二战后钱学森与导师冯·卡门（右）、路德维希·普朗特（左）在德国

（路德维希·普朗特是近代力学奠基人，参加了纳粹导弹计划，是冯·卡门的博士导师）

一代科学巨匠钱学森去世了。但是，媒体在介绍这位伟大科学家的成就时，谈的主要是他对中国两弹一星事业的贡献和1955年回国前对美国航空和导弹研究的贡献。其实，钱学森创立的理论意义非常深远，在他回国后，不仅对中国的两弹一星，而且对世界航空航天、自动控制等领域一直发挥着影响。记得我1992年在英国进修时，钱学森1954年出版的《工程控制论》仍几乎是所有学习自动控制的博士研究生的必读书——在英国大学图书馆里，能够像这样在借阅书架上矗立40年，而且仍然炙手可热的科学经典可以说是凤毛麟角。

钱学森回到中国后，虽然终生未再重返美国，但他1946年提出的特超声学理论，仍对美国50年代至80年代重返大气层技术、载人航天技术和航天飞机等研究起着重要指导作用。美国国家航空航天局（NASA）的一个报告就透露了这一点。这份报告的题目是《面对热障：特超声学的历史》（Facing the Heat Barrier:A History of Hypersonics），是《NASA历史系列》中的一部，作者是出版过多部航空航天著作的科学家贺本海默（T. A. Heppenheimer），发表时间是2007年9月。

下面我把这部报告关于钱学森对“特超声学”所做贡献的段落翻译如下（原文附后。我对航天是外行，如有误译请专家指正）：

“特超声学”在作为一个术语使用之前，已经是一个实际的军事应用课题。战时德国的V-2火箭马赫数已达5以上，但检验所用钢铁以适合结构以及气动力加热的要求，在整体设计中起到的作用有限。德国人曾使用风洞进行测试，以确保这种导弹在飞行能保持稳定，但他们并未认识到这种速度值得采用一个专门的术语。钱学森，一位加州理工学院的空气动力学家，在1946年创造了这个术语(注)。从此以后，特超声学涉及到三个重要领域的应用。

第一个领域是重返大气层问题，在50年代中期这成为一个前沿课题。当时的空军致力于发展“擎天神”洲际弹道导弹，携带针对莫斯科的核弹头。如果不采取措施，这种弹头返回大气层时会像流星一样被加热。它不见得会烧毁——因为它太庞大——但是它肯定会失效。因此，有必要制做一个隔热板，以防止这种强烈的气动力加热。

成功解决这一问题打开了解决其他一大批相关问题的方便之门。从运行轨道携带电影胶片返回地面的太空舱成了家常便饭，从而使针对苏联的战略侦察变成了国防的重要组成部分。载人航天也成为可能，因为宇航员现在已经可以安全归来。此后，随着热保护工程的方法得到进一步改善，航天飞机的设想开始得到突飞猛进的发展，运载工具的外形从而首次设计成可重复使用的。

特超声学在决策者展望航空新时代时也非常重要，在这个时代，战斗机和轰炸机的速度和性能可能会不受限制地增加。这种期望导致了X-15。虽然它是在50年代设计的，但这种火箭动力飞机的研究标志着它的速度和高度直到航天飞机出现后才超过。气动加热再次明确了设计要求——它由镍铬铁合金“因科镍X”制造。当它飞到马赫数6时，可耐受华氏1200度的高温，并且可以飞到足够的海拔高度，以达到让一些飞行员满足宇航员的要求。

方雨注：钱学森创造这个术语的论文是《特超声流的相似定律》（Tsien, Hsue-shen. “Similarity Laws of Hypersonic Flows.” Journal of Mathematics and Physics 25 (1946): 247-251）.

报告封面

附：英文原文

Hypersonics nevertheless was a matter of practical military application before the term entered use. Germany’s wartime V-2 rocket flew above Mach 5， but steel proved suitable for its construction and aerodynamic heating played only a limited role in its overall design. The Germans used wind-tunnel tests to ensure that this missile would remain stable in flight, but they did not view its speed regime as meriting a name of its own. Hsue-shen Tsien, an aerodynamicist at the California Institute of Technology, coined the term in 1946.7 Since then, it has involved three significant areas of application.

The first was the re-entry problem, which came to the forefront during the mid-1950s. The Air Force by then was committed to developing the Atlas ICBM, which was to carry a nuclear warhead to Moscow. Left to itself, this warhead would have heated up like a meteor when it fell back into the atmosphere. It would not have burned up—it was too massive—but it certainly would have been rendered use­less. Hence, it was necessary to devise a heat shield to protect it against this intense aerodynamic heating.

The successful solution to this problem opened the door to a host of other initia­tives. The return of film-carrying capsules from orbit became routine, and turned strategic reconnaissance of the Soviet Union into an important element of national defense. Piloted space flight also became feasible, for astronauts now could hope to come back safely. Then, as the engineering methods for thermal protection were further improved, thoughts of a space shuttle began to flourish. They took shape as a reusable launch vehicle, the first of its kind.

Hypersonic technologies also became important as policy makers looked ahead to an era in which the speed and performance of fighters and bombers might increase without limit. This expectation led to the X-15. Though designed during the 1950s, this rocket-powered research airplane set speed and altitude marks that were not sur­passed until the advent of the shuttle. Aerodynamic heating again defined its design requirements, and it was built of the nickel alloy Inconel X. It routinely withstood temperatures of 1200°F as it flew to Mach 6, and reached altitudes high enough for some of its pilots to qualify as astronauts.