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作者：大国重器专家团队成员 张伟 近一段时间，网络世界中流传着各种有关SpaceX和COTS器件的传说。我曾经写了一篇文章，比较系统的介绍了COTS器件在商业航天的应用问题（商业航天可以用COTS器件吗？）。如果，您看过其他网文，也看过我的上面这篇文章，心中可能还是会有这样的疑问： SpaceX到底有没有用COTS器件呢？ 如果COTS真的没有成本优势，SpaceX为什么会用呢？ 我觉得这样的疑问是有意义的，有必要正面回答一下。 1 目前，从网上可以找到的，得到SpaceX官方认可的，有关SpaceX元器件选用的，唯一资料就是2012年Aviationweek.com对John Muratore的一篇访谈（“大国重器”做了编译）。还有两篇相关的分析文章，分别是https://www./article/from-earth-to-orbit-with-linux-and-spacex/和 https://blog./elon-musk-architectures-leading-systems-falcon-heavy-rocket-fully-aligned（大国重器对部分重要内容也做了编译）。 其他资料，有的是在此之上的牵强附会，有的完全是蹭热点。我对流传比较广几篇文章的信息来源一一查证，结论是：皆不足信。下面我就在以上3篇文章的基础上，尝试做一些合理的推断，推导出SpaceX背后的方法论。 需要特别说明的是，除了对上述3篇文章的引用，和注明出处的资料外，其余都是我自己的推测，读者可以从自己的视角，批判性的阅读。 2 SpaceX有没有使用COTS器件呢？我们再来回顾一下资料中给了我们哪些信息： Q: But there's nothing on thespacecraft in the way of radiation-hardened parts? A: The parts aren't hardened, the design as a total system ishardened.What it is each part does not go through the screening thatis typical of radiation hardened parts. —— aviationweek.com 访谈中只是说了器件是“不抗辐照的”，但没有说是“未经鉴定”的。这个两个概念有很大区别， Muratore应该是故意模糊表述，避免透露出更多信息。 The Falcon 9's onboardoperating system is a stripped-down Linux running on three ordinary dual-core x86 processors.The flightsoftware itself runs separately on each processor and is written in C/C++. —— ZDNet ZDNet的说法，和很多流行的说法一致，但是没有出现在前文引述的SpaceX软件开发人员的原话中。正常的理解是，这个信息是作者或编者自行加上去的。但既然很多媒体披露的信息中都包含了“dual-coreX86 processors”，抱着空穴来风、事出有因的态度，我们可以先采纳这个说法。 Muratore接受访谈是在2012年，CPU的选型比这个时间还要更早，那时候有哪些符合“dual-coreX86 processors”条件的选项呢？ 首先，dual-core这个条件排除了“鉴定合格”产品的可能性。2012年版QPDSIS-38535中的X86CPU是80C86这样的16位产品。 其次，功率因素排除了XEON系列服务器芯片的可能性。虽然在稳定性方面服务器CPU有明显的优势，但是单颗CPU 100W+的功率，是空间应用的供电环境和散热环境无法满足的。 最终，只剩下桌面CPU的选项，下面的列表显示了Intel桌面处理器的发展历程。按照时间推算，SpaceX最有可能选择的是2008年至2009年发布的产品。其中，Celeron系列因为功耗优势，入选的可能性更高。并且，45nm工艺尺寸的Radiation Tolerance潜力很高。 如果放到今天，让SpaceX的设计团队重新选择，ARM精简架构可能是更好的选择。 数据来源：如图中信息 3 SpaceX为什么要选择COTS器件呢？我们可以在访谈中找到答案。 首先，是产品的性能优势。这点是首要的，也是显而易见的。 Q: What's the downside tobuying radiation-hardened hardware or software? Is it expensive, or just notwidely available? A: It's really not the expensethat drives it. We're committed to having the best possible parts in all of ourdesigns. So if it cost a lot and we needed it, we'd go get it. We were alreadyrequired to have all this redundancy in the computers to meet all the differentsafety requirements. Then we started looking at what parts do we want to useand what is appropriate for this design. And what really is more important tous than the cost of the parts is the capability ofthe parts – how much power dothey use, how much memory do they hold, how much do they process, and howphysically big are they. That's the first thing. —— Aviationweek.com 其次，是开发工具的易获得性。以上两点，我在“商业航天可以用COTS器件吗”一文中，做过分析，可以参阅。 