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Comment: Re:Wrong Focus (Score 1) 123

by cjameshuff (#49367097) Attached to: SpaceX's New Combustion Technologies

Chemicals aren't going away. They have too many advantages in thrust to weight, cost, controllability, etc.

SpaceX is in the business of launching stuff to orbit, and bringing boosters and spacecraft back down to planetary surfaces. Ion engines aren't going to do that...exactly how would you suggest they deliver those reactors and ion propulsion systems into orbit? Their focus is right where it needs to be.

Comment: Re: Wrong Focus (Score 1) 123

by cjameshuff (#49367021) Attached to: SpaceX's New Combustion Technologies

The gravitational acceleration of an object isn't a constant at all, it is a function of distance. The relevant "constant" is the "standard gravitational parameter" (should be Greek mu, broken in the preview) = GM, the product of the gravitational constant and the mass of the body. This can be directly measured more precisely than G or M (G being very difficult to measure precisely, and measurements of M generally being derived from measurements of ) and is far more commonly needed in calculations than G or M alone.

More specific quantities are "surface gravity" and "surface escape velocity". These are relatively constant, given a reasonably spherical body, but also only meaningful on the surface. Surface gravity is the specific thing you were looking for, telling you how fast things fall and how much things weigh on the surface, setting the minimum acceleration needed to leave. Escape velocity tells you how deep in the gravity well the surface is, and how much of an overall velocity change is needed to leave.

Comment: Re: Wrong Focus (Score 4, Informative) 123

by cjameshuff (#49365449) Attached to: SpaceX's New Combustion Technologies

The gravitational constant is G, and is the same everywhere...it's a physical constant. The surface gravitational acceleration of Mars is different because of its lesser mass. And apart from the problem of the atmosphere, having surface gravity of about 1/3 of Earth's is nowhere near enough to make ion propulsion useful for launch, an ion propulsion system with a nuclear reactor and propellant would easily weigh around ten thousand times what it could actually lift on Mars. The only bodies where launch could be usefully performed or assisted by ion thrust are asteroids and comets.

Ion engines use very high amounts of power and very low flow rates of propellant. They provide a benefit when you need low amounts of thrust for a long period, and have either plentiful solar power or a nuclear power source. They could be used for shipping bulk supplies ahead of a manned expedition, but a manned expedition itself or any other mission with tighter than usual time constraints will use chemical propulsion, or at most nuclear thermal propulsion. These relatively low-Isp systems require more propellant for a given delta-v, but can achieve accelerations millions of times higher than ion engines, and do so without heavy power systems and gigantic radiators.

Comment: Re:Fair business practices. (Score 3, Interesting) 71

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 that...in 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 aerobraking...it 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".

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