『每日軍事武器鑑賞』－ X-30「東方特快車」國家航太機

克里斯Chris

X-30 National Aerospace Plane (NASP)


There is also the possiblity that the SR-71 follow-on was hidden in plain sight. The program to develop what is called the National Aerospace Plane (NASP), designated the X-30, had its roots in a highly classified, Special Access Required, Defense Advanced Research Projects Agency (DARPA) project called Copper Canyon, which ran from 1982 to 1985. Originally conceived as a feasibility study for a single-stage-to-orbit (SSTO) airplane which could take off and land horizontally, Copper Canyon became the starting point for what Ronald Reagan called:<1>

"...a new Orient Express that could, by the end of the next decade, take off from Dulles Airport and accelerate up to twenty-five times the speed of sound, attaining low earth orbit or flying to Tokyo within two hours..."

The next stage of the program, called Phase 2, with Copper Canyon being Phase 1, was intended to develop the technologies for a vehicle that could go into orbit as well as travel over intercontinental ranges at hypersonic speeds. There were no commitments to undertake Phase 3, the actual design, construction and flight testing of the aircraft. The decision to undertake Phase 3 based on the maturity of the requisite technologies, originally planned for 1990, was currently been postponed until at least April of 1993.<3>

There were six identifiable technologies which are considered critical to the success of the project.<3&g; Three of these "enabling" technologies are related to the propulsion system, which would consist of an air-breathing supersonic combustion ramjet, or scramjet. A scramjet is designed to compress onrushing hypersonic air in a combustion chamber. Liquid hydrogen is then injected into the chamber, where it is ignited by the hot compressed air. The exhaust, consisting primarily of water vapor, is expelled through a nozzle to create thrust. The efficient functioning of the engine is dependent on the aerodynamics of the airframe, the underside of which must function as the air inlet mechanism and the exhaust nozzle. Design integration of the airframe and engine are thus absolutely critical to project success. The efficient use of hydrogen as a fuel for such a system is another crucial element in the development of the X-30.

Other enabling technologies include the development of advanced materials including various composites and titanium-based alloys which maintain structural integrity at very high temperatures. The enormous heat loads associated with hypersonic flight, sometimes in excess of 1,800 degrees fahrenheit, will necessitate the development of active cooling systems and advanced heat-resistant materials.<4>

克里斯Chris

但是，經過幾年的研究分析，科學家們發規，過去的估計過於樂觀。實際上。上述三條途徑知易而行難。需要解決的關鍵技術難度決非短時間內能突破，這些關鍵技術有：

．新構思的吸氣式發動機

　　因為，空天飛機的飛行範圍為從大氣層內到大氣層外，速度從0到M=25，如此大的跨度和工作環境變化是目前現有的所有單一類型的發動機都不可能勝任的，從而也就使為空天飛機研製全新的發動機成為整個項目的關鍵。

　　眾所周知，噴氣式發動機需要在大氣層中吸入空氣，無需攜帶氧化劑，但無法在大氣層外工作，且實用速度較小；而火箭發動機自帶氧化劑，可以工作在大氣層內外，使用速度範圍較廣，但攜帶的氧化劑較笨重，比衝小。目前設想的空天飛機的動力一般為採用超音速燃燒衝壓發動機+火箭發動機或渦輪噴氣+衝壓噴氣+火箭發動機的組合動力方式。但超燃衝壓發動機的研製上存在相當多的技術問題，而多種發動機的組合方式又使結構變得過於複雜和不可靠。

2．計算空氣動力學分析

　　穿梭機返回再入大氣層的空氣動力學問題，曾經耗費了科學家們多年的心血，作了約10萬小時的風洞試驗。空天飛機的空氣動力學問題比穿梭機複雜得多。因為飛機速度變化大，馬赫數從0變化到25；飛行高度變化大，從地面到幾百公里高的外層空間；返回再入大氣層時下行時間長，穿梭機只有十幾分鐘，空天飛機則為l～2小時。

