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


Forgot your password?
Slashdot Deals: Deal of the Day - Pay What You Want for the Learn to Code Bundle, includes AngularJS, Python, HTML5, Ruby, and more. ×

Comment Re:Space-based Economy (Score 2) 273

As usual, "pop science" news overstated the case. We know that there's ppm quantities of water in most lunar regolith, but that's not what people usually talk about. There's also a good degree of confidence that there's a lot of *hydroxyl* group in a lot of places on the moon. But the connection between that and the group being specifically water is much weaker - and many missions sent to detect water in likely areas have failed. The best evidence for water have come from Chandrayaan and LRO, examining craters that were considered likely to find ice. They have both failed to find "slabs" of ice in the crater, but found evidence for ice grains in the regolith - about 5% according to LRO. On Earth that would be considered dry soil, but it's something at least.

Of course, if you're constraining yourself to such craters, you're really constraining where you can go. On the general lunar surface, the sun bakes water out of the regolith.

Iron, aluminum, and titanium are very useful for making things

They're all tightly locked up as oxides, without the raw materials that we use to refine them on Earth being available. There are however tiny grains of raw iron in the regolith, so there is some potential to comb it out magnetically. Still, asteroids present by far better resource options in much greater concentrations.

There really is just no reason to do your work in a gravity well as deep as the moon's, and then have to break out of it, when you can just mine NEOs. Yes, it's "half the gravity of Mars", but it's vastly more than asteroids. Rockets with a couple thousand spare m/s delta-V don't just grow on lunar trees.

Comment Re:Space-based Economy (Score 1) 273

The moon's surface is kind of boring, as far as geology goes. Aluminum oxide, titanium oxide, iron oxide, silicon dioxide... by and large it's stuff that's really common on Earth. And not much of the common stuff that's super-useful, like water. And really, it's way more of a gravity well than is ideal to have.

Comment Re:Cost of access is key. (Score 1) 273

No no, we can get much more than a 1-2% improvement in chemical rocket performance. The issue is that for our needs thusfar (large objects to LEO and GEO, small objects further out with long transit times and gravity assists or ion propulsion), H2/O2 has been fine and it's not been worth all of the headaches of more energy dense fuel mixtures, like Li-(LF2|FLOX|OF2)-LH2 triprop. But we can indeed get a 25% improvement in ISP if we're willing to work with very hazardous, toxic chemicals (at least the resultant LiF isn't as toxic as F2!). It was already done in a lab-scale development back in the late 1960s. And let's not kid ourself, NASA has indeed launched successful missions using toxic, corrosive and dangerous chemicals as propellants. But this would be a new upper bound in this regard. I doubt they'd ever use a propellant like that on a lower stage, but for an upper stage or a return stage... it's a possibility.

Without invoking significant toxicity we can improve the picture somewhat. Burning the lithium with O2 (and of course H2 for exhaust flow reasons) is also a very high energy propellant, but it still means working with metallic lithium in some form or another (liquid, hybrid, slurry, cryosolid, etc), which most people would really like to avoid. But it is possible to do.

A small boost to H2/O2 can be made with aluminum - it only boosts the Isp a few percent (I believe about 4%-ish, though I'd have to double check), but it also gives a nice secondary bonus of really increasing your propellant density. Aluminum is neither dangerous nor toxic, but burning it with the H2/O2, and in a reliable manner, hasn't been tackled yet.

Boron is another high-energy compound one can use. As is beryllium (Be-F2-H2 is even more powerful than Li-F2-H2 by a small margin), but it's hugely expensive and extremely toxic in dust form.

Beyond all of the "familiar" stuff there's a lot of research on more exotic compounds with strained chemical bonds which remain in a metastable state until burned; there's way too many such compounds to list here. But at present they all generally suffer from either production cost issues or problematic instabilities.

Oh, and you can also improve performance by increasing the chamber pressure. That said, it's rather modest - if I recall a doubling of chamber pressure is usually on the order of a 7% ISP boost. But it does mean that advances in material technologies can translate to advances in rocket ISP. And there's also a wide range of other modifications to engine design that could boost rocket ISP to lesser extents.

Comment Re:Cost of access is key. (Score 1) 273

Staging works pretty well to get around the energy density problem, at least early on.. though the rocket equation starts getting pretty tyrranical when it comes to returns from other planetary bodies. It's really hard to conceive of a manned Mars mission with return that doesn't involve at least the return ascent stage being fueled by one of the following:

1) In-situ propellant production
2) Extreme-ISP chemical propellant
3) Nuclear thermal

You can't rely on ion propulsion (even higher power variants like VASIMR) to get you off the ground. Nuclear thermal (1) should work (NERVA showed promise), but the development costs will be huge and it'd face massive public opposition, having that much nuclear fuel on a single craft. It also puts a rather large minimum size for your ascent stage - fission doesn't scale down well, and even as big as it was NERVA only had a thrust to weight ratio of 3 to 4. And the mass of that large, heavy ascent stage imposes significant mass penalties on your earlier stages, partially negating the benefit of that 800-1100 sec ISP.

