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东风DF-26边飞边像“雪糕””一样融化,怎样解决超高温难题?

最近,美国和加拿大通过卫星证实了中国人民解放军在中国东部和西部地区均部署了“东风26”中远程弹道导弹。去年,笔者曾草草计算了《东风DF-26B和DF-21D两枚导弹同时击中南海目标的飞行时间和轨迹》,东风26之所以被称为“航母杀手”,是因为其巡航速度快,达到了10马赫以上,这么快的速度在出入大气层必定会产生非常高的温度,那么怎么解决这种高温难题以不至于导弹还未射中目标就像“雪糕”一样融化呢?本文不仅从效应、原理、复合材料,还通过引用“超音速导弹温度场建模与仿真”论文进行了综合介绍。

来源:EDN电子技术设计

作者:Challey

 

所有的超音速弹道导弹都会遇到这个难题,我们首先来看看俄罗斯的“先锋”高超音速导弹是怎样解决的。

俄罗斯“先锋”导弹的“冰棍效应

俄罗斯先锋高超音速导弹,是目前最先进的一种高超音速武器,对比现在研制出来的非常多的导弹而言,它的性能非常强劲。不仅时速33077公里,而且导弹表面温度高达2000度。

消息称俄罗斯总统普京在谈到“先锋”高超音速导弹时,曾说这种导弹飞行就冰棍,会边飞边融化,是什么“冰棍效应”。这是怎么回事?

2020年9月,俄罗斯总统普京在谈到最新服役的“先锋”高超声速导弹时,确实说过:这种导弹在飞行中会像雪糕一样融化,因此其能承受极高的表面温度。在与俄核工业代表举行的会议上,他又说:“这就像是‘冰棍效应’,(高超音速导弹)一边飞,一边融化。飞行中的温度接近2000度,只比太阳温度低一点儿,但材料可在需要的时间内抵御住这一高温。”

这看似一个玩笑,实际是对导弹弹头抗烧蚀机理的一个非常恰当的比喻。

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俄罗斯先锋导弹弹头高速滑翔飞行

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俄罗斯先锋导弹弹头高速滑翔飞行,其表面会像雪糕一样“融化”,带走表面热量

为了解决这个难题,俄罗斯花费精力研制出了强耐高温耐腐蚀的复合材料,使得“先锋”导弹能够经受2000度高温的炙烤。而普京形容“先锋”导弹的耐高温性能像冰棍一样,其实说的很形象,“先锋”导弹的机体设计有多层隔热结构,在飞行中,这些隔热层能够确保弹头在极端高温下稳定前行,不会被迅速融化。而按照普京的说法,得益于这些复合材料,“先锋”导弹能够在飞行中慢慢融化,在所需要的时间里抵御极限高温。

超音速巡航过程中的“气动加热”原理与热防护方案[1]

导弹在超高速飞行时,由于流过其表面的气流猛烈受压,动能转化为热能,同时导弹与气流之间发生粘性摩擦也产生部分热能,从而使导弹受热。这种现象就是“气动加热”,温度随着飞行速度增大而急速增高。它也和飞行高度有关,因为高度越低,空气密度越大,气动加热也就越严重。例如,射程3000千米的弹道导弹,在穿越大气层起飞、爬高时,因气动加热而升温到几百度,接近目标再入大气层时,弹头温度可以达到几千度。

因此导弹,特别是中远程导弹,在设计弹头时必须考虑热防护问题。科研人员也先后找到了四种热防护方案:热沉式、辐射隔热式、烧蚀式、发汗冷却式。

使用最广、效率最好的是烧蚀式,特别是对中远程导弹弹头,几乎毫无例外地都选择了这种。普京说的“冰棍”,就是这种方式里的一种。

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经过再入考验的美国MK5弹头再入体外壳

烧蚀式防热,是利用防热材料在受热条件下产生汽化、蒸发、升华、流失等一系列物理化学变化,消耗部分质量,同时将大部分气动加热在表面消耗或带走,从而达到保护飞行器的目的。这在各国再入飞行器设计中得到了广泛应用,包括弹道飞行器、飞船、返回式卫星,还有低升力力再入体。烧蚀防热适用的飞行任务范围很广泛,环境条件可在一个极广的范围内变化,从长时间(2000秒数量级)、低气动加热率(低于100千瓦/平方,并伴随着再入体激波层的强烈辐射加热),到几十秒时间、高气动加热率(高于10000千瓦/平方米),都能适用。

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弹头烧蚀试验后可见铸造的端头帽和纤维布包裹的再入体壳体,被烧蚀掉的部分像雪糕融化一样带走了致命的热量

超高温复合材料

可以看出,采用烧蚀式防热的关键,是烧蚀材料的选择与制造。根据烧蚀机理和烧蚀特征,目前烧蚀材料大致可以分成四类。

第一,碳化塑料烧蚀材料,包括单基塑料,以有机材料(涤纶等)或无机材料(玻璃、石英、碳、金属等)增强的塑料。例如,许多国家导弹弹头的端头部分,采用了碳纤维编织材料。

