Increasing tipjet economy

[Hawking]

The tipjet (ramjet version) method has been generally frowne upon due to the poor fuel economy. I'm guessing that this is due to the low speeds at which the ramjet must operate in order to keep the blade speed subsonic. At higher speeds though, ramjets become much more effient (moreso than turbojets).

My question is this: Would it be possible to spin the ramjet faster than Mach 1?

Here's the idea.

The jets are mounted to a support similar to a rotor blade, but with a completely neutral shape (aerodynamically). The hub spins the input shaft of a planetary gearbox, which then drives the lift producing rotor blades. The effect is that the rotor blades' tip speed remains subsonic, while the ramjets' speed is increased to the supersonic realm.

Thoughts? Comments? Savage beatings with a rubber hose?

-Don

[Stuart]

The tipjet (ramjet version) method has been generally frowne upon due to the poor fuel economy.
My question is this: Would it be possible to spin the ramjet faster than Mach 1?

Here's the idea.


-Don

It's possible, but you are fighting a number of problems. One is that the centripetal acceleration is a=v^2/r so that if we have a 5 meter rotor radius and Mach=1.5 (call it 450 m/sec cause I can't remember the actual number for Mach 1.5) you get a=450^2/5 = 40500 m/sec^2

Divide by 9.8 to put this in G Force terms and you get 4132 G, a quite hefty load (much less the fuel pressure).

Say your tipjet weighs 10Kg. Your non-lifting rotor must resist a force of 405000 Newtons. Given the tensile stregth of steel is about 600MPa or 600,000,000 N/m^2 this implies a minimum steel bar of .0259 meters (about an inch square). Multiply by a safety factor of 2 to get the size needed (multiply by sqr root 2 to get the doubled area) and we get .0366 m or 1.44 inches

So, the volume of the steel bar is 5X.0366X.0366 = .0067 m^3
Since steel has a specific weight of about 7.8 => 7800 kg/m^3

That means our support rotor weighs AT LEAST 52 kg, Which must be added iteratively to the calculations.

Now, I probably overstated the mass of the ram jet, but far understated the mass of the support rotor. Still, I think you are looking at a structure that weighs 200kg, plus some kind of gearbox, plus has an incredible fuel pressure head to step down.

Plus, if you lose one of those rotors, lookout baby.

[Mark]

Not a ramjet nor is it geared, but for some reason the engines mounted on poles instead of on the blades came to mind, sort of on topic in that small respect.
Mark