Another Small Step: Demo-2

I started watching NASA’s streaming video coverage of the Demo-2 launch Wednesday noon.

I’d have kept watching for the next three hours or so. But Wednesday afternoon is when I spend an hour at Eucharistic adoration.

Interested as I am in space exploration and all that, priorities are priorities. I got back a bit after 3:00 p.m. — and learned that weather conditions were iffy.

Then, shortly after I got that picture from NASA’s launch coverage, the launch was scrubbed. Electrical field strength in the atmosphere had gone above safe limits. And stayed there.

Nasa SpaceX launch: Big day called off because of weather
Jonathan Amos, BBC News (May 27, 2020)

Poor weather has forced SpaceX to call off the launch of Nasa astronauts Doug Hurley and Bob Behnken to the International Space Station (ISS).

“The two men were due to go up from the Kennedy Space Center in what would have been the first orbital mission from the US in nine years….”

Instantaneous Launch Window


(From NASA TV, used w/o permission.)
(“The SpaceX Falcon 9 and Crew Dragon spacecraft stand on Launch Complex 39A on May 27, 2020.” (NASA))

The good news was that the tech had been working fine. And everyone involved knew what they were doing, and did it. NASA rescheduled the Demo-2 launch for Saturday afternoon.

I gather that they’d have launched a few minutes later Wednesday afternoon, if the Demo-2 mission wasn’t headed for the International Space Station.

Since the SpaceX Crew Dragon has to dock at the ISS, Demo-2 has an instantaneous launch window. Orbital mechanics and today’s technical limits leave mission planners with a single exact time for the launch.1

Otherwise, the ISS would be in one spot and the Crew Dragon would be in another, without the option to match position and speed.

It’s as if an airline’s flight from, say, Chicago to New York had to take off exactly on time; or it would end up over Mount Pocono with just enough fuel left to try landing on Highway 940, between the Pocono Bait Shop and Burger King.

Except there’s no Burger King analog in low Earth orbit. Not yet.

Introducing APAS and IDA


(From CSM11 Docking Subsystem, NASA, used w/o permission.)
(The Apollo docking system: practical tech when we had only two spacecraft at a time.)

The Apollo docking system was an effective technology for American moon missions.

It didn’t matter that astronauts couldn’t dock one Command and Service Module with another, or link two landers. We’d never have more than two spacecraft flying at the same time, and they’d be a CSM-lander duo.

That was fine for the Apollo program.

Happily, Soviet and American engineers were cooperating — I am not making this up — in the development of a universal docking system.

That’s how we got the Androgynous Peripheral Attach System: also called the Androgynous Peripheral Assembly System and Androgynous Peripheral Docking System. Or, if you prefer acronyms, the APAS, APAS, or APDS.

The APAS-75, APAS-89 and APAS-95 eventually led to the International Docking Adapter, or IDA. It’s what connects ISS modules.

IDA conforms to the International Docking System Standard,2 which is another topic for another day.

Similar, but not identical, posts:


1 Exact timing, Demo-2 and NASA TV:

2 Docking systems:

About Brian H. Gill

I'm a sixty-something married guy with six kids, four surviving, in a small central Minnesota town. I mostly write and make digital art. I'm only interested in three things: that which exists within the universe; that which exists beyond; and that which might exist.
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