Advantages of Using White Fuming Nitric Acid (WFNA) as an oXidizer in Rockets
Interorbital Systems Educational Briefing
By Randa Milliron
For over sixty years, White Fuming Nitric Acid (WFNA) (HNO3) has been successfully used as an oxidizer in the United States, Russia, Germany, and France. The French Diamant rocket used a WFNA/hydrocarbon combination to place France’s first satellite into orbit in the early 1960s. In the United States, the early rocket-assisted takeoff (RATO) units, and later the second stage of the Nike-Ajax used WFNA and JP-4. Several German rockets in World War II used WFNA. In addition, the German OTRAG rocket, designed in the 1970s, used WFNA and kerosene. Today, the Russian Small Cosmos Satellite Launcher still uses nitric acid as its booster oxidizer.
In recent years, mostly within NASA and the commercial rocket community, WFNA has been confused with and misidentified as Inhibited Red Fuming Nitric Acid (IRFNA). White Fuming Nitric Acid has been falsely branded as toxic and poisonous. The reasons for this are the launch community’s lack of experience with the propellant, and that same community’s lack of knowledge about it. When used by experienced propellant handlers, WFNA is actually safe and environmentally friendly. It is considered a “green” propellant by the military. It is readily available as an industrial chemical, and can be transported on highways without the armed escort required for nitrogen tetroxide and hydrazine.
Here are some facts about White Fuming Nitric Acid:
WFNA is not poisonous. Like any concentrated acid, WFNA is very corrosive and can irritate the breathing passages and severely burn the skin, but when handled properly, it is safe to use.
WFNA cannot be made to explode or burn by itself. On the contrary, hydrogen peroxide can undergo destructive rapid decomposition, and LOx creates an extreme fire hazard.
WFNA is completely water-soluble. If it is spilled, simply diluting it with water eliminates any danger. If spilled in the ocean, it is quickly diluted. In the desert, diluted WFNA acts as a fertilizer and can be beneficial to the environment.
Because of its high reactivity, WFNA cannot form explosive substances when mixed with fuel, as can liquid oxygen when mixed with kerosene. When LOx and kerosene mix in a spill (a common occurrence when an igniter fails to function and propellants are ejected to the launch pad below), a substance called “cryogel” forms. Cryogel is a high-order explosive, far more unstable than nitroglycerine, and capable of detonating and vaporizing a launch site and the personnel staffing it. WFNitric Acid presents no such danger.
The clean exhaust products of WFNA and all fuels are equivalent to LOx/kerosene with the addition of nitrogen gas (which makes up 80% of the air we all breathe). On the contrary, commonly used solid rocket motors produce tons of hydrochloric acid pollutants.
When handled properly, WFNA is less dangerous to use than both liquid oxygen (LOx) and hydrogen peroxide. Nitric acid is not classified as toxic.
Nitric Acid is commonly available. It is used for many applications in the chemical industry. It is easy to transport. Thousands of tons of nitric acid are shipped annually on US highways in tank cars just like gasoline. Nitric acid can be shipped on the highways and railways without any special requirements. Large volumes of nitric acid are shipped in unguarded trucks or tanker train cars with no more precaution than the usual hazmat placards on the truck and the proper bill of lading info for the driver. Small quantities (under 1,000 pounds) of WFNA can be transported by small trucks without placards or a bill of lading.
WFNA has been shown to have many advantages over other types of oxidizers, including liquid oxygen and hydrogen peroxide. In the late 1950s, after fierce competition, the final two preferred storable propellant combinations selected by the United States military were nitric acid/alpha pinene and nitrogen tetroxide/hydrazine. Due to its superior performance, the poisonous combination of nitrogen tetroxide and hydrazine was selected. If environmental factors had played a role in that decision, the nitric acid combination would have surely been chosen.
When used with an appropriate fuel such as kerosene and other hydrocarbons, WFNA yields a moderate specific impulse and a high density specific impulse. Density specific impulse takes into account the density of the propellants. The size of a rocket’s propellant tanks is related to the propellant’s density. A high density specific impulse will lead to shorter and lighter propellant tanks, a smaller launch vehicle, and a greater payload capacity.
As is indicated by its high density specific impulse, WFNA is extremely dense. Its specific gravity is approximately 1.5. That means it weighs about 12.5 pounds per gallon (water weighs 8.33 pounds per gallon). Using WFNA in a booster stage reduces the length of a satellite launch vehicle (SLV) by up to 25% when compared to LOx/Kerosene or LOx/Methane vehicle, and up to 60% compared to LOx/Hydrogen (hydrogen weighs only 0.59 pounds/gallon).
WFNA with its hydrocarbon fuel is storable at ambient temperatures and pressures. Its use eliminates holds and delays experienced when handling cryogenic liquids, especially when considering the amount of oxidizer required by a booster stage (in the neighborhood of 125 tons). Storable propellants can be safely left in propellant tanks for long periods of time. A launcher can be left in a ready state for extended periods of time. It provides the capability of meeting critical launch windows.
WFNA is hypergolic with Hydrocarbon-X (IOS proprietary), furfuryl alcohol, hydrazine, and other fuels; that is, the propellants burn spontaneously when mixed, eliminating the need for an igniter, thus improving ignition reliability and operational safety.
The combustion temperature of WFNA with most fuels is up to five-hundred degrees F cooler than LOx/Kerosene. This simplifies rocket engine cooling.
WFNA is low-cost. It costs as little as 15-cents per pound. Compare this with hydrogen peroxide (hydrogen peroxide’s specific impulse is almost the same as WFNA), which costs between $3.00 and $5.00 per pound. If a booster stage required 125 tons of oxidizer, it would cost $37,500 to fill the oxidizer tank with WFNA and $1,000,000 to fill the oxidizer tank with hydrogen peroxide. When using hydrogen peroxide, it would be impossible to operate a launcher for under a million dollars. Liquid oxygen costs about the same as WFNA.
Storage of WFNA at the launch site requires only simple low-cost aluminum tanks with no insulation. Liquid oxygen and other cryogenic propellants require special vacuum jacketed tanks and special insulation, special valves, and other processing equipment. Large cryogenic storage facilities cost millions of dollars. For the commercial-sector rocket developer, WFNA is the safest and most cost-effective oxidizer available.
Interorbital Systems currently holds an active AST launch license for its Tachyon sounding rocket. That rocket employs White Fuming Nitric Acid as its oxidizer. Also, the area (the Mojave Test Area) where the IOS Lunar Lander will be flight tested, has an over 50-year history of serving as the launch pad for countless WFNA rocket launches.