[AR] Re: Nozzle shapes
- From: Alexander Mikhailov <avmich@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Tue, 12 Nov 2013 14:51:35 -0800 (PST)
I'd really like to hear Alexander, from http://lpre.de/ , on this matter.
The goal of nozzle optimization is to get best delta-V - optimizing
between Isp and engine weight. Certain assumptions have to be made.
Since the optimum isn't very sharp (rather the opposite), some
secondary considerations could come into play - nozzle heat protection,
overall pressure drop etc. On a first sight, there are plenty of
parameters, which could - historically - be assumed slightly differently
in USSR/USA.
Subsonic region of the nozzle - in 1-dimensional approximation of
gas without viscosity - can follow "the curve of Vitoshinsky", which
in practice can be approximated by two circular arcs. Of course, the
supersonic region is more important and more complex; but the optimum,
again, isn't sharp.
And one more thing - it's strange to hear that US, with the tradition
of using pintle injectors, have more optimal combustion process, while
Russian injectors commonly have many much smaller injectors, matched
by flow. Pintle should have other advantages, I guess...
Alexander
On Tuesday, November 12, 2013 11:36 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Anyone have further info to back this up, or Russian nozzle design
logic translated into English?
From a powerpoint on nozzles by D. R. Kirk of FIT:
Q: Why do U.S. nozzles look more like a polynomial contour and Soviet
nozzles look more conical?
A: (Jim Glass, Rocketdyne)
Interestingly, Soviet nozzle designs have a 'different' look to them
than typical US designs. US designs are ‘truncated Rao optimum’
bells, usually designed by method-of-characteristics methods. Soviet
nozzles, to US eyes, look more conical than ours. Ours have that nice
‘parabolic’ look to them - less conical. One would suppose the
Russians are fully capable of running M-O-C and CFD codes and thus
their nozzles, if optimum, should look ‘just like’ ours. Since they
don't, I've always wondered if they know something we do not. In my
experience, the US is better at combustion engineering (minimal C-star
losses) but has fairly substantial losses in the nozzle (aerodynamic
losses). The Russians tend to reverse this, throwing away huge gobs
of energy due to incomplete combustion and then using a very efficient
expansion process to get some of it back. The bottom line is both
design approaches appear to yield roughly the same Isp efficiency...
One wonders what would happen if one were to mate a US combustor to a
Russian nozzle…
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