23: Rocket thrust balancing

Getting a rocket off the ground is quite a trick. Ever tried balancing a broomstick on your finger, then lifting it up into the air? The thrust is all at the bottom, the weight is all above. Now add the growing rush of downward air as the rocket speeds up – the slightest deviation from course will be exaggerated by this wind as it catches the side of the rocket, quickly sending it tumbling off course.korolev escorting the R-7 rocket

Firework rockets get over this by the long stick that balances their relatively light weight.

American rocket pioneer Samuel Goddard put his motor at the top of an open metal frame, and the weighty fuel tank at the bottom, under a flame shield. – It didn’t work too well.

The Nazi V2 missiles used a gyroscope, a stabilising device becoming widely used in aircraft at the time. This was linked to solid graphite flaps directly in the rocket exhaust. The gyroscope would sense any deviation from the correct course and adjust the flaps to direct the exhaust flow sideways a little to counteract it, just as tiny movements of your finger keep the broomstick balanced.

This would not work for bigger rockets as the carbon flaps would just burn up. The R7 also introduces an extra problem: Instead of one motor like the V2, it uses five clusters of four motors each. Unless the thrust from each motor is exactly the same as all the others, the upward ‘push’ will be off-centre, and the rocket will start to go off course.

Keeping the thrust of all twenty engines exactly equal is not practical, but if you look at the array of rocket nozzles at the base of the R-7 in this spread, you can see the solution: Twelve smaller motors. These are ‘vernier rockets’. The nozzles of these small motors can swivel, shifting the balance of thrust under the control of the guidance system, to counteract any unevenness, and keep the rocket perfectly balanced on its finger of flame.

It really is rocket science.

This entry was posted in Making a Graphic Novel, Sergei Korolev, Vostok Rocket design, Yuri's Flying Machines. Bookmark the permalink.

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