christok

Check the note right at the top of the post. I accidentally said 2000km instead of 2000m because I was tired and when I noticed I put a note there rather than edit the whole thing because, again, I was tired. The value then becomes 1.96J/kg - 1.05J/kg, hence the 0.9J/kg specific energy.

Edit: Oops, I wrote megametres instead of kilometres. That makes the numbers significantly different but the conclusion is the same: Your astronaut escapes the surface with about 0.9J/kg specific energy over and above that required for escape velocity. (Continuing the discussion here rather than in PM because it's relevant to everyone interested.) I may well be wrong about the 20cm/20in confusion. If a NASA scientist said the centre of mass jumps up by 20cm on average and only the feet go up by much, you should use their value rather than some guy on the internet's. We already know potential energy at the top of the jump in each case is equal to the kinetic energy at the bottom. However, there are two important caveats: 1) This is energy relative to the surface and not energy relative to the centre of the planet(esimal), which is what we'll be using for the orbit calculations. 2) g * m * h is an approximation which is only valid if the height is very small relative to the distance to the centre of the planet(esimal). We can use g * m * h for the jump on Earth because 0.2m << 6Mm. Instead of regular kinetic and potential energy, we can use specific energy which is the energy divided by the mass and completely ignore the mass of the astronaut. epsilon = specific energy = 0.5 * V^2 - mu / r, where mu = G * M. So at the starting point, epsilon = KE - mu / 2km on the comet where KE = g_earth * h_earth is the specific kinetic energy at the start of the jump and equals potential energy relative to the surface for the Earth jump. Thus epsilon = 1.96 J/kg - mu_comet / 2km = 1.96J/kg - 3.14 * 10E12 kg * 6.673×10^-11 m^3 s^-2 kg^-1 / 2km = 1.96J/kg - 1.05J/kg unless I made a mistake somewhere. Now we have a little shortcut. I did say energy makes it easy! You don't need to calculate the height you jump to. If (specific) orbital energy = 0, you have a parabolic trajectory. If it's < 0, you have an elliptical or circular orbit. If it's positive, the trajectory is hyperbolic and therefore we know that the astronaut escapes.

More problems with your analysis: - An average man jumps way higher than 0.2m, which is only 20cm. You may have misread 20in. Try this table for better values: https://en.wikipedia.org/wiki/Vertical_jump#Vertical_jump_norms - You need to account for solar tides, especially in borderline cases, so you will have a minimum and maximum value depending on where the body is located and where the person is standing. Nitpicking: - You should add a reasonable minimum mass for the spacesuit since it's significant relative to the mass of a man. (Including movement restrictions due to the suit would be difficult, so I recommend neglecting that on the basis that you may be using a more advanced form-fitting suit.) - Energy makes mechanics easy. Use energy instead of acceleration. Your centre of mass is accelerated over the same distance on Earth as on the asteroid, which is the distance between where you start crouched and the point where your feet no longer touch the ground. Work is force times distance and the force your legs exert is the same in both cases, so work is the same. By the work-energy theorem, (sub-)orbital energy gained is the same. Increase in E_specific ~= 0.5m * g on Earth, so E_specific = 0.5 * 9.8 = 4.9 J/kg is gained on the asteroid as well. Add that to the potential energy between the surface and the centre of the asteroid to work out how high your new apoapsis is.

3: A high albedo would certainly help. Oceans have a lower albedo than dry land, and dry sand higher than wet sand. Water is also a potent greenhouse gas. You can thus make your planet a little cooler by making it a little drier. Don't make it bone dry. That introduces all sorts of problems on geological timescales, mainly due to water binding atmospheric CO2 to minerals. To further increase the albedo, you can consider either making the crust richer in white minerals or look for something cloud-forming that would add a higher anti-greenhouse effect than greenhouse effect. 5: Volcanism requires a large partially molten metal core in the absence of tidal heating. This could be a little tricky to explain depending the ice sheet, as it should have rapidly grown into an ice giant out beyond the frost line. Perhaps a collision between an icy body and a rocky one would result in something reaccreting with a substantial icy crust, or perhaps its orbit could have been disturbed during formation? Or perhaps the hydrosphere is similar in size to ours, but frozen at the surface? Thin atmosphere and dangerous weather don't sound like a good combination to me. I'd rather make the atmosphere thicker, or subsitute things like toxicity/corrosiveness for dangerous storms. A bit of danger from a (possibly rare) thunderstorm is something you could fit in somewhere. How about a normal atmosphere Snowball Earth instead? 6: Your largest planet should form in the first position after the frost line. This looks fine to me. Volcanism is only really possible at the innermost moon or two unless they're ludicrously oversized. And out far from the planet, they likely won't be. Since this planet must have formed outside the frost line, expect mostly icy moons and captured asteroids outside of the volcanic ones. Subsurface oceans are likely common but very deep down. Smaller moons will be less completely differentiated so the more even mixture of rocky and icy material near the surface is probably more interesting--and more nutritious. 10: I'm thinking about all sorts of potental problems with getting it into that orbit depending what you mean. I think you're distracting from the story by adding too many planets. I'd probably lose at least 2 and 8-10 but it's your choice.

