ballooney

Yeah good point, it would be much better if every civil servant in every government department in the entire nation-state all collectively only focussed on one national priority at a time. I get very annoyed when my local authorities fix pot-holes in my street, and collect my bins, when they could be concentrating on energy security.

This is disappointingly poor for Jonathan Freedland. The most remarkable thing about the session he cites was how unremarkable it was; no one expected any great revelations and no one got any, as even Freedland himself said in a radio interview on the BBC after the session. It was just a bit of theatre.

The argument about CCTV cameras as evidence of complacency about surveillance seems silly and tired to me, as silly as an article would be in a British newspaper that concluded that Americans are obsessed with killing each other on the basis of gun ownership rates vs the UK. They're not, of course, the overwhelming majority of gun owners do not buy them with the intention of killing another person.

Likewise, the one camera per 11 people crops up a lot and (as far as I know, having a small interest in the actual figure) no groups trying to find evidence for it since have managed to do so, in fact the most high profile study that investigated the number concluded that there were 1.85 million CCTV cameras in the UK, of which 1.7M were privately owned, 0.03M were fixed and publicly owned, and 0.12 were on public transport. [1]

That's 92% of them under private ownership. They exist, by and large, to deter criminals from breaking into and stealing from stores and commercial premises, just as you might be put off from robbing a convenience store in the US in the knowledge that the store-owner might be armed, here it's completely normal to see signs outside a shop saying "These premises are protected by CCTV" - note the use of the word protected - and indeed most hardware shops will have this sign in stock right next to the 'no smoking' signs. They're there so you can give something to the insurance company and if lucky there will be something to help the police get an ID. I find their presence as much a threat to civil liberty as dash-cams are to the civil liberty of the average russian.

Likewise stuff is called 'Her Majesty's $FOO' for entirely vestigial, historical reasons. When Obama says 'God Bless America' at the end of every speech we don't honestly think that he's designed a policy whose success is contingent upon divine intervention, jokes about healthcare.gov aside.

There are differences in the relationship between people and their state in the UK vs other places just because of different evolutionary paths (socially rather than biologically!). For instance, an American economist friend expressed surprise that we didn't have a legal separation of church and state, and how dangerous it was, and how we should get one like in America. Well, people here are just not that religious in comparison to the US, and my experience of the US is that the church has enormously more influence over US politics than it does in the UK anyway, despite the law. It's just not really a thing here. I'm sure if a modern British Prime Minister were to say 'God Bless the UK' at the end of a speech he'd be sectioned under the mental health act, and the PR people would go into overdrive to explain that he didn't really mean it. Similarly, If UK employers were to switch to a policy of only allowing two weeks holiday per year, and why not add random drugs tests while we're at it, there would be riots at the outrageous and dangerous attempt at enslavement of the population, and the government would likely be overthrown if it failed to act. I use these just as an example to illustrate the differences in what's considered important, and of what people are vigilant, between the two nations.

"Many Brits accept the old securocrat formulation: If you have nothing to hide, you have nothing to fear. " Do they? I don't think I've met that many people who think that - maybe I just move in the wrong circles - but my most prominent encounter with that sentiment was as an argument whose flaws we had to point out in a Critical Thinking class homework at secondary (ages 13-18) school, as part of one of the national school syllabuses.

I'm very worried about state surveillance, it's a constant fight, and while I'm thankful that the most egregious proposals of recent years - national I.D cards and 90 days detention without trial - were rightly defeated, I'm under no illusions about winning a battle vs winning a war. But you have to pick your battles, and the reason I think this article is guff is because it misidentifies what to fight, and plays up to superficial and lazy observations of the UK from outside, as if to appeal to its intended audience. All that's missing is a reference to bad teeth.

Like I said, surprisingly bad from Mr Freedland, who otherwise has been quite eloquent on the subject, I think.

[1] http://www.politics.co.uk/reference/cctv

Edit: typo.

"back in 2007, we were enabling college students to debate issues on their campus. And in the unfortunate theme of that era, we had lots of people using it but nothing resembling a business model."

Let's all be thankful things have moved on so much since that era!

As someone who has designed avionics for experimental aircraft and rockets, one of the satisfying highlights, like the salty sticky bits as you mop up the plate of a roast, is designing and building the switch panels.

