
Background on the Falcon 9 Launch - esm
http://www.spacex.com/news/2015/12/21/background-tonights-launch
======
nathanielc
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.

~~~
yellowbeard
There is an XKCD "what if" that explains this: [https://what-
if.xkcd.com/58/](https://what-if.xkcd.com/58/)

"The reason it's hard to get to orbit isn't that space is high up. It's hard
to get to orbit because you have to go so fast."

~~~
logicallee
How fast do you have to go? Easy. Look out at the horizon far enough that you
can see it drop (for example a mountain etc). You have to go _so_ fast that in
the time it takes you to reach that point, you only drop by the amount the
horizon falls.

~~~
chrisseaton
Could you make a cruise missile that didn't need any wings or other surfaces
for lift - only forward propulsion - that just went so fast that it was
effectively in orbit, but nor far above the ground?

~~~
lordnacho
The loss from atmospheric drag would be enormous. If you were on a planet with
no atmosphere it would be no different from any other orbit though.

~~~
iamjs
You could do this on the moon, for example

------
blizkreeg
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?

~~~
sillysaurus3
This hero worship is utterly stupid. It's also a disservice to children. We're
teaching them that there are heroes, and then everyone else.

Bull. Work as hard as Elon did, and you'd make the same progress. It also
required luck. No PayPal, no Elon. And there could only be one PayPal, at
precisely that time in history.

A better question is, why did Carmack fail where Elon succeeded? Armadillo
Aerospace was supposed to be SpaceX.

~~~
sjburt
Carmack invested about $2 million into Armadillo in its first 6 years. Elon
invested $100 million in that time frame (actually, the first 4 years of
SpaceX).

Armadillo did OK actually, they just were on a completely different
trajectory. It was very much a hobby project for Carmack.

~~~
derekp7
Not only that, but wasn't Armadillo one of the "straight-up-to-100KM-and-
return" (suborbital) designs? If so, then there is no money to be made in that
(except for wealthy people that want a 20 minute thrill ride). It is the
"lateral velocity of Mach 25" type of rocket that gets you satellite launch
contracts.

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

~~~
enraged_camel
Yep, came here to post this. The funny thing is that there was a post about
that video a while back, and the HN crowd criticized it pretty heavily for
being a bad way to explain gravity. But... whatever. :)

~~~
mturmon
Here is the discussion, which was interesting but inconclusive about the
merits of this rubber sheet model:
[https://news.ycombinator.com/item?id=8542244](https://news.ycombinator.com/item?id=8542244)

------
w_t_payne
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.

~~~
ggreer
There are several reasons why landing legs make more sense:

– Any flat chunk of cement is a landing spot. That means more places to land
in case of contingencies. For yesterday's mission, SpaceX had one primary and
four alternate landing zones.[1]

– I doubt industrial robots can withstand rocket exhaust. As helicopter
footage shows, the landing pad got lit-up pretty good.[2] Remember, the first
stage is over 40 meters tall. Those are some _massive_ flames.

…and most importantly:

– Landing legs work on other planets.

1\. Map: [http://www.americaspace.com/wp-
content/uploads/2015/12/LZ1.j...](http://www.americaspace.com/wp-
content/uploads/2015/12/LZ1.jpg) (from
[http://www.americaspace.com/?p=89910](http://www.americaspace.com/?p=89910))

2\.
[https://www.youtube.com/watch?v=ZCBE8ocOkAQ](https://www.youtube.com/watch?v=ZCBE8ocOkAQ)

~~~
w_t_payne
Who cares about what makes more sense ... giant robots, dude! :-)

------
panic
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.

~~~
notdonspaulding
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.

~~~
tim333
I think panic is correct. "from 1000 km/h to 2000 km/h" means from 1000 km/h
to 2000 km/h, not 0-2000 kph. Though presumably Elon was thinking of the
latter.

~~~
rimantas
Read it as "going from target velocity of 1000km/h to 2000km/h".

------
caio1982
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](http://f.i.uol.com.br/folha/homepage/images/1535742.jpeg)

~~~
agumonkey
There are a few more here
[http://imgur.com/r/spacex](http://imgur.com/r/spacex)

------
mrec
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.

~~~
ChuckMcM

       > 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 launch
vehicles and their cost per kilo. And that is how you plan you satellite
design.

Now at the moment SpaceX gets 9 merlin engines and the first stage booster
back for "free" (which is to say that the cost paid assumed it would be
consumed in the launch) but as they learn what they can do they will use that
cost savings to offer cheaper launch services (more business) until they have
a full launch schedule and then keep any excess value for re-investment.

But an interesting question is this, given that they have a "used" first
stage, who would be willing to launch on it? It has no track record and no
reliability statistics other than it worked at least once before. To develop
that information you need to re-launch them. And I'm hoping that SpaceX will
make available some higher risk but lower cost "seats" on those test flights.

