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Comment by roshdodd | Hacker News Reader

As someone who actively works in the field, it was a culmination of:

- Advances in rocket engine design & tech to enable deep throttling

- Control algorithms for propulsive landing maturing (Google "Lars Blackmore", "GFOLD", "Mars Landing", and work through the references)

- Forward thinking and risk-taking by SpaceX to further develop tech demonstrated by earlier efforts (DC-X, Mars Landing, etc.)

Modern simulation and sensor capabilities helped, but were not the major enabling factors.

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>Forward thinking and risk-taking by SpaceX to further develop tech demonstrated by earlier efforts (DC-X, Mars Landing, etc.)

Is this basically a technical way of saying "people realized it could be done"? Like the 4 minute mile, once it was done once, many people accomplished the same feat soon after. The realization that it was possible changed people's perception.

Investors. Investors finally realised that it could be done.

I'm sure engineers and science-fiction writers have known for a long time that it could be done.

Could also be that SpaceX cracked it then other companies began poaching the engineers and the other companies started getting tips on how to address the hardest problems.

Even "Musk realized it could be done". He had a few good ideas. Or pushed for a few good ideas until they were put in practice.

Unfortunately then he stopped taking his dried frog pills and look where he is now...

Did Musk realize it could be done, or did he pay some engineers to realise it could be done?

When companies fail, is it the result of leadership or employees? Companies need great employees and leaders to succeed and without both, they typically fail. Hiring excellent people then motivating them all in the same direction is very difficult. Being such a leader and having access to capital and being willing to risk bankruptcy in very high risk endeavours like this is exceedingly rare.

Musk from the beginning realize to reduce the price, he would have to solve re-usabilitly. So he and the company from the beginning was focused on re-usability. He didn't just say 'this is what we are gone do'. As the company grew and solved initial problems of launch, they slowly figured out the best way to do it. And Musk as CEO was deeply involved from the beginning and and involved in all decisions.

... pay and convince people to give him money to pay ...

I still think he had some merit in the past.

Can you elaborate on the advances in deep throttling?

Not in industry, but: rockets can be like 90% fuel by weight. All engines on 105% can lift the rocket, so if you want to land while the tanks are nearly empty you need to be able to get to less than 1/10th of your thrust. Turning off engines only gets you so far- the Space Shuttle engine could throttle between 67% and 109% of rated power but if you only have 1/3 engines on you can only get as low as 22% power.

One major reason for this is the mixing plate at the top of the combustor. Fuel and oxygen are distributed to tiny nozzles which mix together. The better the mixing, the more stable the burn. If you get unstable burning -eg momentarily better mixing in one area- it will cause a pressure disturbance which will further alter the burning power in different areas of the combustion chamber. At low throttle, this can be enough to cause the engine to turn off entirely.

Fluid simulations have made a huge difference. It's now possible to throttle engines down to 5% because mixing is much more stable (manufacturing improvements in the nozzles have also helped) and combustion is more protected from pressure variations.

The extra stability also just makes it easier to control a rocket period. Less thrust variation to confuse with drag properties, less bouncing, better sensor data.

So I’m assuming the simulations lead to better controls software and/or mechanical nozzle designs? Similar to how CFD leads to more efficient vehicle aerodynamics?

I guess I’m trying to connect the dots on how a simulation improves the actual vehicle dynamics.

There is some improvement in vehicle control, but the biggest impact was inside the engine. Controlling the vehicle at transonic speeds benefits a lot from simulation- control inversion is an example. When grid find pass the sound barrier, the flow through the holes of the grid becomes choked off by shockwaves, and the fin starts acting like it was solid and sideways. Since it's effectively pointed 90 degrees off, it acts like its reversed. Knowing when, how intensely, and how turning affects that is important. Simulation also helps you find unexpected places where flows may unexpectedly become super/subsonic and cause torque. Experimenting at these speeds is... hard.

Simulation inside the engine can find resonances, show where shockwaves propagate, and show you how to build injectors (pressure, spray etc) so they are less affected by the path of reflections. Optimizing things like that smoothly along a range of velocities and pressures without a computer is not very feasible, and you need a minimum of computing power before you start converging to accurate results. The unpredictability of turbulence means low-resolution simulations will behave very differently.

the poster above was very conservative in his metrics and throtteling requirements.

