SpaceX makes progress toward Commercial Crew debut

[Chris Bergin]

https://www.nasaspaceflight.com/2018/04/spacex-progress-commercial-crew-debut/ …

- by Chris Gebhardt

(Nathan Koga renders for NSF)

[Rocket Science]

Great article and renders gentlemen, thank you... Looking forward to part two!

[clongton]

Great writeup Chris - very informative! Thanks.

[Svetoslav]

Loved it. I can't wait to read the article about Boeing.

I'd also love to read an article about suborbital spacecraft - Blue Origin and Virgin Galactic.

[copper8]

Thanks Chris.  It's a great article.

[rcoppola]

Hey, who's this fella Chris mentioned in the article asking NASA questions?  "When asked by NASASpaceflight’s Chris Gebhardt when that operation would begin..."

[Comga]

Good article
Sometimes their progress is so painfully slow, with dramatic presentations of additional dangers, like exceeding capsule structural limits by impacting high waves.
At one point the article describes the installation of "oxygen and nitrox" delivery systems.  In diving, doesn't "nitrox" refer to mixed nitrogen and oxygen, like normal Earth atmosphere?  Does Dragon 2 store oxygen and mixed oxygen and nitrogen, rather than oxygen and nitrogen separately?

[Johnnyhinbos]

Good article
Sometimes their progress is so painfully slow, with dramatic presentations of additional dangers, like exceeding capsule structural limits by impacting high waves.
At one point the article describes the installation of "oxygen and nitrox" delivery systems.  In diving, doesn't "nitrox" refer to mixed nitrogen and oxygen, like normal Earth atmosphere?  Does Dragon 2 store oxygen and mixed oxygen and nitrogen, rather than oxygen and nitrogen separately?

You are both right and wrong. Open circuit divers often dive nitrox as their breathing gas. It’s a fixed ratio of oxygen and nitrogen. For recreational divers it’s often preset to 32% or 36% O2, for open circuit technical divers the ratio is all over the place, and involves multiple tanks of various ratios (and other inert gasses as well), all dependent on your exact planned dive profile.

However, for fully closed circuit rebreathers (CCR) it’s different - and here’s the analog for the Dragon 2. Think of rebreathers as infinite blending machines. They mix pure oxygen with another gas (known as the diluent, or dil). One rule of thumb is the dil needs to be breathable at your target depth on its own, with a partial pressure of O2 set to 1 ATA, which is less than your ideal O2 setpoint - generally 1.3 ATA - but still breathable. On ascent, when you decompress as ambient pressure reduces, you blend the gasses to provide higher fractions of O2, while also feathering up the partial pressure of O2 from your “low” setpoint (1.3) to your “high” setpoint  in the range of 1.6 ATA. You never would have a purely inert gas as your dil, you just need something with a low enough O2 component that at the very max planned depth the partial pressure of O2 is not to exceed 1 ATA. This is important (lifesaving important) because if your O2 spikes you need to be able to rapidly flush the loop to drive the O2 down (and yet still be able to breathe the loop) before you convulse and drown (a bad day).

D2 would have a nitrox mix that best suits the desired partial pressure of O2 (which in turn is based on the set cabin pressure), or more accurately, somewhat less the desired setpoint for PP O2. Then O2 is added to actually hit that setpoint. This allows minimal pure O2 flow, while giving flexibility in purging high or low...