~~~
greglindahl
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...](http://spacenews.com/spacex-early-adopter-ses-ready-to-reuse-
falcon-9-%C2%AD-for-the-right-price/)

~~~
ChuckMcM
That article raises an interesting point I really hadn't considered, who owns
the first stage? If you pay "full price" for a launch and SpaceX recovers the
first stage to re-use, did the person who paid to launch it own it or does
SpaceX? It was presumed lost of course. No doubt there is language in the
contracts about that. Would be interesting to see a launch contract.

~~~
Alain-lf
SpaceX own the first stage.

You pay to get your payload delivered to a specified orbit. You don't actually
buy the rocket.

It's like flying, you buy a ticket, not the plane.

~~~
ChuckMcM
Just for reference, I got an inside look at what it took to get OSCAR16[1]
launched and it isn't like flying :-). Now I don't doubt things are more
streamlined now but every launch starts with a basic contract and that
contract has to cover everything from contingencies to insurance to liability
to disposal.

Trying to search for a boilerplate launch contract I found an article[1] where
it discusses that Spaceflight Industries _bought a Falcon 9 launcher_
[emphasis mine] which suggests that one buys the entire rocket. That would
imply that if they land it, you still own it does it not? Can you then go over
to the landing zone pick up your rocket and resell it for parts to offset your
original purchase price? :-)

I am really confident that ownership of the first stage is covered in the
launch contract if it is returned to the landing field. And the math there
would no doubt be really interesting to an insurance company since you have
the possibility that the launch is a success and the first stage lands, the
launch is a success and the first stage crashes, the launch effectively fails
(second stage failure) but the first stage successfully returns, and both
stages are lost. That is a number of different outcomes to insure.

Frankly my mind is boggling at the potential legal complexity here.

[1] [https://en.wikipedia.org/wiki/AMSAT-
OSCAR_16](https://en.wikipedia.org/wiki/AMSAT-OSCAR_16)

[2] _" SpaceIL has purchased launch services from Spaceflight Industries; an
American space company who recently_ purchased a SpaceX Falcon 9 launcher _and
will manifest SpaceIL’s spacecraft as a co-lead spot, "_ \--
[http://lunar.xprize.org/press-release/israeli-google-
lunar-x...](http://lunar.xprize.org/press-release/israeli-google-lunar-xprize-
team-first-sign-launch-agreement-private-mission)

~~~
lmm
I think the real answer is that there will be a complex contractual
relationship that "ownership" is too simple to describe, in either direction.
E.g. I say I "own" my flat, but actually I am a shareholder in a company that
owns the building and have a lease contract with that company (this is the
standard (only?) way of "owning" a flat in the UK, because multiple flats
stand on the same piece of land - you can't truly "own" something you don't
have the right to destroy, but obviously you don't have the right to destroy a
flat with someone else's above it).

> And the math there would no doubt be really interesting to an insurance
> company since you have the possibility that the launch is a success and the
> first stage lands, the launch is a success and the first stage crashes, the
> launch effectively fails (second stage failure) but the first stage
> successfully returns, and both stages are lost. That is a number of
> different outcomes to insure.

The insurance industry already deals with far more complex scenarios. A ship
on an ocean voyage will often have the hull and cargo insured separately,
different loss layers insured separately (e.g. first 10% of losses, 10-20%,
20-100% all separate), with multiple underwriters on each layer (and sometimes
the same underwriter on multiple stamps).

~~~
arethuza
"this is the standard (only?) way of "owning" a flat in the UK"

Maybe in England - its never worked like that for any flat I've owned here in
Scotland, we have completely separate legislation covering such things:

[http://www.gov.scot/Topics/Justice/law/17975/11023](http://www.gov.scot/Topics/Justice/law/17975/11023)

------
marcosscriven
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.

~~~
zodiac
Both you and the article are right - you are analyzing it in the frame of
reference where the center of the earth is stationary, while the article
analyzes in the frame of motion where the center of the rocket is stationary.

Since the frame where the center of the rocket is stationary is a non-inertial
frame, Newton's law doesn't apply [1]. However a modification of Newton's law
that includes a so-called "ficticious force" applies [2] (I don't think this
modification has a name). This is why the article says there's an outward
acceleration, because in the frame where the center of the rocket is
stationary, the outward acceleration is caused by the ficticious force.

