D.3 Combustion Engines
D.3a Air-Breathing Engines
Concepts 24 through 27 all involve using a planet's (usually the Earth's)
atmosphere as a supply of oxygen to support combustion with a fuel carried
on the vehicle. It should be noted that some vehicle concepts (such as the
National Aerospaceplane (NASP) would integrate more than one engine
concept in a single engine. For example, most NASP configurations would
have ramjet and scramjet propulsion combined in the same engine.
The fanjet is the standard type of jet engine found on passenger aircraft and
military aircraft. The original form of the engine, the turbojet, has a series
of turbine compressor stages to compress the incoming air flow. This is
followed by a combustor where fuel is added and burned, creating a hot
gas. The gas is then expanded through a turbine which is connected by a
shaft to the compressor. The expanded gas emerges at high velocity from
the back of the engine.
The modern fanjet adds a fan which is also driven by the turbine. All of the
airflow goes through the fan, but only a part goes into the compressor. The
air which does not go into the compressor is said to have 'bypassed' the
compressor. The 'bypass ratio' is the ratio of bypass air to combustor air.
Generally higher bypass ratio engines are more fuel efficient (in units of
thrust divided by fuel consumption rate). Also in general, engines that
operate at higher speeds are designed with lower bypass ratios.
Typical modern performance values are engine thrust-to weight ratios (T/W)
of 6:1 for large subsonic engines, trending towards about 10:1 for high
performance military jets. Fuel efficiency is measured in units of thrust
divided by mass flow rate. In English units this is pounds divided by
pounds per second, or just seconds, and is termed 'specific impulse'. In SI
units this is Newtons per kilogram per second, which has the units of
meters per second. In some propulsion systems, such as chemical rockets,
the SI unit corresponds to the actual exhaust jet velocity. In the case of air-
breathing propulsion it is not, the velocity result is just an indicator of
engine efficiency. In English units the performance of subsonic engines is
about 10,000 seconds, trending to about 7000 seconds for supersonic
military engines. Fanjets and turbojets operate up to about 3.5 times the
speed of sound (M=3.5).
In common use on aircraft for aircraft propulsion. The B-52 bomber has
been used to carry the Pegasus three stage solid rocket to 35,000 ft
altitude. The B-52 uses 8 fanjet type engines for propulsion. Numerous
paper studies have been made of using aircraft as carriers for rocket stages.
A fan compresses incoming air stream, which is then mixed with fuel,
burned and exhausted. Compressor is driven by gas generator/turbine. In a
fanjet, the incoming air is compressed and heated by the compressor stages,
then mixed with fuel and run through the turbine stages. At higher
velocities the air gets hotter in compression since it has a higher incoming
kinetic energy. This leads to a higher turbine temperature. Eventually a
turbine temperature limit is reached based on the material used, which sets a
limit to the speed of the engine. In the turbo-ramjet the compressor is
driven by a gas generator/turbine set which use on-board propellant for their
operation. Since the gas generator is independant of the flight speed, it can
operate over a wider range of Mach numbers than the fanjet ( to Mach 6 vs.
to Mach 3)
Incoming air stream is accelerated to subsonic relative to engine, mixed
with fuel, then exhausted. The incoming air is moving at the vehicle
velocity entering the engine. After burning the fuel, the air is hotter and
can expand to a higher velocity out the nozzle. This sets up a pressure
difference that leaves a net thrust. Ramjets cannot operate at zero speed, but
they can reach somewhat higher limits than an engine with rotating
machinery (range Mach 0.5 to about Mach 8).
Incoming air stream is compressed by shock waves, mixed with fuel, and
expanded against engine or vehicle. Tha airstream remains supersonic
relative to the vehicle. The forward thrust is produced by expanding the
exhaust against a nozzle shape. Even though the gas is moving
supersonically relative to the vehicle, the sidewise expansion can act on the
vehicle if the slope of the nozzle is low enough. Thus the vehicle can fly
faster than the exahust gas moves. Scramjets may provide useful thrust up
to about Mach 15, or 60% of orbital speed.
28 Inverted Scramjet
Alternate Names: Buoyant Scramjet
Description: Series of balloons floated in atmosphere through which
projectile flies. Projectile carries oxygen and flies through hydrogen
(oxygen is much denser, so cross section is reduced.
29 Laser-Thermal Jet
Description: Laser is focussed and absorbed in heat exchanger, or laser-
[D19] Myrabo, L. N. "Concept for Light-Powered Flight", AIAA paper
number 82-1214 presented at AIAA/SAE/ASME 18th Joint Propulsion
Conference, Cleveland, Ohio, 21-23 June 1982.
