SECOND OPINION

The following is a short paper by Mark Stacey of Waipu, New Zealand. Mark is a polymath, Engineer, Chemist and Researcher who has worked on many prestigious projects from rocketry to electric vehicles. His is an opinion worthy of respect.



Nitrous oxide hybrid rocket with saturated wick fuel grain and free piston oxidizer tank.
Mark Stacey
Director, CNC Prototyping


Abstract:

A discussion of the many advantages of a hydrocarbon saturated wick feed combustible fuel grain combined with a pressurised piston driven liquid nitrous oxide oxidizer feed.

Introduction:

The use of self pressurising nitrous oxide oxidizer with solid fuel in hobby and commercial rockets is a well known and well developed process however development has plateaued with most development focused on refining the known, i.e. shaping the fuel grain, nozzle development, casing and tank design. This leaves fundamental problems, which have caused accidents and lower than optimum performance, unaddressed. The following describes the significant improvements that have been made to the basic nitrous oxide solid fuel design of rocket.

Oxidizer feed:

Nitrous oxide is a well understood industrial chemical and compared to many oxidizers has many advantages. It is relatively cheap, is relatively unaffected by contaminants, does not require cryogenic handling and is, compared to many other oxidizers, relatively stable. However since the 1930's industrial users have been aware that gaseous nitrous oxide is prone to detonation as temperature rises above ambient.

Most current hobby and commercial rocket motors heat the nitrous oxide to provide self-pressurisation thus putting the oxidizer system into an area where detonation is increasingly likely to occur if shocks or hot spots are present.

Additional problems are:
The density is reduced, so for the same tank volume, less mass of oxidiser is carried.
The tank must have a percentage of free gas space for the initial pressurizing.
Once the oxidiser is flowing, consistent pressure, and hence feed, is uncertain.
As the oxidiser level drops, the flow to the motor becomes a mix of liquid and gas, thereby reducing performance.
The tank always has some residual liquid and gas that is wasted which, in aerospace applications, reduces payload.

The tested solution developed to overcome these problems is a cylindrical oxidizer tank with a free moving piston. The piston is driven down the oxidiser tank by a pressurising gas. The advantages are the nitrous oxide is always liquid both during loading and firing, greatly enhancing operational safety and performance. When discharging to the motor the flow rate is even and the tank is completely emptied.

Fuel grain:

Various fuel grains have been used in nitrous oxide motors, but all previous grains have problems. Hard thermoset plastic grains require careful internal shaping for an optimum thrust profile and are prone to cracking and loss of sections of the grain. In the worse cases this can lead to nozzle blockage and casing failure. Alternative soft fuel grains, such as modified waxes or thermoplastics, allow some compensation for oxidiser flow variations but are prone to erosion with out combustion, reducing motor performance.

The use of a liquid hydrocarbon infused, self-wicking, combustible liner as the fuel grain, overcomes these problems and has additional advantages:

As the motor runs the liquid fuel evaporates from the liner cooling the outside of the liner and thus the case due to the latent heat of evaporation of the hydrocarbon.

The rapid evaporation causes turbulent mixing ensuring the fuel is burnt inside the motor casing.

The motor runs a constant thrust profile as only enough liquid hydro-carbon evaporates to react with the oxidizer flow.

A simple orifice on the oxidiser flow is all that is required for even and complete combustion characteristics.

The liner chosen as the wick is heavy density cardboard tubing. This provides a liner that erodes in a linear fashion for the complete length of the motor. Multiple firings have confirmed this behaviour. As the self ablating and insulating action leaves no hot spots it allows the use of an aluminium motor casing.

Conclusion:

The combined developments of a hydrocarbon saturated wick feed fuel grain, and a free piston liquid feed oxidizer, provide demonstrated safety and performance benefits to the liquid oxidizer solid fuel motor hybrid motor combination.