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Space Propulsion Group, Inc. 2019

65 E. BROADWAY, SUITE 308

BUTTE, MT 59701

Tel: (406) 533-6870

Oxidizer Information

SPG has experience operating with a range of oxidizers, including; oxygen, nitrous oxide, mixed oxides of nitrogen (N2O4 + NO), hydrogen peroxide, and mixtures of nitrous oxide and oxygen (Nytrox). 

NYTROX

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 conceived a new class of oxidizers based on refrigerated mixtures of nitrous oxide and oxygen, called Nytrox. Specifically, the goal was 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.

 

Mixed Oxides of Nitrogen

Mixed Oxides of Nitrogen (MON), which are a combination of nitrogen tetroxide (N2O4) and nitric oxide (NO), are one of the most commonly used oxidizers today. The combination of high propulsive performance, hypergolic ignition with hydrazine fuels, and earth-storable capability make this an attractive propellant.  Uses ranging from launch vehicles to in-space propulsion abound. 
 

Since 2015, SPG engineers have been involved with the Mars Ascent Vehicle development project funded through NASA-JPL, with MON-3 (3% by weight NO concentration) being a primary oxidizer in use. The advantages of MON as an oxidizer include:

  • It is an Earth-storable propellant

  • Able to remain a liquid at low temperatures

  • Hypergolic when used with specific solid and liquid fuels

  • Insensitive to mechanical shock and impact

  • Nitric oxide concentration allows optimization based on mission requirements

  • Less corrosive than pure N2O4 or nitric acids

SPG’s experience with N2O4-based oxidizers for hybrid rocket applications extends from lab-scale engines to greater than 1,500 lbf class motors. 

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 specific impulse performance compared to N2O.

  • Unlike liquid oxygen, it is not cryogenic. Oxidizer at -60°C or -40°C is much easier to manage.

  • 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.

  • Mixture 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 compared to N2O.  A typical Nytrox system with 70% oxygen in the vapor phase requires 10,000 times more ignition energy compared to the pure nitrous oxide.

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 below comparing the size of several hybrid replacements for a commonly used strap-on booster.