　　解決空氣動力學問題的基本手段是風洞。目前，就連美國也不具備馬赫數可以跨越這樣大範圍的試驗風洞。即使有了風洞還需要作上百萬小時的試驗，那意味著就是晝夜不停地試驗，也需要花費100多年的時間。於是，只能求助於計算機，用計算方法來解決，而對那維爾斯托克斯方程的求解目前尚存在，許多理論上和計算速度上的問題。

3．發動機和機身一體化設計

　　當空天飛機以6倍於音速以上的速度在大氣層中飛行時，空氣阻力將急劇上升，所以其外形必須高度流線化。亞音速飛機常採用的翼吊式發動機已不能使用．需要將發動機與機身合併，以構成高度流線化的整體外形。即讓前機身容納發動機吸人空氣的進氣道，讓後機身容納發動機排氣的噴管。這就叫做“發動機與機身一體化”。

　　在一體化設計中，最複雜的是要使進氣道與排氣噴管的幾何形狀，能隨飛行速度的變化而變化，以便調節進氣量，使發動機在低速時能產生額定推力，而在高速時又可降低耗油量，還要保證進氣道有足夠的剛度和耐高溫性能，以使它在返回再入大氣層的過程中，能經受住高速氣流和氣動力熱的作用，這樣才不致發生明顯變形，才可多次重復使用。

4．防熱結構與材料

　　空天飛機需要多次出人大氣層，每次都會由於與空氣的劇烈摩擦而產生大量氣動加熱，特別是以高超音速返回再入大氣層時，氣動加熱會使其表面達到極高的溫度。機頭處溫度約為1800攝氏度，機翼和尾翼前緣溫度約為1460攝氏度，機身下表面約為980攝氏度，上表面約為760攝氏度。因此，必須有一個重量輕、性能好、能重復使用的防熱系統。

　　空天飛機在起飛上升階段要經受發動機的衝擊力、振動、空氣動力等的作用，在返回再入階段要經受顫振、科振、起落架擺振等的作用。在這種情況下，防熱系統既要保持良好的氣動外形，又要能長期重復使用，維護方便，所以其技術難度是相當大的。

　　目前的穿梭機，由於受氣動加熱的時間短，表面覆蓋氧化硅防熱瓦即可達到滿意的防熱效果，但對空天飛機則遠遠不夠。如果單靠增加防熱層厚度來解決問題，則將使重量大大增加，而且防熱層還不能被燒壞，否則會影響重復使用。一個較簡單的解決辦法是在機頭、機翼前緣等局部高溫區，使用傳熱效率特別高的吸熱管來吸熱，以便把熱量轉移到溫度較低的部位。更好的辦法是採用主動式冷卻防熱系統，也就是把機體結構與防熱系統一體化，即把機體結構設計成夾層式或管道式，讓推進劑在夾層內或管道內流動，使它吸走空氣對結構外表面摩擦所生成的熱量。

　　為了滿足空天飛機的防熱要求，目前正在研究用快速固化粉末冶金工藝製造純度很高、質量很輕的耐高溫合金。美國已研製出高速固化鈦硼合金，它在高溫下的強度可達到目前使用的鈦合金在室溫下的強度，這種合金適宜用來製造機身內層結構骨架。

　　機頭與機翼等溫度最高的部位，要求採用碳複合材料，這種複合材料表面有碳化矽涂層，重量輕，耐高溫性能好。此外，還需要研究金屬基複合材料，例如碳化矽纖維增強的鈦複合材料等。這種材料應該兼有碳化矽的耐高溫性能，又具有鈦合金的高強度特性。

