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I was watching the webcast for Falcon 9 Flight 15 (launching DSCOVR) when they scrubbed their first launch attempt due to some issues during the terminal countdown. Before the scrub, the narrator noted that if the terminal count had to be held for any reason, the launch would be postponed to a backup date because the launch window was instantaneous - which is what ended up happening.

I'm curious why the launch window was instantaneous in a mission like this (to Sun-Earth L1). I'm familiar with how launch windows for ISS resupply missions are pretty slim, due to the timing of having to catch up to the station. What factors would cause such a small launch window for a mission of this type?

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    $\begingroup$ I was also curious as to why the launch window is so short for this particular mission. I read elsewhere (on reddit/r/spacex I think) that DSCOVR's mass is only 17% of the F9's total capacity to SE-L1, so you'd think they'd have lots of extra fuel to correct for inaccuracies during launch. One thing I do know is that if there's a hold, they can only recycle the count to T-10 mins, so any launch window that's less than 10 minutes is effectively an instantaneous window (and that's not counting the time it takes for controllers to recycle the count to T-10, which is probably another few minutes). $\endgroup$
    – Nickolai
    Commented Feb 9, 2015 at 18:48
  • $\begingroup$ Interesting. I didn't know that about the count recycle. $\endgroup$
    – Nate Barbettini
    Commented Feb 9, 2015 at 19:14
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    $\begingroup$ This is perhaps partly a SpaceX cultural issue that attempts to maximize profits by minimizing margins; they do much the same with their CRS launches (instantaneous launch window). This concept works great if all goes well. If things don't go exactly as planned? That means scrubbed launches, failed landings on a barge. I suspect that after a few more of these, SpaceX will revisit their tight margins concept. Overly tight resource margins harm rather than maximize profit margins. $\endgroup$
    – David Hammen
    Commented Feb 10, 2015 at 19:13
  • $\begingroup$ I've just asked Why is DSCOVR in a Lissajous orbit instead of a halo orbit (to stay out of Sun exclusion zone)? $\endgroup$
    – uhoh
    Commented Oct 22, 2018 at 14:35

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Found an answer on the SpaceX subreddit from user cwhitt:

Basically one specific moment in Earth's 24-hour rotation is best for the trajectory to get from Florida to the DSCOVR destination (beyond earth orbit) [...] In this case it's slightly different than with Dragon/CRS launches, but the concept is the same: alignment of launch point and destination wrt to Earth's rotational position.

And further explanation from atrain728:

[...] in DSCOVR's case, every second that is in delay or advance of the launch moment is raw delta-v that will have to be made up in flight. There's no amount of thrust regulation that can solve that problem, it will simply take more fuel. And that's why the window is so short.

So the short launch window is a matter of efficiency with regards to the trajectory to SEL1 and minimizing any need to change plane/inclination.

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    $\begingroup$ So is Nickolai's comment above about DSCOVR only being 17% of F9's capacity to SE-L1 incorrect? $\endgroup$
    – Russell Borogove
    Commented Feb 10, 2015 at 19:39
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The issue with launch window wasn't whether the launch vehicle could get the object into space or even to L1, it's whether the last stage can still insert the object into a semi-stable orbit about L1. The c3 of the rocket was way more than enough. If you launch at the wrong time of day, you need to use more fuel but it also means you approach L1 at a "bad" speed and angle, which can cost you lots of fuel for insertion. That cost can directly affect mission lifetimes. It was all about whether the bus rockets/thrusters could still insert and have enough fuel for station keeping maneuvers.

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While I am no expert, one of the reasons is the limited restart capability of the second stage. Once the second lights, it is staying lit. This means it can't fine tune the transfer orbit nor correct any errors with manoeuvres that require another burn.

Basically it is a direct accent to the transfer orbit.

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    $\begingroup$ I do not think that is correct. The Falcon 9 second stage is restartable and has been restarted many times in orbit now. (Basically on every launch to GEO or HEO). $\endgroup$
    – geoffc
    Commented Feb 10, 2015 at 0:09
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    $\begingroup$ Correct, hence why I used limited. The 2nd burn is only ~40 min after SECO which may be the limit of the batteries. $\endgroup$
    – tl8
    Commented Feb 10, 2015 at 2:42
  • $\begingroup$ @geoffc: Ah, but can it restart more than once? I've seen almost every launch and haven't once seen more than 2 starts of the second stage. $\endgroup$
    – Joshua
    Commented Jan 18, 2018 at 17:36

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