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"  There&amp;#039;s an old nursery rhyme that starts &amp;quot;For want of a nail, the shoe was lost&amp;quot; and culminates in the loss of a kingdom. Fortunately, the outcome wasn&amp;#039;t that bad for SpaceX early Saturday morning, but the failure of a part that probably cost just a few dollars has delayed the long-awaited inaugural flight of the Dragon capsule to the International Space Station until at least early Tuesday morning.  SpaceX was all set to go during the Saturday morning launch window when the flight aborted automatically at the last second because of a high pressure reading in the center engine of the nine-engine cluster that constitutes the first stage of the Falcon 9 launcher. The same thing had happened on the previous Dragon flight a year and a half ago, and at first, SpaceX head Elon Musk assumed that Saturday&amp;#039;s abort had a similar cause\u2014an overly stringent propulsion criterion on a new vehicle with which the company is still familiarizing itself. In fact, within 15 minutes of the abort, he tweeted, &amp;quot;Launch aborted: slightly high combustion chamber pressure on engine 5. Will adjust limits for countdown in a few days.&amp;quot; But it turned out to be different this time. During the 2010 launch abort, the pressure had just slightly exceeded the limit and then held steady. This time, it was increasing and continuing to increase until SpaceX shut down the engine. After safing the vehicle, SpaceX technicians went out to the launch pad on Saturday to visually inspect the engine and its plumbing with a bore scope and determined that there was, in fact, a hardware problem: a check valve that had stuck open. So what is a check valve? Check valves are to plumbing what diodes are to electronic circuits. They allow fluid to flow in only one direction, and they do so automatically by their spring-loaded design, instead of having to be manually or digitally controlled. You&amp;#039;ve seen a check valve around the home if you use lawn sprinklers, which have these valves to prevent siphoning and backflow of sprinkler water into the house water supply. Check valves can sometimes be unreliable, and are particularly susceptible to contamination or debris if something gets trapped and prevents full closure. Nevertheless, they are critical for many functions of a rocket engine. The check valve that failed SpaceX on Saturday was in the turbopump; it allowed nitrogen gas to flow into the system to purge it prior to ignition, to ensure that there was no mixture of propellants prior to engine start. According to SpaceX, the valve&amp;#039;s failure allowed liquid oxygen to flow where it shouldn&amp;#039;t\u2014from the main injector of the engine into the injector of the gas generator that powers the turbopump, which provides the pressure needed to force propellants into the combustion chamber. Extra oxygen resulted in higher temperatures and pressure.  Fortunately, the Falcon design allows a lot of maintenance to occur on the launch pad, so SpaceX removed and replaced the valve on Saturday in preparation for the next launch window Tuesday morning at 3:44 am Eastern. (SpaceX not only needs to line up with the ISS&amp;#039;s plane of orbit, which happens daily, but also for the ISS to be in the correct part of its orbit. That&amp;#039;s why the launch windows are several days apart.) And over the weekend, SpaceX says, its simulations show that the flight would have gone just fine even if it hadn&amp;#039;t aborted. As Musk tweeted on Sunday, though: &amp;quot;Still, better to stop &amp;amp;amp; fix. Recalling rockets after launch is not an option.&amp;quot; Yet even as the incident validated the safety of the vehicle, it raises issues about its launch reliability. Every rocket design is a compromise of cost, safety, and reliability. In the case of the Falcon, it has nine Merlin engines in the first stage because this allows it to use the same engine in both that stage and the upper stage, which only has one such engine. This saved a great deal in development costs and allows steady production 24/7 in the company&amp;#039;s factory in Hawthorne, Calif.  While this design is a cost-saver, though, it also increases the chances of an error like this scrubbing a launch. We don&amp;#039;t know what the odds of failure for a single Merlin engine actually is, but for the sake of argument let&amp;#039;s say that it&amp;#039;s 1 in 1000. Using nine of those engines, then, would raise the odds of something happening to any one of them to more like 1 in 100. And that&amp;#039;s just for Falcon 9. SpaceX&amp;#039;s Falcon Heavy would have 27 first-stage engines (made of three Falcon 9 cores), meaning there are even more components that could go wrong. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/rockets/a7735/what-went-wrong-with-spacexs-launch-9035871/?click=pm_latest>