Ukraine’s Kinzhal intercepts should cool hypersonic hype

In the past two weeks Ukraine reportedly intercepted seven Russian Kinzhal missiles – which travel at hypersonic speeds – using its Patriot missile defense system. It is widely believed that Patriot and other current missile defenses could not stop hypersonic weapons, which travel at speeds over Mach five, or five times the speed of sound.

So, what’s going on?

The claim that hypersonic weapons are invincible is one of the many beliefs about these weapons. Here’s why it’s wrong.

Kinzhal is an air-launched ballistic missile with fins that allow it to maneuver as it approaches its target. It is called “hypersonic” since its top speed is reportedly around Mach 10, which would give it a range of somewhat over 1,000 km. This system is not, however, what defense analysts typically mean by the term “hypersonic weapon” since it is not designed to glide over a significant fraction of its trajectory. Its high speed and ability to maneuver, however, mean that it poses a similar challenge as true hypersonic weapons to terminal missile defenses, like Patriot, that are used to defend against weapons of this range.

A maneuvering missile traveling at Mach 10 would be too fast for the U.S. Patriot PAC-3 and similar defense systems to intercept reliably. However, Mach 10 is roughly Kinzhal’s maximum speed, and its speed drops sharply as it reenters and travels through the increasingly dense atmosphere to hit a target on the ground.

Patriot is designed to intercept missiles at low altitudes, and my estimates show that Kinzhal slows enough during its dive that current versions of PAC-3 should be capable of intercepting it. Moreover, reports indicate that at least for the first of the Kinzhal intercepts, Ukraine was not using the most advanced version of the PAC-3 (called MSE , which is 25 percent faster than the previous version).

This analysis has two important implications.

First, Ukraine’s claims that it intercepted Kinzhal missiles are credible, and its defenses may be able to blunt a significant weapon in Russia’s arsenal.

Second and more generally, the medium-range hypersonic glide weapons like those the United States, Russia, and China are currently developing may not be as effective at performing key mission as advocates often claim.

A common argument for building hypersonic weapons is the desire to use them to destroy enemy missile and air defenses early in a conflict, to clear the way for subsequent attacks. Technical modeling, however, shows that the hypersonic weapons the United States has been developing – including the Air-Launched Rapid Response hypersonic Weapon (ARRW) , the Army’s Long Range Hypersonic Weapon (LRHW) , and the Navy’s Conventional Prompt Strike (CPS) – may also be vulnerable to interception by missile defenses.

In particular, reports about the speeds and ranges of these weapons imply that they begin the glide phase of their trajectory with a speed of about Mach 12 or less. Their speed decreases due to atmospheric drag during the glide phase – especially if they are maneuvering significantly – and will decrease even further as they dive into the thick atmosphere on their way to their targets on the ground. My estimates show that these effects will likely make these systems vulnerable to interception by systems similar to current versions of PAC-3, although intercepting them may require the advanced PAC-3 MSE.

The United States must assume it will face defenses like these in other countries — soon if not now.

To be able to evade such defenses, hypersonic weapons would need to be launched with even higher speeds. Doing so would significantly increase the intense heating they experience during flight, which is a key obstacle to developing these weapons. Increasing their speed also makes them larger and heavier, which reduces the number that aircraft can carry.

Adding propulsion, such as scramjet engines being developed for hypersonic cruise missiles, could help the weapon maintain its speed during the glide phase. But these engines are unlikely to be powerful enough to help much against the exponentially increasing drag encountered during the dive phase, which could leave these weapons vulnerable to interception.

Russian and Chinese hypersonic weapons similar to these U.S. systems (such as the Russian Zircon and Chinese DF-ZF and Starry Sky 2 ) are also likely to be vulnerable to defenses like PAC-3. In this sense, these weapons do not represent a revolution in threat beyond that posed by medium-range ballistic missiles.

The Ukrainian experience with Kinzhal may be a wake-up call for Russia. It should also be a wake-up call for the United States. Congress and the U.S. military need to think clearly about the missions these weapons can realistically accomplish and whether they justify the high priority and budget share they are getting.

David Wright is a visiting scholar in the MIT Department of Nuclear Science and Engineering’s Laboratory for Nuclear Security and Policy.

