David F Mayer made some comments concerning ramjets:

>Some engineers seem to believe that specific impulse is measured in
>seconds. It is not....

A standard definition for specific impulse is:

Isp = Vex / g

where:
Isp = specific impulse
Vex = exhaust velocity relative to the vehicle
g = acceleration of gravity, 9.98 meters/sec

Note the units: Vex / g = [meters/sec] / [meters/sec^2] = seconds

Specific impulse is really exhaust velocity rescaled by a gravity constant. Exhaust velocity is physically more intuitive than specific impulse since a rocket works through conservation of momentum [mass * velocity].

>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.

I'm unaware of a turbojet motor / supersonic inlet combination working beyond Mach 3.4 [SR-71]. The first stage of the turbojet's compressor will not work in supersonic flow. The purpose of the supersonic inlet is to compress and slow down the free stream air such that the turbojet's first stage only sees subsonic air. This process also heats up the air. Mach number is the ratio of velocity divided by local speed of sound. Local speed of sound is the square root of the ratio of specific heats (also called the "isentropic exponent") times the gas constant times the absolute temperature of the gas. Note that the local speed of sound is proportional to the square root of the local gas temperature. Therefore the local Mach number for a constant velocity gas scales as the inverse of the square root of the local gas temperature. Obviously if the local gas gets too hot then the turbojet compressor blades will melt.

>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.)

There are some basic technical issues here that need explaining. There are two types of ramjets. One is the classical ramjet which is always based upon a "normal shockwave". A normal shockwave always has a supersonic Mach number upstream from the shockwave and subsonic Mach number down stream from the shockwave. The other type of ramjet is the "SCRamjet" or Supersonic Combustion Ramjet. The scramjet is always based upon an oblique shockwave. An oblique shockwave is supersonic on both sides of the shockwave. The performance of a classical ramjet seriously degrades below Mach 10. The problem with the classical ramjet is the requirement that flow behind the shockwave is subsonic. This requirement means the post shock flow becomes superheated to the point that combustion is impossible, i.e. the carbon dioxide combustion product actually decomposes into carbon monoxide and atomic oxygen. The scramjet was the reponse to this basic problem of the ramjet. However the scramjet also has two serious limitations:

1) A scramjet will not function at Mach numbers below 6. This means an alternative propulsion system is required to bring the vehicle up to the operating Mach number.
2) Combusion inside the scramjet must be near complete before the combusting gas leaves the scramjet.

Limitation 2) is what makes scramjets a nonoption for spacecraft propulsion.

Example: Assume the vehicle is travelling at significantly less than orbital velocity, 6 km/sec. Make the optimistic assumption that the combustion process requires one millisecond to complete (fuel/air mixing and chemical combustion). How far will a particle of air go before combusion is complete. Answer: 6 meters. This means your scramjet combustion duct must be at least 6 meters long from injector to exhaust nozzle [this length does not include the length of the inlet]. The scramjet duct will have to be cooled and withstand the dynamic pressure of 6 km/sec (1/2 * [local density] * velocity squared]. This means you are carrying a lot of heavy and hot metal to get to velocities that are below orbital velocity. So then the designer asks: Which weighs less?

1) A scramjet with its sub Mach 6 propulsion system and extra metal for the combuster at high Mach number.
2) A garden variety rocket attached to a liquid oxygen tank.

The answer is option 2) which is why scramjets are out-of-fashion. Years ago I "did" scramjets for a living but not anymore.

Gary