Just as web-UI design can be an all-consuming obsession to get it 'just-so', you can loose yourself in comparisons between rotary switches - finding one with just the right torque and a satisfying positive click, locking toggle switches, the colour of the panels and the colour of the filled engravings to get the readability good in both direct sunlight and dim red-lit cabins, the pride you take in wiring looms that no other humans will likely ever see. It's of course not all fun and games, people's lives will depend on the quality of that wiring loom, and the spark from an unsealed toggle switch in the oxygen-rich cabin is what caused the Apollo 1 fire ( http://en.wikipedia.org/wiki/Apollo_1 ).

So videos like this are quite a source of design inspiration!

Relatedly: if you wonder what goes into making nice panels and wiring looms, have a flick through this: http://www.rbracing-rsr.com/wiring_ecu.html it's motorsport but it's basically the same story for aircraft.

This is incorrect, there have been successfully flown, actively stabilised, passively unstable model rockets made by amateurs before. E.g. Gyroc in the UK.

michael.sdf-eu.org/Gyroc/

There have also been a few in the states. All passively unstable too.

> Not sure if this is off topic, but shouldn't it be simple to make a high altitude balloon out of a material that doesn't explode so easily when it gets to the lower pressures in low earth orbit?

The very early comms sat experiments in the 50s were just aluminised passive balloons that reflected radio signals bounced off them while in orbit. [1]. However they were there because they were put there by a rocket. I'm not sure if your question is implying that you could float your way up to LEO altitude. I suspect you couldn't with anything that could be engineered.

> Or even some kind of contraption that slowly releases gas from the balloon as it detects lower pressure in the atmosphere to help it maintain altitude and prevent the balloon from rupturing?

This is done already in two ways. One passively does what you describe - a zero pressure balloon - by having a hole in the bottom so helium leaks out as it rises. Eventually enough leaks out that the net lift is zero and it just floats.

The other is a super-pressure balloon [2] which does what you described in your first paragraph - it's strong enough to not explode until the pressure (and so density) of the helium inside increases relative to the outside atmosphere until the lift becomes zero. But we're still a long way from an altitude that could sustain orbit. These operate at relatively low altitudes like 45km/150kft. The drag and heating there would be enormous at orbital velocities!

[1] http://en.wikipedia.org/wiki/Project_Echo [2] http://en.wikipedia.org/wiki/Superpressure_balloon

You can hit those speeds and beyond with a rocket engine. They don't really have a speed limit, they'll keep accelerating something (provided their force is greater than the drag and any other retarding forces) until you turn them off. Eg, a payload into earth orbit, which would be about Mach 25 if there was some atmosphere. Or significantly faster if you're putting a probe on an escape velocity, for example the Pluto probe New Horizons, which was launched with a solar escape velocity that would be equivalent to about Mach 50 (all sea level).

The advantage of the scramjet over a rocket is that you don't have to carry the oxidiser in a tank with you (like a conventional rocket). You get it from the atmosphere. But being inside an atmosphere making thermal management a big challenge, as the article describes.

These things share what is fashionable, not what is important. If I find an interesting, low volume blog, I can subscribe to its feed and I will get an update when content is added. With your scenario, I might get an update if content is added, if I can find it between guff about conference scandals. It's simply not comparable, unless you're forcing worthy content (in my judgement) to have the same value as transitory celebrity gossip.

These services give me a good chance of discovering things, RSS lets me follow them. As a case in point, there are a lot of things in my RSS feed that I have discovered from HN because one single post from them made it to the front page, but no subsequent one has. But I get to see the updates because I subscribed to their feeds. That subsequent ones haven;t made it to the HN front page is absolutely not a value judgement on their worthiness - unless, as I say, you think 5 same-but-different articles on who said what and to whom at a conference is more important than the infrequent technical article by a researcher.

In short, I want to follow the stuff on the right of the bell curve, not in the middle. The stuff that mostly floats to the top of what you're advocating (HN, fb, twitter) is firmly in the middle. A lot of the people posting the content I find interesting don't have twitter feeds announcing their posts - that kind of thing is often the preserve of this week's preferred javascript framework project. My RSS feeds allowed me a much higher SNR than any of these things. So please don't compare them, as if they can serve the same purpose. That's nonsense.

