From: "Jeff Greason" <jgreason@hughes.net>
Newsgroups: sci.space.tech
Subject: Re: Nozzles, that don't melt.
Date: Mon, 10 Apr 2000 16:49:48 GMT

Ian Stirling <Inquisitor@I.am> wrote in message
news:8cq899$l4i$1@uranium.btinternet.com...
> I'm interested in long burning, very small hybrid rockets, probably H2O2/PE
> (maybe initially just H2O2)
>
> I notice that Niobium(aka columbium) is quite cheap, relatively easily
> machinable (compared to Mo or Ta), and quite resistant to melting.

Niobium nozzles are pretty much standard for radiation-cooled space
use -- there are alternatives, but niobium is state of the practice.

> Do I need to, for example, ensure that the nozzle is in a reducing
> atmosphere, to avoid a nozzle-rich-exhaust?

Ah, there's the rub.  Niobium oxidation is fierce at high temperatures
(above a few hundred degrees C, IIRC, but I don't have the reference
handy), so that Nb nozzles are uniformly coated with an oxidation
resistant coating.  This drives the cost up and takes it out of the
realm of "garage shop" fabrication.

Note that it isn't enough to run the exhaust reducing to avoid this,
you also have to burn it only in a vacuum or reducing *ambient*,
which isn't too practical.

The fabrication also isn't that simple, as welding Nb is nontrivial.
However, at the scale of your interest, machining from a solid Nb billet
is probably practical.

One other note -- I found that while the price of Nb is not prohibitive
in a lot of applications, it isn't a "stock" item most places, and the time
from order to delivery of your billet can be quite long.  The obvious
place to go for more info is Teledyne Wah Chang.

> Or are the alternatives (graphite, Ta, Mo) better?

Ultramet has done some very interesting work on Ir/Re chambers and on
oxidation resistant coatings for Carbon-Carbon nozzles and chambers.
This obviously isn't as mature as Niobium (niobium has a flight history
stretching back to the 1960's at least), but I think it's very promising
work.

www.ultramet.com

----------------------------------------------------------------
"Limited funds are a blessing, not         Jeff Greason
a curse.  Nothing encourages creative      President & Eng. Mgr.
thinking in quite the same way." --L. Yau  XCOR Aerospace
   <www.xcor-aerospace.com>                <jgreason@hughes.net>