lang-8.com is well known amongst intermediate/advanced language learners who are self-taught, but I thought I’d explain it for beginners. It is an amazingly useful site where you can blog or upload any text you’ve written in the language you’re learning. Many people use it as a diary, though I’ve seen people check essays and business emails there too.
Once the text is uploaded native speakers of the language will check it for you. You return the favour by checking pieces of work written in your native language and correcting them. In this way everyone gets their work marked by a native speaker (often more than one person contributes so you can get several opinions too).
It is massively helpful in improving all aspects of writing, but natural usage in particular (notoriously difficult for self learners to pick up when they are not in a native environment).
SpaceX Restartable Thrusters Pass Early Tests
SpaceX completed qualification testing for its SuperDraco thruster late last month. The engine will eventually be mounted on the manned version of the Dragon spacecraft as part of its launch escape system. It will also help the vehicle touch down on its return to Earth or on whatever other planet it visits.
SpaceX says the engine produces 16,000 pounds of thrust and can be fired multiple times. In an emergency, eight SuperDracos built into the Dragon will provide 120,000 pounds of thrust to propel the crew a safe distance from the rest of the vehicle.
The thruster’s engine chamber is made through the industrial 3-D printing process called direct laser metal sintering. It is composed of Inconel, a strong nickel-chromium superalloy able to withstand high temperatures.