Hacker News Reader

Comment by elliekelly | Hacker News Reader

Can someone ELI5 why they sometimes need to have an “instantaneous launch window” that can’t be pushed even by a few minutes?

View on HN

To visualize one of the constraints (the orbit inclination) -- imagine holding up a hoop around a globe, with the hoop angled at 51.6 degrees. Now start spinning the globe until Florida intersects with the hoop. That is when it has to launch, to be going in the same direction as that hoop.

Spin the globe a bit further, and now you can see that Florida no longer intersects with that hoop. If you were to try to launch at that point, your trajectory would form another ring that intersects with the ISS's orbit, but at a different angle. So it would take quite a bit more propellant to change direction once in space.

Now, you also have to launch so that you intercept the ISS when it is at the proper position on that hoop. If, when you reach the hoop the ISS is half way around the world, you won't get to it. (Technically, you can spend some time at a lower orbit, then raise your orbit at the right time, or adjust the amount of time it takes to get to the right height, but again that messes with the timing of everything and fuel usage).

> Now, you also have to launch so that you intercept the ISS when it is at the proper position on that hoop. If, when you reach the hoop the ISS is half way around the world, you won't get to it. (Technically, you can spend some time at a lower orbit, then raise your orbit at the right time, or adjust the amount of time it takes to get to the right height, but again that messes with the timing of everything and fuel usage).

This is normal, actually. It's part of why it normally takes several days from launch to ISS rendezvous. (Another part is that the trajectory is very carefully calculated to not leave the spacecraft on an intercept course with the ISS if anything breaks) And it takes essentially no fuel - 100m/s of closing rate will let you catch up from halfway around the world in 2-3 days.

This makes sense. If you aren't in a huge rush to reach your destination and can spend 2-3 days to let your orbit match the ISS. If launch windows were weeks or months apart, a couple days of catch-up orbiting might be okay.

Though, if you can spend that same amount of time sitting on the ground and get there at roughly the same time, you'd waste less resources sustaining the crew in orbit, as well as reducing the "Ugh, are we there yet??" factor by a decent amount.

Here is a really good explanation of rendezvous using a lower and faster phasing orbit and how faster rendezvous works: https://spaceflight101.com/progress-ms-07/russia-to-introduc...

It depends on the target orbit. For launches to the ISS, you have to launch at the correct instant because even a few minutes later means the ISS is so far ahead it will take much longer, and use more propellant, to rendezvous.

I think they could probably delay a few seconds without too much trouble, but I guess the extra complication of recalculating the rendezvous isn't worth it because most of the time a few seconds won't matter anyway.

John Insprucker just talked about this on the webcast and pointed out that with the chilled propellants used by the F9, it's also not possible to wait after prop load has started because the propellants will warm up and change engine performance.

Can you delay the launch window 90 minutes until the next ISS orbit? Maybe you then run into the next on the long list of reasons.

No, because in 90 minutes the cryogenic liquid oxygen would have warmed up too much. The Falcon doesn't have an onboard refrigeration unit; it relies on loading the propellant shortly before launch.

However, "shortly" is also relative, and 90 minutes is not long enough to purge all the fuel from the rocket and refuel it with cold oxygen (it took close to an hour today to de-fuel the rocket), so it can't be cycled in that window.

The orbit moves because the earth is moving, so it will not be in the same trajectory as planned. This requires a change in fuel requirements as well as programming

In 90 minutes the space station is back at the same point in its orbit, but Kennedy Space Center has rotated ~1500 miles east.

Long list but one of the big ones:

ISS travels at 7.6 km/s. Thats a lot of kms in those few minutes

At that speed, you could run a marathon in 5.55 seconds.

Around 17,000mph.

A miss is much bigger than a mile.

Physical constraints resulting in sub-optimal orbits increasing chance of mission failure or FAA rules limiting options for launch.

Reply on news.ycombinator.com