It's not really mentioned in the article but as a starting point they received all the work done for Fastrac [1], a NASA program to build small, cheap, expendable rockets - something like the Falcon 1. That got cancelled as NASA projects are wont to do but it made sense to use that design as a baseline. A clean sheet engine design is very complicated indeed - it's a very high dimensional, highly coupled optimisation problem, so a known good starting point is a very valuable thing to have.

Indeed the turbopump, which is one of the hardest bits of a rocket engine, is made by Barber Nichols, who made the turbopump for Fastrac and who make the turbopumps for the Merlin engines [2]. I've heard rocket engineers describe rocket engines as turbopumps with some extra plumbing. Perhaps a slight exaggeration but not far off, especially since their design is so tightly coupled and dependent upon overall engine parameters. The degree of coupling depends on the topology of the plumbing, or the 'cycle' of the engine, which I'll try and explain:

There are three kinds of rocket engine cycle (well, there are maybe more but these are the three that have been flown historically). The Expander Cycle, the Staged Combustion Cycle, and the Gas Generator cycle. I'll mention the last two.

Merlin, as the article mentions, is an example of a Gas Generator cycle [3]. In this cycle, you take off a little bit of fuel and oxidiser to burn outside the main combustion chamber, to generate some hot energetic gases that you can exhaust over a turbine. This spins the turbine up, which is connected to a shaft with a compressor on the other end. The compressor increases the pressure of the propellents so that they can be injected into the main combustion chamber. This assembly (turbine, shaft, compressor) is called the turbopump. It's necessary because the engines require very high flow rates to get the thrust they need, and that has to be at a high pressure - higher than the pressure of the combusting gases inside the combustion chamber, else you wouldn't be able to inject it!

Back to the bleed-off to drive the turbine. You usually don't want a perfect stoichiometric mix of fuel and oxidiser for this, or even close, because it generates extraordinary hot gases that no turbine would last long in (The turbines are spinning at many tens of thousands of RPM usually so would be subject to much higher forces than the actively cooled walls of the main combustion chamber). For this reason you usually have a large imbalance of one propellent to the other to keep the temperature down. Usually you run with excess fuel, or 'fuel-rich', as the opposite - oxidiser rich - means you have hot oxidising gases which are harder on the metallurgy. I do know of some russian exceptions to this, though, where fuel rich would have left sooty deposits in the plumbing (The materials science employed in the turbines was apparently so witchcraft that when the US got intelligence of oxidiser-rich turbine precombustors, they thought is was deliberate counterintelligence from the russians to get them to waste billions researching the impossible). The gas generator cycle, as the article mentions, dumps this turbine exhaust overboard separately. The problem with this is that there's a load of uncombusted fuel in this exhaust, which you're just wasting, and this hits your rocket performance - the Specific Impulse ( I_{sp} ), as you're not getting as much bang out of a given mass of fuel as you could.

The answer to this is the Staged Combustion Cycle [4], where you also inject the exhaust of the turbine into the combustion chamber to finish off combustion. The performance of these engines is higher but the thermodynamic balance to design a working system is a greater challenge, and some of the engineering is a bit harder too. Staged Combustion engines are mostly russian, although the Space Shuttle Main Engines are a US-design example of Staged combustion.

SpaceX have been gradually and incrementally improving the Merlin's away from their simpler beginnings, and it's been pleasing to watch as an interested outsider. To bring it back to the OP question, "are all engines of that caliber that size or is this one special?", Merlin didn't particularly stand out in terms of power density in the early days, although it's been improving and improving. Now there is the Merlin 1D [5], which claims to have the highest thrust to weight of any rocket engine every made. One should take these claims with a pinch of salt as what counts as 'engine' and what counts as 'plumbing' and what counts as fuel tank is sort of open to debate and you can do some creative accounting to make your numbers look better. However, it's an impressive achievement regardless.

The metric that doesn't lie, from a performance point of view, is the mass fraction of the rocket - that is the fraction of the all-up, fuelled-up mass of the rocket on the pad that makes it into orbit. The higher the better - i.e. you can take bigger payloads for a given size rocket. Note this is just from a performance point of view, not an economics point of view.

