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Spaceflight Now's ULA’s Atlas 5 is NASA’s go-to rocket for nuclear-powered space probes quotes Tory Bruno, president and CEO of United Launch Alliance:

“This rocket is going to leap off the pad with this relatively tiny payload, so do not blink when they say ignition,” said Tory Bruno, president and CEO of United Launch Alliance, the 50-50 joint venture between Boeing and Lockheed Martin that builds Atlas 5 rockets.

Bruno said that the Atlas 5 rocket, set for its 85th flight since debuting in 2002, is healthy and ready for the start of Thursday morning’s countdown.

“Atlas is go, Centaur is go, and we are literally chomping at the bit to take this nuclear-powered dune buggy out to Mars,” Bruno said.

I'm thinking that no matter if it is large or small, the payload is still a very small fraction of a fully fueled rocket's mass, boosters or not, and so initial lift-off acceleration wouldn't necessarily be indicative of the payload mass, but then I realized that since the payload's destination is Mars and the mission requires boosters at lift-off, it may be that the destination is more indicative of lift-off acceleration than the payload mass.

Question: Did the Perseverance rover's Atlas 5 really "leap off the pad"? If so, was it because the payload was tiny, or because it's going to Mars? Which has more impact on lift-off acceleration rate for a given launch vehicle?

Tory Bruno, president and CEO of United Launch Alliance, poses with the Atlas 5 rocket that will launch NASA’s Mars 2020 Perseverance rover. Credit: NASA/Ben Smegelsky

Tory Bruno, president and CEO of United Launch Alliance, poses with the Atlas 5 rocket that will launch NASA’s Mars 2020 Perseverance rover. Credit: NASA/Ben Smegelsky

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    $\begingroup$ The real question is, where can I get a hat like that? $\endgroup$
    – Organic Marble
    Commented Jul 30, 2020 at 1:13
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    $\begingroup$ here? @OrganicMarble $\endgroup$
    – user20636
    Commented Jul 30, 2020 at 19:53
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    $\begingroup$ Ah I thought you were talking about the hard-hat (actually, this one looks like a Vulcan) - but I guess you're actually talking about the logo. $\endgroup$
    – user20636
    Commented Jul 30, 2020 at 20:03
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    $\begingroup$ The commentary during the launch noted that the tower was cleared in about 5 seconds, while during previous launches this was about 17 seconds. Does that qualify as "leaping"? $\endgroup$
    – Ludo
    Commented Jul 31, 2020 at 9:38
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    $\begingroup$ @Lubo 17 seconds sounds unrealistically slow. Is that a typo? A quick youtube comparison with another Atlas V launch would suggest that 7 s would be right in the ballpark... $\endgroup$
    – user2705196
    Commented Jul 31, 2020 at 14:11

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It's the tiny payload that matters--the rocket is lighter than normal so the same thrust causes it to move faster. This is because it's heading to Mars instead of to orbit, they are trading payload capacity for the speed needed to get there.

It's not going to make a big effect in the liftoff speed, though, the weight of the payload is quite small compared to the weight of the rest of the rocket.

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    $\begingroup$ I'm thinking that it's the destination that matters most for the initial take-off acceleration. Since a large delta-v is needed for an interplanetary transfer, they added more SRBs, and that results in a larger thrust to weight ratio excess (how much it is greater than 1:1) and therefore initial acceleration rate. $\endgroup$
    – uhoh
    Commented Aug 1, 2020 at 2:50
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    $\begingroup$ @uhoh Good point, it had more SRBs than a typical Atlas. That's just the configuration of the rocket rather than where it's going, though. $\endgroup$
    – Loren Pechtel
    Commented Aug 1, 2020 at 2:53
  • $\begingroup$ I'm thinking that rocket's configuration is dictated by where its going; the proximal cause may be the number of boosters but the root cause is the destination. $\endgroup$
    – uhoh
    Commented Aug 1, 2020 at 2:56
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    $\begingroup$ @uhoh It's dictated by how much power is needed. A lighter Mars probe wouldn't have needed the strap-ons, a heavy enough satellite to LEO would have. $\endgroup$
    – Loren Pechtel
    Commented Aug 1, 2020 at 2:56

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