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SpaceX Crew Demo-1 Mission [video]

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Re: SpaceX Crew Demo-1 Mission [video]

#81

Post by avmich »

solarkraft wrote: post #79
avmich wrote: post #68

Earlier quoted context omitted.

> 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.

But only the underlying kernel part is actually real-time, the Linux process wouldn't be, right? That leads me to the question: Shouldn't all things in a rocket be real time? Are they "post-poning" stuff like telemetry and other communications?

All critical processes can run outside of Linux, in hard real-time fashion. Linux is used for non-critical things, like networking... The whole RTLinux includes Linux (non-RT part) and a hard RT part, for which you can write your code.

Re: SpaceX Crew Demo-1 Mission [video]

#82

Post by JshWright »

vermontdevil wrote: post #53
sandworm101 wrote: post #50

Earlier quoted context omitted.

Technically they do reach the stations velocity. In fact they exceed it. The lower orbit is slightly faster. Total energy is lower, but they are moving faster than the station. A counterintuitive part of spaceflight is that often you have to first accellerate in order to slow down.

Makes sense. Get up to a higher speed and it’s easier to do a controlled slow down. Than having to speed up after getting into orbit near another orbiting body.

That's not really it. "Speeding up" and "slowing down" are equally easy (or hard). They're both acceleration, just in a different direction.

The reason it's going faster is because it's lower. The lower an orbit, the faster the thing is moving. In order to slow down, you have to accelerate twice (I'm using the term "accelerate" here to refer to "going faster in the direction you're already going").

The first acceleration raises the far side of your orbit. You then have to "climb" up to that new higher orbit. Once there, you accelerate again to raise the low side of the orbit (otherwise you'd pick up all that speed again then "falling" back down to the lower altitude).

At the end, you're in a new, higher, orbit, going slower than when you started.

Re: SpaceX Crew Demo-1 Mission [video]

#83

Post by mandeepj »

420codebro wrote: post #75
JshWright wrote: post #60

Earlier quoted context omitted.

A lot of what they are doing dates back a lot further than that. The innovations SpaceX has brought to the industry are more process related than technical. SpaceX has done a great job of setting a streamlined vertical integration for their production processes, which has allowed them to iterate a lot faster than others in the industry. That isn't to say they haven't been doing interesting things from a technical per…

The resulting cost savings they (SpaceX) are bringing to the launch market is really amazing. United Launch Alliance (ULA) made a fortune on over-charging the United States Government for their National Security launches. SpaceX is now competing for those launches with a price between 1/4 to 1/3 of what ULA is charging.

> SpaceX is now competing for those launches with a price between 1/4 to 1/3 of what ULA is charging.

Is there really a competition for this much price difference?

Re: SpaceX Crew Demo-1 Mission [video]

#84

Post by mandeepj »

joezydeco wrote: post #71
JshWright wrote: post #60

Earlier quoted context omitted.

A lot of what they are doing dates back a lot further than that. The innovations SpaceX has brought to the industry are more process related than technical. SpaceX has done a great job of setting a streamlined vertical integration for their production processes, which has allowed them to iterate a lot faster than others in the industry. That isn't to say they haven't been doing interesting things from a technical per…

So, out of curiosity, was there one person responsible for SpaceX operating in this manner? Was it Musk, or someone else? Unless they accomplished a miracle by building a world-class engineering team from zero, there must have been a catalyst.

> there must have been a catalyst

There is - having visionary, charismatic founder(s). This pattern has been successful at Apple, Microsoft, Amazon, Facebook, Netflix, Tesla\SpaceX, Google.

Re: SpaceX Crew Demo-1 Mission [video]

#85

Post by GhostVII »

Symmetry wrote: post #48

From Eric Berger's twitter[1] > If the first stage lands tonight, SpaceX will have successfully landed 35 of its 69 launches, meaning that MORE THAN HALF of its rocket launches will have safely returned. [1] https://twitter.com/SciGuySpace/status/1101752352317628416

Important to note that for many of their launches they weren't trying to land. They had 42 landing attempts, so over 80% success rate.

Re: SpaceX Crew Demo-1 Mission [video]

#86

Post by utdiscant »

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…

I was a summer intern at CERN a few years back. I had the same feeling of watching thousands of people working on projects together including tons of code, wiring, machinery, and complex physics to measure extremely hard things at ridiculous precision. I could never really shake the feeling that it should have been practically impossible to make such projects work. It is hard enough to not make bugs in personal projects, but when you are hundreds of people writing code and plugging in wires over time, some mistakes should appear that breaks the results.

Re: SpaceX Crew Demo-1 Mission [video]

#87

Post by monocasa »

avmich wrote: post #68
monocasa wrote: post #59

Earlier quoted context omitted.

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.

Wow, that's really silly that someone went out, made a hard realtime system using Linux as the lowest priority level, and then called it RTLinux. RTLinux for years had meant Linux with the PREEMPT_RT patches.

In any case, SpaceX doesn't use Linux for hard realtime systems.

> They do not have hard realtime requirements

https://lwn.net/Articles/540368/

And I probably wouldn't use the RTLinux for a real hard realtime system if I were making the choice. Putting all of the realtime code in the same memory domain as the rest of the Linux kernel is a huge surface area for corruption. I'd probably use sel4 with a virtualized Linux to get the same semantics.

Re: SpaceX Crew Demo-1 Mission [video]

#88

Post by danpalmer »

JshWright wrote: post #61
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.

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

You can also skimp on hardening if the operational lifetime in space is minutes to days, instead of months to years like most satellites.
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