Rocket Science is Easy

Mr. Denbeste has the first two parts up for a series speculating on characteristics of future space warfare based off the past. I have a comment on a particular off-hand comment he makes. Poor Denbeste gets deluged with letters about such tangential issues, so I'm posting it here instead.

"When the Harrier goes into 'hover' mode, the engine has to be overcranked and water is injected into the input air stream to keep the engine from blowing up. A Harrier usually carries enough water to hover for 90 seconds."

I can't say for 100%, but I believe his rationale for the water injection system is incorrect.

The Harrier is a peculiar aircraft, being able to land and takeoff vertically;these types of aircraft are called VSTOL or Vertical and Short TakeOff and Landing vehicles. VSTOL is obviously a desirable characteristic, because you don't need a huge runway from which to fly an aircraft. The airplane could be based almost anywhere; it could function like a helicopter. But there have been relatively few VSTOL aircraft; not counting helicopters, there are basically only the Harrier and Russian Yak-141 that have been built in significant numbers. There is a reason for this, and its part of why the Harrier has a water injection system, but I want to talk about some other things first.

There are several ways to achieve VSTOL ability. The engines can be tilted to provide lift, the thrust can be redirected down to provide lift or some combination of the two. The V-22 Osprey uses the first method by tilting its engines such that it is a de-facto helicopter on takeoff and landing. The Harrier uses the second method, having ducts that direct the exhaust to movable nozzles to direct the thrust down on takeoff. The Joint Strike Fighter uses something of a combination of the two. It directs part of its thrust downward by tilting to the nozzle and uses the rest of the power it generates to turn a fan at the front of the engine. This has many advantages and is a much more efficient system and part of why the Lockheed version won the JSF contract. But I'm going on a tangent to my tangent.

The Harriers direct lift system imposes some constraints on its design. It obviously has to generate enough thrust to lift its weight at takeoff, when it will weigh the most. This means it needs a large engine. The airplane also has to be controllable when taking off and landing, which forces the engine to be centrally located such that the thrust will balance the airplane. This can cause maintenance problems; the Harrier has to have its wing removed to replace the engine, for example. There is a cost associated with making an airplane VSTOL, which is why not every plane is.

Getting to my main point real soon, be patient. For any airplane you want to make the engine as small in thrust as you can get away with because it has a big effect on the cost of the engine and on how expensive it is to operate. However, as mentioned earlier, the engine must be capable of lifting the airplane at its heaviest. Like people, airplanes have a tendency to gain weight as they age, as newer, and heavier, equipment is added. If the engine was not initially designed to allow for this weight gain, then some compromise has to made (reduced payload or fuel). The other option is to increase the thrust of the engine. I'm going to give away some secrets of aerospace now.

Turns out that rocket science is pretty easy (errr at least thrust calculations). The thrust for a rocket is equal to the mass flow rate coming out the end times the exit velocity of what's coming out. That's it (though it does make numerous assumptions and ignores the chemistry involved). The basic equation for a jet engine is pretty similar; the mass flow rate coming out of the engine times the exit velocity minus the mass flow rate coming in times the velocity of the gas coming in. So there are two ways to increase the thrust of an engine, increase the exit velocity or increase the mass flow rate. Increasing the exit velocity makes the engine much noisier, the exhaust hotter, and the engine less efficient, plus it would require major changes in the engine itself. But by injecting something like say, water, into the exhaust the mass flow rate (and hence thrust) can be increased without causing other deleterious effects or forcing the engine to be changed. The disadvantage of this is that the water that is injected wastes volume and increases the weight of the vehicle, slightly decreasing performance. I'm pretty sure this is how and why the Harrier has a water injection system. It needed more thrust, but only for short periods of time, so it was cost-effective to just tack on a water injection system instead of redesigning the engine. A pretty small part, to be sure, but I thought the accompanying explanations could be helpful. And now you can tell people that you know rocket science!