Specific impulse, etc.

One of the best rocket propellants there is is hydrolox – a combination of liquid hydrogen, the fuel, and liquid oxygen, the oxidiser. This might seem a bit unexpected because most (if not all) other fuels performing the same function are compounds, not elements – like 1,1-dimethylhydrazine.

An engine that uses hydrolox as its propellant must technically be a cryogenic engine. This is because storing and transporting gaseous hydrogen, which is its natural form at ambient temperature and pressure, is very difficult. Hydrogen has the lowest density of all elements, and thus occupies a large volume for a given mass; is difficult to pump; and reacts explosively with oxygen to form water. To use it in an engine, then, engineers typically cool it until it becomes a liquid, at -253º C, and store and move it in containers that are also constantly maintained at this temperature. The same condition applies to the oxygen used to oxidise the hydrogen in the engine’s combustion chamber; oxygen becomes liquid at -184º C. ‘Cryogenic’ today typically refers to systems that operate below -150º C or so.

The spaceflight industry goes to all this trouble for hydrogen because hydrolox affords a very high specific impulse among all rocket propellants, together with the engine designs currently in use. In technical terms, specific impulse refers to the efficiency with which the reaction mass – the propellant, in this case, liquid hydrogen plus liquid oxygen – lifts the rocket, a.k.a. creating thrust. The higher the specific impulse, the more efficiently the engine can use the reaction mass, the higher the rocket’s acceleration per unit of fuel consumed.

The concept of a specific impulse applies to jet engines as well, with a small difference. Rockets typically carry the fuel and its oxidiser; on the other hand, jet engines carry only the fuel and ‘breathe’ the oxygen from the atmosphere. So a jet engine’s reaction mass is only the fuel. In these cases, the specific impulse is directly proportional to the velocity at which the exhaust exits the engine’s nozzle. In rocket engines, the relationship is similar with a small modification.

But in both cases, the specific impulse depends on the exhaust velocity, which in turn depends on the difference between the combustion chamber pressure and the ambient pressure. The exhaust velocity is highest when the engine operates in vacuum, because this is when the difference between the combustion chamber pressure and the ambient pressure is highest. Also in both cases, the specific impulse also depends on how the engine itself works.

For example, the CE7.5 cryogenic engine that the Indian Space Research Organisation (ISRO) uses on its GSLV Mk 2 rockets has a specific impulse of 454 seconds in vacuum. The most common way to measure specific impulse is in terms of seconds – that is, “for how many seconds can one kilogram of fuel produce one newton of thrust”. (1 N = 1 kg m/s2). So this means the CE7.5 engine allows one kilogram of its reaction mass – hydrolox – to produce one newton of thrust for 454 seconds in vacuum. The CE20 engine that ISRO uses onboard its GSLV Mk 3 rockets has a specific impulse of 443 seconds in vacuum – and the Aerojet Rocketdyne RS-25 engine, which the Space Shuttle used, has a specific impulse of 453 seconds in vacuum. Yet all three engines use hydrolox as their propellant.

Jet engines typically have much higher specific impulse. For example, the GE GEnx and the Rolls Royce Trent 1000 engines, both built for the Boeing 787 Dreamliner, use aviation fuel and have specific impulses of 12,650 and 13,200 seconds, respectively.

One reason for the higher specific impulse of rocket engines that work with propellants that have liquid hydrogen as the fuel is hydrogen’s high heating value – the amount of energy released when a fixed mass of it is combusted. The theoretically highest heating value of hydrocarbon fuels is 42 megajoule per kilogram (MJ/kg) in air. The heating value of dihydrogen, or H2, is 143 MJ/kg in air. This is the highest of all known fuels, for rockets or otherwise, and is also why hydrogen has been increasingly touted as “key” to the world’s energy transition.

I wanted to see if I could break my writer’s block by writing something. This was it. Thanks for reading. 🙂

Featured image credit: NASA.