Posts by Tim Starling

LOHAN fizzles forlornly in REHAB

All about the temperature

It sounds like most well-informed commenters are agreeing that the problem is the fuel temperature in the fuel contacting the igniter. It's hard to imagine the problem fundamentally being pressure. If the rocket could only burn at 1 bar then it's not going to do well once the blast plug has popped off and the thrust has settled down to ~2 lb, since that will only provide a pressure back on the fuel of ~30 mbar. If it's going to work at all, it has to be self-sustaining in a vacuum.

So the solutions come down to either:

1. Use a blast plug, to contain the flame and help it to heat the bulk fuel to the autoignition temperature.

2. Heat the fuel externally, e.g. with thermite paste.

My preference is for 2, since it won't waste energy popping off the blast cap, so it should give a higher final altitude, and it seems like there's less that can go wrong with it.

Proper job

Well done to the SPB team, for finally bringing this experiment together and getting us some excellent results. We wanted to know if the rocket would burn at low pressure, and now we know. This is a milestone, not a setback.

Numbers don't lie: Apple's ascent eviscerates Microsoft

Reciprocal

Yes, I suppose Apple's market share looks better if you graph the reciprocal of it. It's a pity for the fanbois massaging the numbers that Apple started making an OS before Microsoft, otherwise they could extend the graph back to Apple's founding and watch Microsoft's relative market share plummet from infinity down to 19, instead of from 56.

Boffins pull off room-temp quantum computing with home-grown gems

Re: No safety goggles?

I'm guessing that photo was taken with a tripod and a long, long exposure. The beam looks like a solid line, a long exposure is the only way to get that effect. It usually comes out looking very nice, but there's a bit too much scattered light here.

To be used for paying for stuff, sure

Definitely a promising architecture. I don't have access to the Science article, but I found a preprint from the same group describing it: http://arxiv.org/abs/1012.2864 .

Maybe the boffins hope this technology will be used for securing bank transactions, but I bet their military backers instead want to use it for snooping on civilian communications using Shor's algorithm. DARPA, AFOSR (Air Force Office of Scientific Research) and MURI (a DoD grant program) are listed as funders.

LOHAN sucks 27 inches

Hissing

If there's a leak in the plumbing, you should be able to hear it hissing. It may be easier to hear with the motor switched off.

Review: Raspberry Pi

Re: Desktop performance is not surprising

The price has dropped to $51 so the original product listing was deleted, and when I tried to go back to the same link, it redirected me to a different product. So I suggest going to the seller page:

<http://www.aliexpress.com/fm-store/405092>

Then search for some part of the product description which is "Dropship Free shipping 7 " Epad Android 2.3 RAM 256M ROM 4GB Camera Flash 10.1 WiFi 7 inch Tablet PC".

There are hidden screws under the corners of the self-adhesive screen cover.

Re: Desktop performance is not surprising

"No doubt Android could be hacked onto the device but it will never run 1% as nicely as the cheapest, cheapy Android tablet out there (you can find them for £50 now, with touchscreen and wifi) which can run the Play Store and use the Android logo legitimately."

I recently bought a generic 7" Android tablet from aliexpress.com for about $55 including shipping, to be used as a development platform. I was pleasantly surprised when I ripped the case off it and discovered that its only internal storage was a 4GB micro-SD card, with the whole system stored on it. I rooted it by mounting the card on my Linux laptop and copying in the necessary files. It has ethernet, wifi and USB.

On forums, people moan about how these things suck compared to an iPad, but to me, the possibilities are exciting. I'm planning on making it into a portable touchscreen-controlled media player, by attaching computer speakers. The sound quality from the headphone jack is good enough.

The Raspberry Pi sounded good to me at first, but the price ended up being much higher than they originally advertised, making it comparable to these cheap Android tablets. So it mostly became a choice between HDMI and a touchscreen, and the applications I was imagining favoured a touchscreen.

Sparkfun will sell you an Arduino ethernet shield for $45, a wifi shield for $90 and a 3" touchscreen for $85. If you're a thrifty geek, it's hard to beat repurposed consumer gear.

LOHAN demonstrates impressive sucking skills

Sounds like a risk to me

You want to conduct your experiment at a pressure which is 7.5 times the intended operating pressure? I don't know much about rocket ignition, but whatever effect pressure has on it, I would expect it to be proportional to pressure. A factor of 7.5 sounds like a lot to me.

Yes, it sounds less if you take advantage of the exponential relationship between altitude and pressure, and express it as a height difference instead of pressure difference. But the rocket motor is not going to know how high it is, only what pressure it's at.

To put it another way, sea-level pressure is only another factor of 7 above the pressure you're proposing for the experiment, so in terms of pressure ratios, the experiment will be further from reality than it will be from sea level.

Here's my advice: head down to your local 3rd-year physics lab, and say to whoever runs the place: "hey, that's a mighty fine rotary vane pump you've got there, mind if we borrow it over a weekend? It will be used for the glory of Britain. Of course we will pay for any damage." Then be really careful with it.

UPDATE: GAGA team hunts down grass-smoking ROBOT

Optical obstructions

Gardens aren't just empty space, they have things like swing sets, paddling pools, trees and sheds in them. Obstructions could cause problems for beacon-based optical methods.