D.3b Internally Fuelled Engines
30 Solid Rocket
Description: A solid rocket consists of a high-strength casing, a nozzle,
and a solid propellant grain which burns at a pre-designed rate. The grain
is a mixture of materials containing both fuel and oxidizer, so combustion
can proceed without any external action once it is ignited. Modern solid
propellants have a formulation close to the following: About 15% by
weight organic fuel, usually a type of rubber, about 20% by weight
aluminum powder (which acts as a metallic fuel), and about 65%
(NH3ClO4), which is the oxidizer. About 1-2% epoxy is added to the
powders to hold them together. The epoxy, being an organic material, is
also part of the fuel.
31 Hybrid Rocket
Description: The hybrid rocket consists of a solid fuel grain and a liquid
oxidizer. One combination is rubber for the fuel and liquid oxygen for the
oxidizer. The fuel is in the form of a hollow cylinder or perforated block.
The oxidizer is sprayed onto the fuel and the material is ignited. By not
being self-supporting in combustion, the fuel part can be treated as non-
hazardous when being made and shipped. Only when on the launch pad and
the oxidizer tank is filled is there a hazardous combination. With only a
single liquid to handle, the harware is relatively simple in design.
32 Liquid Rocket
Description: Mixture of fuel and oxidizer are burned in combustion
chamber which leads to a converging-diverging nozzle. The flow becomes
sonic at the narrow part of the nozzle, then continues to accelerate in the
diverging part of the nozzle. A variety of propellant combinations have
been used, including mono- bi-, and even tri-propellant combinations.
Status: This is the most common form of launch propulsion used to date
to put things into Earth orbit.
Variations: Number propellant variants by oxidizer/fuel letters
(incomplete list of propellants)
LIQUID ROCKET PROPELLANT TABLE
Chemical Name Formula Mol. M.P. B.P. Density
Weight (K) (K) (kg/m^3)
a Oxygen O2 32
b Hydrogen Peroxide O2H2 34
c Fluorine F2 38
d Nitrogen Tetroxide N2O4 92
e Chlorine Pentafluoride ClF5 125.5
a Hydrogen H2 2
b Methane CH4 16
c Propane C3H8 44
d Monomethyl Hydrazine CH3N2H3 46
e Kerosine (RP-1) ~CnH2n ~14n
[D20] Cooper, Larry P. "Status of Advanced Orbital Transfer Propulsion",
Space Technology (Oxford), v 7 no 3 pp 205-16, 1987.
[D21] Godai, Tomifumi "H-II Rocket: New Japanese Launch VehicleÊ in
the 1990s", Endeavour , v 11 no 3 pp 116-21, 1987.
[D22] Wilhite, A. W. "Advanced Rocket Propulsion Technology
Assessment for Future Space Transportation", Journal of Spacecraft and
Rockets, v 19 no 4 pp 314-19, 1982.
33 Gaseous Thruster
Description: The propellant is introduced in gas form to the chamber. It
may be a mono-propellant (a single gas) or a bi-propellant combination.
34 Mechanically Augmented Thruster
Description: Velocity of exaust gases is increased by placing thrusters on
end of rotating arm. Adds 200-300 sec to specific impulse based on
structual material capabilities.
D.4 Thermal Engines
35 Electric-Rail Rocket
Description: High voltage electricity supplied by rails is shorted through
tungsten heat exchanger, which heats hydrogen carried by vehicle flying
[D23] Wilbur, P. J.; Mitchell, C. E.; Shaw, B. D. "Electrothermal
Ramjet", AIAA paper number 82-1216 presented at AIAA/SAE/ASME 18th
Joint Propulsion Conference, Cleveland, OH, 21-23 June 1982.
Description: Sunlight generates electricity, which is used to heat gas
passed over or through a heating element.
[D24] Louviere, Allen J. et al "Water-Propellant Resistojets for Man-
Tended Platforms", NASA Technical Memorandum 100110, 1987.
Description: Sunlight is concentrated by a reflector or lens, then heats an
absorber. The absorber transfers heat to a working fluid, usually hydrogen.
The hydrogen is then expanded through a nozzle.
