How many of you have debated fitting rockets to your car? Hmmm, thought so.
In a bit of a departure from the norm, this time I am going to wobble on about the most powerful motive force ever made. I used to think it was the legendary Saturn V rocket that powered the Apollo missions to the moon, but actually it was the Russian Energia – but that was only used experimentally and so apparently doesn’t count, so here are the facts on the Saturn V.
One thing that strikes me about it is its sheer size, at 363 feet tall its only one foot shorter than St Pauls Cathedral and only 6 foot shorter than the assembly building it was made in, a building so big that it has its own weather system.
The rocket was assembled on a mobile platform with the launch tower already fitted, the whole lot is stacked on the ‘crawler transporter’ which is still used for the space shuttle today. The transporter is, as you would expect, big and is basically an oil rig sitting on four giant bulldozers, each of the eight tracks weighs over 50 tonnes.
It wasn’t light either at about 2700 tonnes, mostly fuel, with a payload of 118 tonnes.
The choice of fuel is interesting, well it is to me anyway, the top two stages were fuelled by liquid hydrogen which has excellent power to weight ratio, but is a bit tricky to store and so only gets pumped in when everything is ready to go on the launch pad. The bottom first stage is fuelled by the much more convenient RP-1 which is basically highly refined kerosene which doesn’t leak and explode so easily but weighs more for a given power. There is no air in space so the whole lot uses liquid oxygen to create the ideal mixture. Liquid oxygen is funky stuff, if it soaks into a piece of wood you have something more explosive than dynamite. There was a technician back in the ‘70s at an American university who used to light the annual engineering department BBQ with liquid oxygen, three full sacks of charcoal would ignite, flare off and be ready to cook on in under 3 seconds. He used a bucket on the end of a long plank to deliver it but they have stopped him doing it now. In this country you need an explosives licence to have liquid oxygen, with good reason.
Anyway, back to the subject. Have you ever wondered how you steer a rocket? Well, the nozzle is moved by enormous hydraulic rams, controlled by several computers (all analogue) to keep the whole stack stable, a bit like balancing a broom upside down on the palm of your hand whilst someone lets dynamite off under your feet. Nothing to it really. (For the techy amongst us they vector the thrust angle through the centre of gravity, simples)
That initial blast comes from the first stage and lasts for 2.5 minutes, in that time the rocket has reached 42 miles high and is doing over 6100 mph, it has also got through 2000 tonnes of fuel and oxygen. That’s a rate of over 13 tonnes per second. You need a big pump, a turbo pump in fact, and they look like large jet engines hung off the side of the rocket motor, more about them later.
The first stage of the Saturn V has five Rocketdyne ‘F-1’ engines, the middle one is fixed but the other four are gimballed for steering on one mighty ball joint. When starting it up there is a little more to it than turning an ignition key, first the middle engine is fired up then opposite pairs of outer engines at 300 millisecond intervals to minimise stress on the chassis. When all five were on song they produced over 34 MN (Mega Newtons) of force which accelerated the whole lot at about 1.2g, pinning the astronauts down as if a fat man was lying on them, but this increased to 4g as the fuel load reduced so to limit this crushing force the middle engine is switched off early.
Although thrust force doesn’t really equate to power, if we use the energy released by the fuel to give you a rough idea of the magnitude of this thing it’s about 573 Giga watts or about 770 million horse power. But as I say that is not a sensible way of expressing thrust, it’s just to give you some perspective.
After the first stage finishes and separates from the rest of the rocket, momentum carries it further up to a height of 68 miles before plummeting into the Atlantic ocean, or waste disposal unit as they seem to think of it.
The next two stages are a lot less powerful, but as gravity and air drag reduce you don’t need such a big motor. The second stage produces 5MN of thrust for 6 minutes but raises the speed from 6100mph to over 15000 mph, the final third stage chucks out 1MN and raised the speed to over 25000 mph which is just enough to escape earth’s gravity and glide unpowered for three days to the moon.
These stages used liquid hydrogen as fuel which is much more efficient than RP-1, using Rocketdyne J-2 engines the second stage had five engines and the third stage only one.
More interesting is the first stage F-1 engines which are the most powerful ever put into service. The two turbo pumps that supplied fuel and liquid oxygen were powered by a gas generator providing 55 thousand bhp, it’s basically a really big turbo with a gas generator powering the turbine and the compressor doing the pumping work. The fuel pump delivered nearly a thousand litres per second and the liquid oxygen pumped out over 1500 litres per second at -184ºC. The gas input temperature to the turbine was above 816ºC so the bearings were cooled and lubricated by fuel. In fact the fuel was pumped round 178 tubes at the top of the thrust chamber (rocket nozzle) to stop the whole engine melting.
Each F-1 engine weighed just over 8 tonnes and was 5.79m high by 3.76m diameter
Which got me thinking; if you mounted one in a racing truck chassis it would accelerate at about 50g, instantly killing the driver, and after one second it would be doing about 1000 mph. Don’t try this at home kids.
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