AER 710 Aerospace Propulsion
Introduction
Propellers
Internal Combustion Engines
Gas Turbine Engines
Chemical Rockets
 Non-Chemical Space Propulsion Systems
Introduction to Non-Chemical Space
Propulsion Systems
• The chemical rocket family will continue to contribute to the
exploration of space, and are still major players as regards
to space propulsion
• The vacuum of space, and the large distances involved in
travel from one point to another, encourages innovation
and the consideration of more exotic options, including
systems that are not based on combustion
• On the low-thrust end of the spectrum, monopropellant and
bipropellant thrusters still have their place for attitude and
positional control of satellites and other spacecraft; for
lower cost, short missions, even non-combustion
approaches like pressurized cold-gas or heated cold-gas
thrusters are potentially viable options
Magellan spacecraft (for exploration of
Venus, 1990-1991; NASA/JPL project; all propulsion
systems on Magellan are chemical)
Spacecraft Attitude Control
• Commonly, s/c attitude control is shared between propulsive
thrusters and non-propulsive inertial momentum devices (e.g.,
reaction wheels, magnetorquers, extendible gravity or aerodynamicdrag booms)
• separation of duties for position and attitude control needs to be
established as part of the thruster integration to the space vehicle
Electric Propulsion for Space Flight
• EP systems are becoming more
competitive with spacecraft chemical
thrusters at the newton level; at the microand milli-newton level, EP systems are
becoming the dominant option, given the
commonly superior specific impulse and
low system weight
• EP systems characterized as having their
energy source (batteries, etc.) separate
from the mechanism of particle/gas
acceleration
Performance Equations in Space
m final
minitial
n[(
m final
minitial
)(
1
 exp(
m final
minitial
 V
 V
)  exp(
)
I sp  g o
ue
)(
2
m final
minitial
)]  
3
(Vtotal )
I sp  g o
, ideal rocket equation,
single mission segment
, three-stage example
1
2

