Beyond Cold Gas Thrusters

Transcription

1 Beyond Cold Gas Thrusters Good - Simple Bad - Limited I sp How to increase specific impulse of monopropellant? raise T o Where will energy come from? chemical exothermic decomposition of monopropellant electrical other heat source (solar, nuclear, ) Decomposition Monopropellant Thrusters - 1 Decomposition-Based Monopropellant Thrusters How to use energy? increase in T o also can use to evaporate propellant can store propellant as liquid (less volume) Example propellants hydrazine (N H 4 ) hydrogen peroxide (H O ) isopropyl nitrate (C H 7 NO ) nitric acid propyl ester, Decomposition Monopropellant Thrusters - 1

2 Hydrogen Peroxide Early use: monopropellant to operate turbo-pump on V- rocket jet packs also used as oxidizer in some bipropellant combinations (e.g., with kerosene) hypergolic with hydrazine Composition hard to find pure H O, usually has some amount of H O commercial grade, only 0% H O by mass propellant grade, 85-98% by mass Density ~1.4 g/cm Safety can burn skin contaminated solutions can explode for T>448 K Decomposition Monopropellant Thrusters - Hydrogen Peroxide Decomposition l H O g 1 O g HO Exothermic: 5.4 kj/mol Releases 54 kj/mol of H O at room temperature part of energy goes to vaporizing liquid Rapid reaction either at high temperature or in the presence of appropriate catalyst Less H O in solution means more energy release (higher T o ) lower MW of products At room temperature (e.g., in storage), conversion is ~1% per year for 95% concentration Decomposition Monopropellant Thrusters - 4

3 H O Decomposition Catalysts Originally liquid catalysts (injected) potassium permanganate (V-, K-1 Henschel 9 air-to-surface missile) sodium permanganate More common solid catalyst beds multiple layers of wire screens of silver, platinum, palladium, iron oxide typical size 5 cm long with sufficient cross sectional area can handle 50 kg/m s mass flux Decomposition Monopropellant Thrusters - 5 Hydrogen Peroxide Performance Temperature T o,max ~ K with catalyst heating Possible to operate without active cooling (radiation only) since there are a number of metals sufficiently strong to operate at these temperatures Characteristic velocity c* ~ 1040 m/s (400 ft/s) for 98% H O solution Decomposition Monopropellant Thrusters - 6

4 Hydrazine Mostly widely used, storable and high performance attitude control thrusters gas generators Composition commercial grade: <1.5% H O, < 10 mg particles monoprop. grade: < 1% H O, < 0.5% aniline (C 6 H 5 NH ) high purity: < 0.005% aniline, < 0.00% carbon aniline and impurities can poison catalyst Density ~1 g/cm, T boil ~86 K (5 F) Safety toxic and carcinogenic Decomposition Monopropellant Thrusters - 7 Hydrazine Decomposition (R1) (R) N H4 NH 1 1 N H NH 4 N Exothermic: 11.1 kj/mol Endothermic: 46.1 kj/mol Rapid reaction either at high temperature or in the presence of appropriate catalyst After hydrazine decomposes, ammonia can dissociate lowers T o decreases MW net effect on c* and Isp? Decomposition Monopropellant Thrusters - 8 4

5 Hydrazine Performance From ideal calculations of mixture properties as function of amount of NH decomposition T o : K MW: 17 1 c*: m/s optimum at ~0% decomposition Isp: 55 0 s overall T o reduction dominates Optimum performance requires proper catalyst bed design Decomposition Monopropellant Thrusters - 9 from Sutton 1000 psia =50 N H 4 Decomposition Catalysts Early catalysts (Mariner) N O 4 slugs, one per start (so limited #) Current catalysts bed of ceramic pellets coated with iridium (Shell 405 by JPL/Shell 1965) above 450 K can also use Fe, Ni, Co can achieve 00 kg/m s mass flux limit bed thickness to limit NH conversion to 0-80% (typical value 55%) N NH H from Sutton Decomposition Monopropellant Thrusters