the same time lifting 30 tonnes of fuel into orbit using commercial launchers would set you back probably around $100 million. It would still be cheaper than building an enormous rocket to do it in one go. I've already spoken about rendezvous fuel costs in post #7. It's not much considering that you have lighter spacecraft to begin with (very little fuel) and the delta-v numbers are very small compared to what you'd need in total for a Mars mission for example. It doesn't matter. Spacecraft all over the place use engines after extremely long travel times and fuel does not freeze. Take Curiosity landing or Rosetta's capture maneuvers as examples. And in this example Rosetta uses liquid fuel MMH+nitrogen tetroxide and it hasn't frozen in 11 years! It's just a matter of choosing correct fuel for the mission and engineering tweaks to make sure it stays in shape. If done properly fuel never freezes so it's not an issue here.

I'm not quite sure why this is important. Could you please elaborate? Yes, but if we have a proper multi-layer insulation it will, say, reduce both input and output heat by a factor of 20 (for example). Ratio of input/output heat will stay the same, but the actual amount of heat lost will be lowered by that same factor, wouldn't it? Oooops! W*hr of course. Anyway, once again I want to bring up the fact that kerosene was just an example and regardless of which fuel is used in depot it will require exactly the same handling to prevent freezing/boil-off as it will when it's on board spacecraft on a potentially very long mission. If you're going to Mars, it doesn't matter whether you use fuel depots or go via conventional means, you'd still have to make sure that your fuel is still in working condition by the time you get there and will need to perform maneuvers for capture, and landing, and trip back. And in this case we're talking months. Since long-term missions work and new ones are being prepared, you can safely assume that there is an engineering solution to this issue that you seem so focused on. Anyway, apart from fuel freezing or flying away, are there any major issues with fuel depot strategy? I didn't start this topic to promote any agenda, rather the contrary - I believe that in NASA and other space exploration organisations there are thousands of people who are heaps smarter than me and they are all don't seem to be planning on using any fuel stations, although on paper the idea seems rather good. Hence the question, why don't they?

I really doubt that you can make something like that or even close to that as low as $30-40. A microcontroller alone would set you back at least 20-30 dollars, probably more, then also you need plenty of servos, batteries, camera, sensors and gyros for stability and motion, software to make it all work and then you need to assemble it all together. I can't really give you an estimate, but it'd be a steal for a $100 for sure. If it's for the kids you just need to do some calculations to make sure that they won't cause serious bodily harm when a projectile will (inevitably!) hit a child in the face. And also that would depend on the size of robots themselves. Sturdiest stuff would not be cheap. You can build them from carbon fibre or titanium, but that will add a huge extra price. Probably construction-grade aluminium alloy is the most cost-effective option.

As I said I was describing absolutely a worst-case scenario. And also I will repeat my question, since you might have missed it - wouldn't overheating be a bigger issue than the possibility of fuel freezing? Again, let's look at numbers. Say, we send 10 tonnes of kerosene in a cylindrical tank 2 meters in diameter. It would be 3.89 m long approximately and will have a surface area of 30.72 m2. Say, when the tank is on the sunny side of the Earth, some 30% of it's area is effectively exposed to solar irradiation (best-case scenario). Then, given that solar power in the vicinity of the Earth is 1367 W/m2 we have some 13998.08 W of solar irradiation that the tank is exposed to. By Stefan-Boltzmann law, using a temperature of the fuel at 273 K and full area of 30.72 m2 we have 9675.09 W of radiative power that the tank is emitting (in case of no insulation at all). If it's on a 500-km circular orbit with a period of 1.57 hours, Earth's shadow is something around 2.36 radians, or ~0.376 of the total orbital period. Then, if we maintain orientation of the tank so that it gets the maximum amount of the sunlight at all time when not in the shadow for one orbit we get a total power: PT = (1-0.376)*13998.08 - 1*9675.09 = 8734.8 - 9675.09 = -940.29 W/orbit or -598.91 W/hr And that amount of power will only allow us to cool down our 10000 kg of kerosene by less than 0.1 oC/hr so then it will take some 400 hours or 16 days for it to freeze. And again, this calculation is for the case with no insulation. The real tank will obviously be built with some proper multi-layer insulation and then heat losses will be negligible.

