The current NASA Chief Technologist Mason Peck had a paper describing a way to give cubesats over 2000 miles per hour of velocity change from one kilogram of water to go to the moon or near earth asteroids.
As CubeSats grow in both numbers and capability, the need to extend their reach with integrated propulsion systems is becoming clear. A water-electrolysis propulsion system for 3U CubeSats is proposed that could fill the gap in the available propulsion systems at this scale. Combining the advantages of electric propulsion with those of chemical rockets, the system is safe both to handle and to launch; it is lightweight, and it is capable of providing roughly 1000 meter per second, enough delta V to reach lunar orbit from GTO. The efficiency of the proposed technology is at least 75% for Cornell’s prototype system, consisting of Ni electrodes and 0.5 M KOH as electrolyte. With over 1 km/s of ΔV from 1 kg of water as propellant, sample missions include compensating for drag, orbit raising and lunar exploration.
Electrolysis propulsion system for a 3U CubeSat. The water tanks (A) store propellant and generate H2 and O2 through electrolysis using power from solar cells (B). The gases are combusted in the chamber (C) and expanded through a nozzle (D) to generate thrust. The spacecraft spins passively about an axis parallel to the thrust direction. For clarity, some solar cells are shown only as outlines.
The original standard specified cube satellites with a volume of 1 liter and dimensions of 0.1 m per side (a 1U CubeSat). However, many recently developed satellites have the size of two (2U) or three (3U) of the original satellite specification.
For the most part, CubeSat missions have been confined to low Earth orbit (LEO), their small volume and mass precluding the use of any sort of propulsion system. New, more complex missions will be possible if these small, affordable satellites can perform significant orbit raising. Previously implemented propulsion systems at the CubeSat scale have been low specific-impulse technologies. For example, the CanX-2 mission, a 3U CubeSat, flew a liquid sulfur hexafluoride cold-gas thruster, which attained an Isp of 50 s and a total ΔV of 2 m/s.
Miniaturized solid-propellant rockets barely fit in a 3U CubeSat, with little space for payload or other hardware. ATK’s Star 3 motor, with a diameter of 8 cm, a length of 29 cm, and a loaded mass of 1.16 kg, can provide a 4 kg satellite with just over 330 m/s of ΔV. Solid rockets are also difficult to throttle and usually expend all their propellant at once. Such propulsion systems severely limit the type of orbital maneuvers possible, precluding multiple burns.
New developments in electric propulsion will soon make a small hall thruster, ion engine, or similar system viable, with the constraint that enough propellant for a significant ΔV will require heavy pressure vessels to contain the gaseous propellant. Since in most cases not enough power is available to operate an electric propulsion engine continuously, a burst-operation strategy must be used, where the system is on for only a fraction of the orbit. Throughout the rest of the orbit, the spacecraft uses its solar panels to charge batteries. When this operations concept is taken into account, the total mass of the propulsion system, batteries, power distribution system and propellant storage tanks can in many cases become restrictive for CubeSat applications.
Electrolysis propulsion works by capturing energy from the sun and converting this energy into chemical energy which can be used to propel the spacecraft. The first step in the process is to convert the energy from the sun into electrical energy using photovoltaic cells. The electricity generated is used to electrolyze liquid water into hydrogen and oxygen gas. Water is a good propellant choice because, among other useful properties, it has a high enthalpy of formation per unit mass, 15.86 kJ/g.
The technology described here is based on a 3U CubeSat that spins about its maximum axis of inertia during orbit raising, which provides passive attitude stability and momentum stiffness for managing the effect of thrust-induced torque. The spin also virtually eliminates thrust-direction errors due to mechanical misalignment. At least as important, these kinematics provide a spin field that conveniently collects the electrolyzed gases, due to centrifugal effects, at the center of the spacecraft. They are then passed into a combustion chamber when enough gas has been electrolyzed. The flight software then commands a spark that combusts the gas mixture, which expands through a small nozzle located approximately on the spin axis, generating thrust.
