Catch up on stories from the past week (and beyond) at the Slashdot story archive


Forgot your password?

Slashdot videos: Now with more Slashdot!

  • View

  • Discuss

  • Share

We've improved Slashdot's video section; now you can view our video interviews, product close-ups and site visits with all the usual Slashdot options to comment, share, etc. No more walled garden! It's a work in progress -- we hope you'll check it out (Learn more about the recent updates).


Comment: Re:Fair business practices. (Score 3, Interesting) 67

by cjameshuff (#49355989) Attached to: US Air Force Overstepped In SpaceX Certification

The "established" guys were compensated for having to follow those rules by being given cost-plus contracts that guaranteed profits and provided incentive to inflate costs whenever it could be justified, and actively punished reductions in costs.

So: they were applying the same restrictions to SpaceX, without giving them the same benefits, since SpaceX operates under fixed-price contracts: they sell a product, get paid, and their ability to make a profit and continue existing is dependent on keeping expenses low. What was that about fairness?

Comment: Re:Light going faster than the speed of light? (Score 1) 162

by cjameshuff (#49322589) Attached to: How Space Can Expand Faster Than the Speed of Light

More than the frame of the emitter at its higher location in the gravity well, the light actually appears to slow as it passes through the well (Shapiro delay). Measured locally, the light is traveling at the same speed no matter where you look, but it's taking a longer path through curved space-time and its speed in a distant reference frame can be something different.

Comment: Re:Stupid Question (Score 1) 162

by cjameshuff (#49308213) Attached to: How Space Can Expand Faster Than the Speed of Light

Stated another way, as your travel speed approaches the speed of light, your experienced travel time approaches zero. Note that time dilation actually happens both ways...the ship sees the surroundings as being time dilated just as much as they see it as being time dilated, and any physical measurements taken on the ship while it is coasting will be indistinguishable from those taken "at rest". It's only the fact that it's the ship that accelerates at the start and end of the trip that breaks the symmetry.

However, you reach a point where you have to convert most of the mass of your ship into energy well before you reach really high levels of time dilation. If the EmDrive worked as described, it would break a number of conservation laws and allow production of energy and momentum from nothing, which would be a way around this, but more realistically is an indication that the EmDrive doesn't work.

Comment: Re:Requires Gravity: Won't work in space (Score 1) 95

You'd only need a centrifuge, and only one providing enough "gravity" to confine a shallow puddle of liquid, which doesn't take much. The powder fusion approaches are better suited for the stuff you'd need to print in space, though...just keep that powder out of your life support filters. (not to mention your lungs)

Comment: Re:UV sensitivity (Score 3, Interesting) 95

Paint is almost never the solution. Paint involves additional equipment and manufacturing steps, dealing with adhesion and coverage issues, loss of fine details, sensitivity to wear and scratching, and so on. Plastic parts are almost always unpainted, instead incorporating pigments or other stabilizing additives within the plastic itself. These can't be incorporated into 3D printer resin for obvious reasons.

Comment: Re:The moon is a better idea anyway (Score 1) 228

"This ignores that other space craft on reentry... using your aerobraking method have to take similar stresses."

No, they do not. The heating occurs due to compression in a detached shock in front of the vehicle. Much of it is radiated away immediately, the vast bulk of the remainder is left far behind the craft, and the craft itself only needs to handle a tiny fraction of it.

"You say it will meet with the impact of a tank shell. But we're talking about brushing the surface not impacting it at a 90 degree angle. The translated energy will be vastly lower."

No. The energy is a function of the relative velocity. The angle is completely irrelevant. You are converting the kinetic energy of the cable and spacecraft into heating of the cable and ground via friction, and the cable alone has enough kinetic energy to completely destroy it.

"The issue will be can the surface withstand the friction and heat. A surface similar to diamond should withstand the friction."

