Since the launch of Sputnik 1 by the Soviet Union on 4 October 1957, the world has been entranced by the idea of traveling in deep space. For the past six decades, humanity has watched all manner of rocket and shuttle be launched into space for missions in low earth orbit, to the very edge of our solar system. However, despite the grand ambitions of generations past and present, wide scale space travel and exploration is still not commercially viable, instead relying primarily on government funded space agencies, like the US’s own NASA. Part of the problem is the complicated, not to mention expensive, process of creating and launching a rocket into outer space. This blog post will cover the chemical basis of that exact process, describing in detail the various fuels that make a rocket zoom.
Rocket technology does not consist of one piece, and neither does a rocket’s propulsion system. To get a rocket into orbit, or out of Earth’s influence altogether, requires a combination of different fuels and propellants to best suit the situation. In the following section, I will cover some of the most common fuel sources, as well as some interesting experimental ones.
Liquid Oxygen and Liquid Hydrogen
One of the most popular, and simple, of the conventional fuels is a combination of liquid hydrogen and liquid oxygen, which are kept compressed and cooled in storage buildings on-site until needed at the time of launch, as seen in the white dome in the leftmost edge of the image on this page, where NASA describes in detail some of its transfer and storage methods for this cryogenic fuel. An interesting point to note is that, at least in the case of the liquid hydrogen, no active pump in necessary to transport the fuel. A small amount is simply allowed to vaporize, creating a pressure differential that pushed the liquid fuel out of the tank, through the piping, and into the waiting rocket.
The liquid oxygen and hydrogen are then combined in the rocket. From there, it is a simple matter of igniting the two, setting off a reduction-oxidation (redox) reaction. As might be expected, the oxygen is the oxidizer, and the hydrogen is the reducing agent. Written in expanded form, the reaction proceeds as such.
Hydrogen is oxidized (gives up electrons)
H2 → 2H+ + 2e–
Oxygen is reduced (gains electrons)
O2 + 4e– → 2O2-
Combined, they yield a net reaction of:
2H2 + O2 → 2H2O
This reaction, however, is very spontaneous under elevated temperatures. As anyone who has ever ignited hydrogen will tell you, given the right conditions, it more explodes (or pops) than burns. It is this controlled explosion that powers the first state of the rocket’s flight. From then on, however, different means of propulsion come into play, one of the most prominent of which is solid fuel rocket boosters.
Other forms of liquid propellant include using kerosene instead of hydrogen and using a combination of nitrogen tetroxide and hydrazine as the fuel, but they all rely on the same fundamental process of a redox reaction.
Solid-fuel based rockets come in a wide variety of configurations, depending on the scale, cost, safety, and performance needed for a given operation. They are useful in providing boosts to the main liquid-based propulsion systems of some rockets (i.e. the Space Shuttle), hence the name of solid rocket boosters. While the exact fuel varies, one of the most common general fuel types for large scale rockets (the kind needed to launch spacecraft) is a powdered metal, usually magnesium or aluminum, that is oxidized by a strong oxidizing agent, such as ammonium nitrate or ammonium perchlorate. These are called composite propellants. An example reaction goes as follows:
10Al + 6NH4ClO4 → 4Al2O3 + 2AlCl3 + 12H2O + 3N2
The aluminum is oxidized, yielding the high energy output of the reaction, and the ammonium chlorate acts as the oxidizer, easily accepting the aluminum’s electrons.
Other forms of solid rocket propellants include conventional black powder, adding high explosives to composite propellants, a powdered zinc-sulfur mixture, and using sugar as the oxidized material, amongst others.
Other Forms of Propulsion/The Future:
One promising form of rocket propulsion lies in ion-based systems, pursued by space agencies up to and including NASA. In an ion-based system, the careful position of electrodes is used to accelerate charged particles (the ions) to speeds that can reach a significant fraction of the speed of light. Theoretically, given enough time, the rocket and/or vehicle can accelerate with an upper bound of the speed of its ion exhaust, but other factors (i.e. time, particles in space) make the top functional speed of the rocket significantly slower, but still faster, depending on scale, than the current batch of propellant technologies. Already, ion propulsion is used to make minute adjustments in a rocket’s (or other space vehicle’s) trajectory and/or orientation, but the time of ion propulsion as the primary source of acceleration is still a ways away.
This system relies on a constant source of electrical power, usually solar or nuclear, given the constraints of a system with limited resources in outer space. It also requires a computer and a source of particles to ionize to operate, conditions unique to this system of propulsion.
The future is promising for ion propulsion technology, given the high theoretical velocity vehicles using it can reach and its relatively fuel-cheap design. If may well be that ion propulsion systems come to dominate the industry for space vehicle propulsion, but for now, we just have to rely on conventional fuels, be they cryogenic liquids or metallic powders.