150 Meter SpaceX Starhopper Test [video] | Hacker News Reader

150 Meter SpaceX Starhopper Test [video]

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217 pointsthemgt69 comments

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.

Calling Raptor "really complex" still manages to undersell it a bit, in my opinion. It's pushing the state of the art in rocket engine design pretty significantly. It's the first full flow closed cycle engine to fly.

For folks who are interested in learning more about rocket engines, Scott Manley is a great resource: https://youtu.be/4QXZ2RzN_Oo

It is also worth noting that the first two prototypes (they are building 2 in parallel on different sites in Texas and Florida) are already quite progressed and planned to do their first testing soon (according to Elon possibly in September even, but that is likely to be optimistic).

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.

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?

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

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

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

That's what I linked already. Your link is the same, just two seconds later.

Link broke, other one with slowmo: https://youtu.be/lsoS6C0uzGY?t=659

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?

I'm no rocket expert, but I have some ideas about altitude.

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

GPS is indeed notoriously terrible. They might have better luck with it though, given they can buy high-quality receivers and always have a good view of the sky.

However I think I read (or hope at least) that they use barometers. Even cheap smartphones have barometers for relative altitudes these days, and while the barometer data is influenced by the atmosphere and weather of the day (the focus of like all my startups [1]), the rocket could easily know about the differences in the weather and create MSLP (mean sea level pressure) numbers that would allow for excellent precision at landing.

Your phone barometer can tell the difference in altitude between your head and your feet.

[1] My latest go at this is https://www.allclearweather.com (US, Android) with a goal of eliminating the altitude part of barometer data so that the remaining data can be QC'd, summarized with statistics and eventually hopefully included in weather models for increased forecast accuracy

I think they do also have a radar for distance to pad information but I couldn't cite that. I believe they have a form of instrument landing system, similar to how it is done at major airports but oriented for the rocket of course, and the distance calculating "radar" could be part of that (using signal timing) but I am not sure exactly how they did it.

Yes I think that radar idea is quite correct. I now recall Elon/SpaceX explaining that this rocket (BFR or whatever) should be able to land on any solid body in the solar system. This would eliminate barometers and GPS for long-term solutions, but keeping radar/lidar and other light-based imagery in the running.

Falcon 9 is known to have a radar altimeter; it's quite likely the Starhopper does too.

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

I would imagine they would use a number of different sensors, they probably have barometric altimeters, but probably rely on some kind of radar altimeter for low altitude maneuvers.

Oh yeah - radar! Duh, I was thinking lazers or something.

Modern GNSS can reliably measure altitude at <50cm 2-sigma accuracy using techniques such as RTK and PPP-AR. Modern GNSS is a better source of altitude data than barometers.

Likely SpaceX is using a combination of radar altimeters and GNSS.

Source: I do engineering on modern GNSS

Super interesting info. Sorry for my annoying questions. Does modern GNSS receive that level of accuracy in cities among buildings, or is that an ideal case like what SpaceX has, being outside?

Also, how expensive is a modern GNSS receiver that can get that level of altitude accuracy? Is this something that could make it into phones some day?

To add to the parent comment, super high precision gnss/rtk is often paired with multiple inertial measurement units, not very dissimilar from what you find in a smartphone. Also used in COTS drone flight controllers. Software can combine data from an imu that some vertical acceleration had taken place, with data from gnss sensors that altitude is now 2m higher, for additional level of confidence in what the gnss sensors are reporting.

Not an annoying question whatsoever.

Dense urban environments are challenging. The error distribution is more complex there, its hard to definitively say. The numbers I'm quoting is for open sky environments where you have plenty of satellites and little multipath.

For the average end-user, these are $500 GPS units (this is the one I work on: https://www.swiftnav.com/piksi-multi). The technology is already moving down the supply chain to ~$10 GNSS units from Broadcom and ST (we're doing this: https://www.swiftnav.com/news/swift-%E2%80%8B%E2%80%8Bnaviga...)

The issue with phones is mainly an antenna issue. Your phone antenna is tiny and squeezed in alongside half a dozen other antennas and radios. This degrades the GNSS signal quality a lot. A stopgap here is external GNSS antennas for your phone: you might plug your phone into your car and get lane-accuracy turn-by-turn directions, but it's unclear when we'll have high accuracy GNSS inside phones, but most major phonemakers are indeed working on this.

... $10?! Really? Can we hook one up to a Raspberry Pi?

This would be a game changer for my sport (kitesurfing)

Depending on the update rates and accuracy you need, there are many commercially available GPS breakout boards and small cheap modules under about $20

I've used [1] which can be found for about $18 before on some toy projects.

