Propellants

NYTROX

Unlike the wide range of fuels that are currently available for chemical propulsion the list of potential oxidizers is quite limited. Moreover each oxidizer on this short list is associated with important shortcomings. For example the high-density, storable oxidizer hydrogen peroxide, H2O2, has a tendency to self-decompose explosively. The commonly used nitrogen tetroxide, N2O4, gives good performance but is highly toxic. Inhibited red fuming nitric acid, H2NO3, is extremely toxic and somewhat corrosive.

Two of the most easily accessible liquid oxidizers that are widely used in rocket propulsion are liquid oxygen, O2 or LOX, and nitrous oxide, N2O. SPG has extensive experience with both. At room temperature (20°C) nitrous oxide has a vapor pressure of about 730 psia and is commonly chosen for small rocket systems due to its self-pressurizing capability.

Liquid oxygen is a commonly used oxidizer with a high specific impulse but has to be stored at deep cryogenic temperatures. The common features of both oxygen and nitrous oxide are their low toxicity and cost effectiveness compared to other oxidizers.

SPG engineers have recently conceived a new class of oxidizers based on refrigerated mixtures of nitrous oxide and oxygen, that we call Nytrox. Specifically, the goal is to formulate a high density, self-pressurizing oxidizer that does not have to operate at deep cryogenic temperatures. Note that in the mixture the oxygen is the volatile component which serves as the pressurizing agent whereas the N2O is the less volatile one with the primary function of densifying the mixture. The advantages of mixtures of oxygen and nitrous oxide compared with the pure components are summarized below:

  • Partial self-pressurization possible at high densities (eliminates or minimizes the use of expensive helium).
  • Improved Isp performance compared to N2O.
  • Unlike liquid oxygen, not cryogenic. Oxidizer at -60°C or -40°C is much easier to manage. Composite tanks can potentially be used.
  • Potentially lower freezing point compared to N2O useful for space applications.
  • Large fraction of oxygen in vapor phase allows vapor-phase combustion in blow down systems. Up to 8% total impulse improvement possible.
  • Optimization based on mission requirements. The critical control variables are the temperature and pressure which determine the oxidizer mass fraction in the mixture.
  • Thermodynamic non-equilibrium mixtures can be used in order to increase the system performance.
  • The Nytrox systems are much safer than blow down nitrous oxide systems since the vapor phase of the Nytrox system has a large O2 concentration, in the range of 50-90% by mass. A typical Nytrox system with 70% oxygen in the vapor phase requires 10,000 times more ignition energy compared to the pure nitrous oxide. In summary Nytrox vapor is impossible to ignite with any practical ignition source that exists in the tank.
  • Nytrox is safe for human exposure. A mixture of 30% oxygen in nitrous oxide is a common anesthetic.

SPG has conducted extensive systems studies comparing Nytrox with other oxidizers to show the performance and packaging benefits it offers. The systems studies indicated extensive weight savings and a smaller envelope when compared with nitrous oxide or LOX based systems. As an example, see the figure above comparing the size of several hybrid replacements for a commonly used strap-on booster.