Oftentimes as one reads about boosters and spacecraft, one encounters the somewhat mysterious phrase specific impulse, or the equally mysterious mathematical symbol Isp. What is specific impulse, what does it mean, and how does it affect us, and should we even worry about it?
Anyone who drives an automobile is familiar with the concepts of power and fuel efficiency. Power is a measurement of how much work the car can do – how easily it will climb hills with a full load of your friends aboard, how quickly it will pass slower vehicles on the highway, and how easily it will merge onto a superhighway while towing a camp trailer. This is always measured in terms of horsepower, which allows one to make easy and useful comparisons between different cars. Fuel efficiency is also pretty easy to understand – it tells you how far you can go on one tank of gas before your car comes to a shuddering halt. This is usually measured in terms of miles per gallon or kilometers per liter, which again makes it easy for you to make useful comparisons between different cars.
Power tells you how much work your car can do; fuel efficiency tells you how long it will do said work.
How do these terms transfer to rockets? The usual way of measuring a given rocket engine’s performance is thrust. This is a measurement of how hard the rocket engine can push, how much force it can exert against the remainder of the rocket. This doesn’t say anything, however, about how long the given engine can do all that pushing, so rocket scientists will sometimes use the term total impulse when talking about the performance of a rocket. Total impulse is a measurement of how much net change in momentum that a given rocket engine with a given supply of fuel can produce. It is a measurement both of how hard the rocket engine can push and how much fuel the rocket has on board, or, roughly speaking, thrust multiplied by burn time.
But what does any of that have to do with the mysterious “specific impulse” entity? The short answer is everything.
Virtually all rocket engines work in more or less the same way. A propellant of some sort is burned or reacted in a combustion chamber. The reaction releases energy, which heats up and pressurizes the gases in the combustion chamber and causes them to blast out of the engine’s nozzle, which is how the engine develops thrust. The thrust of an engine can be determined by measuring how much fuel is blown out of the back of the engine and multiplying it by the speed at which the gases depart the engine (determining the momentum of the exhaust stream, if you like). If you want the engine to produce more thrust, you can simply burn more fuel (that is, make the engine bigger) or you can somehow make it burn hotter, which increases the velocity of the exhaust gas.
And that’s where specific impulse comes in. It tells you, in effect, how energetic any given combination of propellants will be. It predicts how much energy is released when the fuel is burned, and thus predicts how fast the exhaust gas will be moving when it leaves the engine. The higher the specific impulse, the faster the exhaust stream will be moving when it leaves the engine, and the more efficient the engine will be.
Specific impulse is usually measured in units of seconds, such as “232 sec” or “443 sec”. What does that mean? Specific impulse is measured in such a way that the answer tells you how long in seconds a given mass of propellant will produce a given thrust. You can do a little math on the answer and determine directly the exhaust velocity, since they are related. The specific impulse is fixed more by the chemical properties of the propellants than by the design of the rocket engines themselves. Barring gross errors in design, a given set of propellants will produce about the same specific impulse in any engine.
The most efficient rocket engine in the world is often taken to be the RL-10 engine in several American upper stages, which has a specific impulse of about 469 seconds. The laws of chemistry being what they are, this is about as efficient as a conventional chemical rocket engine can get. Experiments with exotic propellants produced an engine with a theoretical specific impulse of about 540 seconds, but the combination of propellants presented a veritable smorgasbord of problems, not least of which was the fact that the engine exhaust plume was ionized, which interfered with radio communications. The laws of chemistry and physics suggest that the highest feasible specific impulse a conventional chemical rocket engine can produce is about 470 seconds using the best known propellants, and that’s just that.
William Hamman - I have been professionally involved in the aerospace field for 27 years, most of that time in the linked areas of guidance, navigation and ...
