http://www.dsti.net/News/52691.htm
X-51A專案面臨一定風險
[據英國《飛行國際》2009年3月30日報導] 波音公司X-51A項目官員承認僅僅依靠4次試驗就成功完成高超聲速飛行器的飛行試驗項目面臨著一定的風險。
波音X-51A專案經理Joseph Vogel表示最好能進行10次飛行試驗,但他對只依靠有資金支持的4次試驗就成功完成專案也表示樂觀。他還補充說如果能尋求到新的資金來源,試驗的次數將可能再增加兩次。
美國空軍研究試驗室(AFRL)負責X-51A的專案經理Charlie Brink將高超聲速飛行器比作巡航導彈技術,不過巡航導彈的飛行試驗是出了名的問題專案,之前進行的20次巡航導彈飛行試驗有一半以失敗告終。
重1815kg的試驗飛行器設計可以承擔超高速巡航導彈任務,一枚與X-51A大小相當的“乘波者”導彈能夠在10分鐘內飛行965 km,與之相比,洛克希德•馬丁公司的AGM-165“聯合防區外空對地導彈”(JASSM)打擊480 km外的目標則可能需要一個小時。
“乘波者”具有棱角的鼻錐設計可“騎乘”在超聲速激波上飛行,儘管會產生一些阻力。同時由普•惠公司火箭發動機部設計的SJX61-2超燃衝壓發動機能夠在空氣進入進氣道時利用來流的速度當作天然的壓縮級。
空氣在進入發動機後首先與乙烯混合,然後是JP7燃料,燃燒溫度高達1870°C。為了防止發動機過熱,燃料也用來充作冷卻劑,在發動機結構周圍進行迴圈流動。X-51A項目希望盡可能地依靠傳統燃料和材料,這是決定採用JP7的驅動力,這種燃料也用於洛克希德的SR-71“黑鳥”高空高速偵察機。JP7需要一定的加熱時間,因此X-51A的燃燒室採用噴射乙烯的方法來啟動。能夠容納2.7kg乙烯的儲存箱佔據了載荷空間的大部分,這一要求也是飛行器設計者需要考慮的一個次要因素。Brink表示乙烯儲箱儘管笨重,但目前還是必須配備的。如果能有資金支援後續專案,Brink 和Vogel相信新的“乘波者”飛行器的設計將只依靠JP7作為燃料,這樣就能夠徹底消除對乙烯的依賴。
高超聲速飛行所產生的大量熱量使得即使在發動機結構外部也需要進行熱管理。X-51A的外部採用了由太空梭發展而來,名為波音輕量燒蝕塗層的特殊噴塗處理,能夠經受高達1260°C的表面溫度。這種塗層允許在飛行器結構中採用傳統合金,而不是特殊的複合材料。比如約31 kg的鋁制框架形成了發動機結構的主要部分,儘管這種材料的熔點只比當地溫度高150°C左右。飛行器採用了68 kg重的鎢制鼻錐,而不是更為特殊而且高風險的複合材料。
儘管極為謹慎,但飛行試驗仍具有很高的風險,這絕不僅是因為從沒有人對採用燃料冷卻的高超聲速推進系統進行過飛行試驗。X-51A試驗飛行器的尺寸和重量限制使得工程師只能採用雙餘度的系統來以防萬一。Vogel表示如果X-51A突然離開預定飛行路線,飛行試驗團隊希望能在主自毀系統一旦失靈的情況下能夠擁有第二選擇來摧毀飛行器。
7.8 m長的X-51A試驗器由三級組成:助推級、中間級和巡航級,其中巡航級長4.27m,重680kg。助推級是由陸軍戰術導彈系統改進而來的,增加了空氣動力學整流裝置和輕重量的鈦制噴管。X-51A從B-52H飛機的吊臂下釋放後,助推級將點火約30 s,將飛行器推到20km高空並達到馬赫數4.8的高速。中間級有一個通流管道幫助JP7燃料預熱,在助推級燃盡後,助推級和中間級都將與巡航級分離。分離以機械方式進行,以避免採用爆炸裝藥。
分離後巡航級將在無動力狀態下滑翔數秒鐘,然後開始試驗的高超聲速階段。巡航級的發動機將工作300s,使飛行器爬升到24 km高度,速度達到馬赫數6左右。發動機熄火後,飛行器將在濺落到海上前約500 s長的下降階段內進行機動動作。(中國航空工業發展研究中心 晏武英)
http://www.flightglobal.com/articles/2009/03/30/324520/x-51a-programme-admits-facing-long-odds.html
X-51A programme admits facing long odds
By Stephen Trimble
DATE:30/03/09
SOURCE:Flight International
Boeing X-51A programme officials acknowledge that they face long odds for completing a successful flight-test programme for the hypersonic vehicle with only four experiments funded.
