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

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

#241

Post by shasheene » Sat, Jan 17, 2015, 2:45 AM UTC

grecy wrote: post #210

vladimirralev wrote: post #202

Earlier quoted context omitted.

Not aeronautics, but control systems engineer and I've seen the explanation. The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful. In order to land you turn the engine off and wait to pick up some vertical speed, then in the last N seconds you turn the engine on to kill th…

Thanks, that's very helpful. > The rocket engine they use is too powerful to hover and move towards the destination in a slow adjustment loop. And there is no throttle control on the engine, 70% minimum which is still too powerful I've seen this a lot, and I've also read about how many times each engine can be re-lit. Can you talk a bit about why the engines have a limited number of times they can be lit? If there we…

While there's a limit in the amount of (highly toxic) TEA/TEB hypergolic fuel that's carried in the ignition system I'm not sure it's the bottleneck.

The limited number of times the Merlin 1D can be relit is probably referring to the re-usability aspect (from memory they tested "cycling" the engine something like 40 times in qualification testing, but it's a little unclear exactly what a cycle entails). It's the associated "coking" of the engine with hot and sticky carbon/soot from the incomplete combustion of the kerosene that's been speculated to be a major hinderance to the long-term engines survivability.

Other than consumables, it's probably the high (and variable) latency in the switching time that restarting the engine in quick succession that would make it problematic, in addition to the engine damaging aspects you mentioned.

Hover-slamming a first-stage with a Merlin 1D has been heavily tested by F9R-Dev1/2 - it's a completely workable solution that minimizes engine restarts over the flight profile to 3 (boostback, re-entry burn, hoverslam).

While the switching the engine may be made to be workable solution which can effectively throttle the engines between 0% and 70%, a hoverslam with a vehicle with thrust/weight ratio > 1 gives less stress on the entire (~$40 million F9R first stage) vehicle to be reused.

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

#242

Post by skywhopper » Sat, Jan 17, 2015, 2:49 AM UTC

gamegoblin wrote: post #152

SEJeff wrote: post #140

Earlier quoted context omitted.

Not sure what you're saying reflects reality. When Orbital Science's rocket recently exploded, Elon's words were nothing but polite: https://twitter.com/elonmusk/status/527247155954610176 And his comment on their rocket being the butt of a joke, using engines literally made in the 1960s and stored in Siberia were true. It is upsetting, but still not as surprising that they suffered catastrophic failure having such an…

One example of his being polite doesn't mean he doesn't also make fun of competition publicly. Look at the interview where the interviewer asks him about a competitor's electric car (BMW I think) and he just laughs for a not-insignificant amount of time followed by something like "have you seen the thing?"

To be fair, the BMW electric car is the ugliest car BMW has ever made. It looks like they accidentally welded two different models together in the middle. They deserve to be made fun of for that disaster.

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

#243

Post by sillysaurus3 » Sat, Jan 17, 2015, 2:54 AM UTC

jacquesm wrote: post #240

sillysaurus3 wrote: post #238

Earlier quoted context omitted.

I don't know if you're having a bad day or have gotten into a strange mindset, but I just never expected this level of "personally insulting" out of Jacques Mattheij, especially against someone who freely admitted at the very top of the thread that I'm an idiot who is arguing from a position of ignorance. So I'll go try to find someone else to geek out with an interested newbie. Like, say, someone who finds engineeri…

You're not being insulted, you merely are trying to argue about things that you admit that you have no clue about and you persist in doing so regardless of the effort made to show you that you are wrong. So either stop arguing (I do know a thing or two about mechanical engineering) or start researching what I gave you above as the bits and pieces of the solution. What else do you want? A blueprint? A cad design?

To have an enjoyable conversation with an interested colleague, rather than someone whose tone is "shut up and listen to your elders." Besides, having a crotchety back-and-forth makes for boring reading, and we should be respectful of the community.

Thanks for the pieces you've provided so far. I'll bring up my objections elsewhere. Hopefully anyone who's been having my same concerns will be able to do the same.

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

#244

Post by schiffern » Sat, Jan 17, 2015, 2:57 AM UTC

jacquesm wrote: post #213

sillysaurus3 wrote: post #177

Earlier quoted context omitted.

I realize I'm probably an idiot, but I do try to learn from my idiocy. Thanks for helping me learn. But I'm having trouble understanding how your reply takes everything into account, such as the fact that the hyperloop moves pods, not liquid. Pipelines for liquid are certainly a solved problem, but a pipeline of pods is new. Btw, in railroads (that other long distance transportation method) the expansion is dealt wit…

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.

FWIW, Musk makes no mention of expansion joints in the tube, only at the ends. He's also stated elsewhere (sorry, no source) that you'll probably need to run a polishing robot through the tube to get it microscopically smooth. That doesn't sound like a surfacing requirement that would tolerate gaps.

If you care to, check out the conversation that sillysaurus3 and I had down-thread. I would love to have someone experienced in MechE glance at my napkin math!

TL;DR it looks like simply pushing all the slack to the end stations is actually feasible, and it's easier to deal with there since the cars aren't moving very fast.