Let's not regard the matter of what we're going to grow as anywhere near settled. There are several options and we'll need to do a little research on all of them, as well as work out the thermal design, before we can make a final decision. My current status: I'm going through K^2's maths for the simulation. Expect it to take several days since I'm also reading up on some physics I'm not familiar with.

And arcjet is lame because it wears out quickly. Anyway, I mentioned resistojet simply as an example of one electric thruster that can use a wide variety of propellants. The main reason for using an arcjet is that it produces much more thrust than most electrical thrusters, which is not equally important for all applications.

Of course I won't mind f you use it. But if you put up a link you should probably say that it hasn't really started yet. Otherwise you'd get people excited and they would not come back to the project after the initial letdown.

I'm also against full sunlight but that's not something to worry about yet. The species hasn't been decided but we have asked for more info on which mosses are the commonly used model species.

There are three types of electric propulsion: - Electrothermal is a normal rocket except the propellant is heated electrically instead of chemically. - Electrostatic, AKA ion engines, accelerate electrically charged ions by means of electrostatic fields. - Electromagnetic engines, AKA magnetoplasma engines, accelerate a plasma using the interactions between electric currents and magnetic fields. Besides the important points MoparGamer and Ralathon raised about xenon for ion engines, these general principles apply: - You generally want the propellant to have the lowest possible molecular mass because you get higher exhaust velocity. This favours hydrogen and to a lesser extent helium and is the reason why some electric and nuclear designs use those fluids. - HOWEVER electrostatic thrusters require high voltage/low current electrical systems for high propellant particle masses and low voltage/high current systems for low particle masses. This strongly favours high atomic masses to the extent that there has been work on droplet-propellant ion engines to further increase mass. Xenon wins hands down over hydrogen or helium in this case. - Low density makes for bulky and heavy tanks. - At least in thermal engines you want the propellant to have as few internal degrees of freedom as possible because these suck out some of the energy that would otherwise increase speed. This favours noble gasses such as helium and xenon. - Cost. The ideal propellant is not the one with the highest delta-v per unit mass, it's the one with the highest delta-v per dollar spent. (The latter may tend to be the same as the former for especially later stages in the mission.) This favours, for example, hydrogen over helium. - Toxicity/corrosiveness/fire hazard. You want to make it as easy as possible to produce, store and launch the spacecraft and associated propellants. This tends not to be such a big problem with electric thrusters but favours noble gasses over hydrogen. Some thrusters, such as the (electrothermal) resistojet, can in principle run on almost any fluid. Don't we all dream of ISRU...

I started that. It's mentioned earlier in the thread but I don't blame you for not finding it The idea was to get some sort of outline going. There isn't anything to see yet, as you may have noticed. I also need to rewrite some stuff where I guessed wrong about how it's going to work and upload what I've done. Sorry, but I had a major project at work and have been on call, so I haven't done as much as I should have. (I.e. nothing.)

Sorry I've gone AWOL for a little while. Will get right back on it (work permitting).

Don't tell me someone has a malfunctioning neocortex and then claim that just means "stupid". Don't tell me mild neurosis means temporary anger. I'm not going to bother with this sort of argument anymore.

[citation needed] These do probably contribute to the problem but having several mild neuroses is normal. Unless you happen to be a psychopath, you almost certainly have them.

All of our brains work like that and failing to recognise it can lead you down the same path. The brain builds a complex framework for understanding the universe through everything you experience in your life. When one little fact seems to contradict a large framework covering a broad range of phenomena, it makes more sense (in general) to try and find a way to reinterpret the fact in terms of the framework than to rebuild the framework in terms of the fact. This is exactly the same thing we do when we see a claim about superluminal neutrinos: we assume there must probably be a mistake because it would invalidate to many things we think we know. You and I have had broader and better exposure to the topics at hand and have, as a result, built ourselves relatively decent frameworks for understanding the science and engineering behind spaceflight. Your typical "nutter" isn't so much insane as chronically ill-informed over a period of many years.

Personally, I think Squad is doing a lot for public education with this game. They created what all those stupid "educational games" were supposed to be. And I do not fault the game for simplification nor for being a little too easy--it's a stepping stone to greater understanding.

