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Leonardo Times MARCH 2013T
he constant increase in number ofsmall satellites launches during the last decade has led to a consequent increment of research activities on miniaturization of satellite subsystems. This is one of the rea-sons why “miniaturization” is among the 2012-2020 focus areas of TU Delft’s Faculty of Aerospace Engineering, together with “green aviation” and “planetary explora-tion”.
However, due to limited cost and mass budgets, micro- and nano-satellites usu-ally lack a propulsion capability, therefore limiting their lifetime and performance. The need for high-performance, highly miniaturized propulsion systems for small satellites is explicitly stated in the tech-nology roadmaps prepared by the main space policy makers, including NASA1 and
ESA2. To enhance their performance, the
next generation of small satellites will re-quire extremely miniaturized and highly integrated propulsion systems capable to meet stringent mass, power and volume constraints. In particular, for nano-satellite
applications, such propulsion systems shall be small, lightweight, and capable of delivering very low thrust levels (in the order of magnitude of μN up to a few mN) with a limited power consumption (possi-bly lower than 1W).
In spite of these clear needs, there are still few ongoing activities in Europe on micro-propulsion, especially if compared to what can be seen in the United States and in Japan. In the Netherlands, TNO is active in the development of CubeSat propulsion systems based on a propri-etary solid propellant cool gas generator technology. This research led to the de-velopment, in collaboration with TU Delft and the University of Twente, of the T3μPS
micro-propulsion system3. The system will
be demonstrated on board of Delfi-n3Xt, the nano-satellite presently under devel-opment at the Space Systems Engineer-ing chair4, scheduled for launch in 2013.
The T3μPS is however a cold-gas
propul-sion system, with a limited performance in terms of specific impulse. Heating the
propellant to a higher temperature will lead to significant improvements in terms of characteristic velocity, specific impulse and general propulsion performance. Improvements towards this direction are expected from the silicon-based MEMS resistojet design recently proposed by our group5. This enhanced propulsion system
has been demonstrated to be more prom-ising in terms of general performance than other options at higher specific impulse levels (such as ion and plasma thrusters), especially for missions requiring a total ve-locity change Δv lower than 200m/s. The micro-resistojet technology offers a series of exciting advantages, including: high thrust-to-power ratio, low system specific mass, an intrinsically uncharged plume and the possibility of using a wide variety of propellants. Further steps are however needed in order to investigate and even-tually improve the actual feasibility of the system, presently demonstrated only at a prototype level.
Challenges towards future nano-satellite projects
The Space Systems Engineering department at the Faculty of Aerospace Engineering
is hard at work miniaturizing satellite subsystems to accommodate the growing use
of small satellites. One of the challenges the department has taken on is research into
micro-propulsion. The limitations that come with small satellites make research in
this field such a challenging adventure. An inside look at micro-propulsion research
shows precisely which challenges are faced and which projects are underway.
TEXT Angelo Cervone, Barry Zandbergen, Jasper Bouwmeester and Jian Guo, Space Systems Engineering TU Delft
MICRO-PROPULSION RESEARCH
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THE DELFFI PROJECT
The QB50 program is an international mis-sion under the leadership of Von Karman Institute in Belgium, partly supported by the European Union. The mission is intended to establish a network of fi fty nano-satellites developed by diff erent in-stitutions worldwide, with a range of mis-sion objectives ranging from multi-point measurements in the lower thermosphere to re-entry research.
The Space Systems Engineering chair will contribute to QB50 by means of two satellites, as space segment for the so-called DelFFi project6. The main objective
of DelFFi is an autonomous formation fl ying demonstration between the two satellites, using innovative concepts for their navigation, guidance and control. This is a very challenging objective, since it has never been demonstrated so far with spacecrafts of such a small size. Two almost identical triple-unit CubeSats (Del-ta and Phi) will be used, with an in-orbit
demonstration payload constituted by a micro-propulsion system (for controlling the relative motion of the two satellites) and a radio-frequency navigation sensor (for relative ranging and inter-satellite communications). The formation fl ying experiment will allow the two satellites to fl y at a controlled along-track separation of about 1000km, with a control window of 100km kept with an accuracy of 10km. The relative navigation accuracy required for this scenario is about 1km. The velocity change for maintaining the baseline dis-tance between the satellites, in the low al-titude orbit foreseen for QB50, is expected to be about 0.1m/s per day. Considering a thirty-days formation fl ying mission and accounting for maneuvers, contingencies and margins, a total velocity change of 6.3m/s per satellite is expected to be re-quired.
