# Do rockets leave launch pad at full thrust?

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.

• Curious if Shuttle launches also seem slow to you; they always looked like they got off the pad fast to me. – DylanSp Jan 10 at 16:54
• Shuttle's acceleration off the pad was significantly higher than some -- initial TWR was around 1.4:1 IIRC, as compared with 1.2:1 for many other launchers, so the initial acceleration would appear twice as fast as something like a Saturn V. – Russell Borogove Jan 10 at 17:05
• IIRC, for the US launches, the paradigm is "release clamps / bolts once nominal thrust is reached" - usually 100%. For Russian launches, the rocket is released and starts climbing as soon as TWR exceeds 1, and it takes a moment until the power ramps up to maximum. – SF. Jan 11 at 12:58
• You're probably used to small rockets like sounding rockets (VSB-30, etc) where the TWR can get up to 100:1, so they take off like a bat out of hell (10-100g acceleration) with short burns of 10-20 seconds. They're already starting a second stage burn before something like Atlas or Dragon even clears the tower. As rockets get larger, maintaining such a TWR is not practical. – J... Jan 11 at 16:07
• @RodneyP.Barbati: If you're gonna do that it's better to go full throttle ASAP and throttle down on reaching Mach 1. You want to fly not hover. – Joshua Jan 11 at 23:23

## 3 Answers

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)

• A pettier man than I would argue that 100% RPL is "full thrust" as per the question title, but the body of the question indeed asks about "maximum thrust", so you win a +1. – Russell Borogove Jan 11 at 18:36
• @RussellBorogove only a pettifogging pedant would point that that 109% was called Full Power Level – Organic Marble Jan 11 at 18:45
• @OrganicMarble - so they did turn it up to 11? – Jon Custer Jan 11 at 23:51
• @JonCuster almost - 10.9! – Organic Marble Jan 12 at 21:50

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.

• Not off the top of my head other than generally around 10 seconds; there are many videos of launches available on YouTube. – Russell Borogove Jan 10 at 4:25
• Worth pointing out the shuttle has throttled down to about 70% by the time it reaches main engine cutoff. This is because as the launch progresses, the vehicle gets lighter and lighter, so to prevent excessive g forces is gradually reduces thrust – Innovine Jan 10 at 12:22
• @CharlesStaats No, I was just hedging in case someone pulled an 89% propellant fraction example out of their back pocket. – Russell Borogove Jan 10 at 15:46
• Accelerating at 0.2 g is about 2 m/sec^2. The height after t seconds is nicely t^2 meters. To clear a 100m tower takes 10 seconds. – Ross Millikan Jan 10 at 16:06
• @Alex I suspect what's tripping you is that rockets are really really big, and their acceleration profile is very different from the objects you're familiar with - their initial acceleration appears relatively slow, but unlike a car, it just keeps accelerating faster and faster (until it runs out or is throttled down). – Luaan Jan 12 at 13:47

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...

• To add another interesting number, the Saturn V first stage burns for about 150 seconds, so just clearing the tower takes almost 7% of the first stage propellant. – J... Jan 11 at 16:27
• @JoeJobs Amateur rockets get nowhere near the altitude or speed required to put a satellite in orbit or send a spacecraft to outer space. So they need a lot less propellant (they usually burn for a very short time), and can have a very high thrust to weight ratio. See the example in the last paragraph: same thrust, less weight, much much quicker take off. – jcaron Jan 11 at 21:46
• @JoeJobs ... except that it would probably break from an acceleration that high. – fraxinus Jan 12 at 7:05
• @JoeJobs see en.wikipedia.org/wiki/Sprint_(missile) – GremlinWranger Jan 12 at 8:14
• @JoeJobs If Falcon 9 took off with 40:1 TWR all the satellites and crew inside would be broken or dead. – J... Jan 12 at 15:33