[1]:
[https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion](https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion)
begins the laws with "when viewed in an inertial reference frame"

[2]: [https://en.wikipedia.org/wiki/Non-
inertial_reference_frame](https://en.wikipedia.org/wiki/Non-
inertial_reference_frame) quotes, "One might say that F = ma holds in any
coordinate system provided the term 'force' is redefined to include the so-
called 'reversed effective forces' or 'inertia forces'."

~~~
marcosscriven
Right, of course I understand that - but two points.

1) The whole original post appears to be worded for the layman to try and
dispel myths around what orbit is. I don't think the layman is going to think
about inertial frames of reference, rather I genuinely think the common
misconception that there's really something 'pulling' the astronauts up will
continue, much like many people think a car is throwing them out of a bend in
the road, rather than the vehicle pushing them away from a straight line.

2) In any case, even if Musk implies the frame of reference, there's a reason
a 'centrifugal force' is also referred to as a 'fictitious force' [1]

[1]:
[https://en.wikipedia.org/wiki/Fictitious_force](https://en.wikipedia.org/wiki/Fictitious_force)

------
mannykannot
"[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.

~~~
mikeash
Blue Origin is cool but it's certainly not astonishing to me that a
billionaire is able to send a rocket on a suborbital trajectory and recover
it. It's certainly more than I've ever done by a long shot, but it's not a
very interesting achievement unless you're interested in five-minute space
tourism. I don't think BO's achievement here has many implications for space
travel.

~~~
fixermark
I wonder its implications for terrestrial travel, though? We are in a post-
Concord era, after all, but suborbital flight could get you to any point on
the planet pretty quickly if the price is right.

~~~
thomashabets2
This may be interesting to you: [http://www.antipope.org/charlie/blog-
static/2015/01/why-were...](http://www.antipope.org/charlie/blog-
static/2015/01/why-were-not-going-to-see-sub-.html)

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

~~~
dcgoss
Elon has said himself that he is a fan of WaitbutWhy.com

~~~
sixQuarks
Waitbutwhy did a few in-depth articles about him and his companies after Elon
invited him over for interviews.

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

------
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.

~~~
rimantas
It ment going from 0-1000 to 0-2000, i.e. increasing target speed twice will
increase total energy neede four-fold.

------
IshKebab
> 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!

------
kibwen
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.

~~~
timdierks
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.

~~~
fixermark
Hypothetically, I imagine they could anchor a stable ocean platform out there
if they find it's easier than the barge. No idea if it actually would be
easier than the barge though.

------
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.

------
idlewords
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.

~~~
jholman
I see two aspects to your question/comment: at-rest, and two-objects.

The whole point of that comment was to apply only to at-rest things (he goes
on to contrast it with objects with velocity), though I think maybe you got
that.

If you have a marble and two other objects, the other objects make two
funnels, and there is a saddle (itself curved) where the two objects's funnels
are equipotent. That's the three-body case. If it were possible to balance
perfectly on this line of equipotence, you'd just slide to the lowest point on
that line, and stay there, out of the funnels. This, IIUC, is one of the
Lagrange points... L1, I think. In theory it's possibly-stable, but its
stability exhibits negative feedback, so in practice it's impossible (although
with station-keeping rockets, it's cheaper to hover there than most places).

As you add more funnels, you just get more of these saddle lines intersecting.
There are many infinitely-fussy places in the universe where you could in-
theory-but-not-in-practice hover without falling into a funnel (if it weren't
for brownian motion and maybe some other quantum effects that disturb your
infinitely-difficult equilibrium).

Of course, this whole analogy doesn't account for the fact that all of the
bodies are acting on each other, not just the funnels acting on the marble.

~~~
idlewords
Right, the fact that all the bodies are acting on the other is what makes me
feel the analogy is not kosher. The funnels are going to start moving around
based on their mutual attraction, and I could see the marble being ejected to
infinity, for example. It's less clear to me (but possible) that it could
enter a stable orbit.

------
v4n4d1s
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.

~~~
exDM69
Fwiw, on their public webcast, they had on-screen telemetry showing km/h for
speed and km for altitude (and no downrange distance). Not necessarily what
they use internally, though.

Aerospace engineers traditionally uses knots, thousands of feet and nautical
miles. SI units typically have km/s (not /h).

------
soperj
"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."

~~~
aparent
Attitude is correct. It refers to thusters controlling the orientation of the
rocket.

[https://en.m.wikipedia.org/wiki/Attitude_control](https://en.m.wikipedia.org/wiki/Attitude_control)

