posted 09-05-2005 01:20 PM            

In issue 68 there is an excellent article written by Mr Stephen Bragg called "Reminiscences of the Blue Streak". Mr Bragg worked as a Performance Estimation engineer on the Rolls-Royce RZ2 rocket engine that was to power the Blue Streak missile and later the ELDO "Europa" launcher first stage. Mr Bragg was part of the team headed by the well known UK astronautical engineer and spaceflight champion , Mr 'Val' Cleaver.

One of the memories recorded by Mr Bragg in his fascinating article recalls "wet starts" of the RR RZ2 engine. One of the concerns of the 'Blue Streak' missile team was how to reduce shock loading on the flimsy vehicle. One of the methods was the 'wet start' routine whereby water was used to fill the thrust chamber tubes before starting. These wet starts did indeed give a relatively low rate of thrust build up and reduced shock loading on the missile. But, because the fuel mingled with the water ,the mixture did not reach the whole of the injector simultaneously and this could lead to dangerous one-sideness of the flame.. This could lead to a catastrophic high frequency 'screech' oscillation as the gases sloshed back and forth across the chamber causing enhanced heat transfer and almost instantaneous disintegration ! From then on, dry starts were the norm which allowed the fuel to distribute itself across the back of the injector plate before entering the injection holes.

Another experience recalled by Mr Bragg was that even under steady running conditions the rate of heat transfer from the combustion gases at around 3200 degrees Centigrade to the cold combustion chamber was phenomenal. The walls were partly protected by a self-renewing layer of soot deposited from the fuel-rich outer layer of gas. The injector plate was heated by gas recirculation induced by the fuel and oxygen jets. The arrangement of these jets was therefore critical and one of the areas where RR benefited from collaboration with Rocketdyne,USA

Another interesting memory recalled by Mr Bragg was when the propellants were nearly exhausted the missile would be accelerating at 12 g. For every second that cutoff was delayed the missile would travel another 150 miles ! Therefore it was necessary to get cutoff to the nearest one hundredth of a second and to estimate how much thrust the engine would produce after cutoff as the pumps ran down and the thrust chamber emptied.. A problem that RR engineers had not met on RR gas turbine aero engines.

Finally, in conversation with Mr Bragg when requesting permission to reproduce the above anecdotes from his article, he mentioned that gas brazing was used to help form the RZ2 tubes. But, in the early tests, they were getting terrible results even though they were using basically the same methods and equations that Rocketdyne were using over at Canoga Park. It was only after some deep studying and a lot head scratching that it was realised that the gas used in Britain (at that time) was Town Gas and not the natural gas that was used at Rocketdyne. Once the equation and calculations were adjusted everything was fine !

(Extracts published with kind permission of the author , Mr Stephen Bragg , and RRHT "Archive" magazine Historical Editor, Mr David Birch. The original article or these extracts are not to be reproduced without permission of myself, Mr Bragg and RRHT Editor )

Phill Parker
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