Technology Challenges in Solid Energetic Materials for 

Micro Propulsion Applications

by Eugene Zakar

ARL-TR-5035 November 2009

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Army Research Laboratory

Adelphi, MD 20783-1197

ARL-TR-5035 November 2009

Technology Challenges in Solid Energetic Materials for 

Micro Propulsion Applications

Eugene Zakar

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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Technology Challenges in Solid Energetic Materials for Micro Propulsion 

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Eugene Zakar

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14. ABSTRACT

Micro thruster is a relatively new class of micro propulsion system for micro spacecraft, though there are many other potential uses in power 

generation, actuation of parts, and micro airborne robotics platforms. Propulsion systems of the same order of size for micro scale vehicle 

components do not exist, as the level of thrust and the impulse precision required for maneuvering micro scale vehicles in space orbit is 

deficient. The use of solid fuels in lieu of traditional liquid state fuels is attractive in many ways. Integration of solid fuel on a chip can lead to 

a miniaturized propulsion system. Single micro thruster and micro thruster arrays have been successfully fabricated using standard micro 

fabrication technologies. The purpose of this report is to review the development and challenges in solid energetic materials for micro 

propulsion applications.

15. SUBJECT TERMS

Micro thruster, propulsion, energetic

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iii

Contents

List of Figures iv

1. Introduction/Background 1

2. Trends 2

3. Conclusions 8

4. References 9

List of Symbols, Abbreviations, and Acronyms, 10

Distribution List 11
iv

List of Figures

Figure 1. Micro thruster array fabricated jointly by TRW and teammates Caltech and the 

Aerospace Corp. under a contract from the DARPA................................................................3

Figure 2. Photograph of Tanner Research Inc. array of micro thrusters.........................................3

Figure 3. BSAC silicon wafer with DRIE combustion chambers (left), and chamber design 

(right). ........................................................................................................................................4

Figure 4. Silicon nozzles (left), and Pyrex cylinders with electrical igniters (right) from 

Tohoku University. Pyrex cylinders are each approximately 1 mm in diameter.....................5

Figure 5. Arrays of micro patterned PETN explosive materials are being investigated by 

Sandia National Laboratories as on-chip sources of gas for actuation, locomotion, and 

acoustic signal generation. Center ball pattern is less than 0.5 mm in diameter. .....................6

Figure 6. Schematic view of one single micro rocket by LAAS-CNRS Laboratory......................7

Figure 7. Photograph of a single solid propellant micro thruster by team IPFL Space Center, 

and RUAG. ................................................................................................................................7

Figure 8. Photograph of a MEMS addressable 2x2 energetic device by ARL. Each circle 

pattern is approximately 2 mm in diameter. ..............................................................................8
1

1. Introduction/Background

Because of their simplicity, micro thrusters have received significant attention during the last few 

years, though early development in the microelectromechanical systems (MEMS) field began 

approximately 10 years ago. The principle applications of micro thrusters are for primary 

propulsion and attitude control of micro spacecraft, micro satellites (10 to 100 kg), nano satellites

(1 to 10 kg), and pico satellites (0.1 to 1 kg). These small-scale satellites will require efficient 

propulsion systems that can approach and maneuver around objects in space orbit.

If we consider fundamental requirements for a rocket thruster, assuming that it is used for the 

attitude control of a 10 kg spacecraft with 1 m/s velocity increment to maneuver around an object 

in orbit, a total impulse thrust of 10 N-s would be needed. If each thruster produces an impulse 

thrust of 1mN-s, the total impulse thrust of 10 N-s can be produced by 10,000 thrusters. If each 

thruster occupies an area of 1 mm in diameter, a 100 × 100 array of thrusters (10,000 thrusters) 

requires an area of 10 × 10 cm, which can fit on the side of a satellite. Evolutionary 

improvements in material and assembly techniques are expected to significantly reduce the total 

array size requirements. 

Technological efforts are converging from two directions: the miniaturization of conventional 

thrusters and the development of new solid energetic materials (EM) and device concepts. Most 

of the focus is on the use of MEMS technology and hybrid technology for reducing the cost, size, 

and system weight. MEMS technology allows subsystems (such as micro thrusters, switches, 

phased array antenna) to be reduced significantly in size and mass, and in the longer term will 

enable new classes of extremely small, intelligent, and relatively low-cost batch-produced pico 

satellites consisting primarily of bonded MEMS devices. 

