Section 3.4 - Rocket Fuel

Rocket Fuel Classification

In previous sections I've mentioned that rocket fuel typically consists of two parts, a separate fuel and oxidizer (hence the term bipropellant). This is true, but it's worthwhile to understand the different classifications of rocket fuels as well as some alternatives to standard bipropellants

• Cryogenic: liquefied gases which must be stored at extremely low temperatures. For example, liquid hydrogen is a very commonly used cryogenic fuel due to its high performance, but hydrogen needs to be almost at absolute zero before it liquefies. Liquid oxygen is often paired as the oxidizer of choice, but it also needs to be very cold
 Liquid hydrogen at a temperature of about -423 degrees F

• Petroleum-derived: Many rockets use a highly refined form of kerosene known as RP-1 as fuel, which is popular due to its high density and relative simplicity
 A beaker of RP-1

• Hypergolic: these fuels are like the other two classes of bipropellants (cryogenic and petroleum) except that the fuel and oxidizer ignite immediately on impact, eliminating the need for an ignition system and simplifying the engine design. A common combination is UDMH/iFRNA (unsymmetrical dimethylhydrazine/inhibited red-fuming nitric acid). Watch the below video to see the spontaneous combustion (can skip to 2:50)

• Monopropellant: rather than combusting two substances, a monopropellant can generate thrust through the decomposition of its own chemical bonds. These are more useful for low-thrust applications like attitude control in space, rather than lifting off from Earth. For example, hydrazine produces hot jets of gas when passed over an iridium catalyst
 Mark Watney passing hydrazine over an iridium catalyst, from the movie The Martian

• Solid: solid propellants take a completely different approach and are actually the simplest design, just a metal casing filled with powder. These are discussed in greater detail below

Specific Impulse

Specific impulse is a crucial measurement of rocket fuel efficiency and performance. Higher specific impulse implies a greater amount of energy generated per mole. The formula is given as

$I_{sp} = \frac{F}{\dot m \cdot g_0}$

$F =$ thrust in Newtons
$\dot m =$ mass flow in kg/s
$g_0 =$ standard gravity ($9.8m/s^2$)

The unit for specific impulse is seconds. This seems really strange, but if you do the dimensional analysis you'll see this is in fact the case. The $g_0$ term is present because multiplying by $g_0$ converts the flow rate from a mass basis to a weight basis

Example

A rocket engine produces a thrust of 1,494 kN at sea level with a propellant flow rate of 400 kg/s. Calculate the specific impulse. What was the propellant used?

a) 299 seconds (liquid methane / liquid oxygen)
b) 381 seconds (liquid hydrogen / liquid oxygen)
c) 286 seconds (hydrazine / dinitrogen tetroxide)
d) 289 seconds (RP-1 / liquid oxygen)

Solution: b

$I_{sp} = \frac{1494000N}{400kg/s \cdot 9.8 m/s^2}$

For further reference, this website has an excellent appendix of all the specifications of different rocket fuel combinations

Other Important Propellant Considerations

Liquid hydrogen tends to produce the highest specific impulse, so why doesn't every rocket just use it?

Specific impulse calculates thrust as an output of weight, but it doesn't factor in the volume needed to store the propellants (ie density). Hydrogen is the lightest compound; it takes a huge tank to store an equivalent weight of hydrogen versus something dense like RP-1. The extra weight of a larger tank may or may not negate the higher specific impulse.

In selecting a rocket fuel, as with many other aspects of rocket propulsion and engineering in general, everything comes down to a trade-off. There are many other factors that must also be considered

• Melting / boiling point (can't burn a fuel if it solidifies at 50 degrees Celsius)
• Toxicity (liquid fluorine is in fact the best oxidizer, but it's so toxic that liquid oxygen is much preferred)
• Ease of storage (if the propellants need to be stored for a long time, cryogenic fuels, highly corrosive substances, or unstable compounds that decay are unsuitable)
• Explosiveness (will the compounds spontaneously combust and kill you?)
• Residue (RP-1 and other petroleum derived fuels leave a carbon residue that clogs the engine over time)
• Cost

Solid Rocket Fuels

Although liquid propellants have superseded them as primary propulsion systems, solid rocket fuels are still useful as boosters. The Space Shuttle's solid rocket boosters were the largest ever flown, burning a mixture known as ammonium perchlorate composite propellant (APCP). Solid fuels can be generally classified as homogeneous compounds or composite mixtures

In solid rocket boosters, the propellant is typically ignited in the center, then burns outward. As a result, one of the most important considerations in designing a solid rocket is grain geometry. Essentially, once the solid booster is ignited, the thrust generated depends on the instantaneous surface area of the fuel being combusted. Different arrangements can be set up to produce different time-thrust curves as needed

 Six different internal arrangements of the solid propellant produce six different time-thrust curves
Some other things to consider - although solid propellants have lower performance, certain types do not produce any fumes, making them quite useful for tactical missile systems since fumes can be used by the enemy to trace back the point of launch. Also, unlike with liquid propellants which be shut off after ignition, once a solid propellant booster is ignited, there's no way to abort

Book Recommendation

Almost everything I know about rocket fuel came from this one book, Ignition: An Informal History of Liquid Rocket Propellant. I stumbled across it because it was on Elon Musk's list of essential must-reads, and it's both witty and informative. I think I enjoyed it so much because my foundation in chemistry is much stronger than in physics or math (though still weak compared to real STEM majors)