shynung

Most people imagine space combat would just involve ships shooting at each other, with various spaceborne weaponry. Little attention was given to the scenario I will now describe. The premise: You are an astronaut on EVA, floating a few meters near your ship. Not some sci-fi warship, just a common space capsule, something like the recent Orion spacecraft. Ahead of you floats another astronaut, his suit bearing a flag of a group hostile to yours. Both of you are civilian astronauts, wearing regular space suits rather than some space-grade armor, and have no weapons other than your fists, what tools you happened to have strapped on, and your EVA jetpack. He wants you to be out of action just as well as you want him to be so. The question is, how would you effectively put him out of action (doesn't have to be lethal), while preventing him from doing so to you, using as little effort as possible?

I thought one would 'dock' onto an asteroid, not 'land'. There's barely enough gravity to keep itself from coming apart.

True, there isn't. But it does place quite a strain on the engines' material, so it depends on the engine's particular design and material.

That's because Russian engines typically use kerosene/LOX mix as propellant. In terms of molecular weight, oxygen is much lighter than kerosene (having somewhere between 6-16 carbons per molecule). In a rocket engine, for a given chamber temperature and pressure, the exhaust velocity is higher the lighter the exhaust gas molecules are. For the Russians, this means cramming as much oxygen into the chamber as possible. As a result, some of the oxygen simply becomes hot without actually reacting with the kerosene, then gets quickly dumped out of the nozzle. Oxygen, however, is notoriously corrosive at high temperatures, so the engines using this mix are understandably of non-reusable designs. Rockets using liquid hydrogen as fuel (Space Shuttle, Ariane 5), however, face a backwards situation: their fuel is much lighter per molecule than their oxidizer (LOX). That's why their engines run fuel-rich, in addition to the extra fuel lowering their chamber temperatures (hydrogen has good heat absorption properties), giving them much longer engine service life (both vehicles used their main engines almost all the way to orbit). Also, said Russian engines are able to use a single turbine for both fuel and oxidizer pumps, because the density of kerosene and LOX isn't very far apart.

That's about half. The other half has to do with propellant chosen for the rocket. I'll explain the engineering aspect. For simplicity, let us assume a monopropellant engine, whose primary components are composed of a propellant tank, an ignition valve, a reaction/combustion chamber, and a nozzle. Let's also assume a single type of monopropellant, to ignore the chemical side for now. The simplest type of liquid fuel rockets are pressure-fed rockets. This has no turbopump, so the pressure needed to transfer propellant to the chamber are either supplied by a separate pressurant gas, or provided by the propellant's own vapor pressure, using a heating element in the tank. This design is often used in engines designed to operate fully in vacuum, including the Apollo SPS. While simple and efficient, this system has a limited maximum thrust, because the pressure in the chamber goes up in proportion to the thrust. At some point, the pressure generated by the pressurant gas or propellant vapor pressure will fail to exceed the chamber's, resulting in no more thrust being produced by the engine. At this point, the solution is simple: install a pump. Turbopumps are chosen for their high power-to-weight ratio (the amount of work it does, compared to its mass), to keep the engine's overall TWR high. But at the same time, the turbopump brings its own problems, the most notable of which is that it requires power to operate. Some designs simply route some of the gases from the chamber into the turbopump, some others install a separate chamber for the turbopump. The next question regarding the final performance of the engine is: where does the turbopump exhaust go? One solution used often is throwing it overboard from an exhaust pipe. This gives the maximum thrust available, as the turbopump runs efficiently on its own, delivering the maximum propellant flow rate into the chamber. However, since the propellant used to run the turbopump is thrown out, it is no longer available to produce thrust, resulting in lower specific impulse. This design, called a gas-generator cycle, is used frequently in engines used for lower-stage work, one notable example being the Saturn V's F-1 engines. The other solution is piping the turbopump's exhaust into the main chamber of the engine. This is to reduce the loss of efficiency associated with dumping propellant off to the side, resulting in a more efficient engine compared to the gas-generator. However, since the turbopump exhausts into the main chamber, it has to deal with back-pressure from the reaction in the main chamber itself, needing a separate chamber with a higher pressure. In addition to lowering the turbopump's performance (leading to lower mass flow, therefore lower thrust), it also exposes the turbine blades to much harsher working conditions than a gas-generator engine's, requiring exotic and/or advanced materials. This makes the final engine more complicated and expensive, but gives more specific impulse than a gas generator, at the expense of lower TWR. This design, the staged-combustion cycle, is used for upper-stage engines or in engines that are never staged away even if other engines are, a notable example of which is the Space Shuttle Orbiter's RS-25/SSMEs. Note, there are other engines that work on slightly different cycles that I may have missed (one cycle simply heats the incoming propellant via regenerative cooling of the chamber and nozzle, rather than reacting it, and using the expansion of said heated propellant to drive the turbopump), but the most common chemical rocket engines in service today typically use one of these three cycles. And that's a little bit of rocket science from the engineer's side. For the chemical side, read this book to explore the lair of the beyond-insane-yet-brilliantly-intellectual crackpots, known to the industry as 'propellant chemists'. EDIT: ...aaand all of a sudden I just made a wall of text.

