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.