The mass budget allocated to the in-orbit demonstration payload is 420g per satel-lite, out of a total satellite mass of 3600g.
In particular, a mass of 330g is available for the propulsion system, with a
maxi-mum available volume of 3x10x10cm3.
The power budget allocated to the in-orbit demonstration payload is 290mW per satellite: 100mW are required by the radio-frequency navigation sensor, while an average of 190mW are allocated to the propulsion system. Taking into account the general project requirements, the torque disturbances and the possible mis-alignment and assembly errors, the thrust per satellite can be estimated to be 2mN as a maximum. On the other hand, duty cycle considerations lead to a minimum allowable thrust per satellite of about 0.14mN. Due to the presence of a deploy-able array the maximum allowed accelera-tion is relatively low, 2m/s2.
DELFFI REQUIRES ADVANCED MICRO-PROPULSION
The present candidate propulsion sys-tem for DelFFi is the T3μPS. It consists of a
Along-track separation of satellites 1000km Total satellite mass 3600g
Control window on separation 100km Propulsion payload allocated mass 330g
Accuracy on control window 10km Propulsion payload allocated volume 3x10x10cm3
Relative navigation accuracy 1km Propulsion payload allocated power 190mW
Total mission Dv (baseline) 6.3m/s Thrust range 0.14÷2mN
Total mission Dv (extended mission) 20m/s Maximum allowed acceleration 2m/s2
Table 1. Key requirements and constraints of DelFFi. Figure 1. Cold gas propulsion board
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Leonardo Times MARCH 2013Printed Circuit Board (PCB) and a plenum chamber in which the cool gas genera-tors, the thruster and the thrust valve are mounted. The propellant is gaseous nitro-gen, produced by the cool gas generators and then accelerated in the nozzle with-out any pre-heating. In 2010, a qualifi ca-tion model of the system has been suc-cessfully tested in vacuum at TU Delft. In this model, each cool gas generator was loaded with an amount of usable propel-lant equal to 0.125g, producing 0.1 normal liters of gaseous N2. A total of 8 cool gas
generators were present in the model, for a total system mass of 140 g. The reported vacuum specifi c impulse was 68s. The fol-lowing step in the fl ight qualifi cation pro-cess will be a demonstration of the model on board of Delfi -n3Xt.
A further development of the cool gas generator units, presently ongoing at TNO, will allow for scaling them up to a mass of 16.3g, of which 4.21g are usable propellant. This increased propellant-to-mass ratio will lead, in turn, to a signifi cant performance improvement.
The MEMS resistojet system recently dem-onstrated by our team includes an inlet manifold, a heater section and a nozzle. Also in this case, the propellant is gaseous nitrogen. The gas is heated by an integrat-ed thin-fi lm heater made of aluminium, and elevated temperatures at the fl uidic channel walls are ensured by using high
thermal conductivity silicon wafers. A sin-gle thruster unit has a dry mass as low as 162mg (excluding the propellant and the storage tank). The estimated vacuum spe-cifi c impulse is 73s in the cold-gas mode, and can be increased up to 104s when the propellant is heated to 327°C. The nozzle throat width can be chosen among two values, 10μm and 5μm. A thrust of 0.38mN per thruster unit can be obtained with an available heating power of 190mW. In a paper recently presented at the IAC 2012 Conference in Naples, we have dem-onstrated how advantageous the MEMS resistojets technology can be if applied to the DelFFi satellites7. The total estimated
mass of the satellite propulsion system, including propellant, is 271g if the T3μPS
cold-gas system is used, but can become as low as 100g if this is replaced by a mi-cro-resistojet. Thus, by using a resistojet system, a total of 171g are saved - almost half of the allocated payload mass! This saved mass can be used for additional payload or for upgrades of the other sub-systems, thus increasing the general mis-sion capabilities. In order to achieve this result, however, it is necessary to combine the benefi cial aspects of the resistojet design with a high-density propellant storage system such as the TNO cool gas generator units.
It is clear that DelFFi, as well as the other future fl agship nano-satellite missions
planned by TU Delft, will require an ad-vanced micro-propulsion concept. Our group at Space Systems Engineering is therefore planning to increase the re-search activities in this fi eld in the upcom-ing years, and several challengupcom-ing MSc thesis topics are expected to be off ered to TU Delft students from 2013.
WHAT CHALLENGES ARE WE FACING?