Apart from the fact that a hypothetical diamond super-cable isn't a substitute for the present reality of would not, and the surface of the moon isn't perfectly smooth diamond. Your cable will make first contact with projections such as mountains, hills, boulders, crater edges, etc. It will separate explosively at the point of contact and the portion below will slam into the side of whatever the obstruction was. It may remove the obstruction in the process, but that's of no help in braking your spacecraft. This might be a useful method of landing on very low gravity objects...providing both a deceleration method and a way of securing the payload to the surface...but the idea is completely unworkable at the speeds involved in landing on the moon.

You might be able to engineer a hypervelocity runway landing, a very smooth aluminum surface with a cushion of injected gas to support the craft and electromagnetic braking to reduce velocity until you can make physical contact and stop, or you might be able to put up a lunar space elevator or surround it with momentum exchange tethers, but this gets back to the infrastructure problem, and it might well be cheaper to just land on rockets.

Comment: Re:The moon is a better idea anyway (Score 1) 228

The impact velocity will be nearly double the muzzle velocity of Abrams M829 armor piercing shells. The lunar surface consists of mountains, boulders, craters, and so on. There is no "just brushing the surface", the portions of the cable that actually make contact would be vaporized.

Even if the moon were polished smooth and you were able to lightly drag the cable across the surface, the kinetic energy of the cable itself is 2.88 MJ/kg at minimum just to brake the portions in contact. Friction will convert that to heat. The areas in contact additionally have to brake the portion of the cable that is above the ground, and of course the payload itself. With aerobraking, the excess energy is shed extremely effectively by compressing the atmosphere in the path of the vehicle and leaving it behind as a streak of incandescent gas. With your suggestion, it is your cable that becomes a streak of incandescent gas.

Comment: Re:The moon is a better idea anyway (Score 1) 228

Assuming the cable doesn't snag or break, your suggestion leads to the payload impacting the surface at slightly under the velocity of the lowest possible circular orbit, about 1700 m/s. That's at the end of the deceleration the anchor can provide, your trajectory intersects the surface after that. The cable of course must first survive impact at the initial 2.4+ km/s in order to provide this deceleration. This is not a realistic requirement.

There are other possibilities (momentum exchange tethers particularly stand out), but they require support infrastructure either on the moon or in orbit.

Comment: Re:Terraforming Mars: why? we can do better than t (Score 1) 228

A mass driver has severe limitations in reachable orbits and imposes very strict limits on payload mass and volume. It also means higher accelerations, which makes it a poor fit for launching people. It is also still less efficient, even the highest mountain is far deeper in the atmosphere than first stage separation, so the mass driver is limited to lower velocities. SpaceX would have needed to build several of them in order to launch to all the orbits the Falcon 9 can reach, and it would have needed to rebuild them for the Falcon 9 v1.1. The Falcon Heavy would have been impossible, that payload increase would have needed a completely new, much larger mass driver. And many of the payloads they've launched simply wouldn't have survived mass driver accelerations. And of course, there's the little issue of not having a conveniently located mountain to put those mass drivers on.

Mass drivers are a complex, expensive, difficult, limited, and impractical way of reducing the size of the first stage. SpaceX is taking the much simpler approach of just recovering and reusing the first stage. They have already made drastic reductions in launch costs and are making steady progress on further reductions...all without a single megastructure. Musk seems to know what he's doing.

Comment: Re:At this point Mars is running before you can wa (Score 1) 228

If it worked, the end point of this suggestion is a planet with a viciously toxic ~60 atm nearly-pure-O2 atmosphere, and a surface covered in a thick layer of explosively flammable algae dust. You would be explosively flammable as well, especially if your suit is at ambient pressure, which would require a breathing gas mix that's almost entirely hydrogen. Following the inevitable inferno, you would once again have a CO2 atmosphere.

It doesn't work though, because there's not enough water for plant life to lock more than a tiny fraction of the CO2 away in carbohydrates and lipids. You need to import hydrogen, lots of it...almost 10% of the mass of the current atmosphere in hydrogen. Then you have plenty of water, and get to work on the nitrogen problem...there's about 3 Earth atmospheres worth of it there.

