Practically getting an asteroid full of platinum down to Earth

[shynung]

It wouldn't be that simple an engine. The problem is, the nuclear reaction produces immense waste heat and neutrons. There's supposed to be this swirling layer of water to protect the inside of the engine bell. Except, that water will vaporize, and be lost, reducing your ISP. That 479000 number sounds impossible as a result of this - the water you'd lose would ruin your ISP. The 6.7k is marginal - NERVA engines could do 1k, and you have 36 times the power output by reacting the fuel directly without a heat exchanger. That sounds possible with a lot of engineering work. There would be a set of complex systems to cool the engine, probably using droplet radiators because they are immensely lighter.

Fuel/propellant is a 2% solution of uranium/plutonium tetrabromide in water, tanked in bundles of pipes filled in-between by neutron absorbers. No reactor - fuel/propellant is injected into a long plenum bar (basically a large pipe). Critical mass develops in plenum bar, starting the nuclear reaction, turning it into superheated steam, which then goes out the nozzle. All this, compared to a full-blown nuclear reactor required for NERVA/solid core nuclear thermal rocket. Seemed pretty simple to me.

Just like NERVA, it reacts the fuel directly without a heat exchanger. Main difference is that in a NERVA/solid core reactor, nuclear fuel is separated from propellant. In NSWR, fuel and propellant is stored in the same tank as a solution, going to the reaction chamber through the same pipes. No need for reactor controls; simple valves could be used to control engine power output. Close the valves, and no more UBr4 solution goes to the chamber, and reaction stops.

What's problematic is cooling. As you said, an extensive system of cooling pipes would be needed to protect the plenum bar/reaction chamber and the nozzle assembly, what with those pipes supposed to hold a superheated steam cloud infused with fissioning nuclear fuel moving at past earth's escape velocity.

By the way, 479k was Zubrin's claim, not mine. Though, given 90% enriched uranium, reaching 400k mark doesn't seem impossible to me.

Another issue is powered landings. Assuming you're trying to land on a vacuum moon, what do you do when you get near the surface of the Moon? Won't the exhaust reflected off the ground fry your engine and spacecraft? Maybe you could inject a lot of water into the exhaust right before touchdown, slashing your ISP but making the landing survivable.

I have to say, this is actually a brilliant idea. Slashing the I_sp in half doesn't really matter when one's paying it in liquid water, which is practically everywhere. That, and the fact that this is probably only used for final approach - at most, a few seconds to minutes at a time.

That neutron flux is really annoying. There's no stopping the neutrons from the fission reaction from impinging on your engine and dumping waste heat into all the components. This is why I've been recently thinking that anuetronic fusion, if it can ever actually work, is a winner. Basic concept of the engine : the fuel is injected as plasma and the fusion happens in a continuous flow reaction, where the fuel stream passes through some honking big magnets and charged metal grids and is crushed with enough pressure to fuse. The exploding plasma then travels into the engine bell, which is immense, and is made of a latticework of magnet cables and empty spaces between them. The light from the reaction and many of the neutrons usually will pass through the gaps in the engine bell out to space. You also keep the neutron production down by this single-pass "flow" of fuel, where the reacting fuel only gets a chance to fuse once, so there are less side reactions.

The exploding plasma is allowed to expand in the middle of the engine bell, but the alpha particles and electron products are redirected with a variety of magnetic and electric fields away from the engine components and out the back instead.

In theory, almost none of the immense energy of the reaction actually heats up your engine, since none of the electrons or helium nuclei hit it, and you have all the components coated with a 99% reflective mirror material to reflect away light, and there are gaps in your engine bell that permit more than 50% of the light and neutrons to fly away into space instead of impinging.

In theory, this would let you jack the power output way, way up, possibly to sci-fi levels of thrust. (continuous burns of a decent fraction of a G - you would need terawatts of engine power to do this) Whether this is possible or not depends on things like how good you can really make your superconductors, whether we can get this fusion thing to actually work without needing a fission bomb to create the needed conditions, and so forth.

So, magnetic confined fusion? Looks like a good idea, if we can get past the 'aneutronic' part.

If not, the I_sp boost still looks pretty impressive, if Atomic Rockets is to be trusted.

Edited July 29, 2015 by shynung

[PakledHostage]

I believe it. SOme of the deorbited satellites, certain metal parts came down totally intact. Shape matters.

Does it? Following the extremely high speed crash of Turkish Airlines flight 981, full and unbroken wine bottles were found among the wreckage. Shape certainly mattets a bit but I'm inclined to believe that luck plays a dominant role in cases where something survives against the odds.

Edited July 29, 2015 by PakledHostage

[lajoswinkler]

If the impact is cushioned by something... That's how people have survived falling from enormous heights in planes. It happens. Extremely rare occurence, though.