Modern pressure vessels can reach 5% empty mass, thats a factor of 20

Rockets have stages, a good approximate is to stage half your rocket to get rid of the most empty mass. This also means your first stage has to have double the thrust to lift itself and its stage. Now you're at a factor of 40 just to hover.

Now you actually have to take off, usually around 1.2 to 1.4 thrust to weight.

So a more realistic scenario means your rocket engine has to throttle down to exactly 2% power while the laval nozzle is optimised for takeoff thrust only.

Rocket engines struggle to throttle down to low levels due to combustion instability, injector dynamics, and turbopump limitations. Here are some stats on minimum throttle levels:

SpaceX Merlin 1D: ~40% Rocketdyne F-1 (Saturn V): ~70% Space Shuttle Main Engine (RS-25): ~67% Blue Origin BE-4: ~20–25%

Falcon 9 does the "hover slam" where they have to turn off the engine exactly at touch down, or the rocket starts to go back up again. Throttle is too high for the weight of the booster at that point in flight.

Also didnt spacex do reuse without throttling and only having on/off?

They do throttle, and quite low compared to other comparable engines, but they still cannot throttle an engine below 1 TWR when the stage is near empty. Meaning that they cannot hover a stage, either the engine is on and the stage is accelerating upwards, or it's off and it's accelerating downwards. (And you cannot rapidly turn engines on and off.)

So they need to "hoverslam", that is, arrive at the landing pad rapidly decelerating so that their altitude hits zero just as their speed hits zero. This was thought to be very hard, but I don't think SpaceX has lost a stage due to estimation failure there. It helps that there is significant throttle range and fairly rapid throttle response on the engines, so they can have some slack. (Plan to decelerate at 2.5g for the last ~20s or so, with the ability to do anything between ~1.5g to 4g, so you can adjust throttle based on measured landing speed.)

Their Superheavy has more engines, allowing them to bring the TWR below 1, enabling hovering.

No. SpaceX's Merlin engines use a single https://en.wikipedia.org/wiki/Pintle_injector which has excellent throttling capabilities.

I still don’t understand how its cheaper than disposable. You have groups like Hamas who can make rockets. V2 went to space on 100 year old tech. Seems like a dummy old rocket should be quite cheap compared to one with all these control systems and the need to store descent fuel.

Maybe disposable rocket designs lost the hat and got too overengineered and expensive? Saturn V costs seem absurd to me when the USSR was also making similar rockets presumably far cheaper. Maybe the US defense spending model is just a poor one for getting a lean product developed compared to nations and groups that absolutely must be lean to achieve anything at all.

@roshdodd - Is there a modern reference/book that covers the design of such systems?

> Google "Lars Blackmore", "GFOLD", "Mars Landing", and work through the references

They linked details to look into in their original post.

I recall hearing SpaceX cite manufacturing improvements as well. How do you feel about materials science and the ability to source parts not made of unobtanium?

Many of the hardest problems facing rocket engines are about temperature, heat and thermal density, not structural strength.

This means that 3d-printed copper (alloy) is an amazing process and material for them. You can build the kind of structurally integrated cooling channels that the people building rockets in the 60's could only dream about, and it's not a gold-plated part that required a million labor hours to build, it's something you can just print overnight.

I learned a couple of years ago that the people in the sixties did in fact 3d print, but they did it via electroplating and wax. It took weeks to print a Saturn V rocket bell because they had to build up something like 5mm of material onto the outside of the bell after carving the channels into the outer surface of the inner bell and then packing them with wax.

I don't know how representative it is, but this photo seems impressive:

https://www.voxelmatters.com/wp-content/uploads/2024/08/Spac...

With the block 3 design they have hardened all of the surfaces within the bottom skirt on the rocket so the blowback during reignition can’t melt anything. And the flanges on the turbopump exhaust exist to facilitate redesign and inspection of the system. Once they know the exact shape they need they can construct a single pipe with two flanges instead of three pipes with six. Flanges make bigger failure points than a solid pipe due to the seals.

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