据报道，过去两周，乌克兰使用其“爱国者”导弹防御系统拦截了七枚俄罗斯“匕首”高超音速导弹。人们普遍认为，“爱国者”和其他现有的导弹防御系统无法拦截飞行速度超过五马赫（即五倍音速）的高超音速武器。

所以，到底发生了什么事？

认为高超音速武器无敌的说法是关于这类武器的众多误解之一。以下是它为何错误的原因。

“匕首”（Kinzhal）是一种空射弹道导弹，其尾翼使其能够在接近目标时进行机动。它被称为“高超音速”导弹，据称其最高速度约为10马赫，射程超过1000公里。然而，该系统并非国防分析人士通常所说的“高超音速武器”，因为它并非设计用于在飞行轨迹的大部分距离内进行滑翔。尽管如此，其高速和机动能力意味着它对“爱国者”（Patriot）等用于防御此类射程武器的末端导弹防御系统构成了与真正的高超音速武器类似的挑战。

一枚以10马赫飞行的机动导弹速度过快，美国“爱国者”PAC-3及类似防御系统难以可靠拦截。然而，10马赫大致是“匕首”导弹的最大速度，其速度在重返大气层并穿过日益稠密的大气层以击中地面目标时会急剧下降。

爱国者导弹系统旨在拦截低空导弹，据我估计，“匕首”导弹在俯冲过程中速度会明显降低，因此目前版本的PAC-3导弹应该能够拦截它。此外，有报道指出，至少在首次拦截“匕首”导弹时，乌克兰使用的并非最先进的PAC-3导弹版本（称为MSE，比之前的版本速度提升25%）。

这项分析有两个重要的意义。

首先，乌克兰声称拦截了“匕首”导弹的说法是可信的，其防御系统或许能够削弱俄罗斯武器库中的一种重要武器。

其次，更普遍地说，像美国、俄罗斯和中国目前正在开发的中程高超音速滑翔武器，在执行关键任务方面可能不像支持者经常声称的那样有效。

研发高超音速武器的一个常见理由是，希望在冲突初期利用它们摧毁敌方导弹和防空系统，为后续攻击扫清障碍。然而，技术模型显示，美国正在研发的高超音速武器——包括空射快速反应高超音速武器（ARRW）、陆军远程高超音速武器（LRHW）和海军常规快速打击武器（CPS）——也可能容易被导弹防御系统拦截。

具体而言，有关这些武器的速度和射程的报告表明，它们在滑翔阶段的初始速度约为12马赫或更低。在滑翔阶段，由于大气阻力，它们的速度会降低——尤其是在进行大幅度机动时——并且在俯冲进入稠密大气层攻击地面目标的过程中，速度还会进一步降低。我的估算表明，这些因素很可能使这些系统容易受到类似于现有PAC-3系统的拦截，尽管拦截它们可能需要更先进的PAC-3 MSE系统。

美国必须预料到，它将在其他国家面临类似的防御措施——即便现在不是，也很快就会面临。

为了突破此类防御，高超音速武器需要以更高的速度发射。这样做会显著增加其飞行过程中所受到的强烈加热，而这正是研发此类武器的关键障碍。此外，提高速度还会增加其体积和重量，从而减少飞机可携带的数量。

增加推进装置，例如为高超音速巡航导弹开发的超燃冲压发动机，可以帮助武器在滑翔阶段保持速度。但这些发动机的动力可能不足以有效对抗俯冲阶段呈指数级增长的阻力，这可能会使这些武器更容易被拦截。

与美国这些系统类似的俄罗斯和中国高超音速武器（例如俄罗斯的“锆石”和中国的“东风-ZF”和“星空2”）也可能容易受到PAC-3等防御系统的攻击。从这个意义上讲，这些武器并不代表着对中程弹道导弹构成的威胁的革命性突破。

乌克兰使用“匕首”导弹的经验或许能给俄罗斯敲响警钟，也应该给美国敲响警钟。国会和美国军方需要认真思考这些武器究竟能完成哪些实际任务，以及它们是否配得上目前所享有的高优先级和预算份额。

大卫·赖特是麻省理工学院核科学与工程系核安全与政策实验室的访问学者。