The conventional use of liquid EM for propellant systems is due to their high specific impulse, 

their ability to be restarted, and their high thrust-weight ratio. The main disadvantages of liquid 

EM propellants are the power consumption required for pumps and valves, and the necessary 

cooling system to keep the system from overheating. Additional concerns are the precautions of 

handling toxic liquids, storage, and leakage, making solid EM alternatively more reliable. The 

concept is based on the high rate combustion of one single propellant or explosive stored in a 

combustion chamber. 

Some advantages of solid EM include:

• Propulsion systems of the same order of size as the other components of the vehicle

• No moving parts

• Vastly reduced number of components
2

• No storage tanks, fuel lines, or valves

• Ability to deliver thrust in very precise and predetermined amounts

Disadvantages, however, include the following:

• Single shot characteristic

• Relatively low specific impulse

The lack of restart ability in solid EM is compensated by the fabrication of arrays of thrusters. 

Each thruster could be addressed independently, or in other words, “digitally controlled micro 

thrusters.” 

• Each thruster is a chamber filled with an EM

• The heater element ignites the EM in the chamber

• The content bursts and the escaping gas delivers an impulse force

• Thousands of needle-sized thrusters can be fabricated using MEMS micromachining 

methods

Solid EM can be classified into different classes—propellants, explosives, and pyrotechnics (1).

Propellants and pyrotechnics release their energy through relatively slow deflagration processes 

(combustion). Explosives release their energy in fast detonation processes. Explosives can be 

produced by mixing of oxidizer and fuel constituents (nitroglycerine), or by mixing oxidizer 

powders (ammonium nitrate or perchlorate) and fuel powders (sulfur or carbon) to produce a 

composite black powder. But these are slower reactions due to limited mass transport of the 

reactants. 

So far, several types of EMs have been evaluated by several institutions; achievements are in the 

evolutionary stages, and systems are not reliable for micro propulsion applications. The greatest 

challenges are compatibility of EM and MEMS. A summary of the research and developments 

by the participating institutions is presented to illustrate the technology trends for micro 

propulsion applications. 

2. Trends

TRW Corporation lead a Defense Advanced Research Projects Agency (DARPA)-sponsored a 

large project (US $3.5M) in the late 1990s to investigate digital micro propulsion. The research 

was jointly conducted by TRW, the Aerospace Corporation, and the California Institute of 

Technology (2). They used a MEMS approach to assemble an array of chambers filled with a 
3

propellant (figure 1). The researchers reported that the propellant did not achieve complete 

combustion within the confines of their chambers. Test data can be summarized as the following: 

• Fuel is lead styphnate

• 1 m-s thrust duration

• 0.1 mN-s impulse

• 100 W of power 

Figure 1. Micro thruster array fabricated jointly by 

TRW and teammates Caltech and the Aerospace 

Corp. under a contract from the DARPA.

Tanner Research Inc. demonstrated high specific impulse chemical micro thruster actuators (3) 

shown in figure 2. The institution helped gain a more thorough understanding of the fundamental 

relationship between thrust and nozzle dimensions. They determined that more propellant 

volume produced more impulse. Impulses were roughly proportional to volume, and the specific 

impulse (Isp) was intrinsic to the energetic material (black powder). Isp was found to be

inversely proportional to the nozzle exit hole diameter.

Figure 2. Photograph of Tanner Research Inc. 

array of micro thrusters.

Performance and characteristics of the Tanner micro thruster: 

• Fuel black powder

• Array metal/epoxy composite

• Spacing 10 mm (center to center)
4

• Thrust pulse 4 m-s, thrust ~10 N

• Max Isp record 140 s

• Firing voltage 10 V

• Total volume 800 mm3 (per thruster including epoxy unit cell)

• Impulse/volume 0.05 mN-s/mm3

The Berkeley Sensor and Actuator Center at the University of California conducted research on 

millimeter-scale solid propellant rockets, shown in figure 3, for one-time deployment of wireless 

sensor platforms, known as the Smart Dust program. The investigators found the thermal losses 

to the silicon sidewalls were too high to reliably maintain a burn (4). Successful combustion was 

demonstrated in cylindrical alumina ceramic combustion chambers with thermal conductivities 

five times lower than silicon and cross sections of 1–8 mm2

. Micro heaters required less than 

0.5 W of power to ignite a propellant composed primarily of hydroxyl-terminated polybutadiene 

(HTPB) with ammonium perchlorate (AP) oxidizer. The researchers also added ten-junction 

thermopiles near the micro rocket flame that produced a maximum power of 20 μW in order to 

demonstrate energy harvesting to augment the Smart Dust power supply.

Figure 3. BSAC silicon wafer with DRIE combustion chambers (left), and chamber design (right).