However, increasing mass fraction will be important to SpaceXs staged aim of re-usable rockets. That's because as well as the payload, each rocket stage as to also carry the fuel it needs to land. I believe Falcon 9 can launch about 2% of its pad mass into orbit (i.e. the payload can be 2% of the total mass), and Musk reckons if that could be increased to about 4 or 5%, there's be enough margin to carry enough landing fuel and extra landing hardware like legs.

So my bet, just for fun (I'm not connected with SpaceX, this is just sideline speculation for the sake of interest) is that you might start to see development of a Staged combustion engine instead of gas generator, and a switch to methane fuel which, with LOX, has a slightly higher specific impulse than Lox/Kerosene which they currently use. Maybe 20 seconds extra (seconds being the unit of specific impulse), which is maybe 5-7% more than they might be seeing now, which is worth having. Methane is nice because it has a similar density to kerosene. Lox/LH2 has almost 50% better performance than LOX/Kerosene in theory but LH2 is of a very much lower density, so the tanks must be much bigger (the illustrates my earlier point about the slipperyness of engine-only thrust to weight as a metric - how much bigger and heavier are the tanks?).

[Edit: of course instead of building a higher performance engine you could just built a much bigger rocket with the same mass fraction and have the cargo be a smaller percentage of launch mass. probably cheaper than developing a staged combustion engine. I suspect the rocket science equivalent of more servers vs a rewrite in C]

This post has ended up being a bit longer than I thought it would. Hopefully of some interest if you're new to the subject.

[1] http://en.wikipedia.org/wiki/Fastrac_(engine) [2] http://www.barber-nichols.com/products/rocket-engine-turbopu... [3] http://en.wikipedia.org/wiki/Gas-generator_cycle_(rocket) [4] http://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket... [5] http://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)#M...

If you want potentially revolutionary, reusable space projects with some ninja materials science, there is a project rather closer to home for you, which you might be interested in:

www.reactionengines.co.uk

Lots of industries do this. Patents are useful when it's obvious what your idea is, and where it's easy for people/competitors to have have a poke inside. Like a vacuum cleaner or a non-reflective mobile-phone screen coating, or one-click or something like that.

But for industries where that's not the case, like rockets (my current) or to pick another example from a previous job, magnetic bearings for sub-sea tubo-machinery installations like natural gas compressors[1], it makes no sense to file a patent. It's much easier to keep it a secret, because it's not like your competitors are going to scuba-dive down to the compressor control cabinet, whip out a JTAG and see how you've implemented your control loop on an FPGA.

Likewise, if you do patent it, it's very difficult to prove an infringement for the same reasons.

So essentially, in industries where it's not the case that your competitors can see what you've done, secrets over patents is the norm, in my experience, and has been for while.

[1] Mag bearings use electromagnetic stators and magnetic collars on the shaft to levitate the shaft inside a bit of spinning equipment. This means it's entirely non-contact, dramatically reducing wear, especially useful on difficult-to-service installations like sub-sea.

You've been able to build your own hardware from 74xx series chips for decades now, infact about half a century! Infact, that was the only option you had back in the day. This is retro rather than new, like making a ham radio out of thermionic valves, say. Nothing new, but kinda cool and certainly educational if you're new to the field.

I think that's why the GP took objection to your rather statesmanly proclamation that this was heralding some kind of revolution in open hardware. It isn't, it's just a rather neat project.

However, in your followup comment I definitely agree with you in that as soon as I can fab my own silicon in my garage, the world will be my mollusc. But we're pretty close nowadays with a $20 FPGA from digikey.

  > I don't know what you are complaining about.

Indeed, and the rest of your reply goes on to confirm this, with the additional quite needless, but amusing, psychological analysis. I'm sorry if I worded it poorly in my first reply, although I can't see that it's particularly ambiguous.

The issue is quite orthogonal to him personally and his achievements, for which I have the utmost respect. I am fascinated by the early days of space exploration and am as impressed by this work as I was when I first read about it a few years ago.

The issue is with the journalism, written by the journalists, which attempts to sensationalise the achievements (which can stand on their own two feet without it) by making comparisons with famous mathematicians that are not valid comparisons, as Paul Lutus explains in his first post. It's just apples and oranges, and will at best be lost on the lay reader and at worst confuse the lay reader who looks into the three body problem further. Hence my point that it is unhelpful to the public understanding of science.

