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For main engine burns, nitrogen tetroxide is the oxidizer and hydrazine is the fuel; for
the other reaction control system burns, those are monopropellant thrusters, where just
the hydrazine will decompose across the catalyst beds to give you some thrust.
Jeff Lewis
Spacecraft Engineer, Operations Lead, Juno
So very rarely do we burn things, only when we’re firing the main engine is there
combustion going on. The rest of the thrusters use an exothermic monopropellant
reaction. Hydrazine is effectively double ammonia N2H4 and it will break down
energetically to create ammonia, nitrogen and hydrogen, and in doing so the exhaust
products get hot. That sort of relates back to the rocket equation. The rocket equation
likes high exhaust velocity, and exhaust velocity relates back to having a low molecular
weight [exhaust gas]. You don’t want your exhaust product to be lead or depleted
uranium. You want something with a low molecular weight that you can make go very
fast, and that creates your efficiency, so having a propellant that breaks down into
ammonia, nitrogen and hydrogen works great. You get it hot so that gives you more
energy, and it all streams supersonically through the nozzle which gives you high
velocity, and thus gives you a more efficient approach.
Tim Martin
Spacecraft Engineer, Propulsion Lead, Juno
Chapter 8 – Propellants
A model rocket engine resembles nothing more than a standard “safe and sane” firework–
a hollow cardboard tube whose ends are covered by a clay-like material. Further
investigation with a knife reveals the interior to contain a gray powder, which most
people guess is the explosive propellant. Yet those people who have fortunate enough to
witness a major launch, such as the Space Shuttle or other governmental rockets, and
especially have heard the commentary, know that there are plenty of liquids involved –
for instance, “LOX” which stands for liquid oxygen.
How are there so many different kinds of propellants for rockets, and is there a best one?
These are the questions to be answered in this chapter.
There are many different ways to use the action/reaction phenomenon that Newton’s third
law allows. That is, there are many different ways to throw material out the back end of a
spacecraft; typically (but not always), the material that goes out the back is a gas or
plasma (ionized gas). For convenience, we’ll break these down into the three principal
types of generating the material that goes out the back: chemical reactions, plasma
reactions and nuclear reactions.
Chemical propellants are the most common method of moving a rocket
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Since they are the most common, we’ll start with chemical propellants – this
classification covers both the rockets that accelerate the model rocket and the Space
Shuttle. Chemical propellants are the most used, tested and understood of all propulsion
methods. Moreover, they produce the most reliable thrust and impulse.
Chemical rocket motors are further divided by the state of the fuel: solid, liquid or hybrid.
A hybrid engine uses both a solid and a liquid. Liquid chemical rocket motors are
subdivided by the number of chemical compounds that must be used: monopropellant
(one chemical) or bipropellant (two chemicals).
Chemical propellants, whether liquid or solid, rely on a chemical reaction to occur. At
least one fuel compound must react with at least one oxidizer compound; this reaction is
called combustion (or burning). The fuel is oxidized by the oxidizer and the resulting
products of the reaction, the exhaust gases, are ejected out of the back of the motor at a
rate of about 2 to 5 km/s.
Let’s start with the simplest motor: a solid chemical propellant rocket. The solid
propellant chemical rocket has a long history. The earliest Chinese rockets from the
thirteenth century were powered by black powder, which is a combination of charcoal,
sulfur and potassium nitrate, which was lit by a flame. Black powder burns quickly
because of the presence of oxygen in potassium nitrate. The exhaust gases are carbon
dioxide, and sulfur and nitrogen oxides. Thus, in this case, the carbon and the sulfur are
the fuel and the potassium nitrate is the oxidizer. Currently, the model rocket motors
(classes A through E) use black powder. Clay is shaped to make the nozzle in the back
end of the cardboard tube.
The black powder in these motors is compressed to allow shaping and to make the
burning of the substance more uniform. Once the amount of black powder becomes
approximately E motor size, the compressed black powder becomes too brittle to form
properly.
For high-powered rocketry, a propellant made of ammonium perchlorate (AP), potassium
nitrate and aluminum powder is often used. The ammonium perchlorate and the
potassium nitrate are the oxidizers, and the aluminum is the fuel. The exhaust gases are
aluminum oxide, hydrogen chloride and some nitrogen compounds.
For even higher-powered rocketry, such as the Space Shuttle Solid Rocket boosters, an
ammonium perchlorate composite propellant (APCP) is used. The formulation is slightly
different than the AP propellant above; APCP includes an organic plastic binder, such as
hydroxyl-terminated polybutadiene (HTPB), which acts as the fuel (in addition to the
aluminum powder).
The advantage of AP-based propellant is that they may be cast, rather than compressed
like black powder propellants. This means they can be made much larger than the black
powder propellants. In addition, AP-based propellants are more thermally stable than
black powder, and behave more reliably when ignited.
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The liquid propellants are classified according to the number of chemicals needed for the
reaction. A monopropellant, such as hydrazine, is a single chemical that can decompose
without another chemical to react with it. Hydrazine generates ammonia, hydrogen and
nitrogen gases that go out the nozzle. Hydrazine itself is highly toxic and unstable. One
advantage of hydrazine is the large quantity of gas it generates, especially at high
temperatures, relative to the amount of fuel used.
Liquid monopropellant engines have a long flight history, and do not require a lot of
plumbing to get the propellant to the combustion chamber. They are also easy to turn off
by shutting off the supply of propellant. The problem is that they don’t deliver a lot of
thrust or specific impulse.