It'll work. Given a stoichiometric O/F ratio to maximize energy output, and a large enough airspeed, it'll work similar to a ramjet. Though, one would need some sort of compressor turbines if you plan to use it in low-speed crafts, such as landers.

That's what I was thinking. Airbreathing jet engines have lower actual exhaust velocity than rockets. Their nozzles were not designed for maximum exhaust velocity, but to balance it along with mass flow from the air intakes.

Uh, ramjets are more similar to turbojets than to rockets. They're basically a turbojet minus the turbines. They don't, as a rocket does, carry their own oxidizer. That said, I don't see much improvements over a traditional ramjet design, as they are already more fuel-efficient than pure rockets.

I was under the assumption that the mass driver would act as an engine, i.e. docks to a client spacecraft and pushes them around, using orbital debris as propellant. Yes, I did not completely read your post, but with that wall of text, something's bound to be missed. However, the exhaust velocity problem is still there. It is in the mass driver owner's best interest to not fire a piece of debris into a retrograde orbit, lest his spacecraft would risk get hit by its own exhaust. However, collecting the propellant for the mass driver is a complicated task, to put it lightly. If the spacecraft owners were concerned about mass-efficiency of the system (effective Isp), they'd set the system to fire into a retrograde escape orbit, which brings its own energy requirement problems on the table. Ideally, the exhaust velocity should match the mass driver's current exhaust velocity, so that it would fall straight down into the atmosphere, to present the least risk to neighboring spacecraft. This means adjusting to different payload mass by adjusting the exhaust mass, which can limit the effective Isp, in addition to aggravating the propellant collection difficulties. Here, the mass driver owner is presented with a dilemma: (A)fire the propellant on a retrograde escape trajectory to conserve propellant mass (exhaust velocity > 19.5 km/s, effective Isp about 1950 seconds), requiring massive and powerful (=expensive) powerplant in the process, or (B)fire the propellant at the spacecraft's current orbital velocity (about 8 km/s at LEO), to skimp on the power generation system, at the cost of propellant mass efficiency (effective Isp about 800 seconds), putting considerable loads on the propellant collection system in the process. Which plan do you think is better? Or do you have a better alternative?

If it's costly, get insurance. If it's sentient, backup the programs into another chassis, so that losing the original in a great ball of fire has little effect to the entity. Really, LES is for irreplaceable payloads, like human astronauts (you can't copy human memories and personalities into another body. At least not yet). If humans themselves can be copied in relative ease, LES would be obsolete.

What needs to escape? There's no one inside the spacecraft, so no one dies of the rocket go boom. Launch escape systems are designed to save human lives on manned spacecrafts. One manned spacecraft that interestingly has no LES is the Space Shuttle. The crew can't do much if one of the main engines suddenly quit.

So it's like VASIMR, where the engine can alter its exhaust velocity on-the-go. Though, that also puts a difficult twist on the engine's operations: the exhaust velocity must (A)not be larger than twice the orbital velocity (so the exhaust doesn't go retrograde, smacking into stuff), or (B)be larger than earth's escape velocity (so the exhaust gets flung out into solar orbit). Option B is fine for satellites, since they only need thrust for stationkeeping, but heavy payloads like interplanetary missions would use option A for the need of high thrust. Given that the specific impulse of the engine is limited by that plan, that means a large propellant bin full of scrap, which would go head-on against the rocket equation (all that mass needs to be moved around, too). Alternatively, one can install some sort of grinding device into the engine, so that any orbital scrap used as propellant is ground to particles small enough to cause little damage to other spacecrafts, but it would raise the needed power requirements, and ultimately effective Isp, as that powerplant mass would still need to be hauled around wherever the engine goes, not to mention the grinder itself.

