首页 > 技术 Tech > 火箭发动机的热防护 Thermal Protection for Rocket Motor Casings

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings

译自 http://www.nakka-rocketry.net/therm.html/





A great deal of heat is, necessarily, generated by the combustion of propellant in a solid rocket motor. The hot combustion products are under high pressure and must be effectively and reliably contained by the motor casing to ensure the safe operation of the rocket motor. The casing behaves as a heat “sponge”, continually absorbing heat, as essentially no heat is transferred from the casing outer surface to the surroundings (under flight conditions, however, some of the heat may be convected to the atmosphere) thereby continuously elevating the temperature of the casing walls over the operating duration of the motor. Fortunately, operating durations are usually quite short, as most structural materials suffer a significant reduction in strength at elevated temperature. Despite the short burn times, some form of thermal protection is usually required for the casing, as a result of the rapid transfer of heat that occurs in the “inferno” of high pressure turbulent flow conditions present in a rocket motor.




Thermal protection is generally not necessary, however, if all the conditions below are satisfied:




  1. The motor has a particularly short burn time (typically less than one second)
  2. The propellant has a relatively low combustion temperature (e.g. KN based propellants)
  3. The casing is fabricated from a material that will not weaken greatly at elevated temperature, and the casing wall is of sufficient thickness such that it is structurally capable of containing the chamber pressure at its reduced strength.


  1. 工作时间很短(典型值:<1s)
  2. 燃料工作温度相对较低(如KN系)
  3. 外壳由强度不会因高温而过分降低的材料制成,且壳体足够厚,以至于在强度降低之后仍能耐受燃烧室压力


This is the approach that has been taken for my A-100, B-200 and C-400 motors. For all other scenarios, such as my new kAPPA rocket motor), thermal protection of the casing will be necessary. Practical thermal protection for amateur motors can take three forms:


我在我的A-100, B-200和C-400 发动机上都忽略了热防护。除此之外,比如我最新的kAPPA火箭发动机,外壳的热防护就是必须的了。




  1. Layer of thermal insulating (low conductivity) material on casing inside walls
  2. Heat sink, which may be as simple as using a thick walled casing of high conductivity material
  3. Layer of ablative material which absorbs heat as it burns away (or casing is fabricated from an ablative structural material)


  1. 外壳内壁贴隔热材料
  2. 散热片。例如发动机壳体采用厚壁高导热材料
  3. 烧蚀吸热材料


Item #1 is self-explanatory, which involves installing a heat-resistant liner against the casing inner walls. The low thermal conductivity of the insulator simply reduces the rate at which heat may be diffused into the casing walls. The challenge is to use a material that is sufficiently heat resistant such that it does not simply burn (or melt) away over the operating duration of the motor. Since most practical materials will in fact tend to burn away, it is necessary to size the thickness of the insulating layer such that enough remains to suit the task.


方案1 很简单啦,blahblah挑战在于找到一种材料,热阻足够高且不会在工作周期内烧蚀或者融化。考虑到大部分实用隔热材料都或多或少会被烧蚀,应调整隔热层的厚度以保证有足够余量撑完整个工作周期。


Item #2 is certainly the simplest approach. As will be shown later, materials with a high thermal conductivity (such as aluminum alloys) are capable of rapidly diffusing and “storing” any absorbed heat in such a manner that the overall temperature of the casing will remain reasonably low, as long as sufficient mass (i.e. thickness) is used.


方案2 是最简单的。足够厚的一圈铝合金可以快速吸热。


Item #3 is probably the best approach to thermal protection for motors with high operating (combustion) temperatures and /or long burn times. An ablative material is usually a thermoset plastic or rubber material which decomposes (rather than melts) as it burns away. The material undergoes an endothermic (heat absorbing) degradation shortly after motor start, as the poor conductivity causes the surface temperature to rise rapidly. Pyrolysis gases produced upon decomposition provide additional thermal protection by forming a protective boundary layer.


方案三则最适合那些工作温度高/ 工作时间长的发动机。烧蚀材料通常是热固性塑料或者橡胶材料,它们在烧蚀过程中会分解(而不是融化)。这些材料因为导热不良,表面温度会迅速升高,因而在发动机开始工作后不久就会进入受热分解过程。

高温下的强度 Strength at Elevated Temperature

Both the material yield strength and the ultimate strength are similarly affected by elevated temperature. The yield strength (upon which design is typically based for reusable motors) is the stress level, which exceeded, results in permanent deformation, or yielding, of the structure. The ultimate strength is the stress level at which fracture occurs. The effect of elevated temperature on some casing materials is shown in Figures 1 and 2. It can be seen from these figures that aluminum alloys, in particular, suffer significantly even under moderate heating. For example, at 150 C. (300 F.), the 6061 alloy has only about 80% of the room temperature strength. For comparison, low-carbon (mild) steel retains 80% of its yield and ultimate strengths at 240 C. (465 F.) and 380 C. (720 F.), respectively. For reference, melting points are provided in Table 1.

高温下材料的屈服强度和极限强度都会受到影响。超过屈服强度,发动机就会永久变形,超过极限强度,发动机就会裂 (可重用发动机对屈服强度要求很高)。图1图2是高温对几种外壳材料强度的影响。铝合金150多度就开始受不了了,强度都降到80%以下,其他的钢都还好。


火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第1张  | 科创航天 KCSA



火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第2张  | 科创航天 KCSA



火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第3张  | 科创航天 KCSA



Note that the strength reductions shown are for prolonged exposure (1/2 hour). For very rapid heating, such as that occurs in rocket motors, the effect is somewhat less severe, as illustrated in Figure 3 for 2024-T3 aluminum alloy. Unfortunately, data on rapid-heating strength of most materials does not seem to be readily available. Consequently, the data from Figures 1 and 2 are used for design, which is conservative.


