undefined | Better HN

0 pointsdotancohen1y ago0 comments

Combustion in the chamber happens at roughly the speed of sound in the F/O mixture. I don't know what the speed of sound in gCH4 is, but it's probably within an order of magnitude of air - air at 250 bar.

This is actually extremely important to model. Early F1 engines (Saturn V, not motorsport) were exploding and the engineers pretty much got lucky with the baffle design. Having a suite of sensors and then a computer model it would have saved lots of hardware and time - and really would have pretty much assured success. They were unsure if they'd succeed right up until they did.

0 comments

8 comments · 3 top-level

HPsquared1y ago· 2 in thread

Speed of sound also depends on the temperature (i.e. speed of the gas molecules) which is high and variable. These kinds of coupled interactions make combustion instability really hard to model in CFD hence the importance of full-scale testing.

extraduder_ire1y ago

If you solve the formulas for calculating it, the speed of sound only depends on temperature. Mostly because temperature is dependant on pressure itself.

HPsquared1y ago

Temperature and molecular properties.. which guess what are also highly variable in combustion! Very messy stuff. Especially when you get into the actual nitty gritty of all the intermediate chemical species involved.

frodo8sam1y ago· 2 in thread

Not pure luck though, they tried 15 different baffle configurations. Would of course be easier to filter configuration in simulations.

dotancohenOP1y ago

There was enormous engineering effort put in with close coordination of physicists, but those 14 prior baffle configurations could have just as easily been 140.

For a relevant example, even just the oil formulation for Water Displacement on the exterior of the steel Atlas rockets took 40 iterations. Hence naming the product WD-40.

HPsquared1y ago

I wonder how many of the other baffle designs and water displacement formulations would still have been "good enough" - not optimal but usable?

1 more reply

raverbashing1y ago· 1 in thread

Still way slower than what particle physics throws at you

(Wolphram Alpha gives me 743 m/s @ 250 bar and 1000C - could be wrong but probably the same OoM)

dotancohenOP1y ago

Yes, of course. I was clarifying, not disputing.

j / k navigate · click thread line to collapse