Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Efficient Rocket Nozzles
Name: Chris
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: N/A

Please give a short description (in english (heh)) as to why a convergent-divergent rocket nozzle is shaped the way it is, and what having those particular dimensions accomplishes. In other words, what makes a nozzle efficient?

Dear Chris--In plain English, the convergent part of the nozzle compresses the mass of the expanding gases from the combustion chamber where the fuel is ignited. Due to the expansion of the gases in the divergent part of the nozzle, thrust is created which propels the rocket forward. The same principle, roughly and minus the heated fuel part, is why an inflated balloon flies around when you release it from your grip after blowing it up. When you blow up the balloon, you are creating pressurized air in the balloon (in place of the combusted fuel in a rocket). The elasticity in the balloon is acting on the air inside and forces it out the opening when you release the throat of the neck of the balloon. In case you are interested, if you do not totally release the balloon and instead hold on to the intake port and allow the air to escape in a whine or whistle mode, you are sensing the sonic effects of the escaping air. In certain cases of fluid flow through a nozzle, you actually achieve the speed of sound, which is why you hear some of the things you do.

I cannot make it simpler than that. If, however, you seek to understand the principle a little further, just pick up a basic college book on fluid dynamics, and it will explain things a bit more. Science can actually be fun, despite the complicated math, etc., when you look at the applications upon which the science is based. It always was to me, at least.

John S.

Click here to return to the Engineering Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory