I have mentioned several times the possibility of using a ramjet to send payloads into low Earth orbit. This requires reaching a speed of some 9 kilometers per second, which may be possible. I like the ramjet because it offers a far higher specific impulse than any rocket.

Specific impulse is defined as the thrust (force) produced per unit fuel consumption per unit time: In metric units this is:

newtons / (kilograms * seconds)

Since 1 newton = 1 kilogram meter / second, this gives the metric units of specific impulse as

(kilograms meters / seconds) / (kilograms * seconds) = meters / second which is the form of velocity.

[Some engineers seem to believe that specific impulse is measured in seconds. It is not. They use the specious argument that specific impulse can be computer as:
pounds of thrust / (pounds of fuel / second) = seconds, which is wrong, since the unit of mass in the US system of units is NOT the pound, but the slug = 32 pound weights.]

The specific impulse of a rocket is precisely equal to the rearward component of velocity of the exhaust. For the best propellant combination currently in use (hydrogen + oxygen), this is somewhere (depending on engine design) in the neighborhood of 4.5 - 5 kilometers / second.

The extreme importance of specific impulse in rocketry is due to the famed "rocket equation" which states that, if M0 is the original mass of the rocket, including fuel; M1 is the empty mass of the rocket after fuel is used; Vf is the final speed of the rocket (assuming it starts at zero speed); Ve is the exhaust velocity; exp() represents the exponential function (inverse to natural logarithm function); then

M0 / M1 = exp(Ve / Vf)

For example: if Ve = Vf, then Ve / Vf = 1 so that the ratio M0 / M1 = 2.71828 = exp(1)

To get to orbit requires a delta-V of some 9 kilometers per second (part is needed to gain altitude). If the exhaust velocity = specific impulse = Ve = 4.5 kilometers / second, the ratio Ve / Vf = 2 ; requiring the mass ratio to equal exp(2) = 2.71828 squared = 7.3890. That means that less than one seventh of the launch mass can make it into orbit; that is the entire rocket + payload + fuel + whatever must be less than one seventh of the takeoff mass. In practice, the actual value is somewhat less. No better chemical propellant combination has ever been used in practice.

To reach 3 times Ve, the mass ratio M0 / M1 would need to be at least 20.085; that is the fuel must compose at least 95% of the launch mass, clearly quite challenging. Step rockets are used to make low Earth orbit launches possible, but the rocket law still applies to them unaltered.

The specific impulse of an air-breathing engine can be far higher than that of a rocket. Ordinary turbojets get values in the range of perhaps 60 kilometers per second. (In this case, the specific impulse is not the same as the exhaust velocity. The reason for this is that a kilogram of fuel blasts far more than a kilogram of exhaust out the back of a turbojet, because the exhaust includes inhaled air.)

Turbojets work OK until about mach 5, above which the turbine blades enter the supersonic airflow envelope, and efficiency drops precipitiously. (Even at well above mach 1, the air is compressed by the scoop so that the compressor turbines do not experience supersonic air flow.) Once above about mach 5, the compressor turbines actually produce friction, reducing compression of air, rather than enhancing it.

Therefore, in this regime, the ramjet is actually more efficient than the turbojet. It is also far more efficient than the rocket, since it carries only fuel, not oxidizer and not all exhaust mass.

If speeds of mach 30 can be reached, a ramjet could launch a vehicle into low Earth orbit. (The orbit would need rounding by a rocket at maximum altitude.)

I believe that such a vehicle could be made provided that the materials problems were overcome. I foresee something like a tungsten-iridium-platinum superalloy for the leading edges and the combustion chamber. Properly designed metal alloys are far less brittle than ceramics, and therefore less prone to mechanical failure.

Now, follow me one step further. Suppose that a speed of mach 45 could be achieved in the upper atmosphere. The vehicle could release its payload (safe inside a suitable protective capsule) in such a way that the payload would leave not only the Earth's atmosphere, but the Earth's gravitational influence as well, and enter interplanetary space. This capsule could be launched all of the way to Mars in a single step, without any low Earth orbit phase. The launch vehicle could then change its flight path (using wings) and return to Earth for re-use without having ever left the atmosphere.

If this can be achieved, it represents a plausible future path for interplanetary exploration, far less expensive than the use of rockets.

Comments?