I was watching youtubes of Saturn V and Space Shuttle launches the other day and was wondering how they manage to get all of the rocket engines firing at the same time so the rockets don’t fall over. It turns out the rocket engines don’t all start at the same time so the whole assembly has to be held on the ground until everything is ready.
To get off the ground requires energy, you know this after doing a few jumps and your legs get tired. You’ve had to burn energy so your muscle fibres can contract and propel you upward against the 9.81ms2 acceleration that is holding you on the ground. Do that a few times and you get tired, and of course the higher you jump the more energy you use. So imagine how much energy you’d need to accelerate to more than 11km per second or about 33 times the speed of sound – that would require a big rocket, kind of like the ones I mention in this blog to reach the speed required to escape the Earth’s gravity and head off to Mars or somewhere. So if you want to put heavy stuff into space then you have to have a bigger rocket engine. How big do rocket engines get?
You can have a liquid rocket or a solid fuel rocket, they both have their benefits and negatives. Liquid rockets can have some control once they’re burning and solid fuel rockets don’t offer a lot of control when they’re burning, so good for the initial launch. The engines on the Saturn V rocket were the most powerful single chamber liquid fuelled engines ever flown and the Saturn V needed five of them as shown in this picture below from NASA:
The F-1 engine was a monster, it weighed in at about 8500kg and could generate over 1.5 million pounds of thrust, which gave Saturn V about 7.5 million pounds of thrust at sea level. Saturn V needed all of that power to get 140,000kg into Low Earth Orbit and eventually over 48,000kg to the Moon. The engines would fire for about 168 seconds at which point the rocket would be about 67km. The Space Shuttle was powered by 3 RS-25 engines and two solid rocket boosters (SRBs). Unlike the F-1 and the RS-25s, the SRBs are fuelled by a solid propellant each generating about 3,100,000 pounds of thrust at altitude (a bit less at sea level). The SRBs helped propel the shuttle to about 46km at a speed of about 5000kmh, the RS-25s and the big fuel tank did the rest of the job of getting the orbiter into the right orbit and, if going to the ISS, continued acceleration to around 27,000 kmh. The total thrust of the shuttle was about 7,000,000 pounds of thrust, so not far off what the Saturn V produced. This image from NASA shows all five engines of the Shuttle Atlantis and SRBs just after lift off:
So what I found very interesting was that with so many rocket engines all going off together there has to be a safety mechanism to stop the whole assembly falling over.
Basically what happens is that when the SRBs are fired the whole assemble pitches a bit so the SRBs are held in place with explosive bolts or it would all fall over. Once the main engines of the orbiter have fired up and are maximum power then the bolts are let go and the shuttle lifts off. The rocket has to be held on the ground until all engines have fired up and are successfully producing the thrust that is required. You wouldn’t want to let it go with only half the engines firing or the rocket would fly off in a random direction, creating all sorts of problems. The Saturn V was held to the pad with four of these (courtesy of this website):
Rockets are usually tethered with explosive bolts or big clamps. So thats how rocket assemblies don’t fall over. Imagine the huge stresses on these bolts and clamps when those massive engines are firing up to full power, and then the sudden acceleration when they are all let go.