Implementing a Ramrocket for KSP

[KerikBalm]

So, one thing I've wanted in KSP for a while, is a good Ram-rocket or Air augmented rocket.

http://en.wikipedia.org/wiki/Air-augmented_rocket

The principle is simple, for the same amount of energy it takes to accelerate X kg of propellent to Y m/s, one could accelerate 4X kg of propellant to Y/2 m/s -> generating 2x the thrust for the same amount of energy.

This actually favors low exhaust velocities, pretty much the opposite of what rocket designers normally want.

In an atmosphere, you can use the atmosphere as propellant. *This is the primary reason that Jets are more efficient than rockets* - Using oxidizer in the air is an added bonus that can roughly double the effective Isp on top of the previous gains.

So a Ramrocket/Air agumented rocket burns LF and O to heat and expand the intake air... just like a normal ramjet, except it is supplying the O2 as well.

The result is a rocket that will function like a normal rocket in a vacuum, but its exhaust can be mixed with intake air to provide thrust augmentation in the atmosphere.

So how do we implement such a thing in KSP?

Option #1:

Simply make a normal rocket where the sea level Isp is higher than the vacuum Isp. Give it a high dry weight

Issues: the rocket would work equally well at any speed. In real life, you'd have a mach range, and for a ramrocket, you'd get very little thrust augentation from a standstill... like with a normal ramjet, you need to pick up some speed first before it works. - Or you need a much more complicated engine, that is much like the turbojets on an SR-71 - with compressors used at low speed, and bypassed at high speed.

Option #2: Take a rapier, cut its turbine engine Isp roughly in half, make it burn LF+O in jet mode too. Add an "IntakeAtm" (or similar) resource + intakes for it, that are flagged not to check for the presence of oxygen (so it works in any atmosphere).

Issue: Rapier Isp is constant, IRL, the ramrocket Isp would drop and drop as less air is sucked in, until its simply the vacuum Isp.

It would also pretty much never have its thrust drop below the thrust generated in vacuum (although stationary thrust in an atmosphere would be lower) - something that can't be accomplished (although one could get close) with the standard atmosphere and mach curves.

-Can we still have the jet Isps vary with altitude?

Anyone else have some suggestions for how to model a ramrocket in KSP?

[Red Iron Crown]

I believe OPT includes an air-augmented rocket, not sure on the exact details of the implementation though.