There are many types of
rocket engines in use today for various specific tasks. However, there are only two standard types of
engines currently being used for
space program launch vehicles:
solid-fuel and
liquid-fuel engines. Hybrid-fuel engines are being considered as a third choice for
launch vehicles. In order to appropriately compare hybrid engines against those currently in use, it is necessary to have a basic understanding of the operation of each of these three engines.
All
rocket engines used for
launch vehicles consist of a
fuel and an
oxidizer which are mixed together and burned in a
combustion chamber. As the
fuel and
oxidizer react, large amounts of high-energy
gasses are released. These gasses expand, and as they escape through a
nozzle at the end of the
engine, they create the tremendous
forces that launch the
rocket into
space.
Solid-Fuel Engines
Solid-fuel rocket engines consist of a hollow tube of solid fuel with a
nozzle at the end. This solid core is actually a
fuel and an oxidizer mixed together with a
binder which causes the mixture to solidify. The
hollow center of the tube of
fuel acts as the
combustion chamber. The fuel is usually made from a powdered
metal such as
aluminum or
magnesium mixed with an
oxidizer, usually
ammonium perchlorate.
More information on solid rocket engines can be found here.
Liquid-Fuel Engines
The design of a liquid-fuel engine consists of two
tanks, one for the
fuel and one for the
oxidizer, and a separate
combustion chamber. The
fuel and
oxidizer each have a
pump system which pump the them into the
combustion chamber, where they mix and are
burned. Currently, highly refined
kerosene (
RP-1) and
cryogenic liquid
hydrogen (
LH2) are the most commonly used
fuels in liquid-fuel engines. Most current liquid-fuel engines used in the
space program use cryogenic liquid
oxygen (
LOX) as the
oxidizer. Most
military liquid systems use Inhibited Red Fuming Nitric Acid (
IRFNA).
(Thanks, Jurph, for that info.)
More information on solid rocket engines can be found here.
Hybrid-Fuel Engines
The overall design of a
hybrid-fuel engine involves a hollow core of solid fuel similar to those used in
solid-fuel engines as well as a
tank of
liquid oxidizer and a
pump system like those used in liquid-fuel engines. Possible
fuels for hybrid engines could include
aluminum,
magnesium,
polyethylene,
hydroxyl-terminated polybutadiene (
HTPB), or
beryllium. Possible
oxidizers could include
cryogenic LOX or
hydrogen peroxide.
Performance
Comparisons of hybrid-fuel engines to existing engines are difficult because
hybrids are currently only
experimental.
Full-scale hybrid-fuel engines have never been built and tested. However, enough experimental results have been produced to
extrapolate and compare the
performances of full-scale hybrid-fuel engines to solid and liquid-fuel engines.
Hybrid engines cannot produce the extremely high
efficiency of liquid-fuel engines, but compare quite favorably with solid-fuel engines. In terms of
thrust to
weight ratio,
hybrids can approximately equal
solids. Data suggests that the achievable
specific impulse of a hybrid-fuel engine may even exceed that of
solid-fuel boosters currently in use.
Cost
Hybrid engines combine the
simplicity of the solid-fuel core with the
complexity of
pump systems for the liquid
oxidizer. However, because
pumps are only needed for the
oxidizer, only about half of the pump systems used in liquid-fuel engines are required in hybrid-fuel engines. In addition, because the solid-fuel core is smaller than that used in solid-fuel rockets, hybrid-fuel rockets have less of a chance for
burn-through problems that sometimes affect
solid-fuel rockets. Hybrid-fuel engines must deal with the same
cryogenic handling difficulties and costs as liquid-fuel engines. These costs can be avoided by using
hydrogen-peroxide as he
oxidizer instead of
LOX.
Additional savings can be achieved because, like liquid-fuel engines, hybrid-fuel engines have the capability to
throttle. This allows cheaper testing, and also reduces the cost and difficulty of an
emergency launch abort.
Hybrid-fuel engines should be able to offer a significant savings over liquid-fuel engines, because they only experience half of the difficulties associated with liquid pump systems, and can avoid difficulties with
cryogenics entirely. In relation to solid-fuel engines, they can reduce the problems regarding the solid-fuel core because it is much smaller. In addition, the extra expense of the liquid system may be offset by the savings gained in
safety precautions and
throttleability.
Even with the advancements being made in the field, liquid-fuel engines remain much more
expensive than solid-fuel engines. Though a quantifiable comparison is difficult to make, it appears that hybrid-fuel engines represent similar costs to solid-fuel engines. Because of the additional advantages of increased
safety and throttling capability, hybrid-fuel engines may even represent savings over the cost of
current solid-fuel engines.
Environmental Impact
Liquid-fuel engines are the cleanest burning of the three types of
rocket engines. The primary components of their
exhaust are
water and
carbon-dioxide. Solid-fuel engines produce several
pollutants, including
hydrochloric acid (
HCl). Though recent developments will significantly reduce the amount of
HCl in engine exhaust, some acid will still be produced. Hybrid-fuel engines produce some of the same pollutants as solids, but because they use a
liquid oxidizer rather than
ammonium perchlorate, hybrids produce none of the
hydrochloric acid that solid-fuel engines produce.
Summary
Hybrid-fuel engines are very comparable to
solid-fuel engines. Hybrids can achieve higher
performances than solids. Hybrid-fuel engines also represent an overall savings over solid-fuel engines. Finally, hybrid-fuel engines have a similar
environmental impact to solid-fuel engines, with the additional advantage that no
hydrochloric acid is produced at all. In the future, hybrid-fuel engines could be a viable alternative to solid-fuel engines in
launch vehicle applications. Hybrids would increase performance, decrease cost, and eliminate a major
environmental concern of
current solid-fuel engines.