The second thing is what tools they come with. We run the Linux operatingsystem, we program everything in C++, and that enables us to tap into a hugepool of very talented people and find the absolute best people in the computerand software industry to work with us. If you go into the radiation hardenedparts, they are very limited in terms of what languages you can work in, whatsupport packages there are for them, who knows how to program in them. Itreally limits your ability to work with the parts. And the other thing itreally does is they all take a little longer time to get and they're a littleharder to come by. —— Aviationweek.com 其三，是方便快速迭代。这是SpaceX真正反传统的地方，传统航天为了保证可靠性，对变化的控制非常严格。通常，传统航天会倾向于采用经过验证的，风险可控的产品的产品和技术。SpaceX用2年时间开发出3代控制计算机，而且每一代产品都是用市场主流的软硬件产品，在付出巨大研发努力的同时，也承担了很高的风险。 …… Wehave hundreds of flight computers of different capability levels, and we're inmultiple generations of design. The radiationparts tend not to have growth and upgrade paths. It's very hard to grow, if youdecide you want a little more capability, a little faster, you're reallylimited – it's that part.And we're already in our third generation of flight computer at SpaceX. In the last two years we've worked through threegenerations, we've got people working on a fourth generation computer. So weare constantly looking at what's available in the marketplace, moving with themarketplace so we can use the best software tools, the best people the besttechniques and achieve the most modern, optimized, efficient design. That's whywe don't want to go into these lines, and they are good pieces of equipment,lots of people use them. But they don't open up the kind of possibilities thatwe want to have. —— Aviationweek.com 4 我在“商业航天可以用COTS器件吗”一文中，给出了明确的结论：使用COTS器件不能节省成本。 那么，SpaceX采用X86处理器，总体而言也提高了成本吗？ 问题可能有点复杂。
按照NASA给出的成本模型，采用COTS器件，可以显著降低采购成本（上图红色部分），但会显著增加“风险缓解”成本（上图蓝色部分）和“产品更换”成本（上图绿色部分）。采用的元器件等级越低，增加的后两种成本越高。因此，在宇航项目中采用低等级元器件，综合成本会高于直接采用高等级元器件。 SpaceX也花费的巨大的“风险缓解”成本。从访谈中我们可以看到，无论是JPL专家给出的咨询意见（方案），还是University of Indiana做的辐照测试，成本都是非常高的。 Now that doesn't mean that eachpart can't take the dose that a “rad-hardened” part can, because we've taken all ofour designs and we've tested them extensively, we've had contracts with theNASA Jet Propulsion Lab (JPL) to consult us, and their the world's experts init, and we've gone to the Universityof Indiana and tested all of our parts, and we test them until they fail.We keep bringing the environment up and up and up until they fail. But we testthem as a total system, not each part at a time. We've tested lots of our partsto very, very high radiation environments. So we test them as a total system,and by that I mean a unit with three processors in it, we test the entire unit.We take the cover off and we hit it really, really hard with radiation, and wedo that so we understand how the parts react in the radiation environment. —— Aviationweek.com 5 作者简介：张伟，高可靠元器件从业者，美国ELLEGRO CORPORATION中国区负责人（原航天五院进口元器件采购处副处长，原德国TESAT-SPACECOM中国业务代表）。如果您想和作者讨论或反馈，可以扫描下面二维码添加微信，请注明“大国重器”。作者的其他力作请查看开头专辑。 |
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