m
u
e
kinetic power of jet exhaust Pjet 2
1
1


 u e  I sp g o
thrust delivered
F
m u e
2
2
I sp
ue

go
, vacuum ideal
Performance (cont’d)
t 
Pjet
Pinp
1
2

m
u
Pjet 2 e
FI sp g o



IV
IV
2 IV
s 
Pinp
, thruster efficiency
, specific power (W/kg)
m pp
100 to 300 W/kg typical
range; mpp is system dry
mass
EP Systems
• Currently three main categories for EP
thrusters: 1) Electrothermal, 2)
Electrostatic (Ion), 3) Electromagnetic
(Plasma)
• Lowest specific impulse associated with
first category, high specific impulse but low
thrust associated with second category,
and high specific impulse with substantial
thrust associated with the third category
Electrothermal Systems
• Characterized by use of liquid or solid
propellant that is electrically heated, with
resulting gas expanded and accelerated to
high exit speed
• Resistojets are a longstanding
electrothermal system, using a high elec.
resistance metal like tungsten to heat the
propellant that flows over the metal coil
• Modest Isp (200 to 300 s), newton level F
100-W miniature resistojet
using nitrous oxide as the
heated propellant , producing
up to 0.1 N (Univ. of Surrey)
• Arcjets use an electric arc discharge to
heat the propellant gas as it passes
through a cathode/anode nozzle structure
• Propellant in storage can be liquid or gas,
e.g., hydrazine, ammonia, hydrogen
• Temperature in vicinity of arc can reach
20000 K, while surrounding structure
should remain at less than 2200 K;
depending on this compromise, Isp can
reach 1500 s on high performance side
Diagram of a direct-current (DC) arcjet thruster [2], an electrothermal EP system.
Hydrazine is used as the working propellant
Electrostatic (Ion) Systems
• Characterized by use of heavier positive
ions (mercury, indium, cesium) stripped of
electrons, to produce the desired thrust
• EBT thruster uses electron bombardment
of propellant in storage to produce the
needed ions
• Ion contact thruster uses a hot ionizer
• Colloid thrusters pass a colloid (small
droplet) mixture through an intense electric
field
Electron bombardment thruster (EBT), using mercury
as propellant
• Thrusters accelerate the positive ions via a
strong electric field to a high exit speed
• To maintain a neutral charge on the
surrounding spacecraft, one needs a
negatively-charged neutralizing cathode
seeding electrons in the exiting ion stream
to remove the positive charge from the ion
beam; this tends to reduce the thrust a bit
Schematic diagram of generic direct-current ion thruster and associated
operations . Right photo of xenon-based 13-cm XIPS (Xenon Ion
Propulsion System) ion thruster
Electromagnetic (Plasma) Systems
• Characterized by use of neutrally charged
plasma (mix of electrons, positive ions,
neutral atoms), produced from electrically
heating a propellant in storage, that is then
accelerated by various techniques
exploiting electric and magnetic fields
• Inert gases like xenon and krypton a
common propellant choice
EM Systems (cont’d)
• The plasma is accelerated in a direction
perpendicular to the electric current
passing through it, and also perpendicular
to the magnetic field contributing to the EM
Lorentz force that is accelerating the
plasma
• To be effective, need a high plasma beam
density, which necessitates a high electric
power input to produce the needed plasma
• Hall or stationary plasma thrusters (SPTs)
exploit the Hall effect to strongly
accelerate the plasma; Hall effect is a
force resulting from an axial electric field in
the presence of a radial magnetic field,
with a spiral movement of electrons (Hall
current) aiding the force development, in
addition to keeping the plasma beam
neutrally charged at exit
• Isp from 3000 to 5000 s, with newton level
thrust at the high end (requires lots of
power to do so)
Schematic diagram at left of Hall thruster and associated operations , using xenon as the
propellant. Right photo and schematic diagram of SPT-100 Hall thruster
Solar-Thermal Propulsion for Space Flight
• EP systems noted earlier can use the
Sun’s energy to power spacecraft batteries
via solar cells
• Alternative use of the Sun is to focus its
infrared (IR) radiation into a graphite
collector/receiver, which can heat (without
combustion) a propellant like hydrogen to
produce thrust, or heat a thermionic
converter to produce electricity
Solar Orbit Transfer Vehicle illustration at left, and an inflatable
solar concentrator to be used by the SOTV on display at right.
A specific impulse of around 800 s was anticipated for the SOTV
in propulsion mode
SOTV
Nuclear-Thermal Propulsion for Space Flight
• Instead of using solar energy (especially
for spacecraft that are too far away from
the Sun), one can use radioactive decay,
or nuclear fission or fusion energy, to heat
a propellant medium like hydrogen without
combustion
• Solid-core nuclear fission rocket engines
like NERVA can deliver an Isp around 900
s, while proposed gas-core nuclear fission
engines can deliver an Isp approaching
6000 s (using much higher temperatures)
NERVA = Nuclear Engine for Rocket Vehicle Application
NERVA engine diagram (NASA)
• Solid nuclear reactor is surrounded by a
pressure vessel operating at the desired
chamber pressure for thrust delivery, e.g.,
3 MPa to 8 MPa, where propellant is
heated as it passes through peripheral
channels and exhausted as a hot gas
• More modern improvements in solid
nuclear reactor core design can be
applied, e.g., particle-bed reactors for
enhanced surface area of heating
Schematic diagram of nuclear/thermal rocket engine employing an
open-cycle gas-core fission reactor. A gaseous uranium plasma fuel core
is contained in the reactor by an induced toroidal vortex flow involving
the injected hydrogen propellant . The specific impulse from a gas-core
system may range from 1500 to 6000 sec, as compared to 900 sec for a
conventional solid-core system.
NTR = Nuclear Thermal Rocket
Orion Nebula
(interstellar
cloud)