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One of the things that drew attention from my local group of rocket-watchers with the Falcon 9/Crew Dragon launch is the perceived speed of the actual launch process. We have been conditioned for years to expect rocket launches to start engines, and then there be a readily perceptible gap between that and the rocket moving, and taking a certain amount of time to clear the pad, but Falcon almost literally appears to light its engines and be clear of the tower significantly faster than any of us ever expected.

Assuming that one of the major factors is vehicle weight, what other things in the design, technology and process of launching Falcon mean that its perceived time between "on the pad, condensing vapour" and "clear of the tower" is so much shorter than most of the other rocket examples we've seen? (For comparison, I've watched the Falcon Heavy launch, and while that is a little slower, it's not much much.)

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    $\begingroup$ You think that was fast, try watching a missile launch from the bridge of a guided missile cruiser. BAM!, Flash of light (could even perceive the heat!), then just smoke - I could never say for sure that I ever really saw the missile! $\endgroup$ – Glen Yates Jun 3 at 16:45
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    $\begingroup$ I seem to remember the New Horizons launch also removed my socks. $\endgroup$ – Adam Barnes Jun 4 at 2:52
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    $\begingroup$ You think that was fast, compare that to a Sprint missile which achieves first stage separation at T+1.2 seconds, hits Mach 10 at T+5 seconds, and achieves intercept at an altitude of 30km after at most T+15 seconds. $\endgroup$ – Tom W Jun 4 at 9:54
  • $\begingroup$ @AdamBarnes I saw New Horizons launch as well, the only non-shuttle launch I ever saw in person. Memorable. $\endgroup$ – Organic Marble Jun 4 at 19:01
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Aside from thrust/weight ratio, I suspect at least part of this conditioning came from a peculiarity with the Space Shuttle and its launch process.

The Space Shuttle's launch process was different because it had two different types of engines firing at launch. Its liquid-fueled main engines (mounted on the back of the orbiter itself) were ignited several seconds before t = 0. At this point, its thrust was less than its weight (and the whole stack was still bolted down to the pad!) so it did not move (well, not upward, at least.) At t = 0, its Solid Rocket Boosters(SRBs) were lit and the restraining bolt nuts were blown and then it started moving pretty much immediately, as its thrust was very quickly significantly larger than its weight once the SRBs started firing.

Falcon 9 has only one type of engine on its first stage and they all light at approximately the same time. As soon as they throttle up, the thrust very quickly becomes greater than the stack weight and the rocket starts moving.

Most rockets actually work more like the Falcon than the Space Shuttle, with all first stage engines being of the same type and ignited simultaneously. The Space Shuttle's mix of its liquid-fueled main engines and solid-fueled boosters in its first stage meant that the liquid-fueled engines would have to be ignited sooner than the SRBs in order to be throttled up and ready to balance out the pitching moment of the SRBs once they start firing.

By lighting them before the SRBs, it is also possible to shut down the liquid-fueled main engines and cancel the launch if something goes wrong during their ignition. Once the SRBs light, however, the rocket will go somewhere. If they light and the main engines are not functioning properly, the somewhere it goes will not be where you want it to go and you will not go to space today.

With Falcon 9, there are no SRBs to worry about. If one of the Merlin engines fails to light, they can just shut the others back down quickly and abort the launch. You will still probably not go to space today, but at least you still have a rocket (and payload) and can try again later.

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    $\begingroup$ I love your last two paragraphs and strongly believe they should be in some science textbook somewhere! $\endgroup$ – Pavel Jun 3 at 7:13
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    $\begingroup$ The »you will not go to space today« part is from Randall Munroe's Up Goer Five (he later made a whole book on that premise). $\endgroup$ – Joey Jun 3 at 8:05
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    $\begingroup$ I also think a Saturn V with its iconic launch sequence starts much slower due to the need to rev up the turbo pump. (However I am not sure how Merlin turbo pump design differs from that since both have a similar gas generator design. Maybe it's the pure mass effect which made S-V liftoff slower) $\endgroup$ – eckes Jun 3 at 10:07
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    $\begingroup$ The Ariane 5 also launches like the Space Shuttle: first ignite the liquid fueled Vulcain and do some last-second checks, and then ignite the solids. So for liquid-solid stacks, a different sequence of events makes sense than for liquid- or solid-only stacks. $\endgroup$ – Dohn Joe Jun 3 at 11:52
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    $\begingroup$ @prl Curious about that - what do you consider "most rockets"? Falcon, Long March, Soyuz, none of them use solid boosters (Long March 11 is solid fueled, but no boosters; and Long March 3b/etc. often use liquid boosters). Ariane is the only "big" one I see that does, though I'm not an expert... $\endgroup$ – Joe Jun 3 at 23:03
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What you are seeing is a consequence of Thrust to Weight ratio at t=0.

