Monday, 18 June 2018

Book Review

Ignition!


I have just read "Ignition!" by John D. Clark. It's an informal history of liquid rocket propellants and while it's a great read I'd have probably enjoyed it more if I had a degree in chemistry. Some great authors send multiple characters (or generations of characters) through similar trials, or have recurring themes that affect all the characters. (I'm looking at you Hugo, Dumas, Tolstoy, etc.) In Clarke's book  you get something similar, but the characters are hard to relate to and have difficult names like C-stoff, S-stoff, N2O4, Mixed Oxides of Nitrogen, Inhibited Red Fuming Nitric Acid and Unsymmetrical Dimethyl Hydrazine. The recurring themes are eutectics, equilibrium mixtures, toxicity, corrosion, unacceptable freezing points and the tendency to explode unexpectedly.

Asimov writes a gushingly complimentary foreword and compliments Clarke's short stories handsomely. Ignition is also really fun to read, it contains some great stories, and wicked phrases.

I learned most from the chapter on rocket performance and I now have a much better understanding of Specific Impulse. It's normally defined as the thrust of the rocket (in pounds) divided by the rate that the rocket consumes fuel (in pounds per second). It comes out in seconds. I've never been particularly happy that the quality of a rocket motor is measured in time, but I have manage to justify it to myself by thinking that it's sort-of how long a rocket could hover before it runs out of fuel. Clarke shows us that it's a ridiculous measure brought about by the horrific ambiguity of force and mass in American scientific units. If you just apply Newton's Laws, common sense, and basic algebra you can see that all the fuel being consumed is coming out the back of the rocket (or there's something badly wrong with your design) and that the force applied to your rocket is the same force that's accelerating you exhaust stream.

Specific impulse becomes the velocity of the exhaust of a rocket divided by the standard acceleration due to gravity. Clarke correctly points out building a rocket whose sole purpose is to get away from earth and then expressing its exhaust velocity in terms of the gravity of earth is "parochial and extremely silly". He prefers to think of Specific Impulse as a velocity expressed in units of 32 feet (or 9.8m) and while that works well it offends my dimensional analysis sensibilities. I now think of Specific Impulse as the length of time that a stone dropped in a vacuum takes to gain the same velocity as the exhaust stream.

The book is rather dated. It doesn't cover any developments past the late 60s and it amused me when he talked about Boron Nitrate and discussed the theoretical possibility and the early research that suggests it might have a hard cubic form analogous to diamond.

Richard "8 out of 10 would read again" B

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