Sorry, It's an important distinction in an absolute sense, and not a matter of personal taste. He did not 'solve' it in the sense that 'all the great mathematicians ever' were trying to solve it, as the article and the clip make out.

Similarly, I am not ringing up the Clay Institute to claim my $1M prize for 'solving' Navier-Stokes because I've just run a toy computational-fluid-dynamics problem in OpenFoam.

There is nothing good to be gained from obfuscation and sensationalisation in the public understanding of science.

Agree on all counts, especially 2. Funny you should mention the [LOX/LH2 Single Stage To Orbit] DC-Y! If you can only have one stage, Hydrogen is also very attractive. I agree with you that someone should make a proper go of it because it's possible and it makes a lot of sense. It was actually this realisation that prompted me to change career and work where I now do. (I am British, work in propulsion research, especially Oxygen (be it stored liquid or from the air...) with Hydrogen, am interested in reusable space vehicles, and I think wings are A Good Thing, and I live in the city where you did your MBA. As a fellow space enthusiast that's hopefully enough for you to figure it out :D ).

All valid and I'm enjoying the discussion but I would address the points as:

1) Sure at the moment, and it will be difficult to match economies of scale with the oil industry, but the amount of research money going into hydrogen production at scale is enormous. It's only getting cheaper, and while 'The Hydrogen Economy' has been promised since the 70s and hasn't yet arrived, it's definitely coming. I'm sort of thinking about this debate in a 10+ years out sense rather than right now, I admit.

2&3. Sure, but that only becomes a smaller problem as rockets get bigger (i.e. very heavy launchers such as this one being proposed) and the cube/square law works in your favour (i.e. tank volume increases faster than frontal area to which the drag is proportional). T/W is often limited anyway by payload constraints rather than rocket engineering constraints, especially in manned systems. Doesn't matter if the vehicle can do 5G off the tower, the people inside it can't.

4) 'Embrittels' I would guess? Anyway, it occurs in most metals but certainly not all metals and very certainly not "everything it touches". This is the clue for the way out of this problem. I obviously couldn't show you a hydrogen engine with the same track record as no operational hydrogen oxygen engines have been built with a clean sheet using what we know today. That said, a few are being designed precisely as we speak for very long life operation in reusable vehicles. I don't think the chickens have come in to roost yet.

My comment serves no purose other than to say YES. I love this book dearly, it's probably the best written and most enjoyable niche technical book ever written. It's an absolute gem. I high commend it to people as an excellent short read. One of those instances where you say 'why on earth would I be interested in the history of liquid rocket propellents?' but you should read it anyway - it's massively enjoyable, some bits are laugh-out-loud funny, and a tour de force in technical writing and communicating complex ideas to non specialist readers.

It actually got me into rocket propulsion research, having been a signal processing, ML and control theory person by background. May I offer it my highest commendation to the curious HNer and thank you baq for providing a link to it.

Indeed, as I say it's very much harder to work with, but it's worth a revisit with 2012 technology, I think. I also am involved with oxygen+hydrogen rocket propulsion research but can't really discuss it much here. But things have come on a bit in the last 30 years :)

Re: 'BFR', well I have a few hypotheses about 'MCT' in a similar vein but I reasoned they might be a little bit blue for HN!

Edit: A better reason for first stage hydrogen occured to me over a cup of tea:

> "SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path."

I think the economics change when you try and go reusable in the way that they are. Lox Hydrogen engines and tankage might be more expensive, which is obviously a factor in an expendable vehicle, but if you reuse the vehicle, and have to keep enough fuel back to land, as SpaceX plan, I think actually the equation goes far more back towards optimising for propulsion efficiency in order to get the cost efficiency. The launcher cost metric is basically dollars per kg into orbit, and for a given reusable rocket, lox+hydrogen will get many more kgs into orbit than lox+kerosene - you need to keep much less fuel back in reserve to land the various rocket stages, and the extra fuel you can use on the ascent is also a lot more thrusty per unit mass. I wouldn't be surprised, assuming you sell at a dollar/kg price, if lox+hydrogen ended up being much more profitable in reusables than lox+kerosene, everything else being equal. I have not done the sums though.

If we're playing 'guess-the-acronym', I'm going to depart from something Mars-based and suggest that the 'C' is to do with 'Cryogenic', and that now that they are more confident with engines they might be switching to a harder, but much higher performance, fuel combination like liquid oxygen with liquid hydrogen.

