Rocket

 

 

I’ve written the following as part of a general knowledge program for my grandchildren. Although they are working online with their schools I thought they would benefit from something entirely different. I’ve been interested in rockets and space travel all my life so the launch procedure is well known to me but it may not be apparent to the casual reader. When I was a teenager I joined the British Interplanetary Society. At that time in the late 50’s this tended to be seen as a society of cranks and weirdos .although in fact it is a fairly sober scientific group. By that time with the launch of the first space satellites it was just about gaining public credibility. This didn’t stop the Astronomer Royal of slightly earlier declaring that “space travel is bunk”. I did go to one meeting in Birmingham which described a very futuristic mission using fusion power derived from helium three. I was slightly embarrassed to ask a question in which I didn’t properly aspirate the h in helium.

In a fit of economising after I married I dropped my membership in 1965. Ironically this was just as the tempo of development culminating in the Apollo moon missions was ramping up. After just starting work I was too busy to follow all the detail although I retained my interest.

Many different types of rockets have been flown. The largest ever so far was the Saturn V for the Apollo moon missions fifty years ago. At take off this weighed about 4000 tons. To describe a rocket used a lot nowadays I have picked  the Falcon 9 by Space X. Although not entirely typical it is the most used currently.. All rockets work on the same principle. They hurl mass out very fast and by Newtons  law that reaction equals action the rocket moves in the opposite direction. The energy required to hurl out the mass is from the burning of chemical fuels. The fuel used is rocket grade kerosene and the oxidiser is liquid oxygen. The fuel consumption is very high and rocket engines burn at the rate of many tons of fuel per minute but for a short time, only a very few minutes.

It is usual for the rocket to work in stages. Thus a first stage burns all its fuel then drops away reducing the weight of the whole. Then a second stage until it to drops way and so on In this way the least weight is carried to orbit.. The Falcon 9 has two stages.

The Falcon 9 apparently gets its name from the Millennium Falcon in Star Wars followed by 9 which is its number of rocket engines. The idea is that rocket engines can fail so the remainder burn for a bit longer to compensate.

It is expensive to build complete rockets and then throw most away after staging. The Falcon 9 pioneered the recovery of the first stage for re-use., a concept which is now widely accepted. After the first stage has completed its burn it retains just enough fuel to return to earth and land.( You can watch this on Youtube ; the best view of landings is with Falcon Heavy which is 3 Falcon 9 linked together )

The Falcon 9 is big; In metres 70 tall and a cylinder of 3.7m diameter. Loaded at take off it weighs about 550 tons. Of this height the first stage occupies about three quarters. The rocket most often serves to launch  satellites. Theses satellites are housed in a fairing at a nose of the rocket. The fairing is needed to shield the payload ( satellites ) from aerodynamic forces as it accelerates through the thick lower atmosphere. These aerodynamic forces are substantial as the rocket goes supersonic ( faster than the speed of sound)

The launch sequence is roughly as follows. The engines are ignited and run for a second or so to spin up the turbopumps transferring fuel to the engines. These pumps move the fuel and oxygen very fast at a rate of many tons a minute. A rocket does not use any oxygen from the air; it carries its own on board , often in the form of liquid oxygen which is very. very cold. While the engines are starting and building up thrust the rocket is held down.

If the engines are all working properly  which is checked by computer control, the rocket hold downs are released and the rocket takes off rising vertically before gradually turning over until at very high altitude it is parallel to the earths surface. As it rises if one engine fails then the others burn slightly longer to compensate. The rocket balance is maintained by slightly altering the direction of thrust by the engines. The engines are attached by flexible mounts( gimbals) which allow this movement under electronic control.

As it rises most of the fuel in the first stage is used up and the first stage is separated  and falls back to earth under radio control to direct it back to the desired landing place. Its direction of fall is controlled by  small wings called grid fins. As it falls it first fires its engines to slow down( it’s travelling very fast ) and then just above the ground it fires its engines again and splays out landing legs held against the side of the rocket and lands.

Meantime the second stage has fired its single engine ( it only needs one being much lighter and in thinner air ) and also the fairing around the payload is jettisoned. An effort is made to recover the fairing halves but this is only sometimes successful at this time. The second stage then ascends to orbit attaining a speed of about 17000 miles per hour when the satellite separates. The second stage is also in orbit..

The whole launch process takes 6-7 minutes to orbital height and speed but the manoeuvring to achieve the precisely desired orbit can take longer. Satellites can have small engines to do this. Big orbit changes may occur before separation using the second stage engine to make multiple burns.

The payload depends on the orbit desired and many other factors but is of the order of 5 tons. As can be seen the orbital payload weight is of the order of 1% of the rocket at take off