Many people confuse ramjets with pulsejets and are unaware that although both are quite simple and somewhat similar in design, they vary significantly in the way they operate and the tasks for which they are suitable.
Ramjets have no moving parts -- much like a valveless pulsejet but they operate with cotinuous combustion rather than the series of explosions that give a pulsejet its characteristic noise.
On the face of it a ramjet would seem to be the ideal kind of jet -- no spinning turbines, no vibrating valves and a seemingly ultra-simple construction.
Unfortunately there are two things that work against the ramjet:
1. Operating Speed
Unlike pulsejets and jet-turbine engines, the ramjet will not operate unless it is moving through the air at a speed of at least 400 mph.
If you attempt to start a ramjet while it's stationary or moving to slowly then it will give little or no thrust -- in fact you'll just get lazy, smokey flames billowing out both the intake and exhaust.
This is because a ramjet relies on heating a fast-moving stream of cold air as it enters the engine and then expelling that air at a higher speed out the back. Unless the engine is moving rapidly through the air there's nothing for the burning fuel to heat.
2. Fuel Consumption
As a general rule of thumb, the fuel-efficiency of an internal combustion engine is related to the compression ratio at which it operates. That is to say -- the more the air/fuel mixture is compressed before it is ignited then the more power you'll get from a given amount of fuel.
Diesel engines have a compression ratio of about 20:1, most car engines operate at compression ratios as high as 11:1, a pulsejet runs at a compression ratio of less than 2:1 and ramjets are about the same at low speeds.
As a result, the ramjet (like the pulsejet) is not a particularly fuel-efficient engine. What makes it worse is that since the ramjet needs to be operated at very high speed, it's going to have to burn a lot of fuel just to overcome the drag it creats at that speed.
Can Ramjets Be Home-built?
Yes -- it's quite possible to build a ramjet using the kind of tools you'd find in a reasonably well equipped home workshop -- but why bother?
Unless you're going to be using it at a minimum speed of 400 mph it will be nothing but a paperweight.
There are some companies that offer to sell you the plans for a ramjet and a helicopter which is powered by these jets affixed to the tips of the rotor blades.
Many people have bought these plans, few have built the engines or helicopters and, to the best of my knowledge, nobody has ever reliably flown one.
What is ignored by such designs are the problems of fuel-efficiency and the fact that the rotorblades of such a helicopter would have to be spun up to almost full RPMs before the ramjet would even operate. This would require significant power to "spin-up" the rotor system and add to the cost and complexity of such a project.
So, I'd advise readers that such projects are an interesting curiosity -- but don't expect that you will end up with a jet-powered craft capable of flying safely or reliably.
So Where Are Ramjets Actually Used?
Subsonic ramjets are seldom (if ever) used these days but more modern hybrid turbjet/ramjet engines have been used in the ultra-high speed SR71 Blackbird.
Many of the low-orbit reusable space vehicles are now considering the use of scramjets for their power-plant while still in the earth's atmosphere but even scramjets remain largely the domain of the drawing board.
Scramjets differ from ramjets insomuch as the airflow inside the engine remains supersonic -- whereas conventional ramjets slow the incoming air down to a much lower speed so that its pressure is increased and it is easier to burn regular fuels.
