Slashdot is powered by your submissions, so send in your scoop

 



Forgot your password?
typodupeerror
DEAL: For $25 - Add A Second Phone Number To Your Smartphone for life! Use promo code SLASHDOT25. Also, Slashdot's Facebook page has a chat bot now. Message it for stories and more. Check out the new SourceForge HTML5 Internet speed test! ×

Comment Re:Light Sail vs. EM-propulsion (Score 1) 160

The EmDrive is not EM propulsion, it's pseudoscientific nonsense, confirmation bias in action. As claimed to operate, the EmDrive is a closed system that does not emit anything, and so does not conserve momentum or energy. EM propulsion using on-board power would be a photon rocket, and while it might have some exotic applications in things like precision formation flying, it's not going to be good for any more than that without the ability to convert matter into EM radiation with high efficiency.

Also, even if you can make a compact power source that would be good enough for a photon rocket, it's still probably going to be massive enough that it's better used as a portable beam station pushing on a photon sail. Rather than point it forward and brake the whole thing, point it backwards, cut it loose, and use it to brake a much-lighter payload equipped with a photon sail.

Comment Re:SpaceX landings may eliminate this advantage (Score 1) 44

It's only 1.5 m in diameter, too small to use as a space station component, and as a SSTO is necessarily built with absolute minimum mass just to enable it to reach orbit, razor thin structural margins and none of the mounting points, ports, or other hardware needed for conversion to a space station component, let alone shielding and thermal control systems...even without such things, its payload is only 100 kg. Add them, and your vehicle is now suborbital. (Assuming they can reach orbit in the first place. If their vehicle ends up massing slightly more than they've estimated, it ends up falling short of reaching orbit. After the whole pendulum fallacy thing, I'm a bit skeptical of their ability to competently model and estimate the vehicle mass.)

And a SSTO isn't a more efficient way to launch anything to orbit, a booster stage would greatly increase its payload and mass budget. The whole reason we use staged vehicles is because they give us a huge increase in efficiency by working around the rocket equation's exponential growth. The fact that it lets us use engines tailored to a particular atmospheric environment is just a side benefit.

Comment Re:SpaceX landings may eliminate this advantage (Score 1) 44

Except they have competitors flying two-stage rockets, who are going to be cheaper than an expendable SSTO due to not having to deal with the tight mass fractions and low payloads of SSTO craft...SSTO only makes any kind of sense if you bring the thing back for reuse, which they're not doing with their stated mass ratio. Those competitors are in the $4-10 million range, $1 million is likely wishful thinking. (Keep in mind, ARCA's the group that cooked up a rocket stabilization system based on the pendulum fallacy. These people...well, they aren't rocket scientists.)

Comment Re:Just wait for Falcon Heavy (Score 1) 260

Carbon fiber over an aluminum honeycomb core, which might be problematic, especially if it's vented to allow trapped air to escape. I expect they'll use the helicopter approach.

They just used a medium-heavy lift liquid fueled first stage on its second launch, they'll figure something out.

Comment Re:Just wait for Falcon Heavy (Score 1) 260

They seemed to be planning on a wet recovery for this set, with a ship in place and no news about helicopters. They might not bother with them until they're confident they can stabilize them for reentry and get the parafoils working. That at least one came down mostly intact is quite promising...

Comment Re: Who cares? (Score 1) 317

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) 317

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) 317

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 refining...you'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.

Slashdot Top Deals

Work is the crab grass in the lawn of life. -- Schulz

Working...