For more advanced chemicals (2), there's lots of theoretical stuff, but with stuff that we could do today for a practical cost, it'd probably pretty much have to be some variant of lithium/fluorine/hydrogen triprop. The oxidizer could be LF, FLOX, OF2, or a couple other possibilities... but if you want an ISP(vac) from chemical propellants in 500-550 range and good density, that's pretty much what you have to do (yes, the LM and CSM used toxic, corrosive, dangerous propellants too, and NASA managed fine, but these are even worse). And even still, 500-550 sec is low enough that you'd probably still want some sort of ion "tug" cycler to move you between LEO and LMO, with your fuel only used for ascent.

If you don't want to or can't do either of those two options (#2 and #3), you're pretty much stuck with in-situ production (unless you want to have to launch a LOT of tonnage into orbit!) Which is why that's SpaceX's focus... it probably is the best option. Still, though, it's a challenge and a risk, no question.

Comment Re:Affirmative Action won't take us to Mars. (Score 1) 273

I think his job is more "ticking large numbers of people off". For example, he was one of the leaders behind the "Pluto, Eris, Ceres, etc aren't planets" movement - he had references to Pluto being a planet removed from the Hayden Planetarium years before the IAU vote. He's not exactly popular among those who felt that hydrostatic equilibrium was the relevant constraint and that the "cleared the neighborhood" definition is fundamentally flawed.

Comment Re:The guy aint no Sagan... (Score 1) 273

Which is silly, because the Apollo mission was primarily oriented around the physics of getting a bunch of large mammals into space, keeping them alive on the way to the moon, landing them on the moon, keeping them alive down there while they explore, and then doing all of that in reverse. If they hadn't brought a single rock back the total change to the mission cost would have been almost unnoticeable.

Furthermore, who's focused on mining the moon? Most mining proposals focus on mining NEOs. It's way easier to get material from a NEO to Earth aerocapture. You could do it with a coilgun with no expenditure of consumables, again and again for years on end. They're also far more rich in interesting materials - much better than the best mines on Earth, and with no overburden.

Comment Re: Cost of access is key. (Score 1) 273

Actually they very well might. The strongest individual SWNTs measured thusfar are, what, 60GPa? That's way too weak to make a practical space elevator. And that's for individual tubes. Ropes are only held together by VDW and break at their weakest points, which will invariably exist - as a result it's hard to get ropes more than a couple GPa. There may be some better structures out there, but I wouldn't hold my breath waiting for an Earth-based space elevator.

If you want a physical structure reaching to space, go for a Lofstrom loop.

Comment Re:Cost of access is key. (Score 1) 273

Where are you getting those prices? NASA was paying $3,60/kg for LH in 1980, so that's probably, what, $7/kg for LH today? Remember, this is LH, not gaseous - you not only have to cool it to extreme temperatures, but you also have to catalyze the conversion of orthohydrogen to parahydrogen - which is exothermic, yielding enough heat to nearly boil off everything you just cooled. NASA was paying $0,08/kg for LOX in 1980, so probably around $0,15 today. The Shuttle ET holds 630 tonnes of LOX and 106 tonnes of LH, so $836k.

The SRBs are 70% ammonium perchlorate, which is about $3/kg. 16% aluminum (about $1,50/kg), 12% PBAN binder (about $1/kg), 2% epoxy (about $5/kg), and an irrelevant amount of iron oxide. The total propellant was about 500 tonnes. Total propellant cost, $1,3m.

So the total propellant cost between the two, about $2m. To lift 27,6 tonnes of cargo to LEO, or $72 per kilogram. Now, people shouldn't fall for the fallacy that you just multiply that by how much a person weighs or a little more and that's the per-person cost to go to space - you actually have to launch many times more than a person's weight to get them there and keep them alive. But yes, propellant costs are not the key issue - if costs were close to propellant costs, rocketry would only cost about $25-100k to bring people to orbit in bulk.

Unfortunately, that's not the case.

Mind you, it's even possible to get significantly lower than that, but you can't rely on the rocket equation. And even if Space Elevator unobtanium existed, it wouldn't actually get you down to the levels one wants - there's no practical way to pump the climbing power up the tether, and beaming efficiencies with such a small receiver are unfortunately very low over such long distances. Much more practical is something like a Lofstrom loop - one might get power transfer efficiencies upwards of 50% or so. In such a case, you need about 70MJ per kilogram (19,4kWh). At industrial power rates of, say, $0,08/kW, that's a cost of a mere $1,56/kg. Sending people up in bulk might cost on the order of $800-ish per person in energy costs.

In both cases, though, it's not the propellant/energy costs that are killer, it's the hardware.You're asking structures to perform some borderline magical tasks in terms of the challenges that are put on them.

Anyway, enough Slashdot for now... back to working on simulating my caseless rocket design in OpenFoam and optimizing propellant combinations in CEA. ;)

The trouble with money is it costs too much!