SpaceX: “Close, but no cigar. This time” • Hacker News Board

Re: SpaceX: “Close, but no cigar. This time”

#221

Post by schiffern » Sat, Jan 17, 2015, 12:37 AM UTC

sillysaurus3 wrote: post #216

schiffern wrote: post #211

Earlier quoted context omitted.

Can't reply to sillysaurus3 directly ( https://news.ycombinator.com/item?id=8902595 ), so I'm replying here: You make a great point! I didn't quite grok it before. > Can that expansion really be pushed 175 miles north and 175 miles south simultaneously? > Surely at some point the weight of the tube overcomes the steel's desire to expand, so the result would be the tube either scrunching up like an accordion or going…

The trick is to click on "link" next to the comment and then write a reply. I try not to reveal that or to use it during a heated debate, since the reason the "reply" link is hidden is to discourage flamewars, but it's obviously not intended to hamper tech talk. And I wish HN had a private messaging system so that I could've mentioned that in private. For a sense of scale, 60 mph travels one mile in 60 seconds, or 0.…

Ahh, thanks for the tip, and the wonderful discussion!

I agree, that would not work. I was suggesting that motorized rollers move the track longitudinally instead of sideways. Essentially they would take up the slack and actively push it to the end stations, counteracting the friction from the passive rollers. You could use thermometers and position sensors to model the stress on the tube.

Re: SpaceX: “Close, but no cigar. This time”

#222

Post by rwallace » Sat, Jan 17, 2015, 12:38 AM UTC

jacquesm wrote: post #59

kabdib wrote: post #55

Earlier quoted context omitted.

Cheaper to use drones / cruise missiles / HE carried by a flock of small robot helicoptors; many simpler schemes that are less traceable, unless developing a reputation is your intent. There are uncountable ways to kill people without needing to invoke Hollywood class exotica. [NB: I thought about not posting this, because people who are paid to not have a sense of humor about these things almost certainly read HN, b…

Hard to make any of those work across intercontinental differences. Drones, cruise missiles and small robot helicopters need to be launched from a location relatively close to the target.

A kitchen knife can work across intercontinental distances. Buy plane ticket to city where target lives, buy knife in local store, get taxi to location of target, stab target to death with knife. It's less glamorous than drones and missiles, but it's just as effective in the scenario that kicked off this thread, where one civilian decides to kill another even though he'll be writing off his own life in the process. Shinier weapons don't change that fundamental.

Re: SpaceX: “Close, but no cigar. This time”

#223

Post by sillysaurus3 » Sat, Jan 17, 2015, 12:45 AM UTC

schiffern wrote: post #221

sillysaurus3 wrote: post #216

Earlier quoted context omitted.

The trick is to click on "link" next to the comment and then write a reply. I try not to reveal that or to use it during a heated debate, since the reason the "reply" link is hidden is to discourage flamewars, but it's obviously not intended to hamper tech talk. And I wish HN had a private messaging system so that I could've mentioned that in private. For a sense of scale, 60 mph travels one mile in 60 seconds, or 0.…

Ahh, thanks for the tip, and the wonderful discussion! I agree, that would not work. I was suggesting that motorized rollers move the track longitudinally instead of sideways. Essentially they would take up the slack and actively push it to the end stations, counteracting the friction from the passive rollers. You could use thermometers and position sensors to model the stress on the tube.

Whoops, I see. Sorry for the confusion!

That's an interesting idea, but are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station? 175 miles of steel would be incredibly heavy. It seems like the heating/cooling might even cause the steel to weaken enough such that, rather than rolling the tube towards the station, it would start to scrunch like an accordion. But I wish a structural physicist would step in with some calculations to prove whether or not 175 miles of steel on rollers could be pushed by thermal expansion without compromising hyperloop's structural stability.

Maybe we could ask /r/askscience?

EDIT: After re-reading your comment, I think you're saying the rollers would actually provide the force necessary to move 175 miles of steel, rather than letting the expansion passively push it. But those rollers would have to grip onto something in order to do that. They wouldn't be rollers so much as tank treads.

Hmm... Do you think that might work? Could 175 electric motors working in unison provide enough force to tank treads that bite into the concrete pillar below, and into the steel tube above, in order to successfully shift 175 miles of steel? And the system has to have temperature sensors every mile, and those sensors must not ever report incorrect readings or else the whole tube breaks. And those motors must never break down or run out of power...

I'm sounding pessimistic, but honestly I have no idea whether that would work.

EDIT 2: Ah, I think I've figured out why it can't work. It's not a straight tube. It bends all along the route. http://i.imgur.com/3TavjCY.png

So motorized rollers would actually need to bend the tube in order to successfully transmit the thermal expansion. In other words, when a piece of steel tubing is initially created, it's bent with a certain curvature. But if rollers are to transmit the expansion all the way to the station, the exact location of where the steel is bent needs to be dynamically defined at runtime, rather than statically assigned at compile time. Did I phrase that very clearly? It would help to be in-person, or maybe I could draw a picture if needed.

