SpaceX Crew Demo-1 Mission [video] • Hacker News Board

Re: SpaceX Crew Demo-1 Mission [video]

#61

Post by JshWright » Sat, Mar 02, 2019, 5:49 PM UTC

biesnecker wrote: post #52

danpalmer wrote: post #44

Earlier quoted context omitted.

From what I understand, they are pushing the boundaries on the hardware front, i.e. relatively recent, typical Linux computers onboard, and just relying on having a few redundant ones and short flight times, rather than very radiation hardened systems like those used in satellites or real-time operating systems. However on the SpaceX software side I think they’re fairly conservative and “boring”, so I’d guess C and s…

I'd be surprised if they were able to be human flight certified without all the redundancy and hardening that any other manned spacecraft has, though.

You can "skimp" on hardening if you use more redundancy (which is generally cheaper, and often lighter (which is the real cost savings)).

Re: SpaceX Crew Demo-1 Mission [video]

#62

Post by JshWright » Sat, Mar 02, 2019, 5:52 PM UTC

sitharus wrote: post #36

jessriedel wrote: post #35

Earlier quoted context omitted.

Anyone know why they choose a launch time and trajectory such that the arrival at ISS is more than a day later? It seems like you could choose to arrive at the ISS almost immediately after reaching orbital velocity if you wanted.

So they have time to validate the systems are working correctly. A direct launch would leave little margin for error, especially on a test flight. Edit: now I think more it also allows for variations in launch performance. Soyuz does a three day rendezvous as back when the system was designed the accuracy required wasn’t possible, though now they’ve planned a same-day docking.

Soyuz has done "fast track" (same day) approaches for years now.

Re: SpaceX Crew Demo-1 Mission [video]

#63

Post by JshWright » Sat, Mar 02, 2019, 5:57 PM UTC

azernik wrote: post #38

jessriedel wrote: post #35

Earlier quoted context omitted.

Anyone know why they choose a launch time and trajectory such that the arrival at ISS is more than a day later? It seems like you could choose to arrive at the ISS almost immediately after reaching orbital velocity if you wanted.

tl;dr because the ISS orbit is not equatorial, choosing the right launch time is non-trivial and solutions are only approximate. ISS launch windows are complicated - you have to match both plane (i.e. the Earth needs to turn so that the Cape/Baikonur is under the ISS's orbit path) and phase (i.e. at the time the plane match happens, the station needs to be at the right part of its orbit for the spacecraft to meet the…

If you don't mind me simplifying your answer a bit...

Imagine the orbit of the ISS like a tilted hula hoop around the Earth, with the Earth revolving inside it. Because the Earth rotates once every 24 hours, any given spot on Earth will be directly underneath that hula hoop twice per day (once on either side of the hoop).

When launching to the ISS, you need to launch when that orbit "hoop" is directly overhead. That's the bit that really matters, because you need to be in the same orbit. The ISS itself is almost certainly not overhead (it may be on the other side of the planet entirely), and so it will take some time to catch up to it.

Re: SpaceX Crew Demo-1 Mission [video]

#64

Post by bisby » Sat, Mar 02, 2019, 5:58 PM UTC

primordialsoup wrote: post #31

I am an engineer who builds with a computer. And when I see such things as this, I sometimes spend a minute or so marveling and thinking about the complexity of building and orchestrating such a launch. In fact, I really cannot comprehend how something like this can be successfully done, given how so many things could go wrong. Yes, I understand the idea of redundancy, I understand gravity, I understand that physics…

Last night watching the launch I said to my friend: If I worked at SpaceX I would be second-guessing everything I'd ever done every time there was a launch.

"Did I finish that weld good enough?" "Yes, we checked it 1000 times" "But what if I missed something?"

And clearly, those kinds of mistakes have happened. And that's why we check 1000 times instead of 500.

