
150 Meter SpaceX Starhopper Test [video] - themgt
https://www.youtube.com/watch?v=bYb3bfA6_sQ
======
mabbo
For some context for those who aren't SpaceX fanboys (card carrying member
here)...

Starship is a new two stage rocket SpaceX is building. It's going to have
similar launch capabilities to the Saturn V Moon rocket, but be 100% reusable.
The upper stage is planned to be able to take people to Mars and back.

The test today is a very early prototype. It's just the upper stage, just a
mock up of it (literally built by a company that makes steel water towers,
I've heard) and it had just one of their new "Raptor" Engines. The test was
basically "Can we hover 150m in the air and safely land?". It passed almost
perfectly. Why do this? Because the Raptor is a really complex and new design
and SpaceX need to test the hell out of it.

Next after this will be two higher quality prototypes of the upper stage,
probably with more engines, and probably to go high enough to test reentry.
And then eventually a test of Stage 1, which will be a massive reusable
booster (like Falcon 9 but many times larger).

The whole thing is designed for rapid reusability. No refurbishment between
launches and no new hardware needed (100% reusable) means very low variable
cost per launch.

~~~
woodandsteel
Besides being 100% reusable, the second stage will be able to be refueled in
orbit. That means it can take off to Mars with a full load of fuel, and carry
about a hundred times as much cargo there as a Saturn V sized rocket could.

~~~
mabbo
Yeah, in-orbit fueling will be a game changer. I'll bet good money that early
on, SpaceX runs a ride-share program to send landers/orbiters to Mars for
every space agency on the planet.

How cool would that be in 10+ different countries had rovers on the ground,
collecting different data, different experiments?

------
wcoenen
An RCS tank fell off during landing and went flying :-) (if not visible,
rewind a bit. It's only clear when the video is shown in the highest
resolution, which might not be the case during the initial buffering.)

[https://youtu.be/m0MTtqzzf-U?t=8518](https://youtu.be/m0MTtqzzf-U?t=8518)

~~~
colinthompson
You can see it a little bit better in the feed from Everyday Astronaut:
[https://youtu.be/m0MTtqzzf-U?t=8520](https://youtu.be/m0MTtqzzf-U?t=8520)

~~~
walrus01
caution, there's loud yelling in that video with volume scaled beyond a normal
level, RIP headphone users.

------
monk_e_boy
That was really amazing. I've been following it closely on YouTube but to see
it actually fly - mind boggling! Not quite clean room facilities. The nose
blew over in a storm. Locals streaming video from cameras mounted on telegraph
poles. When it landed various parts came shooting off spraying gas as they
tumbled away.

Full-flow engine - first to ever fly. And boy did it fly!

I love it. Back yard hacking hardware.

Summary - SpaceX are the first people to land rockets for reuse. Then they go
and do the same thing with a different engine design, different fuel, in a
water tower controlled with Falcon 9 reaction control system.

Quick question for the experts:

1 - how do they measure altitude? GPS is notoriously poor at this.

2 - any idea of how the control software was written for a flying water tower?
Could they use the Falcon 9 software?

~~~
leecb
In similar systems, the position is generally tracked with the combination of
interial sensors (accelerometer / gyro / angular rate sensors) and external
references like GPS and star trackers.

Inertial sensors are able to provide high frequency data on change in position
and orientation. Higher frequency means the software can make much more rapid
decisions to steer the rocket, on the order of 1000 a second or so, much more
often than can be achieved with GPS.

The fact that inertial sensors don't rely on external signals (like GPS does)
means that there is some degree of robustness- for instance, if there is a
temporary disruption in GPS signal reception, the rocket will still have some
idea of its position.

Over time, integration and measurement error accumulate from the inertial
sensors. (Remember that they generally measure changes in position /
orientation, not absolute position or orientation). For this reason, it is
usually necessary to use external position and orientation references to
correct the error that accumulates over time. GPS is used for this, and in
some applications, star trackers can be used as an absolute orientation
reference.

On the algorithm side, a Kalman filter combines measurements from all of the
position and orientation sensors to generate a prediction of the current
position / orientation / velocity / acceleration etc.

~~~
Symmetry
A minor addendum, you should feed your Kalman filter the gimbal and thrust of
the rocket too, it'll happily chow down and combine that with everything else
to further improve its estimate.

~~~
tlb
It's dangerous to throw extra sensors into a Kalman filter, because if the
sensor goes bad it'll corrupt the output.

Angular rate sensors are extremely accurate, far more accurate than a thrust
sensor could be, so it probably wouldn't improve the overall accuracy anyway.

~~~
phkahler
>> It's dangerous to throw extra sensors into a Kalman filter, because if the
sensor goes bad it'll corrupt the output.

Not necessarily. A Kalman filter uses a covariance matrix to account for noise
in the input signals. Another technique can be used to dynamically adjust that
matrix while it's running. If a signal doesn't agree with everything else
going on, it's noise level is effectively increased to the point that it
doesn't contribute any more.

