K^2

It's not about energy requirements. It's a simple Hohmann transfer equation. His numbers check out. So a single bottle, no dice. But something with 2-3 stages of coke and metos might do the trick. Of course, if the top stage is a single bottle, you'd need at least 6 for a stage before, which is entirely feasible, but a 3-stager is probably out of the question. At least, as far as launching something from ISS.

That's at the maximum rate of fire, which is almost never used.

Depends on how you build your nozzle. Mythbusters managed to get the fountain to about 29 feet. That is about 13m/s. Or ISP of 1.34s. The total delta-V is probably going to be significantly lower than what NovaSilisko estimates, however, since you'll probably not end up with just a mass of the empty bottle in the end, and you certainly won't have that high of an ISP through the entire run. I wouldn't expect anything more than 20m/s of total delta-V. So yeah, no de-orbiting from ISS, but it'd still be some sight to see it launched.

Yeah, that's a good way of putting it.

Not really. You'd certainly start having problems in hypersonic regions, but you can build a prop plane to match the Mach 2 supercruise of modern fighters. It's just going to be very difficult to design (impossible without modern computers), awkward to control, less efficient, less maneuverable, and prone to all sort of failure modes that aren't even an option with a jet. Thing is, the faster you go, the smaller amount of air you should be pushing for a given amount of fuel burned. A high performance turbojet is already something like 2:1 after you take stoichiometry into account. Running a prop from a turbine at these ratios just doesn't make much sense. A turbojet is lighter, more reliable, and ultimately, more efficient. At lower speeds, however, the more air you push, the higher the effective ISP of your engine. So your best options are a turboprop or a turbofan. Both have their own advantages and drawbacks.

Yeah, I just realized that you are right. I mean, it's not hard to scale so that all motion matches real satellites, but then LEO satellites would end up in atmosphere. So it'd have to run with the real system mod, then. And no, I haven't tried it either and make similar assumptions.

Could be a weight class difference. A10 is massive for a CAS plane. And it's much easier to build a turbofan with good thrust-to-weight for a larger plane than for a smaller one. Though, with advances in VLJ tech, I would bet that even the lightest of CAS are going to be turbofans in the near future.

Well, this would have to either work with the real Solar System mod, or convert all orbital elements and perturbation parameters to scale them down to Kerbin size. Both are entirely reasonable options.

The biggest effect on Earth satellites is the fact that Earth isn't a perfect sphere. So orbital elements evolve over time due to perturbations. It would probably be pretty straight forward to write a mod that looks after that stuff, though. I can write the code for it, but I have zero experience with KSP modding, and I'm not sure I have the time to learn all of the details. But if somebody wants to team up for this, let me know.

That would be absolutely horrible in terms of efficiency. At best, you are getting 25% of the total impulse, while absorbing 50% of total energy. There are no materials that can withstand that sort of energy output while still giving you anything like a decent thrust. For every 1N of thrust you gain with such a drive, you'd have to dissipate 600 megawatts of heat. If you were to build your pusher plate out of Tungsten, it would only radiate 8.5 megawatts per square meter. So you'd have to have a pusher plate that's at least 70m² per 1N of thrust. And that plate would have to be at least 10cm thick to actually absorb the radiation. (This is a very rough estimate, but the order is right.) So we are talking about 140T of tungsten in shielding for every Newton of thrust. The only place that's going fast is nowhere. You might be able to improve on this a little bit by using different materials. Carbon will perform a lot better, but it's going to slowly evaporate at high temperatures. Maybe you could use carbon encased in tungsten? You could drop that value from over 100T to just a few tons. But we are still talking about something that can't even accelerate at 1% of g by a wide margin. If you want a ship that can manage a 1g acceleration and an ISP of a matter-antimatter rocket, you have to build an efficient photon drive (perhaps a variation on q-thruster, or maybe something using Mössbauer Effect) and power it with a very efficient matter-antimatter reactor. How you manage to get a nearly 100% efficient reactor that's light and produces enough power to drive a photon drive like that, I have no idea. Ok, I have some ideas, but only because I haven't done the math on them, and I'm sure they wouldn't work anyways. Point is, we don't know how to actually build something which is both powerful enough and efficient enough to go at these sort of speeds. We know that it's physically possible, but we've got less thank bupkis on this from engineering perspective. This is the main reason I'm keeping my eye on the warp drive development. Even if we can't beat speed of light with it, just getting to .1c with it seems more likely right now than with a rocket of any kind. And when I'm considering warp drive as the more likely scenario, you can see how things don't look good.

Erm... This very effect is derived from Maxwell's Equations in the Electrodynamics text by Landau and Lifshitz. That's precisely what I was talking about earlier. People really need to start spending more time learning theory. It's sad when publications like this actually happen.