　　空天飛機技術難度大，所需投資多，研製週期長，所以將來進入全尺寸樣機研製，勢必也會象空間站那樣採取國際合作的方式。

克里斯Chris

航空與航太兩大技術存在不可分離的聯繫和很強的互補性。因為無論什麼航太器進出太空，都必須穿越大氣層與空氣打交道，後者顯然屬於航空技術範疇。航空與航太緊密聯繫的必然結果，就導致了人們對一種既能在大氣層內飛行，又能在大氣層外航行、水準起飛、水準降落的新型飛行器的構想，這就是“航空穿梭機”，簡稱“空天飛機”。

　　60年代初，就有人對空天飛機作過一些探索性試驗，當時它被稱為“跨大氣層飛行器”。由於當時的技術、經濟條件相差太遠，且應用需求不明確，因而中途夭折；80年代中期，在美國的“阿爾法”號永久性空間站計劃的刺激下，一些國家對發展載人航太事業的熱情普遍高漲，積極參加“阿爾法”號空間站的建造。據估計，空間站建成後，為了開發和利用太空資源。向空間站運送人員、物資和器材等任務每年將達到數千次之多。這些任務如果用一次性運載火箭、載人飛船或穿梭機來完成，那麼一年的運輸費用將達到上百億美元。為了尋求一种經濟的天地往返運或系統，美、英、德、法、日等國紛紛推出了可重復使用的天地往返運輸系統方案。




　1986年，美國提出研製代號為X-30的完全重復使用的單級水準起陣的“國家航空穿梭機”，其特點是採用組合式超音速燃燒衝壓噴氣發動機。英國提出了一種名叫“霍托爾”單級水準起降空天飛機，其特點是採用一種全新的空氣液化迴圈發動機。90年代，他們又提出了一個技術風險小，開發費用低的新方案。德國則提出兩級水準起降空天飛機“桑格爾”，第一級實際上相當於一架超音速運輸機，第二級是以火箭發動機為動力的有翼飛行器。兩級都能分別水準著陸。法國和日本也提出過自己的空天飛機設想。

　　80年代末，這股空天飛機熱達到高潮。也激起了中國航空航太專家的很大興趣。

　　發展空天飛機的主要目的是想降低空天之間的運輸費用。其途徑歸納起來主要有三條：一是充分利用大氣層中的氧，以減少飛行器攜帶的氧化劑，從面減輕起飛重量；二是整個飛行器全部重復使用，除消耗推進劑外不拋棄任何部件；三是水準起飛，水準降落，簡化起飛（發射）和降落（返回）所需的場地設施和操作程式，減少維修費用。

　　

克里斯Chris

REFERENCES
<1> State of the Union Address, February 4, 1986.

<2> According to project manager Robert Barthelemy. Aerospace America, September 1991. page 6.

<3> United States General Accounting Office. "National Aero-Space Plane: A Technology Development and Demonstration Program to Build the X-30." USGAO/ NSIAD-88-122. April 1988. pages 35-40.

<4> GAO ibid. page 38.

<5> "DARPA Chief Notes Potential of Supersonic Combustion Ramjet," Aerospace Daily, 29 March 1985, page 165.

<6> "NASP Moves at Slower Speed," Military Space, 17 July 1989, pages 1, 7-8.

<7>United States General Accounting Office. "National Aero-Space Plane: Key Issues Facing the Program." March 31, 1992. p.

<8> GAO ibid. page 7.

<9> Defense Daily. April 17, 1992. page 103.

<10>Aerospace Daily. March 28, 1986. page 484.

<11> GAO ibid. page 19.

<12> United States Senate Armed Services Committee, 101st Congress, 1st Session, ibid.

<13> Press release. Office of Congressman Dave McCurdy. June 3, 1991.

><14> Williams, Robert M. "National Aero-Space Plane: Technology for America's Future." Aerospace America. November 1986. page 20.

<15> World Aerospace Weekly. November 11, 1988.

<16> Marshall, Eliot. "NASA and Military Press for a Spaceplane." News and Comment. January 10, 1986, pages 105-107.

<17> Williams, Robert, "National Aero-Space Plane: Technology for America's Future," Aerospace America, November 1986, pages 18-22.