A bipropellant is a system of two liquid chemicals that react to generate the gas or gases
that go out the back. An example of this is unsymmetrical dimethyl hydrazine (UDMH)
and dinitrogen tetroxide. The combination of these two chemicals is hypergolic, meaning
that, when the two are combined, they spontaneously ignite. UDMH is carcinogenic but
more stable than hydrazine. In addition, it can be stored for long periods of time in a fuel
tank of a launch vehicle without losing efficacy. On the other hand, mixing two liquids in
the right proportions at the right times poses a significant engineering challenge.
Another example of a liquid bipropellant system is the Space Shuttle main engine: it uses
liquid hydrogen (LH2) as the fuel and liquid oxygen (LOX) as the oxidizer. The exhaust
gas is water vapor. There are over a hundred pumps and valves, as well as a control
system, to deliver the fuel and oxidizer on time and in the proper amounts.
Liquid bipropellant engines can deliver a higher thrust and specific impulse than
monopropellant engines. They are also easy to shut off, and are more efficient than solid
or hybrid engines.
Hybrid systems use two chemicals: a solid and either a gas or liquid. For instance, a
common hybrid system is using solid HTPB (the plastic binder) fuel and gaseous nitrous
oxide as the oxidizer. The advantages of a hybrid system are simpler engineering than the
liquid bipropellant system, more specific impulse and lower explosion risk. The
disadvantages of a hybrid system are that they are more complex than a solid rocket, and
they don’t have a long history; the first one was successfully fired in 1952.
SpaceShipOne, the craft that won the Ansari X Prize in 2004, had a hybrid rocket system.
Plasma reactions use magnetic fields to manipulate charged particle streams
From previous chapters, you know that plasmas are charged particles in gaseous form.
The solar wind, for instance, is a example of a plasma, and it flows outward, following
the magnetic field lines of the Sun. A similar principle is involved on a much smaller
scale with plasma-powered rockets.
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There are three types of plasma-generating rockets; all use on-board electricity to
generate the charged particles in the first place.
Electrothermal systems use the electricity, as the name implies, to heat up a neutral gas
such that it will lose electrons and therefore become charged. But rather than using a
magnetic field to guide the plasma, the heat expanding the volume of the plasma is all
that is required; that extra volume of the heated plasma can be directed out of the nozzle,
and exhaust gases can be propelled at speeds much faster than chemical rockets. NASA’s
Arc Jet project is an example of such a technology. In addition to the complexity of
generating the plasma, these systems are not capable of generating high thrust, so their
use at present is limited to station-keeping.
Electrostatic systems, on the other hand, also generate a plasma using electricity, but do
use a magnetic field to accelerate and direct the ions through the nozzle. These types of
systems are quite efficient and generate high exhaust speeds. In a gridded ion thruster,
such as that used by NASA’s Dawn spacecraft, the ions are accelerated by passing them
through grids of alternating charges.
In the Dawn rocket, the plasma is generated from xenon atoms; xenon is a gas that does
not interact with most materials and thus is safe to store in a chamber for decades. The
xenon atoms are also massive (compared to other atoms), so a comparatively low flow of
these ions will produce as much thrust as a higher flow of lighter ions. The spacecraft,
therefore, does not need to carry as much fuel. Dawn carried 425 kg of xenon for over
five years that the rocket will be firing. The drawback is that the thrust of the rocket is
miniscule – it would take Dawn four days to accelerate from 0 to 60 miles per hour.
However, sustained thrust will result in the interplanetary speeds the spacecraft will need
to reach its objective asteroids.
Electromagnetic systems use an electrical current and a magnetic field to generate a
Lorentz force. Current flowing through a conductor in a magnetic field causes a force on
the conductor that is perpendicular to the direction of the current and the direction of the
magnetic field. It is this force that will propel the rocket, and is successfully used on
maglev trains on Earth.
The pulsed plasma thruster is the oldest of these electromagnetic spacecraft propulsion
systems. In this thruster, the electricity is used to heat a solid fuel (often the plastic
Teflon) into a plasma, then use two charged plates to generate a magnetic field, with the
plasma passing in between the plates. Because the plasma is charged, it is a current that
will interact with the charged plates’ magnetic field and thus yield a Lorentz force that
accelerates the plasma out of the nozzle. It has the advantage of being a fairly simple
design, but it does not generate much thrust nor is it particularly efficient in its use of
fuel, so its uses have been limited to pointing spacecraft and station-keeping.
Nuclear propellants are the realm of science fiction
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The idea of harnessing the tremendous power within the nucleus of an atom for space
travel has been around since the development of the first nuclear bombs.
In fact, the idea of using a nuclear bomb concussive force to move a spaceship was
proposed in 1958 and became Project Orion. The idea was relatively simple; the
spacecraft would hurl atomic bombs out the back and they would explode. There would
be a large and strong concussion plate on the back of the spacecraft to absorb the shock
of the blast wave, and, true to Newton’s Third Law, move the spacecraft forward.
Since the bomb was to detonate about 60 meters behind the spacecraft, the thrust that
each blast would provide was estimated at 1 megaNewton, so any humans on board
would have to be cushioned sufficiently. It was also estimated that the craft could
eventually achieve one-tenth the speed of light, far faster than any spacecraft today.
Though the costs of such a craft would be enormous and the engineering quite difficult,
the Nuclear Test Ban treaty of 1963 effectively killed off the project. The idea has
resurfaced periodicially, most recently in the form of antimatter-catalyzed nuclear pulse
propulsion which would involve the relatively small releases of nuclear energy initiated
by matter-antimatter annihilation that would nevertheless “push” a rocket forward. The
advantage of this type of nuclear propulsion is its relatively small thrust, which would not
require special shock-absorbing protection for humans or their equipment.
Currently, nuclear propulsion has been limited to nuclear thermal propulsion, in which a
nuclear reactor heats up the gas to be expelled from the rocket, and nuclear electric
propulsion, in which a nuclear reactor generates the electricity to power one of the
electrical propulsion methods.
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