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I've seen a few Space Shuttle and rocket launches in-person, and it always seemed to me that the Shuttle would launch a bit slower than rockets. Today, I saw the CRS-6 launch of the Falcon 9 and its ascent seemed similarly sluggish. It wasn't until the press conference that I realized CRS-6 and Shuttle had a thing in common - live creatures on board.

I've always assumed, but never verified, that the reason for the Shuttle's relative sluggishness was to keep G forces within human tolerance. Sure, some Shuttles would launch slower than others and I'd later realize they had a particularly hefty payload on board. But in general, nearly every unmanned rocket seemed like a Ferrari next to a golf cart in comparison.

During the press conference, the SpaceX representative was asked about whether CRS-6 appeared unusually slow. The representative couldn't attest to the launch's appearance, since he was watching status screens rather than the rocket itself. But he did say that everything appeared nominal, and the payload wasn't especially large - so there's presumably no reason for this launch to have been slower than any other of its kind.

Are G force tolerances really taken into consideration when launches are planned with are living things on-board a spacecraft? Or is it just a simple matter of certain rockets having different performance capabilities, and different payloads affecting them? Or, are there other factors simply affecting the perception such as launch trajectories being more/less toward/away from the viewer?

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  • $\begingroup$ The SpaceX CRS launches all look pretty much the same to me. What appears to change from flight to flight is the photography. $\endgroup$
    – David Hammen
    Apr 15, 2015 at 4:55
  • $\begingroup$ G forces generally aren't a factor at liftoff. They can become a factor later in the launch as mass depletes. $\endgroup$
    – David Hammen
    Apr 15, 2015 at 4:56
  • $\begingroup$ Satellites and humans have launch G limits. Warheads, OTOH, are much more sturdy. A Trident D-5 experiences 10G at launch. $\endgroup$
    – Deer Hunter
    Apr 15, 2015 at 5:42

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Launchers generally start at full throttle, so for the most part their immediate performance off the pad is determined solely by the type of launcher and mass of payload. Falcon 9's initial acceleration is quite modest, about 1.5 m/s2 vertically (thrust to weight ratio about 1.15, cancelling and exceeding 9.8m/s2 of surface gravity). Shuttle's initial TWR was higher, IIRC about 1.5:1 for initial acceleration on the order of 5 m/s2.

I don't have great sources for this information; I generally look at the Wikipedia specs, do the math, and keep a grain of salt handy. First stage sea-level thrust in kN divided by total mass in metric tonnes = free acceleration in m/s2. Either divide by Earth's surface gravity (g, 9.81 m/s2 -- or just call it 10 for back-of-the-envelope estimates) to get TWR, or subtract g to get acceleration-off-the-pad.

I did note when watching Japan's Hayabusa 2 launch on an H-IIA rocket that the takeoff acceleration was quite dramatic, and according to Wikipedia's numbers the launch TWR would be about 1.6:1, for 5.6 m/s2 off the pad. An Atlas V 401 with 9 ton payload has TWR ~1.16 (1.6 m/s2) at takeoff.

That initial acceleration, though, is always much lower than the peak acceleration of the launcher, because thrust remains more or less constant while fuel mass is thrown away. As noted in my answer to a related question, some launchers shut down one or more engines in a cluster while others throttle back in order to cap acceleration at a level that's comfortable for the payload. Saturn V goes from 5 engines to 4; Falcon 9 1.0 goes from 9 engines to 7 while Falcon 9 1.1 uses its deeper engine throttle capability; Shuttle drops the solid boosters and throttles back the SSMEs.

In the typical case, peak acceleration is reached at the instant before the first stage engines are cut off, when first stage propellants are depleted. So you need to know the thrust at that point (which is non-obvious, if the engines are throttleable or clustered), divided by the total launch mass of the rocket less the first stage fuel & oxidizer. That information can often be found on Spaceflight101 - for Atlas V this would imply somewhere between 4.2g and 8.4g depending on throttle, most likely on the low end of that.

