他正在递出一个信封。

He is handing over an ___envelope.

请把信放进信封里再寄出去。

Please put the letter in the ___envelope before sending it.

航天飞机的外壳在重返大气层时承受了极端高温。

The ___envelope of the space shuttle withstood extreme heat during re-entry.

这组直线的包络线形成了一个优美的椭圆。

The ___envelope of these straight lines forms a beautiful ellipse.

新设计的引擎突破了性能的极限。

The newly designed engine pushes the ___envelope of performance.