One problem I see with using this kind of propulsion: impact hazards from the exhaust. Exhaust gases emitted by typical rockets, both chemical and electrical, disperse after a relatively short distance (a few kilometers) enough to render them essentially harmless to other spacecrafts unlucky enough to be in their orbital path. Solid objects spewed by mass drivers stay more-or-less in chunks even after thousands of kilometers away from the engine itself. Given that an impact from a single paint fleck produced this crater on a Space Shuttle's front window: It can be said that this mass driver engine is as good as a propulsion system as it is as a weapon. That, alone, brings political problems to the owners of the spacecraft. But I digress.

As of today, the only spacecraft that's being constantly resupplied is the ISS (Even then, they don't currently need things like nuggets of platinum or the like). The other possible customers are space telescopes like Hubble, which occasionally needs repairs. Also, no matter how easy the processing is in the absence of gravity (or practically any other troublesome earthly conditions), even if the machines doing them is much lighter than their earth-bound contemporaries, it's still be quite heavy to be lifted out into orbit. To put it simply, one does not use solar panels designed for GEO comsats to mine asteroids. Anything powering the beast would have significant mass on its own. All that would translate to high launch costs, even before considering that they have to get to escape velocity to get to the prospective asteroids, for none of the rocks have stable orbits near the earth. It's not impossible that the final profit of the extracted resources would be big enough to sustain a company, but it's hard to see anything indicating that condition from here. I'd look at PR's mission report before claiming anything silly.

Here's one.

Jupiter is so massive, the barycenter of the Jupiter-Sun system is outside the Sun's surface, making it wobble around as the gas giant orbits it. If, somehow, Jupiter's mass was to be multiplied instantly, it will pull the barycenter even further from the Sun's surface, possibly disrupting the orbits of the other planets, including Earth. Considering that it may alter the surface temperature of the Earth considerably, I believe anyone living in such an era would be much less fortunate than we are today.

Uh, this? This is a ZM-87, a Chinese laser device designed to damage laser rangefinders, cameras, missile seekers, and human eyes. It's no longer in production since 2000 due to a UN protocol on blinding laser weapons, but the few units that are produced have been reported being used by the Russians and North Koreans.

I recall that Apollo 13 had an oxygen tank explosion, even though caused by a short-circuit from the stirrers, so I'd be careful around those stuff. LOX, as it is, isn't hypergolic, but it is murderously flammable. Whatever cooling system the tank has, it has to be very good. Well, a tank floating in space is, in effect, a thermos flask the size of the universe... Note, nuclear missiles lurk inside steam-heated and air-conditioned missile silos, so they have more leeway in choosing their fuel. Most missiles from the Cold War era used 50-50(Aerozine 50)/N2O4, from what I've read(they used solids these days). Some used kerosene/nitric acid(mainly Russian ones), with some fluoric acid (HF) mixed with nitric acid to stave off corrosion. It's not storable indefinitely, but something like that might stay good for a few decades. In another note, I've got an idea. Why bother with storing liquid fuels that eventually boil off anyway? Why don't, say, store water-ice in the tank, put a heater and electrolyzer unit nearby, and process the ice into fuel as needed? The devil's advocate in me points out the energy requirements, but I'll see if I can get more ideas. There's a reason Soviet rocket engineers, working with RFNA (nitric acid, with some 20-27% N2O4), nicknamed the stuff 'the Devil's venom'. Nasty stuff alright. Nibb pointed out that Mir and Progress operations have included fuel transfer capabilities, so yeah, it's definitely possible. Now it's just a matter of finding out potential customers (there's none as of today) before the fuel in the tank boil away. At least half a dozen flights to the Moon every year, I think. Also, I think this discussion merits a new thread.

Just a small issue, but annoys me a bit. The texture for the core drill disappeared (it became white) when I use this mod alongside SCANsat. Deleting SCANsat fixes the issue, though.

"Into the universe... and beyond!"