Peformance and characteristics of the Berkeley micro thruster:

• Fuel is HTPB/AP

• Thrusts of 10–15 mN were measured for ceramic

• Specific Impulse 14 s

• Burn rate 1.7 mms–1

Tohoku University of Japan, together with the Institute of Space & Astronautical Science (5),

fabricated silicon nozzles (left) and pyrex cylinders with electrical igniters (right), shown in 

figure 4. The prototype device has an array of 10 × 10 solid propellant micro-rockets with a pitch 
5

of 1.2 mm. Boron/potassium nitrate (NAB) propellant was selected over HTPB/AP and glycidyle 

azide polymer (GAP), because of its ignition capability at atmospheric pressure and low 

temperature. NAB is used with/without lead rhodanide/potassium chlorate/nitrocellulose (RK) to 

aid in the ignition. The impulse thrust was measured using a pendulum suspended in ambient 

air. 

Figure 4. Silicon nozzles (left), and Pyrex 

cylinders with electrical igniters (right)

from Tohoku University. Pyrex cylinders 

are each approximately 1 mm in diameter.

Performance and characteristics of the Tohoku micro thruster:

• Fuel is NAB

• Ignition = 4 W with RK, and 6 W without RK

• Impulse thrusts = 2×10−5 N-s to 3×10−4 N-s 

Sandia National Laboratories investigated mesoscopic structure of EM and their response during 

ignition, growth, and detonation (6). Mesoscopic properties, such as particle size, porosity, 

heterogeneity and material interfaces, determine rates of gas transfer, heat conduction, void 

collapse, and frictional heating during ignition and growth. Pentaerythritol tetranitrate (PETN) 

films up to 40-μm thick were deposited on silicon substrates to investigate processing techniques

and determine the resolution of film patterns.

Patterning of PETN using oxygen plasma etching proved to be too harsh an environment,

causing solubility changes and physical eruptions of the film, likely due to thermal stresses in the 

film. An alternative patterning technique that has proved successful involves the use of a “liftoff” procedure in which, prior to the EM film deposition, a silicon dioxide masking layer with

openings was deposited on the surface. The openings acted as anchor points during PETN 

blanket deposition that followed. When the hydrofluoric acid (HF) solution was finally 

introduced, the underlying silicon dioxide film dissolved, leaving only the PETN anchor points 

in place. Coarse PETN features were defined using this method, but finer features in this film 

thickness range were difficult to achieve due to the lack of a continuous mass of sufficient 
6

material to support the remaining structures in the film. A representative patterned film is shown 

in figure 5. 

Figure 5. Arrays of micro patterned PETN 

explosive materials are being investigated 

by Sandia National Laboratories as 

on-chip sources of gas for actuation, locomotion, 

and acoustic signal generation. Center ball 

pattern is less than 0.5 mm in diameter.

The Laboratory for the Analysis and Architecture of Systems-National Center for Scientific 

Research (LAAS-CNRS) in France assembled solid propellant micro thruster arrays with 

reservoir dimension 1.5 mm across using Foturan or silicon materials (7). The nozzle and igniter 

parts were made by micro machining the silicon in figure 6. Ignition energy was very low—tens 

of mJ, depending on the ignition material. Two specific propellant mixtures have been optimized 

and used for the ignition process: a GAP mixed with AP and zirconium (Zr). When a GAP 

propellant is used in the reservoir, the ignition process works unreliably with the nozzle attached 

on top. A more sensitive and easily ignitable zirconium perchlorate potassium (ZPP) propellant 

has since been substituted with improved reliability (8). 

First thrust characteristics tested were: 

• Fuel is GAP-based propellant 

• Impulse force of 6 mN-s 

• Ignition Power 150 mW 
7

Figure 6. Schematic view of one single micro rocket by 

LAAS-CNRS Laboratory.

The Federal Polytechnic Institute of Lausanne Space Center (IPFL) at Lausanne, Switzerland, 

RUAG Aerospace (Emmen, Switzerland), and RUAG (Ammotec GmbH, Fürth, Germany) 

reported at a recent conference that the largest technological barriers to the development of solid 

propellant micro thruster arrays shown in figure 7 were reliable, low power ignition and 

complete combustion of the propellant within the micro cavities (9).

Figure 7. Photograph of a single solid propellant 

micro thruster by team IPFL Space 

Center, and RUAG.

The U.S. Army Research Laboratory (ARL) has developed high energetic reaction in porous

silicon (10). The motivation was on-chip integration of energetic material and MEMS using 

existing processes on silicon wafers, shown in figure 8. The silicon was etched electrochemically 

in HF to create nano pores; those pores were filled with an oxidizer such as sodium perchlorate

(NaClO4), and a heated bridge wire created a self-sustaining energetic reaction between the 

silicon and the oxidizer. Using characterization techniques during energetic reaction and infrared

absorption analysis after explosion, the principal investigators demonstrated an improved 
8

understanding of the energetic behavior. The technical challenges are reliability and 

repeatability; low-power electronic initiation; and protection from the effects of aging.

