No it wouldnt be possible with hobby grade stuff. You need alloys and thermal protections strong enough to resist the flight loads of hypersonic speed (search aerothermodynamics on wikipedia), also you need the correct type of fuel and geometry of the engine and airframe of the plane, which isnt quite something you eyeball out of foam and it has to be air launched from a larger plane and use a rather beefy booster engine to get it between mach 3 and 5 so the ramjet itself can ignite and sustain power.
Also the SR-71 engines aren't quite ramjets, those are turbojets with the afterburners producing most of the thrust at high speed like a stato-reactor. Also its speed was under that of typical ramjet powered aircrafts.
Unless you work at NASA or some stuff like that you wont get to play with ramjet planes.
There are 2 different types of RAM jets. The first is the original design of an internally sculptured hollow tube and the second is the renamed SCRAM jet which is designed to use a similar hollow tube design but with the use of shock cones to slow/adjust the airflow to allow the combustion to occur reliably.
With the subsonic RAM jets which would be easiest to build the real problem is that the molecules of the air travel at around 900 MPH and in order to be able to raise the internal air pressure to a high enough pressure so that a directional combustion can be maintained the air entering the engine MUST be travelling at a speed greater than 300MPH or the engine will not start and no thrust will be generated.
As we are all aware we can model almost anything BUT we cannot scale air! The 300MPH requirement will always exist as it is outside of any possible engine design.
As for the SCRAM jets on the SR71 I was informed of a different set of performance figures and have read the pilot reports of what happened when one of the SCRAM jets lost its thrust/flamed out. Not pleasant!
If someone is determined to play with the fire belching monsters that jet engine engines are in models I think the simplest jet engines that would be applicable and even possibly able to be made in the home workshop, (if skilled enough), would be a pulse jet.
At one time in the 70's you could buy a pulse jet for models but the exhaust flame was multiples of the engine length and both the noise and the temperatures were extreme!
I've seen a fuel turbine RC jet on YouTube go 451 miles per hour. Technically, that would fulfill the 300 mph start-up requirement for a ramjet engine, so it looks feasible. It would be large, expensive, and dangerous. We are talking fiberglass and reaction engines, not electric motors and foam board. The end result would not be a toy, and would best be done with lots of algorithms and fail safes in a proper UAV setup rather than just a flying model you can take to the park.
RC Jet travelling 400+ MPH is not the problem! When trying to fire up the RAM Jet you must remember that the engine itself is nothing but a severe aerodynamic drag until it lights and starts producing thrust! With such additional aerodynamic drag the speed quoted may not be achievable!
I have not heard of any SR71 performance figures for a take off without using the A/B.
As for construction materials the hollow tube used as the compressor and combustion chamber on a RAM Jet does not have any oil circulation or any other method of removing heat from the engine casing. Fibreglass and such composites would need to be thermally isolated from the engine or a flight similar to a meteorite would be the result.
This insulation adds additional weight which would again reduce speed of the test model. For a decent attempt the model due to the requirements of the engine would not be a small or lightweight.
The only actual military use I know of for a subsonic RAM jet engine was for range extension on early ground based missile systems!
The engine would be used in a carbon fiber plane reneforced with carbon steel. Yes it would indeed be possible and again be very expencive, the ram jet has to have exact measurments to work and be efficant enough. I do know what you guyes are talking about and I have done my research. Aswell this an RC!!! It would never be able to get to 50000ft and to mach 3, let alone the SR-71 was only using one engine during its speed record run. Also theoredicly this would work but is unlikly to be made by me or by anyone else due to the amount of money that would be flying at a few hundred MPH.
I don't really get why a ramjet would be something you're after, you could get the same thrust from a solid rocket. (and that's many times less expensive) I understand it's theoretical, but if you want to turn anything like that into a real model you certainly won't be using carbon steel and a thousand mile connection through plasma. imo a rocket plane looks cooler too.