04-07-2012, 02:53 PM
Hybrid Rocket Technology
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ABSTRACT
With their unique operational characteristics, hybrid rockets can potentially provide safer, lower-cost
avenues for spacecraft and missiles than the current solid propellant and liquid propellant systems. Classical
hybrids can be throttled for thrust tailoring, perform in-flight motor shutdown and restart. In classical hybrids,
the fuel is stored in the form of a solid grain, requiring only half the feed system hardware of liquid bipropellant
engines. The commonly used fuels are benign, nontoxic, and not hazardous to store and transport. Solid fuel
grains are not highly susceptible to cracks, imperfections, and environmental temperature and are therefore
safer to manufacture, store, transport, and use for launch.
INTRODUCTION
Any propulsion system that is not entirely homogeneous
can be called a hybrid, but the classic hybrid rocket motor
that interests the propulsion community today might best
be described as a combination of a solid and a liquid
system. More specifically, it is usually a motor using a
solid fuel and a liquid oxidiser as shown in Fig.1. It could
be the reverse, a solid oxidiser and a liquid fuel, but the
availability of suitable fuels and oxidisers favours the
former combination.
EARLIER DEVELOPMENTS IN HYBRID
ROCKET TECHNOLOGY (1930-1970)
An interesting discussion of the history of hybrid
rocket development is presented by Altman.2 The very
first reported experiment on a hybrid rocket motor in the
late 1930’s was conducted by Smith and Gordan. In the
same period, Oberth in Germany did some work on LOXgraphite
rocket2.
The Pacific Rocket Society successfully flew a hybrid
rocket with LOX and a rubber-based fuel reaching an
altitude of 30,000 feet in June 1951. During the late 1940’s,
Moore and Berman tested a hybrid rocket motor with 90
per cent hydrogen peroxide (H2O2) as oxidiser and Polyethylene
(PE) as fuel.2
STUDIES IN HYBRID ROCKET TECHNOLOGY
Regression Rate Enhancement
McDonnel Douglas Aerospace has developed a high
performing second-generation fuel, based on a combination
of amine fillers that enables tailoring of the regression rate
exponent. The characteristics of this fuel include higher
density with higher density-specific impulse and higher
regression rate. Environmental Aero Science also reported
an enhanced regression rate using an azide-based polymer
for the fuel grain12.
Strand, et al. supported the inclusion of particulate
additives like aluminum and coal in solid fuel to enhance
regression rate19. Researchers at Pennsylvania State University
conducted a series of investigations and found that the
addition of ultra-fine aluminum powder having mean particle
size between 0.05 μm and 0.10 μm could significantly increase
both regression rate and mass burning rate compared with
pure HTPB.20 The mass burning rate increased by 70 per
cent, over that of pure HTPB. Based on observations of
the different fuel surfaces, the primary mechanism of regression
enhancement is thought to be associated with aluminum
heat release or particle micro-explosion at or near the solid
fuel surface.
Liquefying hybrids
A fast burning, long chain hydrocarbon-based (nonpolymeric)
paraffin hybrid fuel has been developed and
successfully tested by researchers at Stanford University
in a lab-scale motor27. The results indicate regression rates
3-4 times larger than the rates of the conventional polymeric
fuels. These newly identified high regression rate fuels
burn in a fundamentally different way than the slow-burning,
evaporative-diffusive-dominated combustion process of
conventional hybrid fuels.
Hydrogen Peroxide Hybrid Rocket
The monopropellant H2O2 has attracted the interest
of propulsion community as a possible alternative to hydrazinebased
propellants28. The major advantages of H2O2 are its
high density, storability, non-toxic nature, high oxidiser
to-fuel ratio for optimum operation, low vapour pressure,
and high specific heat. Also, hybrid rockets with H2O2 as
the oxidiser can be used in a dual-mode operation.
The researchers at Purdue University have conducted
a number of experiments using concentrated H2O2 as oxidiser
and PE as fuel and regression rate characterisations have
been obtained29. They developed a consumable catalyst
bed for the disintegration of H2O3 and also demonstrated
the spontaneous ignition of the PE fuel.
APPLICATIONS OF HYBRID ROCKETS
The feasibility of most of the outstanding features
predicted for hybrids has been demonstrated; namely high
impulse performance, start-stop operation, thrust modulation,
temperature and pressure insensitivity, and efficient throttling
over a range of 10 to 1. Successful tests have been
completed over a wide range of thrust levels from < 0.1
to > 250,000 lb and for durations from 0.1 s to 300 s. Some
remaining features, such as suitability for segmenting and
clustering, appear even more probable than were several
years ago, but their demonstration must wait for more
ambitious programmes than those now in progress.
CONCLUSIONS
Though the research in hybrid rocket propulsion was
making rapid developments in the past several years, it
wasn’t given serious attention by vehicle engineers, probably
because of the gap thatexisted between actual versus
theoretical performance. There were many problems which
prevented their entry into the industry in a big way, such
as low recovery of theoretical specific impulse, uneven
fuel burnout along the length of combustion chamber,
unpredictable decay in thrust level with time, high losses
in efficiency with thrust modulation,