In resistojets, when gaseous nitrogen is used as propellant and heated at a tem-perature in the order of 300°C, a specifi c impulse of about 100s can be reached. However, this value can be increased up to 600-1000 seconds by either increasing the heating temperature or changing the propellant (hydrogen, water). But what are the main scientifi c and technological challenges associated with the develop-ment and the qualifi cation of this kind of propulsion device at a micro-scale?
• With the typical nozzle size required to achieve thrust levels in the order of micro-Newtons, large viscous losses are pres-ent at the nozzle throat. This is caused by the low Reynolds numbers (typically less than 500) and the formation of a laminar boundary layer at a micro scale. Alterna-tive geometries and materials need to be analysed to minimize the impact of this problem.
• Traditional component interfacing techniques show functional limitations
µ
°
Table 2. Comparison between the T3μPS cold-gas system and the MEMS resistojet equipped with cool gas generator units, when applied to one of the
DelFFi satellites (total mission Δv = 6.3m/s). Figure 2. Resistojet prototype
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when used at small scale, and are therefore not suitable to micro-propulsion systems. Novel ways to join and interface minia-turized components shall be identifi ed, based on new gluing/bonding techniques and advanced non-silicon materials.
• Resistojets require a signifi cant amount of power for heating the propel-lant: about 500mW per mN of thrust, for the prototypes tested so far at TU Delft. This is an important problem if we think of the strict power requirements of typi-cal nano-satellites. Alternative heating technologies, based for instance on thin-fi lm heater chips, are required to minimize heat losses and achieve higher tempera-tures.
• The maximum temperature
at which the propellant is heated can be further increased by investigating inno-vative materials. A compromise needs to be found between temperature and me-chanical strength. High thermal conduc-tivity materials such as silicon are good candidates in terms of heat exchange properties, but cannot typically withstand high temperatures. Alumina has better thermal and chemical properties, but its machining at a small scale is very diffi cult. Low-temperature co-fi red ceramic tapes, generally used for packaging electronic devices, have been recently proposed for micro-fl uidic components; however, ef-forts are still needed to reduce the costs of their production and machining tech-niques.
• Higher temperatures, when
com-bined with the small size of miniaturized components, lead to large pressure and temperature gradients and thus to high mechanical stresses and an increased risk of failures. A compromise needs to be found between performance and reli-ability.
On top of these technical challenges there are also several critical aspects at a system level, for instance: design and validation of adequate facilities for testing micro-propulsion systems, preparation of a complete verifi cation & validation plan, in-tegration of the system into the actual sat-ellite, and fi nal in-fl ight qualifi cation. All the design activities shall of course take into account the lessons that we will learn from the in-orbit performance analysis of the T3μPS system on Delfi -n3Xt.
CONCLUSIONS
Our research team at Space Systems Engi-neering is continuously working to push the limits towards new frontiers of satel-lite miniaturization. Our future fl agship nano-satellite projects, such as DelFFi, will require exciting research eff orts for achieving the technological improve-ments needed to accomplish their ambi-tious goals. Motivated students are always welcome to join and share with us the emotions of this exciting adventure. If interested, contact us via the e-mail address A.Cervone@tudelft.nl or B.T.C.Zandbergen@ tudelft.nl.
References
1 Johnson, L., et al., NASA Technology Area Roadmap for In Space Propulsion Technologies, 2010
2 ESA, European Space Technology Master Plan , Issue 6, 2008
3 Moerel, J.L.P.A., et al., Development of Micro-Propulsion System Technologies for Minisatellites in the Netherlands, 5th
International Space Propulsion Confer-ence, Heraklion, Greece, 2008
4 Bouwmeester, J., et al., Design Status of the Delfi -Next Nanosatellite Project, 61st International Astronautical
Con-gress, Prague, Czech Republic, 2010 5 Tittu Varghese, M., et al., A Silicon-Based MEMS Resistojet for Propelling Cubesats, 62nd International
Astronauti-cal Congress, Cape Town, South Africa, 2011
6 Gill, E., et al., Formation Flying within a Constellation of Nano-Satellites: The QB50 Mission, Acta Astronautica, Else-vier Science Ltd., j.actaastro.2012.04.029, 2012
7 Cervone, A., et al., Application of an Advanced Micro-Propulsion System to the DelFFi Formation-Flying Demon-stration Within the QB50 Mission, 63rd
International Astronautical Congress, Naples, Italy, 2012
