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Comment Re: Who cares? (Score 1) 303

Considerably more than that. You need enough propellant to fuel the tanker for a trip from the lunar surface to LEO with a load of propellant, and then back to the lunar surface empty. For the tanker itself, that's roughly as much total delta-v as launching to LEO from Earth. In the hypothetical "Mars as stepping stone" scenario, you'd burn most of the propellant you produced delivering propellant to the Mars vehicle. And that's only after burning more than enough propellant to establish a Mars colony to deliver all the needed mining and refining equipment and propellant tankers to the moon.

Refueling on the moon isn't a way around this: you need to get the spacecraft, supplies, and personnel there first, which takes more delta-v than sending them to Mars and would require first refueling them in LEO or launching them from Earth with enough propellant to go straight to Mars.

Comment Re: Who cares? (Score 1) 303

And that "gently landed on the Moon" bit is expensive...more expensive than landing something on Mars. Each tanker landed on the moon will consume more propellant in doing so than an equivalent-mass Mars vehicle would in going to Mars...and we haven't even filled and launched the tanker yet. And then each tanker will have to reserve enough of its cargo to take it to Mars and then some in order to return to the moon for its next load. Propellant is going to be a limited resource, expensive to extract on the moon, and you are proposing to burn huge quantities of it to refuel a Mars craft.

Someday, when we have cities on the moon and lunar mass drivers hundreds of kilometers long that can hurl propellant payloads that can reach LEO with a small burn as they pass Earth, lunar propellant might become an economical way to slightly reduce operating costs for a steady stream of Earth-Mars traffic, but it's not something that's going to help us get there in the first place.

Comment Re: Great idea... But there is a problem... (Score 1) 303

You do the math. Due to the availability of an atmosphere for performing around 6 km/s of the braking on arrival, it takes considerably less propulsive delta-v to go straight to Mars than it does to land on the moon, and that doesn't even include the subsequent launch from the moon. Red Dragon is a Dragon 2 capsule with minor modifications, and it can carry about 1-2 metric tons to the surface of Mars. The surface of the moon is well beyond its reach without an additional stage.

Every launch to the moon could instead be a Mars launch carrying more payload. Every propellant launch from the moon requires a tanker vehicle to spend enough propellant to land that it could have gone to Mars. And that's ignoring all the landings required to set up large scale ice mining and're talking about a sizable colony on a body that's more expensive to land payload on than the moon. The moon is not a stepping stone to Mars.

Comment Re:Would be nice... (Score 1) 75

It also depends on the way the helium is expanded. In free expansion, it'll cool and gain a large amount of kinetic energy. If expanded through an insulated porous plug, it'll gain a small amount of kinetic energy but heat up. It's basically down to where the energy released in the expansion ends up.

As the helium swirls around the tank, turbulence and friction will convert the kinetic energy it gained in expanding into the tank to random heat. However, when it is released into the tank, I'd expect it to cool due to experiencing more or less free expansion, so while the average temperature of the tank may rise, cold spots seem likely.

Comment Re:Not Harvard architecture? (Score 1) 101

It runs at 160 MHz. Processors that run directly from flash are much slower (around 32-48 MHz...ST's Cortex M0 processors run at 48 MHz). The only flash-based processors that run at comparable speeds do so with complex hardware to read instructions ahead of time in large chunks, storing them in SRAM until the processor requests them (ST's ART Accelerator, for example)...which can result in difficult to predict variations in execution speed when branches result in the needed code being something other than what was preloaded. Luis mentioned working on some method to "virtualize RAM" in the other reply, which might be a somewhat similar system, which again would sacrifice determinism for speed.

Comment Not Harvard architecture? (Score 2) 101

The high speed is because they currently don't have any on-chip flash (flash being slower to access than SRAM, and typically being what slows 32-bit microcontrollers down). That means this isn't a single-chip solution like most microcontrollers, though they are working on changing that.

Instead of flash, they store their program in the same SRAM used to store data (which makes that 8 kB of SRAM a lot more limiting than it would be on a Cortex M0 with the same amount of SRAM plus 16-256 kB flash). Most microcontrollers use a Harvard architecture with separate program and data memory, allowing instructions to be fetched from flash while performing reads from and writes to SRAM. If they don't do this, I wonder what sort of performance they'll see when they have to make regular reads from a slow flash memory in between SRAM accesses. Or will they just load the entire program into SRAM? That's not going to be ideal in terms of power consumption, requiring a much bigger memory array than they'd otherwise use, something that's going to get worse as they try to compete with larger microcontrollers.

Also, the Harvard architecture has some advantages in security: things can be set up so a very specific sequence of actions has to be performed to enable writing to program memory. With IoT devices, this sort of thing is becoming more important...not an issue at present, with their 8 kB memory, but something to consider when thinking about this thing's future.