Regarding LH2/LOX boil off, there's another problem: hydrogen molecules are so small, they simply seep through the molecules of the tank's material. No matter how cold the tank is kept, unless the hydrogen is frozen solid, it will eventually leak off into space anyway. Fortunately, there is an alternative. Methane (CH4) has larger molecules due to the relatively heavy carbon atom, so they don't leak off into space as hydrogen does. It also boils off at a higher temperature(111.6K, as opposed to H2's 20.3K), so any active cooling system would have its load reduced. Also notable is that some oxidizers (I'm looking at you, nitric acid) are apparently so energetic, that if kept too long, they degrade the tank it's in, along with the associated plumbing. I'm not sure about LOX or N2O4, but I wouldn't be surprised if those oxidizers does the same thing, seeing that they are powerful chemicals in their own right. So yeah, fuels can't be stored in the tank indefinitely; either they boil off out to space, or they chew through the tank and then leak out to space. Northstar, you mentioned about fuel transfers in Apollo missions. As I read from the wiki, the transfer mentioned between the Apollo CSM and LM meant the transfer of astronauts (people moving to the LM). The CSM and LM each have their own fuel tanks, which was full at launch, so no fuel transfer occurred. There are fuel stirrers in the Apollo spacecraft (along with plenty other safety devices), but no fuel transfer plumbings.

As I said before, such a craft would need a small engine for stationkeeping and disposal (deorbiting), otherwise it could fall out of orbit before it can be sucked out of. The small engine's fuel should be separated, so that a refueling spacecraft cannot accidentally completely empty the reserves and render it incapable of deorbiting. Yes, I understand, which is why I pointed it out. I've read about possible design solutions to mitigate the problems depicted in the article, so I'd leave it as an engineering problem for now. Since you stated your agreeing position on nuclear power plants, I see no reason to defend it. I'm on your side. I think that's a different dogma altogether. Nuclear power plants deal with energy generation, while robots deal with industrial processes. It may take centuries, maybe even a millenium or two, depending on the situation. Point is, having widespread nuclear energy use is an important milestone in human history. Some people will resist it, just as some people resisted the idea of a sun-centered solar system back in Galileo's time. Eventually (however long that may be), either nuke power plants and GMOs will be accepted, or the humans will have to endure some energy(and food)-deficient era before they realize its potential. Not really sure which will happen, looking from my end.

That full fuel tank could be an entire upper stage without the engine. Of course, having engines onboard means the upper stage can now push other spacecrafts around (and be a tug itself in the process). Does hypergolic fuels degrade with time? Anyway, being used to KSP with fuel-transfer-capable docking ports, I realize that the concept would need to be adapted to current conditions, maybe even significantly. However, I don't think it would be long before someone established a safe way to transfer fuel between spacecrafts. Or, one could go with the simpler way, docking a fully-fueled spacecraft with a waiting fully-fueled transfer stage, and use the transfer stage to push the spacecraft to where it needs to go Wasn't it like that when the steam engine first came out? People were afraid of fire and boiler explosions these things occasionally cause, but it was widely used in the end anyway. I still think that the anti-nuke dogma will disappear eventually, for the energy demand will inevitably rise along with the population. I'm not sure how long are we supposed to wait, though; we might as well be nonexistent when nuclear power is finally widely accepted.

An Energiya rocket designed for bulk fuel delivery? Seems promising, although I had to admit that today's heavy-lift rockets could do the same thing just as effectively. Of particular note, one could send a full fuel tank into orbit, equipped with small and efficient engines, some solar panels, and some unmanned craft avionics, in order to do stationkeeping. Other spacecrafts with further destinations could launch unfueled, dock with this flying fuel tank (an orbital support vehicle, as I called it) and take their fuel from there. A single OSV, if large enough, can support multiple outbound spacecraft before running out of stored fuel, and can be simply deorbited later. I'd rather not talk about nuclear politics here (in fact, I hate politics), but in reality, it's difficult to launch a nuclear space tug without provoking the ire of certain people. Yes, I agree that reusable fuel depots and nuclear space tugs would be incredibly useful in intensive space missions, but some people (not me) doesn't like the idea of using nuclear energy to power our spacecraft (or indeed, anything at all). That's one of the reasons solar-electric propulsion is currently being developed as well; to get around this anti-nuclear dogma. Although, Ad Astra did say that their more powerful electric engines would require onboard nuclear reactors (their energy consumption is quite significant), so I think the anti-nuke dogma will disappear eventually. I can only hope their research will yield something very useful within my lifetime, and that I get to ride in whatever vehicle they strap the engine to.

It's not a stupid question, but it's certainly a loaded one.