Jet Engine SS1 vs. Turbofan Jet Engine SS7

[Flyndaran]

Why get the Turbofan if the SS1 version is better and cheaper? At TL8+
Two Turbofans give 1G with one fuel tank giving 1 hour vs.
One Jet Engine gives 1G with one fuel tank giving 1 hour
The first costs $200K times 2 at SM+5 vs.
the other costing $300k.

[vierasmarius]

As written, you're right, there's no reason to get two turbofans instead of one jet engine. However, if you're only fitting a single engine module, the turbofan offers better fuel economy (1 fuel tank per hour instead of 1 per half-hour).

[vicky_molokh]

Originally Posted by vierasmarius

As written, you're right, there's no reason to get two turbofans instead of one jet engine. However, if you're only fitting a single engine module, the turbofan offers better fuel economy (1 fuel tank per hour instead of 1 per half-hour).

What about working at less than maximum thrust?

[Ulzgoroth]

Originally Posted by vicky_molokh

What about working at less than maximum thrust?

Or if you don't need 1G thrust, a half-size jet engine. Much like half-sized MHD turbine vs. fuel cell...

[thrash]

This may be a case where the real answer lies outside the design system. High-bypass turbofans are less efficient that straight jet engines, but they are both quieter and cooler. This leads them to be used in passenger aircraft, where the legal and public-opinion environment around airports can be as important as performance.

[malloyd]

Originally Posted by thrash

This may be a case where the real answer lies outside the design system.

That will be it. A propulsion system using an ambient fluid as reaction mass (jets, propellers, oars) has a thrust per unit energy or fuel consumption which is a function of current speed relative to the fluid. Turbofans are more fuel efficient (or higher thrust) than turbojets are more fuel efficient than ramjets but the speeds they operate at go in the other direction so for the same thrust or fuel consumption a turbofan is slower than a turbojet is slower than a ramjet. GURPS rules usually try to cope with this without complex math by his by capping the airspeed of different kinds of jet engines at different levels.

[Ulzgoroth]

Originally Posted by malloyd

That will be it. A propulsion system using an ambient fluid as reaction mass (jets, propellers, oars) has a thrust per unit energy or fuel consumption which is a function of current speed relative to the fluid. Turbofans are more fuel efficient (or higher thrust) than turbojets are more fuel efficient than ramjets but the speeds they operate at go in the other direction so for the same thrust or fuel consumption a turbofan is slower than a turbojet is slower than a ramjet. GURPS rules usually try to cope with this without complex math by his by capping the airspeed of different kinds of jet engines at different levels.

Er, that really doesn't explain why turbofans as written get the same fuel efficiency as ramjets (the standard jets are ramjets) and are twice as heavy in the bargain. The speed limit part does put in an appearance, though.