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It seemed to me that today's (Dec. 6, 2015) launch of ULA's Atlas V carrying Orbital ATK's Cygnus spacecraft in orbit and towards the International Space Station on the OA-4 mission had a particularly slow liftoff and the previous Atlas V launch of GPS IIF-11 on Oct. 31, 2015 was faster off the pad. I thought at first that it might have been some frame lag of the live stream during webcast, but the launch vehicle appeared equally slow off the pad in later repeats of the liftoff from various angles;

I checked with ULA's mission brochures (for OA-4 mission and for GPS IIF-11 mission, both PDF), and event timeline shows that today's launch was slower to accelerate to Mach 1 and Maximum Dynamic Pressure (Max Q), but that the Common Core Booster of today's OA-4 launch also cutoff nearly 12 seconds later than during the GPS IIF-11 launch, despite having same mission time ignition and liftoff:

enter image description here

So it's not simple maths of heavier payload on otherwise equivalent flight profile and same launch vehicle (this is as expected really), and the OA-4's Atlas V did indeed launch with a smaller thrust-to-weight ratio.

My question is simply, what this T/W was? Additional kudos for pointing me to an online resource where such information can be found for all ULA launches, so I can refer to it in the future.


If it helps visualize what I'm asking about, here are the videos of the two launches:

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  • $\begingroup$ The 2nd stage burn seemed endless too. 0.6 g! $\endgroup$ – Organic Marble Dec 7 '15 at 12:50
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Per Spaceflight101, the Atlas 401 has a mass at launch of 334.5 tons (this likely varies with fuel load) and thrust is 3827kN. Cygnus is "just over 8 tons"; GPS IIF is 1.63 tons.

So for the GPS IIF launch, we have $\frac{3827}{334.5+1.63} = 11.38 m/s^2$ acceleration, less Earth's gravity of $9.81 m/s^2$, gives acceleration off the pad of $1.57 m/s^2$.

For Cygnus, $\frac{3827}{334.5+8.1} = 11.17 m/s^2$, less g, for $1.36 m/s^2$.

The length of the Atlas itself is 55m. Time to travel under constant acceleration is $\sqrt{ \frac{2 d}{a} }$ gives us 9 seconds for the Cygnus launch to travel one rocket-length; 8.4 seconds for the GPS launch.

However, in the videos you link, it looks to me more like 6 and 5 seconds to clear the tower, which is almost the entire length of the rocket, so something's amiss here.

Some rocket engines run above 100% of their so-called rated thrust for portions of their ascent, but I don't know if the Atlas's RD-180 is one of them.

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