1.3 Classifications of Propulsion Systems
1.3.1 External Combustion
In 1902,Louis Gagnon flew a steam helicopter inRossland, British Columbia, called the “Flying Steam Shovel”. Control problems caused a crash.
On 12 April1933, a Travel Air 2000 biplane made the world’s first piloted flight under steam power over Oakland, California. Brothers Besler (George and Wil- liam) flew a prototype steam powered biplane, based on aTravel Air 2000, several times (taking off, circling about, and landing, to show the ease of control) at Oaklandairport. Airplane remained aloft for 5 min at each time. It was powered by a two-cylinder, double-acting reciprocating, 90! V engine, 150 hp (110 kW) weighing about 500 lbs.
Aircraft Engines
External Combustion
Engines
Pulsejet Ramjet
Others
Wankel Reciprocating
Scramjet
Human
Shaft based Reaction Engines
Internal Combustion
Engines
Turboprop Turboshaft Propfan
Turbojet Turbofan
Turbo-ram Advanced
Ducted Turbo-
Rocket Steam
Stirling
Intermittent Solar/
Electric
Continuous Turbine
based
Athodyd or Ram-based Nuclear
Fig. 1.56 Classification of propulsion systems
Steam is generated from water by burning cheap crude oil. Sealed condensers return the steam from the cylinders to the boiler with only 1 % loss. Constant pressure in the boilers is maintained by electric gauges, which automatically ignite the burners when pressure begins to fall. Take-off pressure can be generated in 1 min. It was capable ofSTOLoperation due to the ease of reversing thrust.
The strangest feature of the flight was its relative silence. The advantages of this engine were the elimination of audible noise and destructive vibration; greater efficiency at low engine speeds and also at high altitudes where lower air temper- atures assisted condensation; reduced likelihood of engine failure; reduced main- tenance and fuel costs. However, steam reciprocating engine turned out to be unsuitable for scaling up to the needs of large aircraft. Thus at present days no single aircraft is powered by steam engine.
1.3.1.2 Stirling Engines
Astirling engineis aheat enginehaving either air or other gas as a working fluid. It operates by cyclic compression and expansion of the working fluid, at different temperature levels such that there is a net conversion ofheatenergy to mechanical work. Stirling engines have many advantages like high power density and low cost, quieter, less polluting, gain efficiency with altitude due to lower ambient temper- atures; more reliable due to fewer parts and the absence of an ignition system, produce much less vibration (airframes last longer), and safer, less explosive fuels may be used. However, they have low power density compared to the commonly used piston engines and gas turbine. This issue made them critical for use as aircraft engines.
1.3.1.3 Nuclear Engines
Anuclear aircraftis an aircraft powered by nuclear energy. Research into them was pursued during the Cold War by the United States and the Soviet Union as they would presumably allow a country to keep nuclear bombers in the air for extremely long periods of time, a useful tactic for nuclear deterrence. Neither country created any nuclear aircraft in production numbers. One design problem, never adequately solved, was the need for heavy shielding to protect the crew from radiation sickness. Also, in consideration, was the ecological impact of a crash during operations. Should one of these aircrafts were to crash in a populated area, the radiation fallout could have been disastrous.
1.3.1.3.1 U.S. Programs
In May 1946, the Nuclear Energy for the Propulsion of Aircraft (NEPA) project was started by the United States Air Force. Studies under this program were
done until May 1951, when NEPA was replaced by the Aircraft Nuclear Pro- pulsion (ANP) program. The ANP program included provisions for studying two different types of nuclear-powered jet engines, General Electric’s Direct Air Cycle and Pratt & Whitney’s Indirect Air Cycle. ANP also contained plans for two B-36 s to be modified by Convair under the MX-1589 project; one of the B-36 s was to be used to study shielding requirements for an airborne reactor while the other was to be the X-6. However, the program was cancelled before the X-6 was built.
The Oak Ridge National Laboratory conducted research (Aircraft Reactor Experiment) to produce a nuclear powered aircraft. GE produced the X-39 engine, which was a modified version of its J47s turbojet engine. The USA designed these engines to be used in a new specially designed nuclear bomber, the X-6. The X-6 was eventually terminated by Eisenhower.
The General Electric program in Evendale, Ohio, relied upon direct air cycle program because of its advantages in simplicity, reliability, suitability, and quick start ability. Conventional jet engine compressor and turbine sections were used, with the compressed air run through the reactor itself to heat it before being exhausted through the turbine.
The Indirect Air Cycle program was assigned to Pratt & Whitney, at a facility near Middletown, Connecticut. This concept would have produced far less radio- active pollution. One or two loops of liquid metal would carry the heat from the reactor to the engine. The Indirect Cycle program never approached anywhere near producing flight-ready hardware.
On September 5, 1951, the USAF awarded Consolidated-Vultee a contract to fly a nuclear reactor onboard a modified Convair B-36 under the MX-1589 project of the ANP program. The NB-36H Nuclear Test Aircraft (NTA) was to study shielding requirements for an airborne reactor, to determine whether a nuclear aircraft was feasible. This was the only known airborne reactor experiment by the U.S. with an operational nuclear reactor on board. The NTA flew a total of 47 times testing the reactor over West Texas and Southern New Mexico. The reactor, named the Aircraft Shield Test Reactor (ASTR), was operational but did not power the plane, rather the primary purpose of the flight program was shield testing. Based on the results of the NTA, the X-6 and the entire nuclear aircraft program were abandoned in 1961.
1.3.1.3.2 Soviet Programs
The Soviet program of developing nuclear aircraft resulted in the experimental Tupolev 119 (Tu-95 LAL), which was based on a Tupolev Tu-95M bomber. It had 2 conventional turboprop engines and 2 experimental ‘inboard’ direct cycle jet engines powered by a minimally shielded nuclear reactor in the main fuselage. The Tu-119 completed 34 research flights. Most of these were made with the reactor shut down. The main purpose of the flight phase was examining the effectiveness of the radiation shielding which was one of the main concerns for the engineers.
Massive amounts of protection used resulted in radiation levels low enough to consider continuing development. But, as in the US, development never continued past this point. Budgetary constraints and the development of new conventional aircrafts designs were cited as the main reason for the cancellation of the program in August 1966. Several other projects reached only design phase.
1.3.1.4 Final Comment
As described above, the three external combustion engines are not appropriate for employment in aviation field for different reasons. Steam engines are only appro- priate for small aircrafts while large ones need heavy boilers, piping and other accessories. Stirling engines generate also low power which is also improper for present aircrafts. Nuclear engines have two drawbacks regarding shielding of flight crew and passengers versus radiation, as well as the risk of crash in residence areas leading to catastrophic situation.