8 MICROPROPULSON
Andrew D. Ketsdever
Propulsion Directorate, United States Air Force Research Laboratory, Edwards AFB, CA, USA
KEYWORDS: Micropropulsion, propulsion, microspacecraft, microscale, system specific impulse.
8.1 Introduction
In its broadest definition, a micropropulsion system is one capable of producing a change in momentum of a microspacecraft. (Ketsdever, 2000) The United States Air Force Research Laboratory (AFRL) defines a microspacecraft as having a total mass of less than 100 kg. This broad definition of micropropulsion allows the inclusion of a wide range of concepts ranging from microelectromechanical systems (MEMS) fabricated thrusters with micrometer characteristic sizes to simply scaled-down versions of macro-scale thrusters. Regardless of the characteristic size of a particular micropropulsion system, severe design constraints will be imposed by resource limited microspacecraft, including:
· Mass
· Volume
· Power
· Maximum voltage
Micropropulsion concepts will not only have to address microspacecraft mission requirements, but will also have to fit within the systems constraints imposed. Although this is true of all spacecraft subsystems regardless of spacecraft mission or size, the constraints of some microspacecraft may pose such severe design restraints that new paradigms will need to be developed. A complication to the design process for micropropulsion systems will be the fact that, in many cases, microspacecraft missions will require exceedingly low thrust levels, accurate and very low minimum impulse bits, and low thrust noise. Also, many microspacecraft missions currently under investigation require relatively large DV, indicating that high specific impulse micropropulsion systems are necessary to perform these missions under strict mass constraints.
A wide array of micropropulsion systems have been investigated, designed, built, and even flown. As with larger spacecraft propulsion systems, these micropropulsion concepts incorporate both chemical and electric means of producing high speed propellant species to create thrust. Both solid and liquid chemical thrusters have been developed for use on microspacecraft missions where relatively large thrust levels are required. Electric propulsion thrusters including micro-electrothermal, electrostatic, and electromagnetic systems have also been investigated for missions where higher specific impulse is desired. Electrothermal thrusters use electrical means to heat a propellant, which include arcjets [See 2.4.02 Arcjets] and resistojets. Electrostatic thrusters include a class of propulsion systems that use electrostatic means to accelerate ions formed in a plasma discharge [See 2.4.05 Ion Thrusters]. Electromagnetic thrusters achieve acceleration of plasma propellants through a combined interaction between electric and magnetic fields [See 2.4.03 Hall Thrusters and 2.4.04 Magnetoplasmadynamic Thrusters]. A comprehensive survey of micropropulsion systems under development has been compiled by Mueller (2000). Each system brings unique design challenges including:
· Microscale combustion, heat transfer, and fluid dynamics
· Microscale plasma formation
· Material compatibility
· Micro-valve leakage
· Passage clogging
· System reliability and lifetime
· Manufacturing and integration complexities
Many of these design challenges will be addressed in the following sections.
Micropropulsion is considered an enabling technology for microspacecraft by making possible missions which otherwise could not be performed. However, micropropulsion could also find application on larger spacecraft to compensate for very small disturbing torques (e.g. solar pressure), providing extremely fine attitude control. Several propulsive requirements for potential missions are given in the following section. Reducing the physical size, operating power, and in some cases operating pressure and temperature, may lead to significant reductions in operational performance and efficiency. For example, a microchemical propulsion system will require high pressure operation for efficient combustion at small physical scales. However, a particular microspacecraft mission may require low thrust levels indicating that a micronozzle must be employed with a small throat diameter. High temperatures along with small nozzle throat diameters will lead to a reduction in the operational Reynolds number, leading to significant viscous losses. High temperature microchemical systems will also suffer from high heat transfer rates which will further reduce the efficiency of these systems, particularly in MEMS fabricated thrusters where the thermal conductivity of silicon-based materials is quite high. On a positive note, there is significant potential for improvement in all manner of micropropulsion systems, leaving future micropropulsion developers many fruitful areas of basic research and applied engineering practice.
8.2 Sample Microspacecraft Missions and Requirements
Constellations or platoons of microsatellites working in collaboration have been envisioned to perform the functions of larger spacecraft in a distributed way. Microspacecraft constellations will benefit from increased survivability, flexibility in mission performance, distributed functionality, and more graceful degradation of overall capability. Partitioning the functions of a single large spacecraft into a number of smaller satellites operating cooperatively in close proximity or in widely dispersed orbits could lead to significant improvement in mission performance. In some cases, this approach enables missions that cannot be performed by a single satellite of any size. AFRL has proposed a standard definition of spacecraft with masses below 100kg as detailed in Table 1.
