If you have ever had a bartender with a generous pour, you’ve probably heard someone say their drink is “pure rocket fuel.” The connotation, of course, is that the drink is so stiff, that it is caustic and dangerous. And in the early days of rocket development, this comparison wouldn’t have been too far off. Propellants like hydrazine, nitrogen tetroxide, nitric acid, and fluorine were previously used in liquid fueled rockets, and there was the extensive use of metal cleaning solvents in rocket engines such as trichloroethylene, which was left to run into soils near launch pads and testing stations. These propellants and solvents are corrosive and toxic.
Just as commercial rockets are revolutionizing the industry in areas such as reusability, huge advancements are also happening in propellents used to power these rockets. Rocket scientists can’t change chemistry, but advancements in manufacturing technology, like 3D-printing and enhanced rocket engine designs, have allowed aerospace engineers to minimize and eliminate the use of dangerous cleaners / solvents. Now, rockets better utilize more common oxidizers like hydrogen peroxide and liquid oxygen combined with fuels like methane (natural gas), propylene (basically the same as propane in your gas grill), hydrogen or RP1 (rocket propellant 1, which is a fancy name for a type of kerosene and similar to jet fuel). In fact, some rocket companies are even exploring the use of renewable sources such as plant-based fuels like ethanol, quite literally the same “active ingredient” in a Jack and Coke.
So why haven’t these fuels been used before? Liquid oxygen and RP1 or liquid hydrogen, have actually been used, for many years, with great success. However, for various reasons, they were not always favored by some rocket designers pursuing maximum performance and who had few, if any, cost constraints (for example, military and defense purposes). But for some of the newer propellant combinations using methane, propylene and ethanol, the answer is fairly complex, and has to do with the general design of a rocket and its engines. In contrast to an internal combustion engine like in a car or a jet engine in an airplane, which mixes fuel with gaseous oxygen found in the atmosphere for combustion, rockets fly out of the atmosphere so they must bring their own oxygen source along for the ride. You also need a lot of energy to get a rocket moving fast enough to reach orbital velocity and that requires a lot of fuel with a lot of power, either in the form of brute force thrustto escape gravity and the effects of heavy air or another measure of performance that becomes more important when the air is not so dense (or non-existent) called specific impulse powerwhich is a measure of how effectively a rocket uses propellants.
Now, if you need to put a lot of fuel on the first stage of a rocket to get it and its cargo through the heavy lower atmosphere, you need to minimize the weight of the rocket and you need a fuel that has a lot of power per pound or gallon, so something that is very energy dense. A given weight of dense propellant can be carried in a smaller, lighter tank than the same weight of a low-density propellant. (A pound of lead takes up less space than a pound of feathers.) Liquid hydrogen, for example, is very energetic but for a given unit of energy it is a very bulky substance (compared to kerosene / RP-1) requiring larger (and hence heavier) tanks. The weight of these tanks generally outweighs the energetic properties of the hydrogen when compared to kerosene / RP-1 for a first stage rocket having to fly through the lower atmosphere and escape Earth’s gravity.
As a rocket gains altitude, the physics of gravity and air density change, and it becomes necessary to shed weight from depleted propellant tanks and heavy engines designed to get through the lower atmosphere. So, the first stage is dropped and differently designed engines take over. Sometimes the propellants in the second stage (or for some rockets, the third / fourth stages) are the same as the first and sometimes they are not. For the highest performance rockets today, a second stage uses liquid hydrogen and liquid oxygen due their superior specific impulse power characteristics versus RP-1 / kerosene, but others stick to RP-1 for simplicity and reliability. In the past, sometimes designers used toxic fuels like hydrazine and nitrogen tetroxide in their main propulsion units or solid fuels that had toxic exhausts that negatively affected the upper atmosphere.
The caustic propellants of the past were often used for convenience, like hydrazine, nitrogen tetroxide, and even nitric acid (used as an oxidizer). These fuels were very dense and had high boiling points, meaning they don’t require temperatures of hundreds of degrees below zero to remain a liquid. In the early days of rocket development, these toxic fuels allowed engineers to build their rockets without the complex plumbing and ground systems needed to keep fuels cryogenically stable in liquid form. As material science has advanced rocket stage design with things like carbon fiber, these older caustic fuels are also not very compatible with newer, lighter, stronger materials.
As just noted, some of today’s rockets are built out of more complex materials like carbon fiber that allows their structure to be lighter and stronger than ever before. Propellants like liquid oxygen are more readily available and can be kept below their boiling point of hundreds of degrees below zero for longer periods of time. As a result, nearly all modern rockets use these safer, more environmentally friendly propellants such as liquid methane, liquid oxygen and liquid hydrogen, and/or room temperature fuels like RP-1.
This is the reason that environmental alarmism about water contamination and pollution resulting from launch operations are largely misplaced. For example, the test rocket launched by Vector at Spaceport Camden last year utilized liquid oxygen and propylene, a chemical very similar to the propane in your home grill. SpaceX uses liquid oxygen and RP-1/kerosene for its principle propulsion on the Falcon 9, as does the representative launch vehicle analyzed in the FAA’s Spaceport Camden Environmental Impact Statement. Blue Origin uses methane and liquid oxygen (first stage) or liquid oxygen and liquid hydrogen (second stage) for its propulsion in its New Glenn rocket, as does United Launch Alliance for its new Vulcan rocket. The bottom line – if you are comfortable grilling your steak in your backyard, you can be comfortable with the fuels used by rockets that are proposed to launch from Spaceport Camden.
***You must be 21 to consume alcohol. Camden County reminds you to drink responsibly.***