There are several possible modes of operation, depending on the mission. The most likely is for the satellite to electrolyze water while it is in sunlight and then combust the gases only once per orbit. This approach helps ensure that enough gas has accumulated for a successful firing, and the satellite’s orbit can be raised efficiently, with thruster impulses only at perigee (for example). The process repeats, each time combusting about 1.5 grams of water in the case of a 90 minute LEO orbit.
For this study, it is assumed that the solar cells are at least 30% efficient in converting solar energy to electrical energy and that the satellite is a 3U CubeSat with solar cells on all faces. Deployable solar panels are not considered, although these would certainly increase the power available significantly. The solar cells supply power to a set of electrolyzers, which convert the electrical energy to chemical energy by splitting the liquid water from the fuel tank into hydrogen and oxygen gas.
A propulsion system capable of imparting CubeSats with hundreds of meters per second of ΔV can transform the way CubeSats are viewed and used. No longer will CubeSats be seen only as testbeds for microgravity experiments or neat educational projects. Instead, with a capable, safe and reliable propulsion system, CubeSats will become devices for exploration. Electrolysis propulsion systems allow university researchers to develop and launch them safely and provide CubeSats with much more ΔV than any CubeSat technology to date.
The development of an electrolysis propulsion system for CubeSats still requires several key technology-development activities. The first and possibly one of the most critical aspects of this is to test the lifetime of the electrolysis mechanisms. As described above, efficiency tests have been run that show that the process is feasible and yields sufficient gas in reasonable amounts of time. However, it is unclear how the nickel electrodes will corrode in extended electrolysis operation. Over the lifetime of the propulsion system, and as the water in the water tank is exhausted, it must be clear that the efficiency is not significantly reduced by corrosion, operation in the gas bubble, or extended use.
Several tests were run in which electrodes were operated in a manner similar to the one described in section II, but at much higher voltage, for 120 continuous minutes. In these tests increased voltage led to very fast corrosion of the electrodes. However, it is not clear that this is true if the electrodes are operated near their optimal, much lower, voltage. In this circumstance, the nickel in the electrodes is far less likely to participate in the reaction and will therefore not corrode significantly. In the electrolysis tests run at lower voltage, no significant corrosion was observed, despite cumulative operation time comparable to the tests at higher voltage. However, more work needs to be done to assure that the electrolyzers will survive for the lifetime of the mission without critical corrosion.
The second aspect on which future work will focus is the performance testing of a prototype integrated propulsion system. The prototype will include the electrolysis mechanism as well as the combustion chamber and nozzle. Testing the system in a vacuum chamber will lead to measurements of the impulse per burst, the specific impulse, and the thrust time-history for the prototype system. These tests are expected to occur in late fall of 2011.
Missions and Applications for Electrolysis Propulsion Systems
Once CubeSats are enabled with propulsion systems that are reliable, safe to work with, and provide sufficient ΔV for relevant orbital maneuvers, the possibilities for missions increase greatly. The cost of a lunar mission, an earth escape mission or even a LEO to GTO mission decreases considerably if the whole spacecraft can conform to a CubeSat standard. Including an electrolysis propulsion system among the off-the-shelf tools available to researchers and CubeSat designers will be an important step in the low-cost, private exploration of space. While an electrolysis propulsion system is an enabling technology open to many possible missions, a few of them are described here.
A. Drag Compensation
CubeSats are usually launched to low Earth orbit and therefore have a lifespan limited by aerodynamic drag. A CubeSat with an integrated propulsion system would be able to extend that lifetime and allow the CubeSat to maintain its altitude for longer so that it may continue normal-mode operations. An electrolysis propulsion system would be particularly well-suited for this type of mission. The power with which it operates can easily be varied. So, the propulsion system can provide just enough ΔV for maintaining altitude while accommodating the payload’s power requirements.
An electrolysis propulsion system would be able to extend the lifetime of a satellite in a 250 km altitude orbit by about six months at worst, even more if solar activity is low. As the orbital altitude increases (and drag losses decrease), the propulsion system would provide even longer lifetime. At 350 km, the propulsion system would increase the lifetime by at least 3.5 years. This mode of operation can be very useful for low-budget Earth-observation missions where data is needed over a long period of time or when a constant altitude is necessary.
