Do rockets leave launch pad at full thrust?

Question

Rockets always seem to launch slower than I expect them for the thrust they can produce. Do they really launch off the pad at maximum thrust? Or do they launch at lower thrust until they clear the tower? I'm always surprised when I see how slow they seem to budge from the pad. I know they're massive, but still.

Answer 1

Rockets always seem to launch slower than I expect them for the thrust they can produce. Do they really launch off the pad at maximum thrust?

In most cases, yes.

For most orbital launchers, over 90% of the mass at liftoff consists of propellant. As the propellant is burned and exhausted, the mass rapidly decreases, and the rate of acceleration increases in inverse proportion. In fact, if the thrust remains constant, acceleration would become far too high over the course of an ascent from the surface to Earth orbit. Staging manages this by dropping the big first-stage engines (and empty tankage) and switching to a smaller set of engines. Some launchers shut down (e.g. Saturn IB, Saturn V) or throttle down (e.g. Falcon 9, STS) engines to limit acceleration as well.

The performance (in terms of payload to a given orbit) increases as propellant load increases, so typical launchers have a liftoff thrust-to-weight ratio in the 1.2-1.4:1 range, yielding just enough acceleration to safely clear the tower.

Answer 2

As in many things, shuttle was an exception, the answer for it is No.

At liftoff the Space Shuttle Main Engines were running at a throttle setting of 100% of their rated power level. About 4 seconds after liftoff they throttled up to 104.5%. The maximum emergency throttle setting was 109%, but this was never used in flight.

Screenshot from a Shuttle Mission Simulator run showing the commands from T-4 through the thrust bucket.

(the commands at 1:22 are a response to a failure inserted into the sim run)

Answer 3

I'm always surprised when I see how slow they seem to budge from the pad. I know they're massive, but still.

Remember that when you are in the vehicle, what you feel is acceleration, not speed, but when you are outside looking at it, what you see is mostly speed, not acceleration.

Speed obviously starts at 0, and is the the result of the integration of acceleration.

Based on thrust-to-weight ratios of 1.2 to 1.4, this results in a net acceleration of $0.2g$ to $0.4g$. Let's consider the $0.2g$ case.

Speed will then be $v=0.2g.t$. After 1 second, speed will be less than $2m/s$, about $7 km/h$. That definitely looks very slow.

But that increases linearly (if we ignore the fuel burn, for now), so 10 seconds later it reaches $70 km/h$ and a minute later it's about $420 km/h$. Starts to look a bit faster, but the rocket will have cleared the tower for quite a while already.

We can actually compute at what point the tower is cleared: the height of the rocket is another integration of speed, so $h=0.1g.t^{2}$ and $t=\sqrt{10h/g}$. For a tower height of $100 m$, the tower is cleared at about $t=10 s$ (Saturn V cleared the tower at about 12 seconds).

So 10 to 12 seconds to clear 100 m, that indeed looks very slow. But that's only the beginning, and as speed increases at least linearly with time, minutes later the rocket reaches thousands of $km/h$.

Even better, as the rocket is burning fuel, its mass reduces, while the thrust could remain constant, so the thrust-to-weight ratio and the acceleration increase. In the case of Saturn V, 135 seconds after lift-off, total acceleration (including gravity) had already increased from the initial $1.2g$ to $4g$, and it was actually capped at $4g$ by stopping one of the engines. Add to that the fact that the rocket will switch from vertical (where gravity is deducted from thrust) to nearly horizontal (where it isn't), and you moved from $0.2g$ net acceleration to $4g$ in a bit over 2 minutes!

So, all in all, a lot of thrust, but a lot of weight, so a net acceleration which is initially relatively modest, which explains the pretty "slow" lift-off.

For comparison, if we took the same Saturn V first stage, but without the upper stages, and with only 10% of its fuel capacity, we would have a thrust-to-weight ratio of about 10, so net acceleration would be $9g$, and it would clear the 100 m tower in 1.5 seconds, at which point it would already have a speed over $450 km/h$! Of course, it wouldn't go very far after that, as it would run out of propellant in less than 15 seconds...