## Introduction

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.

4. 工作时间很短（典型值：<1s

5. 燃料工作温度相对较低（如KN系

6. 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:

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)

4. 外壳内壁贴隔热材料

5. 散热片。例如发动机壳体采用厚壁高导热材料

6. 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.

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.

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.

[修改于 6年2个月前 - 2016/07/11 14:26:31]

3

HXKRRRR
6年2个月前

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6年2个月前 修改于 6年1个月前

## 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.

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.

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.

6年1个月前

### 壳表面的热

<code> h为对流系数，单位为Watt/m2-K
Tg为喷射气体的温度，单位为K
Ti为壳内壁温度，单位为K
</code>

• 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-

• 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。

### 对结构尺寸的热学考虑

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