It seems like a no-brainer, even to non-rocket scientists: wouldn’t even one hole in a heat shield be one hole too many?
The heat shield is there to protect the crew cabin from the compressional heating of the air during re-entry into Earth’s atmosphere. When a space capsule returns from low Earth orbit, it rams into the thin atmosphere at 100 kilometers altitude at orbital velocity, nearly 8 kilometers per second (Mach 25, or 25 times the speed of sound). In 1951, H. Julian Allen and A. J. Eggers, Jr., of the National Advisory Committee for Aeronautics (NACA) discovered that a high-drag blunt shape made the most effective heat shield for atmospheric entry. They found that the greater the drag of a re-entry vehicle, the less the heat load. During high-speed atmospheric entry, the blunt leading face of the capsule piles air up in front of itself, pushing the shock wave of compressed, heated air away. Compression heats the gas to over 1,100°C (2,000°F) (ref. 1). Since most of the super-hot gases are not in direct contact with the vehicle, most of the heat energy stays in the shocked gas flowing around the vehicle and dissipates into the atmosphere. Still, the radiant heat from the hot gas could melt the spacecraft’s internal structure (ref. 2). Therefore, most space capsules typically rely on thermal protection materials which burn and char away in the air flowing past the capsule, carrying away the heat (ref. 3). Reusable vehicles, like space shuttles, have advanced thermal protection which insulates the vehicle’s structure from the heat without burning away itself.
|Space Shuttle Orbiter immediately before landing, |
showing landing gear deployed through heat shield
(thermal protection system)
|Gemini-2 capsule reflown with simulated hatch in heat shield|
|Gemini-B/Manned Orbiting Laboratory|
(A) internal tunnel (selected concept)
(B) capsule reposition
(C) inflatable external tunnel
(D) EVA transfer (ref. 8)
Other alternatives have also been considered. Two early concepts for transfer between Gemini and MOL evaluated space-walking between the modules or extending an inflatable tunnel from one module to the other (ref. 8). Both those options were discarded as even riskier than the hatchway in the heat shield.
|Martin Co. concept, 1958 (ref. 9)|
History shows that, as critical as intact heat shields are, resealable penetrations through them have not been uncommon. Still, there are several alternate approaches to permit inter-vehicle astronaut transfers. Spacecraft designers have usually opted for those that go “around” instead of “through”, even if it means heavier spacecraft, more complicated in-flight procedures, and costs more in the long run.
- __, http://en.wikipedia.org/wiki/Aerodynamic_heating, last accessed 23 April 2012.
- __, http://en.wikipedia.org/wiki/Space_Shuttle_thermal_protection_system, last accessed 25 April 2012.
- __, Atmospheric Reentry Physics: Development, Validation and Uncertainty Quantification of Ablation Models, February 3-8, 2013, Four Points Sheraton / Holiday Inn Express, Ventura, CA, http://www.grc.org/programs.aspx?year=2013&program=atmosentry, last accessed 22 April 2012.
- Cosmos-140, Feb. 7, 1967, in Siddiqi, A.A., Challenge to Apollo (Washington, D.C.: NASA, 2000), p. 576.
- Floyd, Jamie, M.D., "Assessment of the Almaz Capsule for Space Station Assured Crew Return (ACRV)", McDonnell Douglas Aerospace MDC 97W5151, January 1997.
- Soyuz 7K-VI, in Siddiqi, A.A., Challenge to Apollo (Washington, D.C.: NASA, 2000), p. 597-8.
- __, "Why Do China's Shenzhou-Series Spaceship Use 3-Module Layout?", www.sina.com.cn in Chinese, 22 July 2011.
- MOL transfer concepts, described in Gatland, K., Space Technology (New York: Salamander Books Ltd., 1981, 1982), p. 77.
- Martin Co. concept, in Miller, R., The Dream Machines (Malabar, FL: Krieger Publishing Co., 1993), p. 474.
- Transformational Space Corp. (tSpace) Crew Transfer Vehicle CXV concept previously described at http://www.transformspace.com/Welcome.html, site under construction when checked on April 29, 2012. See http://www.astronautix.com/craft/cxv.htm (accessed April 29, 2012).