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

#245

Post by Shivetya » Sat, Jan 17, 2015, 3:02 AM UTC

sixQuarks wrote: post #23

Sorry for sounding like such a fanboy, but during my 40 years on this earth, I have never been more impressed with a human being. The guy is pushing the envelope on perhaps the most difficult engineering/technological endeavor ever attempted by a private company - and he's making it look cool and futuristic. As if that wasn't enough, he's doing this in two different industries simultaneously. I'm not saying he can do…

Tell me about it. Close but no cigar, who else even has the gumption to try? NASA has been pushing rockets into space for fifty plus years and I have never seen them trying it; granted I may have missed it but I have pretty much watched most televised stuff since the moon landings.

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

#246

Post by jacquesm » Sat, Jan 17, 2015, 3:02 AM UTC

schiffern wrote: post #244

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.

FWIW, Musk makes no mention of expansion joints in the tube, only at the ends. He's also stated elsewhere (sorry, no source) that you'll probably need to run a polishing robot through the tube to get it microscopically smooth. That doesn't sound like a surfacing requirement that would tolerate gaps. If you care to, check out the conversation that sillysaurus3 and I had down-thread. I would love to have someone experi…

It might be, but if you did want to use expansion joints to alleviate some or all of the problem you most likely could.

There are other alternatives too, such as allowing the tubing to move sideways a bit in a long curve. That would change the overall shape of the trajectory but the air cushion they talk about could take care of that.

If they do end up using expansion joints they could actually use the fact that some air might leak in to purposefully increase local air density to use that as an impromptu air bearing.

Anyway, there is plenty of challenging engineering in hyper loop, but the least interesting part is how to make a long metal tube, either in one piece or out of segments.

And of course even more challenging than the engineering will be the economics and the paper tigers.

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

#247

Post by joezydeco » Sat, Jan 17, 2015, 3:12 AM UTC

taeric wrote: post #162

ctdonath wrote: post #94

Earlier quoted context omitted.

In contrast, from the HN-featured article about finding the Beagle2 probe on Mars: MRO's data confirms that Beagle landed just 5km from the centre of its targeted touchdown zone. 5km, and it was considered a bulls-eye accurate landing.

At the relative ranges we are talking about here, those numbers are probably more comparable than many would realize. Consider, Mars at its closest, is over 6700km away. The space station at its furthest is 418km. (Assuming I looked up the correct numbers, of course.)

Mars is 225,000,000 km away, on average.

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

#248

Post by jacquesm » Sat, Jan 17, 2015, 3:20 AM UTC

sillysaurus3 wrote: post #243

jacquesm wrote: post #240

Earlier quoted context omitted.

You're not being insulted, you merely are trying to argue about things that you admit that you have no clue about and you persist in doing so regardless of the effort made to show you that you are wrong. So either stop arguing (I do know a thing or two about mechanical engineering) or start researching what I gave you above as the bits and pieces of the solution. What else do you want? A blueprint? A cad design?

To have an enjoyable conversation with an interested colleague, rather than someone whose tone is "shut up and listen to your elders." Besides, having a crotchety back-and-forth makes for boring reading, and we should be respectful of the community. Thanks for the pieces you've provided so far. I'll bring up my objections elsewhere. Hopefully anyone who's been having my same concerns will be able to do the same.

Let me try to explain what is going wrong here:

You argue that something is impossible but at the same time you outline your ignorance. The normal position for being ignorant is to study the subject, not to argue that something is impossible.

It's like saying a moonshot never happened because you couldn't do it yourself. There are lots of things that each of us individually can not do, but collectively we can do amazingly complex things. The specialization required takes time and effort. If you're unwilling to put in the time and/or make the effort then you should not take up positions that tell those that did make that effort what they should not be able to do according to you.

If mechanical engineering interests you then maybe you should go and study the subject rather than to take up strong positions like these and to be rigid in your inability to shift on those positions.

> I'll bring up my objections elsewhere. Hopefully anyone who's been having my same concerns will be able to do the same.

Basically that says that you want to argue, not learn and that you'll only take what agrees with your viewpoint.

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

#249

Post by sillysaurus3 » Sat, Jan 17, 2015, 3:21 AM UTC

schiffern wrote: post #232

sillysaurus3 wrote: post #228

Earlier quoted context omitted.

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 rolle…

Heh, I love that we thought of two different but related problems simultaneously! First yours, since it's much cooler. The minimum curve radius is 3.67 km, or a length difference of 18 mm between the left and right side over the length of one pylon. Assuming that all sections start as straight tubes, how much sideways force would that apply on the pylon? For that we need to calculate the "flexural stiffness"[1], whic…

Do you have an email address we could continue this at? I'm going to need a little while to get back into a happy mindset after a rather unfavorable exchange upthread. I'm really looking forward to picking your brain though. Thanks for chatting with me.

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

#250

Post by Patrick_Devine » Sat, Jan 17, 2015, 3:30 AM UTC

downandout wrote: post #176

Could an aeronautics person kindly explain why it's so hard to get this thing to land as it's supposed to? I'm not being judgmental at all here; I am saying that as a software engineer with no aeronautics experience, I simply don't understand the problem. Assuming all of the sensors indicating tilt, altitude, etc. are accurate, the software portion coordinating where and how much thrust needs to deployed to keep it l…

Elon mentioned the problem was that they ran out of hydraulic fluid about 30 seconds too soon. The rocket had to try to right itself using only engine gimbaling instead of using the waffle paddles, which didn't work out so well.

The great thing is they've already fixed the problem for the next flight.