If the vehicle weighs say 6.5 million lbs (Saturn V example) and has a thrust of 7.9 million lbs, then it has about a thrust to weight of about 1.2, so that will be kind of slow but every moment after 0, the fuel is burned off mighty fast, and the stack gets lighter, but the thrust stays the same. Thus it accelerates. If T/W is less than 1, it isn't going anywhere in the rocket example.

In the linked article on T/W it shows airplanes with values much less than 1, but that is ok, since they fly by aerodynamic lift. (F-15/F-22 fighter jets have T/W > 1 which means they can fly straight up, without 'wings' needed, which they do at air shows and it is pretty epic. (Yes they need the wings, yes it is still aerodynamic flight, but they could accelerate upwards just by thrust alone)).

The burning of fuel affects the T/W so much so that most vehicles throttle down as they hit Max-Q the moment of maximum aerodynamic pressure and then either throttle back up, or stay at the lower level. This protects the vehicle from unnecessary stress as they hit the low dense atmosphere at high speeds. Once above that point, the air is thinning out enough not matter.

For a Falcon 9, mass at launch (depends on payload, but general numbers are fine) is about 1.2 million lbs with thrust of about 1.7 million lbs. Thus a T/W ratio of about 1.4. So it will appear to take off faster than a Saturn V.

The T/W article says a Space Shuttle has T/W of 1.5 at launch which is pretty good. I was looking for a better list since I know there is some booster that has a really high T/W and seems to leap off the pad.

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    $\begingroup$ Also, back in those old days, it took several seconds after ignition for the engines to reach full power, and rockets were physically restrained from leaving the pad until full thrust was achieved. For example, google for "Saturn V" and "hold down." $\endgroup$ – besmirched Jun 1 at 14:12
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    $\begingroup$ The 5 first stage F-1 engines of Saturn V were not ignited simultaneously but in sequence, center engine first and then pairs of outboard engines. $\endgroup$ – Uwe Jun 1 at 16:07
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    $\begingroup$ @WaterMolecule geoffc was a bit unclear there: when thrust remains constant but total mass gets less, the derivative of acceleration is positive, meaning the magnitude of acceleration increases ; which leads to velocity increasing as time^(exponent greater than one) $\endgroup$ – Carl Witthoft Jun 2 at 11:37
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    $\begingroup$ If you want to see a rocket that really leaps off the pad, check out the Sprint missile. $\endgroup$ – reirab Jun 2 at 16:00
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    $\begingroup$ @geoffc Oh, yes, totally different missions. Just a really extreme example of TWR on a rocket. It pulled up to around 100 g and accelerated to Mach 10 in about 5 seconds. Mission was shooting down ICMB re-entry vehicles in the last few seconds before they wiped out a city. $\endgroup$ – reirab Jun 2 at 17:00
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In addition to thrust to weight ratio, in simple terms Falcon's engines (Merlin) light much faster than other engines. The space shuttle had SRB that light relatively fast, but the main engines took some time to spool up and get started as well as get calibrated. The SSME would start lighting well before T=0 for two reasons, the first because of this start up time, and the second for safety because once the solid rocket boosters would start they cannot be shut down (unlike the SSME).

You may also notice that the SRBs on the Shuttle took a moment to start up too. That's because the igniter was at the top of the SRB and it would take some time for the flame to propagate through the SRB and then develop a high enough chamber pressure. This is in part because of the enormous size of the SRBs, many missile systems like the Hellfire use SRB to boost and it is nearly instantaneous. Those seem to almost vanish from their launch point. (Again these missile systems often in addition have T/W ratios of over 2 and some upwards of 10).

There are plenty of other rockets that come off the pad relatively quick, but the Falcon's Merlin engines were specifically designed to be able to start and stop quickly for their reentry and landing burns, therefore they do seem particularly remarkable in how fast they seem to liftoff. There is a final reason why it may appear so: the plume of the falcon isn't as dramatic as the shuttles or other large rockets so the time between startup and liftoff isn't as obvious. Check out some LOx LH2 first stage rockets like the Delta IV and it also seems much faster.

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