

SpaceX’s Autonomous ‘Grasshopper’ Rocket Makes Milestone Flight - sk2code
http://www.wired.com/autopia/2013/03/second-spacex-launch-hovers/

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ChuckMcM
Nicely done, nicely done. Reminds me a bit of the early DC-X flights, this
link: <http://www.youtube.com/watch?v=wv9n9Casp1o> is DC-X flight #8 which I
expect GrassHopper to do at some point as well (this was 1995 btw, imagine the
computers they had on DC-X vs what they have on Grasshopper!)

Blue Origin is a bit ahead here in terms of take off and landing. But it has
the challenge of not benefiting from a paid launch contract like SpaceX
currently has.

What I find particularly interesting is that either the Dragon or the Blue
Origin craft already have the delta-V to land an return to orbit from the
moon, if they can get there. The thing that will break that wide open is on-
orbit refueling. The game changes in a particularly compelling way when you
can launch "gas cans" to orbit and refuel existing craft that are already in
orbit. Not only does that extend the life of satellites but it enables Multi-
launch configurations. United Launch Associates gave a pretty compelling talk
about how they would meet some of those challenges in their long duration
vehicle paper:
[http://www.ulalaunch.com/site/docs/publications/Integrated%2...](http://www.ulalaunch.com/site/docs/publications/Integrated%20Vehicle%20Propulsion%20and%20Power%20System%20for%20Long%20Duration%20Cyrogenic%20Spaceflight%202011.pdf)

Exciting times, 20 years late but still.

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jlgreco
Would on-orbit refueling work with cryo? I imagine the fuel/oxidizer would
boil away too fast for that to work well, unless you sent everything up at
once perhaps.

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ChuckMcM
The ULA guys are talking about keeping cryo fuels in depots. They have some
thoughts about hydrogen boil off (which they use in their internal combustion
engine (H2 + O2) to both do station keeping and temperature management.

I have not seen proposals for on-orbit cryo fuel transfer. But I'm sure they
are out there.

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ricw
Have they announced a timeline for actually implementing the grasshopper tech
for real launch purposes?! When that happens, spacex will have won the race to
space. No other company would be able to match their launch price, given how
high the cost of "disposable" rockets these days..

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dasmoth
Not a timeline. However, there have been some reports recently that the first
Falcon 9 v1.1 flight (currently scheduled for this June) will attempt to flip
the first stage around after separation, relight one of its engines, and use
it to slow down before landing on the ocean. IF this can be demonstrated
successfully, it links up with the Grasshopper work very nicely. So, a more
aggressive schedule than I would have expected just a few days ago.

Source:
[http://forum.nasaspaceflight.com/index.php?topic=30385.msg10...](http://forum.nasaspaceflight.com/index.php?topic=30385.msg1023706#msg1023706)
(and generally a good place to look for SpaceX updates...)

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JulianMorrison
Given that they already have thrust vectoring, isn't "grasshopper" basically
just the software? IOW, this is a full "grasshopper" attempt for the first
stage, and the only difference from ideal is that it will land on water.

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dasmoth
Legs (strong enough to land on, light enough not to zero out your payload) are
a non-trivial issue.

There's a Grasshopper 2 plan in the works (using a v1.1 tank) which will
supposedly have rather more flight-like legs than the current one.

But I'm sure that a lot of what they've learned from Grasshopper will be going
into the June test.

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Gravityloss
Relighting / staging while the first stage motor is running at "idle" or
something like that will be interesting.

For a relight, you need some propellant at the feed pipes to restart, though
if you have small enough aero forces, you could just vent some stuff to
accelerate the rocket forward slightly so you can start ingesting liquid
again, instead of the pressurant gas and can then proceed with relight.

Then again, since the empty rocket's bottom end is heavy, it could orient
itself the right way aerodynamically anyway (acceleration vector pointing the
way of the nose) so the liquids go towards the engine end.

If it comes like a dart and not tumbling (which presents its own problems),
such a big stage has lots of mass per frontal area and it will come in quite
fast.

Interesting aerodynamic problems. I wonder if we might see some maneuverable
mini fins or wings at some point.

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Shivetya
Is it worthwhile to get more back than just the motor assembly? As in, is the
tank structure really worth saving?

