RSE #29 RSE #29

Vaporous Substances, Rocket Science for Earthlings # 29

Working with rocket propellants and pressurization systems involves dealing with chemical state changes. There are generally three states of mater. Solid, that includes pretty much anything you can set a beer can on. Only Helium is thought to not have a solid state, but liquid helium is pretty weird anyway. Liquid, if it sloshes it's liquid so that pretty much describes what's inside the beer can. Gas, like air which is a mixture of gases. Gases are compressible. Oxygen is the only good gas, every other gas will poison you or suffocate you. What state a chemical is in depends on what the temperature and pressure is. To get from one state to another involves adding or removing heat energy. Generally a solid plus energy equals a liquid, a liquid plus energy equals a gas. The graph below pretty much gives you an idea how a chemical reacts to temperature and pressure changes.

Consider water ice in the cold vacuum of space, this would be at point A on the graph. The pressure is so low that liquid water cannot exist, the solid sublimes directly into gas when there is an increase in temperature. Point B on the graph is very special, it is called the triple point. It is the one temperature and pressure where a solid, liquid, and gas can all exist together. For water it is at a very low pressure and about zero degrees Celsius. Point B on the graph is the point where a chemical can be a liquid or a gas depending on the temperature. If you have a liquid and lower the pressure it will want to change to a gas and in doing so it will need heat usually from the surrounding environment, this is called refrigeration and is what is happening inside your refrigerator. The liquid to gas state change also cools regenerativly cooled rocket engines. Point C is the critical temperature of the chemical. At temperatures above point C there can only be gas, no amount of pressure will produce a liquid. The pressure at point C is called the critical pressure which is actually a misnomer as only the temperature is critical.

A liquefied compressed gas is a chemical that must be under high pressure to be a liquid, such as propane. A liquefied compressed gas can be used as a self pressurizing rocket propellant. If a tank is 90% full of liquid and 10% gas at the top ( the top gas space is called the ullage ) when liquid is released from the bottom of the tank the ullage gas will expand to fill the space. The resulting drop in pressure and temperature will cause the liquid to boil off a certain amount of gas. This process continues as liquid is withdrawn. For a nearly full tank the process is mostly boiloff, for a nearly empty tank the process is mostly expansion of the ullage gas. To model this process I wrote a basic program. I included all the thermodynamic properties of chemicals, and then ran the program. The results were surprising. Several different chemicals with very different properties gave very similar results. Obviously I had stumbled onto a law of thermodynamics, yet I have not been able to find it stated in the literature. I continue to seek an explanation. Got any ideas??

TANK SIMULATION RESULTS

Final pressure vs ullage percentage for four different starting pressures

This is a graph of the results of several runs of the tanksim1.bas program. This series was run for steam, but the results would seem to be the same for other fluids. Each line is the final pressure at the end of the discharge cycle for the starting pressures indicated and at four different ullage values. While not exactly straight, the lines are not heavily curved so other ullage values can be easily interpolated. With this simple graph you can predict the final pressure in a self pressurized hybrid rocket propellant tank and thus the thrust.

And now a little history; The Sea Dragon proposed launch vehicle made extensive use of self pressurizing liquids in the design of it's propellant pressurizing system. In one case, liquid methane was used to force liquid hydrogen into a tank of RP1. Using a cheap regulator system the designers had the advantages of constant tank pressurization and thrust while avoiding the weight of a gas pressurization system.

Definitions for your edification; Melting point is the temperature at which a solid becomes a liquid. Boiling point is the temperature at which a liquid will change to a gas at standard atmospheric pressure. Cryogenic describes a liquid which boils at atmospheric pressure at a temperature below zero Celsius. A plasma is an ionized gas, either by electrical energy or very high temperature. Neutron star matter is under really huge pressures and temperatures inside neutron stars. All the electrons of the atoms have been forced into the protons and only neutrons are left. It is very dense. Q star matter is something new. If the pressure inside a neutron star is high enough, the neutrons themselves dissolve into pure quark matter. The core of the star is in effect one gigantic neutron.