from Hacker News

Background on the Falcon 9 Launch

by esm on 12/22/15, 10:57 PM with 139 comments

• by nathanielc on 12/23/15, 12:33 AM

  My favorite way to describe orbits is 'constantly throwing yourself at the ground but moving so fast that you just keep missing', hence the constant free-fall/zero-g.

• by blizkreeg on 12/23/15, 1:23 AM

  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 seems involved in and knowledgeable about every aspect of their operations and tech. And finds the time to write a post like this before what is an incredibly important and defining endeavor.

  What can us, mere mortals learn from him? We can't change our baseline raw intelligence (which effects how quickly and deeply you can learn new things), but are there other patterns we can replicate in our lives?

• by dcgoss on 12/23/15, 2:54 AM

  Elon's whole gravity explanation is essentially a textual version of this excellent video/demonstration: https://www.youtube.com/watch?v=MTY1Kje0yLg (19 million views). Highly recommended, very memorable.

• by w_t_payne on 12/23/15, 2:08 AM

  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 rocket over the final few tens of metres of its' descent - a sort of brobdingnagian robotic catcher's mitt.

  Granted, this might be a bit on the expensive / elaborate / bizarrely over-engineered side -- but it would look utterly awesome.

• by panic on 12/23/15, 2:07 AM

  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 way to 2000 km/h takes the remaining three quarters.

• by caio1982 on 12/23/15, 11:50 AM

  Just for the record, the biggest newspaper in Brazil has an incredible picture of the launch/landing on its front page today :-) http://f.i.uol.com.br/folha/homepage/images/1535742.jpeg

• by mrec on 12/22/15, 11:44 PM

  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.

• by marcosscriven on 12/23/15, 9:30 AM

  This all seemed reasonable except this paragraph:

  "The reason they are floating around is that they have no net acceleration. The outward acceleration of (apparent) circular motion, which wants to sling them out into deep space, exactly balances the inward acceleration of gravity that wants to pull them down to Earth."

  There is no "outward acceleration". The weightlessness is because the craft they are in is accelerating towards Earth with exactly the same acceleration. The reason they don't hit the ground is that they have a suitably high tangential velocity.

• by mannykannot on 12/22/15, 11:37 PM

  "[100Km altitude] is the equivalent of the starting line of a race. The race itself is the kinetic energy."

  Did you get that, Jeff?

  The truth is, they have both achieved an astonishing amount.

• by sixQuarks on 12/23/15, 1:16 AM

  Elon is actually great at explaining things in simple terms. The writing on WaitbutWhy.com is very similar.

• by marcus_holmes on 12/23/15, 2:46 AM

  haha they gave the drone ship a Culture ship name! SpaceX just became my favourite company

• by sopooneo on 12/23/15, 3:30 AM

  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.

• by IshKebab on 12/23/15, 4:24 PM

  > Getting back to everyday reality, the impression that most people have is that gravity stops once you reach a certain altitude above Earth, at which point you start floating around in "zero g", but, as we just talked about, this is obviously not true. The force of gravity drops proportionate to the square of the distance between the centers of two objects.

  I'd like to meet the person that is both uneducated enough to think that gravity suddenly stops and after that is "zero g", and also undestands what "proportionate to the square of the distance" means!

• by kibwen on 12/23/15, 2:15 AM

  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's being tossed in the sea. Though it might be hard to find such a site in U.S. territory that's both near the equator and sparsely populated.

• by tempestn on 12/23/15, 3:17 AM

  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.

• by idlewords on 12/23/15, 2:16 AM

  I'm having some trouble with the opening salvo here:

  "Now imagine placing a marble somewhere on that slippery sheet -- it is guaranteed to fall into one of the funnels. "

  This holds for the case where there are two objects initially at rest, but I don't see it as obviously true if there are more than two objects in the universe.

• by v4n4d1s on 12/23/15, 10:45 AM

  Great and easy to understand article, thanks for posting it. Does anyone know it the spacex team uses the imperial or the metric system for development? Elon switches between both systems and it's messing with my head.

• by soperj on 12/23/15, 8:15 PM

  "nitrogen attitude thrusters" Think that was supposed to be altitude?

  edit: just saw this at the bottom and it made me smile; "Apologies for any typos in the above."