Figure 1. The HNF Plant at Aerospace Propulsion Products in the Netherlands.
Contractors: Aerospace Propulsion Products (NL), TNO (NL), Delft University of Technology (NL), University of Delaware (USA).
Funding: Jointly funded by ESA's Basic Technology Research Programme and by NIVR (NL).
In 1992, an article which appeared in Preparing for the Future (Vol. 2 No. 3) described a propellant under development at Aerospace Propulsion Products. Since then, APP and their sub-contractors, TNO, Delft University of Technology and the University of Delaware, have achieved a major break-through in the development of this propellant.
Until now, most composite solid propellants have used ammonium perchlorate as an oxidiser. However TNO identified a more powerful oxidiser, hydrazinium nitroformate (HNF), which in conjunction with modern binders, like glycidyl azide polymer (GAP), or others like BAMO (poly 3,3 bis[azidomethyl] oxetane), PLN (poly 3-nitromethoxy-methy oxetane) or PGN (poly glycidyl nitrate), not only gives a substantial increase to the performance of solid rocket motors, but also has an ecologically benign exhaust, as the combustion gases are free of chlorine. The main combustion products are nitrogen, water, carbon dioxide, nitrogen oxides and, if aluminium is added to the propellant, aluminium oxide. Initially, HNF was produced only on a small laboratory scale; now a pilot plant with a nominal production capacity of 100 kg of HNF per year has been constructed at APP. This plant (Figure 1) produces HNF not only for ESA-related contracts, but also for other customers.
Figure 1. The HNF Plant at Aerospace Propulsion Products in the Netherlands.
A major problem with the initial HNF production was the tendency for crystals to grow in long thin needles with a length-to-diameter ratio in the range 11 to 20 and a large spread in size and shape. This hampered the manufacture of propellant with a high solid loading fraction, for which more or less spherical crystals are preferred. Work on (re)crystallisation by Delft University of Technology and APP led to processes which reduced the length-to-diameter ratio to lie in the range 4 to 11 and also reduced the spread in size and shape distribution. Although not yet optimal, these are significant improvements.
A solid propellant is made by mixing the ingredients under vacuum in what is basically a modified bakery mixer. The pre-polymer is a liquid; most of the other components, such as aluminium and the oxidisers are added as powders. To the preliminary mix, the curing agent (needed to polymerise the pre-polymer) is added and thoroughly mixed with the other ingredients. Subsequently the propellant is cast into the rocket motor. If a lot of curing agent is added, or if the temperature is high, the pot-life is short and there is little time to cast the propellant into a mould. However the pot-life must not be too long, otherwise the polymer may harden only with difficulty.
Hardening is caused by cross-linking of the molecules of the pre-polymer. The curing agent is a reactive compound which may also react with the other ingredients, for example, the oxidiser. During the development, much attention was given to identify suitable curing agents, and suitable mixing and curing procedures to avoid such unwanted reactions. Although at present the curing system is not yet suitable for an operational propellant, it is suitable for making propellants for ballistic tests.
Parts of the propellant surface have to be covered by a liner. The liner has a number of functions:
Thus, the interface between the propellant and the liner, and the liner and the rocket motor housing must have good mechanical strength. Here too, compatibility problems may occur between propellant and liner, because most of the liners are of a type of rubber that polymerizes. For the ballistic performance tests, a short-term intermediate solution was found by applying an unconventional liner material to the propellant.
The present work has focused on the combination GAP (made by SNPE in France) aluminium and HNF. GAP and HNF have inherently high burning rates, but HNF- propellants also tend to increase their burning-rate strongly with pressure. This is expressed in terms of a burning-rate exponent which for HNF propellants is close to 0.8. Propellants intended for use in rocket motors must have a lower value, below roughly 0.6. Decomposition studies, performed at the University of Delaware, allowed TNO to identify suitable burning-rate modifiers.
Knowledge of the decomposition behaviour of HNF and GAP was of prime value for this task and the contractor was able to identify, with relatively little effort, a burning-rate modifier which lowered the burning-rate exponent to 0.59. This is a very important achievement, as it clearly shows that the burning-rate exponent of HNF-based propellants can easily be lowered to acceptable values. Additional work may identify the means to lower the burning-rate exponent further.
The decomposition studies also showed that, although HNF is a highly energetic oxidizer, its decomposition is smooth, even at temperatures above 260°C. This is essential to guarantee smooth and stable combustion for practical propellants.
The primary objective of the tests was to demonstrate that the increased performance, predicted in theory, could indeed be achieved in practice. To make a fair comparison with existing propellants, samples of GAP/Al/HNF propellant were fired in a small test motor, and samples of the best performing HTPB/Al/AP propellant were also test-fired in the same device. Characteristic velocity was used to evaluate performance since this gives direct information about the combustion and performance characteristics of a propellant. Multiplying it with the nozzle thrust coefficient, C(sub F), yields immediately the specific impulse, I(sub sp). Propellants must be compared under similar test conditions to eliminate the particular effects and influences of the test device itself.
Test results were highly encouraging. The average measured characteristic velocity of the new propellant was about 8% higher than for the best conventional propellant. This confirmed the expected increase in performance. The results of the tests are shown in Figure 2.
Figure 2. Theoretical (top) and measured values of the characteristic velocity of AP-based and HNF-based
propellants.
Based upon these preliminary results, one may expect that apogee boost motors, like the MAGE or the EBM, could increase their specific impulse from the present value of 294 s to more than 318 s. There is still scope for increasing the performance of HNF propellants further, since the propellant used for the evaluation tests did not have the optimum composition for the desired performance.
The very promising results of this work, carried out under a contract jointly sponsored by ESA and the Netherlands Agency for Aerospace Programmes, NIVR, led to the ESA's starting the initial industrialisation phase under its General Support Technology Programme (GSTP) Phase 1, with APP as Prime Contractor and Raufoss (N), Royal Ordnance (UK) and TNO (NL) as subcontractors. BPD (I) will also start development of HNF-based propellants, with the intention of fully participating in the next development phase under GSTP-2. In parallel, Bristol Aerospace in Canada, SNPE in France, and in the USA, NAWC and the Phillips Laboratories, have also shown an interest in this new propellant.
Preparing for the Future Vol. 6 No. 1