Friday, September 25, 2009

Nozzle? We don't need no steenkin' nozzle! (Update)

Although I can't quote any equations, I've read several overviews of how rocket nozzles work, specifically the standard de Laval variety, and generally understand concepts like expansion ratios. At least I thought so, so correct me if I'm wrong.

Due to the physical constraints of the motor design, the nozzles on commercial hobby rocket motors are not optimal.  Their nozzles are under expanded due to a combination of their mechanical design and the materials used.  For instance, screw-on closures limit the size of the nozzle and the strength of graphite makes machining a more fully expanded nozzle difficult.  The sugar motor crowd often uses steel for their nozzles and probably get closer to optimal ratios.  However, from what I've read (I don't have a reference and certainly can't back this claim up with theory), the sub-optimal hobby rocket nozzles only have a performance penalty of a few percent.

I have also seen several reports on nozzleless designs.  I know Jeff Taylor of Loki Research has successfully flown one (or more ?).  A recent post on SugPro points to this nozzleless motor, which seemed to work pretty well.

In a later post, the builder, Fori, offers the following to address the performance of nozzleless designs.

"The simplicity,reliability and cost effectivenes due to the avoidance of a nozzle,the simple propellant configuration and the reduced insulation requirements, make the nozzleless rocket motors an attractive concept in spite of its lower specific impulse (by about 20%) compared to nozzled motors. In addition, the elimination of the nozzle assembly is used to increase the overall amount of propellant,that ,in most instances, can compensate for the reduced performance."

Timnat Y.M. ,Advanced Chemical Rocket Propulsion, Academic Press,London,1987,Chap.6

I find this interesting and, if I were making sugar motors, it might be fun to work on nozzleless designs.  After all, home made nozzles generally are heavy or costly or hard to make yourself (people with metal lathes need not reply to this).

Does anybody have additional references on nozzleless motors?


Here's another relevant video.  Hat tip to Randy Dormans!