[SomeGuy12]

So, magnetic confined fusion? Looks like a good idea, if we can get past the 'aneutronic' part.

If not, the I_sp boost still looks pretty impressive, if Atomic Rockets is to be trusted,

What's wrong with it being aneutronic? If you look at a list of known aneutronic fuels, there's several that are pretty common : boron/hydrogen and lithium-6/deuterium. The helium 3 ones are...less common. The aneutronic nature is needed for engineering reasons - in order to get your power level up to "science fiction" levels of thrust, where you need to have an incredible power:weight ratio, of megawatts/kilogram.

The only way that even works on paper to have this kind of power:weight ratio is you cannot have the flare of energy from this engine actually heating your components, since for every megawatt of heat there's only a kilogram or so of matter trying to contain it. It would just evaporate if the heat reached it. But if it's aneutronic and protected by mirrors, you can redirect all the exhaust particles (which are not neutrons and are electrically charged, so that electric and magnetic fields can affect them) away from your fragile engine and into space instead. The light produced reflects from the mirrors, and ditto, to space.

You might be able to make a dual mode engine. In one mode, it can perform one of the other fusion reactions on the list. Note that the one you'd actually want - 4 hydrogens - is thought to be basically impossible to make happen artificially. But you could do deuterium-deuterium.

In that mode, the neutrons which can't be blocked and are cooking your engine force you to throttle back to 1-10% power. You also would put more "engine hours" on your engine and would probably have to repair damaged components more often.

Then when you need to accelerate for realz, you use aneutronic and haul ass. Or, you can inject propellant into the engine bell as a kind of afterburner. The hot plasma is cooled, reducing exhaust velocity, but there's more mass being exhausted. You can increase thrust by a factor of 4, at the cost of slashing ISP in half. If your ISP is normally about 1% of the speed of light (300k ISP) you can get quadruple thrust with 150k ISP, and 16 times the thrust with 75k ISP and so on.

Science fiction novels have always had this standard trope where the spaceship can accelerate harder than the crew can withstand the G-forces. I'm not going to go so far and say you could do that, but you could probably get a full-G if you were willing to slash your ISP enough.

[shynung]

What's wrong with it being aneutronic? If you look at a list of known aneutronic fuels, there's several that are pretty common : boron/hydrogen and lithium-6/deuterium. The helium 3 ones are...less common. The aneutronic nature is needed for engineering reasons - in order to get your power level up to "science fiction" levels of thrust, where you need to have an incredible power:weight ratio, of megawatts/kilogram.

The thing is that aneutronic fusion reaction is harder to achieve than D-T fusion; it takes much more energy.

Going by Atomic Rocket's fusion fuels page, the Lawson criterion of a proton-Boron fusion goes into 500, while a neutron producing D-T fusion gets only 1; D-D gets 50. This means an aneutronic fusion-burning reactor is much harder to start - and keep running - than a typical non-aneutronic fusion. So, the incredible power-weight ratio does come at a steep cost.

In a less-developed spacefaring civilization, it seemed much easier to build neutron-embrittlement-resistant engine components instead. That, or mount the propulsion bus on a long boom, place a thick neutron shield 1/3 of the way to the payload mount, and replace the engine every 50 years or so.

[SomeGuy12]

The thing is that aneutronic fusion reaction is harder to achieve than D-T fusion; it takes much more energy.

Going by Atomic Rocket's fusion fuels page, the Lawson criterion of a proton-Boron fusion goes into 500, while a neutron producing D-T fusion gets only 1; D-D gets 50. This means an aneutronic fusion-burning reactor is much harder to start - and keep running - than a typical non-aneutronic fusion. So, the incredible power-weight ratio does come at a steep cost.

In a less-developed spacefaring civilization, it seemed much easier to build neutron-embrittlement-resistant engine components instead. That, or mount the propulsion bus on a long boom, place a thick neutron shield 1/3 of the way to the payload mount, and replace the engine every 50 years or so.

I'm talking about power output. Amusingly, it turns out that aneutronic barely helps you at all in terms of radiation shielding mass. Reducing the neutron and gamma flux to 1% doesn't do much since radiation shielding follows a halving law, so it saves you about 7 centimeters of lead shielding, when you have to carry several times that no matter what.

So it's not the neutron embrittlement, or the neutrons. It's the heat. Every time a neutron slams into your engine, it releases the heat it was carrying, heating your engine up and limiting your power:mass ratio. With aneutronic, this doesn't happen very often, and the other reaction products you can shield against - you block the light with mirrors, letting only about 1% of the light through with broad spectrum mirrors, and you block the alpha and beta particles with magnets and electric fields. So your engine can have the equivalent of a fusion bomb going off inside it in one continuous flare of light and actually continue to operate, in theory...

The energy to start aneutronic fusion is higher, but it releases far more energy than it costs, it's just very difficult for modern science and engineering to create the needed conditions.

[Findthepin1]

What if we move the asteroid so that it's on a course to move through Earth's atmosphere? Surely it's big enough to survive passing through, if it's big enough to be exploited. Basically it aerobrakes, then it's on a course that brings it back into low space for a while, then back into the atmosphere permanently. At its "new" apoapsis after aerobraking, have it dock with an engine thing and circularize its orbit right there. This requires significantly less fuel than to put it into orbit from a flyby course, and then deorbit it, and then slow it down to land. After it's been aerobraked and its orbit circularized, you can break little pieces off and bring those down to Earth separately. On Earth you can process those pieces to get the valuables out.

[AngelLestat]

Here I found a link from NASA Innovative Advanced Concepts (NIAC) of how to get down an asteroid to earth:

[defunct site link removed by moderator]

[SomeGuy12]

What if we move the asteroid so that it's on a course to move through Earth's atmosphere? Surely it's big enough to survive passing through, if it's big enough to be exploited. Basically it aerobrakes, then it's on a course that brings it back into low space for a while, then back into the atmosphere permanently. At its "new" apoapsis after aerobraking, have it dock with an engine thing and circularize its orbit right there. This requires significantly less fuel than to put it into orbit from a flyby course, and then deorbit it, and then slow it down to land. After it's been aerobraked and its orbit circularized, you can break little pieces off and bring those down to Earth separately. On Earth you can process those pieces to get the valuables out.

I had in mind you instead set up a "straight shot" approach. That is, it would approach the planet and the course would take it on a straight shot through the upper atmosphere right into the ground at the chosen impact location. You would pick a spot far away from other people who have the resources to complain (hence I said kazakstan), make certain that the probable error band doesn't cover any significant population centers, and as a secondary consideration, try to find soil or rocks for the impact point that will soak up the energy in a way that makes it easier to mine.

The problem with your approach is it sends the thing passing over most of the planet, possibly shedding pieces that impact with people on the ground.

[RainDreamer]

related video:

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[PakledHostage]

You would pick a spot far away from other people who have the resources to complain (hence I said kazakstan)

Oh give me a break! You mean this Kazakstan:

Its is as if you think that they're so backwards there that they'd pick up a space rock and think to themselves "the gods must be crazy!"

[SomeGuy12]

Oh give me a break! You mean this Kazakstan:

http://www.independent.co.uk/migration_catalog/article5245012.ece/alternates/w620/travelleadmain.jpeg

Its is as if you think that they're so backwards there that they'd pick up a space rock and think to themselves "the gods must be crazy!"

Less than 6 people per square mile. 17 million total. Less power to object to having their country used as a lithobraking zone...

[PakledHostage]

Less than 6 people per square mile. 17 million total. Less power to object to having their country used as a lithobraking zone...

Then why not Canada? We have a lower population density than Kazakhstan, live in igloos and don't have any allies in the international community to help us stand up for ourselves... Heck, we'll probably even say "sorry" when you come 'round to pick up your lump of platinum.

[SomeGuy12]

Thanks for the idea. Aim north of the permafrost line, and maybe the ice will help cool the meteorite in a jiffy.

[Bill Phil]

Bring it to LEO and take it apart. Then ship it down to Earth.

[SomeGuy12]

Bring it to LEO and take it apart. Then ship it down to Earth.

Too expensive.

[Bill Phil]

Too expensive.

Much safer.

If you're taking it to earth, aerocaptute is an option. Then, once it's in LEO, send up a spacecraft with the equipment to mine up. Then start sending up vehicles to bring it down in pieces.

It would be less expensive if there's an infrastructure already in place.

[Streetwind]

Even safer and even less expensive: leave it in solar orbit. Especially if it already encounters Earth regularly anyway.

[lajoswinkler]

Just throw it on my house. I'll claim it.

[DerekL1963]

Four words. Nuclear Salt Water Rocket.

Keep in mind that the current state of development for the NSWR is... essentially non existent. Zubrin's paper barely reaches the level of "back of the envelope" and no serious analysis has been performed by anyone who knows what they're talking about. Nobody has any idea if it will actually work.

[fredinno]

Ignoring the fact that people will panic over the fact that a space rock is being placed directly over their heads. At least a langarge point location will help people feel safer.

[PB666]

Let's suppose you find an asteroid that is several percent platinum, and work out a reasonable dV pool shot to get the thing into Earth's sphere of influence.

