From: Bruce Dunn <bruce_dunn@mindlink.bc.ca>
Newsgroups: sci.space.tech
Subject: ESA: New Solid Rocket Oxidizer
Date: Tue, 07 May 1996 15:14:07 -0700

Conventional solid propellants use ammonium perchlorate as an oxidizer,
although phase-stabilized ammonium nitrate is being considered as a
future oxidizer which will not have chlorine in the exhaust.  Recently,
the European Space Agency publication "Preparing for the Future" (Vol 6,
no. 1, March 1996) carried an article on research into a new extremely
high performance oxidizer for use in solid propellant grains.

The compound:
- hydrazinium nitroformate, or "HNF" (note that HNF is not the formula of
the compound, merely its abbreviation)
- formula can be written as:

N2H5C(NO2)3

or

[H2N-NH3]+ [C(NO2)3]-

It is the salt of hydrazine (acting as a base) and "nitroform", acting
as an acid (nitroform is a common name for tri-nitro methane, and
is used because of the analogy to chloroform, which is tri-chloro
methane).  Its enthalpy of formation is -17.2 kcal/mole (data from
"Propellant Chemistry", S.F. Sarner, Reinhold Publishing, 1966). It
is an orange-yellow solid, which autoignites after 5 seconds at
165 C.  Its melting point in Sarner is listed as " 123 d. " which I
take to mean decomposition at 123 C.  Its density is 1.86.

This compound has a potentially frightening heritage, but seems to be
stable enough to be given serious consideration as a propellant ingredient.
It is the salt of hydrazine (an unstable compound quite capable to decomposing
into a superheated gas) and an oxygen laden nitro compound (tri-nitromethane)
which is a close relative to tetra-nitromethane which will also explode
if mistreated. Its classification as an oxidizer is a matter of terminology
- it is more akin to a solid monopropellant (or explosive) in that
most of the oxygen in the molecule is internally consumed by the hydrogen
and carbon of the molecule, with little left over for oxidizing something
else.

The ESA article describes development work being done with the oxidizer.
The first problem was how to get granular material or short crystals - the
material normally crystallizes into long needles which are unsuitable for
manufacturing solid propellant grains.  This has now been partly mastered,
and a pilot production plant with a capacity of 100 kg per year has been
set up (a picture of the production plant shows racked piping and equipment
two stories high and about 3 to 4 meters wide for this capacity).  ESA has 
also had to work on binders and curing systems for composite propellants,
and reports that they are not yet to the stage of operational propellants
but can make test samples for ballistic tests. They have also had problems
with the provision of a suitable bonded case liner (insulation)for production
purposes, but for ballistic testing "a short-term intermediate solution was
found by applying an unconventional liner material to the propellant". Finally,
the oxidizer had far to high a burning-rate exponent (the tendency for
burning rate to increase with pressure).  Umodified oxidizer with a glycidyl
azide polymer and aluminum in a composite grain has an exponent of 0.8, while
values below 0.6 are needed for production motors.  A burning rate modifier
was found for the propellant grain which acheived an exponent of 0.59.

Combustion is stated to be smooth.  Measured values of characteristic velocity
from small scale ballistic tests indicated that the HNF/glycidyl azide/aluminum
propellant performed 8% better than a reference propellant using ammonium
perxhlorate.  The article suggests that should these results be acheived in
real-world motors, Isp of solids could rise from "the present value of
295 to more than 318".

The work on this oxidizer is continuing, with the intention of going through
an "initial industrialization" phase, with the company "Aerospace Propulsion
Products" in the Netherlands as prime contracter, and Raufoss (Netherlands),
Royal Ordnance (UK) and TNO (Netherlands) as subcontractors.  Interest
has also been expressed by Bristol Aerospace (Canada), SNPE (France),
NAWC and the Phillips Laboratories (US).  (Apologies for the untranslated
acronymns - the original article does not define these).


The ESA publication "Preparing for the Future" is a 20-30 page glossy colour
pamphlet which I think comes out 4 times a year.  It is free from:

ESA Publications Division
ESTEC, P.O. Box 229
2200 AG Noordwijk
The Netherlands


The publicationt is also available on the net, as a branch off of the ESA
publications home page.  The issue of "Preparing for the Future" with the
HNF information is not yet on the web, although earlier issues are.
The articles reproduced include the original figures as links to GIF files
for display on your net browser.

http://esapub.esrin.esa.it/esapub.html