A preliminary deal for the sale of 24 advanced Russian Su-35 jet fighters to the People's Liberation Army indicates the technological hurdles China faces in developing its own J-20, especially in terms of engine technology, military analysts say. Mainland and Russian media reported last month that Beijing might purchase 24 Su-35s, an updated version of the fourth generation Su-27, for US$1.5 billion. The deal was first proposed by Moscow two years ago. Beijing expressed interest in purchasing only four Su-35s last year, but that was rejected by Moscow, which had originally expected China to buy 48 planes, Moscow's Vedomosti business daily quoted an official from Russia's Federal Service for Military and Technical Co-operation as saying. It also quoted Igor Korotchenko, head of the Russian Defence Ministry's public council, as saying Moscow also asked Beijing to sign an agreement not to make copies of the Su-35. A Beijing-based PLA senior colonel, who requested anonymity, said: "We decided to buy the Su-35 because it's a fact that our home-made engines have failed to measure up to the Russian products." He said China was still playing catch-up, despite recent headlines hailing its progress on military modernisation. "Engines have been the biggest headache and we are still trying to cope with it," he said. "The purchase of the Su-35s might help our J-20 project, but there are too many deeper problems hiding in our military industrial system that are hindering our research and development." Andrei Chang, editor-in-chief of the Canadian-based Kanwa Defence Review , said the possible Su-35 deal was aimed at obtaining 117S engines, the most advanced Russian engine installed in a single-seat, twin-engined fighter. "Beijing wants the engine [of the Su-35]… because their J-20 project failed to solve the most challenging part - the engine," Chang said. The J-20, China's first stealth fighter, was unveiled early last year. Photographs posted on the internet by mainland military enthusiasts show the J-20 prototype has a stealth body shape similar to the American F-22 and F-35, with a hint of the Russian Su-27 in its turbofan engines. Military experts at home and overseas do not expect the J-20 to enter service until 2018 because of the engine problem. Beijing's earlier expression of interest in purchasing only four Su-35s was rejected by Russia because of copycat concerns, the Moscow Times reported. "Beijing compromised on the size of [the Su-35 deal] … because they think 24 is an acceptable negotiation quantity, while Moscow feels it is worth considering," Chang said. The deal could also give a much-needed boost to Russia's military industry, which desperately needs China's business because formerly stalwart customers are proving less reliable. "The Russian side has found their Su-35 production line lacks enough orders to [reduce unit cost and make it a profitable project]," Chang said. "Russia originally expected some sales to Libya, who promised to order some [Su-35] jets under Muammar Gaddafi's administration, while Venezuela also planned to place orders," he said. "But both have now backtracked on those promises due to political upheaval at home." However, the export of Su-35s to China has stirred up a heated debate among Russian defence industry enterprises and military officials, with opponents to the deal highlighting China's history of copying Russian weapons, Chang said. China has long been producing unauthorised copies of the Russian Su-27 - as the Jian-11. It has also designed a two-seat fighter bomber version (the Jian-16) and a stealth version (the Jian-17), and obtained an aircraft-carrier-based version of the Su-30 from Ukraine to produce the Jian-15. But Beijing insists they are all indigenous designs that just happen to bear some resemblance to Russian fighters. "I think the Su-35 deal will be submitted to the Kremlin to make the final decision," Chang said, adding there was still a long way to go before a final deal was signed. "China and Russia have just signed a memorandum this year, meaning both sides have yet to decide on the deal because they need further rounds of negotiation and discussion involving prices, which models of Su-35 will be exported, what kinds of weapons systems will be involved in the deal, technology transfer and other details."