By the AFRL definition, the first microspacecraft launched into orbit from Earth was also the first man-made object launched, Sputnik. Sputnik had a mass of approximately 84 kg. Figure 1 shows Sputnik I, which was launched on October 4, 1957. Since the late 1950’s, trends in electronics, material sciences, and general mechanics have led to advances in micro-scale components. This has led to a significant increase in the performance of micro-scale systems which have been used on microspacecraft, making today’s small spacecraft far more capable then their early predecessors. Today, universities are capable of launching very sophisticated microsatellites. (Sweeting, 2002) Sputnik was a fairly simple radio transmitter launched into orbit on a converted SS-6 nuclear missile, while today microsatellites with sophisticated communication, attitude control, and propulsion systems are being designed, developed, and launched.
Table 1: AFRL satellite classification
Total Spacecraft Mass / Description10-100 kg / Microspacecraft
1-10 kg / Nanospacecraft
<1 kg / Picospacecraft
Figure 2 shows FalconSat 3 designed and developed by the United States Air Force Academy (USAFA). FalconSat 3 was launched in March, 2007 as part of the secondary payload program for the Evolved Expendable Launch Vehicles (EELVs). Fifty years after the launch of Sputnik, a highly capable microsatellite with a mass of approximately 50 kg had payloads which demonstrated an advanced micropropulsion attitude control system and an ability to characterize the ionosphere and local spacecraft plasma environment.
Figure 1: Sputnik I launched October 4, 1957 by the Soviet Union (Photo courtesy of NASA).
Figure 2: FalconSat 3 launched March 8, 2007 (50 years after Sputnik I) by the United States Air Force Academy (Photo courtesy of the Department of Astronautics, USAFA).
Several studies (Janson et al., 1999; Mueller, 2000; Schilling, Spores and Spanjers, 2000; Ziemer et al., 2008) have provided propulsive requirements for many microspacecraft missions including Earth observation, communications, space environmental sciences, satellite servicing, satellite inspection, and technology demonstration. For the proposed TechSat21 mission, which has since been canceled by AFRL but is still a good example of microspacecraft propulsive requirements, the proposed total DV was 390 m/s of which 50 m/s was required for orbit raising, 20 m/s for drag makeup, 200 m/s for stationkeeping over a 10 year mission lifetime, and 120 m/s for a deorbit maneuver at the spacecraft’s end of life (EOL).(Schilling, Spores and Spanjers, 2000) A minimum impulse bit of 2mN-s was required for close proximity maneuvers. An early version of the TechSat 21 experiment called for formation flying maneuvers over a 30 day period with a DV requirement of 30 m/s and a minimum thrust level of 2mN. This would have allowed for a series of formations to be flown with separation distances between 5 m and 5 km. Mueller (2000) also gives a range of propulsive requirements for microspacecraft slew maneuvers with minimum impulse bits ranging from 10-8 to 10-3 N-s.
Janson et al. (1999) gives a good overview of potential micropropulsion maneuvers and the DV required. The DV requirement for a microspacecraft to perform a minor altitude raising maneuver of 1 km at an initial altitude of 700 km was calculated to be about 0.5 m/s for a 10 minute firing time. For North-South stationkeeping in GEO for approximately 1 week duration the DV requirement is over 300 m/s. A deorbit burn at an initial altitude of 700 km requires a DV of over 330 m/s for a low thrust spiral maneuver over a 1 week period. On-orbit servicing or inspection missions may require up to 500 m/s of DV for orbit changing and attitude control. Many microspacecraft missions currently under consideration will have relatively large DV requirements, indicating that the specific impulse and efficiency of micropropulsion systems will need to be correspondingly large. However, a large number of microspacecraft missions will also require a rapid response to changing conditions to be effective. These missions will require relatively large thrust levels, perhaps as large as 100’s of mN. As with their larger counterparts, micropropulsion systems will continue to seek high thrust, high specific impulse solutions.