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maeon3
The goal is to have a vehicle that can land on another planet and return to
the surface of Earth in working condition with the only requirement as add
fuel.

One hindering factor is the problem of having enough fuel to make the return
trip.

There needs to be a better technology to harvest the massive amounts of energy
wasted when decelerating from 30k mph to 1 thousand mph during re-entry.
Convert the heat to a usable fuel. So we store the fuel needed for re-entry in
the velocity of the craft.

A heat to fuel converter. If we could be 100% efficient at this, we would land
with just as much fuel as we left to get into orbit.

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homeomorphic
> There needs to be a better technology to harvest the massive amounts of
> energy wasted when decelerating from 30k mph to 1 thousand mph during re-
> entry.

Or, in the case of of Mars, we can focus on in-situ resource utilization [1].
Zubrin's Mars Direct [2] is built around this, and there are some immense fuel
savings to be had [3].

[1] <https://en.wikipedia.org/wiki/In-situ_resource_utilization>

[2] <https://en.wikipedia.org/wiki/Mars_Direct>

[3] Zubrin, Robert. _The case for Mars._ Free Press, 1996.

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purplelobster
I didn't see the mannequin at first, but that gives a good sense of scale.
Does it keep balanced with some sort of thrusters? I can't see any.

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flyt
The center engine is on a gimbal.

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jacques_chester
Reminds me of the Armadillo Aerospace designs -- relying on fast dynamic
control systems instead of more "stable" rocket configurations.

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otibom
Can someone explain how they control for roll ?

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dasmoth
At least some Merlin engines have vectorable turbopump exhaust. That's used
for roll control on Falcon 9 upper stages. Could imagine it working here, too.

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andrewflnr
Does anyone know what the little vent/flare thing is on the side of the
engine?

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lavezza
It is the exhaust of the gas generator. You can see a good view of it here:
<http://www.spacex.com/press.php?page=37>

You can read about the Gas-Generator Cycle for rockets here:
[http://en.wikipedia.org/wiki/Gas-
generator_cycle_%28rocket%2...](http://en.wikipedia.org/wiki/Gas-
generator_cycle_%28rocket%29)

Basically, a small amount of fuel and oxidizer are bled off into a gas
generator and used to power the turbines that feed the fuel and oxidizer into
the rocket engine.

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EEGuy
Not to be missed is the fuel path of this (and other) liquid fuel rocket
engine designs:

The cryogenic fuel is piped about the "outer surface" of the rocket's nozzle
and combustion chamber such that these essential structures doesn't melt away
under the pressure and heat.

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Gravityloss
Actually, they use the kerosene for cooling, not the oxygen.

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EEGuy
Thank you Gravityloss for your correction.

I do find that a brave integration -- using fuel as a coolant.

Not an engineer in this field, but I'd imagine using fuel flow as coolant
makes for a lighter-weight engine overall, even as it might complicate control
system software, and might place narrower limits on, or complicate stability
control of the engine's net available (throttable) range of power.

Just speculating here, but perhaps a single engine's power range limitation
becomes another reason (along with graceful system degradation, without
mission loss, under single engine loss) for SpaceX's multiple engine designs
for their larger rockets? Switch off additional engines as rocket weight
decreases (due to fuel and oxidizer usage) during descent? Doubtless there is
a great difference in total mass between take-off weight and landing weight,
so it would seem to require a lot less fuel & oxidizer to land it than to lift
it to orbit.

Also wondering about ablative heat-shield placement and arrangement for re-
entry, first and second stage.

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Gravityloss
I don't understand all your questions but here's some.

The fuel is used as coolant since it's a better coolant than the oxidizer for
a range of reasons. One of them is that hot oxygen is very corrosive. Also,
because of that a small oxygen leak from the coolant passage to the chamber
tends to grow larger with catastrophic consequences. With kerosene the problem
is coking.

Yes it's more complicated to design a regenerative cooled engine, but existing
materials can't take the heat. Some maneuvering thrusters are heat sink
designs. Some engines are ablative, with things like evaporating carbon taking
the energy.

Yes it only needs one engine for landing vs nine for fully laden takeoff.

