Comparison of capsules
There is a wide variety of capsules for space applications or parachute test missions, ranging from small vehicles for in-flight testing to large, manned capsules for human space flight. This page provides a brief overview of what different capsules for space applications could look like.
The design of a capsule often is often the result of a multidisciplinary compromise or trade-off. Factors such as aerothermal heating, aerodynamic stability, life-support systems, accessibility, recovery, retrieval, mass, cost, volume, visibility and many others each play a crucial role when designing a capsule.
Spherical capsules
The earliest manned capsules of the Soviet Union used spherical capsules, namely Vostok and later also Voskhod. Due to the poor aerodynamic stability of the spherical capsule, a heat shield was placed on all sides of the capsule. Many of the Venera probes and landers also used a spherical capsule to protect them during the entry into the Venusian atmosphere regarding of attitude. This type of capsule is only used in case a high degree of uncertainty is expected regarding the attitude of the body during reentry.
Blunt body capsules
A blunt body capsule features a vehicle with a spherical heat shield at the bottom. This type of capsule has been very successful and popular for manned missions such as Apollo, Soyuz, Crew Dragon and many other. The vehicle experiences reentry at a fixed orientation such only the heat shield has to deal with the extreme temperature and pressure conditions. This shape allows for a relatively high payload volume for a given diameter, which makes it so attractive for manned missions. The blunt shape of the heat shield makes sure that the vehicle experiences a lower heat flux.
Conical capsules
Many capsules of unmanned spacecraft have a conical shaped heat shield to provide more aerodynamic stability during reentry compared to the spherical blunt body. The cone angle can range from very blunt 140° to very sharp 12° cones. The cone angle and radius of the tip have a major effect on the aerodynamic heating, stability and deceleration during atmospheric entry. Bodies with very sharp-angled cones often have a much higher ballistic coefficient, leading to high heat loads and high velocities at low altitudes. This is generally avoided, but can also be desired to achieve relevant test conditions. Many of the Mars landers use a 140° conical heat shield to protect their rovers during reentry.
Cylindrical capsules
Cylindrical capsules consist out of a cylindrical body with a round or bullet-shaped nose and are frequently used for missions to test EDL systems or flight system demonstrations. These capsules often fulfil the same role as the nosecone of the rockets they are launched with. These shapes are however rarely used for high-speed reentry due to their poor aerodynamic stability. Examples of this type of capsules are the Spica capsule and SPEAR vehicle.