Exhaust velocity of a high ISP ion engine is around 120km/s. 10% of speed of light is 30,000km/s. There isn't enough matter for use as propellant in the known universe to accelerate a single nucleus of cargo to these speeds with an ion drive. Does this put the statement into a little bit of a perspective for you? Edit: It is, in principle, possible to achieve .1c with nuclear pulse drive, but even that is so far beyond feasible that it might as well be impossible. The only realistic way of reaching .1c using reaction drive of any kind is with a high efficiency matter-antimatter photon drive. And we don't have a clue how to make one, let alone get fuel for it.

Because a moving object generates gravity differently from a static object. The analogy is electric charge. If it's not moving, it just creates electric field. But a moving charge also creates a magnetic field, and a moving magnetic field induces its own electric field, and so on. As a result, the electric field lines point towards the current location of the object, rather than to where it was. Again, assuming the source is moving at a constant speed. If source accelerates, other weird stuff happens. Related, but not the same thing. Space-time curvature is frame-invariant. Gravity is not. But if you know space-time curvature and have a chosen coordinate system, you can work out gravity. (Alternatively, you can look at gravity as a gauge field, which ultimately leads you to exactly the same conclusion, with coordinate system choice being your gauge choice.)

Again, you can't ask a question that starts with, "if the Sun were to disappear." For this to happen, entire theory on which gravity is based has to be completely wrong. You are asking a question based on a false premise, and therefore, it has no valid answer. You have to have a change that is physical in order to ask what the physical consequences are going to be. No, it orbits Sun's current position, as I have explained in an earlier post. And WestAir's point is consistent with that. Except for the Sun disappearing bit, which we can't take into account, because that is not itself physical.

We don't have prop fighters anymore, but we still have prop fighter trainers. T-6 Texan T-6 Texan II There is just no contest. Better aerodynamics, better materials, and a turboprop in place of a four-stroke. Take a look at the capabilities at the end of the page. Texan II has higher cruise speed, higher ceiling, longer range, and is still cheaper to build and operate than the original Texan was. If a modern fighter had to be built with prop propulsion, it'd be beyond reach of any WWII fighters. Throw in modern electronics, and even without missiles, modern planes would be able to go against WWII equivalent without losses. By the way, the old Texans are still a lot of fun to fly. They might not have the power or the speed of the modern equivalent, but they are still way more agile than you'd ever think such a machine could be.

I don't know. That's a far more complicated problem. But it's a 3-body problem to begin with, so you expect some weirdness either way.

They are not instant either. Just like electromagnetic equations, Einstein Field Equations are local. Mass only affects space-time where it is located, and gravity propagates further via interaction of space-time with itself. So any perturbation propagates at the speed of light. Actually, no. I have no idea how to derive this result in proper GR, but at least for the case of interaction between planets and stars, using gravitoelectromagnetism is good enough. And just like in normal electromagnetism, there is a very cool effect. All of the fields are oriented in such a way that you feel attraction towards the point where the object actually is, rather than where it was with appropriate time-delay, so long as source travels at uniform speed. This is a very important result in electromagnetism and it carries over to gravity. If this wasn't the case, planetary systems would outright collapse, because the gravity fields would all come in at an angle, causing systems to use energy as if through a very strong tidal interaction. Naturally, if the source suddenly accelerates, that's something you cannot possibly predict until fields propagate, so that's something that's going to have an effect on how things move. But constant velocity allows things to keep working as if all fields propagated instantly.

Yes, the gravitational perturbations propagate at the speed of light. Asking what happens if you just "delete" a planet is a bit pointless, because the very event is a violation of the known laws of physics, so asking what happens according to laws of physics if you break laws of physics has no meaning. But if you were to, say, accelerate the Sun all of the sudden, yes, Earth would only notice the change in 8-something minutes. This goes for both light and gravity. The reason that universe can expand faster than c despite that is that it inflates locally. At no two nearby points does anything move faster than c relative to each other. But over a great distance, this adds up, allowing two remote objects to travel faster than c relative to each other. Gravity propagation is purely a local thing, though. So it can't exceed c. Things get way more complicated once you go from small perturbation to something more complicated. We've been having a lot of discussion about warp drives lately, and that's just one example where things aren't so simple.

I don't see any reason why it would do so. If you can dig up that article, I'd like to take a look at it.

If it's that unstable, the whole thing is useless. Even though practical warp drive would have to dramatically reduce the amount of negative energy required, it's not going to be anywhere near that low. I agree that anything we could build with resources we can get in foreseeable future won't survive through the atmosphere, but it has to be capable of withstanding particle radiation in Earth's orbit or it's too unreliable to ever be used. This requirement puts you in MT ranges easily.