Note that it can be difficult to judge the acceleration of a rocket visually; unless you're very consciously aware of their overall size, larger vehicles are going to seem slower-moving than smaller ones when seen from a distance.

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  • $\begingroup$ The Shuttle's SSMEs actually throttled back (and throttled back up) while the SRBs were attached to relieve max dynamic pressure. $\endgroup$
    – Erik
    Apr 15, 2015 at 1:09
  • $\begingroup$ They did both, actually. Brief throttle down through max-Q, back up full, then progressively back down to keep acceleration limited to 3g. en.wikipedia.org/wiki/… $\endgroup$
    – Russell Borogove
    Apr 15, 2015 at 3:44
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    $\begingroup$ @RussellBorogove Falcon 9v1.0 went from 9 to 7, M1Ds have better throttle capability that they can keep all 9 running. $\endgroup$
    – marked-down
    Apr 15, 2015 at 4:26
  • $\begingroup$ Interesting, I hadn't seen that! Do you have a reference? $\endgroup$
    – Russell Borogove
    Apr 15, 2015 at 4:39
  • $\begingroup$ I like that you've actually called out the Falcon 9's initial acceleration value. Could you perhaps add in others, to compare Falcon 9 and Shuttle to other rockets that aren't generally intended to launch living cargo? Also, the maximum acceleration experienced during launch for each rocket would probably be a more important statistic to consider here. (Of course, that would still vary by payload weight and some other factors. But I'm sure there's certain averages or upper limits that would be useful to compare.) $\endgroup$
    – Iszi
    Apr 15, 2015 at 14:52
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They aren't launched slower but they are launched in different trajectories. This can be to control abort conditions and G-Loads.

The most efficient launch is to get as high as possible before going for speed. The STS however was shallower to allow for the abort modes and control the forces.

The best example I can think of at the moment is the man rating of the Atlas V for commercial crew. The shallower trajectory is one of the things they need to deal with in order to get it approved.

It is also important to point out that rocket launches can be quite deceptive. Rockets are massive and can look quite slow when far away. Even a change in cameras can change the perceived speed.

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    $\begingroup$ "The most efficient launch is to get as high as possible before going for speed." That is obviously not true for extremely thrust to weight ratios very close to 1 because most fuel would be burned just to keep the rocket in the air. In a vacuum the other extreme (accelerating as quickly as possible) should be most efficient, and height gained almost irrelevant. But on earth I assume that atmospheric drag would make this inefficient because then most fuel would be burned to push air to the side. What is the roughly optimal flight path on earth? $\endgroup$
    – Peter - Reinstate Monica
    Apr 15, 2015 at 11:34
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    $\begingroup$ That's a separate question really (but a good one). It would depend on how aerodynamic the vehicle is, how much dynamic pressure it can withstand, whether a low trajectory is preferred for safe abort modes (if manned) and probably a lot of other stuff. By the way launch in vacuum has occurred, on the Apollo ascents from the moon, and they used a relatively early turn as you would expect. $\endgroup$
    – Andy
    Apr 15, 2015 at 12:13
  • $\begingroup$ @PeterSchneider: I had heard that in addition to pointing the antenna at the ground station, the Space Shuttle orbiters rotated dorsal-side-down in order to take advantage of aerodynamic lift properties of the vehicle, and also to have the Earth's gravity pull the astronauts away from the seats a bit, rather than tighter into the seats. Can you confirm or deny either of these claims? $\endgroup$
    – dotancohen
    Apr 15, 2015 at 14:15
  • $\begingroup$ @dotancohen I'm really a bloody layman with some general interest in physics and happend to stumble upon this, so I am not the right person to ask :-). $\endgroup$
    – Peter - Reinstate Monica
    Apr 15, 2015 at 14:29
  • $\begingroup$ This wikipedia article has an interesting detail: en.wikipedia.org/wiki/Gravity_turn. "... gravity drag [...] can be minimized by executing the [...] pitchover maneuver as soon as possible". Makes sense. $\endgroup$
    – Peter - Reinstate Monica
    Apr 15, 2015 at 14:39

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