§ 3.5 Storability of Propellants

Using liquid oxygen (LOX) in space

It has been proposed by numerous sources that lunar, asteroid and/or comet materials could be mined and used as propellants to re-fuel satellites, fuel orbital transfer vehicles and to fuel deep space probes. Since oxygen is a gas above -297.6°F (162°R or 90°K) it takes a very large low pressure tank or a very heavy high pressure tank to store an appreciable volume at normal Temperatures. Therefore oxygen is normally super-cooled to its liquid form commonly referred to as LOX. There are three problems associated with using LOX in space. First is commonality, second is boil-off losses, third is ignition problems and fourth is propellant tank scale, which is related to the boil-off issue.

Commonality

I am not aware of any space craft or satellite that uses LOX as a propellant, but to make sure, I searched Mike's Satellite Library at "http://leonardo.jpl.nasa.gov/msl/". I searched 164 satellite and space craft types. I found no systems that use LOX. The most common was Hydrazine (N₂H₄) monopropellant, followed by cold gas and bipropellant (N₂H₄ and N₂O₄). The deep space probes, which have the highest delta V requirements, generally use bipropellants. This is surprising since LOX has a higher specific impulse, which means less propellant mass is required for the same level of performance. LOX is the premier oxidizer for fuelling launch vehicles, but in a launch vehicle the propellant is all consumed in a matter of minutes and long term storage is not an issue. However, for satellites, which require small intermittent pulses of thrust over their design life of up to 15 years, long term storage is critical. Therefore, there is currently no demand for LOX in space, since no space craft currently use it. We can also state that the requirements for LOX in the near term will continue to be low since rocket engines that burn LOX and provide low thrust, high reliability and long life do not exist today and would take several years to design, build and test.

Space Station Freedom was designed to use Hydrogen and Oxygen propellants, however it was to be stored as water and separated by electrolysis as needed. After separation the Hydrogen and Oxygen were to be stored in gaseous form under pressure. With the switch to the International Space Station, the propulsion function has been moved to the Russian station modules and the propellants are Unsysmmetrical Dimethyl Hydrazine (UDMH) and Nitrogen Tetroxide (N₂O₄).

Boil-off

When liquid propellants are stored at Temperatures above their boiling-point they vaporize. If we contain the vapors in the tank, then the pressure increases with Temperature. Since the tank weight increases with design pressure, a pressure relief valve is generally provided to prevent the tank from over pressurizing and exploding. When the relief valve releases the pressure, some of the propellant escapes from the tank. This lost propellant is referred to as boil-off loss. Propellants have been generally classified as "storable" or "cryogenic" based on weather they remain a liquid throughout the normal terrestrial Temperature range or if they are only a liquid at "very low" Temperatures, however these definitions are not applicable to storage in space. Figure 1 (from reference 1) indicates the thermal equilibrium condition of a well insulated cylindrical propellant tank with effective radiation heat shielding assuming a distance to any object (planet, moon, asteroid, ...) greater than 10 planet radii. The upper and lower curves represent the range of practical designs. The upper limit of each propellant Temperature band assumes a manageable overpressure and the lower limit is defined by the propellant freezing point.

From the graph, you can see that Hydrazine, the most common satellite fuel, is easily storable at Earth and Mars distances from the sun. It also appears that with very careful space craft design, LOX could be stored at Earth distance from the Sun. However, Figure 1 assumes that the tank is not close to the Earth. Figure 2 (from reference 1) provides the thermal equilibrium Temperature of a well insulated spherical tank with an effective radiation heat shielding, orbiting at 2 to 9 Earth radii.

Figure 2 shows that in Earth orbit, the added energy received from the Earth makes LOX storage very difficult. To make matters worse, in actual designs, additional heat sources must also be considered. Heat is also conducted into tanks from onboard electronics, heated compartments such as manned spaces and from solar panels. In addition, heat is gained by radiation from rocket plumes.

Another alternative is to actively control the propellant Temperature, cooling it when the temperature exceeds the boiling point and heating it if the temperature drops below the freezing point. In the past space craft designers have avoided cooling systems when ever possible due to weight and reliability issues.

Ignition problems

Spacecraft and satellites are required to start and stop their rocket thrusters hundreds or even thousands of times over their design life. To eliminate the ignition system as a possible failure, designers prefer to use hypergolic propellants, which means the propellants spontaneously burn when they are combined. Table 1 below defines which common propellant combinations are hypergolic.

Scale

When boil-off conditions exist, the scale of the propellant tank, which determines the surface area to volume ratio, directly effects the boil-off rate. Large tanks, with a high volume to surface area ratio loose a small percentage of their propellant in a unit of time. A small tank, with a volume to surface area ratio will loose a much higher percentage of its propellant in the same unit of time. Launch vehicles with large cryogenic tanks and short storage time can use cryogenic propellants with negligible boil-off losses, however satellites with small propellant tanks and long storage time requirements would have unacceptable propellant burn-off with cryogenic propellants.

Conclusion

Satellites do not currently use LOX and will not use LOX for the foreseeable future. LOX is not storable without expensive and heavy cooling systems in Earth orbit. LOX could be used at about Mars orbit and deeper into space. When boil-off does occur, large tanks with high volume to surface area ratios minimize the rate of loss due to boil-off.

On the positive side, we can use oxygen to produce the oxidizer of choice. Many space craft with high total impulse requirements utilize Nitrogen Tetroxide (N₂O₄) and Hydrazine (N₂H₄) bipropellants because the higher delivered specific reduces the space craft launch weight and this propellants combination is hypergolic. If we obtain Nitrogen from asteroids, or import it from Earth, we can combine it with Oxygen to produce Nitrogen Tetroxide. By weight Nitrogen Tetroxide is almost 70% Oxygen and 30% Nitrogen. If we import Nitrogen from Earth, in what form should be transport and store it? Liquid Nitrogen has a lower boiling point (-320°F) than liquid oxygen (-298°F). Ammonia (NH₃) is an obvious choice, it has boiling point of -28°F and a freezing point of -107.9°F. By containing about 200 psi of vapor pressure we can increase the boiling point to 100°F as shown in figure 3:

For every two ammonia (NH₃) molecules that we convert to Nitrogen Tetroxide (N₂O₄) we liberate 6 hydrogen atoms. We can also combine two ammonia molecules to produce Hydrazine (N₂H₄) and liberate 2 more hydrogen atoms. All these 8 hydrogen atoms can be reacted with 4 extraterrestrial oxygen atoms to produce 4 water molecules (H₂O).

The mass balance based on molecular weights is:

The ammonia makes up 34.7% of the weight of the inputs and the oxygen makes up 65.3%. The propellants make up 63.2% of the outputs with water constituting 36.8%. It is interesting to note that the water produced exceeds the weight of ammonia that is imported. Although this does not appear as attractive as using liquid oxygen from space and imported hydrogen where only 11% of the propellant mass must be imported, Nitrogen Tetroxide (N₂O₄) and Hydrazine are easier to store for long periods and they are currently in use in space.

Another useful fuel to produce in space is Unsymmetrical Dimethylhydrazine [(CH₃)₂NNH₂], also known as UDMH, is used on the Russian MIR space station and will be used on the International Space Station. Because of its use on Space Stations, UDMH probably has the highest demand of any fuel in space. Unfortunately, until we locate sources of Nitrogen in space, only the Carbon appears to be readily available.

Reference 1 "Handbook of Astronautical Engineering," McGraw-Hill Book Company, Inc., 1961, Edited by Heinz Hermann Koelle.

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