I was thinking about it a bit more. There is an interesting possibility. To first order, gravity works like electromagnetism. So one can build an electrogravitomagnetic artificial gravity generator. First, you build two giant mass current loops. In practice, they'd probably have to be particle accelerators. The two loops are in the same plane, side by side, almost touching, and mas rotation is in opposite directions. The region of artificial gravity is going to be located between the two loops. The way you generate gravity is by giving the whole thing a bit of a spin around the axis that connects the two loops. That will generate enormous stress on the structure, as the two gyroscopic effects will fight each other, but no net torque, so this will not require any energy to maintain, other than what you need to run particle accelerators. Why will this generate gravity? Faraday's Law of Induction. Mass current generates gravitomagnetic field. Because that field rotates, the total flux is time-variable, and that's going to induce a gravitational field around the rim of the current loop. Unfortunately, effect isn't strong. Given some surface density àof the current traveling at some proper-velocity u, the total mass current is J = ÃÂu. That gives us the field strength B = (4ÀG/c²)J. The total flux is going to depend on the area of the loop, Φ = ÀBR². Time derivative for a rotating loop (axis of rotation perpendicular to axis of the loop) is dΦ/dt = ÉΦ. And that induced potential is divided along the perimeter of the loop. So we get g = ÉΦ/(2ÀR). Putting it all together, and taking into account the fact that we have two loops, we get the following overall formula. g = ÉR (4ÀG/c²) ÃÂu The biggest problem is the G/c² term. It is equal to 7.425x10-28 m/kg. Units work out because àis in kg/m². In fact, lets talk about ÃÂ. This is density of the particle stream in accelerator times the thickness of the stream. Later is limited by what we can do with the magnets. Former is very difficult to get to a high value. Let me push it to the limits of imagination and say that we can get the stream almost as dense as air, and we have a 10m gap between magnets. That gives us à= 10kg/m². Next, lets talk about É. The limiting factor is the same as in centrifuge. Humans don't like going spinning at high rates for very long. For a comfortable experience, the limit is about 2RPM. If we are prepared to venture a bit beyond comfortable, we can do, maybe, 5RPM. That gives us É = 0.52/second. Finally, the one place where we can make decent impact is u. That's proper velocity and we have a particle accelerator. Woot. LHC launches protons at well over 400GeV. Lets say we can achieve 1TeV per nucleon in our matter beam. Mass of a nucleon is just 1GeV. So we've achieved u = 1,000c. So if we wish to walk away with an Earth-like acceleration of g = 9.8m/s², using all of the above parameter, we get the loop radius of (drum roll, please) R = 6.69x1014m = 4,470 AU = 0.07ly. In other words, a structure with above parameters is not only implausible, but impossible (an internet cookie to whoever figures out why) But if you don't care how much gravity you are generating, so long as you are generating some, and it's not due to the object accelerating with centrifuge, or whatever, you can just get to flywheels spinning in opposite directions next to each other and then start rotating the assembly. Gravity you can generate this way is going to be less than detectable, but it'd be completely artificial. Can this be improved on? Maybe. You can try increasing both density and the proper velocity. You need about 6 orders of magnitude between the two to make the structure plausible. It would still be a megastructure, though, and centrifuge is much, much easier to build.

Hot knife, meet butter. What I don't know is how long the bubble will last before collapsing. Original Alcubierre Drive will punch through and through, but that's just because the amount of energy that goes into one makes even the mass of entire Earth completely negligible. Something a bit more realistic would collapse early on, perhaps even in atmosphere, but that would probably just result in a massive explosion. So depending on how plausible a warp drive is going to be, the effect can range from large nuke to total destruction of the planet.

Alcubierre Drive is prediction of core theory. That's not fringe. Some of the ideas on how to build one in foreseeable future can be qualified as fringe, yes. But yeah. The term fringe is sometimes used seriously, to mean theories still being developed or tested. Ones that are thought to be more likely to be found false than true, but hopefully, with some benefit to understanding why they don't work. Or they can be used to describe various crack-pottery. It all depends on the context. On the topic, to the first order approximation, the only way to generate a constant gravity field is by having enough "charge" to maintain it. In case of gravity, the charge is mass, which isn't practical. But there are many different ways of generating time-dependent gravity fields. They are equivalent to using either an accelerated ship, with everyone walking on the floor while ship accelerates, or a centrifuge, or any number of related concepts. So you don't have to think about complicated gravity stuff. Understanding of classical non-inertial frames of reference is sufficient. Gravity is non-linear, however, so there might be some shortcuts there. We haven't found any yet, but that's not to say that they don't exist.

If we actually find some systems like that, I'd also vote fore naming them "moonlets". Until then, the question is academic, so we aren't likely to have any sort of standardized term.

People would probably be a bit cross about only being allowed someone else's children. But if you allow two children, one of each, then you'll probably have no trouble having people go along with it, and you'd have good diversity.