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

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

#201

Post by schiffern » Fri, Jan 16, 2015, 11:15 PM 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?"

You're conflating two different events.

The "have you seen their car?" comment was from an interview with Bloomberg regarding BYD, not BMW.

He laughed when asked about BMW during the Q2 2013 conference call, but it was a chuckle followed by serious comments. It was only when the questioner began nervously laughing that he laughed for a significant time.

Compilation video containing both events (unfortunately with music, but I don't care enough to track down the individual sources): https://www.youtube.com/watch?v=8Gt6CAswSOY

So, the big criticism here is that… he's too forthright about competitor's products? I'm actually quite ok with that.

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

#202

Post by vladimirralev » Fri, Jan 16, 2015, 11:18 PM 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…

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 the speed you picked up from the freefall just before you hit the ground. Best described as a hover-slam. If your position during the freefall is off, you can't just keep the engine running to hover around and fix it (like they do in their dev rocket), it would fly up quickly if they do. In this video they still have lots of fuel in the rocket so they were trying to hover a little, however in the real landing presumably the rocket will be empty thus the hover-slam will be harder.

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

#203

Post by mcdonji » Fri, Jan 16, 2015, 11:21 PM UTC

iamcreasy wrote: post #80

Someone1234 wrote: post #63

Earlier quoted context omitted.

> I'm not saying he can do no wrong, but I'm just flabbergasted that there are still so many armchair critics and naysayers when it comes to Elon Musk. That's why you're a fanboy. Nobody is perfect, and Elon Musk likes to make fun of the competition quite publicly when they have an issue, while ignoring his project's own failures. It is hard to even discuss the man on HN as anything that could even be taken as critic…

Well, Elon Musk said talking about hyperloop was like shooting his own foot. It was a premature idea. But the interesting part is some(more than some) people are actually trying trying to come up with a real world implementations of hyperloop and so far their verdict is 'it's doable' and they are working on it. And these people are engineers from reputed companies around the world. I don't particularly like the conce…

It seems to me Musk was attempting to get the people who should be pushing bold new ideas to do so. The Hyperloop idea might have been premature but it certainly engaged people in discussing radical new ideas rather than discussing minor tweaks to an existing concept. That general thought is imbued in the culture of the companies he is involved with.

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

#204

Post by seanflyon » Fri, Jan 16, 2015, 11:44 PM UTC

mitchty wrote: post #58

jacquesm wrote: post #42

Earlier quoted context omitted.

The hydraulic oil is not used as a 'fuel' to burn but to control the actuators for the grid fins. So if you lose control because you run out of hydraulic oil the last bit of the descent will be uncontrolled. So they almost made it with enough hydraulic oil, which means that next time the grid fins will work all the way down, which leaves a ton of other possibilities for mishap (too high a descent rate, rogue waves, b…

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.

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

#205

Post by schiffern » Fri, Jan 16, 2015, 11:45 PM UTC

sillysaurus3 wrote: post #160

vidarh wrote: post #146

Earlier quoted context omitted.

And my response is the same: See those sections.

Sure, let's go over it together. Tlb is many things, but "rarely wrong" is probably near the top of the list. He certainly read those sections before making that comment, so there must be something here which made him as an engineering expert say that it doesn't properly address thermal expansion. I'll try to run an emulator of tlb's mind, but mine is far less sharp. The tube will be supported by pillars which constr…

>Is that really feasible, especially given that different points along the route could have temperature differences?

Temperature differences would tend to cancel out, no? If one section is warmer than average and one section is colder, the overall change would be smaller.

>the tube becoming bigger in the circular direction, like a bigger "O"

Yep, by about 2 mm. No problem there.

>But again, where precisely does 0.25m of steel go?

It slides all the way to the station. There trains are moving slowly, so they could add rails to the "expansion section" between the tube and the airlock. The stationary rails slide along the inside of the tube (no seal needed). Wherever the tube ends, the pod is already on the rails!

Am I making any sense here?

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

#206

Post by sillysaurus3 » Sat, Jan 17, 2015, 12:01 AM UTC

schiffern wrote: post #205

sillysaurus3 wrote: post #160

Earlier quoted context omitted.

Sure, let's go over it together. Tlb is many things, but "rarely wrong" is probably near the top of the list. He certainly read those sections before making that comment, so there must be something here which made him as an engineering expert say that it doesn't properly address thermal expansion. I'll try to run an emulator of tlb's mind, but mine is far less sharp. The tube will be supported by pillars which constr…

> Is that really feasible, especially given that different points along the route could have temperature differences? Temperature differences would tend to cancel out, no? If one section is warmer than average and one section is colder, the overall change would be smaller. > the tube becoming bigger in the circular direction, like a bigger "O" Yep, by about 2 mm. No problem there. > But again, where precisely does 0.…

That makes total sense, but I don't think it would work. The expansion doesn't really cancel out. Imagine heating 1 mile of hyperloop by 11C, but only 1 mile. The result is 0.25m more steel that has to go somewhere.

Can that expansion really be pushed 175 miles north and 175 miles south simultaneously?

I'm not going to say that's impossible, because I'd imagine someone having similar objections to, say, the invention of A/C current. "Can pushing electrons back and forth really propagate thousands of miles?"

But if you visualize the implications of that, it's intuitively difficult to believe the expansion can be transmitted to the endpoints across the entire length of a solid steel tube 175 miles away. That would move an incredible amount of mass. 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 in the lateral direction.

(Jacques said sleeves could be the solution so that thermal expansion can't accumulate, but it seems like sleeves would introduce problems for the pod, possibly severe ones. When a pod travels over a sleeve, it would feel like a jolt, if I'm understanding his idea of "sleeves" correctly.)

I'm so happy someone is actually engaging with the comment and tossing ideas around with me though.

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

#207

Post by seanflyon » Sat, Jan 17, 2015, 12:02 AM UTC

Someone1234 wrote: post #63

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…

> I'm not saying he can do no wrong, but I'm just flabbergasted that there are still so many armchair critics and naysayers when it comes to Elon Musk. That's why you're a fanboy. Nobody is perfect, and Elon Musk likes to make fun of the competition quite publicly when they have an issue, while ignoring his project's own failures. It is hard to even discuss the man on HN as anything that could even be taken as critic…

Could you give an example of a constructive comment that was killed? While certainly there is a lot of optimism around what Elon is trying to do, I don't think that "I feel like I at a One Direction concert surrounding by a bunch of screaming teenage girls" is a reasonable reaction.

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

#208

Post by Tloewald » Sat, Jan 17, 2015, 12:09 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…

> I have never been more impressed with a human being

Nelson Mandela.

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

#209

Post by downandout » Sat, Jan 17, 2015, 12:11 AM UTC

vladimirralev wrote: post #202

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…

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…

Thank you. I assume there are legitimate reasons for not having throttle control, but perhaps they should look into adding it. The hover-slam seems risky.

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

#210

Post by grecy » Sat, Jan 17, 2015, 12:14 AM UTC

vladimirralev wrote: post #202

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…

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 were no limit on that, I'd want to just turn the engine on and off a bunch to get the right amount of thrust to hover.