Honda conducts successful launch and landing of experimental reusable rocket

Question why is it so easy today to build reusable rockets? Is it because the onboard cpu speed of the chips can solve more granular control problems with low latency?

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.

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.