The Laser Interferometer Space Antenna (LISA) mission will require precision formation flying to detect gravitational waves produced by processes in the universe. To perform this mission and the precursor LISA Pathfinder mission, a series of propulsive requirements have been derived. Thrust levels from 5 to 30 mN with precision better than 0.1 mN have been demonstrated with colloid and field emission thrusters. (Ziemer et al., 2008; Scharlemann et al., 2008) Thrust noise levels less than 0.1 mN/Hz1/2 are also required, making the LISA mission one of the most stringent in terms of micropropulsion requirements. LISA Pathfinder will fly both the LISA Test Package (LTP) and the Space Technology 7 (ST7) payloads which contain FEEP and colloid propulsion respectively. Although this is not an exhaustive list of propulsive requirements for microspacecraft missions, it is intended to give a broad scope and range of requirements that will need to be met by future micropropulsion systems.
8.3 Micropropulsion System Design Considerations
As previously mentioned, there is a wide ranging definition for micropropulsion which includes concepts from scaled down versions of larger-scale thrusters to specifically designed, MEMS fabricated propulsion systems. This and following sections will investigate some of the design considerations required to successfully scale down existing thrusters and some of the techniques for fabricating unique MEMS designs.
Design issues for micropropulsion systems have arisen from scaling issues, the use of corrosive propellants with new materials, contaminant handling in propellants, limitations in MEMS fabrication techniques, thermal cycling, and environmental interactions. In general, micropropulsion systems will experience decreased performance over their larger-scale counterparts due to losses associated with small characteristic sizes, limitations on system mass and power, and the lagging development of materials and micromachining techniques for propulsion specific applications. Engineers must address these issues by making use of novel approaches that use small-scale properties to the overall system’s benefit. With this in mind, careful attention should be paid to the characteristics that scale favorable with reduced size. These characteristics hold the key to the design of efficient micropropulsion systems.
8.3.1 General Design Considerations
A complication for microsystems of all types is the fact that as a typical device is scaled down, the volume goes with the characteristic dimension, L, cubed whereas the surface area only goes with L2. Therefore, microsystems tend to have large surface area to volume ratios relative to their larger scale counterparts. This has consequences for heat transfer and gradients of temperature, velocity and species. In the heat transfer case, the heat loss in the system will increase as the characteristic length scale is reduced. For example in a 1-D heat transfer case, the energy transferred per unit area through conduction is given by Fourier’s Law as
Q=-kdTdx (1)
where k is the thermal conductivity of the material. For microscale systems, the value of dx will be small while the device area to volume ratio remains relatively large. This indicates from Eq. (1) that the amount of heat transfer in microscale devices can be relatively large. Another way to look at this situation, is that large thermal gradients in small scale devices will not be possible unless very low thermal conductivity materials can be identified and used. For systems (such as propulsion) that rely on temperature gradients for efficiency, heat transfer issues will become increasingly important as the characteristic size of the device decreases.
Heat conduction through relatively thin material will generally produce uniform system temperatures and the thermal response time of a typical microstructure will be low. Heat conduction is complicated by MEMS fabrication in silicon substrates with thermal conductivities that are generally higher than many aluminum alloys. Alexeenko et al. (2005) showed in a coupled thermal-fluid simulation that a MEMS fabricated silicon thruster reached a uniform temperature, from the combustion chamber to the nozzle exit plane, almost instantaneously. In most cases, active cooling of the silicon microthruster was required for survivability. In micronozzle flows, the maximum heat transfer occurs at the nozzle throat (minimum area). The heat transfer rate at the throat varies as Re0.8/D where Re is the flow Reynolds number and D is the nozzle throat diameter. Therefore, in micronozzle flows, the amount of heat transfer between the gas and nozzle walls is a competition between the characteristic size, D, and the viscous losses in the nozzle governed by the Reynolds number.
8.3.2 MEMS Fabrication Techniques
MEMS encompasses a wide range of systems that include the integration of mechanical components, sensors, actuators, and electronics onto a common silicon substrate.(Gad-el Hak, 2002) Generally, MEMS devices have characteristic dimensions of less than 1mm and more than 1mm. MEMS fabrication techniques include silicon micromachining through chemical etching or electro-discharge, lithography, molding, and thin-film deposition. Micromachining selectively etches parts of a silicon wafer whereas thin-film deposition can add new layers all in an attempt to form mechanical and electronic devices. Today, nanoelectromechanical systems (NEMS) processes are being developed which could lead to further breakthroughs in propulsion system design. Although many of these fabrication techniques are currently limited, the development of these areas for terrestrial applications will continue to have a strong impact for propulsion system engineers. An excellent review of these technologies and their applications can be found Gad-el Hak (2002).