Niklas Wingborg

Increasing the tensile strength of HTPB with different isocyanates and chain extenders

Abstract The influence of the diols 1,4-butanediol, BDO, and 1,4-cyclohexane dimethanol, CHDM, on the mechanical properties of HTPB, cured with the diisocyanates, IPDI, HDI and H12MDI, was investigated. The highest tensile strength was obtained by using H12MDI and BDO. When adding the diols the tensile strength increases strongly up to 2 mol of diols per mole of HTPB. Above that point the effect is moderate. The ultimate elongation of the materials has a maximum between 1 and 2 mol of diols per mole of HTPB. If more than 1 mole diol per mole of HTPB is added, all the examined materials begin to strain crystallise and the initial modulus of elasticity increase linearly with increasing amount of diol.

Green Propellants Based on Ammonium Dinitramide (ADN)

Ammonium perchlorate (AP) and hydrazine are today widely used as propellants. AP as oxidizer in solid propellants and hydrazine as liquid monopropellant (Brown, 1995; Sutton and Biblarz, 2001). These propellants are well known for their good performance characteristics, but their limitations and liabilities regarding toxicity, operational handling and environmental impact are also well documented. Perchlorate contamination is becoming a more widespread concern in the United States (EPA, 2005). In 2009, a workshop organised by the US Department of Defence identified AP as one of the key environmental, safety and occupational health issues (DoD, 2009). Perchlorate anions (ClO4–) has been found in drinking water supplies throughout the southwestern United States, and perchlorate may be a problem for water supplies in some regions of the USA (Urbansky, 2002). At high concentrations, perchlorate can affect thyroid gland functions, where it is mistakenly taken up in place of iodide. Apart from impacting the thyroid activity in humans, AP forms vast amount of hydrochloric acid on combustion. For instance the space shuttle and the Ariane 5, generates 580 and 270 tons of concentrated hydrochloric acid, respectively, per launch (Wingborg et al., 2008). Hydrazine is highly toxic and carcinogenic (ATSDR, 1997; Ritz et al., 2006), and handling it requires costly safety measures. A less toxic monopropellant is expected to offer substantial cost savings (Bombelli et al., 2003; Palaszewski et al., 1998; Hurlbert et al., 1998). These economic benefits were analysed and quantified in a study funded by the European Space Agency (ESA) and were considered sufficiently large to support interest in the development of hydrazine substitutes and related propulsion hardware (Bombelli et al., 2004). Propellants of the future must not present major hazards to the crew or ground handling personnel. The use of green propellants would greatly reduce the risks associated with toxicity, operational handling complexity, spacecraft contamination, and hazardous contamination of the environment. Green propellants have also shown promise from a system performance and total life cycle cost perspective. One material that has the potential to replace AP as well as hydrazine is ammonium dinitramide (ADN), NH4N(NO2)2.

Characterization and Ignition of ADN-Based Liquid Monopropellants

Monopropellant propulsion systems for space applications have relied almost exclusively on hydrazine. Hydrazine is however highly toxic, volatile and carcinogenic, and thus costly safety measures are required. In the last few years there has been considerable interest in Europe and in the USA in finding a possible substitute, since a non-toxic monopropellant would offer substantial cost savings. ADN-based liquid monopropellants seem to be a promising alternative to hydrazine, being substantially easier to handle and having a 10% higher specific impulse, and up to 60% higher density-impulse, than hydrazine. To be able to replace hydrazine, ADN-based monopropellants must be as easy to ignite. Hydrazine and ADN-based liquid propellants are very different, and thus new ignition methods must be developed. This paper presents the results from a characterization of ADN-based liquid propellants, as well as the results from electrical ignition experiments in which the propellant was resistively heated to its ignition temperature. It was found that substantially less electric energy was needed than expected. This is due to local phenomena close to, or on the surface of, the electrodes. Very fast ignition was obtained, in most cases below 2 ms.

