CRYOGENIC AIRCRAFT
DEVELOPMENT OF CRYOGENIC FUEL AIRCRAFT
In mid-1970-s of previous century energy strategic dominated in the USSR according to which all atomic energy was supposed to be utilized first while oil and gas should have been considered of minor importance in view of small resources as they erroneously believed at that time.
Realization of Hydrogen Energy Program started. Tupolev’s specialists were involved in the Program. As it used to happen many times in the background of our company – Alexey Tupolev took a courageous decision - to build “Hydrogen” aircraft. Such aircraft was built and successfully tested without any serious incidents. It was preceded by a long-term Program of bench and ground tests intended for testing functioning of new systems (such systems were more than 30 on the aircraft) and mainly for providing safe operation.
Unfortunately mentioned above energy strategy turned to be not very correct. Atomic energy has not become dominating. It was natural gas that turned to be of paramount importance in the Energetic Program of our country. The content of natural gas exceeds 50% of energy balance. That’s why our flying laboratory having status of experimental TU-155 a/c was modified to use not only liquid hydrogen but also to use Liquefied Natural Gas (LNG). This is how the first in the world Cryogenic Aircraft was built.
Remarkable properties of liquid hydrogen as aviation fuel and first of all its high ecological cleanliness, high heat of combustion and high cooling capacity attracted attention of aviation specialists to this type of fuel. Liquid hydrogen allows to improve aircraft performance significantly, to build aircraft operating at speeds of M>6. Therefore our activities on liquid hydrogen served as a scientific and technological work done which will be used in near-term outlook. However extremely high price of liquid hydrogen makes its commercial use impossible for a long time.
If to speak about near future tomorrow task is to introduce LNG as aviation fuel which was reflected in “Program on development of Russian civil aviation for the period from 2002 to 2010 and for the period till 2015”.Oil shortage is growing. During previous 25 years specific weight of oil in worlds energy balance decreased by more than 10%.
Currently price of kerosene is 8000 rubles per tone, LNG price is 3000 rubles per tone. Benefit makes 5000 rubles per each tone of replaced kerosene. The benefit is likely to grow constantly according to opinion of many specialists.
Recently some special scientific “explosion” happened in the world and especially in Russia that provoked a vision that traditional and non-traditional resources of natural gas can be increased by an order magnitude greater and exceed total amount of traditional fossil fuel on earth.
Natural gas is supplied to substantially each airfield via pipelines i.e. transportation issues have been practically solved now. Its high energy capacity, huge cooling capacity make it possible to build aircraft with significantly high performance in comparison with aircraft using kerosene. Fuel efficiency of flight using LNG can make 10 g/pass, km.
When using LNG potential emission of toxic agents will be decreased as follows: carbon monoxide – 1 – 10 times, hydrocarbons – 2.5 – 3 times, nitrogen oxides – 1.5 – 2 times, polycyclic aromatic hydrocarbons including benzapyrene – 10 times.
TU-155 a/c was built on the basis of serial TU-154B a/c. To use cryogenic fuel airframe and some standard systems were modified, cryogenic fuel charging, storage and feeding systems were installed that ensured fire/explosion safety, and data acquisition and recording system as well.In safety purpose experimental cryogenic fuel complex was disposed within special compartment isolated from adjacent fuselage compartments by buffer areas provided with ventilation system. Experimental LNG- or Hydrogen – powered NK-88 engine is located in the rightside nacelle. Cryogenic fuel resource is kept in fuel tank of 17.5 m3 capacity installed in special compartment in rear portion of passenger cabin.
To charge the aircraft with cryogenic fuels special charging complex was made which in view of safety was located on separated site that was also used for a/c parking and maintenance. Cryogenic fuel is delivered to the site by filling truck. Some issues concerning components and systems of experimental cryogenic TU-155 a/c were studied on ground rigs.
Creation of the aircraft was accompanied by serious scientific and research works and elaboration of large amount of regulatory documentation. 15 April, 1988 the aircraft performed its maiden flight using liquid hydrogen. Upon flight testing and development 18 January, 1989 TU-155 a/c performed its first flight on liquefied natural gas. Large flight testing Program was fulfilled, several international flight demonstrations were made including those to Bratislava (Czhekoslovakia), Nice (France), Berlin and Ganover (Germany).
Appearance of TU-155a/c changed dramatically scope of tasks for creation of cryogenic aviation. It was demonstrated in reality that using existing technical aids power plant has been built which allowed to operate Hydrogen- or LNG-powered aircraft with the same safety level than those working on kerosene.
Main result of the works on TU-155 aircraft represented creation of following:
• collectives of specialists skilled in aviation cryogenics in a number of companies;
• ground infrastructure to support cryogenic aircraft;
• complex of ground cryogenic rigs.
A real possibility appeared to turn to creation of LNG-powered aircraft.
In the course of the aircraft building several pioneering technical approaches were developed that were embodied on the aircraft and on rigs.