Comment Re:The 90's called.. (Score 2) 121

Current satellite internet is that way because all the data is funneled through a handful of satellites up in geostationary orbit. This system uses a much larger number of much closer satellites, so latency's far lower, signal levels and link bandwidth are higher and you don't need a big dish to make your link budget work, and system bandwidth is orders of magnitude higher.

Comment Re:Can someone explain this to me? (Score 4, Informative) 99

It's just a really, really terribly written article.
There is a theoretical object called a "naked singularity", a black hole without an event horizon, which stuff actually would be able to escape from. This isn't one of those. The author's calling it "naked" because it doesn't have any of the usual stuff around it...except it's not even that. It's the remnants of the core of a galaxy: a few thousand stars, some gas, and a black hole. The x-rays come from surrounding debris falling into it, not the black hole itself. The black hole isn't hemorrhaging anything, the gas is just debris that the core wasn't able to hold onto after the collision that stripped most of the rest of its stars and gas away. It doesn't even mean anything to say "it may never stop"...stop relative to what?

It's just sensationalized gibberish.

Comment Re: Good attitude (Score 1) 98

Astronomy is better done away from the gravity, dust, and temperature extremes and without the obstruction of half the sky by a giant ball of rock. Power generation is better done in open space where you can have constant direct sunlight. And semiconductor fabrication can be done at whatever effective gravity you desire in orbit.

The biggest reason to go to the moon is to study the moon. When space infrastructure and technologies are more advanced, it'll be a useful source of raw materials in Earth orbit. But at the current early stages of actually developing that infrastructure and technologies, it's an expensive distraction.

Comment Re: Good attitude (Score 1) 98

You could use a silicone binder. Primarily silicon and oxygen, neither of which is exactly scarce. Major downsides include being yet another fuel with solid combustion products (and a pretty terrible fuel apart from that), and requiring a rather complex chemical industry to produce.

And all of these options have the really major downsides of very poor performance, the complexities of producing large solid fuel cores, and inability to refuel the craft landing on the moon. If you want to reuse the same craft for multiple trips, your task is much, much easier on Mars.

Comment Re: Good attitude (Score 1) 98

It's enough atmosphere to be a substantial assist in landing mass on the surface, and actually does provide significant radiation protection while also moderating temperatures. The perchlorate issue is massively overstated: they are not that toxic, and are easy to remove, and there's entire glaciers of water on Mars.

Comment Re: Good attitude (Score 1) 98

The need for huge energy storage systems or nuclear power from the very start is a significant problem for the moon. The game-breaker though is ISRU propellant production. Getting enough water on the moon to supply return craft will require large scale mining and regolith processing facilities...meaning any return propellant will have to be imported until the colony is well established. On Mars, it should involve little more than drilling into a glacier and lowering a heat source to sublime the ice, which makes it a lot easier to get your spacecraft back so you can use it on another trip. The relative ease of delivering mass to Mars and greater proportion of the delivered mass that can be productive colony hardware can do a lot to compensate for the greater distance and travel time.

Comment Re:I'd rather see (Score 1) 348

Lunar helium-3 mining has always been about as plausible a suggestion as strip-mining the moon for green cheese. Helium-3 is a byproduct of storage of the tritium that 1st generation fusion will breed for fuel, and if you can do He-3 fusion, you can do p-B11 fusion. So: by the time we can make use of it, we'll be able to mass produce it far more easily than we could mine it, and we probably won't even bother with it due to availability of far more abundant fuels.

Comment Re:Perpetual motion machine of the first type (Score 1) 532

A "working EmDrive" would be a reversible electromagnetic machine, functioning equally well as a motor or a generator: (page 6, ironically titled Conservation of Energy)

If allowed to accelerate, the microwaves in the cavity would red-shift and lose energy, if accelerated in the other direction, they would blue-shift and gain energy. A "working EmDrive" placed on one end in Earth's surface gravity would thus either continuously create or destroy energy.

The reasonable conclusion is that there is no such thing as a "working EmDrive".

Comment Re:Perpetual motion machine of the first type (Score 1) 532

He claims that, but his analysis is based on velocities relative to a fixed universal rest frame, which he seems to believe is the same reference frame as Earth's surface. For example, he says that it produces thrust most efficiently if "stationary", and is best used to counter gravity and allow a craft to hover, with jets and rockets being used for propulsion (he seems unaware that a hovering craft is one that is accelerating upward at a constant 9.8 m/s^2).

If you really take Shawyer's math and vehicle concepts seriously, he's apparently a stationary-Earth geocentrist. More likely, he's just clueless about physics.

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