I know someone who implemented a system like that and it could drop sensor
inputs and bring them back in real-time. I realized that all the traditional
sensor diagnostics others had used in similar systems might be obsolete with
something like that. It was a pure math-based system, no logic or thresholds
for sensor diagnostics and it just worked (TM).

~~~
tlb
I've tried to make adaptive Kalman filters work, and it's not easy. For
something like a rocket, the covariance of all the sensors is very low while
it's sitting on the launch pad. As soon as you light the engines, the
covariances jump up. So an adaptive filter will try to adjust its weights
during the first few seconds of flight, which is a terrible time to change
everything.

Usually it's better to lock down all the filter coefficients.

Also, an adaptive Kalman filter measures noise on each input to estimate
error, usually by looking at the autocovariance. This turns out to be
maximally bad when a sensor fails and emits a constant value, because the
autocovariance becomes zero and the adaptive filter will decide it's the most
accurate of all the inputs and weight it heavily.

Before a Kalman filter, you need redundant sensors and logic to discard ones
that disagree with the majority.

Angular rate sensors and accelerometers are cheap and light, so I'm sure they
have at least 3 sets. Maybe 5.

~~~
phkahler
A friend of mine used an adaptive Kalman filter to estimate vehicle speed in a
car using the 4 wheel speed signals, and steering angle IIRC. They tested on
special surfaces (alternating ice patches or something) that caused wheels to
lock or slip. He said it was able to track actual vehicle speeds very well
down to a single valid wheel speed. I think the key is not to just measure the
noise of the signal itself, but its deviation from "everything else". Probably
simpler in a car than on a rocket?

Anyway, these things are possible to varying degrees. Nor did I say it was
easy - I've only implemented one, never designed one ;-)

------
jiggawatts
Congratulations to the SpaceX team for once again spectacularly succeeding at
actual rocket science!

------
cryptoz
This is the best prototyping I've ever seen. Small, 1 of 31 engines in the
final config, water-tower contract, built in the open fields in south Texas,
the whole thing.

The end game of this line of rockets is hundreds of Mars trips launching
daily, carrying people and supplies to the Mars colony.

~~~
thecopy
> hundreds of Mars trips launching daily

Not realistic. The minimum-energy launch windows for a Martian expedition
occur at intervals of approximately two years and two months. Worst-case is
four times the distance.

------
throwaway808080
This was just a single raptor engine. I was quite amazed at how stable it’s
thrust control was. Truly remarkable.

SpaceX definitely keeps on making me think I’m doing jack shit with my life.

Here are people prototyping and pushing frontiers in rocket technology, and
all I do day in and out is pump out CRUD UIs.

------
xiphias2
What is special/hard to make for Starhopper compared to Falcon 9?

Hm..I think I found it on the net: The Raptor engines are the same size but
are about twice as powerful as the existing Merlin engines.

~~~
cryptoz
It's also worth mentioning that the Raptor engine is meant to use fuel made in
ISRU systems on Mars. Most (nearly all?) components of Methane fuel for the
Raptor can be extracted from the Martian soil, rocks, atmosphere, etc. A
primary design consideration for Raptor was that it should enable Mars
missions, and Mars missions are nearly impossible to do without manufacturing
your fuel on location, so Raptor is ISRU-first with a Methane fuel.

Everything SpaceX does is moving towards a goal of enabling a colony of 1M+
people on Mars as soon as possible. A lot of people are confused by SpaceX
decisions, but if you view it through the lens of Mars colonization, then it
all usually makes more sense.

I think Raptor might be the first Methane-based rocket engine as well.

~~~
sebazzz
How can methane be created on Mars? It is a gas right, and it isn't that cold
there?

~~~
MPSimmons
Methane is CH4. Carbon from Carbon Dioxide (CO2) and Hydrogen from Water
(H2O).

Stoichiometrically, CO2 + (2)H2O -> CH4 + (2)O2

The process is performed via the Sabatier Reaction, which someone else linked
to.

------
anorphirith
That was glorious. I love the chrome

~~~
kristianp
Glorious indeed. The shape reminds me of Boba-fett's ship from starwars.

Notice the flame turning red before landing? Did the mix become rich, or the
flame cooler?

~~~
londons_explore
Presumably they need to run at a very low throttle to land. My guess is the
engine doesn't run well at such a low throttle, and in fact they might have to
deliberately make the mix very rich to keep the combustion stable.

~~~
hansthehorse
See
[https://www.youtube.com/watch?v=T29ybqjv8-U](https://www.youtube.com/watch?v=T29ybqjv8-U)
Manley goes over the exhaust color changes at the end.

------
abakus
One step closer to the mars.

~~~
Florin_Andrei
> _the mars_

The one and only.

------
B_Throwaway
For a moment I thought it was Boca Chica, Dominican Republic, and was a bit
confused.

------
irjustin
Love itttttt!!! Congrats to the team and am the hard work!!