Figure 8. Photograph of a MEMS addressable 

2x2 energetic device by ARL. Each 

circle pattern is approximately 2 mm in diameter.

3. Conclusions

Promising propulsion technologies have been developed by research groups involved with arrays 

of solid propellant micro-thrusters that are able to deliver small, controlled levels of thrust. 

On the system-level there are issues with low ignition yield, and unsustained combustion in 

small dimensions (quenching); further, filling procedures are not being reproducible, and the 

propellants and igniters are not easily adapted to each other. Challenges related to small length 

scales, heat transfer, and surface tension in micro and nano EM need to be sorted out before the 

technology is ready for its intended applications. 

MEMS technology has been identified by several institutions as key to the development of future 

micro propulsion systems. 
9

4. References

1. Rossi, C.; Zhang, K.; Estève, D.; Alphonse, P.; Tailhades, P.; Vahlas, C. Nanoenergetic 

Materials for MEMS: A Review. J. Microelectromechanical Systems Aug 2007, 16 (4).

2. Lewis, D. H., Jr.; Janson, S. W.; Cohen, R. B.; Antonsson, E. K. Digital Micropropulsion.

Sensors and Actuators 2000, A80 (2), 143–154.

3. Desai, A.; Fuchs, B. Micro (MEMS) and Nanotechnologies for Defense and Security. Proc. 

of SPIE 2007, 6556, 65561A.

4. Teasdale, D.; Milanovic, V.; Chang, P.; Pister, K.S.J. Microrockets for Smart Dust. Smart 

Mater. Struct. 2001, 10, 1145–1155.

5. Tanaka, Shuji; et al. MEMS-Based Solid Propellant Rocket Array Thruster with Electrical 

Feedthroughs. Trans. Japan Soc. Aero. Space Sci. 2003, 46 (151), 47–51.

6. Tappan, A. S.; Long, G. T.; Renlund, A. M.; Kravitz, S. H. Microenergetic MaterialsMicroscale Energetic Material Processing and Testing. presented at the 41st AIAA Aerospace 

Sciences Meeting and Exhibit, Reno, NV, Jan. 6–9, 2003, Paper AIAA-2003-0242.

7. Larangot, B.; Conédéra, V.; Dubreuil, P.; Do Conto, T.; Rossi, C. Solid Propellant 

MicroThruster An Alternative Propulsion Device for Nanosatellite,” presented at Aerospace 

Energetics Equipment 2002, Avignon (France), 12–14 Nov 2002.

8. Rossi, Carole; Briand, Danick; Dumonteuil, Maxime; Camps; Thierry; Phan, Phuong 

Quyˆen; de Rooij, Nicolaas F. Matrix of 10×10 Addressed Solid Propellant 

Microthrusters:Review of the Technologies. Sensors and Actuators 2006, A 126 241–252.

9. Briand, D.; Guillot, L.; Bley, U.; Danninger, S.; Gass, V.; de Rooij, N. F. Digital MicroThrusters with Simplified Architecture and Reliable Ignition and Combustion. Proc. of 8th 

International Workshop on Micro and NanoTechnology for Power Generation and Energy 

Conversion Applications, Sendai, Japan, November 9–12, 2008, pp. 157–160.

10. Churaman, W.; Currano, L., Singh; A. Kumar; Rai, U. S.; Dubey, M.; Amirtharaj, P.; Ray, P. 

C. Understanding the High Energetic Behavior of Nano-energetic Porous Silicon,”

Chemical Physics Letters October 2008, 464 (4–6).
10

List of Symbols, Abbreviations, and Acronyms 

AP ammonium perchlorate 

ARL U.S. Army Research Laboratory 

DARPA Defense Advanced Research Projects Agency 

EM energetic materials 

GAP glycidyle azide polymer 

HF hydrofluoric acid 

HTPB hydroxyl-terminated polybutadiene 

IPFL Federal Polytechnic Institute of Lausanne Space Center 

LAAS-CNRS Laboratory for the Analysis and Architecture of Systems-National Center 

for Scientific Research 

MEMS microelectromechanical systems 

NaClO4 sodium perchlorate 

NAB boron/potassium nitrate 

PETN Pentaerythritol tetranitrate 

RK lead rhodanide/potassium chlorate/nitrocellulose 

Zr zirconium 

ZPP zirconium perchlorate potassium 
11

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