[1] https://www.u-blox.com/en/product/neo-6-series

GPS doesn’t do height accurately :( 10m error

I can’t answer all the questions but I can talk about RTK techniques. I recently used two $75 GPS receivers and two raspberry pis to create an RTK GPS system (real time kinematics). With RTK you have one receiver that is fixed - it does not move. The other receiver is on the thing whose position you want to measure, and the two devices are connected through the network. Because the system knows the fixed station is not moving, any drift in measurement represents the real time error of the current satellite constellation. The moving unit subtracts that error from its own measurements to get very precise figures for its position. In the system I built, it seems to have centimeter level repeatability. That is, if I am looking at the read out in real time and I stop moving the receiver, the drift in the measurement is a couple of centimeters. If I move it 20 centimeters that is pretty closely reflected in the read outs. Currently I don’t know the absolute accuracy of my system but the relative accuracy is very high!

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.

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.

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.

Ah, I guess I'm used to wheeled robots where odometers are often your most accurate gauges of position.

>> 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).

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.

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 ;-)

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

https://www.semanticscholar.org/paper/Lossless-Convexificati...

Reddit blurb about that paper: https://www.reddit.com/r/spacex/comments/7t2tb2/a_paper_by_l...

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

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.

> 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.

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.

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.

The Raptor engine is the first full-flow staged combustion cycle engine to every fly.

https://en.wikipedia.org/wiki/Staged_combustion_cycle#Full-f...

The word you have looked for is ever, not every.

It's clearly an unintentional typo, and also not possible to confuse for something else.

A few things from a non-expert:

-Raptor is operating at previously unheard of pressure, and uses advanced new materials and heavily utilized GPU-powered flow simulations during design.

-it's super efficient. It's the first operational full flow staged combustion engine AFAIK. High thrust + High efficiency == a good time.

-it uses METHALOX fuel, which can be made on various celestial bodies somewhat easily (importantly: mars). Water + solar energy + CO2 == methane-oxygen fuel

-uses more difficult spark ignition w/unlimited restarts instead of more reliable but "cartridge" based TEA-TEB (sp?) igniters that rely on mixing volatile complex to manufacture toxic chemicals to start.

The test hopper has 1 engine. The orbital prototype will have 3. The production booster will have 31 of these bad boys and put out 2.0-2.7x the thrust of a saturn V

I’m slightly worried that at some point they will hit an integer overflow and crash the simulation.

The Raptor engine isn't just "more powerful" but uses an advanced "full-flow staged-combustion" cycle which has never flown before.

The Merlin is highly optimized but it uses a simpler "gas generator" cycle which is less efficient. The Falcon 9 and Merlin is famously optimized for "low-cost" instead of "high-tech" but with Raptor they are now advancing the state of the art in propulsion tech.

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.

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

If I remember correctly, John Carmack's Armadillo Aerospace tried burning methane in an engine. Though I don't think they were the only team - or even the first one.

Wikipedia lists a few methane/LOX engines that have been used in test flights. For example, NASA's experimental Morpheus lander had fifteen or so test flights -- all low altitude, but some higher and longer lasting than StarHopper's hop today. It's also being developed simultaneously with Blue Origin's BE-4 engine, which hasn't flown yet, but has been through an extensive ground test program, and will be used on both Blue's own New Glenn orbital rocket and on ULA's Vulcan. So depending on how quickly the relevant test programs go, Raptor may or may not wind up being the first to be part of an orbital vehicle. (But as neither Vulcan nor New Glenn is currently slated to use it for the second stage, Raptor will very likely be the first to go all the way to orbit.)

Yeah, Raptor is the first full-flow staged-combustion engine to fly, but not the first methalox.

I should point out that John Carmack's Armadillo Aerospace are the ones who actually built Morpheus.

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

https://en.wikipedia.org/wiki/Sabatier_reaction

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.

It's a gas at STP here on Earth. It's a liquid when it's being used as a rocket fuel.

The end game for Starhopper is the Super Heavy/Starship combo, which (if it all works out as planned) will be able to lift around 100 tons to low Earth orbit and be fully reusable with no refurbishment needed between flights.

https://everydayastronaut.com/raptor-engine/ is a great overview of why Raptor is such an interesting development.

That was glorious. I love the chrome

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?

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.

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

Looks like a flying grain bin. On the other hand, it has very nice thrust vector control!

iirc it was built by a company that makes water towers so not too far off.

One step closer to the mars.

> the mars

The one and only.

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

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

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