Re: SpaceX Crew Demo-1 Mission [video]

#65

Post by the8472 » Sat, Mar 02, 2019, 5:58 PM UTC

Gravityloss wrote: post #49

primordialsoup wrote: post #31

I am an engineer who builds with a computer. And when I see such things as this, I sometimes spend a minute or so marveling and thinking about the complexity of building and orchestrating such a launch. In fact, I really cannot comprehend how something like this can be successfully done, given how so many things could go wrong. Yes, I understand the idea of redundancy, I understand gravity, I understand that physics…

Computer engineering is new. We're still at the phase where bridges collapse and ships sink regularly.

It's a matter of tradeoffs. If your webapp is unusable 2 days a year people will just do something else in the meantime. If your flight control software fails for two days a year planes might come crashing down. The former is tolerable even if unpleasant from a customer perspective, thus they don't pay for flight control levels of reliability.

And it's not like this is unique to software. It's the difference between a disposable plastic fork and surgical instruments.

Re: SpaceX Crew Demo-1 Mission [video]

#66

Post by JshWright » Sat, Mar 02, 2019, 5:58 PM UTC

smdyc1 wrote: post #29

Amazing footage. The precision Landing of stage 1 was something I hasn't expected.

This was actually one of the less precise landings. Generally it's right on the X. It looks like it may have slid a bit after touchdown.

Re: SpaceX Crew Demo-1 Mission [video]

#67

Post by mLuby » Sat, Mar 02, 2019, 6:08 PM UTC

twtw wrote: post #55

Gravityloss wrote: post #49

Earlier quoted context omitted.

Computer engineering is new. We're still at the phase where bridges collapse and ships sink regularly.

Computer engineering is not substantially newer than aerospace engineering.

It's true, they just have different focuses. Aerospace (in general) has traded huge costs for safety and reliability. Computer engineering (in general) has traded correctness/reliability for low cost and rapid development.

Re: SpaceX Crew Demo-1 Mission [video]

#68

Post by avmich » Sat, Mar 02, 2019, 6:19 PM UTC

monocasa wrote: post #59

elteto wrote: post #58

Earlier quoted context omitted.

The flight software team did a reddit AMA a while back [0]. They use C++ for writing the flight software (what flies falcon, dragon, etc) running on a customized RT Linux. [0] https://www.reddit.com/r/IAmA/comments/1853ap/we_are_spacex_...

And FreeRTOS. RTLinux is only barely soft realtime.

> RTLinux is only barely soft realtime.

Linux used to run as a non-privileged process in hard real-time kernel, which taken together was guaranteeing responses in nanoseconds range.

If this is still the case (along with what Wikipedia states) , the whole system is by definition hard real-time.

Re: SpaceX Crew Demo-1 Mission [video]

#69

Post by creatornator » Sat, Mar 02, 2019, 7:18 PM UTC

gpm wrote: post #54

biesnecker wrote: post #52

Earlier quoted context omitted.

I'd be surprised if they were able to be human flight certified without all the redundancy and hardening that any other manned spacecraft has, though.

For example, my understanding is that the redundancy approach is basically "do everything on 3 computers, vote on the outcome".

Sometimes you even have different firmware/software teams write code for each of the three different configurations (often targeting different processors/architectures, etc). Pretty cool

Re: SpaceX Crew Demo-1 Mission [video]

#70

Post by mandeepj » Sat, Mar 02, 2019, 7:26 PM UTC

danpalmer wrote: post #44

bsaul wrote: post #41

Earlier quoted context omitted.

I remember reading here a document about C programming guidelines for military / space software ( basically no dynamic memory allocation ever), but that was set a long time ago. I wonder if spaceX reinvented some parts of the software stack, or if they went along the traditional way. I don’t expect seing rust code in the shuttle, but maybe formal proof with custom functionnal languages...

From what I understand, they are pushing the boundaries on the hardware front, i.e. relatively recent, typical Linux computers onboard, and just relying on having a few redundant ones and short flight times, rather than very radiation hardened systems like those used in satellites or real-time operating systems. However on the SpaceX software side I think they’re fairly conservative and “boring”, so I’d guess C and s…

They build everything in-house - software and hardware. You can read about it in detail in Musk's biography - https://www.amazon.com/Elon-Musk-SpaceX-Fantastic-Future-ebo...