

SpaceX's Grasshopper Successfully Flew 325 Metres - cbarnsdale
https://www.youtube.com/watch?v=eGimzB5QM1M

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startupfounder
Elon and the SpaceX team are kicking ass!

2 months ago they hovered at 250m:
[http://www.youtube.com/watch?v=xUvbh-Z8Abk](http://www.youtube.com/watch?v=xUvbh-Z8Abk)

6 months ago they hovered at the height of a 12-story (40m) building:
[http://www.youtube.com/watch?v=Zz-NYeH-
CEY](http://www.youtube.com/watch?v=Zz-NYeH-CEY)

These guys are on a very fast learning curve. These near-ground tests are the
hardest.

Elon said that they will be using a fully reusable first stage in 2015, only
18 months away: [http://allthingsd.com/20130530/tesla-ceo-and-spacex-
founder-...](http://allthingsd.com/20130530/tesla-ceo-and-spacex-founder-elon-
musk-the-full-d11-interview-video/)

[[http://en.wikipedia.org/wiki/SpaceX_reusable_rocket_launchin...](http://en.wikipedia.org/wiki/SpaceX_reusable_rocket_launching_system#Falcon_9_booster_post-
mission.2C_controlled-descent_tests)]

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bpicolo
Not sure I would call them the hardest. Stabilizing while traveling thousands
of miles per hour in atmosphere to get to the point you can land like this
seems much harder.

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grinich
ICBMs already do this, and we've been precisely deorbiting satellites for
decades. Landing safely is the really tricky part.

During the Corona spy satellite days, they used to catch the parachuting pods
with a plane+hook.
[http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_c...](http://upload.wikimedia.org/wikipedia/commons/7/76/Keyhole_capsule_recovery.jpg)

~~~
InclinedPlane
ICBM warheads have about a 90% chance of landing within a circle with a radius
of about half a kilometer, at least according to declassified info, the actual
accuracy is likely higher. Even so, that hardly compares to a precision
landing on a small landing pad.

~~~
mikeash
I think your info on ICBM accuracy may be a bit out of date. Some searching
around shows modern American missiles with a CEP of 100 meters or better. I'm
not quite sure how to translate a CEP (50% chance of landing in the circle) to
a 90% figure, but I wouldn't think it would be a factor of 5.

(I specified American because the Soviets and Russians have historically
lagged behind here, and it looks like that's still the case. I don't know what
the story is for other ICBM-equipped powers. The Soviets compensated for their
lesser accuracy by fielding larger bombs.)

Also keep in mind that the major factor limiting ICBM accuracy these days is
the need to be completely self contained, since they're intended to operate in
the middle of World War III, and no external navigation sources can be counted
on. Slap a JDAM guidance system on the warhead and your ICBM can deliver it to
within whatever square meter you're most interested in.

ICBMs armed with conventional bombs have been considered, since accuracy can
be enough for that to work now. As far as I know, the only reasons these
haven't been deployed are cost (using up an entire ICBM to deliver a chemical
bomb seems wasteful) and, probably more importantly, the fact that there's no
way to tell whether a given ICBM is carrying a nuclear warhead or not, thus
the strong possibility that using such a weapon could trigger a nuclear war by
accident. See:

[http://en.wikipedia.org/wiki/Prompt_Global_Strike](http://en.wikipedia.org/wiki/Prompt_Global_Strike)

~~~
InclinedPlane
The Minuteman III (the backbone of the US ICBM fleet) has a listed CEP of 200
m. The Trident D5 has a CEP of as low as 90 m, but it's an SLBM not strictly
an ICBM, and it varies depending on range.

The CEP figure represents the radius of a circle where there is a 50% chance
of the bomb landing within that circle. The CEP figure is basically 2/3 of a
standard deviation.

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mikeash
I suppose I should have said "some". Anyway, when looking for the state of the
art, we should look at the best value available, rather than the average or
typical. The Peacekeeper missile lists a CEP of 120m, although it's now
retired. As for Trident, with a 11,000km range, I think we can put it in the
same category when looking at reentry accuracy.

In any case, accuracy is gated by navigational accuracy, not so much guidance,
as long as your initial aim isn't too bad. ICBMs have a 100m CEP because
they're using inertial navigation all the way. Switch to GPS and you'll have
no trouble doing better. In the context of SpaceX, getting the rocket to
within a few hundred meters of the landing pad is perfectly fine initially,
because it can then maneuver to the pad more precisely.

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leeoniya
Armadillo did this, and cooler already :)

[https://www.youtube.com/watch?v=9u0qlIoSSkQ](https://www.youtube.com/watch?v=9u0qlIoSSkQ)

[http://www.youtube.com/watch?v=k_Xiq3dYJlM#ws](http://www.youtube.com/watch?v=k_Xiq3dYJlM#ws)

~~~
trafficlight
While it's awesome that they've done this, the scale isn't exactly the same.
The Super Mod is about 12 feet tall. Grasshopper is 106 feet tall and has
considerably more power.