<18> House of Representatives Appropriations Committee Energy and Water Development Subcommittee, Energy and Water Development Appropriations for 1993, 102nd Congress, 2nd Session, Part 6, pages 1669-1670.

<19> "DARPA Chief Notes Potential of Supersonic Combustion Ramjet," Aerospace Daily, 29 March 1985, page 165.

<20> ibid.

<21> Defense Daily. April 20, 1988. page 295.

<22> Aerospace Daily. March 14, 1992. page 408.

<23> Defense Daily. April 20, 1988. page 295.

<24> National Research Council Committee on Hypersonic Technology for Military Applications, Hypersonic Technology for Military Applications, (Washington, National Research Council, 1989), page 12.

<25> Augenstein, Bruno, Assessment of NASP: Future Options, (Santa Monica, RAND, June 1989), WD-4437-AF.

<26> "The Goldin Age" Space Business News, July 6, 1992.

<27> Inside the Air Force. March 27, 1992. page 3.

克里斯Chris

But such reconnaissance and surveillance activities of hypersonic craft are constrained by the high speeds and altitudes at which the X-30 or its derivatives would travel. At altitudes nearly three times that of standard reconnaissance aircraft<21> and a fuel cost 3 times that of aviation grade kerosene,<22> it would certainly seem more economical to get information of comparable (or better) resolution from a satellite in low earth orbit, which could make another pass in 90 minutes instead of being forced to return to base for refuelling.<23>

Although some proponents have viewed these military missions as potentially attractive, a Committee of the National Research Council expressed doubts about the operational effectiveness of NASP derived vehicles:<37>

"Another restriction is inherent in the base support requirements associated with cryogenic fuels. They will require a complete departure from conventional airport storage and distribution facilities. For economic reasons alone, we are unable to envision a network of airfields giving the flexibility that today's aircraft enjoy.

"... sustained cruising flight in the atmosphere roughly between Mach numbers 8 and 20 ... is a very stressful flight environment with high skin temperatures, control and maneuvering difficulties, ionized boundaries through which sensors must operate, and high infrared signatures which would make the vehicle vulnerable to detection. For these reasons, we have great reservations about the military utility of sustained hypersonic flight in the atmosphere above Mach number 8."

A draft analysis done at the RAND Corporation was even more pessimistic:<25>

"Grave doubts exist that NASP could come anywhere near its stated/advertised cost, schedule, payload fees to orbit, etc.... On the basis of current knowledge, it is hard to defend previous DoD plans for NASP on the basis of any singular mission utility sufficiently attractive to operators... NASP could do many missions (but none is singularly persuasive)... No compelling "golden mission" exists for NASP."

NASA was disinclined to significantly increase its share of program costs given its current budgetary constraints<26>, and the Air Force, which has borne the brunt of development costs of Phase 2, expressed doubts about the future viability of the program. According to Martin Faga, Assistant Secretary of the Air Force for Space:<27>

"...these are exciting ideas... but they are not ready for commitment."

Clearly, no single vehicle can serve commercial, civil space and military masters at the same time. In spite of efforts to be all things to all people, the NASP remained without a truly credible mission, and ultimately proponents were unable to save it from termination.

The Hypersonic Systems Technology Program (HySTP), initiated in late 1994, was designed to transfer the accomplishments made in hypersonic technologies by the National Aero-Space Plane (NASP) program into a technology development program.

克里斯Chris

Presumably, an SR-71 follow-on would also be a national program of importance to the entire country. These arguments are, of course, predicated on the assumption that the NASP vehicle could fullfill such a defense mission. Concentrating on hypersonic flight in the upper stratosphere, possible military applications of a NASP derived vehicle include:<14>

space launch;
strategic bombing missions;
strategic air defense;
reconnaissance and surveillance.
While the reconnaissance and surveillance mission would be similair to the SR-71, closer examination reveals that the possible military applications provide a less than compelling rationale for the NASP effort.