I can only think of one practical way to harvest it with today's technology. You launch a robotic mission to the asteroid with some kind of ultra high ISP engine, like that ion engine that has an exhaust velocity of 210 kilometers/second (dual stage 4 grid). It lands, docks by connecting itself to solid rock with a drill (which is harder than it sounds as the ESA found out recently), and then over a period of years performs the burns to redirect the asteroid.

Bringing the fuel along might be impractical, it would be nice if the asteroid itself had a few tons of ice you could harvest. You'd electrolyze the ice, and have an array of several VASIMR thrusters, some using pure oxygen and some using pure hydrogen as propellant. (or you would alternate)

Anyways, so here it is headed towards Earth. Launch spacecraft to dock with it, collect the platinum and send it down? Nope. Spend fuel braking the asteroid? Nope. You'd do just 2 things :

1. Detach the nuclear reactor that powered the engine and send it on a course that will flyby Earth

2. Send the thing to crash on land into the desert of kazakhstan at several kilometers/second.

Lithobraking will hopefully embed the platinum into a crater that is close enough to the surface so you can go scoop it up.

First thing-Purify the platinum. Does not mean smelt platinum to ore, but platinum should be in a molecular form like H₂PtCl₆

Second thing- bundle the platinum into a bolloid (a large plastic bag would suffice).

Third- tug the bolloid back to L2 or L1 using any suitable ion, EM, etc drive.

Forth- Intercept the bolloid at L2 or L1 with some sort of orbital insertion device, remove contents of bolloid and place them into a re-entry craft.

Fifth-begin the long tedious process of aerobraking the re-entry craft in earths upper atmosphere.

Sixth, because of the excessive mass and density the re-entry vehicle would need to deceleration forces a magnitude higher than modern craft. This means that there needs to be extendable drag plates that project from the tail of the central re-entry plate axis. Each of these would carry its own drogue and high velocity chutes. You want the landing to be on land since landing in an ocean means almost certainly you are going to be doing a sea floor recovery (Rel. Density of the salt in sea water is about 10x). The effort will not be to stop the vessel, it will certainly crash on earth but to slow it down to a 10s m/s before hitting the earth. If it were possible to aim the descent (I don't think with all the drag features needed to slow it down, and I think that the re-entry vector at 100km needs to be essentially horizontal to maximize deceleration) I would aim at exposed peat beds in Canada since they offer an intermediate density between water and air, not as hard as soil with a solid foundation. You probably want GPS trackers/transmitters embedded in cells within the platinum so that you can track the pieces that hit the ground, they are likely to be protected but hidden in the peat.

[DerekL1963]

I would aim at exposed peat beds in Canada since they offer an intermediate density between water and air, not as hard as soil with a solid foundation.

At any speed faster than a few m/s, difference in density between peat, water, and soil is essentially moot.

[AngelLestat]

There is something of true behind all this, it will be very inneficient if we need to carry few kg of platinum in a dragon capsule or any other capsule to protect the cargo from reentry and provide a guided soft land... why for?? There is not humans in there, there are just metals.

But.. you can not just drop them either.. depending the purity or mix of the platinum cargo, can be break it in the reentry or just put someone in danger when it reach land.

Also as I said before, if this thing falls free from the sky, anybody can claim it.

So it needs a middle solution.. what occurs to me to fix this is a bag system with thermal cover and tiny parachutes with a tracking beacon and a sound alarm that will activate before touch the ground.

In this case you rise 10 of this reentry bags, you cover 10 ton of material, and then you recover the bag (which may be reusable or not).

The parachute does not need to be big, meanwhile the final speed may be 12 m/s.

These bags can be shooted from orbit, if your ship is tight to the asteroid, it will not be a big change of orbital speed for something as massive as an asteroid. You can recover that deltav using a nuclear thermal engine shooting any gas or compound different than platinum from the asteroid.

So I guess it can be many efficient ways to do it if we just think a little about that.

Edited July 31, 2015 by AngelLestat

[0111narwhalz]

Why bother lithobraking on or circularizing about Earth? Just aim it at Luna (farside to spare people the scars) and recover it there. Roid delivery craft swings by nd returns to the belt from whence it came and grapples a new rock to halve the cost of return. Refine on Luna at your leisure with mobile refineries on the surface. The Pt is alloyed with Fe/Ni/Co &attached to an Al hybrid rocket and railgunned to LEO, where the rocket fires and circularises. A cargo vessel outfits the alloyed lump with transponder, chutes, and heatshield and rails it to the surface. The lump is recovered in one piece and, through metallurgical magic, the Pt is extracted from the ferrous slug. A few more steps, but hey, safety without massive fuel expenditure.

Edited August 4, 2015 by 0111narwhalz