Whereas LOX+Kerosene has a Specific Impulse (crudely the amount of 'go' you get from a pound of propellent) of about 340s, LOX+Hydrogen engines have an Isp of about 450. That's really a huge increase in terms of what you can launch with a given size of rocket.

Tom Mueller, the head of propulsion at SpaceX, previously worked for TRW (the company that did the US's ICBMs in the 50s) and while there he worked on a LOX+Hydrogen engine, the TR106 ( http://en.wikipedia.org/wiki/TR-106 ), which was a beast.

It's harder to work with - both propellants are very cryogenic - but any rocket engine designer will be very aware that LOX+Hydrogen is hovering at the top of the evolutionary ladder looking mighty attractive for those that have the resources to take it on - a position Space X find itself in as they become more mature and heavyweight. They're already looking at it for their replacement Upper stage vacuum engine for Falcon 9 - the Raptor ( http://en.wikipedia.org/wiki/Raptor_(rocket_stage) ).

Also, almost any Mars mission would involve a lot of heavy lifting into low earth orbit to construct a vehicle that would transport people there and back. A space vehicle that can support several people for say 2 years, and move itself to and from Mars, is going to by way bigger than anything that can be launched by a single rocket all at once. A big heavy-lifter whose engines have a much improved Isp (which rather relaxes the expensive mass saving measures you need in the rest of the rocket to launch a given payload, or alternatively lets you launch a much heavier payload for the same vehicle) is consistant with a mars road map.

I doubt they would undertake something like this just for Mars - developing rocket engines is hecka expensive. SpaceX got started by inheriting most of the cancelled FastTrac project from Nasa ( http://en.wikipedia.org/wiki/Fastrac_(engine) ) and indeed still outsource their turbopumps (the hardest bit of an engine) to Barber Nichols, who did the design for Fastrac. That was a great move on their part - starting with a blank sheet would have been madly expensive and very slow - a working baseline engine design is a very high-dimensional optimisation problem, before you even get to cutting metal. My point is, if they're now starting clean-sheet engine design, I'm sure they must have in mind some way for it to pay for itself because it's a hugely costly business.

We live in interesting times!

This must be a spoof. These are the sort of prank letters I would write to newspapers when I was a teenager and thought myself amusing, and had just read The Henry Root Letters* (which actually were amsuing).

* http://www.amazon.co.uk/The-Henry-Root-Letters/dp/0297777629

> I wonder if they can bring the satellite back down, or if the fuels[2] used in the satellite prevent its return.

They can't but not because of the fuels. It's because the Dragon capsule is the only part of the whole launch stack that can re-enter intact, and it's a payload to the rocket. Ther Orbcomm sat is also a payload, but a secondary one. They both get put into their correct orbits (in theory) by the disposable Falcon Upper Stage. There's no way for that Falcon Upper stage to bring anything back, it is just allowed to burn up after it's been used.

A lot of Western aviation qualifying authorities specify a maximum allowable mean time between failure (MTBF) of 1 per 100000 flight hours. I imagine most rocket engine designs would be more like one per hour! That said, they only have to operate for about ten minutes though before they are disposed of, in the case of expendable launch vehicles.

In Falcon 9's case, and totally from memory and probably incorrect and very approximate, we've had 4 launches, each has had 9 Merlin C engines on the first stage, the first stage burn time is 170s or about 3 minutes. There have been two failures - a non destructive engine shutdown on the previous ISS mission on the pad at T-very_little, and the rather more messy rapid disassembly failure of something near the engine or the engine itself on the most recent launch.

So, for this first stage engine the MTBF (remember this is all a bit silly and inaccurate and just for ballpark comparisons with the airline industry) the MTBF has been:

9(engines/launch)4(launches)0.05(3 min burns in hours)*0.5(2 failures) = 0.9, i.e. a MTBF of 55 minutes for Merlin 1C lower stage, vs most regulators requiring mtbf > 100000 hours on commercial jet airliner engines.

So there's a bit of a way to go yet!