Joseph Vogel, Boeing programme manager for X-51A, says he would prefer to have 10 flight tests, but is optimistic that the programme can be successful with funding for only four. Another two flight tests could be added if the programme can find a new source for funding, he adds.
Charlie Brink, X-51A programme manager for the US Air Force Research Laboratory, compares the hypersonic vehicle to cruise missile technology. But flight-test programmes for cruises missiles are notoriously problematic. Half of the first 20 flight tests in previous cruise missile programmes have failed.
The 1,815kg (4,000lb) test vehicle is designed with the ultra-high-speed cruise missile mission in mind. In contrast, the Lockheed Martin AGM-165 Joint Air to Surface Standoff Missile might take an hour to strike a target nearly 480km (260nm) away. A Waverider missile similar in size to the X-51A could travel 965km in only 10min.
The Waverider's angular nose is designed to ride atop the supersonic shockwaves that otherwise generate drag. The Pratt & Whitney Rocketdyne SJX61-2 scramjet, meanwhile, uses the speed of the air flow entering the inlet as a natural compressor stage.
Inside the engine, the air is initially mixed with ethylene, and later JP7, and burn at temperatures up to 1,870°C (3,400°F). To keep the engine from overheating, fuel is also used as a coolant and circulates around the engine structure.
The programme wants to rely on conventional fuels and materials as much as possible. That drove the decision to use JP7, which also powered the Lockheed SR-71 Blackbird. JP7 requires time to heat up, so the X-51A's combustion sequence starts with the injection of ethylene.
A storage tank with 2,7kg of ethylene fuel dominates the payload space, a requirement that is a minor irritant for the vehicle's designers. Brink derisively describes the ethylene storage unit as a cumbersome "scuba tank", albeit a necessary one for now.
If a follow-on programme is funded, Brink and Vogel believe a new Waverider vehicle design should rely solely on JP7 as fuel, thus removing the need for ethylene entirely.
The intense heat generated by hypersonic speeds creates thermal management requirements even outside the engine structure.
The X-51A is wrapped in special, spray-on treatments derived from the Space Shuttle called Boeing lightweight ablative coatings, which can withstand surface heat up to 1,260°C.
The coatings allowed the programme to use conventional alloys, rather than special composite materials, in the structure. For example, a nearly 31kg aluminium frame forms the majority of the engine structure, despite a material melting point of only about 150°C. The nose is formed from a 68kg tungsten block, rather than more exotic and risky composite materials.
Despite such extreme care, the experimental flight tests are highly risky and not only because nobody has tested a fuel-cooled hypersonic propulsion system in flight. The size and weight limitation of the X-51A test vehicle allowed engineers to build in only two redundant systems, both in case of failure. If the X-51A veers off course, the flight-test team wants to be sure there is a second option for destroying the vehicle in case the primary self-destruct sequence fails, Vogel says.
The 7.8m (25.5ft)-long X-51A is comprised of three stages: booster, interstage and cruiser, with the latter measuring 4.27m and weighing 680kg.
The booster stage is modified from the army tactical missile system with aerodynamic fairings and a lighter titanium nozzle. After the X-51A is released from a Boeing B-52H pylon, the booster will ignite for about 30s, lofting the vehicle to about 60,000ft and Mach 4.8.
After burn-out, both the booster and the interstage, which has a flow-through duct that helps pre-heat the JP7, will separate mechanically from the cruiser stage, as designers wanted to avoid explosive charges.
The cruiser stage will then coast unpowered for several seconds after separation before the hypersonic stage of the experiment begins. The cruiser's engine will burn for 300s, as the aircraft climbs to about 80,000ft and about M6. Upon burn-out, the vehicle will perform manoeuvres during a roughly 500s decent phase until splashdown in the ocean.