Is there a flame or spark present in the experiment? If no flame present then look in the link below for autoignition temperatures and see chose what fits your needs: http://www.engineeringtoolbox.com/fuels-ignition-temperatures-d_171.html

I'm pretty sure that hexagons are there because it's the way to cover the entire surface with polygons with equal area and of the least possible perimeter, and for honey bees that would mean that less wax is required to build honeycombs and honeycombs are all of equal size. https://en.wikipedia.org/wiki/Honeycomb_conjecture As far as the other structures are concerned, such as turtle shells, cracking clayey soil in hot weather and others - that should be because hexagon is the regular polygon with the highest number of sides that can tile the entire area and having the highest number of sides means it's the closest to the circle that you can get and circle is the lowest-energy arrangement on a plane, same as sphere is the lowest energy shape in 3d space.

I agree, but why can't we make space exploration vehicles modular so that they can dock on LEO, load fuel on LEO and then go on their way? Also, even if we want to use a big rocket to lift the actual spacecraft we can still use smaller rockets to launch fuel for it, that means that instead of launching something like Block 2 SLS with predicted launch costs in excess of $1 billion, one could launch the fuel using commercial rockets and then launch the spacecraft using a cheaper Block 1B. For the price of extra $500+ million that is supposed to be the difference between Block 1B and Block 2, you could launch in excess of 100 tonnes of fuel into orbit using commercial launchers. Soyuz spacecraft have delta-v between 250-390 m/s depending on the version and they dock with ISS with plenty of fuel to spare (or at least enough for later de-orbit). Found some hard numbers - for Soyuz TMA-19 the entire rendezvous required ~66.1 m/s of delta-v and there-s no hard numbers for the docking but they had to change relative velocity from ~45 m/s to 0 so around 60 m/s probably. So all in all, about 120 m/s for the entire rendezvous and docking, which is not much given that for something like a return mission to Mars you'd need in excess of 10000 m/s of delta-v. http://spaceflight101.com/soyuz-tma-19m/soyuz-tma-19m-flight-profile/ First of all, it's in vacuum which is not very conductive to heat, it's in insulated tank and would probably have solar cells and that way you could use electricity to keep it from freezing. In any case, if you're going on a mission somewhere you'd still have to take fuel with you and you'd still need to make sure that it doesn't boil off or freeze during the course of the mission, and as a result these engineering issues will have to be solved anyway, so I am confused as to why you think this is so important. Also, wouldn't overheating be a bigger issue than the possibility of fuel freezing? Or vice versa. Having functioning fuel depots in orbit will allow for cheaper space exploration, mining and manufacturing, which in turn would lower the costs of creating and supplying more fuel depots at various orbits and ultimately would lower the price even more. At the same time, fuel tanks for non-cryogenic fuels like RP1 are pretty lightweight (compared to hydrogen tanks) and so you won't be carrying a lot of dead weight from the gravity well, most of the weight will be in very useful fuel. EDIT: Back to kerosene. Even if we send a tank with 10 tonnes of kerosene into space and this tank has no insulation, is completely transparent to infrared radiation and completely ignore irradiation by the Sun, we will still end with a total cooling time to freezing point of at least 1.19 days by Stefan-Boltzmann law. Obviously, with all those other factors in place it will take much-much longer. http://www.wolframalpha.com/input/?i=%282.84*10%5E28*1.38*10%5E-23%29*%28233%5E-3-273%5E-3%29%2F%282*1*5.67*10%5E-8*60*60*24%29

The US. History shows that a lot of powerful empires break down from the inside. If we're talking about outside threats then obviously China, Russia (on smaller scale) maybe Iran.