第二,热塑性烧蚀材料,诸如泰氟隆(聚四氟乙烯),在烧蚀过程中直接升华成气体。

第三,耐高温氧化物烧蚀材料,包括石英等材料。它们在高热流冲刷下,会发生软化、熔融、蒸发等现象,并在气体冲刷下流动。

第四,耐高温陶瓷烧蚀材料,比如石墨。它们在烧蚀过程中是氧化和升华过程起作用,受传导介质的影响并受表面发生的多相化学反应的影响。美国航天飞机机翼前缘表面,就是用了陶瓷贴片瓦材料。

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再入体发射前与回收后的比较,可见右侧表面已经被严重烧蚀

上述四种材料都有其最适用的环境条件。弹道导弹、高超音速导弹,飞行时面临的环境条件范围很宽,因此没有固定的某种材料比其它材料特别优越。但相比而言,碳化烧蚀材料,应用范围最广,因为它兼有另外两种热防护方法,辐射式、发汗冷却式的一些最好的特性,表面温度可以达到很高,而隔热问题却由于碳化塑料的分解温度低而极易解决。这就是各国最终都将碳纤维编织材料作为防止导弹弹头烧蚀的最终解决方案的原因。从这点来看,高速导弹在飞行中还真如普京所说,就像“雪糕”一样边飞边融化。

超音速导弹温度场建模与仿真[2]

要解决超音速导弹的高温问题,不仅仅需要结构、材料,还需要前期严谨的设计、建模与仿真,以及后期的试验。下面是海军航空工程学院关于“超音速导弹温度场建模与仿真”的论文概述。

超音速导弹温度场的计算对其红外辐射特性研究具有重要的参考价值。对超音速导弹的两个主要辐射源蒙皮和羽流进行了深入分析,建立了超音速导弹温度场模型,仿真验证了模型的可行性。采用理论模型与半经验公式对导弹温度分布进行了计算,将羽流近似成超音速轴对称无伴随绝热等熵流,利用特征线法计算气流参数分布。此外,建立了超音速导弹尾焰形状的理论模型与计算方法。最后进行仿真,计算了导弹各部分的温度分布,并与实验结果比较,结果表明,该方法是一种计算超音速导弹温度分布的有效方法。

1 、导弹温度分布计算模型

根据超音速导弹的红外辐射将导弹分成 3 个 部分进行温度计算:蒙皮、尾喷管和羽流。文中主要研究蒙皮和羽流的温度分布。

1.1 导弹蒙皮温度计算

当导弹在大气中高速飞行时,导弹蒙皮的温度会由于气动加热而升高,因而会产生相当强的红外辐射。驻点温度的计算公式此处略。

气动加热是一种气动强迫加热过程,所以虽然飞行器表面会有热传导发生,但一两分钟即可达到平衡壁温,导弹蒙皮的平衡壁温可近似为驻点温度 的 0.9 倍,即导弹蒙皮的温度 T=0.9Ts

1.2 导弹羽流温度分布计算

当导弹超音速运动,尾焰在高度欠膨胀的情况下,认为燃气射流起始段包含马赫盘的波节只有一个。

燃气离开喷管瞬间形成膨胀波。膨胀使气流速度加快尧出口压强 P1 降低;在边界处,为满足 P1 和 Pa 相等,就会产生一道拦截膨胀波的冲波。

初始段中粘性和导热性影响只表现在很薄的边界层,这一 段气流结构可按理想流体的气动力问题来确定。在过渡段中湍流度影响显著,还存在一个射流等速核心区。基本段内应用自由湍流射流理论,假设整个过渡段的长度与马赫盘半径之比为一常值(按实验取为 4,则整个欠膨胀流场结构如图 1 所示:

f1

 

1.3 导弹羽流形状计算模型

导弹羽流形状计算模型需要对“射流起始段边界形状”和“马赫盘半径”建模,同时确定”射流过渡段与基本段形状“等。

2、仿真

最后是进行仿真,在 Matlab 环境下袁对某型导弹进行仿真。

通过参数计算结果可以得出初始段气流速度与温度等变化的一些规律。气流刚从喷管喷射出来时速度是逐渐增大的,但是加速度是逐渐减小的。

然后气流由于受到正冲波的作用使得气流速度整体速度瞬间减少。而气流温度的变化与速度变化正好相反。但在马赫盘内,温度急剧地升高很多,故气流在马赫盘将形成温度的一个梯度极点。当气流穿过马赫盘后,在空气阻力的作用下逐渐射开飘散。

详细的计算模型可以联系作者/责编获取论文全文。

参考来源:

[1]:科普中国

[2]:海军航空工程学院 研究生管理大队&兵器科学与技术系 论文《超音速导弹温度场建模与仿真》,作者:吴龙宝 ,谢晓方,王诚成 ,于嘉晖。

 

对军事科技、无人机、芯片等感兴趣的可以加作者/责编微信(同名)加入对应俱乐部。

作者:Challey

来源:EDN电子技术设计

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