As a single-stage-to-orbit vehicle with a claimed turnaround time of as little as 24 hours<15>, proponents of the Strategic Defense Initiative initially saw the X-30 leading the way to faster, cheaper access to low earth orbit, a critical aspect of lowering the cost of any space-based ballistic missile defense systems.<16> However, as it became clear that the time required for the development of an operational capability would extend far beyond the time horizon envisioned for deployment of space-based anti- missile systems, the SDI program soon lost interest in the NASP effort. A similar disenchantment has emerged within the Air Force and NASA, as the high technical risk of the project has become increasingly clear. What has also become increasingly clear is that the claims made for NASP as a space launch vehicle are eerily reminiscent of the initial claims made for the Space Shuttle in the early 1970s. The assertions that NASP will have airplane-like operating characteristics, with lower costs and fast turnaround times on the ground, are assumptions, rather than conclusions based on detailed analysis.

The potential for using NASP derived vehicles for strategic bombardment, as a hypersonic B-3, has not escaped the notice of the Air Force. Gen. Lawrence Skantze, commander of the Air Force Systems Command, observed:<17>

"We're talking about the speed of response of an ICBM and the flexibility and recallability of a bomber, packaged in a plane that can scramble, get into orbit, and change orbit so the Soviets can't get a reading accurate enough to shoot at it. It offers strategic force survivability -- a fleet could sit alert like B-52s."

The idea of reaching targets anywhere in the world in a an hour or two may be a tempting idea, but the challenge of accurately dropping a gravity bomb while travelling 20 times the speed of sound would be non-trivial. This challenge was eagerly embraced by the Energy Department, however. A Hypervelocity Aircraft- Delivered Weapon is among the five new nuclear weapons concepts currently under study by the Energy Department, as phase one or pre-phase one studies.<18>

"The need for the Hypervelocity Aircraft-Delivered Weapon derives from the ability of such a system to rapidly deliver, or threaten to deliver, nuclear weapons into a theater, while maintaining the launch platform well outside potential defenses. Hypersonic velocities enhance defense penetrability and survivability of the weapon and the delivery aircraft against state of the art defenses, while precision guidance can lead to reduced yield requirements, and consequently, collateral damage."

But a hypersonic aircraft would have high visibility to hostile defense due to its enormous heat signature and non-stealth composition of the fuselage, resembling nothing so much as a barn on fire. This is hardly a major selling point for a reconnaissance aircraft. As a bomber, a NASP derived vehicle would combine the worst features of an aircraft and a missile. With the large signature of an aircraft and the limited maneuverability of a missile warhead, it would provide a ready target for defensive systems.

A third suggested mission for NASP derived vehicles would be as a interceptor for defense of the continental United States. Robert Cooper, Director of DARPA, suggested that it could:<19>

"... fly up to maybe 150,000 to 200,000 feet, sustain mach 15 plus for a while, slow down and engage an intercontinental bomber or cruise missile carrier at ranges of 1000 nautical miles..."

But the elaborate preparations needed to maintain a liquid hydrogen fueled aircraft on alert, combined with the limited maneuverability of this type of vehicle, would limit its utility for this mission. And given the relatively low priority the United States has traditionally attached to strategic air defense, it is doubtful that the large investment required by NASP could be justified on these grounds.

A final application of NASP was as an intelligence collection platform. Robert Cooper suggested that it could provide:<20>

"... a globe-circling reconnaissance system, a kind of super SR-71 that would... get anywhere on the Earth within perhaps half an hour of take-off..." (emphasis added).

克里斯Chris

Though the protection of this technological principle may explain part of the secrecy surrounding the NASP program, studies of the missions that such a vehicle might perform remain even more closely held.

