Why variable geometry nozzles are not used on rocket engines

[granzer]

Why is it that variable geometry nozzles, like those found on jet engine(iris nozzles), are not used as rocket nozzle to provide better altitude compensation?

 

[anorlunda]

This video does not directly answer your question, but it talks a lot but altitude compensation.

 

[boneh3ad]

While I'm not a rocket designer, I'd hypothesize that it has to do with the cost-benefit analysis of implementing something like that. They'd get more efficient operation, but it would require increased weight, more exotic materials, and (until recently) would only be used once. That's a lot of cost to justify a relatively small benefit.

 

[granzer]

This video does not directly answer your question, but it talks a lot but altitude compensation.

Haha! A couple of years back I had given a talk on aerospike engine! Recently came across this video and it sparked my interest again it. Lol.
I think Saber engine from UK is going to have a variable geometry nozzle.

 

[granzer]

While I'm not a rocket designer, I'd hypothesize that it has to do with the cost-benefit analysis of implementing something like that. They'd get more efficient operation, but it would require increased weight, more exotic materials, and (until recently) would only be used once. That's a lot of cost to justify a relatively small benefit.

Thank you. Yup that makes sense. If the cost saved by the increase in efficiency does not enough justify the cost of implementation of such a nozzle.

 

[anorlunda]

But consider risk as well as cost. The video emphasized that. If you could double the efficiency at the cost of 33% greater risk of project failure, is it worth it? That is a question for project backers, not the engineers.

 

[Rive]

It is true that different nozzles/bells are optimal for diferent altitudes, but that has been taken care of by using multistage rockets (different stages operates at different altitudes). The engines on the first stage has different nozzle than on the second (third?) stage.
Anything more would just not worth it (on the actual engines/systems).

Maybe it will be different for future SSTO systems.