Comment: Re:At this point Mars is running before you can wa (Score 1) 228

My favorite is the Pioneer Venus Multiprobe, a set of 4 little *atmospheric* probes. One continued sending data for over an hour after hitting the surface, despite not being designed as a surface probe...the atmosphere's just so thick that the terminal velocity was survivable.

We now have silicon carbide electronics that can operate at ambient surface temperature, and numerous other ways of dealing with high temperatures and harsh environments, so we could build Venus surface probes that would have indefinite operating lifetimes. RTGs would be a useful power source, the "cold side" temperature of RTGs that have to radiate waste heat in vacuum is similar to the surface temperature on Venus.

Comment: Re:Works both ways (Score 1) 228

You realize geothermal power involves boiling water and spinning turbines as well, don't you?

And solar panels don't just magically appear in a truck, they have to be manufactured, a complex and high-energy industrial process involving a wide array of hazardous substances. The panels themselves frequently contain hazardous materials, and need to be collected for recycling or safe disposal at the end of their life...provided the operator is willing to pay for it.

They also don't have much available power to convert, generally actually make use of less than 30% of that, and your investment will spend half its time in the dark, not doing anything useful, so you have to cover the landscape with them and build gigantic energy storage systems to handle the variations in production...real "elegant".

Comment: Re:The moon is a better idea anyway (Score 1) 228

Space probes use assists *by planets* to adjust their velocity around *the sun*. The maneuver can not be used for changing the speed relative to the object it's being done around. You can not slingshot around the moon and reduce your velocity relative to the moon. If you are on a trajectory that intersects the lunar surface, your kinetic energy will reach a maximum and potential energy a minimum at impact.

You could use gravitational assists around the moon to adjust the orbit around Earth into one that makes it easier to reach the moon, but you can't do anything to gravitationally brake an object coming in for landing. Such an object is trading gravitational potential energy for kinetic energy, and its velocity will be at a maximum at impact. No gravitational trick can make that potential energy disappear.

Comment: Re:The moon is a better idea anyway (Score 1) 228

"If I can use an orbit to accelerate myself away from a body,"

You can't. The speed relative to the gravitating body in a "slingshot" maneuver is exactly the same on the way out as it was on the way in. The maneuver is useful because it allows for a change of direction and because depending on the approach, the change of direction can mean adding or subtracting the motion of the gravitating body relative to a third body. Slingshotting around the moon can put you in a higher or lower orbit around Earth or allow a cheaper transition between Earth orbit and a Mars transfer orbit, it's not going to help soft-landing on the moon.

Comment: Re:The moon is a better idea anyway (Score 1) 228

The moon has its own gravity, enough to accelerate an object dropped from a large distance to around 2.4 km/s by the time it reaches the surface. It is also in a circular orbit around Earth, and anything reaching it from Earth will be either have a much lower orbital velocity at the high end of an elliptical orbit, or be on a high energy trajectory that has a similar orbital velocity in a quite different direction. And no, it is not possible to slow down a trip to the moon. If you're not going fast enough, you simply don't reach the moon.

And aerobraking at Mars doesn't have to bring you to a stop on the surface to be a benefit. Yes, you still have to carry propellant, but you don't need the vast majority of your craft to be propellant. If 2/3 of the mass you send is payload instead of 1/9th, you're sending 6 times as much payload for a given mass sent to Mars.

The big advantage of the moon is proximity. It's reachable with small craft that only need to operate for days at a time, and it's close enough for equipment on the surface to be monitored and remote operated in near-realtime, with a couple seconds of lightspeed lag instead of up to 40 minutes. Emergency evacuation means reaching Earth in a few days, and delivery of equipment to address failures or unforeseen needs is mostly limited by the time needed to prepare a launch.

Porsche: there simply is no substitute. -- Risky Business