B. Orbit Raising and Lunar Exploration
The full capabilities of an electrolysis propulsion system become apparent when it is used to continually operate at full power in order to increase the altitude of the satellite’s orbit. In this mode of operation, the satellite would electrolyze water the entire time it is in sunlight. Each time the satellite crosses perigee, the system would fire once, raising the satellite’s orbit slightly. Repeated firings at perigee would further raise the satellite’s orbit. In this modality, the satellite would be able to achieve between 1.4 and 1.85 m/s of ΔV per orbit. The total ΔV provided by 1 kg of propellant is 1070 m/s. So, the satellite would be able to raise its orbit significantly. For example, from a 300 km LEO, the satellite would be able to raise its apogee to an altitude of 5550 km in under 670 orbits. This maneuver would take the spacecraft 55 days to complete if it fires once every orbit. If the initial orbit is a geostationary transfer orbit (GTO), then the spacecraft would have enough ΔV to achieve Earth escape.
Perhaps more interesting than LEO orbit raising is the prospect of sending a CubeSat to anther celestial body. The moon would be a prime target, and an electrolysis propulsion system would have just enough ΔV to make it to the moon. To do so, the spacecraft would have to use a low energy transfer, since a traditional patched-conics approach would require too much ΔV in too little time for the current design. A low energy transfer, also known as a Weak Stability Boundary transfer, uses the four-body dynamics of the Sun-Earth-Moon-spacecraft system in order to save significant amounts of propellant. However, the spacecraft would take longer to reach its destination than it would using a traditional transfer. In the context of electrolysis propulsion, however, a longer time of transfer is not necessarily a drawback, as the spacecraft needs the extra time to electrolyze enough water into component gases. Without time spent in eclipse, the spacecraft would be able to electrolyze all of its water in a little over 20 days. A typical Weak Stability Boundary transfer between the Earth and Moon takes 138 days and requires 695 m/s of ΔV after the initial burn in low Earth orbit provided by a launch vehicle. It is therefore within the capabilities of the electrolysis propulsion system to provide enough ΔV in the required time to perform a lunar insertion and circularization maneuver. Such a maneuver might even motive a less-capable power subsystem, saving the cost of some solar cells, since they’re not necessary for propulsion. If the desired orbit is one that is not circular, the required ΔV would be even lower.
Near Earth Asteroid/Lunar Explorer
An electrolysis propulsion system has the ability to be on standby for months and return to operation quickly. A lack of volatile or explosive components adds to the safety and reliability of the system. An ideal application for such a propulsion system would be for the spacecraft to remain parked at a high LEO or GTO, where the orbital decay rate is small, and wait for a target of opportunity. Such a target could be a Near Earth Asteroid (NEA), which a propulsion-enabled CubeSat could photograph closely as well as deliver a payload of dozens of tiny ChipSats. 16
NEAs are usually identified only months before they pass near the Earth and, in some cases, can pass within 36000 km of Earth (the GEO belt). Near Earth Asteroids frequently make close approaches to Earth. As of this writing, in 2011 there have been two NEAs that have passed within 20000 km of Earth (2011 CQ1 and 2011 MD), both within 6 degrees of the ecliptic. A CubeSat in a GTO orbit would have time on the order of a few months to reposition itself to the proper orbit. Once there, the ChipSats would be deployed directly in the path of the NEA. These would impact the asteroid and send back data from their onboard sensors.
The NEA mission described above can be seen as a precursor to a lunar exploration mission. For the latter mission, the CubeSat would use the electrolysis propulsion system to attain a lunar impact trajectory. To minimize onboard propellant requirements, the spacecraft would follow a Weak Stability Boundary trajectory. A lunar insertion from this orbit leads to a highly eccentric orbit, which would require a minimal ΔV for lunar impact. The CubeSat would release its payload of ChipSats before impact, and the surviving ChipSats would then send back data on their location and, if equipped with the proper sensors, about lunar composition.