Experience of ensuring fire/explosion safety of cryogenic aircraft is unique. Principles and technical approaches that were developed when solving this problem (for example gas test system newly applied) will be used on all future cryogenic aircraft.
The same is about power plant of the aircraft which main technical approaches are quite new. Engine scheme and cryogenic components, fuel pumps, pressure maintenance system and cryogenic fuel tanks - all od these could be utilized in future developments.
Tupolev” PSC elaborated Cryogenic Aircraft Manufacturing Program. On the first phase of this Program TU-156 a/c was built.
Cargo-passenger TU-156 a/c was designed for optimization of airborne cryogenic fuel system during long-term operation and its certification and also for optimization of ground infra-structure. Cryogenic components of the aircraft will be installed on consequent serial Tupolev’s cryogenic aircraft.
The aircraft uses two fuels: aviation kerosene and liquefied natural gas which makes it possible to operate the aircraft from usual airfields and from airfields provided with LNG fueling systems. Use of two fuels improves flight safety level significantly.
TU-156 a/c is capable to carry 14 t of payload for distance 2600 km using LNG and for distance of 3300 km using LNG and kerosene. In rear portion of passenger cabin there is a ventilated compartment to receive a main cryogenic tank of 13 t capacity. Nose baggage compartment is provided with ventilated bay wherein trim cryogenic fuel tank is installed composed of two horizontal communicating vessels capable to receive 3.8 tones of LNG.
Substantially all technical publication and records have been issued for TU-156 a/c. NK-89 prototype engine and a number of other cryogenic components have been manufactured and are being tested on benches.
To support activities on TU-156 a/c manufacturing a ground cryogenic power plant rig is now being prepared. On the basis of certified mid-range passenger TU-204 a/c powered by PS-90A engines a mid-range TU-204K a/c powered by PS-92 engines is currently under development. Kerosene is located in wing fuel tanks, LNG tanks are disposed on fuselage within fairing which shape is of minimum aerodynamic drag. TU-204K a/c fuel efficiency makes 19.6 g/pass, km. This aircraft is capable to carry 210 passengers for distance of 5200 km. The aircraft tanks can receive 22.5 t of LNG and 5.5 t of kerosene as fuel reserve. Being developed to the level of technical proposals TU-204K uses main elements of cryogenic system installed on TU-156 a/c.
Works have been launched on manufacturing a new regional cargo/passenger TU-136 a/c powered by two TB7-117SF engines which has optimal layout in view of LNG properties. This aircraft is intended for passenger and cargo transportation. It can be operated from airfields of any class including unpaved ones. It will be operated on routes that connect regional centers of Russia and for cargo/passenger transportation for servicing centers of mineral industry and also to function as flying laboratory for inspection of gas lines in Northern areas and in Siberia. After being certified the aircraft can be used by foreign airlines in the countries with developed infra-structures of natural gas consumers.
This aircraft having takeoff weight of 20 t will carry 53 passengers and up to 5 t of cargo for 2200 km at the speed of 550 km/h and at the altitude of about 72 km. Fuel efficiency (about 20 g/pass, km) corresponds to that of the best aircraft in the world of this class while use of LNG will allow to cut down direct operational expenses by near 30%.
The aircraft was designed in view of special properties of cryogenic fuel. It is of biplane aerodynamic configuration logically interfaced with cryogenic fuel tanks. Structure of the aircraft was made with maximal wing and fuselage load relief. Cryogenic fuel tanks receiving 3680 kg of LNG are located in two nacelles installed behind power plant. Alignment of tank mid-section with engine nacelle mid-section and also supporting pylons agreement with general aerodynamic configuration made it possible to locate cryogenic fuel tank out of fuselage substantially without increasing aerodynamic drag and wing force. Short cryogenic paths are of small weight and do not require superheat insulation. Power plant spaced from passenger cabin increases passenger and crew members safety significantly and facilitates its experimental optimization. Kerosene is filled in wing tanks as in ordinary aircraft.
TU-330 transport a/c powered by LNG NK-94 engine is now under development. The aircraft is designed to carry military and civil cargo of 35 t weight. Aviation kerosene is located in wing fuel tanks, tanks with LNG are disposed on upper fuselage within fairing which shape is of minimal aerodynamic drag and receive 22.6 t of LNG. Engine NK-94 for this aircraft is developed by “SNTK named after N.Kuznetsov” PSC. It represents a cryogenic version of NK-93. Landing on unpaved airfield is provided. Currently the aircraft has been developed to the level of technical proposals. The aircraft includes all main elements of cryogenic system installed on TU-156 a/c and can use both LNG and kerosene. Design works have started to build on the basis of TU-334 a/c a short-range passenger TU-334K a/c powered by BR-710C engines. It will carry 102 passengers for the distance up to 2000 km at the altitude up to 10.6 km. The aircraft is fuelled with 7 t of LNG and 2 t of kerosene as fuel reserve. Main elements of cryogenic system can be taken from TU-156 a/c.