You see video of rockets with munitions in them like surface -to-air and plane mounted rockets and they seem to move so quick. Why do these massive rockets seem to move slow? Is it payload? Do they speed up as they reach the stratosphere?
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It’s due to scale, and a little bit due to acceleration. For a small missile, it can be designed to accelerate at 10G+ because there’s nothing too sensitive in there. Orbital rockets typically shoot for around 3g because that’s what a human can take. For a small missile, it’s a few feet long. It looks really fast because of its size. The large orbital rockets are the size of large buildings. So even though it might be moving at 200kph by the time it clears the launch tower, it doesn’t look fast because it’s the size of a 15-story tall building; and that’s the slowest it’s accelerating because that’s when it has the most fuel and oxidizer on board.
Because they are so much bigger. This effects your perception of it in 2 ways.
First, if you see a big thing and a small thing moving at the same speed, the small thing looks like its moving faster. A 5m long rocket moving at 50 m/s will cover its own length 10 times every second. If the rocket is 50m long it only covers its length once every second which looks slower, even though they are going the same speed.
Second, large rockets do generally accelerate slower than small rockets. A surface to air missile doesn’t have to get to orbit, it just has to get up to speed quickly to reach its target. Therefore it burns it fuel really fast to accelerate as fast as possible then coasts at that top speed to its target. A big orbital rocket has to be efficient in order to reach its goal of orbit at all, and that generally means lower acceleration. Also, big orbital rockets are usually designed for people to eventually ride them so there are limits if they don’t want to kill their passengers.
One part of it is something called parallax: We judge the speed of something that moves across our field of vision by how much of an angle it takes to rotate if we were trying to keep our eyes pointed directly at it. When something is far away, it takes a lot of distance to go through a small angle, but when something’s closer, it doesn’t take as much (compare a plane high in the sky to a fly in front of your face). The bigger the rocket, the further away the cameras are (for safety), which means the rocket is going to look like it’s moving slower.
Another part of this is that when you compare a space rocket launch to munitions, it’s very likely that the munitions are accelerating faster than the rocket in the very beginning. At launch the rocket is heaviest, and the acceleration from the thrusters might only be a couple times bigger than gravity. The rocket eventually reaches a higher speed because it accelerates for longer.
You also don’t usually want the rocket to accelerate too fast because whatever you’re launching to space is usually too delicate to withstand extreme accelerations (even if it’s not humans, satellites and space probes are generally built to be light, which means they have to compromise on robustness).
When they lift off the ground, yes, they’re quite slow. A huge percentage of the weight of the rocket is fuel, so when it first lifts off the pad, the thrust produced is only a little higher than the weight of the rocket. As they get lighter, the thrust to weight ratio gets higher, and the further up in the atmosphere they are, the more thrust the engines make.
The other part of this is perception of speed. The speed is a lot less obvious as they get further away though due to perspective and relativity. If you’ve ever watched a jet fly across the sky, they don’t look like they’re really moving all that fast. However, if you were 35 feet away rather than 35,000, it’d zoom past you in the blink of an eye.
In addition to what others have said, they really are slower, at least acceleration wise. A missile is trying blow something up and get there as quick as possible so its target can’t dodge it so it accelerates fast. A rocket is trying to carry useful, and often fragile, cargo and/or people somewhere so it accelerates slower in order to protect its contents
It has to do with distance. The surface to air missiles are filmed significantly closer to their launch than The rocket launch. We can visualize how distance plays into this by taking a stick and holding it at one end and laying it out flat. Now let’s angle the stick upwards. The middle of the stick is closer to you and as such is lower in elevation than the tip of the stick you aren’t holding. So even though the further part of the stick moved more the change in the angle you are viewing it at is the same as the middle of the stick. When we take this example to larger scales it gives off the illusion of more distant objects moving slower than they actually are.
There’s two different reasons:
Reason 1 – Size illusion. You know how a jumbo passenger plane looks like it’s going too slow to stay up? Like it should be falling but isn’t? That’s because the length of the plane is huge, so in relative plane-lengths, a fast speed doesn’t look fast. 80 meters per second is one plane-length-per-second in an 80 meter long plane. Larger objects going the same speed as smaller objects appear to be going slower even when going the same speed.
So reason 1: Because the space rocket is longer than an anti-aircraft missile, it would look slower even when going the same speed as it.
Reason 2 – Space rockets have as much fuel as they possibly can, lowering their Thrust to Weight Ratio (TWR). Weapon missiles need to get moving fast on their way to the target, so they don’t pack as much fuel weight as possible as that would slow them down.
Let’s say you have a rocket engine that could push a 100 tonne rocket at a TWR of 3. Three times its weight, meaning after you subtract out the 1G of thrust it would take just to hover, the remaining thrust is still giving you 2G’s upward. It can accelerate up twice as fast as a falling object normally accelerates down. That’s super fast.
So what if you added more fuel to use in that rocket engine? Enough to make the rocket 200 tonnes instead of 100? Well then you now only have a 1.5 TWR. The rocket is heavier so it thrusts up much slower. That’s less efficient… But… what happens after that rocket has burned off 100 tonnes of that fuel? When it’s back to the state the previous version was in with the same engine pushing 100 remaining tonnes?
Then it’s just as good at accelerating as the previous version was, but now it’s starting that state already up in the air, already with some speed going. So it can’t hurt to have added the extra 100 tonnes of fuel. Sure, the 100 additional tonnes are less efficiently used, but once they’re gone you’re back to the same state again, with 100 tonnes left, but now you start in that state after having gotten a bit further along.
It’s always worth it to design more fuel into the rocket even though it will be used inefficiently at first, up to the point where the launch is so slow the rocket doesn’t really work at all. As long as it’s able to accelerate up a *little* when it starts, there’s no harm in the extra fuel weight because that weight will eventually burn away and you’ll be right back to the efficient thrust the lighter version of the rocket with less fuel had on the ground.
Therefore typical space launches have a TWR of about 1.2 to 1.4 off the pad. As long as a quick release isn’t needed to hit a moving target, you may as well have the extra fuel that bogs you down at first.
Also, there is actually some value in a slower launch, since it’s the turning the rocket does in the first 20 seconds of flight or so that really locks in the aim for what the rest of the launch is like, and that turn is easier to accomplish if the rocket isn’t going too fast yet. Once it starts going fast you need to have already been turned down a bit or it will be too hard to forcibly turn it. Momentum and aerodynamics will be forcing it to go straight into the “wind”, and deviating too far from that will induce extra drag and possibly break the rocket walls from the forces pushing sideways on it.
So a low TWR at launch makes it easier to get the aim “dialed in” before the real high speed stuff starts and the rocket’s light shell is too fragile to forcibly turn it into the wind. (unlike a missile where it’s worth it to build it strong so it can course correct at high speed without tearing itself apart.)
Some (not all) space agencies are able to slow time down when recording, that’s why some of them look a little slower.
A surface to air missile doesn’t need to go to space, so it has a relatively short burn. As you burn fuel the remaining vehicle and fuel is lighter, so acceleration increases (for the same amount of thrust).
A space-bound rocket is designed to have a liftoff thrust to weight to just get off the pad when full. As it burns and gets lighter the thrust to weight increases. For sending humans up the maximum acceleration is limited to typically 3 G.
A missile doesn’t need that limit.
A lot of it has to do with mass but, If spaceflight rockets accelerated like military rockets did, it would kill the human occupants.
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