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I was reading over the recent Antares launch profile and was surprised to see that the main engine cutoff is at T+230s, but the second stage does not ignite until almost 100 seconds later. Why is this time so long?

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I would have assumed that 100 seconds of deceleration would cost a substantial amount of fuel to recover, making the system heavier, more expensive, lower payload, and all sorts of other bad things. I'm certain there's a good reason (no one burns thousands of extra kilos of propellant 'just because, right?)... but I can't figure out what that reason is.

All my (admittedly basic) physics leads me to believe that you'd want to do as much of the initial burn as you can early in the profile to minimize the amount of total energy you have to generate (or maximize the payload you can carry, which are really two sides of the same coin)...

Edit: thanks for the helpful answers - the key component I was missing is that by the time the first stage cuts off, the vehicle is high enough that there is virtually zero atmospheric drag left, and as such the coast phase costs very little in terms of lost energy (velocity). I found the Antares User Guide extremely instructive, specifically figure 3.2-1, which shows that the velocity reduction between MECO and the second stage ignition to be only ~160m/s. The gravity turn is also a concept that I hadn't heard of, but makes perfect sense now that I read about it. ;-)

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    $\begingroup$ Isn't the second stage solid fuel? That can't be shut off before total burnout so maybe they use the delay and gravity losses instead of engine control to target the right orbit? The lighter payload the later you start the second stage and Cygnus was probably fairly light compared to the max. $\endgroup$ – jkavalik Oct 18 '16 at 4:58
  • $\begingroup$ The Antares A-ONE reached an altitude of 107 km in less than 4 minutes at MECO. That places the Antares in the class of high thrust launch vehicles. High thrust vehicles often use a coasting phase between first stage shutoff and second stage ignition. The A-ONE ascended to 189 km while coasting. I made this a comment rather than an answer because my next action will be to vote to close this question as a duplicate. $\endgroup$ – David Hammen Oct 18 '16 at 11:03
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    $\begingroup$ Note that there are multiple answers to that question. I answered the question raised in the body of the question. Daniel's answer addresses the question raised in the title of the question. That same answer addresses this question. $\endgroup$ – David Hammen Oct 18 '16 at 11:06
  • $\begingroup$ @DavidHammen another option is to point to those two answers also, but take this opportunity to combine them and give a short simple answer - get out of most of the atmosphere ASAP and then do your gravity turn. Of course I'm just paraphrasing, but sometimes a three sentence answer that hits the nail on the head is better than having to read through two answer with a half-dozen paragraphs combined. Stackexchange is not a hardcover tome (exclusively at least). Sometimes it's about how helpful the answer actually is to non-rocket-scientists (like me!) $\endgroup$ – uhoh Oct 18 '16 at 15:30

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