I read it as that they have already deployed rather than will be. But yes, the comment seems to suggest that the idea of trying to put it into the nominal orbit was abandoned, the speculating being that the 2nd stage had to burn for longer to get the dragon/ISS primary payload as it should be, and so not leaving enough fuel left over for the secondary payload nominal insertion. That sounds plausible, but equally SpaceX haven't said anything so it's all just armchair speculation, which is fun anyway.

Note additionally it's being reported that the secondary payload on this flight, an Orbcomm sat, did not get deployed into the correct orbit because the Falcon upper stage insertion burn failed. No info on whether or not this was a related failure. Info at the moment just seems to be social networks, can't find anything more solid: https://www.facebook.com/jsrpage/posts/10151048911726680

Says Wikipedia, and my recollection, the first two flights of Falcon 1 were purchased by the DoD, i.e. the US government, so that's not correct.

http://en.wikipedia.org/wiki/Falcon_1#Private_funding

  What could possibly make the comet, which is very, very small compared to the moon, be that bright?

The tail, which is very, very big compared to the moon.

If you don't think that's the case on HN, just look for a discussion about something that has anything at all to do with Elon Musk.

There exist metal 3D printing technologies, but usually the mechanical properties of the finished material are not as good as an equivalent cast version, although can be as good as about 80%, in terms of Youngs Modulus.

At the cheap end of metal 3D printing, shapeways.com will make you stuff out of stainless steel at $8.00/cm^3. More b2b companies will provide more detailed mechanical info about their process and the results.

A site close to my hobby interests is http://rocketmoonlighting.blogspot.com , where this chap has printed a rocket engine, complete with internal regenerative cooling channels (where you keep the combustion chamber and nozzle cool by pumping the fuel in channels around it before it gets injected in the chamber). I think it's wonderful, and is a reason I'm excited by 3D printing. Also, being able to print Printed Circuit Boards, i.e. printing copper tracks onto a fibreglass substrate, would be a serious boon.

On motor power, note that the power rating of a motor is usually the maximum continuous power the motor is rated for, ie without overheating. Just because a motor is 2000W as opposed to 600W, doesn't necessarily mean it will use any more power for a given job it is being asked to do. Your 600W motor could produce 2000W too if you were to nearly double the voltage, but it might overheat. Regardless, the power they draw is the power asked of them. For example, your skateboard might be 200W continuous when travelling at a constant velocity, and both motors in this case will only use 200W, or rather a bit more because they're not perfectly efficient. Which brings me on to efficiency. Different kinds of electric motors have different characteristics, but it's usually the case that they will have a maximum efficiency at a lower power than maximum power. The designers of this skateboard might have chosen their 2kW motor such that it is operating at maximum efficiency at the power they are expecting it to need to produce to propel humans around. In this respect the range of the 2kW skateboard might actually be greater than the 600W skateboard, everything else being equal.

That said, 2kW is a lot of power! If all that could be instantaneously put down without wheel spin by a skateboard, I image it would shoot out from under your legs like a bullet out of a gun. Fun!

You're taught in faculties just like anywhere else. There is a Department of Computer Science or Department of History or whatever, and those are where everyone goes for lectures. You have 1-to-1 or 1-to-2 teaching sessions in your college (or at least, organized by your college), with some kind of relevant academic in the college (taught by a PhD student, PostDoc, professor, depending on who is availble, who the director of studies thinks is good, what level of undergrad etc).

The analogy usually used for the benefit of American-tourists (not meant patronisingly, it's just empirically surprisingly effective) is that it's like Hogwarts. You have the colleges/houses (like gryffindor and hufflepuff) where you live (at least in 1st year) and eat and play sports for, but lectures and the degree in general (like potions class) are all done in faculties with people doing the same course but in other colleges.

Most lecturers and professors will belong to a college too, but they'll still be lecturers in a faculty rather than a college. In Cambridge there is an academic rank called 'College Lecturer' but like most things that doesn't mean what it says, and is probably designed to confuse and intimidate foreign people, like everything else (for example May Week when all the college balls are held after exams, is in June, and the weeks start on Thursday and end on Wednesday). [Lest the po-faced take me seriously, it's not actually designed to confuse people, it's just there's a lot of vestigial baggage that it's not worth the effort of changing [like removing an appaendix when it's not causing you problems] when you've been around almost a millennium and where a switch to the Gregorian Calendar was more recent than when the desk in your study was built.]