Is this pattern of expanding gas inefficient?

[nhnifong]

When the Falcon 9 1.1 first stage reaches the upper atmosphere, the gas begins expanding in what appears to be a highly non-uniform pattern.

I was wondering if this pattern represents an inefficiency. Would the ISP or the thrust of the rocket as a whole be higher if the gas expanded uniformly?

Could something be done to the profile of the ends of the nozzles to even it out?

[Albert VDS]

It would be more efficient if the gas couldn't be pushed to the sides, for example if the whole thing would be placed in a gigantic cylinder.

Though I doubt that the sideways expansion has a huge impact on it, so doing something about it wouldn't be worth it.

Edited January 13, 2014 by Albert VDS

[Nuke]

i think as you approach space you actually want to spray gas out in as big of a cone as possible, where close to the ground you want to keep a tight column of flame. this is actually one of the reasons for staging.

[Nibb31]

Yes, upper stage engine nozzles provide a longer cone than first stage ones to compensate for the expansion of the gasses in vacuum. That's the main difference between the first stage Merlin and the vacuum Merlin for example.

[NovaSilisko]

All rocket engines gain in efficiency as air pressure decreases, so I'm gonna guess that the expansion, if it does detract from efficiency, has a very minimal effect overall. Plus, if it was really a big problem, wouldn't at least something have been done about it by now? Every rocket in vacuum has this same sort of expansion(varying with the layout of the engines... the octopus pattern of the F9v1.1 is unique to that vehicle, Soyuz has its own expansion pattern, but the principle is the same) , so just going by simple observation I'd say it's reasonable to conclude that it doesn't present enough of a hindrance to be worth attempting to change. edit: Mostly right, see below...

http://en.wikipedia.org/wiki/De_Laval_nozzle

Rocket engines, ideally, use this shape - to "shape the exhaust flow so that the heat energy propelling the flow is maximally converted into directed kinetic energy."

And for more reading: http://en.wikipedia.org/wiki/Rocket_engine_nozzles It helps (and is interesting in its own right!) to read about how rocket engines work, too... they're wonderful products of science, mathematics and engineering all working together.

Probably the most relevant bit: "There is also a theoretical optimum nozzle shape for maximum exhaust speed, however, a shorter bell shape is typically used which gives better overall performance due to its much lower weight, shorter length, lower drag losses, and only very marginally lower exhaust speed.[9]" So, there's some loss of efficiency but making a massive nozzle in order to counteract the effects of gas expansion would end up being less efficient than simply disregarding it.

Edited January 13, 2014 by NovaSilisko

[Nibb31]

They do attempt to mitigate it by designing nozzles specifically for vacuum. But yeah, there isn't much they can do beyond that.

[NovaSilisko]

They do attempt to mitigate it by designing nozzles specifically for vacuum. But yeah, there isn't much they can do beyond that.

Yeah, there's a point where it becomes best to do a tradeoff - the point where the efficiency gained by having a big nozzle offsets the weight of having that bigger nozzle in the first place.

Probably just should have said that in the first place instead of the meandering disaster of a post before, but whatever.