So you're referring to the case where the two rockets are a matching "barrel and rod" (or some variant thereof) firing off each other and accelerating only for the time that the rod is inside the barrel. Fair enough in that case. I only skimmed the article but I think it refers more to two separate structures with an explosion which does not rely on them being in contact, at least not for the entire time they are being accelerated.

All I've done so far is create a few functions to return orbital elements for the conic approximation at a given point in time. I'm going to leave the advanced stuff until I have an idea of your maths, which will affect the code a lot. Not that I did much yet anyway. Ah, yes. I started writing float (force of habit), thought better of it in the middle of the night and neglected to fix it so far. I was beginning to suspect you may call me out on it if you look at the code.

I've pointed out that real rockets can handle more than 18 000g. You claimed they don't count because they're not 25m long. What exactly are you claiming?

Maintaining CO2 concentration is easy. Just add more carbon-rich compost. Container gardens do not in fact kill themselves for lack of CO2 anyway. The problem is with maintaining pressure.

No, they're not. You said it's impossible for the material a rocket is made of to handle the acceleration and I pointed out that's wrong. I see no reason to limit discussion to rockets of precisely 25m. (The gun-barrel component of the spacecraft is itself already assumed capable of handling the same forces it would have to handle when firing attached to some base. Its strenght is therefore not a concern.) In any case, a rocket of that length would be impractically fat if it had to withstand such acceleration. Square-cube law. The strength of the material is proportional to its area while the mass (and hence force) is proportional to its volume. You can make a rocket longer but as you do so the required increase in diameter grows much faster than the increase in length. Ever wondered about all these stories about ants lifting X times their own weight, which is like a human lifting tonnes? Or how fleas can jump so high it's like a man jumping over a multi-storey building? They're all rubbish for the same reason. An ant or flea wouldn't be strong enough to stand if it were scaled up to human size.

We've all experienced that before. It's from stuff you sort-of know but haven't really practiced. Go for an introductory mechanics MOOC.

But then, if it isn't Chinese anymore you've made progress. You can skip over most of the details for now. Ignore stop & start transients. Pretend the walls are adiabatic. Etc. Use only the spherical cows each topic starts with and skip over the more accurate methods until you've improved your skills. Look up terms that don't make sense and if they still don't make sense, put the book down. Anything seems a lot easier when you read it a second time days later.

Some things to consider: - Glass can be quite brittle. I don't think the launch provider would accept any old glass, if they allow it at all. - Most types of glass do absorb UV, but we haven't done the calculation. We don't have much thickness to work with and it might not be doable. - Sunlight up there is very intense. I'm still afraid of cooking the experiment if we get our design wrong. Springs I can live with but any motor adds tremendous complexity. A motor that has to work repeatedly is just asking for failure, and this would be a single point of failure for the entire mission. I am absolutely against such a scheme. Concerning the night side, a forward-facing window will not need nearly as much insulation as it will receive considerable infrared radiation from Earth. LEO is fortunately not as harsh as solar orbit. I was thinking along similar lines. I was also thinking that we need to run a test before we launch anything, to see if we got something experiment-killing wrong. We should build a mock satellite and run the test in a homemade vacuum chamber for an extended period to see if our seals need work. I can't do it because the power goes out every so often. K^2, I created a blank repository called "ksatsim" on Github for the simulator (same username). I added the LGPL licence since I don't really care what anyone wants to do with it but I don't like the "someone slapped his copyright on a bugfix"-type scenarios that more permissive licences enable. If you have reason to prefer something else, speak up. I'm also going to upload a basic C++ project outline (autotools files, main.cpp, etc.). I don't know your preferred toolchain but I certainly won't complain if you add Visual Studio project files.

Fair enough. The problem is that you want to know rocket engineering without the technical details, which contradicts itself. You'll just have to learn them. Rocket Propulsion Elements by Sutton & Biblarz is a pretty standard textbook in the American SOI but I think you may want to start with something a little simpler. (The mathematics required to understand the book isn't all that advanced so you could probably get away with doing some calculus and mechanics on Coursera if you worked hard. You also don't need to understand everything in a book to benefit from it.) If you can't find the books you need, try getting the names of suitable books and get an inter-library loan if you can't find them locally and can't afford them. Microcosm Press has a fairly large collection, as does Amazon. P.S. I think you meant motor. Engines are liquid-fueled and it won't help you to look up the wrong term.

It sounds to me like you basically asked "please tell me everything". That won't work. You'll need to do the research on your own, then you can contact someone with a specific question when you get stuck, along the lines of "I tried this and that as recommended by these books, but I'm not getting it right. Could you tell me what I'm doing wrong, or at least point me in the direction of the right books?" As for publishing your work, a proper scientific journal is not for you, nor is arxiv. You want to find a science journal for kids, of which there are several.