The guy who suggested dead reckoning using wheel motion got downvoted, but it is a serious technique. Check out this article where 0.5% accuracy was achieved:

http://geology.heroy.smu.edu/~dpa-www/robo/Encoder/imu_odo/

Recalibration could be done at close range to a fixed beacon, which would be simpler and cheaper than long-range positioning.

Quantum computing in our lifetime - IBM breakthrough

In theory they can be used for simulating quantum systems such as molecules, which would bring great advances in chemistry and medicine. Unfortunately the research into this application is only preliminary since the vast majority of funding has been focused on breaking public key cryptography. The ethical implications are one of the reasons I got out of the field.

Wikipedia to shut down Wednesday in SOPA protest

Just some JavaScript to cover up the page content with something else, and possibly a tweak to the site CSS to avoid flicker. It'll be possible to work around it. Also, the mobile site will still work.

Quantum computing comes closer as diamonds get spooky

Coherence time

The coherence time really is important, it indicates that they didn't isolate the quantum states from the environment, rather they just did the experiment fast enough that the environment didn't have time to destroy everything. It makes the result a lot less extraordinary, and it brings into question the researchers' claim that the entangled state "persists" at ambient temperature -- it's a funny definition of "persist".

LOHAN: Reader vacuum pump plans really suck

Maybe wrap the battery and electronics in chemical handwarmers, like in this project: http://www.bhaldi.org/1st-launch

New pics of giant black sphere hurtling toward Earth

No more moon!

@Charles Manning: the asteroid is only one trillionth of the mass of the moon, so it would probably not destroy the moon entirely even if it did collide.

Can general relativity explain the OPERA neutrino result?

Re: How do they know

When a pion or kaon decays it produces both a muon and a neutrino. They detect the muons, and infer from the time of the muon detection the time at which the neutrino would have been generated. The original paper is available for free and has some pretty pictures showing the whole process:

http://arxiv.org/abs/1109.4897

Edge of physics?

Gravitational time dilation was identified as a consequence of general relativity in 1907 and confirmed in 1959. Calling it the "edge of physics" is like calling sliced bread the "edge of baking".

LOHAN to suck mighty thruster as it goes off, in a shed

Low vacuum

I've done some work with both low and high vacuums as a physics lab demonstrator. I'm pretty sure outgassing won't be a problem at the pressure required. At 90,000 ft the pressure will be 2 kPa, or 12 mmHg, according to http://www.altitude.org/air_pressure.php . We did low vacuum experiments with an oil-filled single stage rotary vane pump, probably at around 0.1 kPa. At that pressure it was no problem to let students handle the test samples and equipment.

I'm concerned about the rubber seal in the proposed design, it may become brittle at the temperature required, and start leaking. Page 323 of this book

http://books.google.co.in/books?id=sdKAPJh5RgQC

has a table of sealing temperatures, it looks like silicone rubber is the best choice.

LOHAN eyes hardcore partner's impressive girth

Or maybe use a vacuum pump?

You can buy a new vacuum pump for about £250. I'm sure someone would lend one. No need for fantastic water column constructions.

A cheap pump would be slow so it would make sense to keep the volume small. You could close off the combustion chamber with a valve and disconnect the pump entirely if you were afraid of damaging it.

X-COM UFO: Enemy Unknown

Difficult

It became fairly easy after I played it a million times, some of those times being quite recently. I loved it anyway. The secret was to sell almost everything that you recovered from the ground missions, plus a lot of your starting stocks as well. Go straight for heavy plasma, hire 50 soldiers and immediately sack all but the best shooters, then you're ready to kick some alien arse.

Apple to support reps: Don't confirm Mac infections

You don't need root access to install a virus on Linux

@Nigel 11: just about anything that a virus would want to do on a Linux desktop can be done without root access. For example, it could install a Firefox extension in the user's profile which acts as a keylogger, or it could install a cron job which starts a botnet client. On Linux, the browser is the first and last line of defence.

Facebook HipHop serves 70% more traffic on same hardware

MediaWiki

No, MediaWiki is not using HipHop. I recently said on wikitech-l that we should start working on HipHop support for a compatible release in 1.18 or 1.19, and we've started work on it. We certainly haven't finished.

PHP apps plagued by Mark of the Beast bug

assert( (x/y) == (x/y) )

On PHP bug 53632, Rasmus Lerdorf gave us a link to the GCC bug which caused this:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323

"To summarize, this defect effectively states that: assert( (x/y) == (x/y) ) may cause an assertion if compiled with optimization."

WikiLeaks.org resurrected in US of A

.cc

.cc is the Cocos Islands, plural, often called the Cocos (Keeling) Islands to distinguish them from Cocos Island, which is a different place.

NASA: Civilization will end in 2013 (possibly)

Sloppy reporting

As far as I can determine, there is no prediction by NASA or anyone else that the solar maximum in 2013 will be worse than the one in 1991. The conference in question is mostly about contingency planning, not forecasting. The whole story seems to be a worst case scenario confused for fact.

Top-killing, crisp spam and cooling the Tube

2nd law

The second law of thermodynamics unfortunately limits the amount of useful work that can be extracted from a given temperature difference. This is the fundamental reason why waste heat is wasted rather than recycled indefinitely. But before you write to your local MP to have it repealed, I should mention that the second law is also the only thing in modern physics which gives time a direction. Without the second law, nothing would ever happen on a macroscopic scale, and so The Reg would have infinitely less news to report. This, I think we would all have to admit, would be a bad thing.