Combustion of HTPB-Based Solid Fuels Loaded with Coated Nanoaluminum

Abstract

Nanoaluminum powder (nAl, nominal size of particles 50 nm and 100 nm), obtained by electrical explosion of wires, was passivated by air and coated by several different protective organic reagents to assess the effects on ballistics of nAl-loaded hydroxyl-terminated polybutadiene (HTPB)-based solid fuel with respect to pure HTPB baseline. The nAl samples were characterized by transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET), and chemical analysis on active aluminum content (Al°) content and added to HTPB-based solid fuels for hybrid propulsion. Combustion tests were carried out burning central-perforated single-port cylindrical samples in a 2D radial burner. Data analysis was performed to obtain a continuous time-resolved regression rate.

Coated nAl particles may significantly improve the ballistics of HTPB + nAl formulations burning in gaseous oxygen, with respect to pure HTPB. All investigated formulations with nAl exhibit increase of instantaneous regression rate (up to 89% maximum), depending on coatings and oxidizer mass flux G _ox . Fluoroelastomer and fluorine-containing chemicals used for coating show a good compromise between increase in regression rate and low sensitivity to G _ox variation under the implemented operating conditions.

Keywords:

• ALEX
• Fluoroelastomer coating
• HTPB
• Hybrid propulsion
• Nanoaluminum
• Nanoaluminum powder
• Organic coating
• Regression rate enhancement

ACKNOWLEDGMENTS

The authors would like to acknowledge CNES (Commande No. 4700024752/DLA090 + 4700028003/DLA094), Tomsk Polytechnic University (A. A. Ditts for the BET and DSC-TGA analysis, Russian Government Contract 11.519.11.3004, and grant MD-901.2012.8), Advanced Powder Technology LLC's staff (nAl powder production, passivation, coating process), and Donegani Institute in Novara, Italy (TEM analyses), for their support.

Notes

^*Declared by the producer.

^**For spherical particles: .

^***Evaluated by volumetric method (1 h of sample reaction with water).

^*Data for heating up to 1500°C.

^**Initial first mass loss before the first oxidation peak.

^*** ; Al° [%] – metal content in the samples (see Table 2).

^****Probably due to additional products in solid phase, formed from the pyrolysis of coating.

Average percentage variation (Δ _av. ) and standard deviation (σ_Δ) are provided in the last two columns. The lower σ_Δ, the lower the formulation sensitivity to G _ox variation.

Time-averaged regression rate in the last column, [mm/s].

^*Less than 0.001.

^**Negligible.

^***Over the range G _ox from 387 to 244 kg/(m²s), with the exception of 50 nm L-ALEX _NPS (see Figure 9).