First and foremost these propellants are non-toxic and thus are more desirable from the safety point of view compared to the toxic ones typically used for government financed lunar spacecraft. The additional safety coming from non-toxics reduces the development time and costs, making the development feasible for our small team which has only limited finances.
Secondly, both these propellants can be used in mono-propellant mode by passing them through a catalyst pack (here nitrous has the small disadvantage of requiring pre-heating for its catalyst pack). The specific impulse in monopropellant mode is not terrific so we have selected a bi-propellant system for the main lander engine, combing the oxidizer with a kerosene-type fuel. However, the monopropellant mode is quite useful for secondary thrusters for attitude control, which don't require high specific impulse, or for a deep throttling mode of the main engine that might be useful for the final touchdown when the lander's mass is low.
Both the oxidizers have a long heritage in both professional and amateur rocketry so there is lots of information available for guiding the design process. Indeed, we have already complete preliminary designs for engines of both types. However, there is one more factor that is very important for our thinking - the availability.
It turns out that there are virtually no suppliers of rocket grade hydrogen peroxide in the world, and those that do exist are notoriously unwilling to cooperate with small companies involved in rocket development. Some small rocketry groups using hydrogen peroxide choose to manufacture the propellant in-house, which is a relatively simple process but presents significant risks and requires dedicated facilities for production and storage, taking it beyond the reach of our team. Thus, for some time now we have been scouring the world for suppliers, and so far didn't succeed. (If anybody knows a supplier willing to cooperate with GLXP teams, please send us an email.)
Unlike hydrogen peroxide, the availability of nitrous oxide presents no difficulties. It is widely used across many industries, is cheap, and can be shipped almost anywhere when needed.
Another oxidizer option that is frequently discussed is liquid oxygen. This is non-toxic and relatively easy to handle as well as giving great performance. However, we rule out liquid oxygen due to the uncertainties in thermal conditions that will be encountered during out mission, which make it impossible to accurately and reliably predict the boil-off fraction. Also we consider the development of a space-qualified re-condensor system as not realistic given the timeframe and finances available.
As a result of the above considerations we have selected nitrous oxide for our first prototype rocket motor, which we plan to demonstrate in the fourth quarter of this year.
The main downsides of nitrous oxide are its requirement for pre-heated catalyst (if a monopropellant mode is used) and its vapor phase transition behavior, which give extra complication compared to hydrogen peroxide. For these reasons, we plan to keep the option open to go back to hydrogen peroxide later if the availability issue can be solved.