Five years ago in 2002, my senior in the same research field asked me, “Could you devise a safe liquid propulsion system that can be used for student education?” The request was a small burden for me since I had just worked on a liquid rocket engine under the Reusable Vehicle Testing (RVT) project initiated in the spring of 1998, four years before. Nonetheless, I was intrigued because the “safe liquid rocket usable for student education” was a fresh, unique concept. I decided to “give it a try!” I invited Dr. Hiroto Habu and students in our research laboratory to join me. The project started with the first step, the selection of propellant. By the way, the reason why we targeted a “safe liquid propulsion system” is that, under the motto of “practical education by futuristic space transportation technology,” we envisioned an orbital maneuvering system for reusable rocket experiment vehicles or spacecrafts, which would allow us to maneuver any trajectory flexibly.
Selection of propellant
The “propellant" consists of the agents that burn to generate thrust force while “oxidizer” is the agent containing much oxygen to burn the “fuel.” Aircrafts or automobiles only carry fuel because they use air as the oxidizer.
To assure a level of safety that would permit use in student education, the first requirement is non-toxicity. Since our project aimed to prepare an educational program including the actual processes to install an engine into a launch vehicle and operate it, it was desired that everyone could see, smell and touch the agents. Accordingly, oxidizer candidates were limited, while all the commercially available products could be adopted as fuel. As a number of combinations were possible, we narrowed our requirement as follows: nontoxic level that was “basically harmless even if taken internally” and, as an additional feature, “agents can be stored at room temperature” for easy handling. As a result, the final option was limited to the combination of nitrous oxide (N2O) as the oxidizer and ethanol (C2H5OH) as fuel.
N2O is a stable substance that is used as an anesthetic in medical applications or a foaming agent for whipped cream. It is also a certified food additive. On the other hand, ethanol is plentiful in our favorite drink. If we ingest them, no problems occur basically. Their sweet smell makes us feel good or sometimes causes unconsciousness. Thus, we have to avoid “excessive intake.”
In addition to the consideration of safety to the human body, the agents must have the proper features as rocket-engine propellant. Since the N 2O/ethanol propulsion system is categorized in the storable liquid-propellant group, we need to assess its performance compared to the conventional NTO (nitrogen tetraoxide: N2O4)/hydrazine (N2H4) propulsion system. N2O/ethanol is about 10% lower with regard to thrust generated from the same mass flow rate of propellant. In addition, since the density of N 2O/ethanol is low by 20% or more, it is a little bulky. Furthermore, the NTO/hydrazine propulsion system has the convenient characteristic of “self-ignition.” When the oxidizer (NTO) and fuel (hydrazine) are mixed, they immediately ignite. Ignition does not take place when N2O and ethanol are mixed. Thus, we need to prepare a separate ignition system and naturally this makes the engine mechanism a little more complex. The above disadvantages, however, are not so decisive as to deny the practical use of the N 2O/ethanol combination. On the contrary, the advantages of the N2O/ethanol are that it is almost nontoxic and, therefore, easy to handle, and can be used in very low-temperature environments. The freezing point of N2O is -91 deg. C and that of ethanol -114 deg. C. On the other hand, the freezing point of NTO is -12 deg. C and that of hydrazine 1 deg. C. Accordingly, if we were to use the NTO/hydrazine combination on space explorers, heaters would have to be included. On a Jupiter exploration mission, for example, surrounding temperatures are anticipated at -50 deg. C. Thus, by using the N2O/ethanol combination on the mission, we can omit the heaters. As discussed above, we can conclude that the N2O/ethanol propulsion system is superior in operation in low-temperature environments with moderate performance as an almost nontoxic, storable liquid-propulsion system.
Furthermore, we should examine future expandability and potential. Today, since bioethanol is renewable, ethanol fuel is attracting a lot of attention as an alternative to fossil fuel when considering prevention of global warming. The technology for ethanol as automobile fuel is already established. Thus, ethanol is promising in terms of common fuel for both ground and space transportations. If space colonies are built in the future, ethanol should be produced together with agricultural products. On the other hand, N2O is a greenhouse-effect gas with more than 300 times the impact of CO2, which now becomes big issue in relation to global warming, and is subject to emission control. The main source of N2O emissions, however, is nature such as forests and the amount of man-made emissions is relatively small. Of course, there seems no doubt that the out of balance in the generation and decomposition of greenhouse-effect gases is caused by human activities based on deforestation and mass consumption of fossil fuel. We must try to limit N2O emissions. By using an appropriate decomposition catalyst, N2O theoretically decomposes into nitrogen (N2) and oxygen (O2) while releasing heat and generating mixed gas at approx. 1,600 deg. C. This property can be used as a source of energy and oxygen in many other applications while minimizing its greenhouse-gas impact. It is possible to generate electric power, albeit limited, with fuel cells using the oxygen produced and ethanol. Furthermore, if we use the heat mentioned previously to decompose the ethanol efficiently to extract hydrogen (*1), much more electric power can be generated. We organized these ideas into the “N2O/ethanol energy-management system concept” in anticipation of realizing it in outer space. We are confident that with this concept we can offer pioneering, innovative proposals toward the integration of propulsion, energy-supply and life-support systems in manned space stations or space ships.