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xtc
Honestly I would like to see some spectacular failures. Just as Gwynne stated
recently, if SpaceX doesn't generate a few craters with this program during
testing it means they aren't trying as hard as they can. I'm glad Grasshopper
will have its flight ceiling lifted soon with the new testing facility.

Not to say a 100% flight record wouldn't be fantastic, but I'd really enjoy
seeing the trend of doubling height to continue, or even greater.

~~~
mjn
They still have about 10x to go to hit the VTOL rocket record of 3140 meters
[1], which they seem to be aiming for, so I assume some more doubling-or-
greater is in the works.

[1] [http://www.hq.nasa.gov/pao/History/x-33/dc-
xa.htm](http://www.hq.nasa.gov/pao/History/x-33/dc-xa.htm)

~~~
xtc
I can't wait.

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spiek
Could anyone explain why this is a good way to fly a spacecraft/what they are
aiming for with this? It seems somewhat inelegant, considering the challenges
having to do with stabilizing a vertical rocket.

~~~
mikeash
Well, how else do you recover a rocket intact?

Parachutes have lots of trouble. The rate of descent they achieve is still
fairly fast, so you need something to brake the descent at the end.

This is why various American capsules splashed into the ocean, as well as the
Shuttle boosters, because the water acts as a cushion. However, dunking your
delicate rocket in salt water is a bad idea. The American capsules weren't
reused, and the Shuttle boosters required enough refurbishment afterwards as
to make it not really worth recovering them.

You can land on land, but then you need some seriously hefty landing gear, or
retro-rockets. The Soviets use retro-rockets fired at the last moment to
recover their capsules on land. A rocket has problems that a capsule does not,
though, namely that you need to keep it from tipping over once it lands! You
can make a capsule fat, but a rocket really needs to be tall and thin, and any
sort of horizontal velocity will have it falling on its side. You also have
very little control over _where_ the rocket comes to a rest. Again, tolerable
for a capsule, but dropping your rocket into a forest is not going to be
conducive to reusability.

You can add wings, like the Shuttle did, and land horizontally. Trouble with
that is that wings are _extremely_ heavy. Each pound of wing in the Shuttle
was a pound of payload it couldn't carry. If you try to fit every stage of a
rocket with wings so it can come back and land, you'll end up not having much
excess capacity left over to actually carry a payload. Wings are great for
aircraft, when you can use them to efficiently fight gravity for the whole
flight. But they suck for spacecraft, where they don't help at all on the way
up, and are just dead weight for most of the flight.

So landing a rocket vertically on its engines isn't really all that bad in the
end. Inelegant, perhaps, but it has a lot of advantages. You get absolute
control over where the rocket lands. You can land it as softly as you like.
You can ensure that it touches down with no horizontal velocity so as not to
tip over. You use the same engines that you used for the launch, so there's no
extra weight there. The only real downsides are the need for some sort of
landing gear, and the need to carry a bit of extra fuel. Neither one is all
_that_ bad.

When considering the stability, don't make the common mistake of comparing it
to balancing a pencil. When the rocket turns, the motors turn as well, so it
is _not_ inherently unstable the way you might think. Rather, the stability is
neutral: if you perturb the rocket, it will just stay in its new position, not
returning to how it was before, but _neither_ does it run farther away. A
rocket that's slightly tilted will stay slightly tilted (gradually
accelerating sideways, of course), it won't tilt over further the way a pencil
does.

Note that Robert Goddard made this same error. He built his first rocket with
the engines at the top, believing this would help with stability. So if you
were thinking that way, don't feel bad: the father of modern rocketry did the
same thing!

~~~
karpathy
This is great write up, thanks for the explanation.

I've studied physics in undergrad but I still made the mistake you describe in
your last paragraph. The Grasshopper intuitively feels dynamically like an
inverted pendulum but even harder because all you have are rockets at the
bottom and you can adjust their rotation (I assume?).

On the other hand, there is no fixed pivot point around which you get torque,
is this what the difference comes down to?

Do you happen to have any pointers to some articles that discuss this, and how
difficult it is to balance? How does the wind affect things? What are the
biggest challenges?

~~~
sehugg
Here's a good article on Lunar Module stability, which is a similar problem
with the exception that the Grasshopper has a much longer longitudinal axis,
and relies solely on gimbaling (thrust vectoring) with no separate attitude
control thrusters.

[http://www.clavius.org/techlmstab.html](http://www.clavius.org/techlmstab.html)

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GravityWell
I was glad to notice the lack of Johnny Cash drowning out the rocket sounds. I
have nothing against Cash, but these rocket landings are impressive as they
are, without treating them like an MTV video.

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bnchrch
I am really routing for SpaceX, I would love to see the commercial
possibilities (and scientific) if we could reduce the cost of getting to
space. Luckily it looks like they're already making some pretty impressive
steps if you consider that only 6 months ago they were hovering at 40m.

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marcosscriven
Very cool. I'm surprised that the turbulence/vibrations coming back from the
pad on landing don't damage it.

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jloughry
DUCK DODGERS: Whoops! Had the silly thing in reverse.

