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Background on the Falcon 9 Launch

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Re: Background on the Falcon 9 Launch

#41

Post by tempestn »

It's a good summary, but I'm not sure the repeated digs at Bezos are really necessary. I think anyone who would bother to read this already gets the differences. Not sure that he really needs to point out, twice, that height doesn't matter at that stage of testing.

Edit: Just got to the end and saw this was prior to launch, so before Bezos' "welcome to the club" tweet. I guess in that context it's a bit more subtle at least, but still seems like he was making a point of the difference from Blue Origin.

Re: Background on the Falcon 9 Launch

#42

Post by mikeash »

tcheard wrote: post #28
mikeash wrote: post #8

Earlier quoted context omitted.

You can run the first stage longer without any extra second stage fuel. Adding performance to either one (within limits) increases the performance of the system. Using the landing fuel for a launch would allow for a heavier payload without changing the second stage. The first stage would do more of the total work.

Except that the longer you run the first stage, the longer you have to carry the weight of the first stage. It eventually becomes much more efficient to jettison the first stage and use a slightly less powerful but much lighter weight second stage, because you no longer need as much power to get through the atmosphere. Which is the whole point behind staging ( https://en.wikipedia.org/wiki/Multistage_rocket#Optimal_s…

That makes no sense. It's never advantageous to ditch the first stage while it's still producing thrust. At some point when designing a rocket, it's better to add performance to the second stage rather than the first. But when presented with an existing rocket, you'll achieve maximum payload capacity by burning all fuel in the first stage before staging. Every second it burns is more delta-v imparted to the payload. The only reason you'd stage early is if you want to save fuel for e.g. a landing attempt, if the first stage is too powerful and the acceleration or speed would become too great, or if you simply don't need the extra performance to get the payload to where it's supposed to go.

Re: Background on the Falcon 9 Launch

#43

Post by serge2k »

blizkreeg wrote: post #17

Clearly this man's intellect is through the roof. Reading this post and him explaining these concepts in such first-principle terms (despite not being a physicist/rocket engineer) indicates his in-depth understanding (nothing new there). I know he has a Bachelors in Physics but bear with me. But, I'm just in awe and I keep thinking 'how does he do it?'. He's running two intensely technical and risky companies. Yet he…

Is hacker news just the church of Elon now?

Re: Background on the Falcon 9 Launch

#44

Post by sopooneo »

Not trying to nit-pick, just trying to confirm my own understanding. But actually, accelerating a mass from 1000km/hr to 2000km/hr should take three times as much energy as from 0km/hr to 1000km/hr, right? I assume the quantity of "four times as much" was just used to get across the notion of energy being proportional to the square of velocity.

Re: Background on the Falcon 9 Launch

#45

Post by timdierks »

kibwen wrote: post #24

The article mentions that the water landing requires less fuel to return the rocket because it doesn't have to spend fuel overcoming its initial ballistic trajectory. In the water landing scenario, how far away from the launchpad is the landing barge? I'm wondering about the economics of launching from a site where your first stage trajectory is entirely overland, to avoid the complications of landing on a barge that…

I found a reference that the barge was 320 km downrange in the earlier tests. The Bahamas look to be an appropriate distance from Florida, although to the southeast of Cape Canaveral. From SpaceX's planned Texas spaceport, there's nothing at that range but gulf.

Re: Background on the Falcon 9 Launch

#46

Post by gozur88 »

tedivm wrote: post #39
blizkreeg wrote: post #17

Clearly this man's intellect is through the roof. Reading this post and him explaining these concepts in such first-principle terms (despite not being a physicist/rocket engineer) indicates his in-depth understanding (nothing new there). I know he has a Bachelors in Physics but bear with me. But, I'm just in awe and I keep thinking 'how does he do it?'. He's running two intensely technical and risky companies. Yet he…

In most companies the "Posts from the CEO" are a group effort (at my last company I wrote multiple statements that were supposedly from our CEO). What you're attributing to a single person is actually the work of many. This leads to your question: > But, I'm just in awe and I keep thinking 'how does he do it?'. What can us, mere mortals learn from him? We can't change our baseline raw intelligence (which effects how…

>In most companies the "Posts from the CEO" are a group effort (at my last company I wrote multiple statements that were supposedly from our CEO). What you're attributing to a single person is actually the work of many.

If it's a group effort he was being pretty deceptive by ending with "apologies for typos in the above," making it sound like he just dashed it off himself.

Re: Background on the Falcon 9 Launch

#47

Post by grecy »

w_t_payne wrote: post #23

If we can land the rocket accurately enough to put it down on a tiny barge only slightly larger than the rocket itself, then why do we need to tolerate the weight of the landing legs? We already have industrial robots that can move and grasp heavy weights relatively quickly over distances of several metres -- it doesn't take much imagination to conceive of a similar contraption being used to arrest the descent of the…

Every time these discussions come up, people immediately come up with "other" suggestions for landing.

Parachutes, "catching" devices, etc. etc.

It all comes down to one thing, and one thing only.

Whatever the solution, it has to work on other planets with no infrastructure on that planet, and it has to leave the booster in a state that it's ready to go again with only a fuel fill up.

Re: Background on the Falcon 9 Launch

#48

Post by greglindahl »

ChuckMcM wrote: post #19
mrec wrote: post #3

One thing I've not seen mentioned in the coverage so far: how much payload is sacrificed by the need to keep fuel in reserve for the return to base? I'm guessing the sacrifice is roughly equal to the mass of unburnt fuel in the booster at the point of booster separation, but don't much trust my intuition on these things.

> how much payload is sacrificed by the need to keep fuel > in reserve for the return to base? Simple answer: None. A more detailed answer is that building a system which can be reused is an economic proposition. So that the Falcon 9 can lift X Metric tons to orbit for $Y. The way in which they keep the value $Y low is by re-using the first stage. Every satellite project knows the throw weight of all the common launc…

SES already said that they're willing to be the first customer for a used booster[1].

Before they get that far, it'd be interesting to see if SpaceX flies a used engine, since they've got "one engine out" capability. Which has already been tested twice.

1: http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse...

Re: Background on the Falcon 9 Launch

#49

Post by grecy »

kibwen wrote: post #24

The article mentions that the water landing requires less fuel to return the rocket because it doesn't have to spend fuel overcoming its initial ballistic trajectory. In the water landing scenario, how far away from the launchpad is the landing barge? I'm wondering about the economics of launching from a site where your first stage trajectory is entirely overland, to avoid the complications of landing on a barge that…

I was wondering about that too. It would be cool if they could launch from somewhere south of San Antonio, TX on the water, and land in Florida so it doesn't fly over land mass for safety reasons.

Re: Background on the Falcon 9 Launch

#50

Post by notdonspaulding »

panic wrote: post #22

Nice article, but I couldn't pass up a chance to correct Elon Musk's math: It is important to note that the amount of energy needed to achieve a given velocity increases with the square, so going from 1000 km/h to 2000 km/h takes four times as much energy as going from 0 km/h to 1000 km/h, not twice as much. Three times, not four--you already spent a quarter of the energy getting to 1000 km/h. Getting the rest of the…

I believe you've misunderstood his English. If it takes 1 Joule to accelerate the mass from 0-1000km/h, it will take 4 Joules to accelerate the same mass from 0-2000km/h.

Elon is calling that four times as much as because he's considering the total energy required to accelerate from 0 in both numbers. I think you're just accounting for it differently by saying it takes 3 times as much as the original energy input to go from 1000km/h to 2000km/h.

I'm a complete physics novice, I'm open to being schooled on this if I'm completely missing both yours and Elon's concepts here.

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