It doesn't seem like rollers could dynamically bend the steel all along the 175 mile track in order to carry the expansion.

Re: SpaceX: “Close, but no cigar. This time”

#224

Post by mitchty » Sat, Jan 17, 2015, 12:51 AM UTC

seanflyon wrote: post #204

mitchty wrote: post #58

Earlier quoted context omitted.

Open loop is lighter than closed. No need for a closed loop if you have a set time of operation. I thought I heard they were also pumping the fuel through the system as well so I imagine this amounts to just a bit more propellant and another go.

> I thought I heard they were also pumping the fuel through the system as well I think they use fuel as the hydraulic fluid for the gimbaling of the motors, but not for the grid fins.

That might be true, can't seem to quickly find a source for that so I'll just eat crow and look like a muppet on that one.

Re: SpaceX: “Close, but no cigar. This time”

#225

Post by avmich » Sat, Jan 17, 2015, 12:56 AM UTC

tsotha wrote: post #198

threeseed wrote: post #173

Earlier quoted context omitted.

Tesla didn't start the electric car resurgence nor is it likely to take us to Mars. The way I see it Elon is rising up to those around him not the other way around.

He may not ever get to Mars, but he's doing something far more important, which is to drastically reduce the cost of putting things into LEO. If he can get it low enough a whole list of things pencil out that weren't economically feasible in the past, like space-based solar power generation. Without SpaceX it's likely the business would have muddled along without putting any effort into reducing costs for decades.

Yes. The biggest achievement of SpaceX so far was economical and organizational. Musk cleverly employs latest - which are considered common sense in other industries - approaches to the industry known for its conservatism. That gives them - eventually - proverbial kick in the sensitive parts, making them care more.

Re: SpaceX: “Close, but no cigar. This time”

#226

Post by [deleted] » Sat, Jan 17, 2015, 12:57 AM UTC

sillysaurus3 wrote: post #136

vidarh wrote: post #133

Earlier quoted context omitted.

> The steel expands in summer due to heat. Where does it go? Roads can expand using those interlocking teeth things, but hyperloop has to remain a closed system. Feel free to disagree with whether or not the approach outlined is feasible, but this is specifically covered in the whitepaper. See page 5, section "Earthquakes and expansion joints" as well as page 28, section 4.2.3 "Pylons and tunnels".

To clarify, tlb's comment was in direct response to the Hyperloop Alpha whitepaper: https://news.ycombinator.com/item?id=6201728#up_6202327 He doesn't properly address thermal expansion of the entire system. A long straight steel tube from SF to LA will expand a total of 300 meters over a 40C temperature change. Where does it go?

[deleted]

Re: SpaceX: “Close, but no cigar. This time”

#227

Post by schiffern » Sat, Jan 17, 2015, 1:12 AM UTC

sillysaurus3 wrote: post #223

schiffern wrote: post #221

Earlier quoted context omitted.

Ahh, thanks for the tip, and the wonderful discussion! I agree, that would not work. I was suggesting that motorized rollers move the track longitudinally instead of sideways. Essentially they would take up the slack and actively push it to the end stations, counteracting the friction from the passive rollers. You could use thermometers and position sensors to model the stress on the tube.

Whoops, I see. Sorry for the confusion! That's an interesting idea, but are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station? 175 miles of steel would be incredibly heavy. It seems like the heating/cooling might even cause the steel to weaken enough such that, rather than rolling the tube towards the station, it would start to scrunch like…

>are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station?

Nope. That's why you may need to push it! :)

Assume the rollers have the Crr of a passenger train wheel (0.0020). If each section weighs 193 tonnes (23 mm thick by 2.23m diameter by 100' by 8 g/cm³), then each roller will provide ~400 N of friction. Over 175 miles (worst case) that works out to 3.56 meganewtons of resistance to motion.

For a thin-walled cylinder, buckling stress is modeled using the large deflection approximation[1] as:

σ_x = 0.238 Et/r

Where σ_x is the bucling pressure, E is the material's Young's modulus, t is the wall thickness, and r is the radius. Plugging in 190 GPa, 20 mm, and 1.125 m yields…

58 meganewtons, or about 16 times as strong as the predicted frictional force

I guess they may not need active control after all!

But what about curves? Won't it result in a lot of sideways force? Needs more calculations…

[1] The weaker of the two. http://www.dtic.mil/dtic/tr/fulltext/u2/a801283.pdf

Re: SpaceX: “Close, but no cigar. This time”

#228

Post by sillysaurus3 » Sat, Jan 17, 2015, 1:15 AM UTC

schiffern wrote: post #227

sillysaurus3 wrote: post #223

Earlier quoted context omitted.