[D25] Gartrell, C. F. "Future Solar Orbital Transfer Vehicle Concept",
IEEE Transactions on Aerospace Electronic Systems, vol AES-19 no 5 pp
Beam is passed through window in rocket engine. It is then absorbed by a
heat exchanger or is focussed to create laser-sustained plasma. Hot gas is
then expelled through nozzle. By using an energy source external to the
propellant, specific impulse increases of 100% can be achieved by using
hydrogen rather than oxygen/hydrogen.Ê One method of doing this is with a
large, ground-based laser to heat the hydrogen. This concept is applicable
from the ground to orbital velocity, and may be used in conjunction with
another concept. Use of laser propulsion only in an upper stage would
allow smaller lasersÊthan are required for a first stage laser rocket, hence a
laser upper stage has nearer term technical viability than a first stage.
[D26] Abe, T.; Shimada, T. "Laser Assisted Propulsion System
Experiment on Space Flyer Unit", 38th International Astronautical
Federation Conference paper number IAF-87-298, 1987.Ê
[D27] Abe, T.; Kuriki, K. "Laser Propulsion Test Onboard Space
Station", Space Solar Power Review vol 5 no 2 pp 121-5, 1985.
[D28] Jones, L. W.; Keefer, D. R. "NASA's Laser Propulsion Project",
Astronautics and Aeronautics, v 20 no 9 pp 66-73, 1982.
39 Laser Detonation-Wave Engine
Propellant is a solid block with a flat bottom. First laser pulse evaporates a
layer of propellant. Second, larger, pulse creates plasma detonation wave,
which shocks and heats the propellant layer. Layer expands against base of
[D29] Kare, J.T. "SDIO/DARPA Workshop on Laser Propulsion, Volume
1: Executive Summary" Lawrence Livermore National Laboratory report
number DE87-003254, 1987.
40 Microwave Thermal
Microwaves are absorbed by engine, which becomes hot. Hydrogen is
flowed through engine, gets hot, and is then exhausted. A large phased
microwave array on the ground can focus onto a rocket-sized area over a
range of hundreds of kilometers. Given a way to couple the microwave
energy to a working fluid such as hydrogen, this type of propulsion could
provide significant launch vehicle velocities. High power microwave
amplifiers exist in a variety of forms with efficiencies up to 75% and power
levels up to one megawatt. This concept uses direct heating of the engine
structure, which acts as a heat exchanger to heat the working fluid.
Example: 10 meter diameter receiver, 5 cm wavelength, 1 km phased array,
range = 200 km.
41 Solid Core Nuclear
Hydrogen is heated by flowing through nuclear reactor, then exhausted in
rocket nozzle. Although the nuclear rocket program was stopped a number
of years ago, more recent work at Brookhaven National Laboratories on
fluidized particle bed reactors warrants their consideration for launch
vehicles. The small particle size (.3 mm) allows high heat transfer rates to
the working fluid, hydrogen, and hence potentially high thrust to weight
[D30] Thomas, Ulrich "Nuclear Ferry - Cislunar Space Transportation
Option of the Future", Space Technology (Oxford) v 7 no 3 pp 227-234,
[D31] Holman, R.R.; Pierce, B. L. "Development of NERVA reactor for
Space Nuclear Propulsion", presented at AIAA/ASME/SAE/ASEE 22nd
Joint Propulsion Conference, Huntsville, Alabama, 16-18 Jun 1986, AIAA
paper number 86-1582, 1986.
[D32] Thom, K. et al "Physics and Potentials of Fissioning Plasmas for
Space Power and Propulsion", Acta Astronautica vol 3 no 7-8 pp 505-16,
Jul. -Aug. 1976.
[D33] DiStefano, E. "Space Nuclear Propulsion - Future Applications and
Technology", 2nd Symposium on Space Nuclear Power Systems,
Albequerque, New Mexico, 14 January 1985, pp 331-342, 1987.
42 Liquid Core Nuclear
Description: In order to attain higher performance than a solid core
rocket, the reactor core is raised to a high enough temperature to become
liquid. Hydrogen is bubbled through the liquid, then exhausted out a
43 Gas Core Nuclear
Description: The reactor core is hot enough that the core is gasseous in
form. The hydrogen flow is seeded with an absorbent material to directly
absorb the thermal radiation from the core. The core is kept from leaking
out the nozzle by a transparent container (nucear light bulb), a flow vortex,
which uses the density difference between uranium and hydrogen, or
magnetic separation, which uses the ionization difference between the
uranium and the hydrogen.
44 Muon-Catalyzed Fusion
A beam of muons is directed at a deuterium/tritium mixture, where the
muons catalyze mutiple fusion reactions. The heated gas powers an
electric generator to power an ion or neutral particle beam thruster.