# Units

The past being another country, they did things differently. I have kept almost entirely to the units of the day. Thus feet and inches. 1 metre is 39 inches. Weight was used where today we would (I hope) use mass. Mass is difficult to define, but can be thought of as the inertia of a body. Weight is the force of gravity on that mass. 1 pound, abbreviated lb (derived from Latin libra), is 0.45 kg.

More annoyingly, force was also expressed in lb, meaning in this context the weight of an object with a mass of 1 lb. Modern usage takes the unit of mass as 1 kg and of force as 1 N. The force of gravity on 1 kg is 9.8 N on Earth at sea level. Thus rocket thrust was defined in pound force, sometimes abbreviated to lbf, or plain lb, which would now be written as 0.45 kg x 9.8 N = 4.4 N. A metric tonne is 1000 kg, an imperial ton is 2,240 lb. By a lucky coincidence they are almost identical.

Pressure was usually given in pounds per square inch, psi, or atmospheres. ‘Atmosphere’ is still sometimes used today under the name ‘bar’ (and by extension, the millibar or mb found on weather charts). The modern SI unit of pressure is the newton per square metre (N/m2) or pascal (Pa). 1 atmosphere or 1 bar is approximately 15 psi or 100,000 Pa (100 kPa).

Orbit heights are usually given in nautical miles above the Earth’s surface. A nautical mile is, strictly speaking, one second of arc of a Great Circle on the Earth, or around 2000 yards. A statute mile is 1,760 yards. Thus a nautical mile is a little less than 2 kilometres.

After all this, the merits of the metric system appear obvious!

Acronyms and Abbreviations

AWE – Atomic Weapons Establishment, situated in Aldermaston, Berkshire, and responsible for the development of British nuclear weapons. Now the AWRE or Atomic Weapons Research Establishment.

BND(SG) – British Nuclear Deterrent (Study Group). An ad hoc group set up to report on the deterrent, and which recommended the cancellation of Blue Streak as a military weapon.

BSE – Bristol Siddeley Engines. Armstrong Siddeley merged with Bristol to form Bristol Siddeley Engines, which was later taken over by Rolls Royce.

CGWL – Controller of Guided Weapons and Electronics. A senior post within the Ministry of Supply. The Controller was responsible for overseeing guided weapons projects.

DRPC – Defence Research and Policy Committee. A high level committee which recommended particular lines of research and development.

ELDO – European Launcher and Development Organisation.

GW Department – Guided Weapons Department at the Royal Aircraft Establishment (RAE). Responsible for missile development.

HTP – High Test Peroxide, a mixture of 85% hydrogen peroxide and 15% water.

JIC – Joint Intelligence Committee. Provides advice to the Cabinet related to security, defence and foreign affairs

OR – Operational Requirement. This was the specification for a new weapon. The requirement would be issued by the Air Staff, and the Ministry of Supply was responsible for circulating the requirement to the various aircraft manufacturers, choosing the winning design and overseeing the development of the project.

RAE – Royal Aircraft Establishment, situated in Farnborough, Hampshire, and responsible for Government-directed research into aeronautics.

RPE – Rocket Propulsion Establishment, situated in Westcott, Buckinghamshire, and responsible for research into rocket motors.

S. I. – Specific Impulse. One of the measures of rocket motor performance. The higher the S. I., the better the performance.

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