Whoops, I see. Sorry for the confusion! That's an interesting idea, but are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station? 175 miles of steel would be incredibly heavy. It seems like the heating/cooling might even cause the steel to weaken enough such that, rather than rolling the tube towards the station, it would start to scrunch like…

> are you sure that thermal expansion imparts enough force against 175 miles of steel in order to push it all the way to the station? Nope. That's why you may need to push it! :) Assume the rollers have the Crr of a passenger train wheel (0.0020). If each section weighs 193 tonnes (23 mm thick by 2.23m diameter by 100' by 8 g/cm³), then each roller will provide ~400 N of friction. Over 175 miles (worst case) that wor…

Sweet calculations! Thanks! Check out my above comment again, since I was apparently writing out an "EDIT 2" while you were doing these awesome calculations.

Basically, what do you think about the fact that it's not a long, straight tube? The rollers would need to dynamically bend the steel tube in order to carry the expansion all the way to the station, wouldn't they? Is that possible?

Even if there aren't any rollers, the tube still needs to re-bend itself somehow. But how? That would mean steel curves near the endpoints would move a whopping 150 meters in total. So it seems like part of the tube manufactured as a curve in summer would need to become a straight length of pipe by winter.

Re: SpaceX: “Close, but no cigar. This time”

#229

Post by jacquesm » Sat, Jan 17, 2015, 1:39 AM UTC

sillysaurus3 wrote: post #218

jacquesm wrote: post #213

Earlier quoted context omitted.

You could simply slide one tube into the next and leave the 'gap' between the tubes sit inside the sliding portion. That way you could have ridiculously close clearance to the tube except for that tiny bit (a few cm) every hundred meters or so. Just anchor one end of each tube and let the other slide freely.

Each pod weighs 7,700 lb and is traveling at 760 mph. Are you sure a few-centimeter speedbump isn't a big deal? I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems. Remember, the pod comes into direct contact with the wall while banking, so it's going to take the full force of any few-centimeter bump.

> Are you sure a few-centimeter speedbump isn't a big deal?

Depends on lots of things, such as the angle of incidence of the pod relative to the direction of the wall of the tube.

> I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems.

I don't know how to calculate rockets landing on barges either, but SpaceX seems to be able to pull it off. There is nothing in the realm of engineering that says you can't make a bunch of tubes sliding over each other relative to the direction of travel for some vehicle to travel within that tube as long as you respect a couple of basics. One simply solution would be something like this:

    seg2----- --seg1 
    ----- ----------
      0000000

Where the '0' are the elements of the pod in contact with the wall (I would assume they're using rollers there, anything solid would wear out the wall in no time at all).

That way there is no speedbump at all... It's not as if you're going to do right angle turns at the joint of two segments.

Anyway, I really think you're chasing a ghost here, the problems with hyperloop are not basic engineering.

Re: SpaceX: “Close, but no cigar. This time”

#230

Post by sillysaurus3 » Sat, Jan 17, 2015, 1:52 AM UTC

jacquesm wrote: post #229

sillysaurus3 wrote: post #218

Earlier quoted context omitted.

Each pod weighs 7,700 lb and is traveling at 760 mph. Are you sure a few-centimeter speedbump isn't a big deal? I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems. Remember, the pod comes into direct contact with the wall while banking, so it's going to take the full force of any few-centimeter bump.

> Are you sure a few-centimeter speedbump isn't a big deal? Depends on lots of things, such as the angle of incidence of the pod relative to the direction of the wall of the tube. > I don't know how to calculate whether or not it would be, but I'm interested in knowing why you feel it won't cause problems. I don't know how to calculate rockets landing on barges either, but SpaceX seems to be able to pull it off. Ther…

That solution is interesting, but does it account for the fact that the tube curves all throughout its route? Some parts need to be curvy, some need to be straight. Are you sure that expansion can be coaxed into a curve shape rather than linear? How?

Also, curves still result in a speedbump with that solution. The pod will be traveling directly toward the gap just below "seg2" if it's banking against the wall. You could make the edge a sharp edge, like a sword's edge, but that doesn't help because due to expansion that edge will still protrude out into the path of the pod.

I know it seems like it shouldn't be a hard problem, but it's looking like one. Remember, 300 meters of expansion over 350 miles is 0.85m of expansion per mile. Hold out your arms to the length of a meter and picture how much steel that is. That's what your proposed joints need to deal with: 0.05cm of expansion per meter. So if you have your joints every 20 meters, that's still a 1cm gap, which means a speedbump around curves. Wouldn't that make curves very unpleasant for the passenger and the pod? And you can't have them every meter without increasing the engineering complexity and weakening the structural integrity, right? Yet whatever solution they come up with would need to be placed less than every 20 meters, otherwise they'll get a 1cm gap.