Wikipedia:Reference desk/Archives/Science/2018 January 21

= January 21 =

TGV
Were there any cases when a TGV train had to stop on a steep grade for some reason (the steepest grade on the Paris-Marseilles line is a whoppin' 3.5%!) and then couldn't get moving again unassisted? Or is the TGV powerful enough that this cannot happen? 2601:646:8E01:7E0B:C81D:59C2:9A63:53DF (talk) 07:00, 21 January 2018 (UTC)
 * They can reach 300 kph and electric motors have high torque from 0rpm, they probably could start on a grade. Sagittarian Milky Way (talk) 08:46, 21 January 2018 (UTC)


 * It's an electric loco, not a diesel-electric (or turbine-electric). So the limit is almost always the torque available from the traction motors, not the total power of the prime mover - for a TGV at least. The TGV isn't designed for slow speed haulage, but it does have sufficient surplus, and at low speeds, to be able to do this. Also the Sud-Est sets have 12 powered axles per set, not just 8, as there's a powered bogie on the adjoining passenger carriage, not just the power cars.
 * I don't know what you mean by "a whoppin' 3.5%", but for railways that's (1 in 28) very steep for any main line.
 * This also brings in the issue of rated speed, the lowest speed from which full power is usable - usually limited by the torque capacity at low speeds. Diesel-electrics have often been restricted here (as your question implies - railway traction motors do not have particularly high torque and their power rating is severely restricted at low speeds.). Diesel-hydraulics have better ability to use their power as low speed torque. Andy Dingley (talk) 10:49, 21 January 2018 (UTC)

Since we're talking about the original line, let's look at the SNCF TGV Sud-Est trainsets. The spec sheet linked from that article gives the mass of the trainset as 385 t. Multiplying by 9.80665 m/s&sup2;, its weight is 3775 kN, and 3.5% of that is 132 kN. The force/speed graphs at the bottom of page 3 show a force of 210 kN available at low speed: more than enough to get moving on a 3.5% grade, provided only that the wheels do not slip. --70.29.13.251 (talk) 20:13, 21 January 2018 (UTC)


 * So, the TGV can't get stuck on a hill? Thanks! 2601:646:8E01:7E0B:0:0:0:64DA (talk) 07:38, 22 January 2018 (UTC)


 * Compare this with the Eurotunnel Class 9 locomotive’s design specification to be able to start a shuttle train on a 1 in 160 (0.625%) gradient. 82.38.221.49 (talk) 10:30, 22 January 2018 (UTC)
 * The main issue here is that the TGV is a railcar set, not a train-hauling locomotive, and a fast railcar at that. It simply weighs less than a heavy freight train, and it's also (because of the need for speed) powerful. Two other potential limits would be low-speed torque from the traction motors, and also adhesion limits. 3.5% is very steep in railway terms, but even so, the numbers for the TGV sets supports it being able to make a start on such an incline.
 * Plenty of freight trains would have difficulty in starting on such an incline, or may require banking assistance. Andy Dingley (talk) 17:28, 22 January 2018 (UTC)

What is the source of this "chapter 43" that found in a lot of sites on the internet?
I've got this document about "Antihyperlipidemic Drugs" which is "chapter 43". It is found in a lot of sites (such here), but the issue is that as much as I looked for the name of the book that it came from, I couldn't find it. What is the name of the book that it was taken from? 2A02:ED0:6F4E:6D00:F198:54F3:E7C4:F74B (talk) 15:34, 21 January 2018 (UTC)
 * It is:
 * Or possibly some other edition or format. DMacks (talk) 15:43, 21 January 2018 (UTC)
 * Indeed, it is accurately this book! Thank you. 2A02:ED0:6F4E:6D00:EA:29A2:D26A:5986 (talk) 01:04, 22 January 2018 (UTC)
 * Indeed, it is accurately this book! Thank you. 2A02:ED0:6F4E:6D00:EA:29A2:D26A:5986 (talk) 01:04, 22 January 2018 (UTC)

Any spaceship prototype based on capturing dust and using it as propulsor?
Could a spaceship be based on the capture of matter found in outer space - like dust and asteroids - and posterior use a propulsor? As least, could a satellite use this matter to correct its trajectory? --Hofhof (talk) 22:44, 21 January 2018 (UTC)


 * Like a Bussard ramjet? Sagittarian Milky Way (talk) 23:55, 21 January 2018 (UTC)


 * I think OP is proposing to use dust as reaction mass, not as fuel. —Tamfang (talk) 08:51, 22 January 2018 (UTC)


 * You mean like a Ram Augmented Interstellar Rocket? (Mentioned in the above article.) Nil Einne (talk) 14:54, 22 January 2018 (UTC)


 * All of these Bussard based designs are for interstellar travel, so they're very theoretical, no prototypes. ApLundell (talk) 16:00, 22 January 2018 (UTC)
 * …could a satellite use this matter to correct its trajectory? You can't cheat physics. Conservation of momentum dictates that if the satellite hits something else to "scoop it up", no matter how tiny, it's going to lose momentum. Colliding with matter will slow down the craft, not speed it up. Even if you later eject the material, you're never going to get back more than the momentum you lost, and in practice you'll always get less because no real system is perfect with zero energy losses. The aforementioned Bussard ramjet is based on scooping up hydrogen and then fusing it to harvest the nuclear potential energy. Solar sails are more promising for non-rocket spacecraft propulsion, as you get essentially "free" energy if you're close enough to a star, and in addition you can use lasers to transfer more energy to the craft, beating the tyranny of the rocket equation. --47.157.122.192 (talk) 10:38, 23 January 2018 (UTC)


 * No, you won't lose momentum if you eject the scooped particle at a higher speed that it hits. — Preceding unsigned comment added by 31.4.136.130 (talk) 11:36, 23 January 2018 (UTC)


 * 47.157, That is a misapplication of the conservation of momentum.
 * Momentum is conserved because the vectors are in opposing directions. (ie: You eject the dust particle at fantastic speed 'backwards', and your craft accelerates 'forwards'. Add those vectors up and you get zero.)
 * So long as you eject the particle faster than it was going when you first encountered it, the particle will have a net 'backwards' increase in momentum, therefore your craft will have a net 'forwards' increase in momentum.
 * ApLundell (talk) 16:48, 23 January 2018 (UTC)
 * But don't you need to expend energy to accelerate the particle out the back? That has to come from somewhere. And if you're carrying some kind of propulsion mechanism that ejects stuff, well, just use an ion thruster. I guess harvesting dust to use as propellant might be an option for some spaceship intended to operate indefinitely, since even an ion thruster will use up its propellant eventually. --47.157.122.192 (talk) 02:24, 24 January 2018 (UTC)
 * Yes, you would still need energy. (Perhaps from a nuclear reactor.) But using a Bussard collector would cut down on propellant needed, which for an interstellar journey might be non-trivial, even if you're using an ion thruster. ApLundell (talk) 17:13, 24 January 2018 (UTC)


 * I was thinking the description reminded me vaguely of a magnetohydrodynamic drive on a submarine, which acts on the water it is moving through/around. That article actually talks about using them on spacecraft, but Variable Specific Impulse Magnetoplasma Rocket and magnetoplasmadynamic thruster both seem to be about things that carry their own propellant rather than trying to do something to a handful of particles in empty space.  I know nothing about this, just throwing out some terms for comparison. Wnt (talk) 23:48, 24 January 2018 (UTC)