火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第4张  | 科创航天 KCSA


Thermal protection is of particular importance for motors with free-standing propellant grains. Not only are the combustion gases in constant and direct contact with the entire casing walls, more importantly, convection of the gases greatly increases heat transfer to the casing walls.



几乎所有从燃烧气体传递到壳表面的热都是通过对流的机制实现的。这个过程不仅涉及到分子运动或扩散产生的能量(热)传递,还涉及到液体牵连运动(速度)所传递的能量。对流热传递的方程可以表达为:q = h (Tg – Ti )  [式1]





• 初始壳温度为20摄氏度

• 燃烧气体温度为1450.摄氏度

• 燃烧时间为1.5秒

• 热传递对流系数为 1000 Watt/m2-

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第5张  | 科创航天 KCSA


要注意的是,例子中的热分布是以整体壁厚的一半为前提展示的。 为了更好地理解上图所示的结果,要注意[式1]中传递到外壳壁上的热量是一个关于内壁温度的函数,Ti. 传递的热量的速度随着内壁温度上升而下降。同时,热的传递受壳材料的扩散率的影响。扩散率(alpha)是瞬时热传递的决定性因素,定量式子如下

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第6张  | 科创航天 KCSA

其中k为导热系数,ρ为质量密度,Cs为外壳壁的热容量。这三个参数都受到温度的影响而改变,虽然密度的改变可被忽略。室温下这三种材料的 a值如下:

• 铝合金(6061-T6), a = 690

• 不锈钢(AISI 304), a = 40

• PVC塑料, a = 3.4


PVC中的温度分布十分地有趣。管壁内外存在着巨大的温差, ,这是因为PVC材料的扩散性非常差。同时PVC的低密度决定了它储热性差的特点,除了内壁最内的部分(那里会非常地热),整个外壳的温度都比较低。内壁的急剧升温进一步减弱了热传递(Ti),图表中温度曲线之间越来越窄的间距就是证据。分析中没有考虑到的是,实际上PVC材料在大概摄氏250度开始分解(碳化)。然而分解会降低传递到管壁上的热的量,因为就像上面提到的那样,热量都被热烧蚀过程吸收了。


壳降温过程中的热隔离层的效率可以轻易地在图5a,b中看到。在这个例子中,隔热层的厚度是0.5mm,且有着一些纸或热固性隔热层的典型特性(alpha=1.0). 壳是2mm厚的6061铝合金,热学状态与上一个例子相同。

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第7张  | 科创航天 KCSA



在这个例子中,目标是把壳的温度保持在150摄氏度以下。在这个温度,材料强度值(Ftu,Fty)相当于室温下的80% .例子中的这个值是可以接受的。当然,如果需要保持壳在一个更高的强度的话,可以用更厚的隔热层来进一步降低壳的温度。由于隔热层的质量密度相当低,所以副作用主要不是增加的质量,而是因此下降的外壳直径,进而使得能容纳的推进燃料变少。尽管可以通过把壳造得更长来弥补,但是更严重的是,推进燃料的厚度会因此下降,进而减少燃烧时间。


我在两种材料上分别作了一系列的测试,分别是是卷起来的纸和聚酯涂层。做测试的目的有两个,一是在真实的加热条件下研究这两种有成为隔热层潜力的材料的表现,另一目的是为THERMCAS提供一次验证的机会。测试中的壳是用一块6061-T6511铝合金做成的薄壁型壳, 跟kAPPA火箭发动机上的完全一样。(直径63.5mm,壁厚度1.65mm)。用的纸是平均厚度0.15mm的棕色信纸。一共做了两次测试,一次用了两层(0.29mm),另一次用了7层(1.0mm)。




火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第8张  | 科创航天 KCSA



火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第9张  | 科创航天 KCSA



此测试的数据将会与THERMCAS结合来决定纸质和聚酯材料隔热层的热传递率的实验值。至关重要的是,用无隔热层样本实验的结果来估算加热过程中的对流系数(h). 这种分析是通过不断尝试——出错——纠错——尝试的循环实现的,不断地更改输入THERMCAS中h的值,知道预测出来的温度——时间曲线与实验结果吻合。把1800摄氏度作为假想火焰温度输入(在理想的空气/丙烷混合比下,丙烷火焰的温度为1967摄氏度)。预测结果对这一参数并不敏感。

分析得出h=55 Watt/m2-K.(一定要注意的是这个值比实际中火箭发动机的值要低很多,因为实验是在标准大气压下做的。而在火箭发动机的高压环境下,对流系数很可能会是前者的20到30倍那么大,加热时间也因此短很多)。图8展示的是无隔热层的测试结果和用来估计h的加热曲线

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第10张  | 科创航天 KCSA




表2- 公开可查阅的材料数据

火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第11张  | 科创航天 KCSA


火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第12张  | 科创航天 KCSA


火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第13张  | 科创航天 KCSA


火箭发动机的热防护 Thermal Protection for Rocket Motor Casings - 第14张  | 科创航天 KCSA





而设计一个PVC材料的发动机 外壳就涉及到另一种方法了。壳的内表面附近剧烈升温,而温度沿着外表面方向快速下降。这里使用优先考虑短板的设计方法,例如壳壁的厚度。有效的壳厚度指的是能将温度保持在某个阈值以下的时候的壳厚度。由于PVC从摄氏100度开始变软,这个值就可以被当做阈值了。用图4c中无隔热层PVC外壳作为例子,那么有效厚度将会是teff = (1-9/22) * 3.9 = 2.3 mm.