Volker Gettwert

This paper gives an overview of new composite propellants with environment-friendly ingredients investigated at Fraunhofer ICT which is believed could lead to a new generation of high performance halogen-free (ammonium perchlorate free) propellants. In particular two formulations, which can be prepared from commercially available substances, were characterized, tested and compared to the actual state of the art AP/HTPB/Al in a small scale motor campaign. The HISP project focused mainly in obtaining maximum performance, and this led to an ADN/GAP/Al propellant. It revealed stable combustion and high performance but high burning rates in the order of 24 mm/s at 7 MPa, improvable mechanical properties and some concerns about the shock sensitivity. The aim of GRAIL was to overcome these drawbacks introducing the low burning ammonium nitrate as a second oxidizer accepting some specific impulse losses. The output was an ADN/HEM/PSAN/GAP/Al propellant. The burning rate was reduced to around 9 mm/s at 7 MPa and the shock sensitivity test was passed. The mechanical properties were still poor but can be improved by coated oxidizers. The use of a dual-oxidizer let the pressure dependence of the Vieille´s law rose till 0.6, which is borderline for a stable function of a rocket motor. Unfortunately no ballistic modifier exist for ADN propellant up to the present day but Alane as a high-energy metallic fuel is able to adapt ballistic properties and could be a potential propellant ingredient if commercially available.

Solid rocket propulsion enjoys unique properties favoring its use in space exploration and military missions still for decades to come. Yet, it also suffers a limited performance especially in terms of gravimetric specific impulse. Although new high-energy materials have been identified, most of them are far from being practically usable in the short range. Presently, no integrated vehicle designs make use of these new ingredients. A broad overview is discussed in this paper and attention is paid to Ammonium Dinitramide, ADN to overcome the current limitations of Ammonium Perchlorate, AP. The latter imply not only a limited gravimetric specific impulse but also a negative impact on the environment and personal health. ADN-based dual-oxidizer formulations, with Al-based dual-metal fuels and inert or energetic binders, are promising solutions for a variety of solid rocket propulsion missions aiming respectively at minimizing environmental impact (ADN + AN) or maximizing performance (ADN + AP).

ADN/GAP propellants with or without metallic fuels are most likely candidates for a green alternative to AP/HTPB solid rocket propellants. These propellants are not only interesting in terms of environmental friendliness but also to overcome the intrinsic performance limitations of AP/HTPB. This paper describes some properties and the burning behavior of ADN/GAP-based propellants as a function of different ADN particles sizes, and metallic fuel (Al, AlH3) regarding burning rate, pressure exponent, flame temperatures, particle temperatures and agglomeration phenomena. Furthermore, the formation of pores during propellant preparation is discussed and a small-scale motor test of an ADN/GAP/Al propellant demonstrates the basic applicability of ADN/GAP-based propellant formulations.

The fire behaviour of light-weight material used in structural applications is regarded as the main challenge to be solved for mass transportation. The task is to perform realistic experiments, including a mechanical test scenario under fully developed fires, to improve the material’s reliability in structural applications. Our approach utilises an intermediate-scale test set-up (specimen size 500 × 500 mm) to apply realistic compressive loads and fully developed fires directly to one side of a carbon-fibre-reinforced sandwich composite. Three different intumescent coatings were applied to sandwich structures and compared to a bench-scale study. The results emphasise intumescent coatings as a promising method to sustain fire resistance, multiplying the time to failure. Nevertheless, the realistic intermediate-scale test using severe direct flame application underlines the extremely short failure times when the actual composite components are tested without any additional insulation.

We have studied changes in the hole transport properties of a smectic liquid crystalline semiconductor resulting from high-energy electron irradiation. The “radiation doping” results in sharply increased shallow trap densities and the onset of nearly trap-limited hopping conduction. A simple semiquantitative model provides estimates of the trap lifetime and the irradiation energy required to create a single trap. Experimental techniques to overcome parasitic effects due to increased molecular ion densities in irradiated samples are also discussed.