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


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:Cost of access is key. (Score 1) 208

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) 208

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) 208

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) 208

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) 208

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) 208

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. ;)

Comment Re:Yes! (Score 1) 251

I switched to a Mac in 2012 for my personal shit and about 6 months ago went to a Mac for work too. With the release of Office 2016 for the Mac, I honestly cannot find a single thing I cannot do comfortably on my Mac anymore.

If you have a serious problem with it, Parallels has been running Windows apps for me better than any native PC installation since version 7 back in 2012.

I mean, I know you're probably trolling or trying to be funny, but it's a dead joke in 2015.

Comment Re: Good! (Score 1) 356

The only way to fix this problem is by taxing the products when they enter the country.

Except we have treaties that forbid us from doing that. If we violate trade agreements, other countries will retaliate, and the world economy will spiral downward. For an example of this scenario actually happening, Google for "The Great Depression".

It's ridiculous to allow corporations to hide billions overseas.

It is ridiculous for America to tax profits on a product made in England and sold in France. It is ridiculous to have absurd tax laws that encourage companies to move jobs overseas. We should tax domestic sales, or domestic revenue, or domestic payrolls, or even domestic profits. But instead we tax worldwide profits, of only companies domiciled in America, giving them a huge incentive to go elsewhere. No other country has a tax like that. It is economic self-sabotage.

No, that's the price these companies need to pay if they want to enjoy the strongest IP laws in the world. If they want to have HQ in China or India, let them. Viagra is about $25 a pill here in the US, because Pfizer has patents and strong laws to back it up. The same pill is about 30 cents in India. Some of that is due to "what the market will bear" and some is probably due to inflation caused by most patients having no idea what a drug actually costs. But India's policy on drug patents (they don't recognize them as valid) has quite a lot to do with it also.

Neckties strangle clear thinking. -- Lin Yutang