Defining the mission of NASP to attract maximum support and funding has been a tricky business for program proponents. Original cost projections of $3.1 billion dollars have more than tripled, now at approximately $10 Billion total cost for the development of a pair of single-stage-to-orbit vehicles.<7>

A decision to undertake Phase 3 flight testing would have brought total program costs up to as much as $17 billion<8&t;. The target date for the first test flight of the X-30 was pushed back to the 2000-2001 period<9>, 11 years behind schedule and 500% over budget. Many years and a further $10 to $20 billion would have been required for the development of an operational vehicle. Funding this significant increase in a time of general budget cutting is not easy, and program cost overruns and delays in scheduling have made the project less attractive to many supporters.

Though the X-30 was originally touted by the Reagan administration for its civilian commercial applications and as a possible follow-on to the Space Shuttle for NASA<10>, the funding structure of the program tells another story. The Department of Defense was scheduled to fund approximately 80% of the project, or $2.65 out of $3.33 billion over the 8 years of the original project.<11> Budget allocations come primarily from the Air Force, which has seen NASP as potentially having a range of military missions.

The mystery remains of what military mission would justify this level of effort. Or perhaps there is no mystery at all. The X-30 may have been the purloined letter of military aircraft, an SR-71 follow-on hidden in plain sight. This would certainly jibe with the statement of Senator John Glenn, noted earlier and repeated here,<12>

"...what you are talking about on that system, I know what you are talking about. That is many years down the road and is still a very speculative system..."

Such a possibility would also explain the tenacious position of Congressman Dave McCurdy, the only member of Congress at the time to sit on both the Armed Services Committee and the Space and Technology Committee. From 1989 through 1992, McCurdy fought hard for continued funding for and Air Force involvement in NASP.<13>

"It's important to remember that NASP is not a NASA program. NASP is not an Air Force program. It is a national program. We believe that it is important to the country."

克里斯Chris

The mass saved by eliminating the final rocket propellants had to be balanced, however, against the mass of the active thermal management system. This system became more complex and massive at higher speeds. At some point, the additional mass of the thermal management system needed to continue the acceleration of the air breathing scramjet would become greater than the mass of the rocket motors and propellant needed to continue the ascent to orbit.

As the NASP effort began, analysis suggested that this transition speed, at which rocket propulsion would be more efficient than continued scramjet operations, would be quite high, above Mach 20. Although this fell short of the initial promise of Copper Canyon, it nonetheless suggested that a scramjet vehicle might offer superior performance compared to conventional rockets. Over time, however, as the complexity of the active thermal management system was better appreciated, estimates of the transition speed declined to below Mach 17.<6> This diminished performance significantly reduces the attractiveness of scramjet propulsion compared with all-rocket vehicles.

克里斯Chris

X-30 NASP

克里斯Chris

Although the NASP effort was announced by President Reagan in his State of the Union address, much of the project remains shrouded in secrecy. Indeed, the paucity of publicly available information on this project is remarkable, given the scope of the effort to date. This very high level of classification derives at least in part from the core technological innovation that was the genesis of the X-30 project.






Prior analyses of scramjet propulsion systems had concluded that they would only be able to achieve speeds of about Mach 8. At this speed, the thrust emerging from the rear of the plane would be balanced by the heat generated by atmospheric drag and the high temperature of the air as it entered the front of the engine. Thus limited to a maximum speed that was only one-third the orbital velocity of Mach 25, a scramjet-propelled vehicle would need rocket motors to achieve the remaining speed needed to reach orbit. Analyses concluded that such a vehicle would be heavier and more complicated that a conventional rocket.

However, the Copper Canyon project discovered that higher speeds could be achieved through the imaginative use of active thermal management. By circulating, and thus heating, the scramjet's hydrogen propellant through the skin of the vehicle prior to injection into the engine, energy generated through atmospheric drag was added to the thrust of the scramjet, enabling it to accelerate beyond the Mach 8 thermal barrier. Initially, there was optimism that this active thermal management approach would permit speeds of up to Mach 25 using air- breathing engines alone, eliminating the need for rocket propellants to achieve orbit.<5>