Development of ADN-based Minimum Smoke Propellants

urrent minimum smoke double-base propellants used in tactical missiles contains lead compounds, such as lead resorcylate and lead maleate, to obtain the desired ballistic properties. Exposure to lead is associated with a number of acute and chronic health effects, even at very low levels. The US Department of Defense (DoD) organized workshop on advanced strategy for environmentally sustainable energetics identified lead compounds used in propellants as one of the key environmental safety and occupational health issues . Hence the DoD’s Strategic Environmental Research and Development Program (SERDP) have stated a need to develop new environmentally benign, insensitive, castable, high-performance, minimum smoke rocket propellant formulations . The formulations must meet all of the performance requirements associated with current minimum smoke, doublebase propellants, but must not contain lead compounds, ammonium perchlorate or RDX. There has been considerable efforts to develop such a propellant and many different energetic materials have been studied . One of the most promising energetic material for this application is ammonium dinitramide (ADN). This paper presents some of the properties of ADN and the ongoing work in Sweden to develop a minimum smoke composite propellant based on ADN and glycidyl azide polymer (GAP).

Solid Propellants based on ADN and HTPB

The aim of this work was to perform an initial evaluation of ammonium dinitramide, ADN, as substitute for ammonium perchlorate, AP, in solid rocket propellants for large space launch boosters. This paper includes performance evaluation, curing and compatibility assessments, propellant formulation and determination of ballistic properties. The results show that the theoretical specific impulse increases from 262 s to 270 s by replacing AP with the same volume ADN in a typical HTPB/Al-based formulation. ADN is found to be chemically compatible with HTPB. However, ADN seems to accelerate the oxidative degradation of HTPB and thus a suitable antioxidant is required. ADN/HTPB/Albased propellants have been formulated and cured successfully using isocyanates. The formulations were found to be thermally stable. Ballistic properties were determined using a strand burner. ADN/Al/HTPB propellant with a solid loading of 80% had a burn rate of 12.8 mm/s at 6 MPa and a pressure exponent of 0.9. The pressure exponent was high, but no burn rate modifiers or ballistic additives were used.

Electrical Ignition of New Environmental-Friendly Propellants for Rockets and Spacecrafts

Monopropellant hydrazine is today widely used as propellant for missiles and spacecrafts. Hydrazine is however highly toxic. ADN-based liquid monopropellants seems to be a promising alternative to hydrazine, being substantially easier to handle, giving a 10 % higher specific impulse, and up to 60 % higher density-impulse, compared to hydrazine. This paper presents the results from electrical ignition experiments of liquid ADN-based propellants were the propellant is resistively heated by conducting an electric current through the propellant. Due to local phenomena close, or on the surface, of the electrodes, the required electric energy for ignition is small. The limited amount of energy required for ignition shows that this type of ignition method can be used in a flying vehicle as a missile or a spacecraft where strong weight and volume requirements apply.

Influence of water content in an ADN based liquid monopropellant on performance characteristics

Hydrazine is advantageous for attitude control systems of satellites because it is space storable for long times and the developed thrusters are reliable for long term operations. Unfortunately hydrazine is very difficult to handle on ground due to its very high toxicity. Especially with regard to the REACH regulation of the European Community, which has passed some years ago, Hydrazine is on the candidate list of substances whose use could be limited in future. Thus strong research efforts have to be conducted today to find and to qualify alternative propellants, which have significantly simpler handling characteristics, are less toxic, environmentally benign and have similar or even better performance characteristics. One of the most promising candidates to replace hydrazine is the ADN based ionic liquid FLP-106, which is a monopropellant and has been developed by FOI. This monopropellant has a higher Isp in comparison to hydrazine and to LMP-103S which is currently used by the ECAPS company from Sweden. Also FLP-106 has a lower vapor pressure than LMP-103S. Thrusters using ionic liquid propellants are working with heated catalysts to decompose the propellant. One of the main drawbacks of this design is the lack of cold-start capability. Additionally, the high combustion temperature is a concern, possibly requiring high-temperature alloys like Iridium/Rhenium. FLP-106 consists of 64.6 % ADN (ammonium dinitramide), 23.9 % water and 11.5 % of a low volatile hydrocarbon fuel. In order to use simpler materials with lower melting points the combustion temperature of FLP-106 can be decreased by increasing the water content in the propellant. However, the increased water content will decrease the specific impulse and may also influence the ignition properties.

Solid ADN Propellant Development

Ammonium dinitramide, ADN, is studied in Sweden in order to develop castable high performance minimum-smoke propellants for tactical missile applications. In 2010, 3 kg grains based on 70% ADN and 30% GAP were cast and test fired successfully. This paper presents on-going work at FOI concerning ADN propellant development which includes upscaling the prilling method, improving the mechanical properties and studying the influence on the ballistic properties when ADN is partially replaced with a low sensitive energetic filler.