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hwillis parent
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.

bumby
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.

hwillis OP
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.

Out_of_Characte
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.

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