Why variable geometry nozzles are not used on rocket engines

Click For Summary

Discussion Overview

The discussion centers on the reasons why variable geometry nozzles, similar to those used in jet engines, are not commonly implemented in rocket engines for altitude compensation. Participants explore technical, economic, and risk-related factors influencing this design choice.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the cost-benefit analysis of implementing variable geometry nozzles may not justify the increased weight and complexity, despite potential efficiency gains.
  • Others hypothesize that the use of exotic materials and the fact that such nozzles would typically be used only once contribute to the reluctance to adopt this technology.
  • A participant raises the consideration of risk alongside cost, questioning whether increased efficiency at the expense of greater project failure risk is worthwhile.
  • It is noted that multistage rockets are designed with different nozzles optimized for various altitudes, which may mitigate the need for variable geometry nozzles in current systems.
  • One participant mentions the potential for future single-stage-to-orbit (SSTO) systems to change the landscape regarding nozzle design.

Areas of Agreement / Disagreement

Participants express varying viewpoints on the feasibility and practicality of variable geometry nozzles in rocket engines, with no consensus reached on whether the benefits outweigh the costs and risks involved.

Contextual Notes

Discussions around the specific trade-offs between efficiency, cost, weight, and risk remain unresolved, with participants acknowledging multiple factors influencing the decision-making process.

granzer
Messages
15
Reaction score
1
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?
 
Physics news on Phys.org
This video does not directly answer your question, but it talks a lot but altitude compensation.

 
  • Like
Likes   Reactions: billy_joule, granzer and berkeman
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.
 
  • Like
Likes   Reactions: granzer
anorlunda said:
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.
 
boneh3ad said:
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.
 
  • Like
Likes   Reactions: berkeman
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.
 
  • Like
Likes   Reactions: granzer
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.
 
  • Like
Likes   Reactions: granzer

Similar threads

Mass of the rocket engine's nozzle and combustion chamber as a percentage of the total engine
  • · Replies 8 ·
Replies
8
Views
2K
Rocket thrust equation in under-expanded nozzle
  • · Replies 2 ·
Replies
2
Views
5K
Rocket nozzle thermal protection
  • · Replies 14 ·
Replies
14
Views
5K
2D vs 3D Method of Characteristics for Rocket Nozzle Design
  • · Replies 4 ·
Replies
4
Views
3K
What are 1st stage and 2nd Stage engines of an airplane?
  • · Replies 2 ·
Replies
2
Views
3K
Understanding High Thrust of Vacuum Rocket Engines
  • · Replies 2 ·
Replies
2
Views
4K
Momentum Thrust of an Over-Expanded Rocket Nozzle
  • · Replies 1 ·
Replies
1
Views
2K
Why Not Titanium Alloy Rocket Engine Nozzles?
  • · Replies 19 ·
Replies
19
Views
6K
Compression Nozzles on Model Rockets
  • · Replies 14 ·
Replies
14
Views
8K
Graduate Optimal Size of Rocket Engine Nozzle for Vacuum Performance
  • · Replies 26 ·
Replies
26
Views
4K