What happened

For six straight trading days, Boeing (BA -1.55%) stock has moved in only one direction: down. And I fear Thursday is going to be no different.

As trading begins to wind down for the day Thursday, shares of aerospace giant Boeing are down in the dumps once again -- off 6.6% as of 2:55 p.m. ET. Two explanations for this fact suggest themselves.

Artist's depiction of Boeing CST-100 Starliner docking with the International Space Station.

Here's an artist's depiction of Boeing CST-100 Starliner docking with the International Space Station -- but when will it happen in real life? Image source: Boeing.

So what

The first issue concerns Boeing's flagship commercial aviation business. CNBC is reporting today that the rising cost of plane tickets is depressing demand for air travel, and that flight bookings dropped 17% in April as a result. Further up the supply chain, decreased demand for air travel has the potential to depress demand for new airplanes, which would obviously affect Boeing's sales.  

That being said, rising ticket prices are more likely tied to rising oil prices than to anything Boeing has control over -- so this is not really a problem that the company can be blamed for, or that it can fix. In time, travelers will adjust to the new reality that prices are higher, and I'd expect prices, and demand, to stabilize.

Now what

Of greater interest is the negative press Boeing is catching this week over a Reuters report on its "clash" with space stock supreme Aerojet Rocketdyne (AJRD), which manufactures the propulsion system for Boeing's CST-100 Starliner space capsule.

Last year, stuck valves on the Starliner's steering jets forced Boeing to scrub a planned launch that would be a key milestone toward getting Starliner certified to carry astronauts to orbit. Boeing blames Aerojet for the mishap, citing design flaws. Aerojet says Boeing got the valves stuck itself, by using the wrong kind of cleaning chemicals -- and Aerojet is refusing to redesign the system unless Boeing antes up more money for the work.  

You might think this is just a run-of-the-mill contract dispute, and one that will eventually be resolved by Boeing paying Aerojet some money, or suing Aerojet to fix the problem for free -- but the stakes here are actually much higher, and much more time sensitive.

Boeing, you see, has been trying to get its CST-100 certified for human spaceflight for more than two years now, and so far unsuccessfully. If a planned rerun of its test flight on May 19 is delayed again -- or fails! -- because of a sticky valve, this would be a real disaster for Boeing's space business, potentially forcing it to admit only SpaceX has the ability to put Americans in space for years to come.

It's not too much of an overstatement to say that the fate of Boeing's space program now hinges upon a single sticky valve -- and unless it gets this problem solved in the next seven days, its stock could have even further to fall.