Reusable spacecraft include mechanisms to deorbit and reenter the atmosphere in a controlled fashion. For this purpose, the 穿梭機 included 航天飛機軌道機動系統, and the 龍飛船 included its own engines, used for deorbiting. Deorbiting slows the spacecraft down, lowering its perigee to inside the atmosphere where the vehicle descends to Earth.[1][2]
As a rough rule of thumb, 15% of the landed weight of an atmospheric reentry vehicle needs to be 防熱盾ing.[3]
穿梭機隔熱系統s (TPS) can be made of a variety of materials, including reinforced carbon-carbon and 再入.[4] Historically these materials were first developed on ICBM 多目標重返大氣層載具s. However, the requirements of reusable space systems differ from those of single use reentry vehicles, especially with regards to 防熱盾 requirements. In particular the need for durable high 發射率 coatings that can withstand multiple thermal cycles constitutes a key requirement in the development of new reusable spacecraft. Current materials for such high 發射率 coatings include transition metal disilicides.[5]
Ablative heat shields are reliable, but they can only be used once, and are heavy. Reinforced carbon-carbon heat tiles like those used on the Space Shuttle are fragile, and this was proved on the Space Shuttle Columbia disaster. Making a resistant yet lightweight and effective heat tile poses a challenge. The LI-900 material was used on the Space Shuttle.
Landing and refurbishment
[編輯]Spacecraft that land horizontally on a runway require wings and undercarriage. These typically consume about 9-12% of the spacecraft mass,[來源請求] which either reduces the payload or increases the size of the spacecraft. Concepts such as 舉升體 offer some reduction in wing mass,[來源請求] as does the 三角翼 shape of the 穿梭機軌道器.
垂直起降 (火箭) can be accomplished either with parachutes or propulsively. 龍飛船 was an example of 宇宙飛船 with parachute reusability. Its derivative, 龍飛船2號, was originally intended to propusively land on land. However, such concept of reusability was canceled in 2017 and now Dragon 2 uses parachutes to land in the ocean.
After the spacecraft lands, it may need to be refurbished to prepare it for its next flight. This process may be lengthy and expensive, taking up to a year. And the spacecraft may not be able to be recertified as human-rated after refurbishment. There is eventually a limit on how many times a spacecraft can be refurbished before it has to be retired, but how often a spacecraft can be reused differs significantly between the various spacecraft designs.[6][7]
